I did the same on a 28ft 2 ton yacht. I sikkaflexed a marine ply backing plate inside the transom and used a plastic backing plate on the front with bolts through. I may even have drilled out the boltholes (from the inside without going through the outer glass) and filled with epoxy prior to drilling the actual boltholes.
I also took out the leg and glassed the hole in.
Its been 7 years and I'm very pleased with it. I use a Tohatsu 9.8 2 stroke as its the lightest engine I could get for the power.

I have 9.9 Yamaha 4 Stoke on smaller boat. It's about twice heavier, Previous owner implement large
white plastic chopping board as a internal bolts support , have to say, after 5 years no problem
experienced. This engine does not have heavy displacement props. inquire that option , for your boat
is a must. The only problem I experience , when is rough , forget engine, you on your own, cavitation too much.
the well would be better option but can't lift the engine, compromise , limitations, that's life.

FabulousPhill said..
I discarded a 40 year old diesel also, and installed a 27kg Tohatsu M9.8B two-stroke motor on the transom. I have had issues twice with the propeller coming out of the water, when the waves were 2m high, reflected waves and were a short chop, but those conditions aren't common.
My OB bracket on the transom has some 50x19 or so timber, (hardwood timber for a garden picket fence) and it seems fine.

The other thing to get is a high thrust propeller, so the engine revs at the correct RPM to deliver the maximum HP. For my 3500 kg boat that meant the Tohatsu 8.75 diameter, 5" pitch 4-bladed propeller as in the following photos.






I am currently thinking of going back to a diesel motor or making an OB well inside the lazarette area. To avoid the prop coming out of the water when the yacht is see-sawing on such rough waves.

The fuel tank set-up. A 11.3 litre tank, with hose to the motor. It could be a better set-up in future.

Good idea re prop. Do you have a long shaft outboard and still have cavitation.

Might want to add 80 kg of lead ingots where the engine was after this conversion. Leave the shaft and bearers in place too. If you decide to sell the Bukh there would be plenty of buyers.

On the Top Hat with the transom bracket, I had a long shaft, but had to swap it out for the ultra long shaft, Toehy 9.8 2st, with the 5inch 4blade prop. Too much cavitation going over bars etc, the ULS is very good though, no cavitation yet.
I set the OB bracket as high as I could(tenob bracket, pricey but worth it), as I didn't like the idea of dunking the head of it in the water when the boat hobby-horses. When I'm motoring at full power the bottom of the timber of the bracket where the OB clamps to is 10mm above the water, seems good.

FabulousPhill said..
Not cavitation as such. It was more the case of pitching/see sawing in steep short chop and thus losing grip in the water. As the boat leaned forward the prop came out of the water. It is a 28' Compass yacht. So it was not nice waves.
Yes a long shaft OB. Not the extra long version.
I haven't sailed for a year (been too busy) but I had reflected waves off the granite faces coming out of Refuge Cove in Bass Strait, from 10-15 kt SE breeze and swell. After going to the Melb side of the peninsula it was smooth sailing al the way home.

thanks Phil, some great info there!
I'm getting more confident.

KS, I have a Top Hat 25 that came with an 8HP 4 Stroke with a standard shaft, on a bracket over the transom. The previous owner changed to the 4 stroke, but didn't realise it wouldn't fit in the well, so he re-fitted it to a transom bracket.

After a few days out I noticed the transom flexing. Careel Bay Marina's shipwrights soaked some 12mm marine ply, cut to shape across the lower transom. They epoxied up the original holes and re-drilled, bolting the bracket lower down so that, like BlueMoon, the head is just above the water.

We've had the boat for 9 years this year, had the standard 3 blade prop all this time. Until mid last year when I couldn't reverse thrust enough to hold off hitting a jetty in a 30knot Northerly. I got the Solas 4 blade and haven't looked back. A corts nozzle would make it even better, but time and lacking in skills to make one..... The only bar I've crossed is Lake Macquarie, twice each way. There was some cavitation, but not enough to worry. In short chop you get a little here and there as well, but I've also motored from Norah Head to Careel Bay in 2 metre swell with no wind and got home fine. We go between the harbour and Pittwater a couple of times a year as well, so it's not just all bay sailing.

