PostApocalyptic Boat Building

Or….  How to Save the Money that You Need to Build a Boat During a Time of Recession; Strategies and Substitutions to save Money on your Boat Construction.

As the designer of some several hundred boat building projects, I find myself lately often being asked to advise on material substitutions and building strategy changes.  My work is entirely multihulls, but this applies to any boats that benefit by being careful about weight.

Back in the 70’s when we were building our early multihulls, they were basically hulls, beams and the hardware we scrounged on and above the deck.  Compared to multis now, they did not cost much.  We built them ourselves, probably we did not take out a loan to build them and usually did not insure them.  If you built it yourself and it crashed, you could fix it yourself.

In the go-go 90s through maybe last year, that changed.  Newly wealthy sailors used the boats as a matrix to attach cool toys to.  The value of the electronics and gadgets might have exceeded even the value of the basic boat.  These boats were more often than not owned by the bank and heavily insured.

For many sailors and sailors-to-be, that might have changed with the recent economic excitement.  What can be done now to get a boat?

First, turn off the news.  Next, put Gimmie Shelter on the player.  You’re still alive.  You need to get away and go sailing.  Maybe you can get a boat without the bank.

What is the boat like?

Sorting out what the boat will be like and what it needs to do is an important first step.  What do you really need?  Does it need to have the giant flat screen television, the air conditioner and the powerful electric generation system?  Expensive cabinets and extensive systems can vastly increase the cost of a boat. Downsizing the cost does not always mean a boat that is too small to use however, not to disparage small boats.  In multihulls, longer waterline can be less expensive than a boatload of gadgets.  That gives more safety, more payload and better speed for less money than one might first think.

Building or Buying

While there are many bargains for sale out there, they often have hidden surprises.  I maintain that you can build a boat that you could not afford to buy, with better quality than most of the ones you could buy.  For example, the boat you build will probably be out of epoxy, unlike most of the boats for sale.  That provides much better quality.

If you buy a boat, you are paying a 10% commission.  Plans cost a lot less than that.

Also, according to the tax agencies in my area, you labor is not worth a cent in boat value.  Only the materials and equipment involved in a boat you build will become its stated value.

Highest and Best Use

Conventional wisdom says that building light is costly.  If building light means carbon fiber, that is true.  If building light instead means using superior engineering to configure the materials in efficient highest and best use, it saves cost.

By now everybody knows that rounded hull shapes give better performance, better water pressure resistance, and more payload than slab sided, flat bottom boats.  Occasionally the thought creeps up that if one builds just such a slab sided, hard chine, under performing boat, you can save time and money.  For several reasons, that is not actually true.  Look at it this way.  Suppose one has a 1000 kg of stack plywood or cored sandwich to make into a pair of hulls.  The same stack of material will give a longer, stronger and more easily driven roundedhull than the dory bottom one will.  The rounded hull can have thinner skin for the same strength due to the shape advantage. As Buckminster Fuller would have said, one way gives more performances than the other.  That is highest and best use of the materials.  That is one way to save money on building a boat.

Also, a traditional slab sided dory hull requires that you build a strong-back first of all.  A rounded efficient hull built with CM (Cylinder Mold, developed ply for big boats) or another developed ply system does not need a strong-back to be built at all.

Plywood vrs. Composite

Composite materials have leapt up in price over the last few years.  I hear stories that it is the military buying up all the carbon fiber, or the windmill manufacturers buying all the epoxy and

foam, or some other reason.  Plywood has gone up also, but not by nearly as much.  At my office the sales of plywood/epoxy boats have surged recently.  A new 65’ passenger ferry prototype is being built near Seattle here using CM plywood.  It will be sent to Africa where others like it will be built using local materials where possible.

A kilo of ply/epoxy boat structure costs some $9/kg. ($4/lb.) compared to a kilo of foam/glass/epoxy structure costing some $13/kg. ($6/lb.) You can now get even more boat for the dollar in plywood than in composite.  That does involve several assumptions, but it is true.  Plywood often takes a beating at the resale market, for no good reason, but it happens.  Epoxy can successfully encapsulate and preserve a wood boat if it is designed for it.  Properly repaired after crashes, the ply/epoxy boat will last as long as a composite one.

