Ways to minimise bow down moment on existing boat

I have a small outboard powered open boat.
It does drives OK, until I hit about 21 knots.
From cruise speed up, it keeps pushing the bow down more and more. At 21-23 knots it becomes unstable, that speed is lower with people in the front. At full blast (alone the instability is scary)

On one occasion, I did 21 knots with 2 persons in the bow. Flat calm sea. A tiny ripple was seen from a small, very distant boat and I was slightly too late reducing power.
The result was a full hard 90 degree turn. And 1 person in the front was launched violently. No injuries. Anyway, that's how bad it is.

Supposedly, this is the effect of quite aggressive inverse chines.

Engine is normally trimmed negative to neutral, and further trim positive results in excessive aeration and blowout.

AFAIK, trim tabs don't work to get the bow up when it's already down.
Shifting weights isn't an option, apart from a small anchor, there's nothing I could shift aft.
Hull if FRP, so modification of the hull is also out if the question.

Any ideas?

 

Are you able to place some heavy steel/angle bars of 25kg each at or near the transom below the sole, or on the hull bottom?

 

I would be.
But the boat in question is pretty beamy and flat bottomed at the stern. A lot of weight would be required.

2 full fuel tanks all the way in the aft vs 1 nearly empty does exactly nothing to keep the bow down at speed. Even A person with me at the stern only helps a little but only slightly raises the speed at which the bow down becomes an issue.

 

But that is an assumption.
We just don't know...so try experimenting by adding 25 kg bars at a time...
If it ends up requiring way way too much additional ballast weight aft, to correct or at the very least be acceptable behaviour...then it would suggest either you sell the boat, or do major hull modification.

 

You have a bow steer issue. Define "small"...

 

Well, that's why I mentioned placing a person (90 lg) with me in the stern. A person seated on the aft bench is pretty much as far aft as one can go. And with 350kg of boat weight (including engine) placing a lot of weight on board isn't very practical.

And yes, the boat is up for sale. It's one of the things that never bothered me much, I'm happy to cruise and relax. A potential buyer might be put off by it though.

So, nothing to do about it then. Shame.

Regardless, thanks for your insights.

 

Small: 5m open boat weighing 350kg including a 60hp engine.

 

5m @ 21 knts, you are way outside most normal hull forms. You need all the weight (people) as far aft as possible.

 

Yeah, but it also happens with 2 people in the stern. Just at slightly higher speeds.
The problem is that even getting close to full speed is outright dangerous.

Like I said, I'm fine with it, I know boats, someone else who doesn't feel the instability might get hurt.

The engine is certified for 60hp max by the way so not overpowered.

 

.....aaand you have adjusted the engine trim angle correctly? ......and the propeller/antiventilation plate at the required level?

Asking because you should be able to trim out the rig without ventilation or blowout. If that's not possible, something is out of place. You might also try with a prop with increased rake; that will change the thrust line to the better.

Could you post an image or two, showing the hull and chine shape?

 

Yes.

Had the propeller replaced for the right size, and moved the engine up a notch as per manufacturer advice.

As for engine trim, there's really not much I can do in that regard, see my first post.

 

See my post after editing.

 

Sorry, I cannot post pictures, as I'm abroad, for 2 more months. (at sea) Don't have any on hand either.

The boat in question is a poseidon fish 510. There are a few pictures floating around on the Web, but not a lot.

 

Ok, have checked images. Very similar used in "my" archipelago for zipping around, should be reasonably "stable" up to about 28-30 knots with a 60 hp. Which points to something else: is it possible that there is a leak somewhere, causing water to enter below inner liner? How's the trim when she is lying empty?

 

No experience of planing boats here - but if you search google images you'll find a two view drawing of this boat, if you click on the camera symbol bottom right of image you get a better image. The boat appears to have a hollow in the deadrise panels in the side view, could this mean the rear half of the boat is acting as a giant trim tab pushing the bow down?