what size outboard

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jwilky

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after suffering saildrive failure (lobster pot) and sailing back up the mersey against an ebbing tide and failing wind.A night
on the moorings at tranmere gave me time to reflect. An
auxillary outboard could come in handy. what size would I need? Something that would give us about 3 knots. Our boat is
11 mtrs cruiser racer with retrouseu stern.The inboard diesel is 29hp.would appreciate all advice <span style="color:blue"> </span> to size/fitting etc
 
I did on a 31ft with swimming step/false stern. We had a hinged bracket just about cockpit floor level that the outboard could be lifted onto. I had a short shaft 2.5hp hp so I only tried it out in flat calm or harbour but it accelerated very slowly to about 3 knots.

If the engine had failed with some winds, even light, then I could have got the boat into shelter under sail then parked with the outboard. However if there wasn't much wind but it was choppy then I'd have been stuck.

Well worth giving it a go, I think even if you can't find a perfect solution.

Cost of the bracket and bolts - about a tenner.
 
I've made a bracket that drops over the rungs of the boarding ladder on our 32 footer. It sort of 'clips' into place so can't come off in normal use. We drop our 2 hp Yam onto it and away we go. Whole lot takes about 3 minutes to rig and get going.

So far we've managed about 2 knots in flat water while 'testing' so we know it's a useful (if feeble!) way of moving in an emergency.

Haven't tried it in windy conditions or with a strongish tide yet!! /forums/images/graemlins/crazy.gif
 
There is plenty of information (e.g. Calder and others) about how to calculate the hp needed to drive your boat at hull speed, I have not yet found any reference to the hp needed to drive the boat at less than hull speed....I would like to be able to construct theoretical fuel consumption/speed curves base on the fixed parameters of my boat and the specific fuel consumption of the engine (curves available from the manufacturer).

You will find that you need surprising little power to drive your boat at 3kt through the water. You could try to estimate the hp by measuring how long a litre (or better, 5 litres) will run the engine for at 3kt BS. Now get the specific fuel consumption figures for your engine and work it back to hp. You would need to do this in zero wind and calm sea conditions. BTW, your 29hp engine will only give you about 15hp useful power. So 15hp gives (presumably) hull speed. I would guess that for 3kt you would need around 5hp useful, so a 10hp outboard. BUT - do make sure that you select the correct pitch of prop as most 10hp outboards would have a prop designed for a planing craft as standard and you will end up making your engine labour, producing lower max hp and probably causing damage.
 
every yacht that has a tender has a means of driving their boat if the main engine fails. merely lash the tender alongside and operate the engine in the tender. worked for me for abt 2 miles even with a 2hp yamaha (did get a touch warm though)
 
Don\'t fit outboard to yacht - lash dinghy alongside!

After a similar experience this summer we bought a Seagull Forty Plus which, clamped onto the stern of our 9ft tender, lashed alongside, moves our 10 ton displacement boat quite creditably.

I recall that Don Street gets "Iolaire", larger and engineless, about, using just this method.

The reason for lashing the tender alongside rather than trying to fit the outboard on the yacht is that a little wash or small waves will swamp an outboard attached direct to the parent yacht.

Seagulls are favourite for this as they give a lot of thrust, don't overheat when used in this way, and can sit in a locker for years until needed.
 
Re: Don\'t fit outboard to yacht - lash dinghy alongside!

I have an OB bracket on Jissel's transom and have used it on a number of occasions, either to help an ailing MD1 along or to get home after it died. My 3.5 short shaft pushes a 24ft snapdragon along happily at around 3kts at half throttle.

The big problem is that as soon as the boat starts to roll, the prop lifts enough to draw air in and cavitate. It's also quite entertaining coming into a marina berth singlehanded, because the only way to control your speed is to lean over the back of the transom when you should be looking where you're going...

Big improvement on nothing, though!
 
Re: Don\'t fit outboard to yacht - lash dinghy alongside!

I must admit that I have not tried berthing Mirelle in a marina singlehanded relying on the Seagull! It's bad enough with the MD2!
 
Re: Don\'t fit outboard to yacht - lash dinghy alongside!

[ QUOTE ]
IIt's also quite entertaining coming into a marina berth singlehanded, because the only way to control your speed is to lean over the back of the transom when you should be looking where you're going...

[/ QUOTE ]Just as food for thought rather than any sort of 'safety lecture', I would suggest that whenever one is faced with handling a boat in conditions of reduced manoeuvrability for WHATEVER reason, in a marina, that one would be best advised to call the marina office and ask for assistance. Otherwise if there is any damage caused as a result, it might be expensive. Just a thought, and hopefully no responsible marina would charge for this service.
 
Confession

I haven't actually done it in a marina, the club pontoon's bad enough, so by extension, well, err, gulp /forums/images/graemlins/tongue.gif

I have to agree with the safety lecture!

The OB does have one advantage though, you can stop the boat then drive the stern in any direction you choose!
 
Re: Confession

It's not so much a safety issue as protecting one's own interests. The way marinas pack the boats in these days makes it near impossible to berth without some form of outside assistance in some conditons. Trouble with UK marinas is that they don't seem eager to help whereas I have found in Spain (so far) that there is always someone there to help you dock - all part of the service. And many of the Spanish marinas are far less expensive than the British ones....and the weather is better too /forums/images/graemlins/smile.gif
 
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