Boat and motor planing problem

[andy capp]

I recently bought a new boat and the problem I am having with it is the nose likes to dig into the water. The motor is already on th second to last hole on the motor adjustment. If I move it to the top it starts to cavitate. Also if i put it down one from where it is now it digs worse. It is a 40 hp merc on a 16 foot crestliner jon boat. The prop is a 13p, is that a speed prop or torque prop. I need something to pop up the nose and keep it up, speed is not a factor. Any ideas of problems, or suggestions? Please help.

[7MM Magnum]

A set of trim tabs might be your answer.

[andy capp]

On a 16 foot aluminum? I don't think so.

[outdooralex]

Look into buy a Dol-fin that mount to the cavitation plate on the lower unit. Pick them up at Gander, Cabelas, or a marine store.

[7MM Magnum]

Sorry Andy,.. I guess I'd better get another cup of coffe in me I mis-read your post. :o I'd have to agree with outdoor alex, get one of those Stingray fins for your cavitation plate. Cabelas has them for around $50. ;)

[andy capp]

Is that going to bring the nose out of the water? I have no problem planing out.

[outdooralex]

Thats a good question. I was assuming that you were having planing problems. That seems wierd that it gets out of the hole ok, but doesn't stay there. I am guessing this would still help your problem. The concept behind those things is basically to provide lift on your bow. If you bought one at Cabelas, they are really good about returns. They are really easy to install. Drill four holes in your cavitation plate and your done ( I know if you return it you will have four holes in your cavitation plate :mad: )

[andy capp]

Sorry all, The motor planes out fine, the problem is it keeps going down into a dig. I was told the boat was fast, but can't go past half throttle. It starts to over rev, and suck air.

[Steven Arend]

It sounds like it might be two problems.

1) To much weight in the front of the boat.

2) The center (or top :confused: ) of your prop. is not 1.25" to 1.50" below the bottom of the boat. (these dim's might be for a 15hp or a 35hp motor??? I don't remember. If you have the manual for the motor it as this info in it.) If the prop is not down far enough it with dig and cavitate.

[andy capp]

There is nothing in the front of the boat except 3 lifejackets. I sit in the back.

And the transom is set up for short shaft. The cavitation plate is even with the bottom of the boat. I think when the front digs, the rear comes up a little and causes the problem.

Now i'm stummped.

[Crawfish]

So it's planing ok, but once on plane the bow is riding too low, resulting in excessive drag since the entire bottom of the boat is in contact with the water.

Is it doing this when you are in the boat alone? Any weight in the front obviously would just worsen the situation.

If it's a short shaft motor, are you sure it's long enough (cavitation plate within an inch of the hull bottom)? I would think a short shaft would be sufficient for any flat bottom, but maybe not for shallow-V jon boats.

[andy capp]

Exactly, it planes fine. Too much drag in front. The transom swoops down in the middle for the short shaft motor. It does it when I am all by myself, yes.

I am positive the motor is correct. I do know that much. I can take a pic tonight.

[Steven Arend]

what about the angle of the transom?

is the motor still tilted back on the second to last hole?

Your not running with your anchor still down??? :eek: :D :D

[coverdog]

Prop damage?

[andy capp]

The prop is brand new! i don't know what you mean by the motor being tilted back? No the anchor isn't down.

Trying to answer a previous question, Is a 13 pitch for speed or torque?

[FishTales]

The 13 in pitch prop will give you higher rpm's but won't have as much 'push' as you would get with a 15 or 17 in pitch. Theoretically the pitch of the prop will move the boat forward 13 inches per revolution. If you go with a larger pitch you should get more movement per rev. Some places will loan you a prop to try out before you buy it, if not find someone that has a different pitch prop that fits your motor and give it a try.

[IT pimp 2002]

Do you have trim/tilt?
May want to try to adjust this as you plane out to get the best perfomance, all the way down to get up on plane then adjust to get best perforamance.
With no trim/tilt, you may need to adjust your fixed position of the motor to acomodate this.

[Michigan Birdhunter]

Sorry all, The motor planes out fine, the problem is it keeps going down into a dig. I was told the boat was fast, but can't go past half throttle. It starts to over rev, and suck air.


Hi Andy - Sounds like you are having a ventilation problem. It occurs when air from the water surface or exhaust gases are drawn into the prop causing it to lose its thrust and causing the prop to overrev and cause cavitation. I would talk to a Mercury dealer about the prop. Some places will let you try different props to see what works best I'm told. Check Mercury's website, I think they may have a list on there. I know you know moving the motor away from the transom will cause it to push the bow up and that didn't work so I would think it is the prop. Wrong pitch and or diameter.

[TONGA]

Andy can you pull that prop off and snap a pic of it,,,also any info you can read off the hub,,the pitch is just like your final drive ratio so to speak,,what we need to know is what style of prop you have,,some styles generate bow lift and others don't some should run deep and some should cut the top of the water
also have a look at the bottom of your boat with a long strait edge,,,just run the strait edge right down the center of your boat at the very bottom from front to rear and look at the back half ,,what you are looking for is rocker ( or hook) turn those symbols on their side and you will get the idea,,,I really hope this is not the case

[suppa roosta]

Heres the link to the Merc Marine site Prop selection guide.
Merc Props (http://www.mercurymarine.com/prop_selector)
Looks like it would call for a 14 pitch wheel, the 13 will give you a better hole shot though.
But I'm more inclined to go with Tonga and check the hull for a hook.
Especially if the boats been riding on a roller rigged trailer.

[andy capp]

Iy pimp There is no trim or tilt. I have already tried the fixed position adjustments and that makes it worse either way. I am going up north till sunday, I am going to take it to be looked at next week. I will holler at you all later.