transom repair advice

[CIG29]

Have rotten transom in my 29" cigarette, could someone please advise best method of repair, have included pics of boat outlining damaged areas and current transom core construction.
The transom in this particular vessel is extremely thick (no one knows why) and special extension studs were made to bolt the leg in place.
The transom core is as follows starting from the inner skin

12mm inner fibreglass layer
12mm plywood
2mm fibreglass layer
17mm plywood
3mm fibreglass layer
17mm plywood
1mm outer fibreglass layer (very very thin)

It was this last thin outer layer that was worn and allowed the ingress of water, not the exhaust cut outs as first thought.

The seacast method of repair was considered but dismissed after reviewing comments on this forum especially those of member PAR from florida.
New plan now is to use 50mm structural plywood with an additional glass ply glass sandwich to bring transom up to the required 65-70mm thickness.
Have good fibreglass repair experience, so any polyesther glassing proceedure is not a problem, limited experience with Epoxy.

[jimslade]

I'm doing a 29 ft magnum. My work is as follows. 1.Cut out old glass inside
2. Remove wood to outer skin.
3. Grind outer skin with 24 grit. must be level across.
4.Make sure its totally dry.
5.lay 3 layers of 1810 down and wet out completely.
6 wet out 3/4 nidacore precut to fit. fasten,clamp fix whatever.
7. 1 layer of 1 oz. mat and fix another piece of 3/4 inch nidacore on top.
8. 3 layers of 1810 glass across the full width of the transom and tabbed into the motor stringers.
9. cut out outdrive opening and cleanout core around bold holes by 2".fill in with waterproof fiberglass mix. drill holes and install.NO MORE ROT.and strong.

[tja]

Jayslade, If you are going to use Nida core make sure that the glass on the outside of the transom is at least quarter in. thick and go with four layers of 1810 on the inside. The holes for bolting the drive unit on should be sleeved. I

[tja]

Now the rest of the story. I use a lot of nida core in smaller boats that i build and like how it performs. However for a boat that size I would go with Greenwood products Xl panels. They make preasure treated plywood that is void free and dryed to 13 persent moisture. There products are guarenteed not to rot or delaminate for life. If it does they will replace for free including labor. You can't beat that.

[jimslade]

I will NEVER EVER, EVER put wood in a boat below the water line again EVER,EVER. Where the bolt holes line up for the outdrive assembly I use a hard filler. I don't know what you mean by a sleeve. A sleeve used in a mounting boss would compress the core layer to a severe stress that you would have a failure in the skin. I have used pressure treated wood before, but it takes a long time to dry it out to the point of use for adhesion, and then sometimes it begins to warp if not pressured in place.

[understand]

Hello This is nikhil i would like to know that from where i can got liner digram for making a GooD Boat .

[tja]

Jimslade, If you would take the time to go to www.greenwoodproducts.com and read about there XL panels you might understand what the product is and just how good it is. These aren't sheets of plywood that you buy at Home Depot. Also a sleeve is a tube that the drive unit bolts go through that keep the core from being compresed. Nida Core recomends the use of them in these type of aplications. I believe they show it on there web site. I know i have printed liturature from Noda Core about it. Wood isn't a bad thing if you use the right kind. Check around and you will see that there are a large amount of boat manufacturers using Greenwood Products. Good luck. Sincerely, Tom.

[PAR]

The transom on these boats had to withstand tremendous loads and even though they had big support knees, they had delamination issues. Imagine the stresses it is subjected to as the boat dances on is transom with several hundred HP blasting through. Also consider the amount of athwartship stiffeners (or lack of) other then the transom. It does a lot of work other then providing a place to hang the outdrive.

I also agree the XL panels are very nice, but I've used non-pressure treated materials too. This isn't an area to cut corners, the transom on this boat is a huge structural member. Solid construction is a must.

