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Old 03 March 2009, 11:24   #1
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engine hight on searider

I've a question for the hardcore seariderboys.
Others may give their opinion as well

I've a searider 5.4 with a honda 90 hp. (dryweight 170kg/374pounds)
Main 'problem' is that its often starts to chinewalk around 37 mph/60 kmh.
Sometimes it's so bad I've to decrease speed.

When I trim in chinewalking stops but speeds decrease also.
I always put the petrol (sometimes 40gal/150 liters) before the console in the nose. Makes no difference. Same with almost no petrol left.
The topspeed is also a bit low I think (41 mph / 67 kmh when current is favorable)
The gearcase and prop have been changed for another problem, but that also didn't do nothing to the chinewalk.
The hull has only some minor scrathes so that can't be the reason.

I was thinking that the problem could be solved by mounting the engine a bit higher, but someone told me that mounting the engine higher will lift the boat further out off the water en thereby increase the chinewalk.

As I've no tools to lift the engine myself it's more a kind of theoretical question.

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Old 03 March 2009, 12:15   #2
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My guess...

My guess is that because your at the top end of the transom hp rating and if the boat is running light then youll allways have a problem with chine walk at high speed unless you trim in or add weight to the boat.
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Old 03 March 2009, 13:14   #3
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Try the fuel a bit further back-you may be able to trim in a bit more without loss of speed. Mine sits from the transom to the back of the console and no chinewalking probs.

From the 1st pic it looks like your engine might be slightly too low.
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Old 03 March 2009, 13:18   #4
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Quote:
Originally Posted by K&S View Post
My guess is that because your at the top end of the transom hp rating and if the boat is running light then youll allways have a problem with chine walk at high speed unless you trim in or add weight to the boat.
I'm somethimes thinking it's because the engine is heavy rather than HP-max.
I've also a SR4,7 with Yam 70 hp (also max rathed) and that one never chinewalks.
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Old 03 March 2009, 13:22   #5
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Originally Posted by Nos4r2 View Post
Try the fuel a bit further back-you may be able to trim in a bit more without loss of speed. Mine sits from the transom to the back of the console and no chinewalking probs
I intend to change the jockeyseat for one with a fueltank inside.
But I already tried 2 x 25 liter-tanks directly behind the jockey and it wasn't better.
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Old 03 March 2009, 13:58   #6
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Your engine does look low, in the photo the anti ventilation plate looks to be below the keel. mine is above the keel by a small amount. I would raise it level with the keel and try it there. may not work to stop the chinewalk but it will do the MPGs and top speed some good
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Old 04 March 2009, 07:26   #7
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rudyc - engine height can have a real influence on effects of chine walking. ..changing, if possible can improve performance as well as changing the severity of chine walking. Also weight re-distribution can improve the onset of chine walking. Check out the article on Chine Walking here.
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Old 04 March 2009, 08:32   #8
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Engine Height on a Searider

Raising the height of the motor will raise the polar moment of inertia (mass at a radii from the point of rotation) or distance the motor is from the fixed point being the planing pad of the keel.MAKING CHINE WALKING WORSE.Too much anti cavitation plate in the water will increase drag and reduce efficiency but will increase stability.Fit Nauticus gas ram trim tabs problem solved,improved hole shot and top speed/efficiency.
If you want to raise the motor for your own interest,fit a packer below motor skeg on a stable surface with engine trimmed to the vertical with trailer wheels chocked.Disconnect the battery.Undo all engine clamping bolts.Get a friend using the jokey wheel to lift the bow of the boat,shim the motor stern clamp as the transom goes gently down,rebolt and test.
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Old 07 March 2009, 19:50   #9
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Quote:
Originally Posted by AVONMON View Post
Raising the height of the motor will raise the polar moment of inertia (mass at a radii from the point of rotation) or distance the motor is from the fixed point being the planing pad of the keel.MAKING CHINE WALKING WORSE.Too much anti cavitation plate in the water will increase drag and reduce efficiency but will increase stability.Fit Nauticus gas ram trim tabs problem solved,improved hole shot and top speed/efficiency.
If you want to raise the motor for your own interest,fit a packer below motor skeg on a stable surface with engine trimmed to the vertical with trailer wheels chocked.Disconnect the battery.Undo all engine clamping bolts.Get a friend using the jokey wheel to lift the bow of the boat,shim the motor stern clamp as the transom goes gently down,rebolt and test.
That's certainly not true, if you put the motor a few inches higher, less shaft is causing drag in the water so the chinewalk get's less worse.

I saw it myself on many boats and all times the chinewalk became less worse when putting the engine higher. An inch can cause a total different driving experience.

In the case of the topic starter, i would suggest raising it by an inch or so, to level the anti-cavitation plate with the bottem line. An inch above or so would be ideal for speed but with an alluminium prop it will cause cavitating.

Chinewalking with a 5.4 meter boat with only 60 km/h makes no sense at all. That's all caused by a wrong prop or wrong mounting of the engine. To illustrate: I can drive 80 or 85 km/h without any chinewalk in a small boat of only 3.6 meters... The engine's anti-cavitation plate is 4" or so above the bottom line, that's not what the topic starter or most of you here should do, but, when I lower it by an inch or 3, the boat starts chinewalking like madness at only just above 60 km/h.
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Old 07 March 2009, 20:03   #10
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reinier - yes you are correct. the onset of instability that leads to chine walking can result from several contributors (check "chinewalking" article). raising the engine often helps situation as the lower unit (torpedo) drag is reduced significantly. this changes the "dynamic CG" and alters the behaviour of the hull, changes the dynamics of the "hump zone" and can mitigate chine walk.
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