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Old 07 October 2013, 18:12   #71
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What were the dealers comments other than using a larger engine? Did he have the motor tilted in as shown in the sketch above? If tilted in, that would cause bow steer.

You need less HP for a light load, so the comments to use a larger motor seem out of line.

You would not see the transom twisting except when power was applied. If the hull caused bow steer whether a new boat or not, it could still have the same problem. The video does indicate a bow down attitude.
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Old 08 October 2013, 00:44   #72
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The driver gets the boat up on plane very easily and all is well. Then, right at 0:20, when he accelerates from that slow plane, you can see the bow get pushed down! That can't be right...
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Old 08 October 2013, 00:50   #73
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SIBRIDER:

I have no tach, but it feels like last 25-30% of throttle provides no more speed, if that's worth anything. How could it: the bow is being pushed down and there is so much drag in the water?!

The prop says 10.3x12 on it.
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Old 08 October 2013, 01:32   #74
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SIBRIDER:

I have no tach, but it feels like last 25-30% of throttle provides no more speed, if that's worth anything. How could it: the bow is being pushed down and there is so much drag in the water?!

The prop says 10.3x12 on it.
There is so much drag because bow is pushed down. Was engine trimmed-in heavily?
When you open last 30% of throttle do RPM increase without gain in speed? Up to ventilating possibly?
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Old 08 October 2013, 08:03   #75
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The motor should have the propeller shaft parallel with the surface of the water, but if it is tilted facing up as in the sketch, it will force the bow down.

Imagine a boat fitted with trim tabs which are tilted down, ie, the back of the tabs are lower than the front of the tabs. That would force the bow down. If there are no tabs but the bottom of the hull is hooked so that the back of the boat is lower than the area of the bottom directly in front of the transom, you would effectively have the same problem.

As the line of force of the propeller is below the bottom of the boat, it will tend to force the bow up with a good bottom on the boat. That combined with the boat being forced forward against the surface of the water should allow the boat to be driven forward in a normal planeing attitude.

If the propeller cavitates, the boat would still move forward on a normal plane. It would not attain sufficient speed, but that would not interfere with the boat running at a normal attitude.

Either the motor is improperly tilted or the bottom has a hook in it. If the dealer was really experienced, he would have spotted this when he ran the boat. He would have trimmed the motor out to prevent bowsteer. You can feel the stern of the boat rising and the bow steer when at speed.

Your video and explanation of what you saw indicates that the bow is being forced down on that hull. That is dangerous and could cause an accident.

Putting a larger motor on that hull will just cause worse problems than what you have. He is banking on putting a heavier motor on the boat forcing the transom down, but should recognize that there is a hull problem instead. The hydraulic forces will only increase with speed and offset the weight on the rear of the boat.

Is the motor fitted with hydraulic trim control? Tilting the motor out would raise the bow, but a problem with the hull would still be evident.

It appears to me that it was run with improper motor trim or there is a problem with the hull.

I have a slight hook in the bottom of my fishing boat. With 15 HP, it runs just fine. When I had a heavier 25 HP on it, it would bow steer slightly at high speed. My not so little weight plus the heavier motor did not cure the effects of the slight hook. The extra 4 MPH and weight was offset by hydraulic forces on the hull. No bowsteer with the 15; bow steer with the 25.
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Old 08 October 2013, 13:18   #76
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Thanks FrankC, but its a brand new boat and I've examined the bottom of the boat and its pristine, so I think that's unlikely.
Assuming its pumped to rated pressures...

The hull is defective. The floor fabric is too loose and when under way its creating a cup which is causing your bow steering. On the trailer/ out of the water you won't be able to detect this.

Slightly longer floor boards would fix the issue, you might be able to make a spacer to keep the floor fabric tighter.

A heavier engine won't fix the problem at all.
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Old 08 October 2013, 14:21   #77
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Assuming its pumped to rated pressures...

The hull is defective. The floor fabric is too loose and when under way its creating a cup which is causing your bow steering. On the trailer/ out of the water you won't be able to detect this.

Slightly longer floor boards would fix the issue, you might be able to make a spacer to keep the floor fabric tighter.

A heavier engine won't fix the problem at all.
From what I gather bow riding only happens when engine is trimmed-in heavily. Which is expected of course. Sound like only ventilation/cavitation is a problem at neutral trim. If I'm not mistaken.
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Old 08 October 2013, 15:01   #78
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From what I gather bow riding only happens when engine is trimmed-in heavily. Which is expected of course. Sound like only ventilation/cavitation is a problem at neutral trim. If I'm not mistaken.
Bow riding occurs on trimmed in motors OR when there's a cup in the hull. Since SIBs have fabric hulls, they depend on the exact length of the floor boards and tube inflation to stretch the hull fabric tight enough so that it doesn't have cup. Underinflation, short floor boards, or stretched hull fabric can all lead to cup. And a cupped hull experiences bow riding no matter how far out the engine is trimmed.

Ventilation can occur at any trim setting, not usually when trimmed in however.

From the description of the problem and the video, I strongly suspect the hull is cupped. Since its new I can only conclude its either improperly inflated, the floor boards are not installed right, or the hull fabric is too baggy. Any of which can create a cupped hull which will never work right.
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Old 08 October 2013, 15:13   #79
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We speak the same thing. From what I see. Look at the trim pin position.

Engine is at furwest trim-in position. So boat beheaves as expected. Heavy bow riding. I don't understand why this "rep" didn't pay attention to such obvious issue.
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Old 08 October 2013, 15:37   #80
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Oh jeesh. I thought the engine was trimmed out to the point of ventilation and it was still bow riding?

After 8 pages I might have misunderstood...
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