HP is often the easiest and/or only number we have a handle on, so speed calculators always use HP. And my program talks about the HP required to move the hull and the HP delivered by the engine to propshaft.

This is misleading. It's really all about drag and thrust. The hull and drive cause drag and the propeller overcomes this with thrust. At top speed drag and thrust are equal. The problem is we can't measure drag and thrust directly. So we fall back on the measurements we have...HP and torque.

What the onboard dyno really needs to measure is thrust. We need a wireless pressure transducer inside the thrust washer. The thrust (pressure) at different speeds is the hull curve. The HP needed to make the propeller generate thrust at different RPM and pitch is really the engine curve, what the engine has to have the power to do. If you had those direct measurements you wouldn't base your speed calculator on HP.

Send me an email when you finish experimenting.

Tom

tomcat,
Your technical topics lately have been great! Just wanted to say the effort you have put into them is very much appreciated.
That's all, Thanks!
Jerry

Thanks Jerry, I've got a bee in my bonnet this time for sure. I've been doing a lot of web searches on this topic and I'm still at "the more I learn, the less I know" stage. A lot of the sites are for airplane props, but a few are for boat props. The math seems to be the same anyway. I have found some typical prop efficiency curves so I know what to ask prop manufacturers for now.

These sites all talk about thrust horsepower.

Thrust HP = Thrust(lbs) X Speed(ft/sec)/550 (ft-lbs/sec)/HP

Thrust horsepower is what's left of the propshaft horsepower after you lose some of it due to the prop efficiency. Prop efficiency can't get much better that 80% and it varies from zero to a maximum value then falls again as the advance ratio increases.

Advance ratio = boat speed/(prop RPM X dia)

Since diameter doesn't change much for stern drives, we can ignore it. So advance ratio is really just boat speed over prop RPM, which is the same as actual speed over theoretical speed, which is how we calculate slip, which is closely related to angle of attack.

Mercury says propeller efficiency peaks at a certain angle of attack for a given prop,so it's all connected, and if I can get some propeller efficiency curves I think I know enough of the math to put them in the program and adjust for their effect.

Hey Marty, I found a couple sites with prop thrust and prop torque strain gauges, but they were research labs offering services, not a product.

Tech Question of the Day: The base of the blade near the hub has more pitch than the tip of the blade. Do you know why?

Answer to todays tech question,
The pitch is greater towards the center because the speed of the blade in relation to the water is slower in the center, the speed of the tips of the blades is much faster. They are spinning at the same speed and the outerpart of the blade has a further distance to travel in the same amount of time therefore it is traveling faster.


Or maybe its just because!

Please send my new boat to...........

Caleb

Is that your final answer? You are correct, sir! It's so the blade tips don't get ahead of the hub, which would be a bad thing.

Prop science what a topic. As for a wireless transducer on thrust washer that would be great! Ever thought about using a pressure transducer from trim cylinders to try and fiqure your thrust. Not sure how the math would work but seems there has to be some differance in the pressure on rams depending on load. Just a thought.