can i make a mould this way?

i want to make a mould of my 19 foot fiberglass boat os i can extend the hull 3-4 feet.
i intend on making a plywood cradle somewhat simmelar to the shape of the hull.
shrink wrap the hull
suspend the hull in the cradle, but the hull not touching the cradle 4-5 inches above.
fill space with expanding foam!
when dry pull hull leaving shrink wrap behind.
this should give me a mould to work with when the hull is extended.and smooth enough to spray gellcoat.
my question....
will this work? or is it just a stupid idea?

have atter guys!

 

Good concept, but I suggest that you will get wrinkles in the shrinkwrap, and do check that your shrinkwrap does not dissolve in Gel / resin.

My suggestion would be to pull the shrinkwrap off after the foam is set and then lay up a good thickness of GRP in the foam to make a substantial mould. Gel and sand this back to make a good mould. Expensive, but most good things are>

 

thank you!
im still new at this stuff so all help is appreciated!

 

it might,, but i dont think so,,alot of things to go wrong,, wouldnt it be easier to cut your transom, then add the footage, then re-connect the transom?. that was my very first job in a boat yard, we were adding 10 feet to a 55 foot power yacht, and thats the way we did it.

 

i was thinking of cutting amid ships. i was worried about the strictural integrity of the joint approxatly at the front on the motor. i thought a lot of pressure would be put on that hull section. i was going to add two more stringers.
If youve done this before i would highly value your ideas and knowladge!
how did you re align the hull perfectly? that was the whole reason for this contraption i dreamed up?
thanks,
oops

 

the biggest worry about doing it is keeping your hull from "going out(stretching)" like the tops of a V spreading out \_/,,, so cross bracing BOTH pieces is the HIGHEST priority. then re-attaching you'll need to use batton,,or anything long and straight,,start as far to the bow as you can with ya straight edge,, and run it back,,and when re-attaching,, you need to make a WIDE and LONG feathering,, and theoretically your new part should be stronger then the rest of the boat just because its newer material and un-stressed.,, and i dont know what shape ya boat has( a boat shape dummy) but couldnt you add the stringers and still add to the back instead of mid?

 

You might like to look at a site advertised as

"Alternative method of boat building!!!!" at
www.bourneboats.com.au

he suggests the foam method, and has actually built a boat along similar lines,

From personal experience, foam is a very expensive and labour intensive way of mould building. Bits of soft wood and a hot glue gun will get you to the required shape quicker, then plaster over it for the final faired shape.

 

If you are just following the lines back 3-4'. You could just build a wood form
screwed into the boat where necessary and supported out the back with simple framing. For 1 shot simple molds I use 3/4" mdf for ease of milling and relative expense, (cheaper than AC fir) and a nice smooth surface, which i wax and PVA.
Lay up right up on the existing transom or as suggested cut the transom out and extend stringers and deck accordingly.

 

I like Aromarine's plan the best; straightforward and very little to go wrong.

 

Could you not make a mould of the last 6', release it from the hull and slide it back 4'. Cut off the original transom, and layup into the new hull mould..... Job done....

 

if you did that, your boat would have either a flat "dead" area, or a funny shaped boat. almost every boat has curves going bow to stern and topside to keel, adding on like that would actually give ya the "wave" effect. least thats the way my eye see's it happenin

 

You would have to kerf the mold to make the geometry "fair". That could make a lot of sense if you have a good eye. I wouldn't try to do four feet, but You might try repeating the last two feet of the boat......