Boat Plows....Why?

[patvetzal]

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???

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

[Sinker]

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.

[Bandwagon]

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

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

[Bandwagon]

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?

Anyway you can take a picture of your setup?

[standup]

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

If it even starts the beginnings of porposing watch corners

[JoePa]

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 -

[wheelin_ca]

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?

[baby123]

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

[dutchhunter]

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.

[sawbill]

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.