Mine is also a pull start and has no morse control, but I don't really need it. On a Compass, it may be worth converting the diesel cables and key system to an electric start so you don't have to reach over for controls.

My transom has a fair curve in it running horizontally and I need to epoxy a ply backing board to it for strength. There is about a 10mm gap in the middle of the ply board when I sit it up against the inside of the transom. Here is my plan;

- find best position for bracket and drill holes through transom and ply backing board
- using an epoxy resin with some filler (Epiglass HT450) to give it a very thick consistency. Now plaster the back of the ply with a thick coat (>10mm) of the epoxy and position board in place. Temporarily secure with bolts. Epoxy should squeeze out link vegemite between two Vita wheat biscuits ;)
- using epoxy and some biaxial cloth cover an area which is about 150mm bigger than the ply board and glass to the transom. Maybe repeat a second time.
- have a stainless plate the footprint of the outboard bracket made up with rectangular piece welded on for the later attachment of a knee which will be epoxied the hull.
- finally bolt bracket through the ply backing plate and SS plate.
- now the boat sinks with the extra weight ;-))

what do you think Seabreezers!

twodogs1969 said..
You are in Jarvis Bay lots of short sharp chop.
You are going to get cavitation.
Why don't you look at utilizing the hole from the sail drive to make it a well some how.

Planning not to have too much cavitation with the cavitation plate being 300mm below water line and thrust angled down into water slightly. But I think it may be an issue at times.
Building a well around hole from saildrive did cross my mind but I don't think there is enough clearance for top of motor under galley entrance.

Ramona said..
I would like to hear a bit more about the Bukh.

These are some of the issues with the motor only having been reconditioned by bukh Taren point four years ago.
A small channel has been burnt into the block face where the head gasket blew. There is movement in the flywheel whilst piston is still, you can see an imprint of exhaust valve in top of piston. The head has been machined and surface still looks pretty chatty.
I could go ahead and get barrel machined, replace the big ends etc etc but at the end the day it is a 20+ year old raw water cooled motor. These are some pics of already machined head, piston and block face.

twodogs1969 said..
Rather then wasting money on a rebuild that adds nothing to resale value. Have you priced a replacement?

The only replacement is another "reconditioned" motor from bukh for 6k like this one.

twodogs1969 said..
Why can't you go a cheaper Kubota based?

It's a sail drive. Would have to change engine bed at very least.

twodogs1969 said..
Keep in mind if you do look at Kubota based the same wanker at tarren point is the beta dealer .

what's the story with that mob?

twodogs1969 said..
What I am getting at is it will ad value to your boat.
Diesel inboard is much better then an outboard bolted to the ass lifting out of the water.
Worth looking into new mounts is going to be easier then filling the sail drive hole.
Keep in mind if you do look at Kubota based the same wanker at tarren point is the beta dealer .

The beta saildrive is 15k, already enquired about that. And to convert to drive shaft plus motor would be 15k. Regardless of what motor you have in these 80s boats the value is decreasing yearly anyway.

keensailor said..

Ramona said..
I would like to hear a bit more about the Bukh.

These are some of the issues with the motor only having been reconditioned by bukh Taren point four years ago.
A small channel has been burnt into the block face where the head gasket blew. There is movement in the flywheel whilst piston is still, you can see an imprint of exhaust valve in top of piston. The head has been machined and surface still looks pretty chatty.
I could go ahead and get barrel machined, replace the big ends etc etc but at the end the day it is a 20+ year old raw water cooled motor. These are some pics of already machined head, piston and block face.

20 years old but a real marine diesel. Mate is a marine diesel enthusiast. He is away up in Nth Qld at the moment buying a diesel. I'll forward this page to him for his opinion.
To me there is no choice. If I owned a Bukh I would be buying my parts from the UK agents and using the PBO forums for technical assistance.