Did you note the grain orientation when you removed the core? I have a tendency to cock the panels at 22 1/2 degrees to the centerline (two layers of ply). This provides more cross grain strength and athwartship stiffness. I don't think I'd want to cut a 2" thick panel of ply. I wouldn't laminate any type of wood thicker then an inch except balsa. It is a good idea to build up the thickness of the ply, rather than using a big chunk in one shot. This also permits grain orientation, which is free stiffening.

I use epoxy for this type of repair, though reasonable results (as good as the manufacture) can be had with polyester. The price difference between polyester and epoxy isn't that much if you use one of the discount epoxies available. Vinylester is a much better choice then polyester, but both will not rival the strength of epoxy (and why I use it on transoms).

Hear the warnings about plywood. There's a huge difference in construction and material quality in marine panels, compared to land based products.

I also "sleeve" all holes, but mine are epoxy then drilled for the proper diameter fitting or fastener (which is smaller). This way water can get past the bedding, but not into the core to cause trouble.

[CIG29]

Thanks Gentlemen for all the good advice, PAR I have specifically followed your diverse range of advice, on this forum, since viewing a response to a seacast post by FOCA a while back. I have determined, from your general experiences in this field and technical postings throughout this forum that any information or recommendations made will be adhered to. I have reasonable experience in working with and repairing fibreglass, from the manufacture of automotive body kits to complete GT40 replicas, working in both poly and vinylesther. I had purchased 200 litres of vinylesther for this transom repair but if epoxy is recommended and an advantage, so be it, the last thing I would want to happen is a transom related problem at 100MPH.

My thoughts on the repair are as follows:

Since the inner sKin is considerably thicker than the outer 12mm (1/2") against 1mm I would first remove all layers of the transom up to this layer leaving a 2" band of original outer transom as a support.
If this support is thinner than expected I would build it up to a thickness of say 1/4" on the transom side and say 3/4" on the hull.
As the original transom has not been removed, I will note the grain orientation of each specific layer and advise accordingly, would again appreciate your advice on the orientation of the specific layers.
lay up each individual layer of transom separately using either 12mm or 17mm as these sizes readily available here, not sure whether to use structural or marine grade, and need advice on type of glass between layers chopped strand, woven cloth etc.
when installing each new layer of transom, where transom meets hull, using a mix of fibre rienforced resin as a fairing filler and also using screws from the original inside layer at 8" spacings to compress laminate.
Once the desired new sandwich constructed transom is achieved say 6mm lower than original, glassing the outer skin to the new transom fairing into the original 2" band left from the old transom.
Cut out all required apertures oversize ie outdrive including bolts, exhausts etc by about 10mm and glass the edges to the required inner diameter.
sand all repaired area shaping inner cut out to clear related items then 2 pak epoxy and top coat or anti fowl depending on area.
Please advise add or alter, Thanks in advance

[jimslade]

A boat does not ride on its transom! A boat is the sum of it structurall integrity. Why are most builders going with core transoms. We need to get out of the middle ages with our thoughts about new materials. Wood transoms are for termites to lunch on. No wood for me, ever. If you put a material that is stiff with one that is flexible, you will have delamination issues.

[Martin B]

I use wood. I use solid fiberglass in all mounting areas and the wood is completely encapsulated. Unless someone drills holes in the thing it is never going to rot. It's not the wood that is the problem it is the way it is being installed. It does take a considerable amount of extra time to do it this way though, and I have yet to see one from the factory that was encapsulated. If there was a product out there that was as easy to use as wood but did not need to be encapsulated it would make transoms easier, but what is the additional cost of the coring material? I think also the stress on the bottom is much greater than the transom.


www.bowkersfiberglass.com

[PAR]

I agree Jim, that these are engineered structures, but in the case of long narrow very fast powerboats of this configuration, the only athwartship structural member, aft of midship, is the flange of the deck cap, some minimalist cockpit furniture and the major member of course the transom. The racking loads imposed by the sea or landing on the contact patch (if your lucky) after a brief airborne jaunt from a wave, places surprising stresses in the aft structures of these boats. They are built light to get the speed and their structures have a minimum safety margin. After ten years of steady use, I've seen this style of boat badly distorted. Additional structure would have prevented this, but also required much more power for the same performance envelope. From the engineer's point of view, it's cheaper to build them light then to power it up.

