Outboard motor size

Tell us the type of boat, and displacement maybe?

My Achilles 24 (slender, 1150kg) had a 5hp two-stroke, which was plenty.

Other owners reckoned a 3.5hp would suffice. Some people fitted an 8hp four-stroke...rather overweight.

50157013426_1913f48498_c.jpg


There were even a fair number built during the era of petrol inboards.
 
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As well as the power, do absolutely make sure you get one that is geared and propped to drive a bigger boat. (There used to be a common outboard model-name suffix for this, but I can't remember it. These will also often have more (or any!) in the way of 12v electric output.)

Most outboards of the sorts of sizes you will be looking at will be aimed at the lightweight inflatables, ribs and dinghies market. They will be geared/propped for high speed, rather than grunt. Typically you can never get these up to their most powerful (and economical) rev range, and hence never get full power from them. Better to have a 5hp made to drive a bigger boat than something larger that isn't: it'll weigh more, use more fuel and even then not necessarily drive you any better.

The other thing is that modern boating expects higher performance auxiliary motors than days of yore. You may find with an older boat, though, that you may have to content yourself with more modest performance to be able to use the size and weight of outboard it was designed to accommodate. If you have an outboard well in the boat you may struggle to get the size of outboard expected today in it (see below), and if your outboard is transom mounted, the last thing you want is additional weight that far back.

Note also that 4 strokes have become much more prevalent, and though generally quieter and much more economical (and less smelly!) than 2 strokes, they tend to be heavier than the equivalent power 2 stroke.

I had an 8hp twin-cylinder 2 stroke (Yamaha?) in my Hurley 22 (a fairly heavy boat for its size). It did fit in the well, but only just, and it was so difficult to get in and out of the well due too the tight fit and weight that I had to leave it in when the boat was on its mooring, which didn't do the outboard a lot of good, let alone remove it and put the plug in when sailing to avoid the drag of the leg and propellor. It also meant the outboard couldn't be turned at all in the well - a distinct manoeuvring disadvantage in an arrangement with the prop behind the rudder: the boat couldn't be turned until it was moving forward at significant speed. I would willingly have traded it for a smaller engine.

I also had (actually, I still have, but that's another story!) a 23 footer - a heavyweight, long-keeler - with an inboard diesel (luxury!). The engine in that was 14hp, which was more than enough. The 10hp version of that motor would have been perfectly adequate for even modern tastes. I imagine that 8hp would have been quite acceptable, and one could have managed with 6hp more sedately but without too much trouble. (I have just checked and they were originally marketed (late 70s) with Vire 7hp petrol engines or a Petter (6hp?) diesel.

As Dan Crane indicates, the hull/keel form and the weight of the boat, rather than just its LOA, should be taken into account in deciding your power requirements.

p.s. Don't forget you will almost certainly need a long shaft outboard.
 
westernman said:
5hp per ton is generous. 3hp would be more than enough.

Big container ships have less than 1hp per ton.
Click to expand...
Everyone works on 5hp per ton. 3hp might work in flat water but wind and waves...your reference to container ships is beyond irrelevant.
 
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dancrane said:
Tell us the type of boat, and displacement maybe?

My Achilles 24 (slender, 1150kg) had a 5hp two-stroke, which was plenty.

Other owners reckoned a 3.5hp would suffice. Some people fitted an 8hp four-stroke...rather overweight.
Click to expand...
Yes about the same weight as your A
LittleSister said:
As well as the power, do absolutely make sure you get one that is geared and propped to drive a bigger boat. (There used to be a common outboard model-name suffix for this, but I can't remember it. These will also often have more (or any!) in the way of 12v electric output.)

Most outboards of the sorts of sizes you will be looking at will be aimed at the lightweight inflatables, ribs and dinghies market. They will be geared/propped for high speed, rather than grunt. Typically you can never get these up to their most powerful (and economical) rev range, and hence never get full power from them. Better to have a 5hp made to drive a bigger boat than something larger that isn't: it'll weigh more, use more fuel and even then not necessarily drive you any better.

The other thing is that modern boating expects higher performance auxiliary motors than days of yore. You may find with an older boat, though, that you may have to content yourself with more modest performance to be able to use the size and weight of outboard it was designed to accommodate. If you have an outboard well in the boat you may struggle to get the size of outboard expected today in it (see below), and if your outboard is transom mounted, the last thing you want is additional weight that far back.

Note also that 4 strokes have become much more prevalent, and though generally quieter and much more economical (and less smelly!) than 2 strokes, they tend to be heavier than the equivalent power 2 stroke.

