Re-enforcing the transom

[GuidoAmala]

Hi all,

I am in the process of completing my Coronet Seafarer 21ft.
Fitting an outboard Yamaha 130Hp and i think i need to re-enforce the transom.
Any ideas ?

What i intend to do is to fit a 2in thick Iroko plank across, well tied to the bottom supports (running the length of the boat) and then having adjustable tightening 'vents' from the outboard bracket bolts to the iroko plank.

Any ideas greatly appreciated.

[UNCIVILIZED]

There are of course the standby's; cores, & ribs. But for something that heavy, & with that kind of torque generation, you're likely best to talk to a professional. Unless you have some other boats of the same design, to use as "guides", to compare it against when building her.

Obviously, Iroko is stout stuff... but, it has to be bonded properly. And also, you don't want to build it into the boat's structure in a way which is going to generate hard spots & point loads.

Plus, before any meaningful advice can be offered up, it'd help to know what she's made of. How thick. Quality of build, & quality of materials. Design of the supporting structures in the boat, etc., etc.

As, even if she's built in GRP, one brand of triax can literally be 2x or more stronger than another. And a laminate could test out at 15,000psi or 60,000psi (or more), strength wise. So, yeah, a Lot of variables.

[GordMay]

Greetings and welcome aboard the CF,Guido.

[boatpoker]

So if I understand ..... you are going to hang a 400lb. outboard on the transom of an single skin FRP boat. The entire transom would have to be reinforced all the way from top to bottom and whether or not you use plywood or iroko the core would have to be completely bonded to the existing transom. You'll get a much better bond with quality plywood than oily iroko. I think this would be very difficult unless you remove the deck first which is the only way I can see you getting a homogeneous transom. If you have the skills and knowledge about bonding materials it could be done. I would guess you are doing this because the engine/drive is toast and this is your cheapest alternative.

[jimbunyard]

Assuming you have a sterndrive or inboard now.

Attached quick drawing shows what I would consider a minimum. An outboard exerts tremendous forces at the top of the transom; that is no place for 'adjustable' reinforcements or anything else not fixed and permanent. The transom must be rigid and well bonded into the hull, such that it becomes and integral part of the hull.

While iroko is reasonably strong and rot resistant, it is an oily wood and must be properly prepared if you are going to try and bond it to fiberglass. That means it must be properly dry, not too smooth, and washed with acetone before you begin.

The wood should be bonded to the fiberglass skin with resin and some sort of thickening material, I just you glass beads, others will tell you to use milled fibers and/or fumed colloidal silica, but glass beads are quite strong enough if you do the rest of the tabbing and laminating properly...

Any of the structural plastic foams would be a better choice for this application, my current favorite is Coosaboard. It is about three times the price of say, 3/4" pressure treated plywood, and doesn't hold screws, but it is lighter and doesn't rot or absorb water.

Isophthalic polyester resin would be my choice for a project of this size, the real strength comes from the structural members. Any bonding, however, must be done to solid, ground fiberglass substrate.

For proper strength, the transom should extend all the way across the boat, and should be filleted, bonded and tabbed into the sides of the hull. The gussets shown in the drawing should also be bonded in place with whatever you use to bond the transom to the existing skin, filleted and tabbed.

The stringers should be shortened so the new transom goes between the existing skin and the end of the stringer, so that the outboard pushes against the stringer. This joint too should be bonded, filleted and tabbed.

The attached pictures might give you some ideas.

The second and third group of two show a transom reinforcement using plywood where the transom itself was sound but the stringers had become detached. A splash well, battery box and livewell were incorporated into the reinforcement.

The fourth picture show a failed factory reinforcement.

The fifth shows the beginnings of a more proper reinforcement, to replace that shown in four.

The sixth picture shows the proper amount of grinding to bond a new transom element in place.

The seventh shows two layers of 3/4" pressure treated plywood, bonded in place with new stringer ends also bonded. No filleting, tabbing or inner lamination have yet been done. The gray section in the middle is two layers of 3/4" Coosaboard, installed to isolate the plywood from water intrusion (this is a sterndrive application). If you use wood in your transom, it is good to do something along this line when you throughbolt the outboard to the transom.

