prop/leg position

[Madhatter]

One of my obscure questions again
Does a prop/leg have to be centrally mounted ?, I ask this as most aux outboards seem to be mounted to one side with no side effects ( that I have noticed )

 

[VicS]

Outboards mounted to one side do have side effects. The boat will turn far more easily in one direction than the other.

Mine, with the outboard mounted to port, will easily turn in a tight circle to starboard. To make an even a not so tight turn to port I have to build up the steerage way and then reduce the power.

The engine was on the stbd side prop walk might even make matters worse. I dont know, but large outboards have trim tab that is adjusted to offset that effect.

 

[Seajet]

Rudder 'Authority'

My boat has an outboard well just to port of the centreline; she turns - spins more like - very strongly in either direction, mainly because the prop is still ahead of the large rudder.

The ultimate asymmetric outboard example must be the canoe stern dayboats which mount an outboard on a gunwhale aft; apart from the risk of dunking the thing when heeling, I expect it's a case of applying power gently until way is on, but hammering into a marina and slamming on full reverse is an even worse idea than usual !

 

[TQA]

Some wooden boats had the engines mounted and angled in such a way as to allow the propshaft to exit well to one side to avoid cutting through the keel or stern timbers. This was often termed a wing engine and gave some unique handling problems especially in reverse.

 

[prv]

Some wooden boats had the engines mounted and angled in such a way as to allow the propshaft to exit well to one side to avoid cutting through the keel or stern timbers.

See eg http://www.boatdesign.net/forums/at...169121-roberts-spray-36-too-slow-img_3457.jpg

The owner designed the boat like that; he reckons it's faster under sail than cutting an aperture in sternpost and rudder.

Pete

 

[Tranona]

That sort of setup was very common in the early days of adding auxilliaries. Avoids boring the stern post which was often long and narrow and does not cut chunks out of the rudder, many of which were marginal anyway. Downside is getting the area where the shaft log is situated strong enough to take the thrust - well known point for leaking seams and cracked frames. Engine also often has to be set well forward because it is at an angle. Not so much of a problem if you have a wheezy old Stuart Turner, but more a problem if you want to have any real power.

 

[prv]

The one I posted was designed like that in (I think) the 70s. Fairly powerful engine; apparently she's poor to windward having a wide flat hull, so he mostly motorsails if going upwind. I guess being by design, the structure is probably strong enough. Instead of being at an angle, the engine is off-centre - its box forms the saloon table, with the dry exhaust pipe wrapped in fibreglass lagging passing through the table and up out of the cabin roof!

Pete

 

[Madhatter]

Thank you all for your replies.
A while ago we all entered a debate about inboard/outboard engines when I posed the question of what to do on my project boat, financial restraints have pushed me towards an outboard in a well but fitting it in the centre line is quite a large undertaking (but achievable )

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As you can see it could be done with some keel modification

 

[Tranona]

That hull shape does not lend itself to having an outboard in a well. It will need to be significantly offset to be able to swing a big enough prop to avoid the keel and unlikely you will be able to turn it for steering. Difficult to make it run in a straight line with all the thrust on one side of the keel and rudder - and fun in reverse. Lot of work building a strong enough false transom to take the thrust and you run the risk of fumes coming back up the well. If you can't get the inboard to work then an outboard on a bracket is the most practical alternative.

 

[Madhatter]

That hull shape does not lend itself to having an outboard in a well. It will need to be significantly offset to be able to swing a big enough prop to avoid the keel and unlikely you will be able to turn it for steering. Difficult to make it run in a straight line with all the thrust on one side of the keel and rudder - and fun in reverse. Lot of work building a strong enough false transom to take the thrust and you run the risk of fumes coming back up the well. If you can't get the inboard to work then an outboard on a bracket is the most practical alternative.

