Opinion/help on design I have 3d model pics

[PatP]

That is some serious "pink (blue?) plywood thinking" (an engineering term for using the wrong design technique for a composite structure).

What you'd drawn is a badly designed jonboat filled with foam. Congrats.

Imagine if you were challenged to build a catamaran with foam board and wood beams. What kind of design would you make? Just out of curiosity and so i can borrow from your design

 

[PatP]

One inch foam is plenty bendy for what you have, so you don't need to sand anything except possibly the bow contour. You need to coat and paint the foam, and a thin veil of glass will add a lot of surface protection. Four ounce deck weave would be plenty to allow you to handle the boat without denting the foam. You should shoot for about 4 inches of draft with a single crew, and adjust the width of the pontoons accordingly. That suggests about 7-8" draft with 2 and gear, so 14" hull depth would be about the minimum practical. As others have mentioned, your hull has a lot of surface area. It isn't the most efficient to propel, so plan on a beefy trolling motor at least. Or a little gas kicker.

You appear to show double bulkheads. They aren't necessary, nor are the wood headers. Just scab the beams to the face of a single bulkhead, glue them, and tab with two layers of 4 once glass extending abut 6 inches down the bulkhead. glue the bulkheads to the hull, fillet the joint, and tab the bulkheads to the hull with 3 inch 9 ounce tape. An old lightbulb makes a good filleting tool for this.

With the small size of the panels the deck has to span, you can easily get away with 6 mm fir or meranti ply. The fir is cheaper, rougher, and will soak up way more epoxy, so I think the meranti will probably work out better in the end. Consider polyester Dynel fabric as it is nonskid if not totally filled epoxy. Or just seal completely with epoxy and apply boat carpet.

Make sure your center of buoyancy and center of gravity match. You want this puppy floating as level as possible, not down by the stern. This doesn't look to be the case at present.

Thanks nice input. Im going to take some of the things people have said and do a redesign. And repost it. If the consensus still seems Grimm then I'll abandon the idea.

 

[PatP]

I have just been doing some very rough / approximate calculations - and if you just had a catamaran form (ie do away with the centre hull) then you are probably looking at approx. 100 lbs / inch immersion for the two hulls (and this will be fairly constant, as the hulls have no flare).
If say you and your friend weigh 150 lbs each, and the boat optimistically weighs 150 lbs, and you have another 50 lbs in the coolers, then you have a displacement of 500 lbs already, and a draft of perhaps 5".
Even with a hull depth of 10", you will only have about 5" of freeboard - and it could get 'interesting' if both of you venture away from the centerline re your stability.
And your transverse beams connecting the hulls might only be an inch above the water surface - in any slight chop (that is more than a ripple) then the wavelets will be hitting these beams, creating even more resistance.

Thanks very useful info. I'm going to do a redesign and see if I can get better results. No center pontoon a bit smaller overall but taller and symmetrical side pontoons

 

[JamesG123]

Imagine if you were challenged to build a catamaran with foam board and wood beams. What kind of design would you make? Just out of curiosity and so i can borrow from your design

Replicate a pontoon boat at the scale you desire. But from your description of intended use, I'd say make a monohulled flatbottom skiff. Or better yet, just find an old aluminum jonboat that will cost less, weigh less, and be easier to car top than anything you are likely to build.

But if you insist.... first you need to do a lot of reading on building composite structures. You can't build a boat out of foam. It doesn't have the strength. What you build is a GRP (fiberglass) boat with a core of foam.

 

[PatP]

Replicate a pontoon boat at the scale you desire. But from your description of intended use, I'd say make a monohulled flatbottom skiff. Or better yet, just find an old aluminum jonboat that will cost less, weigh less, and be easier to car top than anything you are likely to build.

But if you insist.... first you need to do a lot of reading on building composite structures. You can't build a boat out of foam. It doesn't have the strength. What you build is a GRP (fiberglass) boat with a core of foam.

I know a John boat from a utility standpoint would be way easier but I just think a custom stand up cat would be cool. Well I was starting to doubt it would be possible but check this out. How the hell are they floating with such small pontoons with 2 people and a motor and those thick wood beams. I feel like my design is better than theirs. Im not trying to make an offshore boat for going miles out or anything.