Vacuum bagging table platen molding is recently popular as a composite quicker build.  I say that a developed ply hull is still faster to build than the heavily marketed bagging table technique, and does not require building a bagging table.  I strongly believe that a boatbuilder should be required to build as few structures that will never see the water as possible.  Developed plywood actually provides its own “bagging table”.  Unlike with the heavily publicized bagging table hull construction methods, plywood vacuum bagged into cylinder-molded hulls becomes both the hull and the bagging surface.  This is also true in a different way for the deck and cabin structures.  A ply/core/ply sandwich or a ply/core/glass sandwich can provide the same results as a bagging table, without the time and expense of building one. Its like this tilt-up system, but with ply/core/ply.

Finally, a CM mold is easily broken down and can be sold or traded to another builder. It can be carried in the back of a compact pickup.  A full-length bagging table is not easily broken down and shipped.


Plywood has gone up in price, but again, not as quickly as the materials for composite boats.  Twenty years ago using 3mm thick doorskins to build good boats was viable.  Tested samples gave bending strength of around 10,000 psi. That’s not true now.  Still, I see good quality, reasonably priced plywood for sale.  Plywood/epoxy boats will cost less than composite boats and take less time to build.

fusion between plywood and composite is often a cost saving.  I have long advocated a fusion of plywood and composite materials where they are the best solution.   Holo Holo in Hawaii has foam/glass/epoxy hulls, but her bulkheads are ply/epoxy.  She got the toughness advantages of foam/glass with the faster build time and lower cost of plywood/epoxy beams and internals.  Her cabin is a ply/core/ply sandwich that allows the house to be bagged to itself on the floor and assembled like a tilt-up building.  That is also faster and cost saving.

Cheap plywood is not a bargain.  Patching up cheap C grade plywood is not an actual cost saving, even if it had the needed strength properties.  After the knot-holes are wetted-out, filled and sanded, the cost will be as much as the grade of plywood you should have bought in the first place, not to mention the time spent.  Get that right at first.

However, inexpensive good plywood may be available from China.  One link is  They have exterior plywood for under $4 per sheet there, but I do not know if they have local vendors or what shipping costs are.


For plywood boats, epoxy resin is the only choice.  For composite boats, that could be true also.  Epoxy is much more expensive than polyester or even vinylester.  Vinylester is running about $3.40/lb at this time.  Epoxy is about double that.  Still, the lower cost might be expensive in the long run.  Polyester does not belong on any modern boat for several reasons.  I do know that a few very famous designers have declared polyester just fine for your boat, but the data does not support that.  Or you have to say what the meaning of “fine” is.  I will state that no boat worthy of the time you spend building it should use polyester resin.

You do often hear “my cat is polyester and it hasn’t cracked”. That may be true, but note. It then either has never been stressed, or it was designed with an excessive safety factor. It is then far too heavy. Note that a well designed fiberglass boat has critical parts designed with safety factors of usually not more than 5 to 1.  Polyester resin has a stretch to failure of about 1%.  E-glass has a stretch to failure of about 6%.  The result is a highly tension loaded part could become a necklace of fiber with polyester chunks on it.  More than that, in the marine environment, polyester leeches out into the water as it bonds with the water molecules.  It loses strength its entire life.  After several years it will have about 20% the strength that it started with. Also don’t forget gel coat blisters. With polyester, it is not if it will get blisters, but when it will.

Vinylester has stretch to failure and other mechanical properties similar to those of epoxy, at a lower cost.  I have had many successful boats done with vinylester.  It is tougher to get right though for a backyard builder.  The initiator and catylist proportions must be accurate to fractions of a percent of volume, and must change during the day as a function of temperature change.  With epoxy you can be, as I often say, brain dead.  If the temperature is 15 C, it’s a 5 to 1 or 2 to 1 ratio.  At 25 C, it is still 5 to 1 or 2 to 1.  Further, if the vinylester cure is not complete, it will never get better.  With epoxy, the part can be post-cured later at any time and get a complete cure.  None of this is visible, but it is real.

Vinylester has a shorter shelf-life than epoxy.  If your project gets interrupted, that can cost you if you did a bulk-buy to save money.  In a recent issue of Multihulls Magazine, Tom Pawlaknotes several other shortcomings of not using epoxy resin, including excess shrinking of the part.

