Category Archives: Design Questions

Properties of Hat Sections

To get more room in a hull space people often want to substitute a hat section layup for a bulkhead. So how do they compare structurally?
First, ABS ORY tells me that I can use some of the hull skin in the laminate calculations. But only a flange width of 18 times the skin thickness, plus the base width of the internal part.  The skin thickness is assumed to be 0.15″.
The basic bulkhead is assumed to be an inch wide so I’m allowed to use about 4″ of the flange width. I will assume the bulkhead is 6″ tall and has 0.1″ laminate faces.

bulkhead and a slab of the hull, in section

For a hat section I assume that base is about 6″. I’ll again use a skin thickness of 0.15″, giving me about 3″ so a total flange width of 9″. I give it a 3″ wide inside flange and sides at 45 degrees. Those parts are also 0.1″ thick. I assume all parts are of equal strength.

hat section and a slab of hull, in section

The original bulkhead gave a section modulus SM of 2.1 inches to the third power (2.1 in^3).
The hat section only gives SM of 1.33 in^3. If I thicken all the hat parts to 0.13″ (4 times 17 oz biaxial) it still only gives a SM of 1.6 in^3.  5 layers of biaxial will give a SM of 2 in^3.
Conclusion. Like magical catamaran beams, hat sections take more material to achieve the same strength as a bulkhead.

Thoughts on the African Catamaran

As I was laminating the lunar lander bits on Tuesday, I got some perspective on the Earthwise ferry. I often take amazing things for granted, and I have done so with this. What Rob Smith has put together is really amazing. My 25 years of doing plywood/epoxy passenger cats were just what he needed for this venture.  He knew just where my skill set would fit in to the big picture.  And more than a boat, he has organized the entire venture where they will be built there in Africa with local materials. Showboats magazine has featured a mega-yacht with bamboo flooring and called it “sustainable”. This is the real sustainable deal.  It will be positively changing the lives of thousands of people. I’m surprised that Wooden Boat or even Pro Boat are not all over this. This is not 17′ lapstrake canoes for dilettantes. This is plywood/epoxy/core fusion creating big positive change.
I did try to interest Workboat in this a year or so ago. They were so disinterested. “If its not a tin boat built in Houma by a guy named Skeeter, keep moving.” was the sense I got.  It’s humbling to be part of this.


Elderly CM Hulls – How They Hold Up

Uber surveyor Dennis Smith got to run survey on a couple of 25 year old KHSD CM multihulls. Both had minor rot places. Both spots were inside the hulls at remote locations where epoxy was skimped. The trouble is inside, not the outside. The take-away lesson is coat inside thoughly and make sure any damage is repaired.  Cove all the joints.  Epoxy hulls still need to be ventilated.  Note they were both the $6 door skins and not the 6566 that we use now.  But coat it good inside!

Trimaran Hull Alignment

On a catamaran, you simply measure centerline to centerline front and back, and you’re there.
On a trimaran, if the amas have no angle, or if the sheer line is parallel to the water, the same.
But who wants amas with no angle and no shape on the sheer? On these, if you match centerline forward with centerline aft, your hulls will not be parallel. This slightly ancient graphic shows why. The CM Construction Manual has this and the new Composites Manual underway now will have a newer graphic of the same thing.

functional centerline

Away Laminating

We had the only two sunny days in weeks this last Friday and today. I took advantage.  Forecast is again rain for as far as we can see. The lunar lander panels were taking over the shop. They had to be epoxied and glassed so they could be moved outside. The dozen panels were arrayed in the sun all over the yard outside as they were glassed.   Laying glass did give me ideas for topics here.  Not as Zen as sailing, but close.  Back in the office tomorrow catching up on designs again.

The Most Important Thing about Installing Hardware

As I face the prospect of disassembling my trimaran, I am reminded about the most important thing to do with installing any hardware with epoxy. And I hope to find out that I kept this in mind when I built Geko. MOLD RELEASE
Every fastener set into epoxy should have already had mold release was rubbed into it and buffed. Or even green soap. New builders don’t believe it, but much of your hardware will move or be replaced over the years. And I will admit perfidy on this. The evidence is when is forced to use the battery and jumper cables to heat the fastener. Mold release!

Earthwise Ferries Maiden Voyage

Ok I’m still mumbling about the top level house on my 65′ plywood/epoxy passenger ferry being more like would be found on an 85′ cat, but I’m impressed. It sits well on the water and looks to move well. I can start breathing normally again. It should be the first of 10 vessels.  Rob is a visionary.

Amani side view
I like this with the freighter. The old and new.

this Blog gets Noticed

I always have the notion that I am the only person who ever reads this blog. Matthew tells me however that it is gaining about 20% every month. And it just got a nice write-up in the French Multihulls World magazine! Thanks everybody. I have a list of interesting upcoming topics that I will do.