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Old 14 November 2016, 11:10   #1
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High speed 'wobble'

Hi all, I've just got a Narwhal 4.8M rib with a Mercury 75hp 2 stroke - all in great shape and well maintained. At above 20 knotts with the engine trimmed slightly up she can start to 'weave' slightly from side to side feeling like she is tipping slightly from one side to the other. trimming the engine down reduces the effect but reduces speed and starts to drag the tubes towards the bow. I've had to move the engine directional trim quite a bit to stop her pulling to the right which is now much better at higher speeds (at low speed she still wants to pull right). Extra weight and 'towing' seems to help stability. I'm concerned that the feeling of instability could lead to her hooking up. Other than always towing a ringo with the 18 stone mother in law behind - any ideas welcomed!!!! Coops
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Old 14 November 2016, 11:17   #2
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Its called 'Chine - walk ' have a google / search and read all about it
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Old 14 November 2016, 11:44   #3
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Hi Lee, thanks for the lead and I can see its a big subject! Will immerse myself 'metaphorically speaking' only hopefully....Cheers
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Old 14 November 2016, 13:50   #4
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What bolt hole is your engine bolted through?

It may benefit you to raise the engine by one hole position...that way, you can reduce the drag of the lower unit without causing excessive bow lift.
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Old 14 November 2016, 13:56   #5
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surely it should do more than 20 knots before it starts getting lairy . I would have expected a 4,8 rib with a 75hp should be cruising comfortably at 20 knots
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Old 14 November 2016, 14:28   #6
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Hi there, it starts to counterbalance (Chinewalk) at slow planing speeds when the water is very flat - I'' guessing at just over half top speed and we gps's her at 38 knotts. Its really gentle and controllable at low speed and can be dialled put with engine trimmed in. The tubes are definately under inflated based on what I've read also - could this contribute???cheers
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Old 14 November 2016, 14:33   #7
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Hi - I will check at the weekend - from memory the motor is bolted flush to the transom top - is that what you are asking or are there mounts on the outboard mount (sorry - I'm a newbie!!) Thanks
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Old 14 November 2016, 14:38   #8
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Image of the vessel.....
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Old 14 November 2016, 14:52   #9
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Hopefully now attached

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Old 14 November 2016, 15:00   #10
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check your engine mount. grad hold of the outboard and see if you can turn it and by how much without the steering turning, just the play in the mounts,, the rib should NOT chinewalk at 20 ffs
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Old 14 November 2016, 15:02   #11
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Could be steering play or worn engine mounts.
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Old 14 November 2016, 15:08   #12
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Quote:
Originally Posted by matt h View Post
check your engine mount. grad hold of the outboard and see if you can turn it and by how much without the steering turning, just the play in the mounts,, the rib should NOT chinewalk at 20 ffs

Snap
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Old 14 November 2016, 15:15   #13
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Hi Matt, thanks for the advice, will check. could low pressure in the tubes be an issue?? Thx
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Old 14 November 2016, 15:31   #14
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slightly. the tubes will go down a bit in the cold so ideally should be 2-3 psi or firm to touch.
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Old 14 November 2016, 15:49   #15
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Thanks guys for the help - got a good list to check through now, transom height, play in mounts and tube pressure. Once I get back out will post my findings. Great community - cheers
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Old 14 November 2016, 18:18   #16
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Damaged prop ?
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Old 15 November 2016, 04:05   #17
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Hi Last Tango, that's a good question - the prop on appearance is a little chewed but I'm not getting any significant vibes through the rev range. Keen to understand how the prop could create this kind of instability? Thanks for your help.
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Old 15 November 2016, 06:31   #18
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Can't see on the pic, but you don't have hydrofoils fitted to the engine do you?
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Old 15 November 2016, 06:37   #19
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Hi there, no we haven't got hydrofoils on the motor......Thanks
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Old 15 November 2016, 07:17   #20
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has the boat got a medium v hull with a planing pad at the stern [flat at the stern not a v all the way through] if its gota a planing pad and the boats badly set up all as mentioned above it will be very unstable. your prop needs sorting if its chewed blades they could be bent all adding to your problems and will cause more.tubes need pumping up to pressure too
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