I understand that all the epoxy resin sold here comes in the same tanker from China.  The various companies come up with creative hardners to mix with the same resin.  I don’t think there is much chance to get a lower resin price.  I also place a premium on vendor’s experience in creating the hardners needed for the particular jobs.  As a example of both, I just received some samples of a low cost epoxy. The parts I coated looked ok.  However, when I tried to pop the wasted epoxy out of the plastic cup the next day, the resin shattered like glass.  I would not want to take a chance on my boat shattering the same way on an impact.  The big boys may not have bargain epoxy, but they have the experience to create resin toughness that is so important in the bumpy real world.  Negotiate with them on price.

Resin Fillers

Epoxy fillers like WEST 407 make excellent fast fillet joints.  That is a phenolic (brown) microballoon product with additives to help resist slumping.  Mixing your own filleting blend with microspheres (white) will cost about 1/3 the price.  The compression strength is about 60% that of microballoons.  It does have less weight and absorbs less moisture than the balloons.

Wood powder is strong and less expensive, but very heavy and hard to sand.

I am experimenting with another product, perlite, by the brand name Dicaperl.  It looks and feels like microspheres, but costing $20 for 34lbs, costs far less.  It does not absorb moisture and sands easily.  I am using the highest grade, HP2000.  I don’t yet have any structural data on it being used as structural filler.  This tip comes from Alex Mazurkewycz building one of my 45 cats.


Spar grade spruce is the best material for structural lumber on your boat.  Other substutions have been used in the past.  Firis less expensive but heavier and tends to split more easily.  It does however have better structural properties and could result in slightly smaller stringers needed for example.  Wood like fir can be ripped into thinner strips then laminated back together to make more robust lumber.

Many kinds of lumber have been substituted for spruce by builders over the years.  By comparing modulus of rupture or an E x I equation, your designer can vett a stringer substitution.

There is a website that has properties of hundreds of woods, maybe too many,

Foam/Glass Construction

Many construction situations do require foam/glass construction.  Many good ways are possible to save construction cost; still, some ways are false savings.


Infusion is the best way to get a superior laminate product, this side of an autoclaved pre-preg.  Once the system is worked out, high quality pieces can be turned out one after the other.  Infusion does however take considerable setup time and testing of the system.  On a production run, it is the only way to go.  On a one-off boat, I’m not so sure.  Specialized core and or fabrics must be used.  If the process fails in any way, the part probably has to be scrapped.

In my opinion, wetting out and aggressively squeeging a laminate can give almost a good of a product, with better control of where the resin is or is not.  It is more grunt work during the wet out, but that goes fast.  Then, the part can be compacted with vacuum bagging after the wet-out is done.


Compared to ply/epoxy construction, the resin is an even larger component of composite construction.  The same resin criterion as noted before do apply.

Core Materials:

Core materials like CoreCell grain balsa/index.htm (SEN) or Divinycell (crosslinked PVC) are state of the art.  They are the rock stars of core and consequently are shockingly expensive.  The last time I checked, a cubic foot (0.3 cubic meter) of 100 kg foam core costs $65, bought in bulk.  That’s about $5.50 per square foot of 1” thick core.  Moreover, they might have long wait times to get.  What about alternatives?

One tool I use to compare core materials is a simple equation of shear strength in psi over weight in lbs./cubic foot.  (Shear strength/weight=x)  I call it shear specific.  For example 100 kg Divinycell gives a number of 36.  A heavier core with the same strength will give a smaller number.  There are many other considerations, but this is a good first start.  For example, honeycomb will give a higher number, but has other problems.  Corecell will give a slightly lower number as the shear number is lower, but it behaves differently and will perform as well or better.  In testing lab results with Corecell samples, the laminate always blew up before the core was significantly stressed.  That is a good outcome.

What are the alternatives to foam core?

Balsa core is less expensive than the foams above, with slightly more weight and better shear strength.  It must however be used with epoxy only and with best epoxy construction practice to keep moisture out.  There may be problems with vessel value if built with balsa core. If properly built, that should not be a problem, but that is a market situation.

Balsa does not absorb impact as well as the foams do.

