Outboard motor size for dinghy

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Posse

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I just bought an inflatable dinghy, 2,7 meters long. Any idea how small an outboard would be sufficient? I'd rather go just-large-enough than too large due to weight and handling, and preferably be able to plane the dinghy with two light adults (e.g. 2x 60-70 kg).

It is intended as a dinghy for use when anchoring in Scandinavia/Northern Europe, not for round the world cruising.

I am hoping 4-5 HP would be enough, but any thoughts or experiences would be greatly appreciated before I buy.
 
I bought a Suzuki 2.7m airdeck dingy a Year ago with a 2.5 for pottering around.
Then decided I wanted a bit more power, so bought a 5hp for it.
Still not enough to plane one-up (I am heavy though) so am on the lookout for a 2 stroke 10hp.
 
Unless it's a rib, anything over 4hp would be overkill. My 3.3hp drives my 2.4m inflatable well at half throttle. It has an air deck and an inflatable keel rib. One up with no other cargo it'll sort of plane after a fashion but you really have to be sensitive to the balance of the craft and the throttle response to keep directional control and speed. I've not been able to maintain either comfortably for any length of time.
 
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Unless you have a shaped hard floor like a mini rib it's not going to plane, flat bottomed dinghies are displacement designed. I have a the same size with a 3.5hp tohatsu and that at full speed is more than adequate. If you start going to big they are too heavy to handle easily.
 
Tammany said:
Unless you have a shaped hard floor like a mini rib it's not going to plane, flat bottomed dinghies are displacement designed. I have a the same size with a 3.5hp tohatsu and that at full speed is more than adequate. If you start going to big they are too heavy to handle easily.
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Contrary to your view .

I have two inflatables.

A round tail flat floor 2.3m air deck. It will plane(eventually) with my 13 stone evenly positioned with a Honda 2.3 Four stoker

An Achilles 3.5M hyperlon with a full length rigid jointed lightweight plywood floor and a slow punctured inflatable keel.

This will happily plane one up with an Johnnyrude 3.5hp 2 stoker

It easily planes with 25 stone plus gear and a Johnnyrude 6 hp 2 stoker

It will plane at over 25 knots with a Tohatsu 9.8. 2 stoker but nobody is stupid enough to get in it with me!

It helps with directional stability of there is a bit of a vee under you but I really can assure you that flat floors plane!

The critical bit is to make sure any flat floor is sufficiently stiff to allow you to rise over your bow wave and get up and over the "Hump".
 
My old 2.7M flat-floor with airdeck floor planed steadily at maybe 10-12 knots one-up with a two-stroke 3.5 at full throttle, but you had to get the weight balanced right, sitting amidships. Add another person and no planing.

The same engine on a 2.6M non air deck struggles to plane, the floor ridges and "sucks" it down, best speed maybe 6-7 knots even one-up.

I'd expect the inflatable vee-floor plus airdeck 2.9M I've recently bought will plane fine one-up - maybe even with two, but that might be marginal with 3.5 hp. The newer dinghy is rated to 10 hp but I've no intention of going anything heavier to lift on and off than a 3.5 two-stroke.
 
Thanks for your valuable input. The dinghy is a Bombard Typhoon 265 Aero. It has an inflatable bottom with some V-shape, and inflatable floor.

Sounds to me like I should aim for a 4 HP (probably 4 stroke), maximum 5 HP, which are often same weight as the 4 HP's. And then live with it in case it will only plane with one person.
 
jwilson said:
My old 2.7M flat-floor with airdeck floor planed steadily at maybe 10-12 knots one-up with a two-stroke 3.5 at full throttle, but you had to get the weight balanced right, sitting amidships. Add another person and no planing.

The same engine on a 2.6M non air deck struggles to plane, the floor ridges and "sucks" it down, best speed maybe 6-7 knots even one-up.

I'd expect the inflatable vee-floor plus airdeck 2.9M I've recently bought will plane fine one-up - maybe even with two, but that might be marginal with 3.5 hp. The newer dinghy is rated to 10 hp but I've no intention of going anything heavier to lift on and off than a 3.5 two-stroke.
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Similar story but my 2.6 wooden slatted floor Seago dinghy will plane at 12-13 knots with my 3.5 2 stroke and just me on board (14 stones). I can even throttle back to about 3/4 throttle once it's up.
 
