Foam boat question

[haha49]

I was debating on building a boat. I figured just do a small dingy first my question is what type of foam should I use. I wanted to do what ever was simpler to work with and the right type. My other question would be do they normally make the back of the boat out of foam or do they use wood in that area instead? I plan on building something small with a kicker.

[UNCIVILIZED]

I'd suggest one of the flavors of Corecell for this. With your strips machined into a bead & cove profile. Such "should" make for the least amount of sanding, & fairing.

In terms of the transom, some of how you build it depends on how much HP & weight you're planning on hanging back there. But unless you're looking to power this up, & make it a full on planing dink, then go with a bit higher density foam than you use in the hull, sandwiched between 2 layers of good marine ply. Say 1/4".
With a stiff layer of glass on either side. Say, something in the 16-20oz range. A triaxial, or quadraxial.

But if there's any doubt at all (about weight being hung back there), then at a minimum, put a solid insert into the transom where the OB will go. Possibly with transverse wooden cross members @ the transom's midline, top & bottom as well. With foam filling in the "holes" in the structure, in between the skins.

Although, unless this is a design of your own, in which case, you'd have enough experience not to be asking these questions. Then, what materials to use, where, should be specified in the plans. So what's the story then?

And, BTW, there are plenty of building resources @ the WEST System site, as well as their magazine www.Epoxyworks.com

[GRIT]

I don't fully agree with Uncivilized, but there are always several ways to skin a cat. (Why you'd want to skin a cat is unclear, but I've heard it's been done multiple ways.)

We're using corecell "M" foam for our build. It's more expensive than many other products, but is heat formable, and is very tough in the real world. On a boat concerned with resale, and wanting a top tier product, I'd suggest corecell. As a small project, just to get used to using foam core, I'd suggest Divinicell, or some other cheaper product. The layup, shaping, and glass layup are close enough to the same, that it makes no difference.

I wouldn't go through the trouble of coving the foam, unless you're doing it yourself and have the tools to do so. For a dinghy, using 6-10mm foam (1/4-3/8"), I don't see you saving any time, or sanding; and I have a hard time imagining coving such a thin piece of foam.

Divinicell should be 20-30% less expensive than corecell, and has a "softer hand" for the same density foam. A small bonus is that the sanding dust doesn't make you itch as much as corecell can. For a rowing type dinghy, with less than 3-5hp I'd use 4lb foam.

Either way, any non friable, structural, foam will do for your first experience.
They'll all shape well, sand well, and bend easily enough for the size of your project.

For the transom, for less than 4hp, I'd use 6lb density foam, of suitable thickness, with two layers of 18oz biax on each side. For 5-10hp, I'd use 12lb density foam and move to triaxial glass.

I'm not a fan of plywood on boats, but it's a very stiff, and forgiving structural material. Uncivilized's suggestion for the transom would work very well, and has less of a chance of deformation, than foam, if you get the glass wrong. Although I don't see the need for anything more than biaxial with plywood, as the plywood already acts as a multi-axial stiffener.

Good luck, and enjoy your build. If I can help in any way, let me know. I've made enough mistakes in the last few years to help you avoid a few.

Cheers.
Paul.

[haha49]

I looked at several plans for boats sort of what I want most are plywood plans. I've worked with wood and I've worked with foam. Foam would be allot easyier to get the hull shape I want as you can attach it to it's self in anyshape you want as you can cut it and build it up in layers. The glue I will use is the stuff they use in the foam shop it makes the foam 1 piece it's really good you can't tell were they glued it and if you damage the foam the glue is just as strong as the regular foam. With plywood you still have the seams so foam is the way to go for sure. I'm currently working on plans and thinking it out I've seen plans but I simply want to take a big boat hull and shrink it down to see how it preforms before spending the big bucks on a larger version of it. If foam and glass is tough enough for what I want I will go that route if it's not then it's alunium welding the next hull as long as the shape worked out which is why I will build a mni version first.

[RaymondR]

Build it as a multi chine in ply with stitch and tape then take it to a refrigeration insulation place and get the to spray the outside with polyurathane foam, sand it to shape and glass it over, varnish the exposed ply inside, classy dingy with built in bouyancy.

[UNCIVILIZED]

Originally Posted by RaymondR

Build it as a multi chine in ply with stitch and tape then take it to a refrigeration insulation place and get the to spray the outside with polyurathane foam, sand it to shape and glass it over, varnish the exposed ply inside, classy dingy with built in bouyancy.

With this, you have to be careful. As there are a good variety of foams out there where are hydrophillic. Meaning that they absorb water... even when well sealed as you suggest. And if they do, then you have quite the mess on your hands.

[UNCIVILIZED]

svquintana, not to dismiss your reasoning, but here's why I stated earlier, what I did.

First, to me, it kind of sounds as if the OP has some dreams about building a bigger boat, assuming that the dinghy project goes well. So if that's the case, then to me, it makes sense to use materials which one would use on other projects down the road.
And while yes, CoreCell is more expensive, in something the size of a dinghy, I don't see the small cost difference as being significant. Especially as, as you say, CoreCell is heat formable. A handy trait to have in a small boat, which likely will have a fair bit of curves which will need forming.

Also, CoreCell is a Tough foam, perhaps the toughest out there. And dinghies (when used as tender's especially), Really take a beating. On par with a pickup truck used daily, to build a house. So to me (having lived aboard, on the hook for several years) every bit of toughness which one can build into a dinghy is a perk.