Urethane foam costs much less than the rock star foams.  There are many manufacturers, but one is General Plastics Several times a year I get asked if urethane foam is a good substitute for PVC or SEN.  It is not.  To get adequate shear strength, Scott Lewett at Structural Composites tells me that the urethane core needs to have about double the density of the PVC or SEN to do the same job.  What’s more, it crumbles much more easily under repeat load.  It is called being friable.  Your boat could see a lot of repeat load.  Friable core is thus not acceptable.

Coretex, a fiber reinforced urethane foam is still weak, but it has relatively high weight.  I see no use for it in multihulls or light monohulls.

Nida Core has evolved in the last few years.  Look into the latest offerings.  The original Nida-Core is a polypropylene honeycomb used as a core in sandwich panels.  It comes covered with a non-woven polyester scrimm that is thermofused to the cells.  This covering helps bonding properties and keeps resin out of the cells.  The density is 4.8 pounds per cubic foot.  The shear strength ranges from 70 psi to 130 psi.  That gives a shear specific of 14.6 to 27.  It is available with foam filling.  I see it as being most useful for interior furnishings, but the weight is a bit high for that.  Corners cannot be easily faired into a radius.  Nidacore has superior sound deadening properties to foams or balsa of similar densities.  Again, lately they have greatly expanded their product line.

Polycore looks similar to Nida, but claims better mechanical properties.  Density is 80 kg (4.8 lb/cf) and core shear strength is 0.8 MPA or 109 psi.  It gives shear specific number of 21.8.

Plascore has a variety of honeycomb core materials.  The most useful one to boatbuilders has polypropylene core and seems to be like Nidacore or Polycore.

Styrofoam has core shear strength that is so low it is measured in psf. (pounds per square foot) not psi as the usual foams are.  Enough said.

Norcore (Raven Industries) and

A plastic honeycomb made from a variety of plastics including PEI, PC, ABS, HIPS, and SURLYN.  Available in thickness from .5 inch to 3 inches.  Strength unfortunately goes down with increased thickness.  One inch thick Norcore has a weight of 10# per cubic foot but a compressive strength of only 224 psi.    It is a very rigid “live core” and cannot be bent.  Corners cannot be easily faired into a radius. In a 1” thickness the polystyrene core weighs 9.7 lb/cf. and has shear strength of 105 psi.  In polycarbonate the core weighs 13.3 lb/cf and gives shear strength of 142 psi.  In polyprophelene the core weighs 10.68 lb/cf and gives 57 psi.  The best shear specific number is 10.67.

Nomex A hexagonal honeycomb made from DuPont aramid paper sheets.  A thermosetting resin is used to bond these sheets at the nodes.  After expanding, the block of paper is dipped in a phenolic resin.  A variety of cell sizes, core and paper thicknesses, and densities can be produced.  Flat panels can use the hexagonal cell, but doubly curved surfaces such as the side of a hull must use an over-expanded cell to conform to the curves.  While the paper appears to be very waterproof, work I have seen by the Boeing company suggests that water will condense in the cells over time.  I do not recommend honeycomb for the primary structural parts of a multihull.  I do use honeycomb for bunk boards, soles, shelves and cabinetry.  Corners cannot be easily faired into a radius.  Some tricks are required to vacuum bag glass onto bare honeycomb.

One nice product that uses honeycomb is Tricel.  It is used for interior surfaces.  It consists of thin plywood faces bonded to a honeycomb core.  Weight is very good.  See

Core Craft FRP Honeycomb

Custom fiber reinforced plastic honeycomb panels of 1″ thickness to 30″ thickness.  Panels may be flat or slightly curved and up to 12′ wide with unlimited length.  Compression strength of core can range between 100 psi and 1000 psi.  May be good for large charter boat bridgedecks.  I don’t know what price is like.

Penske Marketed as a replacement for plywood.  A glass reinforced urethane.  Now owned by Alcan, it is called PXC.  It seems to be too heavy for multihulls and light monos.  Cost is about like pvc foam.

One client is using Paulowina wood for core in place of balsa as it costs much less.  It is heavier than balsa at 15 to 19 pounds per cubic foot ( balsa averages 11 lbs/cuft), but has better structural properties.  The bending modulus is double that of balsa.  I don’t find a shear strength printed, but I’m confident it is much better than balsa.  It would need to be hot coated with a tie-coat like bare balsa must be.  I have no personal experience with it yet, but it looks very interesting.  See a starting place.

