measuring outboard prop hp?

[kenfyoozed]

I hope i posted this in the correct area, this is my first post here. How do yoy measure the hp or thrust at the prop of an outboard? Im sure if you know the power head hp and the gear ratio, and prop pitch you couls calculate these. Right? So is there a formula?

[jehardiman]

As i have said before (see HP to WATT to THRUST (http://www.boatdesign.net/forums/showthread.php?t=6572) thread) it is the prop that converts a given HP to a given thrust at a given speed and you need to know something about the hull also.

There is no simple formula that is correct. There are many rules of thumb that all depend on the particular application. But why depend on non-precise formulas when for less $250,000 you can buy and install the hardware to measure it directly real time? Several companies make RF strain collars for just this application.

Really, why do you need this information? The answer you can calculate for low horsepowers has a margin of error greater (unless you have access to measured engine and prop curves) than the effort involved so it is rarely done.

[kenfyoozed]

well how does merc/nissan/tohatsu/suzuki/ johnson/ all calculate the HP for each of their small outboards? all i want is the specs for a particular outboard. i know the power head hp, the powerhead rpm ,the gear ratio. So let me ask this. Knowing whati have can i calulate the shaft hp?

[jehardiman]

well how does merc/nissan/tohatsu/suzuki/ johnson/ all calculate the HP for each of their small outboards?

They dyno the prop shaft and report (based upon company policy) the Hp. Note that this hp is a specific rpm. As horse power is torque*RPM/33000, and most IC engines have a torque curve that drops off at lower RPM, most engines only develop rated HP at maximum RPM (there are some differences between 2-cycle and 4-cycle engines in this however).

all i want is the specs for a particular outboard.

The manufacturer is the best place for those, though you sometimes can find some published data in the industry magazines. That type of exact data is expensive to get though and most people who have it keep it close to them as all sorts of business advantage can be gained by having it.

i know the power head hp, the powerhead rpm ,the gear ratio. So let me ask this. Knowing whati have can i calulate the shaft hp?

Of course, hp at the top is identically equal to hp at the bottom less gear and bearing losses. Mechanical losses generally total less then 10% for a well designed system, but again getting the exact value is expensive. The real effect you have to worry about is the change in torque due to the change in RPM. When stepping up you get less torque, when stepping down you get more. This is important when selecting a prop because it is possible to bog/stall an engine at low speed with a prop it could swing a full RPM (see the discussion about torque curves above).

[gonzo]

Outboards have advertised HP measured at the prop.