Outboard Nacelle?

[ducked]

Thinking about this as a substitute or supplement for my inboard, given that an outboard on the stern is vulnerable to drowning and/or lifting the prop out of the water in waves, and I would be nervous of the boat butchery required to create an outboard well, which seems to be the standard workaround for those problems.

Surprisingly, I've only seen it discussed for multihulls, where the problems are less severe because you can relatively easily mount the engine further forward, typically at the back of the bridgedeck. The nacelles discussed and illustrated seem to simply protect the engines from waves from ahead.

I'm thinking a full outboard engine enclosure for the back of a monohull could allow the engine to be mounted low down but continue to breathe in following seas.

I suspect it may not be practical to take such an enclosure below the static external water level using a watertight seal around the outboard leg, (though I''m not sure on that point) but think it should be possible to get pretty close to the water level.

It might also be possible to build buoyancy into a nacelle to compensate for the weight of the outboard on the stern, but perhaps not very necessary, and it might increase the structures wave loadings.

I would think something along these lines must have been attempted, but havn't seen any reference to it.

 

[VicS]

Exhaust gases, and water, from the back pressure relief port(s) is likely to be problem with any type of enclosure unless you can route them outside the enclosure.

.

 

[Boathook]

Originally the outboard on my cat was right at the stern. It did cavitate at times but mainly when going very slow in biggish waves. It seemed to spend time under water if really rough but always carried on working. We eventually had it moved further forward. We had a nacelle moulded that the engine is bolted to.

What you are proposing is a sail drive. Sealing round the outboard leg will be difficult due to exhaust relief ports as mentioned by VicS, and that the engine always slightly moves as it is mounted on rubber bushes.

 

[ducked]

Originally the outboard on my cat was right at the stern. It did cavitate at times but mainly when going very slow in biggish waves. It seemed to spend time under water if really rough but always carried on working. We eventually had it moved further forward. We had a nacelle moulded that the engine is bolted to.

What you are proposing is a sail drive. Sealing round the outboard leg will be difficult due to exhaust relief ports as mentioned by VicS, and that the engine always slightly moves as it is mounted on rubber bushes.

 

[ducked]

Sealing round the outboard leg was a tentative speculation, and not an essential part of the idea, though it could perhaps allow the prop to be deeper. Engine movement would be a problem for such sealing if it couldn't be accomodated by a flexible seal.

I'm unclear why or how the exhaust relief ports mentioned would be a problem, but then I've never had an outboard. My understanding was that the exhaust was typically discharged down the leg, by the prop, so I'm guessing maybe the exhaust wont discharge or avoid engine flooding if subjected to an increased external hydrostatic pressure, so the discharge might need re-arranged

In the absence of outboard leg sealing what I'm proposing doesnt seem to be fundamentally different to an outboard well, except that it would be external to, and not an integral part of, the hull.

 

[Tranona]

Sealing round the outboard leg was a tentative speculation, and not an essential part of the idea, though it could perhaps allow the prop to be deeper. Engine movement would be a problem for such sealing if it couldn't be accomodated by a flexible seal.

I'm unclear why or how the exhaust relief ports mentioned would be a problem, but then I've never had an outboard. My understanding was that the exhaust was typically discharged down the leg, by the prop, so I'm guessing maybe the exhaust wont discharge or avoid engine flooding if subjected to an increased external hydrostatic pressure, so the discharge might need re-arranged

In the absence of outboard leg sealing what I'm proposing doesnt seem to be fundamentally different to an outboard well, except that it would be external to, and not an integral part of, the hull.

Outboard wells are a poor solution unless very open. They then usually create a huge amount of drag. The exhaust problem is partly the gases coming out of the lower outlet and then if the leg is sealed from the back pressure vent in the upper leg.

If this is for a sailing boat with an inboard diesel I cannot imagine why you think you need an second engine let alone an outboard. I can see why many small power boats have an emergency second source of propulsion - but a sailing boat?

What boat and engine do you have?

 

[Sailing steve]

Outboard wells are a poor solution unless very open. They then usually create a huge amount of drag.

They'll create a huge amount of noise too in any sort of chop with water sloshing and glopping in and out all night when you're trying to sleep as well.

Ask me how I know.