I must admit I had forgotten about eggcrateplywood.  We used to use that back in the 70s when foam was not really available.  That is where a grid of thin plywood bits are interlocked together to form a kind of cellular core.  Testing is being done to determine the best frequency and depth.  It is much more work, but far less cost than core.  Also, new considerations pop up.  All the spaces must be vented, but pests could then enter.  If the boat is traveling, customs tends to be very interested in voids that they cannot see into.  That could be harsh.

This is not intended to be an exhaustive list of cores.  It is simply a few of the possible choices.  I’m sure there are many more.  I could not make it to the last IBEX so there may be new cores that I do not yet know about.

Composite w/Stringers

Many, but not all composite parts can be redesigned to use solid laminate and composite stringers.  That substitution design must be done by your designer.


Many long, slender parts like masts, beams or boards are really best done with carbon fiber.  Have I mentioned that it is expensive?

One work-around that can actually give even better results is a hybrid part.  All the longitudinal, deflection-driven fibers must be carbon.  The off-axis fibers however are not nearly as highly loaded, depending on the part.  They often can be glass.  As an added benefit, they give the carbon part some extra resilience while not sacrificing overall stiffness.  Again, consult your designer to see if part involved can benefit from that substitution.

Be careful about yard-sale fiberglass deals.  Water damaged fabrics must be avoided.  If the binder or sizing is washed away, it might not bond properly to the resin.

I have heard about great prices for fabrics from China recently.  Do test a sample or have assurance that the binder or sizing coating on the fabric is compatible with epoxy.

Peel Ply:

I have heard designers say that dressmakers nylon can be used as peel ply to save money.  No.  That is a bad substitution.  Peel ply must be free of contaminants.  If your resin seller supplies it for example, it will surely be free of those contaminants, unlike the dressmaker’s nylon.

Scraps of 4 oz. or 6 oz. cloth make the best peel ply on compound shapes or odd corners.  Save those scraps.

Vacuum bagging and Infusion Equipment:

An excellent vacuum bagging kit can be purchased from various suppliers costing from $300 to three times that.  They have excellent pumps, nylon bagging film, and dum-dum edge sealer.  These low volume, high vacuum pressure pumps are excellent for composite laminates, especially with infusion techniques.  They start at over $300 for the pump alone typically.

You probably have a compressor already.  In that case, get a venturi vacuum pump that plugs into your compressor for just over $100.

If you are not planning to use infusion, there is an even lower cost option.  A standard shop vacuum will pull one third of an atmosphere (4 to 5 psi) and cost, well, you probably have one already.  Bagging film can be inexpensive polyethelene film, 4mil (0.004”) or 6mil (0.006”) thick.   Compare the low cost of poly. film  to nylon bagging film costing $3/yd.

The bag edges can be sealed with inexpensive storm window sealer (Frost King by Thermwell Products instead of the expensive but vulnerable high-end edge sealer products.  The joints or corners take a shot of plywood panel adhesive (Liquid Nails).

Designer Involvement:

Involving the designer can often save a lot of materials and work.  Running a finite element study on a change or a substitution can possibly save more than its cost in materials. Even a simple section property simulation on a change can cost very little from the designer, and save a lot of materials.

Typically multihulls builders don’t like to pay designers to do that sort of work, but it can save more than it costs.


Two part polyurethane paints like Awlgrip or Stirling will resist stains and look great for years.  I have walked out of the paint store carrying a box that has a thousand dollars worth of paint in it.  Are there any less costly substitutes to those top of the line paints?

First, you must keep the gloss.  It’s not a vanity, it keeps stains from building up.    In my experience one part paints will not keep gloss for more than a summer.

To save money, find an industrial paint supplier in your area and see what they have.  I learned about Jarvie (Now Farwest  paint here in Seattle when I worked for a boatyard while I was in architecture school.  For example, they sell a primer that is a fraction of the price of Awlgrip 545, yet seems to be virtually the same product.


Don’t build outdoors.  If you don’t have a shop already, there are several options.

