Back up engine

[dartmoor]

Just thinking about a back up outboard to keep in a locker on my 22ft Dockrell. Would a 3.5hp two stroke (Tohatsu) be ok as emregency use? I am as long as long shaft, and not battling a big tide/wind would be ok?

 

[ctva]

I have moved a Jeanneau 34 with a Honda 2hp at about 2kts.

 

[coveman]

Just thinking about a back up outboard to keep in a locker on my 22ft Dockrell. Would a 3.5hp two stroke (Tohatsu) be ok as emregency use? I am as long as long shaft, and not battling a big tide/wind would be ok?

Should be fine - I have a 2.5hp Mercury 2 stroke and pushes my 22 ft yacht along ok.

 

[neil_s]

Sounds like a job for a Seagull 40+ ! Already has the high thrust prop!

 

[dancrane]

This summer I've watched substantial (though not very long) yachts being pushed along at a useful pace in calm conditions by 2.5hp motors that looked better suited to their tenders.

Assuming a very small engine's prop can be properly immersed when mounted on the yacht, it may do useful duty there and on the tender too, though if it has a long shaft for the yacht's benefit, I reckon the revs will need keeping low during use on the tender.

I haven't actually had the nerve to run my 5hp long shaft two-stroke on my ancient Redcrest, so I don't know how bad it would be.

I believe a tender-mounted outboard can be used to tug or push the mother-ship, even significant distances in fair weather.

 

[Homer J]

I once read a 2hp would push an oil tanker (eventually) in benign conditions (no wind or tide). The answer depends on the conditions. In a tide, against the wind, up a steep sea?

We had a Drascombe Luger 17ft with a 4 HP and it coped with almost everything, including the Hurst Castle race (terrifying though).
3.5hp? Probably enough until you get caught out.

 

[DownWest]

Friend used to move his 34ft 13 tonne gaff cutter with the dinghy lashed to the quarter and a Seagull Century..
3.5 sounds fine.

 

[Daydream believer]

Many years ago they used a Seagull Century to move a 100ft barge along a canal. It is not so much the HP, I believe, as the way the thrust is delivered. My 40+ would push my Silhouette with ease and I suspect it would also push the OP's boat as good as a modern high revving outboard with small prop, but higher HP

 

[Stemar]

My 3.5hp pushes my 24ft Snapdragon along fine. It was a bit iffy with good(?) F6 on the nose once, but apart from that it was fine. A short shaft isn't ideal in bumpy conditions, but for emergency use, definitely OK

 

[Rustyknight]

My 3.5hp pushes my 24ft Snapdragon along fine. It was a bit iffy with good(?) F6 on the nose once, but apart from that it was fine. A short shaft isn't ideal in bumpy conditions, but for emergency use, definitely OK

I had a Tomas "Ailsa Craig" 4hp longshaft o/b on the transom of a Jaguar 25 as a back up to a 15hp petrol inboard. The inboard was quite a thirsty engine, so I often used the o/b in very calm conditions. It was good enough to cross Lyme Bay in, although top speed was 4 knots.

 

[dancrane]

3.5hp? Probably enough until you get caught out.

I wonder how many small yachts have any kind of reserve engine?

Wouldn't it be lightning striking twice, first to have the main engine totally unwilling to start, or its propeller inextricably entwined in lines, and the reserve propulsion unable to make way against the conditions?

 

[TSB240]

I haven't actually had the nerve to run my 5hp long shaft two-stroke on my ancient Redcrest, so I don't know how bad it would be.

Some people have reported problems of running long shaft two stroke outboards on small dinghies. When you mount your two stroke on your flimsy metal cradle on your Redcrest you will have the power head very very close to the water line. The outboard leg that transmits the exhaust is filled with a large volume of heavy water that has to be displaced so the exhaust can exit the propellor. This causes sufficient increased back pressure to make it either impossible or very difficult to start your engine. I suspect poor tickover issues could also be a direct result of higher than normal back pressure.

Remember a two stroke is an exhaust scavenging engine so it will naturally draw a small amount of exhaust back in for combustion. With the head so close to the water and increased back pressure you may also draw water back into the head which is really not a good idea!

An outboard leg is the worst possible design of exhaust for a two stroke.