I had an 8hp twin-cylinder 2 stroke (Yamaha?) in my Hurley 22 (a fairly heavy boat for its size). It did fit in the well, but only just, and it was so difficult to get in and out of the well due too the tight fit and weight that I had to leave it in when the boat was on its mooring, which didn't do the outboard a lot of good, let alone remove it and put the plug in when sailing to avoid the drag of the leg and propellor. It also meant the outboard couldn't be turned at all in the well - a distinct manoeuvring disadvantage in an arrangement with the prop behind the rudder: the boat couldn't be turned until it was moving forward at significant speed. I would willingly have traded it for a smaller engine.

I also had (actually, I still have, but that's another story!) a 23 footer - a heavyweight, long-keeler - with an inboard diesel (luxury!). The engine in that was 14hp, which was more than enough. The 10hp version of that motor would have been perfectly adequate for even modern tastes. I imagine that 8hp would have been quite acceptable, and one could have managed with 6hp more sedately but without too much trouble. (I have just checked and they were originally marketed (late 70s) with Vire 7hp petrol engines or a Petter (6hp?) diesel.

As Dan Crane indicates, the hull/keel form and the weight of the boat, rather than just its LOA, should be taken into account in deciding your power requirements.

p.s. Don't forget you will almost certainly need a long shaft outboard.
Click to expand...
The Puma 23 in Coruna comes with an inboard engine but fro the photos on internet it shows an outboard bracket which could indicate unreliability of the inboard,I would prefere an outboard as not gifted in mechanical department.will check prices of 5hp outboards today prior to seeing the boat
 
Often 4/5/6 hp engines are the same engine and weight, but with slightly different innards .
Mainly different carburettors or carburettor jets. They weigh around 26kilos which is probably the most you would want to move about.

Many in this range, if a 4 stroke, have a better propeller option suited to yachts that don't plane.
 
A distinct advantage would be reverse and that is usually at 4hp or up.
I have a 3.5, but am eyeing a 4hp for that reason. (My boat is shorter and lighter than the OPs)
 
My 3.5 Tohatsu from the tender pushed my Snapdragon 24 along very nicely in benign conditions, but motoring into a F6, even in smooth water was difficult. Add a bumpy sea and I'm not sure how much progress we'd have made. I reckon a 5 or 6 HP propped for low speed would be about right. I'm no expert, but I'd expect a long shaft engine to have the right prop. Big +1 for having reverse, if at all possible
 
Stemar said:
My 3.5 Tohatsu from the tender pushed my Snapdragon 24 along very nicely in benign conditions, but motoring into a F6, even in smooth water was difficult. Add a bumpy sea and I'm not sure how much progress we'd have made. I reckon a 5 or 6 HP propped for low speed would be about right. I'm no expert, but I'd expect a long shaft engine to have the right prop. Big +1 for having reverse, if at all possible
Click to expand...
Displacement 2.5 tons?
At 3hp per ton, that would be 7.5hp.
 
Wansworth said:
The Puma 23 in Coruna comes with an inboard engine but fro the photos on internet it shows an outboard bracket which could indicate unreliability of the inboard,
Click to expand...

It may, and I used to worry about the same thing boat searching, but it could also indicate a supplement to an original inboard engine installation that was rather underpowered for modern tastes, or for a home port with challenging currents or other conditions. Might also even be a convenient place to carry the tender's outboard where it could actually do some good and keep it's weight lower down than carrying it on the pushpit.

Wansworth said:
I would prefere an outboard as not gifted in mechanical department
Click to expand...

A decent inboard diesel is far, far preferable, in my view, to an outboard for a boat of this size. If the engine is a good runner (you can get an engineer to do a simple check - as recently described on these forums - starts easy, runs OK, doesn't smoke too much) there is nothing at all complicated about basic diesel engine maintenance and it is easily learnt. (A unreliable knackered dog of a diesel, especially if spares are hard to come by for that make/model is, by, contrast an expensive liability.)

Good luck with your search. Keep us posted on progress!
 
LittleSister said:
It may, and I used to worry about the same thing boat searching, but it could also indicate a supplement to an original inboard engine installation that was rather underpowered for modern tastes, or for a home port with challenging currents or other conditions. Might also even be a convenient place to carry the tender's outboard where it could actually do some good and keep it's weight lower down than carrying it on the pushpit.



A decent inboard diesel is far, far preferable, in my view, to an outboard for a boat of this size. If the engine is a good runner (you can get an engineer to do a simple check - as recently described on these forums - starts easy, runs OK, doesn't smoke too much) there is nothing at all complicated about basic diesel engine maintenance and it is easily learnt. (A unreliable knackered dog of a diesel, especially if spares are hard to come by for that make/model is, by, contrast an expensive liability.)

Good luck with your search. Keep us posted on progress!
Click to expand...
All will be reveled??
 
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