Posse said:
Thanks for your valuable input. The dinghy is a Bombard Typhoon 265 Aero. It has an inflatable bottom with some V-shape, and inflatable floor.

Sounds to me like I should aim for a 4 HP (probably 4 stroke), maximum 5 HP, which are often same weight as the 4 HP's. And then live with it in case it will only plane with one person.
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Then the recommended and max engine Hp along with the max engine weight can be found on this web page http://www.bombard-ribs-boats.co.uk/acatalog/info-Z15040.html
 
My large inflatable is an Avon Rover 3.41 with an air-deck and inflatable keel (giving a shallow vee bottom). With myself and one of my children up, it'll plane at all of 6 knots with an old Ailsa Craig 4 (Tomos 4) 2-stroke. I had a smaller Zodiac, with a slatted flat bottom but I came to the conclusion that it was just too short, and had too flat a bottom to plane well, even with the same engine.

Image of the manufacturer's plate here...

Your inflatable appears to be rated to 6hp, which is very much the upper end of what I'd have anticipated. I'd agree with the general consensus here, that between 3 and 5hp should get you on the plane if you sit well forward, but stick with a 2 stroke. Failing that, perhaps you have a friend with a slightly longer inflatable (say 3 or 3.1m) and try your engine with that.

All the best,

David.
 
Why the strong 2-stroke preference? I am aware that it has more torque per HP, but in my experience a 4 stroke is much quieter and more reliable. Also, a 2 stroke will be quite old, since it has not been legal to sell them here for some time.
 
Posse said:
Why the strong 2-stroke preference? I am aware that it has more torque per HP, but in my experience a 4 stroke is much quieter and more reliable. Also, a 2 stroke will be quite old, since it has not been legal to sell them here for some time.
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Weight.
Honda 4 stroke air cooled are noisier than my Johnson 3.5hp 2 stroke.
I don't need to bother how I lay my 2 stroke down as you do with 4 strokes.
 
Any of the small 2 to 3.5 hp 2 strokes will do just fine They are considerably lighter than the step up to 4 My preference is for the Tohatsu 3.3 or 3.5, sold under many different badges.

My experience of small 4 st outboards is that they are quieter but if anything goes wrong they are much harder to get running again.

If tempted by the aircooled Honda have a test drive and check to see if you can put up with the noise.
 
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My 2.9 m inflatable with v keel will go on the plane with Suzuki 2.5 hp and Tohatsu 3.5 hp. both are four stroke (for simplicity and guarantee to start). It would not go on the plane with the Honda 2.3.
 
We have a 3 m. rib eye which I power with a Yamaha 8 Hp two stroke twin which weighs 27 kg , the Suzuki 4 Hp 4 stroke I had before just didn't compare either in weight, fuel consumption ( the Yamaha is better ) reliability and first pull starting of the two stroke . I'd plump two stroke every time and if I could find a tohatsu 9.8 short shaft I'd be seriously tempted as I think that engine weighs less than my Yamaha .Easily planes two adults with 8 Hp as you would expect , the 4 Hp Suzuki wouldn't get two of us on the plane although one up was no problem .
 
It depends on how far you are planning to travel. I use oars to get out to my mooring about 200m. For longer trips I use my little Honda 2.3 (and ear plugs) but I still have to carry oars for safety reasons.
 
We have a small 2 stroke 3.3 mercury which has served us well for 10 plus years on both small Seago plus larger Quicksilver dingy . Real question would seem to be one of weight and whether you prefer smaller 2 stroke you can easily carry about and lift on and off without assistance or the weight of larger 4 stroke which might need a crane etc . I have found even the small 4 stroke Honda quite heavy in comparison to Mercury which they still sell in Alderney. Clearly there are a number of outboard specialists around but if you can escape from Soton a trip to Fareham creek will lead you to a well known one who might be a source of info for your search and advise on reliability and servicing etc and might have a few second hand ones in stock
 
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