Extending fiberclass cabin aft

We are striving for a surface that is a fair continuation of the existing deck-within the thickness of a heavy gelcoat.A quick and dirty mould is the easiest way to get there.Been there,done it,got the T shirt.I do like to see enthusiasm,but I have grown weary of forums full of posters with lots to say and not too many miles on the clock.

 

Got a lot of hours building parts, even in place, etc. Not so many with gelcoat.

Stick to substance.

 

Gelcoat finishing is a boring and laborious job.It absorbs a lot of time.To achieve an imperceptible blend on this project,it will be less work to create a simple plug with a fair surface than to achieve a good result in any other manner.The process of "building a mould" involves a couple of gelcoats and about a 2.4Kg/sq.m layup with a few bits of plywood or mdf to hold it steady while laminating.The mould surface can be compounded to a high quality prior to laying up the new component.The mould itself can be used as an assembly and alignment jig,which will leave a narrow band of new gelcoat to bring to a good level of finish.Minimal finishing and a surface that matches the shape of the existing deck-simple and efficient.

Building a mould in place from sheet material works with flat surfaces.Getting a fair and continuous surface with curvature and feature lines matching is a lot more difficult.The radiused top corner of the cabin will be hard to get right and the lower angle where the top of the coaming meets the cabin side will also be a challenge.Building a male plug means that you get to see the finished shape before any resin is mixed.The fifteen hours or so of mould making will seem like time well spent compared to fixing a mis-match that has arisen because the result of one's efforts couldn't be seen.

 

Agree with needing a male plug or jig.

I'm not sure what I described that cannot be seen. All I said was that I would not build a female mould. I would build everthing in place for the very reason that it can be seen well.

If the purpose of the female mould is for gelcoating, then okay...but still yuk.

 

First, he is going to refinish the entire deck, and he doesn't mention if it's going to be gelcoat or paint. Even if this would not be the case, the area in question is well defined by the coaming and will be under a sprayhood, super precise color matching is not essential.

There are several ways to do this, but he asked for the easiest one, and that is to laminate over a sturdier version of his cardboard mockup then blend the lines with filler and longboard. Everything else takes more time and effort for the dubious benefit of having less filler on the finished piece.
I advocate that he builds the plug directly in place anchoring it to the inside faces of the existing laminate. This way all the laminating work can be done from the outside and hand down, all he has to do on the inside is grind the cut edges of the old laminate smooth.

 

What is the layup of the top you fellas propose?

I would use a core.

 

Building in place over a male former and longboarding will damage the existing surface.It takes a long time to achieve a good finish on the outside of anything and while a pro surfboard shaper might be able to do it quite quickly,most of us won't.The new area and the old will both be visible with a sprayhood in place as sprayhoods normally attach to the outer face of the raised coaming and when lowered the new area and the join will be in daylight.As will the large radius along the corner and it is highlight lines on these corners that betray blemishes.
A sheet of 12mm MDF and a piece of alder/lime/jelutong will probably be sufficient and about 25cl of shellac sealer will almost certainly be sufficient.There are those who insist that only after 8 coats of Duratec and a few days of polishing can a mould be made and while it is a good procedure for production,it isn't essential for a one off.A couple of coats of shellac and a rub down with 320 grit before moulding will work and the fine scratches can easily be compounded out of the mould before making the part.

The layup ought to be easy to deduce once the cuts are made and I would agree that a core would be good.Having seen the state of the sliding hatch I would tend to use PVC foam perhaps about 75kg/cu.m and I would strive for a balanced laminate.

 

There is certainly some truth to the problems of fairing in place. This is why reliefs would be needed. Then there is almost zero longboard work. And existing finish can be protected with double 2" masking tapes. Break them and add another..

I'd also use a marine core, so easy to form up and see what you are going to build and then relieve it for tabbing on the outside.

 

OP is back here. Thanks everybody who has posted here their ideas. It's been busy 1.5half years rebuilding the front cabin and replacing bulkheads, cabin sole etc. Now I'm back to the project of extending the cabin.
I decided to build the plugs, making moulds and laminating the extension pieces inside the moulds. Then use the mould as a jig to align the new piece with existing cabin top.
I started yesterday gluing the XPS blocks together and shaping them.

 

Have you identified a filler that won't attack the XPS?You may need to apply a sealer before moving on to filler but it is time well spent to strive for the best surface you can possibly achieve before making the mould.That way you will only need to do minor cosmetic work after grafting the new piece to the existing moulding.With care you might only need to finish a 10mm strip after the new part is in place.I would suggest covering the gel of the existing cabin with tape or similar while the preliminary shaping is done and only to remove it when the final blending and smoothing is carried out.The scratches that you will avoid creating will be a big time saver.

 

I suppose epoxy won't attack XPS? I'm gonna give it a try on some leftover XPS pieces.
Then I can use microballoons mixed with epoxy resin for the rough job and fine epoxy filler for final smoothing.

You mean connecting the new part from the underside only?

I was thinking doing like this:
1. align new part in place with the help of the mould.
2. connect the new part from the under side to the existing part using a few layers of fiberglass "tape" , each one wider than the previous obviously.
3. remove the mould
4. bevel the top edges of the new part and existing part
5. connect from the top using a few layers of tape as usual.

It will make more work to finish the bevelled and taped connection of course.

The whole deck and topsides as well as hull will be painted later on anyway.

 

The epoxy ought to be fine.I would only add material to the outside if I absolutely had no alternative.By tapering the laminate on both the existing section and the new part you are effectively locating the fibres in the correct place and avoiding the need for a massive refinishing project.By only using a gelcoat and maybe one layer of glass you will get the structure correct and can build up the laminate over a narrow bonding area.I would estimate that 4cm on either side of the join would be a good starting point with each successive layer extending a further 5cm so that there is a good tapering of the bonding strips.Add core at the correct point and then glass over the core in the same manner.Once cured you can remove the mould and hopefully there will be minimal finishing of the surface.Don't be too shocked if you find minor dimples in the old surface as you progress with the finishing as it isn't uncommon.It may be a challenge to get a perfect colour match for the older gelcoat and it is worth trying to buy a custom pigmented batch if you can find an expert at matching colours.I don't know if the world of gelcoat mixing has gone quite as far with the use of spectrometers as the car repairing world and even some home repair warehouses will now match paint colours from a sample they scan,but a word of warning they don't like to scan a phone screen due to pixelation so maybe taking the sliding hatch or similar to a gelcoat supplier would be a possibility.