Quote:
Originally Posted by erebus
. . . I know it's been mentioned many many times by a bunch of you guys in the know (looking at you Bushwacker.  ), that the motors Anti-Cavitation plate needs to be approx 1-1/2 to 2 inches above the bottom of the keel at the transom for optimal Seacraft performance. . . |
I think you'll be pretty close. As Friz said, what really counts is getting the AV plate up above solid water when on plane. That's sort of step 1 in getting optimum performance.
The other variable is what prop you're running, as some props can run higher than others. Forum member Egg Sucking Dog, who hasn't posted here for some time, used to run a Stilleto on his 20 MA, I believe at 2.5" above the keel with motor on transom and no jackplate or extra setback. Props with a lot of cup in the tips seem to have lots of grip and can typically run very high, so once you get the AV plate above water, the next check is, at your normal cruise speed, you want to check handling as Friz recommends. To do that, trim the motor up to where the prop starts to ventilate, then trim it down a bit, and then try some quick hard turns to see if it ventilates. If it doesn't, you might be able to go up a bit more; if it ventilates, you may want to try a prop with a little more cup, or just live with it, knowing that you may have to trim down a bit before doing any quick turns or running in big swells.
The pics below show what it looks like when motor is too low and just right. 2nd pic is with AV plate at 3.25" above keel, which is as high as it would go without a jack plate. (I had to focus on the black fin I added to AV plate, because it's pretty hard to see the white AV plate in white water!) Since I have a bracket with a lot of setback, I ended up installing a small 3" jackplate because the widely quoted rule of thumb of "raise motor 1" for every foot of setback" appeared to be totally bogus! Last 2 pics were taken after I raised the JP as high as it would go, and AV plate is now at about 6 1/8" above keel with 33" of setback, so the correct "rule of thumb", at least for my boat, is about 2.23"/foot of setback!
BTW, I'm going to move this thread to the Performance section because that's the main reason to adjust motor height.