outboard size for heavy displacement sailboat?

Hi all
Would appreciate any insights from anyone who has used an outboard to power a heavy yacht. I appreciate that it is not ideal but I would like to hear from anyone who has the experience.
I have a 10 ton full keel 35ft yacht with an unknown Chinese inboard diesel which I want to lift out to make use of the space.
I will install the outboard in a well about 3 feet from the stern just above the water line adjacent to where the rudder shaft goes through the hull. The outboard leg will be offset to port by about a foot but I would hope that with the long keel and prop walk that this won't be a problem.
My thoughts are that a 15 to 25hp with the larger prop and reduction gearing will give reasonable performance in low winds for coastal work.
I intend to remove the outboard motor completely and plug the well for voyaging.
Any comments appreciated.

 

Hard to get hold of a diesel outboard. Next best is a 4-stroke. It will be considerably less efficient than the diesel so more fuel for the same job. You are dealing with a flammable fuel rather than combustible. Hence you need to be careful with petrol fumes, venting and location of the fuel tank.

If the boat has a tendency to hobby horse then there is a risk of lifting the prop out of the water. I would go for the larger motor so you can can reasonably way and get the hull wave forming and resulting sinkage if you need to in heavy weather. No doubt a 15HP would move it in good weather but you will always get caught out somewhere and need good motoring capability.

You also need to make sure the motor gets good air so it is runs smoothly. Some cockpit mounted outboards are sealed up to reduce noise that they do not breathe well. A simple labyrinth intake will dull noise and still allow good air flow.

The offset thrust is a negligible item. The bigger issue is having the prop come out of the water if there is a beam sea and the wind causing some roll.

Overall you will not have the get out of jail capability of an inboard with a decent prop. I expect you will lose ability to motor off a lee shore in heavy weather.

Make sure it is electric start with a decent alternator so you can charge battery with a little running unless you have other means of charging. Irrespective you do not want a pull start. Also set up remote control so you are not fiddling inside the well to adjust speed or change gear.

I had an outboard on a moderate size yacht for a few years and installing a small sail drive was the best thing I ever did. It made it a real boat rather than a day sailor. Diesel makes a lot more sense for a small yacht.

Rick W.

 

Hi azymuth,

In ideal conditions I would think you could get about 5 knots on 15 hp, maybe 6 knots with 25 hp. (1467 and 880 lb/shp respectively, with approx. 50-60% propeller efficiency.)

So the outboard conversion would be able to putter around the marina, and probably handle some light coastal weather. But those numbers will drop- a lot- when beating into wind.

From your description of where you plan to put the well, I think it's a reasonable approximation to say you'll get the top of the prop 15 to 20 cm below the surface of the water. Maybe 30 cm at best.

Now think of how big a wave it would take to lift the stern 30 cm clear of the water. 35' LOA, let's say 30' LWL.... an ocean wave with a 30' wavelength travels at about 7.3 knots and might be 2' to 3' peak-to-trough. Beating into, or running with, seas much larger than that, would be very likely to lift your stern by more than the foot or so (max) that your prop would be immersed. You'd be losing thrust completely every five seconds or so.

In the end, I think the conversion would lose the inboard's biggest advantage- its reliable, always-there thrust that can pull you away from a lee shore, or a reef, or whatever else.

Is the space the engine occupies in the boat really so obtrusive that getting rid of it would be worth the huge step down to a kicker OB?

 

Thanks Matt and Rick

I can get a 25 hp 4 stroke big foot extra long shaft which will set the prop 2 feet below the water line - the stern is quite full and doesn't pitch up and down much up to moderate conditions.
The rational in removing the engine is the engine beds are not structurally extensively braced and it being a ferro boat and a big agricultural Chinese diesel, I think that could be an issue and difficult to reinforce.
I worry about being able to find spares for the 290C Chinese diesel and new diesels are over 10 grand - second hand being almost not available.
Most important I have a big lazarette which is unused and the engine space would make an ideal aft berth.
I agree that a good inboard is a great help for fighting off a lee shore, I will do the best I can with a cutter rig with a good flat cut staysail and having to be more cautious. But I do take on board your advice.

