Tri-purpose "Micro-Cat" concept

I'm new here, so hope this isn't redundant. I did have a nice long troll through the archives, but didn't see anything directly related.

I'm a programmer by day, but for the last week I've been playing with the idea of designing a "Micro-Cat" that will serve three purposes. Loosely modeled on the Hobie Cat, it would be a 8'-9' catamaran (with a 5'-6' beam) that could be configured as a one-person sailboat, as a two-person stowable tender (for a 25' sailboat), or as a "mountain lake boat" that could be pulled like a trailer (with the pontoons stacked vertically) behind a mountain bike into small lakes that are only accessible by trail. The basic construction would be fiberglass over foam (the pink insulation board from my local building supply) for the pontoons, with two aluminum crossbeams and an optional two-piece mast (from a sailboard?). My initial sketches have the pontoons being 12" wide by 12" high, with a "Lightning"-style streamlined prow and a hard chine on the outboard edge for improved tracking when under sail. A 5'x5' mesh "trampoline" will be stretched between the crossbeams.

The total target weight would be 100 pounds or less. I want to have a minimum of 250 pounds of flotation in the lower 8" of EACH pontoon, giving me 500 pounds of total flotation with 4" freeboard on the pontoons. To achieve this I calculated (based on 60 lbs of flotation per cubic foot of foam) that I will need at least 4 cubic feet of foam in the lower 8" of each pontoon, for a total of 8 cubic feet submerged. I suspect that I will have to compromise in the area of streamlined design (and thus, sailing performance) to get this flotation in. But I'm willing to accept some compromise as part of the price to pay for a multi-purpose toy.

The only boat I've built prior to this was a spruce stringer/canvas/aircraft dope ultralight kayak (wasn't pretty, but worked well). I don't have any software to model this new design and make any perfomance predictions, so I'd like to get some "gut feeling" input from more experienced boatbuilders. Does it sound like a feasible design? Have I made any glaringly obvious oversights/ erronious assumptions?

I have more thoughts on how to configure the boat for each of the three applications, but wanted to get overall input first. One particular area I'm uncertain about is finding a simple but rigid way of joining the square aluminum tubing crossbeams to the pontoons (preferrably without requiring tools). Have considered a number of different possibilities but haven't had that "Eureka!" moment yet. Any ideas on that?

Looking forward to any suggestions that are offered!

Another hair-brained scheme brought to you by. . .
VBAjedi

 

This sounds just about feasible but you should consider that the benefits of a multihull configuration become less significant in very small sizes, which is probably why there are few multihulls less than about 14 feet overal length. Very small boats usualy have to carry a large payload relative to their overal dimensions and a monohull can carry more weight for a given plan area and draft and also has less wetted surface for a given weight.

One of the main advantages of a multihull sailing boat is more righting moment for a given weight. In very small boats the weight of even a single crew member moving accross the width of the boat usually provides all the righting moment needed to hold up a normal size of rig, making the twin hull configuration superfluous.

But if you must have a boat which will pack up into a narrow package a catamaran is worth considering. Alternatives include single hulled folding boats (with flexible panels and watertight canvas hinges), and inflateables of course. You could also consider joining your two hulls end to end rather than side by side, then you could have a 16' - 18' canoe or rowing skiff which would probably row or paddle better than your proposed catamaran. There have been small boats made in two halves which can nest one inside the other.

As for bolting on cross beams you could consider 'big head' female fasteners to provide a flush fitting socket into which you can screw something like M8 or M10 stainless screws. With a grp covered foam hull you would need some internal strengthening, or at least a thickened skin, at the attachment points.

John

 

"Eureka!" moment?

I think I might have just gotten a first-rate idea for how to attach the crossbeams (and mast stays, etc) to the pontoons. :idea: Has anybody ever seen those "Square U"-shaped aluminum extrusions? What if I installed two long pieces of that, with the open part of the U's facing each other and spaced about 5" apart, on each pontoon? This would form a track into which 5" squares of aluminum plate could slide. It would look something like this in cross-section:

[===]

where the braces represent the aluminum tracks, and the "====" represents a plate sliding between them. The tracks could be secured to the pontoon by panhead screws countersunk into the lower side of the track every few inches, with a hole slightly bigger than the screw-head directly above each screw. These upper holes would allow the screw to be installed, and would double as "stop-holes" through which a pin could be inserted to lock a sliding plate at that point on the track.

The crossbeam ends, the sidestay attachments (which support the mast to the side/rear), and the oarlocks would all have identical aluminum plate "feet".

Using this method, I could experiment with the placement of my crossbeams (which would allow me to play with the placement of my mast, too), my sidestays, and my oarlocks. Everything would be easily adjustable.

Anybody have thoughts on this idea (or on other aspects of my design concept)?

 

your sketches

If you want, upload your sketches. That way we can see your ideas. I can take them and make a quick model and give you the marine calculations from the program. I send you a .dxf file.

I would be interested in the results of your XPS construction.

I want to do something along the same line as what you are talking about. Did the quick idea in the rendering attatched. It is twin electric trolling motors with the batteries in the hulls. It may handle waves better than an inflatable boat.

Perhaps a small proa design would be cool?