Category Archives: Design Questions

Alex’s Unstayed Mast

Its kind of a mini Sarabi.
We just got the bids in on the new carbon fiber unstayed mast for Alex’s KHSD 45 catamaran being built in Blaine, WA. It is what we call an aeroesque mast. Same general idea as the aerorig, but updated.
I’m sure the safety of these masts will make them the future of USCG certified sailing cats. The Coasties just need to see the FEA study to accept them.

aeroesque mast on 45 cat

“Magical” Catamaran Beams

Every couple of years I have someone ask me why my cat designs need to have aft beams. This occured again last month. They seem to have the tone that there must be a structural secret I just have not figured out yet. I did just get this picture. “Why can’t you do a beam like this? Ask them how they did it.”

"why can't you do a beam like this?"

People. Remember where I always say “with enough carbon and enough money, you can probably do anything you want to.”? There are limits. They probably think a sleek cat beam looks like this.

they think the beam looks like this

It actually looks more like this.  The boat salesmen hide a beam like this so it will look thin or even gone.

what the beam really looks like

That is compared to a standard beam. Like this…

the standard beam

So why don’t I catch up and do the flat beam? Remember how I always talk about efficient cats? Look at this.

The tools we designers use to compare beams are the moment of inertia “I” and section modulus “SM”.  (assuming shear is satisfied) Suppose the regular beam is 12″ x 36″ by 0.1″ thick laminate.  That regular beam has a SM of 99 in^3 (inches to third power) and a moment of inertia I of 2104 in^4.

In the horizontal mode beam, it only has a SM of 44 and the I is only 272 in^4. To get more strength and stiffness it must be made thicker and heavier.  Even if the horizontal beam laminate is made 7 times thicker and 7 times heavier, it only has the same strength as the regular beam  and still only a third of the regular beam’s stiffness. The flat beam boat vendors hope you don’t know or care enough to appreciate the difference.  They can’t claim efficiency and still have horizontal beams.

Soft Board Control

I have usually always used blocks bolted to boards to allow the hold-down lines or the lift lines to function. In the same way that spectra wraps now can replace a track and car on the boom, I propose similar for a board. On a smaller board you can use a Harken C 8882. On a slightly bigger board you could use that part cut down the center and attached to both sides of the board. On big boards, I’m experimenting with a faired hole using high strength bog. I will see if teflon coating or graphite coating helps. I assume it will not work quite a well as a block, but be much cleaner.

an off-the-shelf fairlead for the board
create your own board control fairlead

New Pictures of JD’s 30 cat.

Some new pictures of Jim Dory’s 30′ cat have appeared at

This boat was built and launched near Nome, AK. After it was launched, it was tied up in the harbor. During the night, in a gale, that barge broke loose and came to rest against it. There was damage, as you can see. But try to wrap your mind around how little damage actually occured. It was the fender for a barge during a gale.

JD's 30' cat

KHSD Lake Victoria Ferry Launched

Earthwise Ventures launched unit #1 on Lake Victoria recently. The KHSD designed ferry is the first of 10 to be built. They have cylinder molded plywood/epoxy hulls and ply/core/ply epoxy house structure. This boat was built here near Seattle, disassembled and shipped in containers to Africa.
I’m pleased that it sits on its lines OK. They made the top house larger than designed, and were not concerned at all about weight. Unit #2 is underway and will be more elegant. The rest of the units will be built in Africa.

ferry launch

Quick Boatsheds

I bought one of these sheds last year for $200. Moto-Shade.  10′ x 20′.

At that website they list stores where you can buy it. The UV breaks the plastic fabric down in less than a year, but tech there told me the trick is to get a new one under the 1 year warranty before the year is up. 

Its not big enough for a complete multihull, but many components can be built in one before the multi gets too big.

inexpensive boatshed

USCG 185 lb. Passenger Rule

I am learning more about the new USCG passenger weight and how it affects your stability letters. The changes are on:
170.170 metacentric height 

170.173 righting moment
171.57 intact stability
170.050 passengers heel
Boats with more than 49 passengers will also need their damage stability studies updated.

The added passenger weight does make most single hull vessels less stable.
It does however makes a catamaran more stable. Typically when I submit the stability calculations to MSC, I do the worst case; lightship. Lightship has no passengers included. The extra passenger weight makes the catamaran more stable. MSC understands that but advises that local offices may not.
So I have to run all the spreadsheets for both lightship and fully loaded with 185 lb passengers. And the attendant graphics illustrating what I’m doing. It looks like I can do the lot for 2 hours per vessel. I’m not sure yet how much time revising damage stability will take. I do need a copy of your stability letter and stamped sail plan (unless I already have them. I do have most.)
Due date is December 2011, but don’t wait until its too late.