Boat Plows....Why?

We just picked up an older boat for a lake that we fish often. Its an older aluminum beast, 14 ft, transom measures just over 16 inches and it has a beam of 64 inches. So I'd consider it a deep and wide. Probably rated for a 20 or 25 hp.
I've put my Yamaha 9.9 on it which has a shaft length of 18 inches.
No matter how far out I mount the motor from the transom the boat plows. I've put the gas can right next to the transom, keep as much weight rearward as possible yet the boat still wants to nose dive when I go about 1/2 throttle.
It wallows all over the place, I can't keep it in a straight line.

You guys any ideas?

My best guess would be not enough horse power to get the boat up on plane.

Perhaps insufficient transom angle? If the tilt adjustments on the motor itself cant resolve it, try experimenting with shims to position the motor as it is mounted on the transom?

The boat goes up on plane in a heartbeat but immediately goes beyond even at half throttle with my 9.9.
Yesterday I shoved a 3/4 inch shim behind the mounting bracket (on the outside of the transom) to increase the angle and it helped somewhat but the problem still exists to a point where the boat is completely unmanageable.

Would a longer shaft motor make a difference??

Have you tried going in the opposite direction with your adjustments? Bring the motor closer to the transom and adjust the loading.....Some boats are just weird.

What is the pitch on the prop ?
It should be stamped on it.


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Sound like it's trimmed down too much, trimming the prop closer to the transom will only make the experience worse. A longer shaft will not make a difference, you would only get better water pressure and better hole shot. I would keep trimming the motor up.

Trimming it up hasn't worked.... Might be that you are "past the point" and it is time to try the other direction. As you increase the gas, it tries to raise the bow, but it might be set beyond the point where the motor can do it.
Load the boat as normal ( level), then find the " horizontal line" on the motor cowling and adjust the trim until that line is parallel with the water line.

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Originally Posted by sawbill

Would a longer shaft motor make a difference??

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It could.
From the info I got so far on this thread a longer shaft may help.
If your motor shaft is too short when the boat pops on plane it may now be getting starved for water to push causing the nose to drop back down.
When the boat is on a trailer with the motor set at medium trim if you look along the plane of the bottom of the boat is the prop fully lower than the bottom of the boat. If it is not than you need a longer shaft motor.

The cavitation plate should be level with the bottom of the boat.
It is possible that the motor is not reving to it's potential, [to high a pitch prop] check the pitch of your prop, if too high your motor then does not rev. up to it's highest capability.
Look around perhaps beg or borrow a prop with the next lowest pitch , this may solve the problem, it has for others.
Another problem that occurs, is when the boat is under way, and if the floor/keel is weak or broken/cracked, this allows the boat to flex at the floor just ahead of the transom and causing it to plow.

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Originally Posted by jaycee

The cavitation plate should be level with the bottom of the boat.
It is possible that the motor is not reving to it's potential, [to high a pitch prop] check the pitch of your prop, if too high your motor then does not rev. up to it's highest capability.
Look around perhaps beg or borrow a prop with the next lowest pitch , this may solve the problem, it has for others.
Another problem that occurs, is when the boat is under way, and if the floor/keel is weak or broken/cracked, this allows the boat to flex at the floor just ahead of the transom and causing it to plow.

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sounds like the problem right there, the cavitation plate should be even or slightly below the hull, the motor will always try to run perpendicular, tilt it in to far, it will push the bow down, tilt it out the far, the boat will start to porpoise, if your prop is not letting your motor run at the max rpm the boat will drop off and plow,

Not correct about the cavitation plate. The cavitation plate while at WOT should be just out of the water. Where it lines up with the boat while not on plane does nothing. The comments about moving the trim back towards the transom are incorrect. If you are trimmed up too high your boat will start to porpoise "bounce up and down". You have not experienced this yet so either you lack enough horsepower lift the bow or you are not trimmed up high enough. I'm thinking your not trimmed high enough. Or the boat is just a tank and just runs like that


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Check out this link; explanations with pictures;http://media.channelblade.com/boat_g...tipstricks.pdf

When you say that the boat plows, you mean that the bow rides low and the stern high so the boat always seems to be going downhill, right?
Is there any chance that the hull has been deformed somehow?

When I say plow, I mean it wants to nosedive. The 9.9 Yam 2 stroke has plenty of power to get it up on plane so that's not a problem. And the boat is in good shape with no leaks, bangs or dents. That's why we want to get it up and running properly but mainly because the deep and wide is, or should be safer in the big rollers that that lake gets.

I've tried just about everything...shifting as much weight to the stern as possible, shimming the lower outside motor mounting bracket to increase the trim even more.
The boat is at a camp right now so checking things--cavitation plate orientation to hull, depth it sits above or below the transom, etc isn't possible.
But you guys have given me some ideas of what to try and what to look for.
Thanks to all who've responded.

Quote:

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Originally Posted by sawbill

The boat goes up on plane in a heartbeat but immediately goes beyond even at half throttle with my 9.9.
Yesterday I shoved a 3/4 inch shim behind the mounting bracket (on the outside of the transom) to increase the angle and it helped somewhat but the problem still exists to a point where the boat is completely unmanageable.

Would a longer shaft motor make a difference??

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Originally I thought it may just need to be brought in closer to the transom, but reading your measurements, it might in fact be that there's cavitation. That size boat is going to displace a fair amount of water when it's load is at the very rear and moving at some speed.

A longer shaft motor may help, but I'd try dropping the trim right tight to the transom 1st.