Some manufactures are moving towards core structures, partly from maintenance issues, but mostly to increase the performance with a lighter structure of adequate substance.

I'm doing a transom now and have encapsulated the two pieces of ply (each set at opposing 22 1/2 degree angles to the LWL) and the holes are bonded. This will not rot, none of mine have, if the owners don't drill holes in the damn thing, which is the usual problem on transoms. A swim ladder is installed with mild steel lags, a sounder is screwed directly to the boat, and engine change forces new holes to be drilled which aren't bonded and the old ones often filled with bath tub caulk, etc. These are the things that cause issues, not the wood, the lack of maintenance or proper repairs or equipment installations. Every so often I'll see one that has rotted from the inside out. The screw holes from former equipment, since removed, spent bedding, leaking sole, etc., permits water to enter the hull, drain aft and live against the inside bottom of the transom, where it eventually will get at the ply. If it's encapsulated, then it will not matter much, but I've yet to see an encapsulated production job.

A boat's structural integrity is the sum of it's maintenance, which few folks understand any more, let alone perform.

There wouldn't be any problem using vinylester at all. There are differences with strength and wood bond, but it's marginal enough not to worry about it. There would be far fewer transom issues on older boats if the manufactures would get off the polyester band wagon. Some are seeing the light, but many still don't care about anything, except selling as many boats as possible.

I usually cut the transom off as you are suggesting. I normally leave as big a flange on the hull as possible, 2" seems a touch on the small side. I remove the outer skin because it saves time and on many boats, often the inner skin has to be cut up into many pieces (structures and equipment get in the way) to get at the wood, which makes my life more difficult. I can put the outer skin back on and paint with reasonably close color matching. Most of the boats that need a transom, have finishes that are shot anyway and I can talk them into a paint job too.

CIG29, if you'd like drop me an email (click on my name) and we can go over the laminate schedule options, but you're on the right track, so keep grinding away.

[tja]

I'll have to agree with Par. I like some of the new core materials but good old wood correctly applyied is the best way to go. I use NIDA core in the transoms of some of the boats that i build but not at the hp or stress you are talking about in this thread. I agree that untreated plywood works well. I'm personally sold on Greenwood Products XL panels. If you do the research one would see that the product is rock solid as is the warranty that you will never have to use. I've always said that the best warranty is one that you never have to use. If you already have vinyl ester resin use it. Good luck. Tom.

[tja]

I see Martin B. is from Michigan. GO BUCKS

[Martin B]

:p......

[roob76]

I usually cut the transom off as you are suggesting. I normally leave as big a flange on the hull as possible, 2" seems a touch on the small side. I remove the outer skin because it saves time and on many boats,

just out of curiousity and to gain knowledge for possible future projects. when you cut out the outer skin do you cut out the sides and bottom leaving say a 3 to 4 inch flange on the sides and bottom or do you leave a flange on the top edge of the transom also?

[tja]

I stand corrected I agree with Par accept for the cutting off of the back of the transom. No mater what always go from the inside. Cutting from the back takes a tremendous amount of strength out of the boat that I don't think that you can get that back buy glassing the old piece back on.

[PAR]

I sometimes cut the top flange to gain access and drop in the new material, but many transoms require it be fitted with pieces, in which case I usually don't cut the deck cap flange. Much depends on how the boat is built, how difficult the inside of the transom is to access, if the deck cap is part of a liner or interior furniture or if there is a separate liner or other structural members needing addressing (the usual case) like stringers or transom knees. It really boils down to how important the inner liner/skin of the transom is, in the structure. Older boats have a very thin skin inside, newer boats have more structure built into the liner. Customers, aesthetics, duplicating details and matching color also play a part determining if I cut the inside or outside of the boat.

Most boats needing a new transom core also have worn out gel coat, have been painted, repowered, soft decks and structural members that also need addressing. Combinations of these issues contribute to the entry point of the saws-all.