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Old 25 April 2003, 03:12   #1
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Twisting Scorpion

My Scorpion 6.5 Optimax 150 twists quite sharply when she comes out of the water on a wave. I quite like getting air but I am not keen on the twisting bit so I guess trim tabs might be the answer.

Any advice on what tabs would be suitable would be most welcome
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Old 25 April 2003, 03:23   #2
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JK can you delete this thread for me please. I did a search for trim tabs and am following Dirks advice and having a look at the Bennett website

< I think I'd better leave it as there's quite a lot more here now . . . JK >

ta
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Old 25 April 2003, 03:48   #3
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Twisting

wavehumper

Does the boat twist to the left on take off, if thats the problem just shift some weight around to the right hand side, ie move the batteries across and that will help keep the boat level in the air, and on take of turn the wheel to the right to conterfit the torque from the engine, should find it lands level then. You may not need tabs.

Julian
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Old 25 April 2003, 04:32   #4
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Or alternatively put a left-handed prop on. That cured my (Scorpion) problem.
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Old 25 April 2003, 06:21   #5
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Thanks guys I have been trying Julians Ideas and it hasn't really helped much, but I could probably try a little harder to redistribute the weight. I am couneracting with the steering wheel, but possibly not enough.

Brian I was talking to Andrew Cox a day or two ago and he was telling me about your solution so I might well give that a go
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Old 25 April 2003, 06:46   #6
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Top man that Andrew.
Came up with the solution that worked for me.
Anyone who needs a top-flight marine engineer in the West Country, contact Andy on:

andrewcox@marineenginering.fsnet.co.uk
Work: 01326 279001
Mobile: 07974 250533
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Old 25 April 2003, 06:59   #7
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Brian, this might sound rather odd but how do you get a left handed prop to make a boat go forwards? run the drive in reverse ?

Another solution might be off setting the engine, think its 1" for every 100 hp. JF might be able to add to this.

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Old 25 April 2003, 07:25   #8
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When you put a LH prop on you also have to take out the control box and reverse the lever attachment points. If you didnt do this you would have to pull the gear lever back to go forwards. (If you see what I mean).
Moving the engine one inch is a non-starter with an inboard diesel (and possibly any other inboard as well).
Weight shifting does not always work either because:
-you can't shift enough weight, far enough
-you can't move anything that weighs enough (engine compartment "full", for example)
I believe the torque effect is caused by the flywheel rotation within the engine.

I will now stand back and let someone who actually knows what they are talking about, comment.
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Old 25 April 2003, 07:48   #9
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Brian, thank you. Stu has an Optimax 150 on the Scorpion which is why I suggested moving it to starboard, but yes I agree moving an inboard is a non starter.

Out of interest (as this quite facinating) if you reverse the control box lever does the gearbox still run with the same cogs driving forward ? I am thinking about the way the cogs are machined with a curve (can't think of the correct name for it).
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Old 25 April 2003, 07:55   #10
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Obviously I dont know (because I am a pillock) but if I were to guess............
I would think that the bits which matter in the gearbox are cog wheels and it doesn't matter if they turn clock or anti-clockwise.
But hell, surely someone is reading this who actually knows?
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Old 25 April 2003, 09:35   #11
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Hi all

Wouldn’t recommend fitting a left handed prop on a standard rotation gear box, Effectively you are running the box in reverse the whole time. The Counter rotation boxes have different gears in than the standard, Must be the case otherwise why would engine manufactures sell two different outboards. I can’t see what you would achieve in doing this anyway, torque effect is created by the single prop spinning in one direction. Sounds to me like offset, The boat manufacture would know what offset you should be running.

Cheers

J
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Old 25 April 2003, 13:57   #12
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Gears which turn the drive through 90degrees are called bevel gears. The ones with a twist are spiral bevel gears. There are a few different profiles for these.

Put a left hand prop on a right hand gearbox on an outboard motor. You are joking, aren't you?

JW.

Alpha stern drives... now that's a different matter.
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Old 26 April 2003, 03:03   #13
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Quote:
Originally posted by jwalker
Put a left hand prop on a right hand gearbox on an outboard motor. You are joking, aren't you?

JW.

Alpha stern drives... now that's a different matter.
Oh no it isn't. Under no circumstances should you put a left handed prop on an Alpha, (unless of course it's a contra rotating)
Your probably confusing it with a Bravo, whose rotation is decided at the control box end.

An Alpha lower unit is basically a V6 outboard gearbox.
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Old 26 April 2003, 03:56   #14
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Agree with the others absolutely not to a left handed prop on an outboard - reverse gear is not up to being run constantly flat out and the box will blow up sooner or later!

The solution to your problem is driving the boat in choppy conditions more and getting used to what it is going to do and counteracting it - it is something you have to learn, and will come naturally eventually.

Trim tabs don't work in the air either!
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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 26 April 2003, 04:38   #15
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Humble pie

Yeh, DD's correct. Bravo drive it is, not alpha. Sorry.
Jeez, I'm getting old.

J (with the long white beard) W.
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Old 26 April 2003, 07:15   #16
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Well on first reading this I thought it was simple weight distribution, off set engine mounts and driving style.

I dont see that changing props from left to right has much to offer, it will turn the other way! But I ll accept there could be a technical reason from an expert, by Id avoid spending the money and do simple things first

I would keep to basic principles of boat trim / balance and driving skills. This is not using the engine trim or trim tabs but stuff in the boat.

Weight distribution can change the ride of the boat at low and high speeds. Having someone sat up front makes a difference, infact on a calm day on a boat upto 6m the movement of a large person can be felt and even seen on the ride.

More so, I would be thinking about backing of the power if getting the whole hull out the water as its the engine torque through the prop twisting the boat when the hull provides no resistance (and you dont want to over rev the engine too often).

Additionally I would advise not to saw at the wheel at the top of the wave as that can flick you over if you are attacking the wave at an angle or across the wave.

Hope this helps and it should cost very little!

Tiger
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Old 26 April 2003, 12:31   #17
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Forgot to mention that trimming in too much can cause an overpowered boat (a Phantom 18 with an XR2 200hp on the back) to constantly fall on the left - I know I raced as a navigator in one for far too long before I took up driving!

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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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