Auxiliary outboard power

[robertt]

Hi
I am toying with the idea of buying a Bayliner 1802 with 3.0 petrol inboard for exploring the sea lochs around the Clyde Estuary area but was thinking it might be prudent to have a small outboard as a standby in case of engine failure. Can anyone advise me as to what type of outboard wd be suitable in this type of boat to get me out of trouble during coastal use? There would likely be 3 persons aboard.
Thanks as always
Robert

 

[neale]

Hi Robert

I have been through this with my 25ft Bayliner and found the following.

In calm still waters a 4hp will move the boat around perfectly adequately.

To counter a current, or to keep control of the bow in a breeze, I really needed a 10hp. A 10hp would give me around 5 knots with no current, so a strong current would still cause significant problems.

In lumpy seas it was very difficult keeping the prop in the water so a long shaft is really required, although I only ever had short shafts on the back of mine.

Four stroke outboards are heavy but you have the benefit of being able to use your main fuel tank to supply the aux engine giving you a much better range with a little bit of plumbing.

Two strokes are light and easier to handle but will need a separate tank of pre-mixed fuel and finding somewhere to store it safely is an issue on a small boat. Mine was strapped to the bathing platform.

I found that a 10hp 2 stroke mariner at 35kgs be be about the limit of what you want to be manhandling on a regular basis. For four strokes, this weight means a max of maybe 6hp.

If you can find one, the 2 Stroke Tohatsu 9.8 at 27kgs offers the best power to weight you will find in an aux outboard.

An aux outboard may get you home but don't rely on it 100%. Have a radio to call for help, an anchor to hold your position if in shallow water, and consider a sea anchor to slow your drift if in deep water, as a backup to your backup.

Make sure you have lots of consumable engine spares on board, fixing the main engine is preferable to trying to get home on an aux outboard if at all possible.

 

[robertt]

Thanks Neale, that's very helpful. You could say that just about covers every point!
I would need to go on an engine maintenance course as I would be useless at diagnosing and managing an engine fault which is why I hoped an aux wd allow me to get home and then worry about it.
I have been reading your excellent atricles in MBM but am not very confident as yet.
Last time you gave me advice I hadn't realised you were the Neale from MBM!
I am exploring all my options in my struggle to get afloat in any way possible.
I looked at your boat for sale and it does look well equipped. The photos on yachtworld were partic good, better than those on moonfruit I thought.
I would love to buy a small diesel cruiser but my options are limited currently by financial constraints (as with everything!), partic as my wife is not keen on me spending much money at present. I'm hoping she will come round

 

[ontheplane]

Also, if at all possible seek out a "HiThrust" or "Sailpower" type engine.

These are designed to offer more thrust (which you need) at lower speed.

A small outboard is designed to push a small boat (dinghy, inflatable etc) at a reasonable speed - when fitted to a large heavy boat it "bogs down" as the prop cant turn at it's full speed, and ends up slipping in the water and so on.

The High thrust engines are designed to only achieve a few knots, but to achieve their peak power and revs - hence why they fit them to sailing boats and so on.

 

[whisper]

Where will it fit?

Hi
I am toying with the idea of buying a Bayliner 1802 with 3.0 petrol inboard for exploring the sea lochs around the Clyde Estuary area but was thinking it might be prudent to have a small outboard as a standby in case of engine failure. Can anyone advise me as to what type of outboard wd be suitable in this type of boat to get me out of trouble during coastal use? There would likely be 3 persons aboard.
Thanks as always
Robert

You need to consider this as you will need to find somewhere that will take an outboard bracket with the engine being low enough for the prop to be in the water. On previous boats I've used a bracket that could be raised and lowered with the assistance of springs. Very handy bit of kit as it means the prop stays clear of the water when the engine is not in use.
Like others on here I'd try to find a 9.8/10hp 2 stroke if possible. Failing that then an 8hp 4 stroke may just about be handleable,weight wise. I would think that a 6hp would give you steerage way in calm conditions, but if the weather turns for the worst you may have to make for another destination that is downwind.

