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snowleopard

snowleopard

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OK, here's the problem: my boat sails about on its mooring and at anchor. The cause is the wing mast which means there is 100 sq ft of 'sail' area set at all times. For those who haven't seen, here's a pic...

snowleopard.jpg


In moderate winds she swings from side to side but when it gets really strong she starts to sail from side to side until brought up by the bridle whereupon there is sharp deceleration and her head whips round to sail off in the opposite direction. It's uncomfortable, alarms the neighbours and of course the jerks on the mooring with the risk of breaking it.

So far I have 2 solutions. The mast maker recommends setting the mast atwartships. It does work fine at low wind speeds but puts more tension on the mooring and if the wind shifts suddenly it can develop forward drive with potentially disastrous consequences. The other I thought up at 6am today with 45 knots of wind over the deck - I put both engines in reverse at tickover so the boat couldn't accelerate. While they were running she just swayed 20° either side of the wind.

I have tried to think of ways to cut the surging that don't involve engines or excess loading on the mooring.

One obvious option is some sort of riding sail but of course we don't have a backstay so that would be tricky. Nor do we have space for a mizzen with riding sail.

So - I'm looking for some ideas.

p.s. other than that, it's a brilliant rig!
 
can you put a small 'sail' at the outhaul end of the boom, so that it 'weather cocks' the mast into wind ? Or does the mast have to be fixed / locked when you are away ?


Fine anchor ball BTW !
 
There is a fair bit of friction in the mast bearings so the rig wouldn't necessarily lie exactly into the wind. It could end up developing a lot of drive. When used deliberately, I can get 1 knot per Beaufort force from the mast alone (I've had 9 knots out of it).
 
you might then get the tide bringing the boat round down wind :eek:


I am beginning to think you need an active control system like a wind genny connected to a drive unit and wind angle sensor which a) provides power and b) required trimming angle to a motor to turn the mast chord to the minimum wind angle.


Walker Wingsails had this system:o
 
I believe the problem is mast position; so far forward. I have a similar problem with my junk rigged Pioneer. In effect, in strong winds, the boat has strong lee helm whilst moored and tries to turn her stern to the wind. I think the only way of reducing this effect is to arrange some lateral resistance/drag at the bows such as a drop-plate right forward - but I've not managed it myself and have spent several anxious stormy nights fending off from other yachts swinging out of phase!

Good luck
 
No, any system which relies on an underwater input will at some inevitable stage place the mast at a cross-wind position. It has to be related to the wind angle.
 
Aerofoil or CLR issue?

I think the solution probably depends on the cause: sorry, this sound trite, but let me explain.

I can think of two causes of such oscillations, but don't know which will dominate in your case.

1. The windage is forward of the centre of lateral resistance. So a puff pushing causes the bows to pay off. This is seen more or less in all boats, especially modern hull shapes with cut-away forefeet, especially when combined with knife like fin keels.

2. Your aerofoil x-section mast provides forward drive.

As I said, maybe both apply.

The cure for 1. is to put more resistance forward, eg a bucket or more chain or a kellet over the bows, or put more more windage aft, eg a riding sail. I don't think I can make any useful suggstion here which you won't already have thought of.

The cure for 2. is more subtle and so maybe this is an idea. Destroy the 'lift' by causing the flow around the wing to break away from the spar - ie destroy laminar flow. This could be achieved in principle by roughening the surface, or blowing air through tiny orifices, but both these seem impractical for you (and/or would dameage sailing performance).

What might work is hoisting a temporary structure, maybe akin to baggywrinkle, on a hallyard so it lies to the side of and touching the mast. Or contrive to hoist a rigid spiral around the mast like tall chimneys have (blue plastic water pipe)? This feels more use as an experiment than as a permanent solution but might tell you something!

Another 'brainstorm' idea would be to control the rotation of the spar and rig so as to damp out oscillation. The transition from stable to oscillatory is often really abrupt, so not much damping may be needed. But how you can contrive a way to swing the rig, and a make a control system to ensure it's got the right transfer function maybe rather high tech even if doable in principle. To discover if it's useful in principle, do you find you can affect the sheering/oscillation by manually swinging the spar to counteract and damp it?

edit: damn, planteater beat me to it!
 
anything hoisted up the mast is going to increase drag forces and lateral thrust if the wind/tide functions conspire to bring the long x axis of the mast near to 90 degs.
 
snowleopard said:
OK, here's the problem: my boat sails about on its mooring and at anchor. The cause is the wing mast which means there is 100 sq ft of 'sail' area set at all times. For those who haven't seen, here's a pic...

snowleopard.jpg


In moderate winds she swings from side to side but when it gets really strong she starts to sail from side to side until brought up by the bridle whereupon there is sharp deceleration and her head whips round to sail off in the opposite direction. It's uncomfortable, alarms the neighbours and of course the jerks on the mooring with the risk of breaking it.

