Advice needed for foam job

[Frosty]

Hi I was hoping some one could guide me in my project of altering the shape of my transom. Just to give you the picture, I have a catamaran with surface props, now the surface props in order to reverse need a 45 degree transom. This is at present achieved by stainless steel plates, these are a pain causing electrolisis problems and I have difficulty keeping paint on them, I want to take them off and throw them as far as I can.

They are small transoms about 1 meter wide and only 1 foot up to the point where it goes up to the built in swim platform. Sorry I am rattling on a bit here but I want to fil in the transom to 45 degrees, leeving a kind of arch in the middle for the shaft drives.

I can get the polyuerothane foam, --I assume that I just glue on bits of foam to make the shape i want ? Do I glue with epoxy or normal fibre glass resin? should I mix in a bit of talc to make a paste? Do I need to remove the gell coat where the foam will be attached? Would it be stronger if I 'bricked' smaller bits interlocked or just glue on the biggest peice I can?.I was thinking I would then fare it with resin and talc mix?

When the shape is complete I assume that I would then grind back around the hull on the join by say a couple of inches to mate up flush with the new shape, then fibre glass a 4 inch strip all around and then a full one over the whole job. How many thickeneses? I shall be trying very hard to keep all this work under the present water line.

Thank you for your time,--- Jack

[Frosty]

I wonder if I am thinking about foam in the right way. Searching of other 'foam' threads encourages me to think that the foam is of no strength what so ever and that the strength is from the glass surrounding it. In my application described above there is no strength needed as I am basically filling a void area. In this respect I assume that the method of glueing the foam to the hull is of no consiquence, but the glass surrounding the foam is. It is therfore arguable that the foam could be initially stuck on with tape.!!
Yes I am having a conversation with myself.
As usuall I think about some thing for 6 months or more before going for it. i think I am doing it right.

[Alik]

How are You going to control the shape of foam? I would make an attached plywood transom or skeleton first, and then fill the gap with foam and shape it.

[Frosty]

I was just going to glue on blocks of it and knife it off to the shape. I was then thinking that I could then sand or rasp it to its final finish before glassing?

[SamSam]

Originally Posted by jack frost

I wonder if I am thinking about foam in the right way. Searching of other 'foam' threads encourages me to think that the foam is of no strength what so ever and that the strength is from the glass surrounding it. In my application described above there is no strength needed as I am basically filling a void area. In this respect I assume that the method of glueing the foam to the hull is of no consiquence, but the glass surrounding the foam is. It is therfore arguable that the foam could be initially stuck on with tape.!!
Yes I am having a conversation with myself.
As usuall I think about some thing for 6 months or more before going for it. i think I am doing it right.

Yes, the foam is mostly to give you a shape to glass. The foam will have some structure but you will probably have to design some more into the 'lumps' you want to attach. The arch for the shaft drives will help a bunch. You can attach the foam to the end of your transom and then lay up the lump in place where it becomes one with your hulls, or you could make them completely separate and then bolt or somewhichway attach them to the hulls and swim platform. I wouldn't think they would be under a whole lot of strain back there, the main thing would be to minimize turbulence. Sam

[Frosty]

Thank you for your input. I havnt yet got as far as what the shape of the lumps will be, it isnt neseccarily a complicated shape. The main Idea was to form nice rounded shapes so as to reduce underwater mainenance,ie keep anti fouling on!! I dont have any trouble at all with the rest of the hull.

Am i right in assuming I can just glue (epoxy, or resin mixed with talc for a paste) the lumps to the hull. Then just knife it into a shape?

As far as turbulence is concerened its not really an issue as the transon is clear of water at running speed,--

[SamSam]

Originally Posted by jack frost

Am i right in assuming I can just glue (epoxy, or resin mixed with talc for a paste) the lumps to the hull. Then just knife it into a shape?
--

As long as the finished glass is securely attached to the hulls with fasteners or laminations, the foam is mostly just there as a form for the glass. Whatever you can stick it to the hull with will work. Knife, saws, sandpaper or anything else that will remove or shape the foam is legal to use. If you remove too much or change your mind about the shape, flatten an area out, stick on some more foam and try again. Sam

[Frosty]

SamSam, This is great thank you very much. So the lumps are glued and I knife it off. I was then going to grind a, say 4 inch wide strip( 2 inches each ) all the way around the 'lump' and the hull giving a 2inch attachement to the hull?
Should I glue the lumps with epoxy or resin or does'nt it matter?

The join will therefore be allong the bottom of the hull up the side of the hull at the transom then up and aroung the underneath of the swim platforms.

Tell you what! I will try to post a picture of the transom, I am not good at this so forgive me if it does'nt appear.

[SamSam]

4" would be enough. No less, more would be stronger. If you are making the thing in place the laminations of the piece would just continue forward onto the hull and platform instead of a separate 4" band of strips. Use epoxy. If you are making it in place, doing the 'arches' and around the shaft will be real hard unless all that stuff is removed. Don't make anything permanent that might get in the way of doing any future work back there. I don't know if that is your boat or not, but if you have rudders like that, how well do they work? Sam

[Frosty]

Yes that is my boat going on the ship from aus to singapore. That was just a week out of the factory, she has considerably more stainless work and awings now.

The rudders,-- well-- when the props are so far back in the hull, rudders will always be a problem, those were modified/upgraded 3 times after that picture. i was infact trying to cure a problem that I cured by altering the bulbous bows, tending to wander at speeds over 20knts. The bulbous bows were badly shaped and at 20knts was trying to dive. Alteration of the bulbs increased top speed by 1 kt and cured the steering,-- well improved dramatically.

The rudders now today look very similar but are much stronger, there operation is no different than a normal rudder/spade. However when entering the marina for instance, I dont use them at all, Providing I have good strong reverse thrust. Ahh back to square one.

[iksok]

hi, could you alik answer me a question about your Albatros800 design i left you on it forum, please. Thanks a lot

[jimslade]

Sandblast and powdercoat the plates. Alot less work and structural rearrangement of the transom.

[Frosty]

You know what Jim, a week ago I would not have tolerated one thought in that direction but now, -today after discussing it, I might be inclined to agree.

Infact I might try a new thread 'antifouling stainless'

The thing is I always remove them, and with a 4inch grinder and 80 grit disc give it a reall good clean up. I had better success with coal tar epoxy as an undercoat, but last time trying to improve I used Jotun penguard primer I think it was, follwing that by Jotun Sea Queen. You are probably not familiar with these products as I know Jotun are in many counries but name thier products differently. Any way I have 6 months only,-- serious break down of paint adhesion on the stainless. Consiquently I have really wierd spongy things and crabs living on the inside of the plates. ( Can you immagine treating Arnesons in this way)

I also have to have zincs hanging over the side attached to the shaft to keep voltages down.
The alteration would have cured most of that.