I have built a boat at a storage complex.  The cost is quite reasonable.  Usually it must be built in components and assembled later as noted below.  Similarly, one of my builders has boatbuilding spaces available.  He is in Portland, OR, but the same kinds of spaces could be in your area.

Inflatable pool covers can be bought new for typically less than 6 months rent in a shop space.

The market is full of portable temporary buildings that can work very well.

Clubs, Craigslist, Ebay

Multihull clubs are in the wane, apparently. They used to give builders the price advantages of bulk buying. Maybe the fact that many of us have our own business licenses now and can get just as good prices contributed has contributed to the decline of clubs.

If your boat is built as a business, your tax number will get significant discounts.

Ebay and Craigslist are great places to buy things for your boat.  Don’t expect to find basic materials as often, but you can find masts, sails, hardware and the unexpected.

For price comparisons on boatbuilding products there are many distributors.  Among them Jamestown Dist., and Fiberlay

Building Components First

Many multihulls can be built in smaller components, in a smaller shop space than will be needed later on for a final, full size vessel.  The designer may need to do some plans revisions to facilitate that remote assembly.  That will save shop rent.

Miscellaneous Tips

Save paper cups from fast food or Starbucks trips.  These are especially good for mixing small batches of bog.

Buy the black foam rollers (Jen Manufacturing), not the yellow ones.  They cost a fraction of the cost of the yellow ones.  Buy them by the box.  Cut them in half for smaller jobs using the half roller.  Cut into smaller bits they can be used as foam brush substitutes.

Mold release wax rubbed onto plastic mixing buckets or squeeges helps keep epoxy from sticking to them for the next use.  That will make them last longer.

Very small epoxy batches can be measured by dribbling epoxy and hardner into a pair of properly sized adjacent circles printed on a piece of paper.  Mix the two. This trick is from J.R. Watson, presented at IBEX.

In an emergency, plastic sandwich bags can be used as gloves.

When vacuum-bagging with plywood, regular mailing bubble pak can be used as a diffuser inside  the bag.

Material waste is typically 20%. Think about how you can reduce that.

Always have another part ready for coating or bogging if too much resin or bog was mixed for the main part.


Despite bleak economic news, with some of the above tips, a reasonably priced boat can be built.  In fact, there may be no better time to negotiate the best prices on boat-building materials, services, and dare I say, even plans than now.

51 thoughts on “PostApocalyptic Boat Building”

  1. This is probably a ridiculous idea. What if you took an existing monohull, shaved off the ballest and attached amas, would this make any sense at all?

  2. The Euro Yacht of the year 2018 uses polyester resin. Is this the yacht construction equivalent of global warming denial?

    It costs around $1 million. Could you give a ballpark of your Triamaran 63 high end cost?

    1. they never tell me. all depends on how much carbon and how many goodies on it. Being simpler and more elegantly engineered that the Rapido, it should cost less. And be much faster.

  3. Sr, after read this i can think that plywood is a good core material for a fiberglass/ vinyl ester hull, will no need core on small boats, just wood as frame and fiberglass lay up. core will be need on boat`s that need be ligt and strongh?
    if i will build a 23 ft cat,will not really need core for the hull, as the sides may be will be 1/4 tick

  4. I was following the cat you designed that was being built in Washington to be sent to Africa. they mad mention on there web site you or they were going to design/build a narrower version. I was wondering if they built it or were going to build it in seattle area as I would like to go see it . it sounds like the boat iam wanting…kevin

  5. iam very interested in the passenger cat you said was being built up here in the seattle area to go to Africa. iam very interested in the builder and where to get pics and or design info kevin 360-301-2220

  6. Nice post. I learn something totally new and challenging on blogs I stumbleupon everyday.
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  8. Very interesting article. Thanks for publishing it. I just wanted to underline the advantage that can be delivered by applying fiberglass, what most of the boatbuilders naturally do nowadays. In their latest issue the Professional Boatbuilder Magazine e.g. describes all the advantages, and furthermore mentions the exact mold lamination schedule with vinylester resin. I outlined their article a little bit here

