what is the actual hp rating 1980 50hp

I have a 1978 and 1980 50hp 4cylinder merc . What is the actual hp rating at the prop? I've heard its the actual hp at the crank on older motors .

Re: what is the actual hp rating

Mike, prior to the mid 1980's (and there was no definative year), outboards were rated in Brake Horsepower (BHP) which is measured at the crankshaft. For Merc and OMC between 84 and 86 they started rating all their new motors in Shaft Horsepower (SHP).

That led to some "strange" horsepower models for a year or two. The 40 Merc became the 35, the 185 OMC was the old 200 horse etc.

But to my knowledge, no official or industry standard was ever established and tested to "go back" and re-rate older motors that were orignally sold with their BHP rating.

Additionally, there is no super easy way to exactly convert BHP to SHP. A multitude of factors must be considered, measured etc to try and convert one to the other with a pocket calculator at home.

So, you kinda have two choices:

you can accept the "rule of thumb" conversion which is generally accepted as about 10% less (although it could actually be between about 5 and 15% depending on the total number of bearings and gears between the base of the flywheel and the prop itself - with a "direct drive" (always in forward gear only losing about 5% because of less resistance due to less bearings and gears than a forward/neutral/reverse gear set-up), OR

you could have the particular motor dyno tested.

Today a shaft horsepower rating is based on "what" horsepower that particular motor would put out if it was operated at sea level, with an air temperature of 70 degress and a relative humidity of 30%.

Since many recreational outboards are not operated under those exact conditions, most would dyno test out somewhat below what the horsepower sticker on the cowl says it is.

In 80% humidity with the thermometer touching the 90 mark, a 100 SHP outboard would be lucky to see 85 horses at the prop, if actually measured.

So unless you plan on carrying testing gear, the rule of thumb rules are more than close enough.

In the case of the 50 horse, you can safely call it a 45