So far I have 2 solutions. The mast maker recommends setting the mast atwartships. It does work fine at low wind speeds but puts more tension on the mooring and if the wind shifts suddenly it can develop forward drive with potentially disastrous consequences. The other I thought up at 6am today with 45 knots of wind over the deck - I put both engines in reverse at tickover so the boat couldn't accelerate. While they were running she just swayed 20° either side of the wind.

I have tried to think of ways to cut the surging that don't involve engines or excess loading on the mooring.

One obvious option is some sort of riding sail but of course we don't have a backstay so that would be tricky. Nor do we have space for a mizzen with riding sail.

So - I'm looking for some ideas.

p.s. other than that, it's a brilliant rig!
Click to expand...

remove the mast and fit bigger engines of course!!

No?? OK thought not. :D
 
snowleopard said:
OK, here's the problem: my boat sails about on its mooring and at anchor. The cause is the wing mast which means there is 100 sq ft of 'sail' area set at all times. For those who haven't seen, here's a pic...

In moderate winds she swings from side to side but when it gets really strong she starts to sail from side to side until brought up by the bridle whereupon there is sharp deceleration and her head whips round to sail off in the opposite direction. It's uncomfortable, alarms the neighbours and of course the jerks on the mooring with the risk of breaking it.

So far I have 2 solutions. The mast maker recommends setting the mast atwartships. It does work fine at low wind speeds but puts more tension on the mooring and if the wind shifts suddenly it can develop forward drive with potentially disastrous consequences. The other I thought up at 6am today with 45 knots of wind over the deck - I put both engines in reverse at tickover so the boat couldn't accelerate. While they were running she just swayed 20° either side of the wind.

I have tried to think of ways to cut the surging that don't involve engines or excess loading on the mooring.

One obvious option is some sort of riding sail but of course we don't have a backstay so that would be tricky. Nor do we have space for a mizzen with riding sail.

So - I'm looking for some ideas.

p.s. other than that, it's a brilliant rig!
Click to expand...


Thinking outside the box...

At first I thought about flying a kite from the back rail of a hull - that'll keep her sort of head to wind, but then thought better of it.

Flying an old-fashioned kite from the back of the "boom" will keep the aerofoil head to wind - if you leave the mast free to rotate.

This only helps if you are aboard though...

Do you recall the V-shaped riding sail design that someone posted on here? I cant remember who it was and at the moment don't have the time to search for it. It was a) simple and b) very effective. IIRC he was the forumite who posted a video of blasting downwind on Loch ness and the desing of sail is Norwegian. A modified one of them should keep her head to wind and minimise any yawing.
 
Try keeping the boom off-centre and use a longer mooring strop on one hull so that she sits biased to one side...?
 
duncanmack said:
Do you recall the V-shaped riding sail design that someone posted on here? I cant remember who it was and at the moment don't have the time to search for it. It was a) simple and b) very effective. IIRC he was the forumite who posted a video of blasting downwind on Loch ness and the desing of sail is Norwegian. A modified one of them should keep her head to wind and minimise any yawing.
Click to expand...
The Forumite used to be called Piota, but has now changed his screen name since he sold his boat. I can't remember it for the moment.

These are his photos.

CopyofPiotaRidingsail1.jpg


CopyofPiotaRidingsail2.jpg


Ash
 
I agree, re flying a V shaped sail aft, the skipper was Piotaskipper. If you search on 'riding sail' I think. If you can't find it pm me I have it bookmarked somewhere.

Not quite sure whether you have the rigging at the stern to fly such a beast but you're a clever fellow I'm sure that you'll find a way.

Chris
 
perhaps a very small riding sail flown "in reverse" from the backstay/topping lift (or whatever it is coming down from your mastehad to the boom end), with the clew of the sail fastened to the gooseneck area. As it could provide some drive opposite to the drive of the mast. I'm sort of thinking like being hove too (in reverse formation), but using the aforemention sail instead of a headsail.

OK, I'm struggling to explain it but hope you get my drift.
 
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