My suggesting going the Bead & Cove route stems from practical experience, & this article Foam Strip Plank Boatbuilding | Epoxyworks Where a couple of the virtues of machining one's planks in that profile come to light.
They adapt to fitting around curved mold sections very well, with little to no gaps. And due to that, plus their built in auto-aligning profile, very little sanding is involved.

And while I doubt that the dinghy in question needs 3/4" foam as is used in the article. I think that it would likely need 10mm foam, as a minimum (SIC). Unless the construction is to be of the wood, foam, wood variety.
For given how thin the composite skins tend to be in a dinghy, they need thicker cores in order to maintain sufficient stiffness.

My suggestions regarding transom scantlings on the dinghy are based on real world experiences. And having a feel for how much torque loads, & the subsequent flexing which OB's put on things in that region of the boat.
They're an area which are probably the mostly highly repaired one on dinghies, due to people underestimating the loads there. So, I don't at all feel as if I'm over spec'ing things with my comments on such (possibly the opposite).

If one puts 10-15hp onto the transom of a dinghy (for which I was NOT suggesting scantlings), & then proceeds to crank on the power in order to get 600lb (conservatively) up onto a plane, then you're looking at a couple of hundred foot pounds of torque being exerted upon the dinghy's transom.
On top of which, there's then the shock loading which everything endures each time you punch through a wave. ALL of this is felt both by the transom (material/construction) itself, & also where it attaches to the hull sides & bottom.

Now it’s a guess on my part, but a fair one, that when you’re planing, the load on the transom & it’s connections to the hull, likely equal the weight of the boat & it’s contents. Plus being magnified by X number of G’s, due to the shock loads of punching through & or over waves.

Just as a common sense/reality check on this. Inflatable dink's, with hard floorboards & or inflatable keels, have transoms made out of solid plywood in excess of 1" thick. This, for the same 10-15hp outboards.
My suggestions on scantlings, explicitly did not cover things to this HP level. As designing & building a dinghy to handle that kind of weight & power hung off of it's aft end, is another animal entirely, from building a light weight, thin skinned, foam cored tender.

Also, my suggestions regarding “the how” options & thoughts on how to build a transom for a low powered OB, also stem from the fact that there are significant crushing forces from the clamps on an OB motor. And that regardless of it's size, these foces have to be dealt with via materials more substantial than thin fiberglass skins overtop of relatively low density foam.
Plus again, there are the shock loads generated by even small OB’s on the transom, each time a dinghy strikes a wave. One’s which I can guarantee won’t be handle able by light weight skins on top of foam. Or even thing ply alone, on top of foam.

There are good (simple) reasons why light weight, thin skinned, plywood dinghies are only rated for such low powered OB’s. And why RIB’s, & Hard Bottom Inflatables, which carry bigger OB’s have such beefy transoms. To try & circumvent such proven construction rules, & laws of physics wouldn’t make much sense.

[jimbunyard]

The dinghy shown in the pictures below is made mainly with a product called Coosaboard, available from composites suppliers. It is a plastic foam (I think PVC but I'm not sure) that has fiberglass embedded in the foam matrix, just below the surface on both sides. It is strong and light.

I used 1/4" thickness for the hull and deck. The centerboard trunk is 1/2" coosa I had left over from another project, the transom is a composite of scored 1" PVC and 1/2" unscored 5 lb density PVC foam, also left over from other projects. The dinghys' construction required 2 1/2, 4' x 8' sheets.

The dinghy is 10' 4" long and 54" wide. It is set up for sailing and rowing, but the transom is quite strong enough for a 15 horse engine (which would be way too much). It's pretty heavy at about 150 lbs, but it is very solid.

I used polyester resin with a sprayed waxed gelcoat. It was built from a design in my head by drawing the outline on a large wooden table, erecting five molds or bulkheads and running stringers along the chines to facilitate marking the panels making up the hull.

I then cut the panels to fit the frame work and screwed them to the molds, keeping the screws away from the keel and chines, because the seams were then glassed with a 4" and 6" wide layer of 6 oz glass. Once that had kicked, the screws were removed and the whole hull was coated with a layer of 6 oz glass and a layer of 1.5 oz mat, applied simultaneously. There is also a layer 6"wide of 18 oz roving along the keel for beach resistance.

Inside the seams are all taped with either 6 oz cloth or 18 oz roving and then the entire inner surfaces were covered with 1.5 oz mat. The transom, inside and out has 2 layers of 18 oz roving and 1.5 oz mat. After the construction was finished, the whole boat was skimmed with a mixture of polyester resin and microballons, then sanded to a smooth finish, and then gelcoated.

This is just an illustration of one way of doing it, there are as many ways as there are people...

The last picture shows were it stays on the stern, in its' combination cradle/boarding ladder/(future)solar panel mount.

[GRIT]

Uncivilized.

I agree that overbuilt is better than underbuilt, and respectfully withdraw my statement "disagreeing" with parts of your suggested method of build.

I might do it differently, but shouldn't have stated that I disagreed with your suggestion.

Your last post was terribly civilized for someone with your moniker. Perhaps you came about your sobriquet in a similar fashion as a big man is sometimes called "tiny".

Whatever the case may be; Peace brother.

Cheers.
Paul.

[GRIT]

Jim, nice job.

Paul.