Making of Negative Mold

Hey Folks,
I just wanted to start off by clarifying that I am not a boatbuilder, nor will I be.

The Project:
I have a backlog of projects in my head at all times; my wife is pregnant with our first and she made me sell my beloved track car. I've been fine for the past six months after selling the car, but now it's spring and I happened across this forum and I have been reinfected by the project germ.

I've decided that my buddies 14' skiff with outboard motor is the perfect fishing boat for lakes and bays, and that it would be the perfect platform to build from. I plan to recountouthe hull slightly, but basically perfect the way it is.

I plan to make a negative mold of my friends boat and build mine out of foam-cored kevlar. I'm hoping to be able to make this boat wieght in at a maximum of 180#'s so that I will be able to transport it on the roof rack of my car - I have a Yakima bike rack with a 200# capacity.

I plan to make a fiberglass 1pc mold of this aluminum boat. My question to is; will typical mold release leave any residue on the hull of the aluminum boat. I've also thought about platic-wrap. I would appreciate any and all suggestions. Also, I'd appreciate if anyone could point me in the direction of a good composite engineering reference?

 

Welcome aboard the forum

Oh, we'll change that all right.....

Congrats on the upcoming kid And yes, we all get that project bug....

The first point to note: Direct copying of an existing hull falls somewhere in between "questionable" and "flat out illegal" depending on where you live; somewhere out there is a designer or builder who owns the rights to that design.

Second point: Building a mould for a one-off is a huge waste of resources. Moulds are significantly more difficult and more expensive to build than the hulls that are produced from them. Economically speaking, your best option is probably to find something similar on the used market; sound hulls in need of a bit of paint can often be found for less than you'd pay for the raw materials to build them new. If you do want to build your own, finding a set of stock plans designed for rapid construction in plywood/epoxy is the way to go.

Third point: 180 lb for a 14-foot skiff hull should be achievable with relatively common building techniques (no expensive foam-cored Kevlar needed). The weight savings of high-tech composites are not nearly as substantial as is sometimes believed (for example, including impact resistance in the design calculations often nullifies much of the perceived weight advantage).

A moot point in light of what I discuss above; but no, mould release wax does not tend to damage aluminum and is usually fairly simple to clean up.

Dave Gerr's "Elements of Boat Strength" should probably be on your list if you're thinking of a project like this. If you really do have your heart set on exotic composites, you'd better brace yourself for some weighty, math-heavy engineering tomes.

 

Thank you for the great input Mr. MarshMat. I just ordered "Elements of Boat Strength".

Regarding duplication of another persons design. I agree with you entirely.... i guess that I am justifying it to myself that by using a proven geometry I'm keeping the scope of my first composite project managable.

You are also absolutely correct by saying that I could probably find another man's project for half of what the materials on a high-end composite project would cost. But for some reason, and as tight-wadded as I am regarding everything else, I have no problem spending money on a project. Especially if that project is learning experience which is why I'm also still steering towards a foam-cored kevlar build.

Oh, and I do intend on altering the profile some..... I know, I know, that it doesn't change the fact that it's still pretty much another man's design. Is it enough that I feel bad about it?!

Thanks again for the help, I really appreciate it.

-Evans

 

This may sound odd or counter-intuitive, but moulds are actually quite an expensive and tedious endeavour. Production boats can be built from female moulds because the cost of the mould can be spread out over dozens or hundreds of hulls. For a one-off, more money and time are likely to go into the mould than into the actual hull.

On a solar car project I worked on a while ago, we spent probably close to two hundred man-hours of tedious, messy work over more than a month to build and prep each of two 2 x 6 m female moulds. Laying up the actual parts took less than 30 man-hours for each.

If you want to learn how to do cored Kevlar, go for it. (Kevlar's nasty stuff to handle, I'll warn you- it destroys most cutting tools.) But I think you'll learn more, have more fun, and get on the water cheaper and more quickly if you use one of the many one-off construction methods designed for foam-cored composite, rather than fussing around with single-use fibreglass moulds.

 

marshmatt is right, the mould costs about 3 to 4 times the cost of the boat made from it, so no point wasting money on a one off project.

You can use PVA though for the alloy and make injection points for garden hose attachments to "blow" it off with water if you insist on the mould bit.