Help with One-Off Method

I am planning to build a 20-21' flats boat, using the One-Off Method, that will be powered by a motor between 130 and 150 hp. I've studied this method and it seems fairly easy using foam and a wooden form. My question is, once you flip the hull and remove the wooden form, there are no structural members. I understand that you have to apply the foam and laminate to the inside of the hull once the form is removed but it just seems like the hull wouldn't be very strong. I've seen where people have used this method on very large sail boats with no structural support at the keel or anywhere along the bottom of the hull....just wondering if I'm under estimating the strength of the sandwiched foam and glass. Of course you would have to apply some structural members to the upper hull for decking, blocking, etc., but I'm still not trusting the strength. Any help will be greatly appreciated.

 

You'll be impressed how strong and rigid the hull comes out to be on a 21' boat when using sandwich construction. If you use 1/2" foam core, the rigidity approaches the rigidity of 1/2" of solid laminated fiberglass. That's not to say that you couldn't put a couple structural longitudinals and maybe a transverse frame in there while the hull is still in the mould. I doubt that there is a reason to though.

 

You will require some structure outside the hull to roll it over onto. That will support the hull shape while you lay up the inside.

Bill H.

 

And if you don't feel confident, take out the frame one by one while replacing them with the permanent structures.

But Bill is right, you need some structure on the outside to roll it over and that will give all the strenght you need.

 

One Off

Request for a back issue or article reprint issue # 10 Professional Boatbuilder magazine. Just sent one to Jfblouin.

 

If you can find them, there are two books that I would recomend:1. One Off in Fiberglass by Allan H. Vaitses and 2. From a Barehull by Ferenc Mate`.
If you don't support the half finished hull when you remove the molds you have no assurance that it will hold its shape. Make a small test sample of foam and fiber glass covered on only one side and check to see how easily it can be distorted. Then glass the other side and marvel at the change. But even a 2x12 plant can be distorted. Until you have most of the pieces in place you should be fanatical about preserving the original shape.

 

I'm building from a one off situation that requires a mold table to creat vacuum bagged, foam cored sandwhich panels. This is a popular system in our area. A builder down the road used a similar system to build a 72' motor yacht. He took a mold and sold the one off at a really great price. The most important part is to tranfer all information to the panel so it can be rechecked as the build progresses.
I had an old uncle that always told me to take time to insure straightness. He built a 42' lobster boat that he had to continually steer to port for the boat to track straight.
I have a small site posted below that shows my build.It may help you out.
http://www.geocities.com/TheTropics/Shores/1748/
Regards
Bob C.