Hull Extension Lay Up

Hi There,

I am planning an extension on my 23 foot cabin cruiser. I plan to extend the hull by approximately 3 feet.

All the work i plan to do using regular general purpose poly resin and was wondering what sort of reinforcing to use.

My plan was to cut the hull, separate the two pieces, grind a 300mm taper on each section, make up an in situ female mould using laminex sheets and then glassing the extension in situ. gelcoat then layers of glass.

My thought was to do this using 600 gram double bias reinforcement with a layer of 225m csm inbetween each layer. I had thought about 6 layers of double bias (and 6 layers of csm)? Would this be sufficient?

Thanks very much.

Nick

 

You are on the right track just make sure the join up is Fair and straight . Put about 3 layers of csm on the outside of the new section first so if you have to sand the bottom a lot you wont sand into the double bias and weaken the structure.
Just carry on any internal framing and make a good job of what you do !!The inside over laps need to be about 200 mm onto the well ground clean older laminates of the hull everywhere !!
I would also do a vee join on the outside as well once you have removed the outside panels . Just csm is acceptable but the joins need to be 300 mm wide and at least a 3 mm deep feathered to nothing .
6 + 6 of double bias and csm is a bit over kill ! half that would be close to what i would be looking at !

 

Nick,

Did you extend your boat? Are you happy with the results?

I'm planning on extending a 22-foot C-Dory, adding four feet. The project will also involve modifying the inward taper of the sheer line toward the transom to widen the transom from 70 to 80 inches at sheer line.

My one thought on using poly resin for a retrofit job such as ours is that it doesn't have the secondary bonding power of epoxy or vinylester. I'm planning on using West System epoxy for my extension and a 24:1 scarph joint as recommended by the naval architect with whom I discussed my project. I also have had a marine surveyor review the project who suggested rather than creating a mold to use C-Flex. C-Flex is a 12-inch wide fiberglass cloth with fiberglass rods running the length. One can staple C-Flex to a framework and it will hold the fair shape. You add the resin of your choice and once it's set up continue to fiberglass as you would over a mold. I've not done this yet but read a bit about it and it's in my plan.

Hope your project is going well or, better yet, you're on the water and happy.

Tom Herrick

 

Id be using Vinyl ester rather that epoxy that way you can poly or epoxy over it if you want . But if you use epoxy then epoxy is forever more .
Cflex will give you a lot of fairing and filling to be done . Mdf with a malamine finish one side and waxed to me is a better option then 90 percent of your surface will be nice and smooth on the out side and you can still use the c flex just make sure you carry on any stringers and girders that the hull has already .
Best of luck , just make a good job of what you do !!!

 

??? did you use these materials successfully ???

 

Just to clarify, have not started anything as yet but am planning to proceed with the job. I have to agree with Tunnesl on the use of a mould for the layup and will take your advice on laying a couple of layers of CSM for a base before the double bias. As far as number of layers go, I am planning to just lay up an equivalent thickness of the existing hull and considering that double bias is being used, the extension will be the strongest part of the boat.

To be honest I had planned on just using poly for the extension but maybe Vinylester would be a better substitute – although more expensive the resultant bond will be much better.

As for the stringers, bearers, bulkheads etc. I am rebuilding the entire sub floor system so I am very confident that the new structure will be much stronger than the existing.

Tom,

There is a c-dory that has been extended before... I think if you do a search you will find it on google and they used plywood as a mould - highly recommend the viewing and it was an impressive job.

Cheers,

Nick

 

Vinylester resin is much easer to use than epoxy and has good bonding to older laminates and superior strength over polyester . It is noticably differant to sand cut and bond with over poly . plus it is ok to poly over the top if you are adding things . If you start useing epoxy then thats all you will be able to use for ever more :idea: