Transom reinforcing 34 Silverton for outboard conversion

I am converting a 34C Silverton from twin inboards to twin 250 hp outboards, using a bracket/swim platform that I will have custom built (Armstrong, Stainless Marine,?). The transom now is only about 1/4" thick, not designed to hold more than the swim platform. It is curved (lay a 6 ft. straightedge inside and the center has about a 2.5" gap). I need to reinforce the transom to hold the bracket. What is the best way to do this?

From reading old post, I know to clean and roughen the existing surface, put several layers of glass mat dripping with epoxy resin on first, then...

I was thinking of using something like multiple layers of 1/4" plywood in order to fit to the curved transom. The transom is about five feet high - I don't think I need to reinforce it all the way up - I was thinking I could add 2" of thickness from the bottom of the hull up about four feet (width of plywood).

To get good bond between sheets, my plan is to put drywall screws from the outside into the plywood to pull it in tight to the existing hull - I think I have to screw on the first sheet, add a sheet, screw it on, then screw the rest from the inside, so I don't have so many holes to patch on the outside.

Am I on the right track here?

 

In searching more, I found the Seacast website. Any comments on Seacast? If I formed a new inner transom skin with 4mm marine plywood then filled with Seacast - maybe easier and stronger than trying to bond 8 sheets of plywood...

 

seacast works, so there website says, what it seems to be is a filled resin system no better then bondo with some strand in it, love to see the replaced transoms with seacast in 10 years, go with plywood and glue the sheets with a layer of glass and resin make sure the glass is resin rich this way the new transom will be strong as hell, drywall screws are the best to pull it in tight, dont worry about the holes, just fill them with milled fiberpaste and sand and respray the transom this way u will never know the transom was repared

 

SeaCast is a core material made from poly resin and ground up old boats. Without sandwich construction on your transom (which you don't have) this product isn't going to work for you.

You can "suck" the thin sheets of plywood in with drywall screws, but I wouldn't. 1/4" bends pretty easily so does 3/8" and 1/2". three layers of 1/2", or 4 layers of 3/8" is a lot easier then 6 layers of 1/4" (you only need around 1 1/2") and there will be less mess, glue, fasteners, etc.

It should be possible to cut up temporary bracing to wedge into the interior of the hull to force the play against the transom, which will eliminate the need for a bunch of screw holes needing to be plugged. After the ply is in place, cover the inside of the ply with a layer or two of cloth (no mat with epoxy), tabbing it well into the hull sides, making a nice water tight fit and strengthening it up a lot.

If you do go the suck it down with screws route, use square drive deck screws instead of the black drywall screws. They're much stronger and you'll probably not rip their heads off as you're driving them. They're available at the local hardware store. You're better off using a pan head screw with a fender washer under it to spread the pressure over a larger area. Good Luck.

 

Thanks for the advice. It sure would be easier to go with three layers of ½ inch… is 1½ inch thick enough to handle the 500hp?

The transom is about 11 feet wide – so I’ll have to piece each layer in. I need to stagger the joints – is there any particular layout that would be better for the seams?

I guess that I should put one layer of plywood in at a time – let setup and put the next one in. How should I tie in the transom to the sides and bottom – multiple layers of cloth?

I'll take some scrap 1/2" plywood I have laying around and try to fit it in - I would guess that marine ply would be a little stiffer than the AC plywood that I have now.

I'll have to cut around the stringers - or should I cut the stringers loose from the transom and slide the new plywood down in the gap - glass it to the last layer of ply?

 

AC grade fur is fine for skinning a roof on your house, but doesn't have the quality in it's construction, to work particularly well in a boat. Ideally, you should cant the layers a touch, say about 10 degrees (long grain), referenced to the LWL. Each layer would cant opposite the previous, providing additional cross grain strength and seam stagger. You'll have to space out the seams as far as you can, with each layer covering previous layer seams. Scarfing the panels would eliminate weak butt joints, but could be a hassle installing them. Often times the stringers aren't tabbed to the transom, but stop just short. Maybe a 2" thick transom isn't a bad idea on your boat. The weight penalty isn't substantial and the stiffness will be welcomed.

This is a large and difficult job for an amateur, based on the size of your boat. Yes, you tab everything back in with cloth to the hull. Nice strong fillets under the tabbing would be nice too. You transom will be stronger if you place each layer while the epoxy is still curing on the previous layer. This creates a chemical bond between the layers and is stronger then a mechanical bond (sanded surface). This usually means considerable prep and setup, but it goes fast if you plan well.

You should consult with your bracket fabricator, to get a feel for their requirements.

