Canoemaran? CanoeTri?

The shape I had in mind is very similar to the Hobie 16 hull shape actually. It will differ from this profile in that it will be notched for about the bottom four inches or so on the outboard side. The vertical cut will be tilted inward unlike the main profile of the hull, the idea being that it will be nearly vertical when the boat is heeled. I don't know what inclination will be ideal, and the idea is that the amas will at least initially be set up so I can adjust their attitude. Below is a crude sketch, based on the hull profile of the Hobie 16. Note that I do not work well with computer drawing tools, and this reflects the general idea, not the actual angle and contour. The vertical portion of the notch will be the central backbone of 1/4" plywood that will extend from keel to deck providing vertical stiffness. I drew it more or less vertical, when it should have been drawn with a slope to the right (inboard) at the top.
As to the use of blue foam, I suggest that you explain that to the folks who are flying foam aircraft. Foam is just a core material, a shear web of sorts, with unidirectional glass strategically placed to provide the stiffness and strength necessary. Blue foam actually has impressive properties if used correctly. One aircraft I have flown has a 30' wingspan, and the wing spars are blue foam with thin wood cap strips glued to it. A cantilever wing no less. Anybody who characterizes the structural properties of blue foam as c**p clearly has no idea how to utilize it properly in a structure.

H.W.

 

What airplane.
Why would you cut the notch in the bottom?

If the outside of the ama is on the left in your sketch, then I just didn't understand the description.

I was a structural design engineer for military aircraft.

If the aircraft you talk about just used the blue foam to establish a shape and then used other material to make the strength, that's fine.
I actually tested a blue foam "control surface" for bending. The foam did nothing for strength or stiffness.

 

My reason for notching the bottom is simple.... it creates a "bite" like a keel as the boat heels. Remember I intend this boat to heel significantly. The original contour loses most of it's bite. The theory of using a wing section on an ama or cat hull has pretty much fallen from favor. While it may provide some lift in the lateral plain, it's not very much in real life apparently. The chord being what it is, and the density of water what it is, one would think the Reynolds number would be well up there, and it should be possible to generate quite a bit of lift, but of course the span (depth of submergence) being trivial, tip losses both to the surface and around the bottom would quickly kill most lift......... Imagine an aircraft with an 8 inch wing span!! The keel effect created by the notch is probably of far greater value than any horizontal lift created by an airfoil shape. This is a situation where aviation technology does not in my opinion cross over to marine technology, unlike hydrfoils and airfoil shaped rudders, dagger boards, keels, and leeboards. The widely known designer of trimarans Dick Newick built an experimental ama that had the entire leeward face in a half moon shape. The ama design worked as he intended. The downsides were the difficulty of construction, and the loss of volume due to the shape. This design could be faired along the bottom edge of the notch with a shaped wood strip to create more of a cup shape, and of course the top of the notch will be faired by shaping the foam.
I'm well aware of the structural limitations of blue foam (extruded polystyrene). I've used it as a core material in several projects. It's not the easiest foam to work with, but it's the most readily available, and fairly inexpensive. The plywood central rib will be locked in plain by the foam glued to it, and because it cannot distort in any direction, will result in a lot of strength / stiffness. Stiffness on other axis can be provided either by wood strips at the surface, or by unidirectional glass roving.
At this point I'm toying with ideas. The foam it would take to produce amas of the displacement I'm looking at would be around 20 pounds if they were solid foam. Since you are structural engineer, I'll let you do the math ;-)

I've been flying all my life, ( I'll be 62 this year) and owned more than one aircraft, and built one myself. But this group is not about aircraft. The aircraft I was referring to was an ultralight single passenger craft with two axis control, 30' span lots of dihedral, rudder and elevator only, called a Skypup. I didn't own it, I only flew it once, but it flew quite well for what it was, and it's a design with a very good safety record. I was extremely impressed with the engineering..... you probably wouldn't have been. I've flown "right seat" so to speak in several of Burt Rutan's designs (there is no "right seat"), and am extremely familiar with the structural engineering of many civilian and home built aircraft. I've never flown any military aircraft and ridden in only one (C47). My only structural exposure to military aircraft has been world war 2 era aircraft being restored. It's not an area of interest for me. Only small planes are.
Let's drop the aircraft stuff......... that's for another forum.

H.W.

 

Thanks for letting me know the aircraft.
I'll look into it.
Military aircraft have very little to do with light aircraft, the lessons don't translate well.

Good luck with the boat.

 

I had a canoe cat for a while, I fiberglassed tubes which a 2×4 slides thru to join them together.
Main problem was that every thing in the canoes got soaked. There was not enouph freeboard when fully loaded for a camping trip.