Thanks for your input Tranona,but a stern mounted out board is out of the question as access to it when in operation is almost impossible and I have lost one outboard too excessive cavitation. Modifying the centre keel (and rudder)at the rear is no problem for me to do along with a full width bulkhead strong enough to take the outboard ( and the sea if it comes to it ), as for fumes they should be no problem to get rid of .

 

[Tranona]

All I can say is good luck! The boat was designed for an inboard so hacking it about to fit an outboard in a well is going to be a gamble. Fumes is a difficult one - you won't know whether you have a problem until you have done it, but fumes coming back up from the underwater exhaust and the relief port is a common problem with wells. If you do have fumes problem there is little you can do about.

An extra long shaft 8-10hp outboard on a swingdown bracket with remotes and possibly an electric start would be a better route to follow.

 

[Seajet]

In praise of engine wells

I just can't understand the fixation with 'fumes in outboard wells' on this forum; Tranona usually has good wise words, but I have to say I think has a coloured opinion of outboard wells having suffered a poorly set up one.

I have a 2 stroke Mariner 5hp in a well using 100:1 oil, before it were Evinrude & Yamaha 4hp 2 strokes at 50:1; at no stage have I ever found the engine or self choking on fumes, and I've used these engines to motor 70-80 miles cross Channel in calms several times.

A well has many advantages for a boat say 25' and less;

it keeps the prop immersed when pitching

has the engine controls easy to hand, not facing backwards to adjust when entering marinas, as transom mouted jobs require unless remotes are set up...

it places the prop in front of the rudder for tight manouvering

one can feel the coolant outlet by hand to check all is well temperature wise

if one picks up a lobster pot etc it's easy to pull it up and breadknife off the offending article

it can be pulled up and stowed on sailing trips ( inserting a fairing plug ) vastly reducing drag and likelyhood of picking up lobster pots in the first place and markedly reducing / speeding up passage times let alone racing

the engine is handy to work on, no crew or tools disappearing over the back end

the weight distribution is much better than a transom bracket

much less vulnerable or tempting to thieves

it won't foul shore lines when rafted up

It looks a hell of a lot better too !

 

[misterg]

... a stern mounted out board is out of the question as access to it when in operation is almost impossible and I have lost one outboard too excessive cavitation. Modifying the centre keel (and rudder)at the rear is no problem for me to do along with a full width bulkhead strong enough to take the outboard ( and the sea if it comes to it ), as for fumes they should be no problem to get rid of .

I wouldn't worry about putting the prop off centre. Our previous boat had an outboard in a well that was way off centre, and that was no problem at all going ahead. I used to give it a very slight offset so that the boat tracked straight with no hand on the tiller. Going astern was entertaining (in a bad way), but not unpredictable - you just had to learn to deal with it.

I would prefer to do that than hack through the keel, etc. to get the well on centre.

YMMV.

Andy

 

[Tranona]

I just can't understand the fixation with 'fumes in outboard wells' on this forum; Tranona usually has good wise words, but I have to say I think has a coloured opinion of outboard wells having suffered a poorly set up one.

That is precisely why I made the comment - it is in the context of the OPs boat. I know wells can work, but also know that if they don't work there is nothing you can do about it having done all that work to create it in a hull shape that does not lend itself to a centerline well, and an offset one will likely create different problems for handling.


There are of course all those benefits if you could get the prop in a well between the back of the keel and the rudder, but it is not just a case of cutting a hole in the bottom of the boat.

 

[RichardS]

This reminds me of an Ionian charter many years ago - we hired two indentical boats with a group of friends. Both had the same off-centre outboard in a well and were steered through a tiller and rudder.

We were each given a 20l jerry can of fuel and followed each other for two weeks either sailing (i.e. racing!) or motoring at the same time.

On the first day we noticed that my boat "crabbed" sideways under power whilst the other boat tracked true.

By the end of the holiday the other boat were still using their first 20l jerry car. We were on our third jerry can having filled it up every few days. At the end we ran out again about a mile from our home base and were towed home!

A mis-aligned motor can apparently prove expensive on fuel!

Richard