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Old 11 January 2011, 11:18   #91
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Country: UK - Scotland
Town: Aberdeenshire
Boat name: Kikican
Make: Vipermax 7
Length: 7m +
Engine: F250 / FT9.9 Aux
Join Date: Nov 2006
Posts: 250
Never raced but spent a lot of time thinking about what is going on back there.

I am getting 49 knots with one up + a full tank of fuel. I have the engine offset by 30mm + 90 kg me + 50 kg aux + 30 litre aux tank all on the right - medium battery on the left which cancels out the aux tank. I had to move the aux from the left to the right to get the balance right - she was OK in the water but tended to screw to left when leaving the water. When there is a second bod on the boat she starts to fall to the left again a bit however I tend to keep it in the water when there are others on the boat so prefer it is optimised for one up.

The planing pad on mine is 1250 mm long x 300 mm wide at the back and looks like it rises by 70 mm or 3.2 degrees. One point to consider is that because of the planing pad, your effective engine height (for comparing against set-ups without a planing pad) must be a bit higher that what you are measuring - i.e. the water right under the planing pad is less deep and also less clean due to being bent round the point of the pad than it would be with a conventional vee hull ending 70 mm lower. This would lead me to think that all other things being equal it would be better to have the engine a little lower relative to the edge of the planing pad than you would have for a normal vee.

For the planing pad to do any good I would have thought you must have to trim out a good bit to make it approach the horizontal. 3.2 degrees does not sound much but if you crouch under the boat as I have just done you can see how far the nose would have to come up to move it towards the horizontal. As you would start to trim out to make it sit on the pad you would start to lose the longditudinal stability of the keel. Also you then have to consider the sides of the planing pads that are about 7 degrees to the keel. I would have thought that if it were sitting on the pad and the boat turned to one side (due to steering or wave action), the back of the keel just in front of the pad and the deeper edge of the pad would tend might tend to steer the boat in the direction of the turn - perhaps there is a point at the speeds you are running that it becomes unstable.

Mine is rock steady laterally at WOT however as you trim out to sit on the pad to creep the speed up to the max you can definately feel the boat starting to slip sideways slightly and the steering getting heavier - you have to run with the steering very slightly offset to compensate. Its weird at first but now I am used to it. Only appears in the last 5 knots or so perhaps it gets more significant at your speeds.

I tend to not put it into a hard turn at those speeds so have yet to find out what happens. It has never shown any signs of instability at any moderate turn at speed or a hard turn at any lower speed.

You can bank the boat up at an amazing angle if you want to - perhaps the higher tube fixing permits this. I have always found it self limiting - once it goes past the 45 degree point the prop will surface and there will be no more force to heel the boat any further over. My favorite party piece at the end of a spin is to bank her hard over and then put her into a sideways slide. Always wondered what would happen if I suddenly reversed the steering and let the keel or skeg sudden bite the water though......

Richard
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Old 11 January 2011, 11:27   #92
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Country: UK - England
Town: Dorset & Hants
Boat name: Streaker/Orange
Make: Avon/Ribcraft
Length: 4m +
Engine: 50Yam/25 Mariner
Join Date: Apr 2007
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Quote:
Originally Posted by Richard Selman View Post
My favorite party piece at the end of a spin is to bank her hard over and then put her into a sideways slide. Always wondered what would happen if I suddenly reversed the steering and let the keel or skeg sudden bite the water though......

Richard
Or a wave came up a little ? I'd guess you and the boat part company as you smartly leave on the outside of the turn ?? I hate 'sliding' a boat like that - its a huge amount of risk ......( but great fun )

The Shearwater just would not do it no matter what I did - the Avon.... all over the place !
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Old 11 January 2011, 13:13   #93
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Country: France
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Boat name: Alea Jacta Est
Make: Osprey
Length: 7m +
Engine: mercury 300xs
Join Date: Jan 2011
Posts: 27
[QUOTE=Richard Selman;384137] Never raced but spent a lot of time thinking about what is going on back there.

The planing pad on mine is 1250 mm long x 300 mm wide at the back and looks like it rises by 70 mm or 3.2 degrees. One point to consider is that because of the planing pad, your effective engine height (for comparing against set-ups without a planing pad) must be a bit higher that what you are measuring - i.e. the water right under the planing pad is less deep and also less clean due to being bent round the point of the pad than it would be with a conventional vee hull ending 70 mm lower. This would lead me to think that all other things being equal it would be better to have the engine a little lower relative to the edge of the planing pad than you would have for a normal vee.


First, thanks for your reply.

I wonder when was build your boat ? Because I wonder if we have got the same pad dimension or not.
Then that is the longer of your shaft and how high is your engine ?

Have you check the the hull and rule it in or out ? In order to see if it is perfectly flat ? Because I suspect mine to have a mould defect.

Thanks.

Regards.

Jean-Charles.
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Old 12 January 2011, 04:02   #94
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Country: UK - Scotland
Town: Aberdeenshire
Boat name: Kikican
Make: Vipermax 7
Length: 7m +
Engine: F250 / FT9.9 Aux
Join Date: Nov 2006
Posts: 250
The hull is just over 3 years old - built around Oct 2007

Fully trimmed in, the anti-ventilation plate projects to 45mm above the bottom of the transom (trailing edge of the planing pad).

The engine is a Yam F250 ultra long shaft. Just hanging on the transom cutout as normal and bolted straight on with no mods made to the transom.

Peter - would only skid the boat in calm waters - would not want to go over a wave sideways! Takes a lot of effort to make it do it. Won't do it by itself if the power is on no matter how hard the turn I think.

Richard
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Old 12 January 2011, 04:22   #95
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Country: UK - Scotland
Town: Aberdeenshire
Boat name: Kikican
Make: Vipermax 7
Length: 7m +
Engine: F250 / FT9.9 Aux
Join Date: Nov 2006
Posts: 250
.....don't see any significant issues with my hull strakes (virure) - I would not have thought that little differences of of the size you seem to have would be the problem.

I am using a Yam 14.5"D x 23"P ported stainless prop. Only problem so far is that if I have not been out a while I tend to forget that it sounds funny at low revs so I often lift the eng to check the prop for seaweed. Not so good for tight manouvering work though (especially in reverse) since it is spinning in bubbles. If I am on rescue duty at the sailing club I will change it back to the 21P stainless or 23P aluminium normal ones I have.

There was a post on here a while back that compared various props for a 7m boat. This one came out the best. Too far north to get access to test props and there was only one boat in the area with anything like this size of engine to borrow a prop but he was running a lower pitch prop for a heavy 8m sea-safari rigged humber so just bought this one and stopped worrying about it.

Richard
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