 

[robertt]

Thanks for reply
Is this an outboard bracket in the pic? I thought that was a bracket that was already fitted (not sure!) but doesn't look to be adjustable in any way?

 

[andy59]

Yep , thats a hi / low outboard bracket , some have spring assistance and basic ones dont , they just lock in place .

 

[ontheplane]

I would disagree you need as much as a 9/10 hp

If you get a standard o/b then yes - you will need that much to make any headway - but you will need to change the prop and it will still bog down.

You do need to look at a sailpower engine - it's designed to push large heavy boats slowly rather than small lights boats fast (i.e an inflatable).

To give you an example - we did a test once with a 28ft Rinker - I can't remember the EXACT figures but roughly...

We fitted a 30hp yamaha 2 stroke and got 5knots (ish) - it struggled to rev much over 1/3 throttle

We fitted a mates 9.9 hp Mercury and got 4.8 knots (ish) and it STILL struggled to rev much over 1/3 - 1/2 revs

We then fitted a 4hp Sailpower. It revved out to it's full revs (or at least sounded like it) and we got 4.9 knots.

It's not power or speed you need - remember you're not going to get more than 4-5 knots on a 21 ft boat - it's THRUST you are after.

 

[robertt]

Thanks for info
I noticed the speeds were similar with all 3 outboards you mentioned, so is the amount of revs an important issue? Is it better to have high revs as opposed to getting the same speed at lower revs?
I saw the Mercury Sailpower engine for sale at around £800 new, doesn't sound too bad?

 

[thomshap]

There's a new Yamaha 9hp 2stroke long shaft on eBay at the moment, that would be perfect (not mine though I do have a used one for sale)

 

[ontheplane]

Outboards produce their power at about 80% of peak revs.

If for example a 9.9 hp can only get to 30% of peak revs (lets say 1500 - 2000 rpm) then it may only be producing 4hp...

The issue is - a 9.9 is designed to run a small inflatable at planing speeds - and the prop is set up for that.

You can get a lower pitched prop to allow the engine to spin up but the real solution is to get an engine designed for what you are trying to do - move a heavy boat at a slow speed. You are NEVER going to get "over the hump" and on the plane with an auxiliary, so why try???

Remember on the canals they used to move a hundred ton boat with 2 horses...

It would be good if MBM could run an article on this and actually test some sailpower outboards against ordinary outboards to show the difference on a 21' boat - c'mon guys - that would be a very interesting test I think.

 

[neale]

It would be good if MBM could run an article on this and actually test some sailpower outboards against ordinary outboards to show the difference on a 21' boat - c'mon guys - that would be a very interesting test I think.

We have run this test a couple of times, the last was back in 2004 but the principle won't have changed. (Although I agree it is probably about time we looked at it again).

On that occasion we used 3 boats, an orkney 20, an Arvor 25 and Failrine 26 Sun Fury. On the back of each we put a mixture of outboards from a 3.5hp 2 stroke Tohatsu weighing just 13kg to a 15hp Mariner two stroke weighing 35kg.

Because this was back in the day when two stroke was king, we only tested one four stroke motor, but from your point of veiw it was an interesting one in that it was an 8hp Yamaha High thrust. It weighed a whopping 45kg so you are not going to be taking it on and off, it would need to be permanently mounted.

The interesting thing is that on the Arvor, the 8hp Yamaha high thrust achieved 4.8 knots at 2/3rds throttle. On the same boat we tested a 9.8 Tohatsu 2 stroke which weighed just 27kgs and that achieved 4.9 knots at 2/3rds throttle. (We did change the props on some of the engines for one better suited to the task, although I cannot remember if this included the Tohatsu 9.8)

We also tested a 2 stroke 6hp on the Orkney which gave us 4.2 knots at 2/3rds throttle and 5.1 knots at WOT compared to a 4hp Sailmate which gave us 3.9 knots at 2/3rds throttle and 4.5 at WOT.

I would agree that high thrust motor is better designed for the job, but for power to weight I was sold on the Tohatsu and went straight out and got one.

I would add that in spite of covering serious miles in my single engines Bayliner, including trips from Essex to the Channel Islands, I never once had to use the aux engine in anger.