building a crab boat

Hey all, i've been looking into building my own boat for a while now. Originally I wanted to build a fiberglass on ply cabin cruiser/fast fisher that I could use at weekends to set a few lobster pots and do a bit of rod fishing. However I want to take up lobster fishing full time so rather than build a cabin cruiser, I'm looking into building a lobster/crab boat. Round here crab boats range from 15 to 30 feet, here's a link to a local firm to show the style. http://www.gsmythboats.com/ I would like to use the hull of an older wooden boat as a plug, stripping off the old hull planking and attaching a foam core to the frames, then laying over it with grp. that way I could flip the hull over, glass in plenty of internal bracing, remove the old timbers then lay up the inside of the foam core. I'd like the boat to be about 20ft long and quite beamy, I have a good 1.5 BMC Marine Diesel inboard i'd like to use. I've never made anything out of grp with a foam core so if anyone has any links that show this process and maybe a few similar build threads I'd greatly appreciate it!

 

Are you building because:
a) you want to build a boat
b) you want a boat

it is almost always cheaper to buy an existing boat than to build, particularly as you often get extras thrown in, like tools, spares, sheets, anchors, gear.

If you want to build, this is probably a fast way if the hull was good to start with, but remember that your hull will only be as good as the hull you use as a mold. I'm guessing you will probably use an old hull, that is not sea worthy, and that you can get cheaply. It may end up being a big project to get this hull to an acceptable level to use as a mold, and even then, dont expect it to be symmetric. Boats move over time. The timbers may be rotten and require replacement. Also, It is better not to have to fibreglass over timber as moisture can get in and de-laminate the whole thing, like if you were glassing the framing to the sandwich.



There is a big debate (which i'm sure others will correct me on) as to weather a sandwich is strong enough without using a vacuum to consolidate the laminate and make it stick to the core, particularly over time. It may be good for a few years but it does make it prone to delaminating. Besides this, on a boat of only 20 foot, you may be better off just using solid fibreglass (chopstrand) straight over the old hull, then pulling it off and framing the inside. This is far from perfect though.


Timber is a great material for boat building, they have used it for thousands of years, is there a reason you want to remove the timber planks to replace them with fibreglass and foam? (just curious, its a lot of work and $$)

 

Before you do anything you should ask the manufacturer of those lobster boats for the price of a Bare hull. Then you will know what your budget will be.

As far as building a new boat off an old boat, its possible. Last year I saw a fisherman haul his old tired wood planked 20ft double ended chug chug net boat out....strip the boat of all hardware...flip it over, straight it up...then strip plank a skin on top of the old hull ' form' .... glass epoxy an outside skin on this new strip plank...remove the new boat from its "old boat " plug, then epoxy glass an inside skin to the new strip planked hull.

The end result was a nice boat just like his old boat at an economical price.

 

cheers for getting back to me guys! I'd like to build my own boat as i love a project and building your own allows you to customise it to your own requirments. Plus the price of boats round here is criminal! Originally I was looking at building a Tolman skiff or similar grp over ply boat, but couldn't find a suitably salty design. I don't know if it's possible to get those bulbous 2 way curves using ply, so have been looking into building a hull using frames covered in foam then glassed over. It makes sense to use a stripped down hull to save time and money making frames, though i'm not sure how best to strengthen the inside before glassing over it. I'd still prefer to use a plywood design if anyone knows of any similar to the boats in the link? closest I can find are the Devlin Dipper and Rufus, though both would need major modification which is a bit risky for a first time builder!

 

Many ways to build a boat.

before you even consider designs and materials you need to have a realistic budget. With this budget you approach a designer. A good designer will be able to recomend a design and materials that fit your budget. The best way to determine a budget for a workboat is to first price a barehull .

Also worth concidering metal plate workboats.

http://www.alnmaritec.co.uk/boats/category_boats_07.shtml

http://www.pacificworkboats.com/aluminiumworkboats.html

 

Dudley Dix (and I'm sure others) has a construction technique called "radius chine" construction. Basically you use plywood for the hull, but instead of a sharp chine, you strip plank that area to create a nice curve. Some of the hulls look great and nothing like a typical plywood hull. A quick look on their website looked like they only had plans for sail boats in this construction method but it could be worth contacting them.

http://www.dixdesign.com/

 

cheers guys, would it be possible to modify a tolman skiff or Roth Bilt 18 to give it a blunt bow, and less taper on the sides and transom before they are attached to the lower hull?

 

Anything is possible, but if you are inexperienced it would be very difficult to get the hull fair (smooth). The best way to do it is in a computer model and re-draw the lines of the hull, you can also do this by hand. As each surface is developable (made from flat sheets with curvature in one direction only) a computer model is much easier.

The easiest way to build it is probably just to eyeball it during the build and measure it all a lot so its symmetric, but there is no guarantee this will work. If its your first build, i would stick to the plans to be safe.

 

cheers hambamble, i'm thinking of taking the existing tolman skiff plans and slightly modifying the bow to reduce the deadrise so it doesn't look as sharp and skiff like, and squareing the transom so the sides sit at 90 degrees to the water. these are only slight alterations and shouldn't drastically affect safety or stability. people had carried out far more extreme modifications!