Trimaran basic questions

Kudzu,

Do not give up on the skin-on-frame concept for a tri. traditional kayaks are designed to be flexible on purpose because they behave better when they have a little flex, like the suspention on a car. What you want to do to accomodate the higher loads is build the structure more like a truss, you will get a lot more stiffness than just the added stiffness of just the stringers. if you use plywood use them to make bulkheads, these will be very stiff and strong.

By adding diagonal bracing between the gunwale and the chine stringers for example, you make the hull very stiff (use small plywood gussets to attache them). Also by adding extra diagonal bracing at the location of the cross beams, rig attached points, etc. you will make it plenty strong. As far as skin goes, use the heavier weight skin (12 oz or more) and space the stringers a closer than you normally would on a kayak, this will support the skin so it will not distort under the higher speeds of a multi-hull sailboat.

IT will be a lot lighter and cost less in materials than if you resort to all plywood construction. Exploring skin on frame construction for a multihull is one of the things I would like to try some time. I have built a dozen or so skin-on-frame kayaks, and three skin-on-frame sailing dinghys. I would like to see the concept explored for larger and faster boats, and I will do that some day.

 

Petros,

Great to hear that there is a solution.
Please explain how you would suggest taking the torsion load from the rudder.

I seriously doubt that heavier skin would make a difference given the very small deflection difference between unskinned and skinned I quoted above and that was with a very light load - 50#.

 

I am still playing with the idea and tossing around ideas. If I went with a flat bottom or v-hull I could always add a thin layer of plywood on the bottom. Once bonded to the frame and that would/should add a lot of stiffness and spread out the load. That is part of why I was asking about flat bottoms.

I have also been thinking more about Plat Monforts's methods of using Kevlar roving on the frame. I have never been a fan of his methods of building so light it had to be reinforced like that. But I did a little looking at it again and I am tossing around ideas of using it. I can't think of better way to reinforce the frame and add virtually no weight to it.

I needed a stand to display kayaks at a show and racked my brain for a simple stand, quick to set up and light weight. Finally came up with the idea of a slip together stand that was stabilized with a couple of nylon cords in an X pattern and was amazed at how well it worked out. Same principle as on Plat's boats.

Here is a image of the design I have been working on. Lots of unknowns and things I don't like. But it is a work in progress. Plus I just started restoring a old Chris Craft Sea Skiff that is going to keep me tied up till spring and probably into the summer.

 

Upchurchmr,

the skin does not stiffen the structure at all, it just holds the water out. At higher speeds it can pucker and ripple unless it is stiff enough, so that is the reason for the heavier skin. Torsion loads can be taken by a boxy truss structure, consider say a truss type radio tower, it is four truses with the corners as common cords. Torsion loads are carried by the diagonal members of the box, the vertical members (corner cords) take tention and copression loads and single axis bending.

Now if you envision a skin-on-frame as 4 truses, tapered at the ends, and with bulk heads or built-up frames about every 24 inches, the torsion loads are again taken by the diagonal members in the sides and bottom. Another approach is to use what is known as geodedic construction, used on some wood aircraft (google it). where flexible stringers are affixed diagonally on the inside of the stringers in both directions at about 6 or 8 inche spacing.

the rudder is not really where the large torsion loads would come from anyway, but I would think the mast and rigging would want to twist the center hull against the beams.

In either case you design a truss type frame by determining all of the forces on the hull, and than design frame details to accomodate those forces until they are spread out far enough so the basic frame is not overloaded. This what how all of the early "tube and rag" aircraft were built, many of the early wood truss designed wings and fuselagues are some of the lightest weight per square foot of area ever built, even lighter than modern (and more costly) high tech composites. No reason the same approach could not be done a a boat hull.

 

Petros,

I'll be interested to see your multihull SOF.
Your response is that the rudder doesn't have large torsion loads. You and I have not sailed the same boats.

You certainly can make diagonal truss members instead of typical SOF kayak style construction (I am thinking Yost/ Kudzu or Greenland/ Aleutian as described by Morris). To get a good torsional load path you will need to get trusswork on the top and bottom of the hull.

One of the reasons early wood truss wings were so light is that they were lightly loaded. When they got higher loads they went to a plywood skinned D section to provide greater torsion resistance and efficient bending capability.

We completely agree on the contribution of the flexible skin.

With either geodetic or double diagonal trusses, the effort to build the boat may well be more than a hybrid monocoque ply and strip planked hull (ply on decks and sides and strip on the bottom if you want circular sections).