boat planes then bow dives

[dixieturk99]

Hi all, bought an old 18' aluminum with a 70hp evinrude. Can't open the boat up all the way as when it reaches a certain speed the boat excessively pulls to the right and the bow starts to dive resulting in everyone getting wet. Can anyone provide some clues here on what to adjust to fix it. Already adjusted the trim tab to the right all the way. I'm hoping the hull isn't causing it but it is pretty rough and leaks quite a bit (glad we have a bilge pump).

thanks

 

[rockyrude]

Re: boat planes then bow dives

If you have power trim, trim in until the bow stays up. If no power trim you will have to play with the trim pin on the motor to get the best position to keep the bow up on plane. The down side is it won't be the best position for getting out of the hole.

 

[_chris_]

Re: boat planes then bow dives

My boat does the same thing and it is the trim, if I trim it up about 1/4 of the way it will lift the bow and ride great.

 

[Bart Sr.]

Re: boat planes then bow dives

Have you checked the hull for dents and/or hooks???

 

[QC]

Re: boat planes then bow dives

If you have power trim, trim in until the bow stays up.

Backwards . . . Trim out (up) until the bow rises if you have trim.

By any chance does the lower unit have a hydrofoil on it? I agree with the other posters here on causes etc, but if this has a hydrofoil on it that could cause the same issues.

 

[rockyrude]

Re: boat planes then bow dives

Good thing QC is keeping an eye on us, I knew that, really late night.

 

[Splat]

Re: boat planes then bow dives

Just FYI its called porpusing.

Bill

 

[QC]

Re: boat planes then bow dives

No, not porpoising . . . Porpoising is the rhythmic rise and fall of the bow at moderate to high planing speeds.

What the OP describes is the bow being driven down to the point of fear and a free bath Porpoising usually happens due to weight balance being too far aft coupled with excessive positive trim. What the OP describes is probably due to too much weigh forward combined with too much negative trim, the exact opposite . . . This can also be caused by hydrofoils that are not setup properly or are misapplied.

 

[dixieturk99]

Re: boat planes then bow dives

nope, no hydrofoils on it...just a plain old aluminum open hull fishing boat. Tried trimming it up until the prop almost comes out of the water and still won't bring the bow up so I can increase speed. I don't know if the the hull is bent but it is fairly old y'know. I suppose the easiest way would be to snorkel underneath it to check that out.

 

[tashasdaddy]

Re: boat planes then bow dives

could it have waterlogged foam under the deck. there can be up to 5-600 lbs under there.

 

[dixieturk99]

Re: boat planes then bow dives

nope, nothing at all under the floor except some bracing.

 

[Splat]

Re: boat planes then bow dives

I apologize. I thought we were talking about a continous rise and fall from being on plane to off plane.

Bill

 

[rockyrude]

Re: boat planes then bow dives

Trimming the motor like you said with the prop almost out of the water is forcing the bow down into the water. Bringing the prop closer to the boat should raise the bow.

 

[QC]

Re: boat planes then bow dives

Trimming the motor like you said with the prop almost out of the water is forcing the bow down into the water. Bringing the prop closer to the boat should raise the bow.

You did it again rocky . . .

Trim in, bow goes down, Trim out, bow goes up . . .

With trim too far out, weird stuff will happen for sure, but the basic description you give is still backwards.

 

[RWilson2526]

Re: boat planes then bow dives

nope, no hydrofoils on it...just a plain old aluminum open hull fishing boat. .

Just to make sure your clear...what QC was asking about was a hydrofoil or "whale tail" mounted on the outboard itself just above the prop.

 

[QC]

Re: boat planes then bow dives

Good catch R . . . he could've meant literally hydrofoils.

Don't ignore Bart Sr.'s comments either. May very well be a hook or a dent/hook only on one side.

 

[Home Cookin']

Re: boat planes then bow dives

A 70 should push it no problem. Try starting with the trim down (motor in) and as you get up to the plane, raise the motor. Starting with it trimmed up/out might not get you on a plane either. Do this by yourself; your passenger weight can be an issue, too.

 

[dixieturk99]

Re: boat planes then bow dives

how big a dent or hook/dent am I looking for here??? This is a pretty old boat. Bound to be a few dings in it...

 

[QC]

Re: boat planes then bow dives

Put a straight edge on the bottom longitudinally. If there is a gap that spans a couple of feet, then start to be concerned and that still might not be the issue. Dings or small dents are not the concern, it is larger concave deviations that would affect performance in the way you describe.