 

[CocoonCruisers]

Why wouldn't you look for some run-down 50$ beach cat on craigslist, cut the beams down to the size you want and add a makeshift platform out of plywood or the remains of the old trampoline ? That would give you 'slippery' hulls that can actually be paddled, solid and reasonably light structure also allowing to mount a small outboard or a comfy seat, a bridgedeck clearance allowing to operate in a little chop, and more clarity on the adequacy of the concept for your fishing, all within a weekend of work. (Sizing tip: if you'll use it with 2 people, look for a single-hand cat - they are designed to carry the whole load on one hull when flying the other one).

 

[PatP]

Why wouldn't you look for some run-down 50$ beach cat on craigslist, cut the beams down to the size you want and add a makeshift platform out of plywood or the remains of the old trampoline ? That would give you 'slippery' hulls that can actually be paddled, solid and reasonably light structure also allowing to mount a small outboard or a comfy seat, a bridgedeck clearance allowing to operate in a little chop, and more clarity on the adequacy of the concept for your fishing, all within a weekend of work. (Sizing tip: if you'll use it with 2 people, look for a single-hand cat - they are designed to carry the whole load on one hull when flying the other one).

Damn. Lol I just found exactly that on Craigslist. I actually did see the posting before for some loose cat hulls. I just thought they might be too heavy but i guess since i can take them apart that would be much better. They are 16 ft so I'll need to see if he has a title for them I guess. Here in texas they say any sailboat over 14ft needs a title. Do you think that applies even if i don't put a sail on it? Thanks

 

[philSweet]

At this stage, if you can show the receipts for purchasing the materials, you can file for a new title as a homebuilt, which is probably best all around. You are actually supposed to report any major changes in the type and arrangement of existing craft that would materially effect the description on the title. For instance, changing a hull from a sailboat to a powerboat.

 

[CocoonCruisers]

Here in texas they say any sailboat over 14ft needs a title.

I'm not familiar at all with your local regulations but if remaining under the radar seems easier ... the average 'family' (i mean not too sporty) 14-footer beachcat should be enough to carry the weight for your usage, and it will be a tad easier to cartop than a 16+. You might want to find one that doesn't have too much rocker, else you will, well, rock where you're already bound to roll.

Yes these cats are a little heavy, because overbuilt for your usage (one doesn't want them to snap under sailing loads, and they endure years of torture in sail schools). But any multihull experiences large torsional loads, and self-build or not, your boat will have some pretty rough beach encounters. Lightweight/strong/cheap is one of these pick-any-two dilemmas, even for experienced composite builders with an adequate workshop already set up, so doing better than a series build won't be easy.

 

[PatP]

I took some of your suggestions and made it. It functioned great. Its very buoyant it weighs about 70lbs give or take. Held me and my girl and a cooler full of beer easily. It actually paddles very easily and its pretty stable. I can stand on the edge and bounce up and down. Thanks for the tips and eat that to the naysayers hehe

 

[PatP]

The epoxy process didnt go as well as planned but not too bad tho

 

[Squidly-Diddly]

congrats, you are now one of the few people who designed and built your own boat entirely from scratch. not bad for what it looks like you want to use if for, easily transportable and stable fishing platform. Length of paddle goes by beam of boat, not strength of paddler/circumference of biceps. You are going to want a really long double ended paddle. These guys SHOULD sell themhttp://wavewalk.com/blog/kayaks-boats/
, or figure out a way to make your own. If you make your own you might be able to make a combo paddle that converts to long fish-net pole.

 

[CocoonCruisers]

Cool, love your radicality, these hulls are really slim for easy paddling, you went much lighter than anything you could have recycled from a sailcat, and it works as intended, including super easy cartopping ! Just don't put it in waves ... unless you're ready for a v2.0

Guess you should cover it or store it inside when not in use: UV tends to eat epoxy and most foams at an amazing pace.
Congrats !!

 

[clmanges]

Good on you, Pat, now you can fish. Just curious, though: did you fiberglass it, or just coat it in epoxy? If it's just epoxy with no glass, be sure to treat it very gently. Epoxy (or any glue or coating) by itself doesn't add any strength, and that foam isn't very strong.

While you're enjoying this one, you might consider starting a little fund and soaking up boat-building know-how for the next build.