  9. thanks Kurt for the quick reply… (started the company stateside with Andrew) & did all the fiberglass work for Simpson/Wild in Kemah Texas back in 1975,1976… (12)-thirty two foot tri’s (Shifters) and (2) forty five foot cats (Tropicat)
    have recently purchased one of your designs a 44′ Catamaran in Fredonia Wisconsin…
    looking forward to the build out on it… just got the plans sent to me, their dated back to 92′ 93’…
    I’m sure you knew the original builder, the workmanship looks excellent…
    Looking forward to being a part of the “Team Hughes Multihull Family”…
    after my time with Simpson/Wild… moved to Hawaii for 25 years,then Missiisssippi for the past ten years on the river… Looking to finish up the Cat, in Wisconsin (about 85% complete as she sits)
    your current cabin lines look so much more streamlined… may make some modifications so as to up date it…my e addy is (…
    back in those days “multihulls” were a dirty word in most sailing circles… but no one could deny “performance reigned supreme”… your designs an excelelnt extention of what true multihull should be

  10. hey Kurt,
    excellent blog… excellent… have you ever had any business associations with Andrew Simpson of Simpson/Wild Multihulls?

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  12. When I built my KHSD23, I built it for 3500 dollars for the boat, excluding rig (1K), trailer, shop space, and motor. I sometimes think about it again, and feel I could do it for that again. Epoxy costs number for number are either the same, or down, over 34 years. Cloth is maybe up a little, but there are great deals. Plywood is more expensive, but when I built the skins were cheap, and the bulkheads were expensive. The reverse is true today I would have to buy fancy skins but there is great exterior grade foreign ply at half what I paid. I have my own place, so that would save 350 a month I had to pay on rent for 6 months.

    When I built my boat I was sceptical about some of the details on the set. But over time I am a believer on all of it. I mean, I never take a double berth too seriously on any multihull set 🙂 One thing I can say for the new set is that it is a far better plan, so for your 3500 dollars, or maybe a little more, you will get an even more beautiful boat.

    The best way to save money is to look at how we did stuff back in the day, and not everyone build a 40 foot cat. A lot of folks I know who built huge boats over long periods and went out cruising, had sold the boats within a few years. Small boats are a lot easier to deal with, and more fun to sail. The plans are worth every penny.

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  14. Kurt,

    I have been interested in CM construction for some time…just seems logical, and agree that plywood is a logical material for low cost, expedient, strong construction. I am currently building a cold-molded 20-foot trimaran here in the Philippines, where I have dropped the hook or, perhaps, swallowed the anchor. Local 3mm “marine ply” costs about $11 a sheet. 5mm is only slightly more. Local epoxy is readily available from $25-50 a gallon, depending on type. And labor runs around $5-6 dollars a day. So, when someone expressed an interest in my old Morgan 461 “mothership”, I realized that perhaps it was time to move on and finally pursue an idea I have had: a modest catamaran with easily-driven hulls that could be pushed in no wind conditions by a small four-stroke outboard, have lots of solar, but with a honda generator as backup. Now, in a monohull, carrying gasoline is dangerous, even on deck. But, gasoline in a compartment on the bridge deck designed for it (venting to air, over the water) would be safe. I recently repowered my leadmine, and it cost nearly $20,000 all told, and required a month of my life in a tropical boatyard. In contrast, 4-stroke outboards and generators are inexpensive, easily worked on, and very efficient when combined with easily-driven hulls (less likely to need it in the first place, and will use very little fuel when you do). The whole idea is to return to the simplicity of the boats I sailed when younger, with no inboard, light weight, minimal draft (I remember jumping off in the NC sounds to push her off if I found a sandbar!) and fewer systems, while maintaining the comfort and space I have gotten use to in my older age. I do not need 4 staterooms, and absolutely do not want 4 heads!!!

    So, after looking through your designs yet again (not to mention dozens of other designers) I think the 36 foot CM bridge deck design is the most suitable for my ideas. Here are 3 better photos of Bagalut, the boat you picture on your site: (scroll down a bit)

    So, to the point (besides sharing photos of a beautiful modest cat): How many layers of 3mm goes into the hull of this design?

    Is there a designed method for mounting the outboard(s)? Single or double? I use to captain a 40′ daysail cat for 32 passengers that used a single outboard on a sled that was steerable, and found it remarkably maneuverable as I daily went to several docks for loading and offloading passengers. There is a learning curve to deal with two steering controls (one for the rudders, one for the motor), but once over that, it is such a good system I can not imagine wanting anything else on a small cat).