 

Par,
thanks for the help and advice. I plan to tackle this job myself, cause I don't have the money to pay someone, and I've built lots of things, even done some fiberglass/composite construction with Burt Rutan, designer/builder of numerous world record aircraft (Experimental Aircraft Assn. convention at Oshkosh). Anyhow, I like a challenge. I plan to discuss this with the bracket manufacturers - with the help of this forum (especially your help) I can discuss this project more intelligently - Thanks!

 

I plan to get some marine plywood - just wanted to see how difficult it may be to bend plywood to this curve. It was fairly easy with the scrap piece I had - I understand that the marine ply will be a bit stiffer. I'll need to rig up some bracing to push the ply against the transom. The transom has some hand laid cloth and some not so smooth work - I figure I'll have to grind off some of this to even it up a bit, then I'll have to do something to smooth it out to fit the layers of ply without gaps. What is the best way to insure no gaps between the first sheet of ply and the existing transom?

 

grind the glass on the transom well, there is a good chance that the resin used had wax in it dont be shy with the grinder any unground resin left the new resin will not stick, best way for the first layer is to wet the transom with resin and wet out a couple layers of glass nice and sloppy this way any voild from grinding will be filled , then place the ply and brace it in i often drill some 1/8 holes in the ply to let the excese resin out just wipe it clean, some people will use a core bond putty too, i like the resin and matt myself, done around 50 transoms that way and no redos

 

What spacing on the 1/8 holes? What kind of glass and resin?

 

1/8 holes random dont need to many say a couple a sq foot, the resin most people will tell u to use epoxy , its ur call myself i would go with a VE resin and 1.5 chop strand matt, what you are doing is letting the matt act like a bonding agent

 

Jack has some good pointers and they work well. I'd use epoxy just to be sure of a good bond, but vinylester will work (don't even think about polyester). I use both filler and fabric to get continuous contact with the rough transom surface. Jack is very correct, don't be shy with the grinder, knock it down, at least put a very rough tooth on the whole of the contact areas. Release agents and other contaminates will likely screw with the bond if you don't. I usually space relief holes on 6" centers.

 

Here is our side of the story

We have supplied Seacast for over 20years, In all these years there have only been 2 failures. The user in the first case disregarded instructions and extended a transom without fiberglass skins on the outside of Seacast. The second case is where the customer over catalyzed Seacast. We replaced the material and had no other problems. As to test information, It has been available on demand, except in 2004 when our website was down while switching to new web hosting company. We lost about 2 months of emails. We have built a complete boat using 35 percent recycled material which was tested in 1995 by the Swedish Institute of Composites and was equal in all respect to a hand laid-up boat. Except for fatigue, were our material was over 10 times better. We replaced a 6" plywood transom on a 55' Ocean Racer with a 3" Seacast transom with 10,000 horse power installed and he is still racing. We also had a customer install 5 big outboards on a 23' boat with a 1 1/2 " Seacast transom that was 6 years ago and had no problems. Anyone who would like the opinions of our customers can visit the Seacast Forum @ www.transomrepair.com and read what our customers have to say. On our website you can request samples and there is a Seacast Manual and Test results that you can view online. We did extensive testing before we ever put our product on the market. We continue this testing with each and every batch we make before shipping. The resin we use is very aggressive and we have tested adhesion on a 40 year old boat laminate with excellent results. Please call or email with any questions you may have about Seacast Toll Free:877-716-4820 info@transomrepair.com

Best Regards
Wolfgang Unger

P.S. In my opinion wood has no place in a fiberglass boat. Encasing wood in fiberglass is an open invatation to rot.

 

Reinforcing knees

Hello naturewaterboy,

I know Key Largo well having spent 16 years living in the Keys. All the above advise is good, but I don't think I saw any mention of reinforcing "knees" from the transom to the stringers. These are a must for the proper transfer of thrust. Usually triangular pieces from the transom to the stringers, the more the merrier.

BTW, I would go a minimum 2" thick and all the way to the deck and then reinforce to the deck with mucho tabbing.

Maybe I'll see ya in da Pirate House Tiki Bar sometime

Steve aka Keysdisease

 

The forces exerted on the transom by the engines has to be transfered to the rest of the boat - the sides and bottom of the hull. I'm aware that it is possible to transfer forces to an area without enough strength to handle the forces. I did plan on glassing in some triangular pieces to tie the transom to the stringers, as the two inboard stringers are about 11" hi x about 1 1/2 thick. I think I can tie them in to the transom without any problem.

Pirate House Tiki Bar.... haven't been there... live by Mandalay MM98