UCD Boat Dyno project; Any help is appreciated!

[bean]

prop specs

Yeah, that is a little bit of a fuzzy area. We do have an old Aerospace Engineer for a senior faculty member and a really strong fluids prof here, so I think we'll get at least a good idea of a model that will be close enough to fit within the errors of the rest of the system.

We are considering multiple props as well, however. Possibly a straight, true-pitch prop as a control first, to see how our model works, then we can move it up to the more complicated designs to compare.

thanks

[bean]

Sorry!

Everyone, I appreciate the help you given. We are turning in our mid-term report on this project today, so we're very busy finishing it. I will be back in contact soon, to get some more advice on details.

Thanks!

Ben

[26V]

Good luck on the midterm. Let us know how it gos

Gene

[bh/]

Re-reading this a few times I now have an idea of what u are up to. Some of us may be missing your first point, that power is work done over time, so we can measure it by simply knowing the thrust in pounds and velocity.
HP=(F)(V)/550, where V is in 'fps'. This of course is the applied, or effective HP that the prop transmits to the water.
If one knew the efficiency of the prop at that instance,
[ie;how good is it in converting torque into thrust] and the losses occuring in the gearbox/water pump, then we know true crank shaft HP.
Detemining that prop efficiency is the $64 question, and one all racers struggle with.
Measuring 'RPM' is easy, so if u could get an accurate torgue reading on the power head, u would then know the
the gross crank shaft HP of the motor, and comparing that to the net HP at the prop, determine the efficiency of the set up, ie; how good is this prop in applying the HP to the water.
I an told that six cylinder Mercs deflect almost a tenth of an inch at the top of the power head due to torque, and if one strain gaged that, then we would have a measure of torque. One would have to calibrate this instrument using a real shaft dyno.
All in all quite interesting stuff, but you will find, and no doubt be surprised how little science there is to prop design, and how much it is a black art.

[johnsonm50]

Im glad to read that your working on this although the many variables. The developing props being a black art may soon be science due to your work. A curiosity for me is would this turn out to be a point where different design begins or find out that 90 + percent efficiency is as far as itll go.

[f8andbethere]

Well propellers are much less of a black art today then 20 years ago thanks in part of a fellow named Steve Vincent who developed a digital prop guage that allowed reading pitch on a prop to .01 of an inch. This was done efficiently and quickly and allowed a driver to see the difference between two supposedly props that were the same.
Mr. Vincent also developed a number of other pretty nifty items (with the help of another skookum racer John Adams) that would collect data from a number of inputs -- I do not remember the number of channels but more than the average boat racer would normally need.
This included the ability to take information simultaneously for RPM (and other engine functions such as engine temperature) and an accelermometer if one was so inclined.
Steve is the guy to talk to; he can be reached at racehydro@aol.com.

So how are you going to accurately determine the exact prop pitch? While this may seem fairly elementary, just take the manufacturer's word that a 19 inch pitch prop is really 19 inches, it would seem that this isn't necessarily accurate enough for your needs. Many people have gauges that can measure pitch at numerous places on a blade, how do you integrate those individual numbers to find one simple pitch to identify that blade? Are you then going to assume that all blades are identical? A true-pitch prop would simplify things here but most all racing props have progressive pitch and some cup at the trailing edge to confuse a simple one number pitch description.

Most small outboard racers don't know much about pitch and none are worried about efficiency. Put it on a boat and let's see how fast it goes, how good it accelerates and how good it handles the boat and maybe even what kind of lap times you can get. Now a map of pitch numbers on that prop may give some insight as to how another prop may work or maybe where you want something different. Propellers at the racing level are a black art. Trying to quantify all that for one of your baseline measurements will be an issue.

[bean]

So, BH/, when you're mentioning "HP=(F)(V)/550", you're saying that barring the losses in the the age, etc. of the motor, the 85HP that is listed as the effective power at the prop can be divided by the boat's theoretical velocity at the RPM that the power is acting, and multiply by 550, you should get the maximum thrust force applied by the prop to the water (before cavitation)? Right?

[bh/]

Yes, Force x Velocity/550 = HP [effective, at the prop]
If one measured[dyno] or knew the true HP at the prop shaft, and the speed the boat was going, the Force calculated would be a bit lower[~10%] due to the in-efficiency of the prop [at converting rotational power into thrust] Prop efficiency is not the 'slip' one hears of. Some of the power is lost in spinning the water, rather than accelerating it rearwards, creating thrust. We hear that modern race boat props are 90% efficient.
So the formulae in practice would be some thing like;
F x V/550= HP(.9)hp