Racing two stroke engines were matched to tuned expansion exhausts so that the exhaust was actually working as a form of supercharger as unburnt fuel in the exhaust pipe is drawn back in for combustion once the transfer port had closed.

https://en.wikipedia.org/wiki/File:Arbeitsweise_Zweitakt.gif.

The fuel efficiency and power output of two stroke engines were dramatically improved. The problem was this power band or being "on the pipe" was over a limited rev range with this type of exhaust.

It is a shame this simple technology has not been exploited on any production line 2 stroke outboard.

 

[dancrane]

Thanks for all that.

I'll find a bigger boat to use my 5hp on.

 

[DownWest]

Back in ln my formative years, I built a 6ft sailing pram when I was 11. A year or so later, my father gave me a Clinton 5 hp out board. It was seized and he said he would pay for the parts if I could fix it. So I did.
Point of this, is that when it was on the transom of my little pram, the thrust line was very low for the length, so sudden changes of power settings tended to ship water fore or aft, if we were three up (I know.. but it was on a river and we could swim) The bailer scoop was handy..

 

[northwind]

Why do you need a spare engine? you have an engine & sails. Maybe consider carrying spares for your main engine? Seems a bit mad to add another 13kgs of weight to a 22footer.. Let alone the hassle of trying to change engines, when you probably have other concerns

 

[QBhoy]

It will do the job...but so important it is mounted and set up right in terms of how it sits in the water.

 

[BobnLesley]

Just thinking about a back up outboard to keep in a locker on my 22ft Dockrell. Would a 3.5hp two stroke (Tohatsu) be ok as emregency use? I am as long as long shaft, and not battling a big tide/wind would be ok?

When we had the Albin Vega with its tired Volvo MD6A, we arranged 'back-up': We mounted a two-piece hinged boarding ladder on the stern with a plywood board built around the bottom step that'd accept the outboard; the hinge height was set above water so that with the lower half of the ladder set at 90* and tied-off to the pushpit, our 3.3 Mercury sat at the perfect depth in the water. As a bonus, the outboard-board on the bottom step made the ladder more comfortable to use and being a Vega, the boat actually manouvered better with the outboard than using the main engine.

 

[Stemar]

Why do you need a spare engine? you have an engine & sails. Maybe consider carrying spares for your main engine? Seems a bit mad to add another 13kgs of weight to a 22footer.. Let alone the hassle of trying to change engines, when you probably have other concerns

I always say, only half joking, that if you've only got a plan B, you're already in trouble.

I'll repeat what I said before, a 3.5HP engine will push the OP's boat along quite happily in any conditions he's likely to want to be out in and give a useful helping hand in plenty that he wouldn't. It will need a good mounting at the appropriate height - as deep as practicable. I've got something like this

Which means I can keep it ready to go but out of the water until needed, at a convenient height for refuelling, etc. You really don't want to have to be grovelling in a locker for it when you need it in a hurry, and if there's no hurry, you don't really need it, as Northwind says, you've got sails, so if you've got the time and space, use 'em!

 

[dancrane]

Why do you need a spare engine? You have an engine & sails. Seems a bit mad to add another 13kgs of weight to a 22footer.

Not sure I see the sense in this point of view. The OP has sails, but there are undoubtedly occasions when auxiliary power is required...

...if on even one of those occasions, that auxiliary (whose reliability may be in doubt) quits, the small weight penalty and insubstantial cost of a reserve outboard will have repaid the bother of its acquisition and storage, many times over.

Not to mention the confidence it inspires even if the main engine never fails, to know that you can cope if it did. Plus the option to power the tender.

 

[northwind]

I don't see the logic in having 3 means of propulsion. Ask the question, what is likely to go wrong, and plan accordingly. modern engines are very reliable. If you are fearful of the reliability of the one you have the money would be better spent on ensuring it is reliable, rather than keeping the reliable engine in a locker... An auxiliary to the auxiliary.

Reminds me of the Goon show "we couldn't find any oars, but found a pair of outboards so rowed with those instead."

Not sure I see the sense in this point of view. The OP has sails, but there are undoubtedly occasions when auxiliary power is required...

...if on even one of those occasions, that auxiliary (whose reliability may be in doubt) quits, the small weight penalty and insubstantial cost of a reserve outboard will have repaid the bother of its acquisition and storage, many times over.

Not to mention the confidence it inspires even if the main engine never fails, to know that you can cope if it did. Plus the option to power the tender.

That's what sails are for