    What is the fuel and water capacity for this design?

    Last question: modern coring materials are difficult at best to acquire here in the Philippines, except that a type called “Coremat” is available. Is this suitable for the cored aspects of the deck house in this design?



    1. Doug,

      I finally got chance to catch up on this.
      Wow thanks for the photos. I had no idea.
      Hulls are (3)3mm. Can be varations on that too with more fabric.
      outboard swing arms are nice. on the transom seems to survive the salt the best.
      Everybody has their own idea on fuel and water. I never declare that.
      Coremat is pretty heavy and thin. Track down some core from china.
      Or even balsa for the house sandwich.
      can i put the pics on the blog?
      Hit me on the regular email and I see it sooner.


  15. whoah this blog is wonderful i love studying your articles. Keep up the good paintings! You realize, a lot of people are looking round for this information, you can aid them greatly.

  16. . First let me say that I have been involved in building many very light weight insanely expensive carbon boats. Although I do love the techy stuff dearly, building a strong relatively lightweight structure with ordinary products in a simple shop has always seemed like just plain fun.
    As a pro builder I have taken part in using the CM method. One of the things that struck me about its use was that as the hulls got larger it seemed to get increasingly difficult to maintain shape accuracy. This began to be a problem at around 40 ft. I see now that you are building much larger hulls with the method. Have you developed methods that have helped manhandle and control the hull parts? (I mean besides armies of men). I intend in the near future to build for myself and would certainly consider a design of yours for many reasons, bang for buck ratio being one of the biggest. However, considering the size hulls I have in mind, it concerns me that I might struggle with controlling shape to to my satisfaction. I realize that I cannot expect to get the kind of accuracy we would get from CNC cut molds. Can you give me some sense of how close to design shape a careful builder can get? Lets say on a 55 to 60 ft hull

    1. Dennis,
      It seems like the guys with 55′ to 65′ CM hulls are managing a great hulls shape and pretty close to what I expect. Tricks have been evolving all the time. If you near Seattle you should see the Earthwise builders doing the 65′. On their website they have more process pictures than you can stand.

  17. Kurt,
    Thanks for opening a blog.
    Which kind of plywoods are suitable for CM? What are the properties that one should look for? E.g. what is the max. weight of sq/ft of ply?

    BTW. on the website one can find boatbuilding blog of F39 folding trimaran. Complete work took over 6000 man-hours and 200 000 EUR to cost.
    What would be the time and cost of building comparable boat of Your design using CM?


    1. BS (british standard) 6566 is an example of a good plywood to use. Ply epoxy is slightly less cost compared to epoxy composite.

  18. So impressed with your knowledge Kurt. I have admired your designs since the beginning(s), and especially appreciate your mantra to provide the best quality for the least investment.

    Material choice is always unfortunately related to marketing hype as you suggest. I completely agree with you about the benefits of plywood, and will always perfer it as my favorite building material due to its incredible shear strength and a personal comfort level in being able to see the grain in a natural product vs the plastic fantastic.

    In my 60’s now, and a packaging engineer by trade, I started delivery crewing as a teenager, and had had enough of mono’s lethargic motion by the late 60’s. I raced a cat across the Pacific in the solo transpac around 1970 and was hooked.

    Now I dig around the world for abandoned projects that I can pick up as donations or on the cheap, get them seaworthy enough to transit to Costa Rica, and there we fix them up and put them to work in dive/snorkel/beach business to support my children’s charity (

    Questions of the day are:
    1. Do you have any comparisons of structural/seaworthy for heavy sea sailing between cats and tri’s.
    2, Any input on square foot of usable space in interiors, cats vs tri’s.
    3. Comments on electric propulsion.

    As most of my boats date from the 70’s when the world belonged to Hortsman/Cross/Piver, I have limited experience with Hughes/Prout/Fontaine at sea.

    1. Frank,

      Thanks. After I survived the Bass Strait in a gale, I asked Lock Crowther if we were in any danger there. As far as I know, he did the only tank testing of multis in beam-on waves. Cats better in that, but tris better in high wind survival. In nutshell.
      Usable space is all in how you use it. Tri seems to have less floor for same size, but with flare, it still has space that is not floor. It’s really how you feel about the space.


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