I have had endless problems with my inboard diesel sail drive and am considering converting to an outboard. The boat is a Roberts 26 that is only used around calm estuarine waters and at the moment is stricken on a mooring and I can’t even find a diesel mechanic to look at it. I have spent more in the last few years on this thing, than the cost of a brand new outboard 9.9hp outboard.

Just posting on here to see if anyone has feedback or comments on this plan.
cheers

Been there. Had those issues.
When my diesel began to die, I put a 5HP OB on the transom, but it was underpowered for the yacht (28 foot, 3 500 kg). Later I put an 8HP on the back which was about the correct power, and then lastly I put on a Tohatusu 9.8 HP 2 stroke OB because it was 2HP stronger and about 10kg lighter. Then one day at the yard I put the OB into the transom well to see how much it would protrude (i.e. above the deck level would equal the depth of the leg below the transom well if I cut a hole).

The story is here with photos: groups.google.com/g/compass-yacht-group/c/4U5qWlntOoo
The inspiration came from James Baldwin here: atomvoyages.com/improved-outboard-well/

Issues: fuel consumption will be about 3 litres/hr compared to a diesel's 1 litre/hr for approx 10 HP. But it addresses your main issues of cost and that you can relatively easily put the outboard into a dinghy and take it home or to a mechanic to work on it. Recently I (had to) learn how to clean out a carburettor and fuel bowl while in the cockpit. I'm happy to have learned how to do it, as well as I buy a service kit online each year and service it myself (spark plugs, impellor, greasing, lower leg oil change, etc). It allows you to clear weed from your propellor and it puts the noise of a motor far to the rear and outside of the yacht.
That's the main points I can think of now. For me it was mostly an economic choice, but I do like to sail with no propellor drag now, and serviceability of the motor.

The OB well was made from 18mm plywood, I think. It's in the Compass Yacht post above. To join the plywood to the f/glass of the hull, I sanded away the paint and really roughed up the surface to allow a good bond between the epoxy/ thickened epoxy, the hull's f/glass and the plywood. Although it is above the waterline a few inches, it has never leaked. It has survived 8 years now of winter storms, some with wind against tide. No leaks.

PS thanks Graeme, I was writing the post ))

I also would like to hear what diesel you have.

I have a cat and have only ever had outboards. I would recommend you NOT use a 2 stroke. The Yamaha 9.9s are a great little motor and very reliable. There are some great things about the Yammie as opposed to any other outboard.

Gear ratio - the 9.9 high thrust has a 3-1 reduction ratio. This means the props spins slower (which is great and)
- you get a much larger prop - prop efficiency on slow boats like ours is directly related to prop size. You just can't get around the Physics. To get thrust we have to push a mass of water backwards. You can push say 10 litres of water at 1m/s or 5 litres of water at 2 m/s. The change in momentum (m x v) is the same and you get the same thrust BUT and this is a big but, the ENERGY required to move the water is different |

E = 1/2mv^2
Energy for 10 litres = 1/2 x 10 x 1 = 5 energy units -
Energy for 5 litres = 1/2 x 5 x 4 = 10 energy units

So going for a typical outboard with fast spinning prop is not good for efficiency. I swapped from a single outboard with a 9.5 inch prop to another with a 12 inch and the 12 inch one has SO much more grip

The Yammie has super reverse - probably 3 times the reverse thrust of a two stroke - we needed to change our anchor technique when we changed to a Yamaha.
4 stroke is much more efficient that a 2 stroke. Especially when lugging. Our 38ft catamaran motored across Bass Strait for her longest motoring trip. We did about 4 litres per hour at 7 knots in a calm with my 25 Yamaha. I also have a 9.9 and it doesn't sip much at all when we sit on 4 or 5 knots (which is rare). Nowhere near 3 litres per hour. I hardly have to fill the tank as we use the other hulls 25 most of the time.

So I would recommend the Yamaha 9.9 Hi Thrust - it is a proper pusher donk. Put it on a slide or pivoting bracket. Look up how cats do this and get it out of the water when sailing.

Get good at stripping a carby - I can do this in about 15 minutes. I have stripped carbys for about 30 years and it is very easy. The 9.9 wasn't starting last Sunday which is really rare (It has never happened before). So I pulled off the carby, laid it on a white piece of cloth and pulled it down. Couldn't find anything wrong, blew through everything I could with my mouth, put it back together and she hummed right up. In my 26 years of owning this boat with four stroke outboards there have been 2 times the carby needed cleaning because it was not running. Sometimes it gets a clean to run better and sometimes for a service but replacing plugs and doing the carby will take 15 minutes when you are practiced.


Cheers

Phil

I put a 9.8 on the back of my 22ft Space Sailer - which is way overpowered, but it was all I could find on the 2nd hand market when I needed to buy, and the extra horse power does give some confidence when passing under the bridges in Freo against the rush of the tide. The only time I get any cavitation is when I have to ride over the steep bow wave from a passing stink boat.

The engine is around 40kg, so I had to upgrade my bracket to something that could handle the extra weight. The Tenob one did the trick. I also had to cast an angled pad out of epoxy for the transom so that the mount fitted on level. I used a large tupperware box for the mold and propped it at the correct angle while the epoxy set to the right level.

Even with the heavy duty springs in the mount it's a bit of a pain in the... back to reach through the pushpit and lift the 40kg engine up out of the water, and then tilt it so that it stays clear when healed. It's also noisy and smelly.

My next boat will have an inboard! (and I'll probably learn to despise that too ;-) )

I like this boat’s outboard conversion, and many other things a lot.

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