Advice for my transom modification appreciated!

Hey Guys.
I'm new to the forum, but I've been reading you great advice for a long time!
I'm planning on starting my transom modification, from OMC Stringer to Volvo penta stern drive and motor.
I've attached a pic of the existing 'hole' in the transom.
My plan is to completely remove and replace the existing 2 1/2" transom and I'm wondering if I should add in extra structural support inside the transom once I replace the old one?

 

Welcome to the forum.

Nope, no additional reinforcement is necessary, you have stringers and perpendiculars attached, which will hold things fine. The engine beds do most of the load carrying work on stern drive setups anyway, so make sure these are in good shape, especially around fastener holes.

 

Thanks for the advice Par. I've had a good look at the stringers and they're in good condition (engine mount holes have not had water sitting in them)and so is the transom, it's really a pity I gotta change it out because it's in great shape (No rot, or cracking GRP.)

 

Don't make more work then you have to. If you just need to change the outdrive hole, plug the one that's there and cut a new one.

 

Hi PAR. That's definelty another option I've been thinking about,however I've been worried about whether the transom will retain its strength in that area. That said I'm am planning on the Volvo 290 which has a large cutout and so the area to be filled would not be as much as let's say a mercruisers cutout. I'll work on some ideas on the best way to do a fill in of that hole and post them here and would appreciate if you would help me choose the best one, unless you have any ideas?

 

Gazzainnorway,

You could mark the outline of the hole in the transom on cardboard, cut out the
pattern & verify it fits the hole. Use thickened epoxy resin to laminate several
marine ply layers to match the thickness of the transom. Allow to cure, cut to
size using cardboard pattern & trial fit. Cut out hole for new I/O leg & again use
thickened epoxy to bond insert into place. Make good the appearance of the
transom.

See Gougeon Brothers book. Good luck.

P

http://www.westsystem.com/ss/assets/HowToPublications/GougeonBook 061205.pdf

 

Thank you for the advice Perciles.
I'm thinking about a tongue and groove solution.
I'll be posting a drawing of the concept soon for comment.

 

Instead of a tongue and groove insert, consider a very wide tapered scarf instead. Tongue and groove and other hard corner joints, like a ship lap (for example) will generate a stress concentration around the perimeter of the insert, which will the place it'll crack or break out.

The typical approach is to grind the perimeter of the opening, say with an 8:1 sloped feather edged taper. A reciprocal beveled edge is made on the repair insert and bonded in place. This spreads the loads over a large portion of old and new laminate, eliminating these stress risers and maintains the integrity of the laminate strength and stiffness.

Log onto westsystem.com and download their free user's guides about how to do these types of things. It'll nurse maid you through most of the decisions.

 

Hi Par.
OK, I can see this being a far superior solution as compared to the tongue and groove approach. Also I was a bit concerned as to how I'd cleanly remove the 'groove' and insert the 'tongue' without the chance of voids with nothing in them, and as you clearly pointed out obvious weak points running parallel to vertical sides of the drive (I had planned on only doing the sides and leaving a relatively small open area in the centre as the VP 290 drive only lacks in width with regards to this hole) therefore forces applied especially during hard turns would find these joints, and they could fail. From what I gather the scarf joint will be exceptionally strong, because of the amount of surface area used for bonding. I have downloaded the users guide from west systems as suggested. My only concern is that the thickness of the transom, and the shape of the hole would in all likelyhood make the manufacturing of a single scarfed plug fairly difficult, so I guess I'd have to break the plug up into let's say 1/2" layers and then build these scarfed layers up from the smallest to the biggest epoxying them in and together as I build up to the complete 2" piece. This would all be done from the outside.
Ok ok, this is starting to work in my head as I write this, so let me read up a little more and then come up with a proposal for comment. Please find attached a photograph of the exsisting transom ply taken from inside of that damn hole.
And thank you again.

 

3 layers of 1/2" (13 mm) plywood works fine and usually how I do it, though the bevel isn't that bad really, about a 4" (101 mm) wide taper around the edges of each piece on 3/4" stock isn't so tough. For those quick with the math, this is a 6:1 slope, but sufficient with this application. A 6:1 on 1/2" is still a 3" wide taper all around the inserted pieces, so abotu the same amount of work.

Since this is an outdrive setup, the loads aren't that high so two layers of 3/4", with a 4" bevel around all edges will do. You don't have to be especially precise, just so the pieces fit flush and any gaps will get filled with thickened goo. I use an angle grinder with a 16 or 24 grit disk and simply plow out the taper around the edges. I plow along, testing the fit as I go, until they are close enough. It's all going to be hidden by the drive flange anyway, so don't get real anal, just insure it's a solid bond.

 

Rather than making a scarfed plywood joint, I've seen a lot of through hull holes, including I/O transom holes, plugged using the Payson butt joint type of method, where you glue a plug in the hole and then grind a taper on both the inside and outside of the hull and then fill in the taper with glass.

Par has good illustrations of them here in post #8...

http://www.boatdesign.net/forums/wo...tion/joining-plywood-puzzle-joints-35333.html

and his modified version here in post #26.

http://www.boatdesign.net/forums/boat-building/how-do-you-scarf-12mm-ply-bulkheads-37001.html

 

A scarf is a wood working joint and the Payson butt joint is the wood butcher's version. Both work, for the same reasons. Sam is correct, with this type of thing, you're not looking to be neat or especially precise, just flush and stiff.

 

We've just had cyclone NIna rip the west coast of Norway a new one....! Its been a stressful night, to say the least... My boat is still on its' blocks and where it should be, but I can't say that for most of the guys with their boats in the water.... I sympathize with you all that have this as a 'common' occurrence..

 

I've made a plan for the payson repair that SamSam suggested with the help of PAR's fantastic webpage. i hope the drawing makes sense.

 

Transom repair with suggested payson technique.

OK, so in the drawing you'll notice that the exciting cutout is very close to the hull and it would not be possible to to get a decent ratio eg 1:6 on the lower sections of the hole. The drawing shows the gradiant cutout of the 'V' grind out So I've proposed the following.