Prop efficiency … a point of interest.

[RotorHead]

An interesting discussion was begun on a different thread relating to the efficiency of the Trophy propellers. I chose to start a fresh thread, the other one is going in a direction I do not care for.

We have been having an awful run with the weather up here and today was no exception. So I could not pull any big numbers and could barely keep the camera from jumping out my hand. But if you grant me some poetic liberty, perhaps + or – 1mph and maybe 100rpm leeway due to the parallax on the gauges, here is what I’ve come up with.

Given; 24p Trophy (un-massaged) and 1.62 gears


@ 5000rpm = 65/66mph, my prop calc puts that at; 90%

[RotorHead]

@ 5500rpm = 72mph, again 90% efficiency

[RotorHead]

@ 6000rpm = 78mph, this time an 89% efficiency

[RotorHead]

This is the last one. It’s the best I could do in the conditions…
@ 6400rpm (the parallax is bad on this one, I was reading where the shadow of the needle hit the gage???) if we can call it 87mph =93%,

If we read it as 6500rpm @ 87mph = 92%
Or 6500@86mph=91%

[Markus]

Bear in mind, though, that the stated pitch of the prop is not exactly the same as the average actual pitch of each blade.

Therefore, that kind of prop. efficiency numbers are not very meaningful for comparing different props.

[blkmtrfan]

I will see if I can do a similar test this weekend ;)

[WILDMAN]

I use Trophys on my 21ft Rallysport. Same hull as Rotorheads. My 225 Optimax, with 1.75 gears, with a 26" Trophy, runs 73.2 at 5600rpm. This = 7% slip. Last year I put a 1999 300 Promax on it. 1.75 gears, 28 Trophy, 86.8 mph at 6200rpm. This + 6.2% slip. Trophys work great on a padded V-bottom.

[Markus]

Either that, or the actual pitch is higher than the stated pitch.

[Reese]

How are you guys getting those large 26 and 28 trophy's to take off from a dead stop. Last year I was helping a friend with his 225 efi...we tried a 26 but it was bitting so hard is was like pulling an anchor and it really lugged the engine.

Yes, we took all the vent plugs off but it still was too much for the bottom end. I later heard that some folks are using small 4" hub trophy's to get them to vent upon take off?

[PhastBoat]

Read this thread:
http://forums.screamandfly.com/forums/showthread.php?s=&threadid=58979

[RotorHead]

Reese,

We ran into a similar interesting problem the first day we tried the boat.

Not knowing what to expect we put the 28p prop on…

The big Merc. could not make enough rpm to bring the water pressure above 6psi., as a result the computer would not release the motor to make hp. and vicious circle perpetuated. End result the boat would not plane off. Finally we tricked the pressure meter to read above 6psi and allowed the motor to make power, but the rpm range was way off the mark.

I don’t know about the 225efi’s whether they have such a safety feature or not. Perhaps some of the more knowledgeable guys can comment.

[WILDMAN]

My Trophys are small hub with 2 plugs removed. It spools right up on take off!