Ways to minimise bow down moment on existing boat

I’ve seen fiberglass boats that develop a hook in the stern from improper trailer supports.
Place a straightedge along the bottom to check that.
Simply selling the boat (and the problem) is pretty rude, take some responsibility for at least warning potential buyers!

 

Trim with empty boat is a tiny bit by the stern.

Speaking of boat speed, you're right, it should by all accounts do 30kn with 60ho, easy, considering is pretty much a flat bottom, but it doesn't. It tops out at 25, that's wlso what the manufacturer states.
The reason for this must be the extreme bow down effect, any additional hp just pushes the bow down further, negative any speed increase.

I can't find the drawings you mention, but I think you're referring to the inverse chines?
And yes, that's what creates the bow down moment. Perfect at cruising speeds, but anything faster, and the bow down moment gets too much

I'm selling the boat because I've upgraded, not because of this issue, and yes, will inform potential buyers of this issue, but I thought I'd try to address the issue before selling by asking here.

Also, this is how the boat behaves at default, it has been certified, thousands have been sold.

I just suspect most have been powered with 40hp max and seem to be popular a a cheap "sloep", for pottering around. It's not a proper speedboat, but can be pretty zippy. But again, 40HP would normally be a good match for this boat. It only becomes an issue due to the aerating at any positive engine trim, and if you look at the pictures on the Web, they all have negative trim, or neutral at max.

Anyway, regarding the hook, you may be right and it would explain the excessive bow down moment. Hard to check though.

Would that be possible to address by filling in the hook in some way?
Also, would this by by design? (boat isn't tralered)
Would it explain the aerating due to diverting the flow of water away from the propeller at speed?

 

Hook can “develop” in a weakly constructed flat bottom boat.
As power is applied to the outboard, the increasing pressure under the hull pushes up on the the bottom, causing the hook or concave to occur. It may not be evident when the boat is at rest.

 

While that maybe so, this behaviour has manifested itself from the beginning, and I bought the boat new (2021)
Also, in all pictures at soeed, the engine have a negative trim. So I'm thinking the mold might be to blame, not discounting deformation though, as the bottom is pretty thin.

Untill I get home and able to measure the bottom, I won't be sure if a hook is the (only) reason for the bow down effect.
I still have a feeling the inverse chimes might be (partially) to blame.

The question remains, how do I fix it as the boat could be much faster at the right angle. Not to mention safer and economical.

 

I see quite a few images of the boat at speed, none exhibiting a bow down attitude, but no stern views to reveal bottom configuration.
Your boat may well have developed a unique warped bottom for unknown reasons, from improper storage support to shoddy repairs, or other unknowns.
The propellor could still be at fault, many are designed to help lift the stern to get the boat on plane, and would be unnecessary, even detrimental on your boat.

 

Like I said, bow down only becomes a real issue at higher speeds, doing 20-21 isn't an issue and wouldn't show in any picture anyway, and most of these boats came with smaller engines and might not be able to attain the necessary speed for the bow down moment.
Coming onto a plane, the bow rises significantly as one would expect, then gradually comes down with more power applied, and just keeps doing so)

Also, again, boat the boat newly built and it had this behaviour from the start. It could have developed a warp during storage and transport, who knows, but that would be awfully quick. (boat built in 2021, purchased in April 2021)

It came over propped, replaced the prop with a original Yamaha one with a slightly smaller diameter, and at the same time had the engine raised slightly by manufacturers advice, which initially made it aerate more, but that somehow sorted itself.

Still strange how any positive engine trim makes it aerate so much since engine trim alone could negate the issue.

Anyway, it's all theory crafting until I can get to the boat and see if, or how badly the hull is "deformed".
I can pull the boat up to the beach behind my house no problem, and measure, and the area where the hook must be is under the bench and easily accessible, and no hook is appearant, but would be hard to spot anyway, looking top down.

Just want to stress an oddity though. Acc to manufacturer, top speed should be 25 knots with a 60hp,and that is spot on.
Using grouch's formula, the top speed should be over 30 knots, the bottom is pretty much flat at the stern and lightweight, yet tops out at 25 probably due to the entire hull being in the water slowing it down, which might indicate the bow down is intentional, or just how the hull behaves, and is known by the manufacturer.

Just thought of something just now.

As I mentioned, the boat is lightly built and the bottom isn't very thick either.
Would it be possible the deformation happens while at speed with the pressure on the bottom increasing?
Iirc, there are no strengthening on the flat bottom in the aft, only where the transom meets the planing surface, and no (linker) chines.

Would stacking canisters of water on the flat part exert enough counterpressure to prevent any warping and thus a viable way of testing this theory?

 

You should move he engine down to be able to trim it down without ventilating the propeller. Also, it will lift the bow by changing the ange of thrust.

 

Quietly, my hunch is the boat lays over one side due to the thrust of the prop creating stern lift.

Call a prop shop and give them a speed/rpm chart and boat specs and tell them what is wrong and they’ll be able to offer you a better prop with some more rake that’ll keep the stern from lifting so much. You really need a tach or cheater tach to resolve the issue.

I’d like to hear any comment back from @baeckmo because his expertise in this area is exceptional. He might also tell me I’m wrong or shed light on bow lifting propellers of which I only have a basic, limited knowledge. Your prop is probably really not right, in my opinion. You ought to be much more detailed in describing the operating characteristics. Is the hole shot super fast, for example? Which side lays over, etc.

 

Just thought of something just now.
As I mentioned, the boat is lightly built and the bottom isn't very thick either.
Would it be possible the deformation happens while at speed with the pressure on the bottom increasing?
Iirc, there are no strengthening on the flat bottom in the aft, only where the transom meets the planing surface, and no (linker) chines.

 

Doesn’t pass my gutcheck. For a vee hull that flattens at the stern, in order for deformation, no longitudinals? I think he is propped wrong, especially considering the other boats pictured look fine.

 

The issue was the same, with engine higher or lower, no difference in bow down moment was observed.
Also, see my comments about ventilating when trimming anywhere near positive.

No prop shop, or anyone with relevant expertise here.

As for operating characteristics, what would you need to know?

Yes, holeshot is super fast, it's on a plane in a second. Bow rises rather aggressively coming onto a plane, but that's not unexpected, and levels out at cruise.
Position is as it should be at cruising speed, 17 to 19 knots.
Any faster and bow trims down aggressively, unstable @21 knots single handed, empty boat.
Bow down moment is so strong, that I doubt that a little bit less stern lift from a propeller would make a lot of difference.

Also, what do you mean by "which side lays over".

Other boats generally have 40 installed. 40hp would probably push it to not much more than 20 knots.

I think the deforning at speed might be a thing. The construction is such that it makes sense, flat bottom, lightly built and no stiffeners.
It's easy enough to check I think. I could glue a stiff straight edge on the inside of the boat hull with non flexing glue, if it comes off or loosens at speed, it means it de forms. Could even check it while underway while someone else is driving.

Currently abroad though, will be able to check it in 6 weeks.