Marine Engine Beds and Structure Bourne Noise

Some 15 years ago I attended a lecture by acoustic expert Joseph Smullin on structure bourne noise and how to attenuate it. Our light multihulls are particularly vulnerable to that. Off top of my head, this is what I remember.
As I recall, the features needed to attenuate the noise include, more core weight, more laminate elasticity, add cross structure in the direction of engine rotation, and isolation of the engine and beds from the hull.
I revised my engine bed drawings to take advantage of that then new information. To best of my knowledge, none of my builders until now has ever followed my plans. It doesn’t seem to matter if the builders are highly regarded like Schooner Creek or the inept like Shaw, they slam in a pair of wood 4 x 4s and call it done.
Use minimum 100 kg foam. Use the more elastic type foam. The cross bracing is critical to reducing the noise. Joe preferred the engine beds not to sit directly on the hull, but on a layer of elastic like trevera. I expect that hull bonding adhesive would work as well. This engine bed from the Arteran Catamaran builders in Estonia look great.  OK, I’d like to see a bit more cross connection, but these are the best so far.

motor mount for the Arteran powercat

Edge Seal Roundup

There are many ways to seal the edges of a vacuum bag. A survey:
SEALANT TAPE AT200Y YELLOW from Fiberlay works very well, but is one-time only. I could never get it off the bag so the bag was damaged getting the part out.
PVC pipe snap bead. I recall seeing this in the Gougeon’s magazine. A pvc pipe is ripped and then snapped over another, with the bag plastic in between. I resisted that way as the ripping is one more job to do.
Storm Window Bead. I prefer the inexpensive storm window sealer (Frost King by Thermwell Products http://beatlas.com)1-800-299-5700.  They are inexpensive, but the shipping is expensive.  Maybe I should keep a few hundred here in the office to go out with the plots.
Rope Caulk. I understand some have used rope caulk, but I could never get it to work well in the cold, which is most of the year here.
Stick. Matthew Reynolds uses a stick or pvc pipe on the bag end and rolls the bag around it. He then clamps the roll off.   Along the long side he uses mailing tape with good results. The bag has to be even and smooth for that.
A guy could use a few tubes of Liquid Nails each bagging, but it takes some time to squirt out.
The joints or corners on all the systems take a shot of plywood panel adhesive (Liquid Nails) to seal.

Please add any others people have used with success.

I can see a published technical paper here with the results including vacuum hold duration comparisons and cost comparisons.

Metric

Virtually all new work is done in metric now. The work is created real world size and then plotted at some scale, metric or not. However there are some exceptions. I recall that some 15 years or so ago the US Coast Guard declared that all work submitted to them for review must be in metric. Within a month or so, the plans reviewers had told me that any plans submitted in metric would be returned without any action.

So, on the chance that a plan set is different than you prefer, I propose buying a dual format tape measure and this website, http://www.hocltd.com/metricalc.html
Again, most of the new work is metric and most of it has dual dimensions.

Latest

Between rush deadlines and supporting some 50 ongoing projects, the blog has fallen behind. As soon as I can surface for air, some of the topics will be:
Revisiting Post-Apocalyptic Boat Building.
Always new projects submitted.
Latest on sealing the vacuum bag.
Looking into PET foam.
The greater Seattle area has about a dozen relatively exotic KHSD multis building and sailing- lets look.
The cat projects in Africa and Estonia are ongoing.
I should catch up soon.

Plywood/Glass strength Comparison

How strong is glass compared to plywood? Here is a table comparing strengths with out of plane loads (like water pressure).

Plywood/Glass Thickness Conversion comparing bending strength. (Assumed 60,000 psi bending strength for glass and 10,000 for plywood)
Assumed that triaxial amount is on both sides of a core

Ply thickness Triaxial thickness Core

3mm ply equals     (12 oz)  400 gsm w/12 mm core

4mm ply equals     (17 oz) 600 gsm w/12 mm core

6mm ply equals      (22 oz ) 750 gsm w/12 mm core

9mm ply equals      (34 oz)   1150 gsm w/19 mm core

12mm ply equals   (2) (22 oz ) 1500 gsm w/19 mm core

USCG certification

I understand that the USCG stability rules are catching up with the increasing girth of Americans. The official passenger weight for COI boats is to go up from 160 lbs to 180 lbs as I understand it. For monohulls, that can be a crisis. For catamarans, it will not actually change the stability at all. I expect that everybody will need to revise their stabilty calculations and stability letters. I have spreadsheets ready and can do that for everyone who needs it.
Related, I can’t believe that with all the budget cuts, a plans submittal and inspections for a COI vessel is still free. Where else can you get a building permit for free? I can’t imagine it will be free for long. Submit those COI plans soon.

A little hull layup…

Upgraded the video on this a bit: The layup on a 68′ cylinder molded hull currently under construction in Roatan: it’s speeded up quite a bit, this is about 25 minutes of real time. the half hull panels are approximately 11′ x 71′ – 4 layers of 3mm plywood and 33 gallons of epoxy.

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Kurt Hughes on Catamarans, Trimarans, and Boat Design