Is there any truth in this?

[pavlosimou]

Whilst admiring my chums brand new 4 stroke outboard this morning he related to me "what the dealer told him"
it went along the lines of " Nowadays on your nice new 4 stroke engine guv'nor the horsepower is calculated at the prop, whereas on yer old two stroke blue smoker it was calculated at the flywheel so your new 60 horses is probably more like 65-70 of yer old horses". now I said mmmm AT the prop? maybe, but not in terms of thrust because that will depend on the prop you fit, and anyway any small gain you might get from this dubious sales pitch you have probably lost in the extra 250 kilos you are now carrying. (I may have exaggerated the weight gain a little)
so could there be a hint of truth in the dealers words or is it truly just a pitch?

 

[Gemini]

25yrs ago plus Mercury, blue band gold band etc where measured at fly wheel. Same as Johnson
Yamaha's arrived on the scene auto lub and these where prop rated
Rough guide was 50hp Mercury was 46hp at prop against Yams 50hp at prop
We used to run 4m Avon sea riders and had both engines. Out the hole very litte difference, WOT Yam by about 2knots
Fuel Yam slightly better not not massive difference

Whilst admiring my chums brand new 4 stroke outboard this morning he related to me "what the dealer told him"
it went along the lines of " Nowadays on your nice new 4 stroke engine guv'nor the horsepower is calculated at the prop, whereas on yer old two stroke blue smoker it was calculated at the flywheel so your new 60 horses is probably more like 65-70 of yer old horses". now I said mmmm AT the prop? maybe, but not in terms of thrust because that will depend on the prop you fit, and anyway any small gain you might get from this dubious sales pitch you have probably lost in the extra 250 kilos you are now carrying. (I may have exaggerated the weight gain a little)
so could there be a hint of truth in the dealers words or is it truly just a pitch?

 

[Spi D]

All true, even for sterndrives. Term is propshaft rating.

Inboard engines always rated on flywheel since the manufacturer never knows what driveline it will be mated to.

 

[pavlosimou]

this is however a 4 stroke Yamaha replacing a 2 stroke Yamaha which I reckon is probably about 1980

 

[Beamishken]

25yrs ago plus Mercury, blue band gold band etc where measured at fly wheel. Same as Johnson
Yamaha's arrived on the scene auto lub and these where prop rated
Rough guide was 50hp Mercury was 46hp at prop against Yams 50hp at prop
We used to run 4m Avon sea riders and had both engines. Out the hole very litte difference, WOT Yam by about 2knots
Fuel Yam slightly better not not massive difference

that's how I understand the situation very early engines were crank rated then enter Japanese into the arena & they prop rated theirs so US manufacturers had to follow suit

personally Id expect a fit 2 stroke to have better hole shot than the equivalent 4 stroke but I guess an old one could be down a little on power so maybe evenly matched

 

[Beyondhelp]

I said mmmm AT the prop? maybe, but not in terms of thrust because that will depend on the prop you fit,

Firstly as said, the statement is true. Mercury changed over at staggered periods in the 80's. For a while it was possible to have two 90hp mercury's a year apart that were different HP's mercury had to release a dealer note to make sure dealers didn't supply different gen engines on a twin setup or problems would happen.

As for your comment on thrust, not quite true. An engine with greater pitch prop would if had already been correctly pitched not have enough power to spin it at the same speed so trust can actually reduce. As an example: I can turn a 21p at 5800 ish rpm @ 56mph. I tried just out of interest a 25p prop (same type) and could only get 5200 rpm and 52mph.

 

[VicS]

The standard referring to the measurement of outboard ( and inboard) engine power is ICOMIA 28-83 http://continuouswave.com/whaler/reference/ICOMIA28-83.html

Obviously dating from 1983 although I believe it is correct that some manufacturers were quoting prop shaft power earlier