Big outboard as main engine.

Conachair

Conachair

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Will this work? Or any suggestions.

Severely bust inboard engine, and ain´t no way is any more money going to get thrown at it. So I´ve just laid my hands on a suzuki DT 20 long shaft outboard. The cunning plan is to strap this to the transom so at least I can move the boat until such times as a new inboard gets fitted. Shouldn´t be too difficult to fab up a bracket, steel hull and there are some rod steps welded already I can get a solid fixing to. Boat is about 10t, long keel steel cutter. No intention of moving in more than a calm day. Not quite sure about stopping yet..

Any flaws in my cunning plan.. ??
 
20hp is far too much. It will likely wizz around making lots of noise but not getting you very far. You need a very substantial bracket to take the thrust. Most brackets are rated to 15hp if of the type that swings down. You will need to weld on a substantial metal framework and pad to take the weight and thrust. Don't think you will be able to lift it on and off singlehanded without a tackle. If it is just for low speed work you are probably better rafting your dinghy alongside if you have an outboard.
 
Needs must.

Just watch/limit the pitching if the cowl is going to be immersed for more than a few seconds when motoring into a nasty head sea.( reefed main helos a lot)

I had DT10 l/shaft on 28ft and it was just lovely. But the boat was quite sporty and I had an extended, open-ish transom walkthrough so it was very easy to mount a rigid bracket at footlevel and lift/drop the motor or remove for servicing..when tilted the skeg 'just' kissed the water. I don't think the prop ever ever cavitated.
Maintenance and rip off spares became a thing of the past
 
Wot Tranona says but if you have the engine and can make a sturdy enough mounting I dont really see a problem, at least not in calm conditions.

The smallest available pitch prop would be sensible.

Swing down brackets come in different power ratings ... mine is limited to 8HP IIRC but up to 15 Hp ones are also common.
There may be even higher rated ones on the American market
 
Make sure it's swinging an appropriate prop for a heavy boat, don't be surprised if it drinks a large amount of fuel even over short distances, or when the prop lifts out of the water on the wash of other boats. I'd guess a mid-ships spring would be more effective at stopping the boat than the outboard...
 
In dead calm conditions, a smaller outboard will do.
Remember those canal barges? They were 1hp. One horse to be exact.
20hp is a lot of thrust at low speed.

The bracket needs to be strong, particularly when the motor is in reverse!
 
Well, boys, don´t you worry about the bracket, will be well over engineered and won´t cost a penny. Lifting the thing won´t be a problem either. :cool:

But sounds like it should work.

If that little sporting event hadn´t put the London berth prices through the roof the old girl wouldn´t be moving at all.

Ta V much :cool:
 
Many catamarans use outboard main power without problems, but also use Ultra long shafts (27") to ensure the prop does not lift out too much. As has been noted the 2 strokes can be thirsty (20HP - probably average 1 gall per hour) and a bigger pitch prop helps (would be better bigger diameter but normally not possible) although you cannot really go below the bottom of the RPM band or the engine will labour.

My 20HP stops me quite well but only 4tonnes/27ft cat.

I would equate it in performance terms to a 10HP inboard, which is small for your length/weight but perfectly viable as long as you accept its limitations.
 
The prop is the most important part of the equation.

Most outboards are designed to push light boats at high speed.Yours is a heavy boat doing slow speed. A high thrust propeller will make all the difference.It will grip the water at manouvering speeds instead of just thrashing the water like an egg whisk.

I have an 8 hp outboard on my heavyish 25 footer. With the high thrust prop it pushes an pulls quite well.
 
Outboard HP is rated at the prop, not the engine, so a 10hp ob is equivalent to about 12hp inboard. Displacement is less important than you think, the principle issue is inertia, you'll take a lot of effort to get going and a hell of a lot to stop! 15hp will be more than ample to manoeuvre at speed, a bow thruster for our 14t yacht is only 3hp and it has no problem pushing us sideways. As others have said, get a high thrust prop, the Mercury big foot came with a beefed up gearbox and prop and they come up second hand. A call to Bill Higham marine will source a good quality engine, they'll swap the prop for you and after you've fitted the new inboard you'll get most of your money back on the o/b. The Yam 9.9 came with ultra long shaft but they are priiiiiiicey.
 
Had similar problem. Had a Volvo Penta MD7B as original inboard. Eventually reliability was becoming a problem. Decided to fit a 15 HP 4 stroke long shaft Suzuki as an auxiliary. Got a bracket made to fit the transom and attached a heavy outboard bracket to this bracket. Due to the geometry had to get remote cables to operate the outboard. Eventually diesel died and for two years the outboard was the main engine on my 10m sailing boat. It worked perfectly well, even in reverse. The only down side was that despite being long shafted if the seas were quite rough the propellor was more out than in. However, this only curtailed sailing very slightly. Oh, aesthetically not too pleasing to the eye ;-)
 
mdonnelly said:
Had similar problem. Had a Volvo Penta MD7B as original inboard. Eventually reliability was becoming a problem. Decided to fit a 15 HP 4 stroke long shaft Suzuki as an auxiliary. Got a bracket made to fit the transom and attached a heavy outboard bracket to this bracket. Due to the geometry had to get remote cables to operate the outboard. Eventually diesel died and for two years the outboard was the main engine on my 10m sailing boat. It worked perfectly well, even in reverse. The only down side was that despite being long shafted if the seas were quite rough the propellor was more out than in. However, this only curtailed sailing very slightly. Oh, aesthetically not too pleasing to the eye ;-)
Click to expand...

Was it a standard prop?

Been thinking of sorting out some sort of bracket on the back for a while anyway, been let down by the engine before, bit stressy :(

Wonder how a 20hp would go on the back of a tinker tramp :D :eek:
 
Yes, used the prop that came with o/b. Didn't know any better! Described the requirements to the dealer and didn't have any problems. 5 knots under almost full power was normal. Hope this is helpful.
Mark
 
I was in a similar situation a few years ago with my heavy Bruce Roberts. The engine (2.5 BMC London Taxi diesel engine) block cracked and while I was getting ready to fit a 3 cylinder Yanmar, I put at the back of the yacht a 25hp long shaft Suzuki outboard on a bracket.

It worked very well and I got used to it. But the engine was far too powerful, I reckon that a four stroke 10hp outboard will be adequate; and a 15hp will be plenty of power. You may need to have a course propeller.

My current yacht, Macwester Wight, has a old Volvo 28hp, and if ever breaks down, I have installed a bracket and I have a long shaft 6hp Tohatsu, to get me out of trouble. It is interesting to see how fast the yacht goes with only a 6hp.
 
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