On board dyno for boats?
 

Marc at Bam raised an interesting point on the speed calculator thread. "Wouldn't it be nice to have an on board dyno for testing." I've wondered about this too and did a web search for answers last year. I think it could be done.

You can have a dyno in your race car. It's a wireless strain gauge that mounts on the driveshaft yoke and transmits a signal to a receiver shaped like a doughnut, mounted around but not touching the shaft. The minute torsional twist in the shaft is directly proportional to torque, the receiver can also sense shaft RPM. HP is just a calculation away.

I think to mount this kind of system easily and temporarily on a boat I would try to measure strain in the hub of the prop instead of the propshaft. The receiver could fit inside the back of the bullet. Because the strain gauge must be calibrated, you would have to have a set of test props with strain gauges installed. That could become expensive real quick for anyone offering the service.

One of the things this kind of test equipment might shed some light on is the %efficiency (not % slip) of the prop and the ability of the prop to absorb torque at different RPM. This could help you select the correct combination of gear ratio and pitch. We all know of cases where one ratio and pitch perform better than another ratio and pitch even though they have the same mathematical final drive ratio. Maybe prop efficiency is part of the reason why.

Isn't technology wonderful?

Sounds like a great idea. I am not an engineer, but. What numbers could you use for a baseline of measuring torque in the hub? Is there such a transmitter that would hold up in front of a prop under water?

Maybe it would be easier to attempt to measure the torque in the upper shaft that goes into the coupler.

That would be a great tool. Maybe an aircraft mechanic could come up with something like they use on turbine powered airplanes.

mike

Tomcat, fill me in on the strain gauges. The only ones I remember had a long "S" shaped wire which changed resistance as it stretched.

Do these new ones work by a light beam or what? Also, what is the source, Jegs?

I would think that it has to be passive to avoid commutation problems.

Anxiously awaiting your reply.

I can't remember who sells the on board dyno for race cars, may be Land & Sea. If you do a search for strain gauges there are lots of manufacturers selling them for industrial applications. Sorry, I don't remember the principle of measurement. Its been a while since I did the search.

Land and Sea makes a unit that attaches to the yoke comming off the trans. I measures 'strain' or twist and thru previous calibration can figure the amount of torque being applied to the yoke. The premise is that any shaft having torque applied to it will twist or deflect a slight amount and if we can measure that amount and compare it to a known amount of applied torque and it's produced twist.
Having said all that, I have looked into this quite extensivly and can't come up with a way of mounting it with a stock bravo. But I am buying a stellings box which I believe I can make it work on. My IMCO box won't work either. Imagine a 2 line digital readout on the dash which will display HP and TQ at your current driving rpm- on the fly! This is only some R and D away. So how much would you pay for this?

Again, I am not an engineer. But, how do you get those much needed and relied on baseline numbers????? If those numbers are not accurate, the entire concept will not be accurate.

Good idea marty! You could send the extension box driveshaft to Land and Sea and have them install and calibrate the strain gauge. Then install the receiver inside the box and your done. Very smart.

From a theoretical point of view IIRC, you just need the material properties and physical dimensions to determine the deflection at some given torque. You'd have to assume a few things like homogenous material and the dimensions remain constant under the deformation but I think you'd be close enough for a baseline.

Tomcat, That's it but the L+S product is expensive, connects to computer, no dash digital display, decent accuracy. I have another company which has all the best features- reasinabky priced, dash digital display with memmory for maximum values, excellent accuracy and can input to a data logger or computer. Allways thinking.