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PeterInSA
21-01-2013, 12:42 PM
Thinking about an upgrade to a more economical smaller motor and wondered about the real HP of my current 85HP Blue band Mercury that is rated at the power Head compared to newer model engines rated at the prop.

Mate suggests that there is a 20% loss so my old 85 is onlly equivalent to 68HP in the new money and with age/wear probably only equal to a later model more economical 60HP 2 stroke 3 cylinder Mercury engine.

Comments appreciated.

Noelm
21-01-2013, 12:56 PM
20% might be a bit optimistic I think, maybe 5% or so, then add in the manufacturers stretching/bending the figures a bit, but anyway you look at it, a new motor will always be better (in a way) than a tired old 20 year old thing.

Moonlighter
21-01-2013, 03:20 PM
I Am pretty sure when they changed ratings fom power head to prop, most motors decreased by 10 to 15%.

Add to that 20 years of wear and tear and it wouldnt surprise me to find a easy net 20% reduction. In other words, if you are happy with the performance of your old 85hp, a new 70hp would be fine.

The other way to look at it, and probably the beyter way, is to find out the weight of your boat and apply the appropriate hp to current boats that way a similar amount. Take it to a weighbridge at a tip, and ask them what it weighs on and off the towbar. Extract the trailer tare weight and you have your boat/motor weight.

Or, If the manufacturer is still around, a phone call may get you an answer as well as recommendations on hp.

What sort of boat is it on? Length? Construction?

Fed
21-01-2013, 05:03 PM
I was told by a very reliable source it was about 7% so that would make your flywheel 85 HP a prop rated whisker under 80 HP.
If it's running ok with reasonable compression I doubt old age would have any great effect on it.
80 down to 60 is a lot to lose.