Horsepower Test for a 9.9Hp Outboard Engine

I googled 'dyno testing' and came up with www.revsearch.com . This looks like a possible fit.

9.9 hp engines are peculiar in that they are really supposed to be less than 10 hp, so they can be used on lakes with hp restrictions. Measuring to 1 percent reliably and repeatably with an average dyno is not all that easy. You may need to research a little (or perhaps your customer knows) how 9.9 engines are certified (or if the regulating authorities take it on faith that they are really less than 10 hp).

The measurement itself could be pretty easy. You could replace the prop with a disc brake and a reaction arm, or with a hydraulic pump (torque being directly related to pressure) and accurately measure the torque and rpm. Driving an electric motor (as a generator) is another option: you'd need about 7.5 kw of load and a possibly a water tank to absorb the heat. With a DC motor, the amperage is directly related to torque.

RPM can be measured easily and very accurately by many devices you are probably already familiar with. Torque is not quite as simple to do accurately, but a load cell on a reaction arm is probably a good bet (or less directly, current or pressure).

Of course, if you find someone already set up to do this, they would be able to run a power curve in less than an hour's time. That would be quick, cheap, but less fun than making your own dyno. The standard chassis dynos found all around the country for cars and motorcycles are not well-suited for outboard motors.

Depending on accuracy required, a very simple method is to put the motor on a test tank and measure thrust against a known 9.9 using the same prop pitch and diameter. (This assumes the two engines are similar in characteristics, though.)

Just as simple: put it on a boat, measure the boat's top speed, and then do the same with a known 9.9. This would be close enough for marketing purposes but not for legal requirements.

The owner has no doubt warned you to be darn sure you provide water for cooling.

Hi Ken,

I used to build swinging arm dynamometers to test my model steam engines and full size stirling engines, they are by far the most simple method of getting a very accurate measurement! They are also very easy to construct, much simpler than a water brake.

The arm has to be graded accurately to get the best results to give ft/lbs, and the maths is quite simple.

Spencer.

I agree.

Perhaps the most fun, though, would be to bolt a mountain bike wheel/tire to the prop spline, get the thing zinging along, and then press the tire against the ground, a little like using an oversized chain saw. Mount two handles a convenient distance apart, and equip one with a scale...

What could be better? Tire smoke, noise, danger...

Take it to a Harley Davidson Motocycle shop most have a Dyno machine.

I've designed and built a couple of these motorcycle dynos. The difficulty, for testing an outboard motor, is that these are chassis dynos, and building an adapter to go from the prop spline to the wheel rollers, a means for supporting the motor while it runs, and a means to provide water cooling without interfering with that adapter is a project somewhat more time consuming than building a dyno from scratch. Measuring rpm is simple. Measuring torque can be as simple as bolting up a car-sized disc brake and measuring the force against a reaction arm which can bear directly on the motor's housing, with load cell between. Easy, fun project that can be done for far less than $3000 - unless the PO is charging a lot for his time. If so, then he should take or ship the motor to a facility that is set up for testing outboard motors, such as one that owns one of these.

If such a facility charges more than $100/hr for dyno time, I'd scream "highway robbery!" (After all, you can rent a car of higher value than the dyno for a whole day for $49.)

yEAH i just figured the motorcycle shop could tell him where one was for a boat motor.