Are all outboard's HP equal?

Tim Good

Tim Good

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Short question:
Are all outboards equal when it comes to power? Will a 6hp suzuki give the same Ooomph as a 6hp in something else? Likewise will a 6hp 2 stroke act similar to a 6hp 4 stroke?

Reason for the question:

I have 3.5hp Mercury and I've been looking for a Yamaha 2 stroke 5 or 6hp for ages and can't find one. So I'm looking at a 6hp Suzuki 4 stroke as it is only 23kg compared to my current 18.5kg Mercury. I can plane in my dinghy with my 3.5 but add another person, dog, shopping or crab pot and it won't. So I'm looking for more power to do adventures in the dinghy up rivers, around islands and so forth. Every one raves about Yamaha 2 Strokes but they appear to be rather rare!
 
northcave said:
Short question:
Are all outboards equal when it comes to power? Will a 6hp suzuki give the same Ooomph as a 6hp in something else? Likewise will a 6hp 2 stroke act similar to a 6hp 4 stroke?

Reason for the question:

I have 3.5hp Mercury and I've been looking for a Yamaha 2 stroke 5 or 6hp for ages and can't find one. So I'm looking at a 6hp Suzuki 4 stroke as it is only 23kg compared to my current 18.5kg Mercury. I can plane in my dinghy with my 3.5 but add another person, dog, shopping or crab pot and it won't. So I'm looking for more power to do adventures in the dinghy up rivers, around islands and so forth. Every one raves about Yamaha 2 Strokes but they appear to be rather rare!
Click to expand...

As pvb says new small two strokes are effectively banned by the emissions regulations that became part of the Recreational Craft Directive a few years ago. People with the "good" ones are hanging on to them

But in answer to your question about power they should all be equal regardless of make and type. Since the early 1980's the power should be measured at the prop in accordance with the Icomia 28/83 test procedure. Before then it was usually measured at the crankshaft which of course gave a higher figure ( A current 6 hp would previously been rated at about 7.5). However actual performance will be affected by the choice of prop.

Modern large two strokes, which do comply with the emission regulations, are popular for their better acceleration "out of the hole".

Generally four strokes are heavier but some are not that much heavier than some two strokes of the same power.
 
A 6hp engine will only put out 6HP if the prop loads it correctly, so it needs full throttle to achieve peak power RPM.
Outboard props are a huge compromise between getting enough thrust to get a tender on the plane at 5 knots or whatever, and having enough revs left to go faster once it's planing.
So the torque or power curve of the engine matters.
If the engine has less torque at say half revs, it is less able to get the boat on the plane and may not ever get to peak power.
I've heard claims that the lighter 6HP engines will have less cc's and therefore less torque at mid revs, so therefore be more picky about having exactly the right prop. I think this is, at best, not the whole story, but it's certainly true that one 6HP motor does not give the same performance as every other one.
Particularly if you are not able to select the optimum prop. Some engines there is no choice at all.
(although you can always reduce the diameter of a spare prop.... a device called a 'beach' can do this for you...)
 
VicS said:
As pvb says new small two strokes are effectively banned by the emissions regulations that became part of the Recreational Craft Directive a few years ago. People with the "good" ones are hanging on to them

But in answer to your question about power they should all be equal regardless of make and type. Since the early 1980's the power should be measured at the prop in accordance with the Icomia 28/83 test procedure. Before then it was usually measured at the crankshaft which of course gave a higher figure ( A current 6 hp would previously been rated at about 7.5). However actual performance will be affected by the choice of prop.

Modern large two strokes, which do comply with the emission regulations, are popular for their better acceleration "out of the hole".

Generally four strokes are heavier but some are not that much heavier than some two strokes of the same power.
Click to expand...

And dont forget that he needs to know where in the rev range the hp is being developed
 
Equal HP outboards should all deliver the same power, but the small props cavitate quite readily so you need to get something appropriate to the vessel you wish to drive. My Yamaha 4hp drove my rubber duck at planing speeds but when I passed it on to a mate with a Silhouette, he needed to re-prop. Also short shaft vs long shaft depending on what you want to drive.
 
Interesting about the different prop types. I wonder if I'm getting all I can from my current Mercury... but I suppose if I can plane when it's only 3.5hp then maybe it's a fairly suitable prop.

Someone on this forum might have a 2 stroke Yam 4hp... I'm assuming based on this thread it wouldn't give me much more than my 3.5hp.
 
macd said:
As Vic implied, there was and is no ban on new two-strokes. The regulations placed a limit on exhaust emissions, which effectively barred stinkwheels not fitted with sophisticated emissions controls. (Viable on large engines but too expensive for cheap little tender-pushers).
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Slightly different viewpoint. The ban is on the sale for recreational use of new 2 strokes which don't meet emissions, not a ban on ownership or using one purchased outside the EU.
 
Graham376 said:
Slightly different viewpoint. The ban is on the sale for recreational use of new 2 strokes which don't meet emissions, not a ban on ownership or using one purchased outside the EU.
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ITYWF that the "ban" is not specifically on 2 strokes or on outboards. I believe in theory it applies to four strokes, including inboards and other auxiliary engines, as well. In practice of course 4 strokes do more easily meet the emission regulations.

I seem to remember a separate set of emission standards for diesel engines.
 
Graham376 said:
Slightly different viewpoint. The ban is on the sale for recreational use of new 2 strokes which don't meet emissions, not a ban on ownership or using one purchased outside the EU.
Click to expand...

No, the ban is on the sale (for recreational use, as you write) of any new outboards which don't meet prescribed emission limits. In practice, as Vic states, it mainly affects two strokes, although a particularly dirty four-stroke could certainly fail. As said, there are two-strokes which meet the regulations, although none are of a size likely to be fitted to a small yacht tender.
 
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It's been touched on, but some of the perceived differences in performance of engines with similar power ratings is caused by the match between the speed of the engine, the propeller and the hull it is driving. An engine that is optimized for driving a planing hull won't be performing at its best when driving a displacement hull, and vice-versa. The ancient Seagull performs very well to drive heavy displacement hulls despite being of modest power rating; the low revs and large propeller give good thrust at low speeds. A high revving engine with a high-speed propleller will simply cavitate and waste a lot of power under those conditions. Matching the engine's power, propeller and speed to the performance of the boat will make a significant difference.
 
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