 

[MisterBaxter]

What sort of boat is it?
I always got on ok with an outboard in a bracket, in three subsequent boats of 24' and under. It's a big advantage to be able to get the prop out of the water and the effect on performance under sail was really noticeable. And they never got swamped, in the choppy waters of the Bristol Channel.
But I suspect an outboard is only really worth it in a slippery boat with good sailing performance that can manage with 5hp or less, with a skipper who chooses to motor only when absolutely necessary - and who plans trips so that it's necessary as rarely as possible.

 

[westernman]

Adding a backup engine to a sail boat seems very strange.
If the engine is broken then you have sails.

If you really do think you need another engine, then consider an engine for the tender and then use the tender strapped along side to move the boat.

Or if the boat is not a 30T 50footer, then may be a sculling oar?

 

[andsarkit]

I have tried to scull 5 miles home on a Soling (27ft easily driven and no engine) and it was hard work and we eventually accepted a tow. Look up Yuloh if you want to investigate further

 

[thinwater]

The other problem is the motion of the transom. On a planing boat, the transom level is fixed. With a displacement hull it moves up and down and the water flows vertically, adding to prop inefficiency. The ONLY solution is to move the prop closer to the center of gyration, which is typically about 2/3 of the way back. My PDQ has wells in the cockpit, which allow the motors to function well in considerable waves, like an inboard. My F-24, on the other hand, has the outboard on the transom, which is bad. Motor sailing helps stabilize the pitching, but the right solution is to move the motor forward. The problem is NOT flooding, it is the vertical motion which ruins the flow over the prop.

 

[ducked]

This is a Trident 24 with a Yanmar 1GM10. Its been abandoned for about 5 years and I dont currently know if the engine will be recoverable, so I'm thinking of alternatives.

I take the point that an engine on the transom isnt in the optimum place even if its in such a "nacelle well", but it seems it might be slightly better off than if simply exposed.

Engine flooding and prop aeration are both frequently reported problems. I suppose a nacelle might help damp vertical motion to some extent also.

 

[Tranona]

Yanmar 1GMs are really simple little engines so hopefully you will be able to get it running reliably. An outboard on that design is always going to be sub optimal with the overhanging stern and aft deck, If the boat is in good nick otherwise it is probably worth putting in the money and effort to get the engine working.

 

[ducked]

Yanmar 1GMs are really simple little engines so hopefully you will be able to get it runnonstructed n good nick otherwise it is probably worth putting in the money and effort to get the engine working.

I understand some Tridents have had outboard wells constructed in the stern locker, or the rear of the cockpit, but I will be trying to avoid that if possible.

 

[garymalmgren]

Get the Yanmar back in action.

 

[simonfraser]

you'd prob do better to spend your time and money on making the inboard as reliable as possible

 

[Boathook]

This is a Trident 24 with a Yanmar 1GM10. Its been abandoned for about 5 years and I dont currently know if the engine will be recoverable, so I'm thinking of alternatives.

I take the point that an engine on the transom isnt in the optimum place even if its in such a "nacelle well", but it seems it might be slightly better off than if simply exposed.

Engine flooding and prop aeration are both frequently reported problems. I suppose a nacelle might help damp vertical motion to some extent also.

In 45 years I've never had an outboard flooded by water and I've never read of an incident except when a dinghy overturns or the outboard falls off.

I would spend money on sorting out the yanmar rather than mucking around trying to fit an outboard.

 

[KevinV]

As above, fixing the 1gm is both simpler to do and much easier to live with than messing around with an outboard/well whatever.
I have an outboard in a well and it is a long way from ideal - I'd love to have an inboard diesel.

 

[William_H]

As said fix the inboard. However my little 21 fter has been used in a lumpy water with an old 6Hp Johnson long shaft. It is mounted on a bracket which can allow for lowering by about 4 inches from middle or raised about 4 inches. The load of crew in the cockpit do affect the transom height . I have been amazed how it keeps running even when a wave from stern will put water level up around the power head casing. The casing seems to act as a nacelle you envisage. I guess if I was really concerned a snorkel extension of carburetor air inlet might help. Perhaps giving a few more inches of under water capability. Leaving spark plugs as next vulnerable point.
My point is don't worry about the engine in a sea way just keep it low enough to avert most cavitation. ol'will