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Old 20 June 2010, 13:57   #1
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Size of auxiliary engine

I have a 6meter RIB with a 115 hp petrol outboard main engine which I use for cruising around the Greek islands. Up to now I have seldom been more than a few hundred yards off shore but now that I am used to the boat and the waters I want to undertake journeys that will take me 4 or 5 miles off shore. For safety sake I want to fit a backup engine to get me to shore if the main one fails. What is the smallest 4 stroke that will suffice please?
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Old 20 June 2010, 15:09   #2
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the general rle of thumb here seems to be 1HP per m of boat length (i.e. 6m = 6HP) but there are also many people who make do with less than this (usually because they can't fit anything bigger on or because they happen to have a spare engine kicking about).
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Old 20 June 2010, 18:11   #3
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I recently bought a second hand Yam Malta 2S,( think its about 4hp) which seems just right on my 5mtr tornado, I had been looking for a while and in terms of size a 2S is definately better but you need to buy 2nd hand now. Its much slimmer that any 4S I have seen and therefore clamps directly on to the transom and allows the main engine full movement without using a additional bracket.

My advice is to go 2S but then again you need to carry mixed fuel but that's a good idea considering its then a completely seperate fuel source.
Just a few thoughts.

Davej
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Old 20 June 2010, 19:06   #4
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Quote:
Originally Posted by davej View Post
you need to carry mixed fuel but that's a good idea considering its then a completely seperate fuel source.
Just a few thoughts.
The counter argument to that though, is that you'd have to carry a lot of fuel for an auxiliary to get you home.

From where I'm sat, it's all about the odds.

Your odds of having to come in on an aux aren't high,but the odds of having to come in on an aux because you've got contaminated fuel or no fuel(ie one specific type of breakdown) are even slimmer.
Sure, carry a separate fuel source,but if your auxiliary will run from what you've got in your main tanks as well, so much the better.
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Old 20 June 2010, 19:26   #5
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The counter argument to that though, is that you'd have to carry a lot of fuel for an auxiliary to get you home.
Not really, i'd have thoughts a 5-6m boat with a 5-6HP aux would go something like 10 miles on a 5L spare fuel can. In reality you probably don't want to sit with the aux for 2 hrs limping home if you can avoid it. I suspect that the need to refill the inbuilt tank several times (if not external option) and noise will mean most people will head for the nearest safe haven rather than home.
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From where I'm sat, it's all about the odds.

Your odds of having to come in on an aux aren't high,but the odds of having to come in on an aux because you've got contaminated fuel or no fuel(ie one specific type of breakdown) are even slimmer.
true but it is apparently one of the most common reasons - so to not carry a "separate" source of fuel would be diluting the benefit of the aux.
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Sure, carry a separate fuel source,but if your auxiliary will run from what you've got in your main tanks as well, so much the better.
indeed but a 2str will run on 4str just by adding the oil - the reverse is not true (so a 4str aux would be less useful on a 2str pre-mix engine).

The one downside I see of a premixed 2 str aux if your main engine is not also premix is that cycling the spare fuel for the aux isn't going to happen properly and so if it rarely gets used the fuel goes off quicker and you have a fuel problem in the aux.
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Old 20 June 2010, 20:14   #6
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Not really, i'd have thoughts a 5-6m boat with a 5-6HP aux would go something like 10 miles on a 5L spare fuel can. In reality you probably don't want to sit with the aux for 2 hrs limping home if you can avoid it. I suspect that the need to refill the inbuilt tank several times (if not external option) and noise will mean most people will head for the nearest safe haven rather than home.
true but it is apparently one of the most common reasons - so to not carry a "separate" source of fuel would be diluting the benefit of the aux.
indeed but a 2str will run on 4str just by adding the oil - the reverse is not true (so a 4str aux would be less useful on a 2str pre-mix engine).

The one downside I see of a premixed 2 str aux if your main engine is not also premix is that cycling the spare fuel for the aux isn't going to happen properly and so if it rarely gets used the fuel goes off quicker and you have a fuel problem in the aux.
You're reading the post in parts though.

What I'm saying is by all means have a separate fuel source, but make sure your aux can run off the fuel in your main tank too, just in case.

I'm not sure there is a real value in having a 2 stroke aux apart from size and weight. If you've ever tried to premix fuel while at sea, it's bloody difficult to get right. Bear in mind you need to measure the fuel as well as the oil.

I'm lucky in that my aux will run on the same premix as my main engine,so I've got an old fuel connector attached to a bit of fuel line meaning I can refuel from my main tanks by pumping the primer bulb.
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Old 21 June 2010, 08:40   #7
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Engine size

Thank you to everyone for your kind advice so far. I have had experience of a 2S auxiliary engine and a 4S main one and found it is a difficult combination - mostly because if no emergency arises ( and so far touch wood none has ) the 2S fuel goes stale and one is eventually faced with the problem of how to dispose of it. For these reasons I would rather have a 4S auxiliary with a good sized portable tank. That way I can every so often pour the fuel from the auxiliary's portable tank into the tank for the main engine and refill the portable tank with fresh fuel.

Which brings me back to my main question. What is the minimum size 4S auxilary engine that will get me to a shore say 4 miles away if the main engine fails? Only suggestion so far is 6m boat = 6hp engine "but less will do" so I'm still a bit unsure.

Thanks everyone. Peter (aka Petros of Glinad) :-)
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Old 21 June 2010, 09:10   #8
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Originally Posted by petros of Glinad View Post
For these reasons I would rather have a 4S auxiliary with a good sized portable tank. That way I can every so often pour the fuel from the auxiliary's portable tank into the tank for the main engine and refill the portable tank with fresh fuel.
I understand your problem as whilst I have a 2str main engine it does not need premix so I have a similar issue (albeit I can just stick it through the main engine and risk fouling the plugs a little). One solution is to store a pre-measured (100 mL for 50:1 in a 5L fuel can) amount of oil that just needs added at the point of use - or a 20 mL syringe if you add it when you fill the 1L tank on the engine. I don't do that because I am concerned I might forget in the heat of the moment.
Quote:
Which brings me back to my main question. What is the minimum size 4S auxilary engine that will get me to a shore say 4 miles away if the main engine fails? Only suggestion so far is 6m boat = 6hp engine "but less will do" so I'm still a bit unsure.
OK to explain a bit more, with 1hp/m of boat you should be able to go most places against normal wind / tides etc. Those of us who have gone for less (e.g. I have a 2HP on 4m boat) accept that if the wind and tide are against us we may not be able to go in the direction we wish - but at least we will be able to maintain steerage and usually get to some sort of shore.
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Old 21 June 2010, 09:43   #9
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Originally Posted by Polwart View Post
OK to explain a bit more, with 1hp/m of boat you should be able to go most places against normal wind / tides etc. Those of us who have gone for less (e.g. I have a 2HP on 4m boat) accept that if the wind and tide are against us we may not be able to go in the direction we wish - but at least we will be able to maintain steerage and usually get to some sort of shore.
Yep, what Polwart said.

With a 6m rib I'd want a 6hp aux in the Med, bearing in mind how little the tidal flow is.
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Old 21 June 2010, 12:04   #10
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A couple of weekends ago I was doing canoe rescue in Loch Etive. (Google 70 wild miles Glencoe) In an 11' (3.3m) Dory.

Due to the vast speed(!) of the canoes, I ran on the aux, and got approx 4 miles on 1.5L of premix.

That was a C. 1974 4Hp Twin, running between approx 3/4 - full throttle.



As for fuel going off etc, you really should give the aux a blast on a regular basis anyway - e.g use it to get in & out the marina.
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