Advise please: Aux motor size.

R

RichardtheBoffin

Well-Known Member
Joined
12 Oct 2012
Messages
72
Visit site
With a 26 foot cruiser, 6000lb displacement, worst case scenario, Poole Harbor entrance at mid-tide with a stiff westerly and an engine failure, what size aux motor should I be looking at?

I would like to store the motor onboard, so it would ideally be one man luggable...

I read that 1hp per 500lb displacement was a rule of thumb, but 10hp sounds a little light to me.

My boat has a bathing platform, so mounting would need to be on that, if possible.

Any advise most welcome!

Cheers.
 
I had a 25ft boat which weighed about 3 tonnes. It had a 9.9 Evinrude Auxiliary which would have been a waste of time if I had to use it at sea as it would only push the boat along at around 2 knots.
However it was very useful in maneuvering around a crowded marina a the 140hp yanmar diesel with a large 4 blade propeller when put into drive, at tick over would push the boat along at 5 knots. i.e too fast. so I would go out of the marina on Auxiliary then switch to main engine when out at sea.
On one occasion when it was very windy and I was on my own, I had my trailer at the bottom of the slipway and with the main engine pushing me along at 5 knots towards the trailer, and with the wind behind me helping the boat along, I was approaching the trailer too fast for comfort, so I put the Auxiliary in reverse, and the boat slowed to a better speed to drive onto the trailer.
 
RichardtheBoffin said:
With a 26 foot cruiser, 6000lb displacement, worst case scenario, Poole Harbor entrance at mid-tide with a stiff westerly and an engine failure, what size aux motor should I be looking at?

I would like to store the motor onboard, so it would ideally be one man luggable...

I read that 1hp per 500lb displacement was a rule of thumb, but 10hp sounds a little light to me.

My boat has a bathing platform, so mounting would need to be on that, if possible.

Any advise most welcome!

Cheers.
Click to expand...

Have you tried lifting and positioning a 10hp+ engine whilst on your boat? It won't be Easy! You would do better to either have it permanently mounted so it's ready for use as soon as you need it, or better still, make sure your main engine is in tip top condition so the likelihood of needing a spare is more or less negated.
 
longjohnsilver said:
Have you tried lifting and positioning a 10hp+ engine whilst on your boat? It won't be Easy! You would do better to either have it permanently mounted so it's ready for use as soon as you need it, or better still, make sure your main engine is in tip top condition so the likelihood of needing a spare is more or less negated.
Click to expand...

Ha! Yes, manhandling 35-40kg will be interesting. I may rig up a small davit.

My mooring is mid river, so I have a choice of either getting the outboard into my tender - very iffy, or getting it into the cabin area - hard work but a lot less likely to be lost overboard!
 
RichardtheBoffin said:
Ha! Yes, manhandling 35-40kg will be interesting. I may rig up a small davit.

My mooring is mid river, so I have a choice of either getting the outboard into my tender - very iffy, or getting it into the cabin area - hard work but a lot less likely to be lost overboard!
Click to expand...

You could get a 9.8hp 2 stroke tohatsu. It's only 27kg. Best power to weight you will find around 10hp
 
RichardtheBoffin said:
But will a 10hp move the boat?
Click to expand...

That depends

A 1hp outboard would move the boat. If you want proper control in choppy seas and strong tides you would probably need quite a bit more than 10hp. If you just want a means of moving the boat in light winds, gentle tides and flattish water, 10hp will be plenty.

I have had various outboards on the back of a 2500kg boat as an auxiliary. Never had to use one in anger, but messed around with them a few times so speak for some experience.
 
I've used a 8hp while on a tender and used the tender to try and position a 8 ton boat. It is certainly doable in calm water but building momentum to do anything happens very slowly
 
RichardtheBoffin said:
Yes but what size engine?
Click to expand...

Its a hard question, the obvious answer is as big as possible but that doesn't fit your other criteria and if you get just big enough then you break down on a spring tide, well it's not big enough.
My feeling is a small outboard can still keep you out of trouble even if it cant get you home. A ten is all you would want to handle on deck at sea. But have you heard of Mercury 10 hp bigfoot? Worth studying
 
BruceK said:
The point to appreciate is it's not really hp you are after but torque.
Click to expand...

So given that Tohatsu's website doesn't say anything about torque, which 10hp motor is going to give best torque? I would assume 4 stroke over 2 stroke?

Is there an optimal prop configuration for moving a larger boat slowly rather than planing a rib?
 
RichardtheBoffin said:
But will a 10hp move the boat?
Click to expand...

A 10hp will be limited on that size of boat. It will move it well in flat water but will be quickly overcome in adverse conditions. The constraints are weight of boat, windage, and difficulty of control with a small engine pushing from the back and a boat with poor directional stability at low speeds.

Have a look around and you will find very few boats of similar size and configuration have an auxiliary outboard, for all the reasons mentioned here, plus the fact that the necessary longshaft high thrust engine that might work is expensive and has no other use on the boat or tender. Better to spend the money on making sure your main engine is reliable.
 
We ended up having to use our 8hp standard shaft Yamaha outboard to propell our 26foot Bayliner 3 miles on to a bouy last year. It was man enough to push the boat to around 6 knots once up to speed. I used the outboard bracket that had been fitted to the bathing platform. It was a calm day and if there had been any chop then the shaft would have been too short so if it is going to be a dedicated auxiliary I would look for a long shaft.
 
Tranona said:
A 10hp will be limited on that size of boat. It will move it well in flat water but will be quickly overcome in adverse conditions. The constraints are weight of boat, windage, and difficulty of control with a small engine pushing from the back and a boat with poor directional stability at low speeds.

Have a look around and you will find very few boats of similar size and configuration have an auxiliary outboard, for all the reasons mentioned here, plus the fact that the necessary longshaft high thrust engine that might work is expensive and has no other use on the boat or tender. Better to spend the money on making sure your main engine is reliable.
Click to expand...

Well, the motor runs great and I carry spare ignition parts and fuel filter. Obviously there's 2 batteries too.

Then there's spare belts and water impeller.

So that leaves the possibility of a carb failure (unlikely to fail, but could possibly suffer a blocked jet), alternator failure, catastrophic engine failure or outdrive failure. The last two being impossible to repair in situ.

It's quite an old motor (early 80's) but hasn't been run hard at all. Mostly pootling about on the river at just over idle.

I guess the question is, is a 10-15hp outboard better than nothing if the main engine fails catastrophically?

Given that I'm not about to cross the English Channel, just run about in Poole Harbor, the risks are low.
 
Somerset Jim said:
We ended up having to use our 8hp standard shaft Yamaha outboard to propell our 26foot Bayliner 3 miles on to a bouy last year. It was man enough to push the boat to around 6 knots once up to speed. I used the outboard bracket that had been fitted to the bathing platform. It was a calm day and if there had been any chop then the shaft would have been too short so if it is going to be a dedicated auxiliary I would look for a long shaft.
Click to expand...


Thanks for the info. So how low is your platform? Mine is probably 6 inches from the waterline.
 
Somerset Jim said:
We ended up having to use our 8hp standard shaft Yamaha outboard to propell our 26foot Bayliner 3 miles on to a bouy last year. It was man enough to push the boat to around 6 knots once up to speed. I used the outboard bracket that had been fitted to the bathing platform. It was a calm day and if there had been any chop then the shaft would have been too short so if it is going to be a dedicated auxiliary I would look for a long shaft.
Click to expand...

Do I know you!
 
You must log in or register to reply here.

Share this page

Top