Marty - I'm answering your question about shaft torque and prop efficiency in this thread.

Tieing the boat to the dock and taking the engine to WOT would cause a lot of slip. With the prop completely submerged the engine would likely come to an RPM less than Max ( like stall speed on an auto trans converter). Without any forward boat movement, the only water flow across the blades is what is drawn into the prop, so the slip calculation is high. This test might be useful if you were trying to set up a boat for a quick launch.

The shaft torque reading has as much to do with how well the prop hooks up as with how much torque is available from the engine at that RPM. So if the prop is not completely submerged, or starts sucking air, the RPM will take off and the readings will be meaningless.

Prop efficiency is a curve. Maximum efficiency occurs at a given angle of attack/slip % for a given prop. When you look at the equations, the biggest variable that affects slip and therefore efficiency is blade area. The simple explanation, (the only one I can understand) is that as blade area goes up too much, slip goes down, but too much power is wasted turning the blades through the water, so the net thrust is lower. Too little blade area, slip goes up, but less thrust is produced to begin with, never mind that it's easier to turn the blades through the water.

Your on board dyno can help explain why some props are faster than others. The "right" prop for speed is the one that generates the most thrust. That means it is just big enough to absorb all the engine power, with the least amount of blade friction. It often seems that spinning a bigger prop slower is the fastest setup, even though the Max RPM is reduced. A prop that converts 80% of 500 HP to thrust would be faster than a prop that converts 70% of 550 HP to thrust.

As an example, I can go 78 mph with 29 Mirage Plus props at 4300 RPM, and 78 mph with 26 Bravo four blades at 5000 RPM. My engines make a lot more power at 5000, but the extra blade soaks up that extra power. My dyno sheets says 413 HP @ 4300 and 483 @ 5000. Assuming the 29 Mirage is 80% efficient (maximum efficiency achievable in most props), the 26 Bravo is only 68% efficient. It is much better at getting on plane, but after that the extra blade is a liability. These results are prior to raising the drives 2". Now I run 82 mph at 5000 RPM with 28 Bravos, but I want to test a pair of 29s again. Since they are 15" diameter, bigger than any other Mirage prop, they may still bite well enough to prevent blowout at the higher drive position.

I know what you're going to say, it's torque, not power, that is important. But I say it's max prop efficiency (more thrust out for every HP in) that counts. Build the damn thing and settle this argument once and for all!

CMarty

Been looking at the L&S site. Appears that, as you say, they use a "normal" strain gauge buried in a tailshaft or something similar. The noncontact aspect of their sender works off of inductance and power source for transmitter same (via AC current from some sort of switching power supply). Cool idea.

I also see why you say the setup won't work on the IMCO box as it sits. No room for junk near the Ujoints. Could probably be made to work on the coupler end, though, with mods.

I am liking this whole realtime dyno stuff. Too cool.

About the only way I can see to get it on the stock brave is to use a 'mini' cutdown 'stern jack' and have it housed inside it. Actually L+S is impenging on my electrical engineers pattents. They don't have it quite right and they don't make any money so he is not going after them.

Tomcat

Speaking of props, what about a four blade with the two opposite blades cut short to help get the boat on plane, once its up, their out of the water an the larger blades take over?

JSV - Interesting idea. I'm not sure if it would work any better than reducing the diameter of all blades, but it sure would look different. The outboard guys have this all figured out. Adjustable blade area (hydraulic jackplate).