Prop Tuning Theory
05-24-2023, 08:14 AM
#1
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Prop Tuning Theory
Guys,
I recently bought a prop that turned out to be modified. I'm not upset, but I didn't get the result I thought I might.
Thus far, a Bravo1 28P has been the best prop for our application. I am on the search for another one as a backup. My nephew turned up a 29P on FBMP, and the seller and I worked out a deal. I had my suspicions about it being a modified prop, as I know Merc doesn't offer Bravos in odd pitch increments. I got it in hand and sure enough, the pitch number had been welded in and a "29" stamped in it's place. The leading edge had been thinned and sharpened significantly, as well. I did not bother to check the model number at the time. I was excited to maybe see that 80MPH mark, so we immediately installed it on the boat and headed for water.
Initial takeoff was fine. While I didn't hammer the throttle, holeshot was respectable; not at all what I thought we'd see. I expected it to be a bit more sluggish than the 28P, but it launched fine. Then, as the boat nosed over onto plane, the prop blew out some. We didn't lose it, but it tached up a bit before it hooked up again. We never see that with the 28P. So we were off. We cruised around a bit as I trimmed the drive out to normal cruising attitude. Everything seemed as expectable from the 28P. Then I eased it up to WOT, trimming it up to where I typically trim it for hammer time. The engine tached out right at 5000-5100 RPM, just like with the 28P, not struggling to get there nor trying to over-rev at all. we were actually a couple MPH shy of expectable top speed; probably 73MPH max. It also seemed to want to porpoise a bit more than with the 28P. We headed for the ramp.
Getting back into the shop this morning, where I store the boat, I pulled the prop off and compared it to the 28P we run full time. I also noticed the model number was different, and checked out as a Bravo1 30P. I also noticed that the trailing edge had been trimmed some. Where as the 28P had a slight curve to the profile of the trailing edge, and a significant cup, the 29" was rather straight and significantly less cup. The leading edge profile also seemed reduced some, but this is very difficult to "measure, as you have to just glance past the two standing one on the other.
I'm curious as to conventional wisdom on modifying props. I have modified hundreds of props for high performance model boats, but never for full sized boats. I know they won't scale evenly, but I'd think the theory would hold. Typically, we would pitch props UP, not down, and yield better results. Is this also the norm for our full sized boats?
Thanks. Brad.
I recently bought a prop that turned out to be modified. I'm not upset, but I didn't get the result I thought I might.
Thus far, a Bravo1 28P has been the best prop for our application. I am on the search for another one as a backup. My nephew turned up a 29P on FBMP, and the seller and I worked out a deal. I had my suspicions about it being a modified prop, as I know Merc doesn't offer Bravos in odd pitch increments. I got it in hand and sure enough, the pitch number had been welded in and a "29" stamped in it's place. The leading edge had been thinned and sharpened significantly, as well. I did not bother to check the model number at the time. I was excited to maybe see that 80MPH mark, so we immediately installed it on the boat and headed for water.
Initial takeoff was fine. While I didn't hammer the throttle, holeshot was respectable; not at all what I thought we'd see. I expected it to be a bit more sluggish than the 28P, but it launched fine. Then, as the boat nosed over onto plane, the prop blew out some. We didn't lose it, but it tached up a bit before it hooked up again. We never see that with the 28P. So we were off. We cruised around a bit as I trimmed the drive out to normal cruising attitude. Everything seemed as expectable from the 28P. Then I eased it up to WOT, trimming it up to where I typically trim it for hammer time. The engine tached out right at 5000-5100 RPM, just like with the 28P, not struggling to get there nor trying to over-rev at all. we were actually a couple MPH shy of expectable top speed; probably 73MPH max. It also seemed to want to porpoise a bit more than with the 28P. We headed for the ramp.
Getting back into the shop this morning, where I store the boat, I pulled the prop off and compared it to the 28P we run full time. I also noticed the model number was different, and checked out as a Bravo1 30P. I also noticed that the trailing edge had been trimmed some. Where as the 28P had a slight curve to the profile of the trailing edge, and a significant cup, the 29" was rather straight and significantly less cup. The leading edge profile also seemed reduced some, but this is very difficult to "measure, as you have to just glance past the two standing one on the other.
I'm curious as to conventional wisdom on modifying props. I have modified hundreds of props for high performance model boats, but never for full sized boats. I know they won't scale evenly, but I'd think the theory would hold. Typically, we would pitch props UP, not down, and yield better results. Is this also the norm for our full sized boats?
Thanks. Brad.
Last edited by Brad Christy; 05-24-2023 at 08:19 AM.
06-14-2023, 12:38 PM
#2
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No thoughts....?
