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Old 16 October 2011, 17:12   #1
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Steering Issue

I have a 2.9m standard SIB with an inflatable keel, 5HP yamaha outboard. When on the plane the bow of the boat has a rapid left to right "S shaped" movement. I think the correct term is yaw. This is not chine walking as I understand it. The nose/bow seems to have a rapid left/right, left /right movement. What is the cause? Any remedies? I was travelling with the current in very little wind.
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Old 16 October 2011, 17:23   #2
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I doubt you would ever get true chine walking on a sib. The hull just isn't hard enough and their isn't a deep enough vee to make it happen. However, I suspect you are suffering something akin to it which could possibly be reduced by trimming the engine in a little to lower the bow.
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Old 16 October 2011, 19:42   #3
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Is it bad enough that it causes a problem (i.e. you have to fight the changes, or it might get bad enough to be unsafe)? You can get a lot of feedback through the hull in less-than-glass-smooth water, and a little meaningless twitch may still be felt.

That said, changing weight around will likely help, as will slowing down in that particular condition.

Just thought of another possibility: are you talking about hull steer as you run faster than a following sea?

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Old 17 October 2011, 00:30   #4
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Thanks for the replies Erin and Jyasaki. I think I probably had some bow/hull steering and the answer may be to to trim the engine out? My motor was fully trimmed in so I guess too much bow down increasing the drag and causing the instability. It felt pretty unstable on the plane ( the twitching of the bow was too rapid too try and make corrections) and seemed to be plowing at slow speeds before this. If the motor was not properly centered could this also add to the problem? Jyasaki I was in a river mouth pretty flat with small chop traveling with a following current from a pushing tide.
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Old 17 October 2011, 01:44   #5
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The twitching was probably due to chop hitting the hull at slightly different angles (from varying waves.) I would guess that it's not really anything to worry about; just something that takes some getting used to. My SIB used to feel like it was a small levitating disk being bounced side to side and fore-aft by the small chop. Actually, in certain conditions my larger RIB gets a similar ride.

Trimming up may help; it will bet the bow up a bit which will put the force more towards the stern, which should be a little more stable side-to-side.

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Old 17 October 2011, 01:55   #6
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I am pretty green to this - it felt very nervous and twitchy, I guess with more experience I may smile at this post some time in the future . I will fiddle with the trim and see what happens. What effect will an poorly centred motor cause?
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Old 18 October 2011, 16:29   #7
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Often motors are mounted slightly off-center to counter the rotational torque of the prop, which tends to turn the motor, and hence the boat, at high power settings.

I suspect that a motor mounted off-center will simply cause the boat to steer to one side. Never tested it though (all my boat motors have been bolted on.)

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Old 18 October 2011, 16:40   #8
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Can I ask where are you sitting in this SIB and what else have you O/B (and where is it stored? i.e. fore/aft)
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Old 19 October 2011, 07:05   #9
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If a motor is tilted too close to the trasom, you can experience bow steer which may be the twitchy you noticed. That can be dangerous and result in the boat tending to spin around uncontrollaby. The cavitation plate should be parallel to the surface of the water at speed or tilted slightly out so that the boat rides slightly bow high.

As far as centering the motor on the transom, it should be centered. The only reason for it to be intentionally offset would be to make up for a defective hull. If you see an outfit with the motor not centered, that should be a red flag that there is something wrong with the hull.
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Old 20 October 2011, 11:58   #10
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Willk I had nothing in the boat except my "massive" 65 kg frame . I was sitting as far forward as I could within the limits of remaining "attached" to the tiller about 1/2 way in a 3m SIB on the port side - this was the best way to get her up on the plane. The only luggage on board was my safety gear and a 3 kg anchor stored aft in front of the motor.
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