Plate even with bottom of boat or not?

[TrackerTom]

Ok so everywhere I read and from Mercury the plate should be even with the bottom of the boat. Then, I call Tracker Marine who built my boat and ask them about my exact motor and they tell me it should be mounted on the transom sitting directly on the top rail which puts the plate 1.25" below the bottom of the boat. What's up with that? If best performance is obtained with it even with the bottom, why in the world would they mount it lower? I'm considering getting a 4" manual jack plate and setting it even to see what happens. Though my performance is respectable with it setup as is. It's a 1654SC Grizzly with a 4 stroke EFI 40 with the prop that Tracker recommends, a 10.38 x 13P factory one, and it runs an occasional 30 MPH with just me and usually 28 MPH. Of course I would like to get all the speed out of it that I can, but I'm not sure I would get much more considering that the calculated speed with no slip on my current prop is only 36.8 assuming that it reaches 6000 rpm. I'm trying to come up with a smartcraft tach to check this, but it takes a bit of $$$ to get a tach hooked up to the mercury EFI's so I figured I would just go with the smartcraft. Anyway, do you think it's worth jacking the motor up?

Thanks,

Tom

 

[crazymanme2]

If your motor is clamped not bolted just put a 1.25 spacer under motor & take it out a try it.Make sure motor is pumping water a WOT.This way you can see if there's any difference without spending the money on a jack plate.

 

[Quackrstackr]

And if it's bolted, just raise it one hole.

Most manufacturers (boat and motor) don't recommend that you sit one all the way down on the transom. My boat has probably an inch and a half gap between the mount and transom and could really stand to go up one more hole.

 

[Zum]

And if it's bolted, just raise it one hole.

Most manufacturers (boat and motor) don't recommend that you sit one all the way down on the transom. My boat has probably an inch and a half gap between the mount and transom and could really stand to go up one more hole.

Really?First I'd heard of that.
Thanks for the heads up,I'll have to check it out.

 

[Quackrstackr]

Really?First I'd heard of that.
Thanks for the heads up,I'll have to check it out.

That's on newer stuff and may not be the case for every application. Today's transoms are quite a bit stiffer than what is found on old boats and the engine mounts are usually a bit different.

A cavitation plate anywhere from 1" below the bottom to even with it are pretty standard starting points. Higher performance boats/hulls like mine may even benefit from going higher.

 

[TrackerTom]

Well I finally got around to moving my motor up. I took more careful measurements and it was a full 1.5" below the bottom before. When I tried to trim the motor past neutral steering it would pour water over the transom. It ran good, but I thought it might be low. You could see the plate, but it was in a hole with water swelling around it on both sides. Anyway, last night I moved it up 2 holes and set it dead even with the bottom. What a difference. I took it to work with me and on the way home I dipped it in the lake. It now runs 32.4 MPH on gps and would run more but it still porpoises if I trim any more. The porpoising is better than before because I can trim up now and improve the speed before it occurs. Acceleration is improved big time. Overall the boat performs much better. Now when I look back the water coming off the back of the boat is much smoother, but it is going over the top of the cavitation plate. It looks like I could actually go up another hole which would put it 3/4 above the transom. It doesn't blow the prop out in turns at full throttle. I'm tempted to raise it again and see what happens. What do you think?

Tom

 

[Quackrstackr]

You can try it and see.

So long as you aren't experiencing excessive prop slip, blowing out in corners and have good pump pressure, you are good.

 

[TrackerTom]

Well I don't have a pressure gauge, but it has water coming much more smoothly off the back now and its going over the cavitation plate so I assume it has plenty of water. It doesn't blow out in turns at all.

 

[ben2go]

Most of the motor manuals I have read state that the plate should be 1 to 1.5 below the bottom of the hull.