Raised my motor, but questions...

[erebus]

So before launch for 2016 I decided to raise my motor up a bit on my 1975 SF18 to see if I can help with my high-planing speed and general drag-ass performance I had last year. (see this thread from last August)

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.

As the boat was set up from last year the Anti-Cav plate was just below the bottom of my keel by about a 1/4 inch, so I raised it up 2 holes.
Now the Anti-Cav plate is about 1-1/4 inches above the bottom of the keel, but it looks kinda weird.
Theres still one hole to go up so its not maxed out, but it just looks kind of odd.

Couple of my co-workers at the boatyard here were assisting me and they all thought that what I was doing couldn't possibly be right.
Here are a couple of pics.
I know she needs a sea trial to really know whats going on but Nantucket Sound has been a lumpy mess all week with winds consistently out of the southwest.
When I finally get out there, I guess I should be looking for prop blow out, and the Anti-cav plate running just about at the surface? Anything else?

Can you guys take a look and let me know if this looks right? Again, this is on an 18 footer with a 115 Mercury 2 stroke.

[FLexpat]

While I have been wrong a lot and occasionally right, I'm thinking you are very close to right on. Whats the the center to center distance on the holes - 1/2"? You probably have all the adjustment range you need. After you dial the height then dial the prop. The prop gurus are way smarter than me on that - I would call Ken Reeves at PropGods.

[flyingfrizzle]

I second that, looks like a good height for sea trial. Just make sure you can live with the handling up high. My motor is way up and has less drag and better speed but if you get too high you see your boat not do as well making tight turns when trimmed up high. Feels like the boat wants to turn like a flat bottom skiff and gives you that sliding felling if you don't trim it down when up real high (2 inch plus). You can get too high and loose your handling but can still trim down a little and she locks in and will turn find tho when up too high. You got to find that balance point you can be happy at. Just make sure you get the AV plate up out of the water when on plane.

[Bushwacker]

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.

[uncleboo]

The person that mounted mine back in '03 has the av plate about 3/4 to1' below the keel. I'm assuming I should raise mine as well. Running a Suzuki df140 on the transom.

[Bushwacker]

Originally Posted by uncleboo

The person that mounted mine back in '03 has the av plate about 3/4 to1' below the keel. I'm assuming I should raise mine as well. Running a Suzuki df140 on the transom.

I usually check the E-Tec Owners forum every couple days, where the engine techs often mention the importance of correct mounting height, and it seems like almost all dealers mount the motors too low! I can think of several reasons they might do that:
1. The average boater, especially a newbie, has no idea how much motor height can effect performance.
2. If motor is too high and the prop ventilates during hard turns, even a newbie will notice that, and complain!
3. The typical dealer could care less that you aren't getting optimum performance! He's very busy during the peak boating season, so he doesn't want to be bothered by come-backs! The safe option from his perspective, particularly if he's dealing with a newbie, is to just mount the motor on the low side of normal to minimize complaints. If you find a dealer that pays attention to mounting height, that's a good sign of a much better than average guy that really cares about his customer!

I agree, your motor is mounted way too low if AV plate is BELOW the keel! Just make sure you don't have some oddball combination, like a 25" motor on a 20" transom or vice versa! I think the AV plate set at 1.5 to 2" above keel is about right, but a sea trial to insure the AV plate is above solid water and handling is good is what's important. I sort of like to get it too high initially so I know where the "edge of the cliff" is and then drop it down a hole, but when doing that, it's a good idea to monitor water pressure to make sure you don't create cooling problems! I believe the basic design of the prop and how heavily it's cupped is a more important factor than size in determining running height. For example, some bass boat and race props are designed to be run "semi-surfacing", but that's a very specialized application, not not likely what you'd want on a typical SeaCraft!

[Old'sCool]

Erebus is prrob close. The Suzi 4S runs a larger dia prop and will limit you

[76Red18]

That's a good starting point. Mine is 2" above on my 18. The prop I have had no problems at 2 3/4", but being that high caused cooling issues. Pay attention to you temp when experimenting with motor height.

[otterhound]

I went through this almost to the letter on my 18 last year. My motor is a '92 Evinrude 120 hp and it was mounted in the top set of holes with the ventilation plate 1" below the bottom. It took forever to get on a plane with spray flying everywhere and in general was a dog. I raised it to the bottom set of holes which put the ventilation plate 1" above the bottom. I also put a cheap Apollo 4 blade prop on following advise given on this forum. The difference was really quite amazing! it hops right up on plane now, rpms are where they should be, spray is under control and minimum planing speed is down to around 15 mph. I think I could raise it one more hole but I'm out of adjustment. The boat really came alive with these changes and well worth having to play with the trim a bit more.
Rod

[Capt Terry]

I never had to consider where the A/C plate was during the first 37 years of running Mercs on my '76 20' Seafari. Why? probably because both engines were sold & installed by Frank Brown, one of Moesly's race mechanics. Three years ago I bought an ETEC, then I started hearing about about height over the keel from the ETEC owners site. My ETEC dealer raised my ETEC twice and I raised it again. Last week Skippertee was riding in the stern jump seat as I cruised at 30-32 mph. He said the A/V plate was positioned just right and said Moesly recommended 1.5" above the keel, which is what I measured after the last height adjustment. BTW, I have no engine bracket like Bushwacker. As others have mentioned the propeller characteristics play into this too. I have tried a few props, but got my best results for WOT rpm and best wakeboarding speed control from a 5 blade High Five. The other props worked pretty well too, but didn't quite get it to the recommended WOT rpm.