grp hull modification

[patrick c]

Hi to all sea dwellers

I have a 4 mtr grp hull rhib medium V hull 10% its stripped bare ready to spray new gel coat
can this hull be modified to a 20% or 25% deep V and be sea worthy
any advice please
regards patrick

[Mr Efficiency]

Of course it "can be done", the real question is whether it be a worthwhile practical exercise, and that would involve considerations of cost (your time and/or money), and perceived gains in functionality. I doubt you would add much value in the process.

[patrick c]

thanks for the reply
i am not looking to add value just a better ride on the water
and i would enjoy the work involved
i have just finished the moulds for my console and jockey seat
so i can cast them now and i put alot of hours in them
as for the cost will be using matrials to cast many of these anyway.
i need advice on how to do the hull the right way the first time around
and be safe .
do i need to cut the hull off start from scrach new bulk heads ect,
or can i go glass on top of hull to make double hull if you no what i mean.

[PAR]

Weight is a huge preformance determent in small craft, you'd be better removing what you don't need.

I'm not sure what you mean by 10% V, but lets assume you mean 10 degrees of deadrise. Since you don't know that this is 10 degrees of deadrise, you're going to need a hull design to work from. Changing to a 20 or 25 degree V bottom isn't as simple as inclining the new bottom panels to 20 degrees and having a go at it, particularly if you plan on going into deep water, where a safe, sea worthy craft is necessary for survival.

What year, make and model RIB do you want to do this to?

[Mr Efficiency]

Superimposing a second bottom seems the most likely way, IMO, but you now have more bouyancy and your inflatable tubes might not touch the water then ! Also, your transom height is now out of whack.

[PAR]

Superimposing presents several unnecessary issues, not to mention suggests a less then reasonable approach, to a new hull form for the RIB. As I hinted at previously, this isn't as easy a task as it might initially seem. Where to place the new displaced volume, maintaining a reasonable CG and CB for the new hull form, incorporating the benefits of the remaining portions of the original hull form, etc., etc., etc. . . .

[patrick c]

thanks for the info
well today i have been sizing the job up
will keep the boat as it is
10% deadrise on hull
18" transom
would have liked to take it down to 22"
to many factors way over my head
if cb and grav are wrong would not feel safe
and don t want a fluding hull
so will not trash the boat its a good boat.

i will start from scrach new mould

[Mr Efficiency]

For a tiny thing like this ( 4m), movement of crew fore and aft constitutes the main changes in COG, and being a planing hull small shifts are of little consequence, the leading edge is where maximum dynamic lift occurs, which means sensitivity to lengthwise COG is somewhat reduced. It might also be possible to superimpose a deeper vee on to the existing structure ( say 20 degrees ) and retain the transom height by having a triangular flat pad along the keel. A lot depends on what the current shape of the bottom is.