Viper Coral with Mercury Promax 300 Engine Height

[Basnova]

I am the original owner, and here are some specs:
1998
20'1"
Promax 300
Torquemaster LU, pickups at bottom of nose cone
10"-setback manual jackplate, 30" from transom to front of prop
Trophy 4-blade 25p with Exhaust Ring

In the beginning, holeshot required finesse feathering of throttle to plane. Burned even more gas and oil than the rated 30gph because of this, but I chalked it up to normal until a service shop suggested I try the exhaust ring. Holeshot improved trememdously, and I found myself with more money in my pocket from the savings of gas and oil!

Over the next few years, I have to admit that I played with the engine height a bit, without really knowing what I was doing, and not noticing much of anything, likely due to being a bit naive coming from a Ski Nautique to the bass boat world.

Now, 18 years after custom ordering this boat, which is still in brand-new condition, I have gotten serious about dialing in the engine height properly, even though visually it looked fine, as judged by hanging my head off the back and clearly seeing the cavitation plate above water at 45mph.

I recently leveled the hull on the trailer, and lowered the trim until the LU was also level as indicated by placing a level on the cavitation plate. After taking measurements, I discovered that my previous adjustments 15 years ago landed the prop shaft centerline (as measured at centerpoint of nosecone) at 3-5/8" below the pad, which I learned on this board is pretty close to a good starting point. Not bad for just some trial and error those many years ago, not even thinking about measuring anything.

I ignored the suggestions I read on this forum regarding only moving up 1/4" at a time, and promptly raised the engine so the prop shaft is now 2-3/4" below the pad. Took it out a few days ago and easily and immediately noticed some dramatic changes, as listed below.

1) The boat practically jumped onto plane.
2) At low planing speed, I feel like the boat is WAY higher than it used to be. Not like the bow is in a wheelie position or anything, but like the entire hull is riding MUCH higher.
3) Boat seems to move with much less engine effort.
4) Gained over 300rpms at WOT, as well as significant mph increase over previous speed at same rpms. Seemed like it could've gone more, as it was still accelerating when I ran out of lake real estate due to low lake water level.
5) Only needed to trim less than 1/2 what I previously had to trim to throw a rooster tail.
6) Noticed that bow is lower at high speed, but not plowing.
7) steering seems more neutral at a lower trim now, whereas I previously had to trim higher to get neutral steering.
8) Noticed some porpoising after bringing speed down to a low planing speed again (although it did not porpoise previously), but not sure if it was because I've not yet found proper trim position when decelerating, and/or I may have been crossing some leftover boat wakes, as it was pretty close to dark by that time.
9) Believe it or not, even on trolling motor the boat moved faster and more effortlessly through the water. Partner on back said it looked like less of the engine mounting bracket was in the water. Could that be true, even though I only moved the engine up 1-1/8"?

I'm looking for some thoughts and opinions as to whether or not I did everything right, whether I need to adjust up or down, or any other suggestions. I also specifically want to know thoughts on #6 above. For example, I've heard that if the engine is too high the bow will fall, but I wonder if I've been so used to the bow being angled up, and now it's actually dropping to where it's supposed to be, if that makes sense.

I'd also like to hear some thoughts on the idea that the farther back the prop is from the transom, the higher the engine should be. Remember, the front of my prop is 30" from the transom, and according to the 1" for every 10" of setback means my prop shaft would end up ABOVE my pad. Could that be right?

Oh, one last thing: this is a big motor, and now I only have 1-1/2" between top of motor and garage door height, so if you think I should raise the motor more, I might have to remove cowl, or tilt motor down to get in and out of garage. When I think about it, I think removing cowl is a better option because I might forget if I had tilted engine down when leaving, which could result in a disaster to LU, whereas I would at least see engine cowl in rear view mirror.

Thank you in advance.

[flatbackcanoe]

Mark the jack plate so you can return it to where it is now.
Level the boat and the engine again.
Mark the jack plate,, for prop shaft relation to the bottom of the hull (pad if you have one), with increments of 1/2 inch or 1/4 inch through the full range of jack plate movement.
You can then move the jack plate up and down, without all the hard work of measuring, to see where the boat performs the best.
Make sure to keep an eye on the water pressure gauge, when running the engine higher, for proper water pressure.

[Basnova]

Thanks for that tip. I have a vertical bolt at the bottom of my jack plate for height adjustment, and 2 bolts on each side that lock in the position. I think I can just mark where the locking bolts are now, and make further adjustments based on their current position, correct?

Also, what would you suggest I use to mark the plate?

With my lower unit having water inlets on the bottom of the nose cone, I can't see having an issue with water pressure without my prop being completely out of the water, but I will be sure to watch my pressure gauge nevertheless.

As far as height is concerned, what do you think about the rule of increasing height based on distance from transom to prop? Also, is it even possible that the boat might run well with a prop shaft positioned above the pad?

Btw, anyone know what rpms this motor should run at WOT?

[flatbackcanoe]

I marked my jack plate with a black magic marker.
I didn't mark the bolts, I marked the point where the 2 plates slide past each other, but up to you.
You can measure from the current position, yes.
I don't know much about bass boats or what would be a good engine height for your boat. That's kinda something your going to have to spend time experimenting with, set it the way the boat handles best for you.
Start lower then the rule of thumb would suggest, and go from there.

[Basnova]

Ok, marking the plate sounds like a better idea. What might be symptoms of being too high?

[JPEROG]

If the motor is too high then you will have three things potentially.

1. loose water pressure
2. slip numbers go way up
3. the boat will want to "paddlewheel" - walking he rear end to the right

Joe

[lance]

I have a somewhat similar setup. It is a fish/ski style boat with a large horsepower heavy motor. Get a power adjustable jackplate and you will save you a ton of time dialing it in. You can lower the engine for holeshot, skiing, and heavy loads. Once on plane, you can raise it up based on weight speed and water conditions. It will also solve the garage door clearance problem. I think you would be surprised how high you can raise the engine to improve your speed. Just watch the water pressure. Also, try a larger diameter prop like a Bravo with PVS holes.