 

Honda Make a 20 hp LRU ( £2000 ) with a high thrust propeller specifically for displacement boats they are a super reliable economical and may do the job for you I had a breakdown on the Humber estuary England on a 10 ton wooden cruiser we strapped an inflatable with a 5hp Honda on to the starboard quater and made 3 Knots. The owner was so impressed he had an aux bracket fitted with a 5hp Honda which gves him 4 knots in flat water.Having said all that I woud probaly go for a Kubutu based inboard ( Nanni diesel) of say the 29 hp 3cyl for about £4000 or convert one from a mini digger ,note the industrial engine differs from the marine but will work.

Jack

 

25 hp TO 30 HP IS A GOOD SIZE they also have electric start and charging circuts plus you can get a 25 inch leg . Yamaha has a 3 cylinder model
Its even possible to get them in a diesel and in the islands Kerosene as well !. Go for a big diameter prop rather than smaller dia , it has a bit more push with the bigger blade area :idea:

 

Get a high trust engine not a regular. Yamaha makes several models and I think Honda may make one. The 10hp high trust yamaha I saw on a 30' very heavy fishing boat. (12-15,ooo lb. )The owner said it would push him right along with no problem even up river. He used it as a kicker engine. Yamaha makes them up to 60hp I believe. http://www.yamaha-motor.com/outboard/products/subcatspecs/4/specs.aspx

 

The problem comes in heavy seas when propeller is in the air and you are throwing up trying to hurrying to safety and wish you would spend the extra money ... Buy a good old used diesel motor, I can get a 20hp for about $500 then rig exhaust and keel cool system for another $300.

 

I'm confused. Would'nt the prop be in the air whether it is a gas outboard/inboard or a diesel inboard? If it is an emergency I'll take modern Japenese engineering (proven) versis some old diesel. Also a lifting transom can be designed so that there is no drag when not needed. that is if there no room to fully tilt up.

 

Outboard in a storm in heavy seas

By necessity the prop is going to be what 20 inches below water or less, and at the rear most point of the boat. The prop is also small in comparison to inboard and has less grip in the water. It is ok for moving boat around marina, but in heavy seas will probably be sucking air instead of pushing the boat. Following seas hitting boat from rear, perhaps engine going underwater and high in the air....

I have no problem with outboard on a planning boat or in good weather close to shore. Heavy seas in the middle of the ocean, get me an inboard, diesel prefered.

 

I have no problem with what you are saying, however, the High Trust outboards have much larger power props and they sit as deep as an inboard prop. I am an inboard man for all seasons but he needs a simple kicker engine/s, I believe, and he can buy 2 High Thrust outboards for the price of a new small (40hp. diesel.) Now that is security and he does not need a genset. I'm not a sailing man so expect from me no expertise re: that. If he put in two, one on each side of the keel, (placement where ever??)
he would not need thrusters and could glide into port easily. 2-10hp would do the job and be triple back-up

 

"10 ton full keel 35ft yacht on a small outboard...."
"My thoughts are that a 15 to 25hp with the larger prop and reduction gearing will give reasonable performance in low winds for coastal work.
I intend to remove the outboard motor completely and plug the well for voyaging. "

Your are setting yourself up for a bad day. This is a good backup engine in good weather. Not for traveling or perhaps even manuevering in the bay. What is the boat going to do 4knots. The tide will carry him around, to say nothing of a little wind. Boats are cheap, motors aren't, if you really need a bigger boat buy one. Don't risk your life and boat by being so grossly underpowered.

 

I use following rules of thumb:
1HP per 1 ton of displacement - just enough to move boat in marina
2...4HP per 1 ton of displacement - reasonable range for monohull sailboats
4...6HP per 1 ton of displacement - recommended range for blue water cruisers, boats with some service loads and minimum for sailing catamarans with high windage.
6...10HP per 1 ton of displacement - motorsailer's range

 

Alik, I will not question your judgement as your education exceeds mine in the general field of Naval Architecture, how ever,these new High Thrust engines give more bang for the buck than your post. They are extremly powerful and I salute Yamaha for their enginering. If I mount 2-40hp I feel I have 2-70hp. thats all, but sometimes an outboard is the answer, sometimes not. You will get no hard argument from me. As always, price is a main consideration with safety and dependibilty always # 1,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

 

resorinc, I agree, NA always wants to put bigger engine to be safe on speed/service power/safety, boat owner would prefer smaller one for purchase cost/fuel saving. The thruth is somwhere between...