[GuidoAmala]

Hi Guys,

Your input is greatly appreciated. I thank you all very much.

Would like to point out that the boat is made from playwood and then covered with Fibreglass.

I will take some pictures of how it is now, and post them here. At the moment it is very similar to onw of the fotos, where you have three thick fins coming out of the transom to meet the hull of the boat. Instead of three there are two nice thick ones. So the strength is already there.

I will sketch what i have in mind and i will explain in detail what i intend to do. It would be greatly appreciated to hear you comments.

With regards to my abilities to do the work...... i am a very technical person. Till now i did all the work on her, fibreglass, top cabin, upholstery, electrics and also welding of the trailer.

I bought the boat without an engine and the stern drive, i closed the stern drive hole using fibre glass 600grms/sq mt and marine ply layers. The material was approx 4 to 5 inch thick.

Instead of the engine bay, i already constructed the seat in the middle which incorporates the live bait tank (self Made and tested).

[GuidoAmala]

Hi all again.

In the attached sketch, you can see exactly what i intend to do.

Just a small note - the engine that was mounted on this boat was a Volvo penta V6 with a 290 stern drive. The ECU of the engine did not function properly anymore and apparently not even Volvo had any replacement. I have seen a video of this boat with 8 persons on board doing 35 knots.

Any suggestions/warnings greatly appreciated.

Thanks to all, Guido

[jimbunyard]

Could you advise a little more information?

What is the thickness of the hull skin?

What are the thicknesses and height of the main keel, the side stringers and the gussets?

Are the existing structural members only mechanically fastened, or are they glued/bonded and/or laminated with glass and resin?

Do you have the ability to weld stainless?

[GuidoAmala]

Hi jimbunyard,
Thanks for your interest.

Hull Skin is 5/8 in marine plywood covered with Fibreglass. Hull was built in Denmark.

Main Keel is 4ins wide and 6ins high
Side Stringers are 2.5 ins wide and 3ins high
Gussets are two 3/4 ins marine plywood glued and bolted together and are 12in high.

the structural members along the length of the hull are bonded mechanically and glassed.
The extra structure support which are shown in my diagram are a 5 ft in length bolted to the structural members and glassed

yes i am able to weld stainless steel with arc welding, not in possession of TIG till now.

I can assure you that the boat structure is very sturdy, i had it surveyed by a professional marine surveyor before i bought it.

Thanks
Guido

[GuidoAmala]

Hi Again,
Just thought of also posting this info.

The boat has a planing hull, please see sketch with measurements.

[jimbunyard]

Hello Guido,

Thanks for feeding my addiction to building stuff...

From your description it seems the boat is very sturdily built, my only concern would be flexing at the upper edge of the transom.

Not knowing what the construction of the gunnels actually is, but relying on it being along the lines of the rest of the boat, I've drawn in on your drawing what I would do if it were me, with the goal of being strongest and simplest.

The white is stainless steel, the brown is laminated wood and is meant to be tied into the existing gunnels. The red dots represent bolts going all the way through the vertical plate to the outboard mounting bracket, and on the plate mounting to the keel they represent bolts going through the keel. The stainless pipe is welded to the mounting plates.

Obviously these are just suggestions since I don't know all the details, but I hope they might prove useful in some manner.

By the way, the forward lines of that hull are pretty nice, it looks like the boat will be reasonably fast and handle short chop well without too much pounding. However, if I remember correctly those Volvo V6s, were rated at from 190-205 HP, so I'd expect a top speed of something more like 25 or so, still plenty fast in my book.

[GuidoAmala]

Hi Jim,

Impressed, did not think of that.

Thank you once again for your input.

Yes the first thing i did as soon as i got it was removing those stupid Yahoo stickers on both sides.

Well in the coming month i just need to finish off your design, hull paint Dark blue and white topside....... anytime you are in Malta, it will be my pleasure to take you out fishing or island tour or both.
Take care and thanks

Regards
Guido