Quote:

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Originally Posted by sawbill

When I say plow, I mean it wants to nosedive. The 9.9 Yam 2 stroke has plenty of power to get it up on plane so that's not a problem. And the boat is in good shape with no leaks, bangs or dents. That's why we want to get it up and running properly but mainly because the deep and wide is, or should be safer in the big rollers that that lake gets.

I've tried just about everything...shifting as much weight to the stern as possible, shimming the lower outside motor mounting bracket to increase the trim even more.
The boat is at a camp right now so checking things--cavitation plate orientation to hull, depth it sits above or below the transom, etc isn't possible.
But you guys have given me some ideas of what to try and what to look for.
Thanks to all who've responded.

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All I got to offer is ,man that's weird. The problem is usually just the opposite of what you got. Problem is if you go to extreme in tilting that motor up bad things can happen in corners. Is the 9.9 long or short?

The shaft measures 18 inches. The transom is just a tad over 17 inches.
And you're right about mean things happening. At half speed it's a constant struggle just to keep the boat in a straight line because it wallows all over the place while trying to maintain some sort of trim.
I suppose all I can do is shim it more and see what happens then.

Is your cavitation plate below the hull bottom? If not then you will lose thrust when the boat starts to plane.

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Originally Posted by Bluebulldog

Originally I thought it may just need to be brought in closer to the transom, but reading your measurements, it might in fact be that there's cavitation. That size boat is going to displace a fair amount of water when it's load is at the very rear and moving at some speed.

A longer shaft motor may help, but I'd try dropping the trim right tight to the transom 1st.

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Wouldn't that make things worse? When you want to get on plane you trim the motor down, as you get on plane you trip the motor up to lift the bow in a traditional boat with power trim. To lift the bow in my Ranger I trim the motor up the lift the bow, I would not trim the motor down???

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Originally Posted by Bandwagon

Wouldn't that make things worse? When you want to get on plane you trim the motor down, as you get on plane you trip the motor up to lift the bow in a traditional boat with power trim. To lift the bow in my Ranger I trim the motor up the lift the bow, I would not trim the motor down???

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What happens when you trim it too far up and the prop gets beyond its operating zone?

The hull of the boat could be hooked. I've seen it before.

Have you tried trimming it all the way down? Just to see?

weird situation for sure.

S.

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Originally Posted by patvetzal

What happens when you trim it too far up and the prop gets beyond its operating zone?

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The boat porpoises if the power is not there and then you would have a situation of low water pressure with your outboard.

Quote:

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Originally Posted by sawbill

We just picked up an older boat for a lake that we fish often. Its an older aluminum beast, 14 ft, transom measures just over 16 inches and it has a beam of 64 inches. So I'd consider it a deep and wide. Probably rated for a 20 or 25 hp.
I've put my Yamaha 9.9 on it which has a shaft length of 18 inches.
No matter how far out I mount the motor from the transom the boat plows. I've put the gas can right next to the transom, keep as much weight rearward as possible yet the boat still wants to nose dive when I go about 1/2 throttle.
It wallows all over the place, I can't keep it in a straight line.

You guys any ideas?

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Anyway you can take a picture of your setup?

Quote:

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Originally Posted by patvetzal

What happens when you trim it too far up and the prop gets beyond its operating zone?

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If it even starts the beginnings of porposing watch corners

Applying engineering principles to Mr. Sawbill's problem - the conclusion is that the force exerted by the motor when not at full throttle is not enough to overcome the force exerted by the drag on the forward part of the boat - this indicates that aside from the trim issue either he needs a different propeller or the 9.9 isn't powerful enough for the size and shape of the boat - prof. Joe

When the boat is resting flat on the water the only forces present are the force exerted downward on the center of gravity of the boat and the upward buoyant force - we have a static situation - then when the motor is started it exerts a horizontal force (assuming that the motor is parallel to the water) - as the motor speed is increased forces exerted by the drag on the front on the boat develop in the opposite direction to the force exerted by the motor - because of the shape of the hull the drag forces increase as the boat speed is increased - these forces gradually move further towards the back of the boat - they finally reach the flat part of the boat and the boat is on plane - the force exerted by the weight of the boat now have a component that is in the same direction as the drag forces - if you decrease the speed of the boat the drag forces move up towards the front of the boat and bring the front down and if the motor isn't producing enough force to overcome these drag forces it plows into the water -

we had the same problem with a harbourcraft 14'

9.9 wouldnt do it, neither would a 15hp... boat was bent (looked like it was overloaded on the trailer at one time)



another possibility is soft transom that is flexing, therefore tilt is being cancelled out?

I had boats that did this///My mechanix who also does motors on big race boats..like 200 + told me to shim it up...Like undo the bolts and lift it up and place like a paint stick on top of the transom and then sent it back down..tighten up bolts and try it...or buy a fin that bolts on top of the cavitation plate..I did that and it solved all of my problems...Iv had to do this for 4 boats...works every time....Im not here to argue with anybody...but this worked for me

I am sure you have checked for water in the bow floatation foam under the seat.it can add a ton of weight to the nose.

That parts OK, dh.
Originally I did a quick check along the keel to make sure the boat wasn't bent just forward of the transom and as I recall it seemed fine. I've shimmed the motor mount out about 3/4 inches and that's helped considerably. I can now run the motor about 3/4 throttle and the boat is just on the verge of being controllable. So hopefully a bit more tweaking may stop the wallowing it does now and make it a decent boat to fish from.