Thinking caps on chaps!

[pyrojames]

SL has a mast of approx 100 ft sq. No amount of aerodynamic spoilers are going to prevent that having a considerable driving force through drag alone, if the mast does not trim easily to face the wind, and if the cat is turned either by tidal action or by being aground, at an angle to the wind.

If it were a question of generating assymetric lift then spoilers might well help.

Just think of a structure about 50 ft long and 2 feet wide with the wind blowing at 90 degrees to it.

The only solution is for the mast to be able to trim autonomously to minimise drag and negate any lifting component. The boat must be capable of doing that even if the hulls are pointing away from the wind through action of the tide and the mooring arrangement, or by being on the putty.

I don't think that it is the drag that is the problem. It is the lift that is generated as soon as the wind is not directly ahead of the mast, and there is some angle of attack to generate lift. Breaking the profile and interrupting the lift with prevent the mast being able to "sail" and should limit the shearing about. If it was only drag, SL should sit comfortablely enough.

 

[bignick]

If it were a question of generating assymetric lift then spoilers might well help.

It is generating assymetric lift, that's why the LIFT force is sideways. If it was a pure drag force it would pull the boat back on its mooring, or serve to renormalise the boat head to wind, as the drag would always be aft of the pivot point about the mooring.

Just think of a structure about 50 ft long and 2 feet wide with the wind blowing at 90 degrees to it.

That's the drag force, and it wouldn't cause the boat to sail on its mooring, any more than a conventional boat with a conventional rig does.

The only solution is for the mast to be able to trim autonomously to minimise drag and negate any lifting component. The boat must be capable of doing that even if the hulls are pointing away from the wind through action of the tide and the mooring arrangement, or by being on the putty.

You said it - its the lift that's the problem. Drag isn't an issue, just as it isn't on a regular yacht.

Consider the difference between this boat and rig, and a conventional boat and conventional rig. The main difference is the rig - which is generating a driving force as it is at an angle of attack to the wind. You're never going to reduce it completely, as it will always be an aerofoil, but you could reduce its effectiveness of being an aerofoil by spoiling its shape.

 

[KristiferColumbus]

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!

Try flying a small helium balloon off the stern,cause it might alarm the neighbours

 

[ianat182]

If there is enough reserve strength in your mooring would a third warp between the hulls to the stern ,bringing the pivot point of the mooring more aft, alter the balance instead of trying mooring only by the stern?
Retrieval of the aft warp outside the hull would revert to the original, and only used when strong winds expected.
This at least could be tried as an experiment before tackling steadying sails and other gear additions.

ianat182

 

[Baggywrinkle]

The boat lies head to wind at low wind speeds and starts to 'sail' as the wind increases. From the photo the cat is moored symmetrically with a bridle and the mast straight fore-aft. This symmetry makes it equally efficient on both tacks ... as the centre position will generate equal lift on both 'tacks', and when the boat first strays off-centre then the oscillations will start. This happens with monohulls too, you often see yachts shearing on their anchor line - just your mast is a much more effective sail than a monohulls hull.

Another way to stall an aerofoil is to increase its angle of attack to the wind. (like flaps on an aircraft, or flaring when landing).

Looks like the whole rig swivels so why not lock it off at 45% to the boat so the boom exits over the stern of one of the hulls. This means the aerofoil may remain stalled in stronger winds, if it does generate lift, it will be more effective on one tack and no longer symmetrical, so spoiling the ability to oscillate and preventing the sailing back and forward. It might be that there is an angle for various wind speeds where it remains stalled, or it might be that only 90% works ... what do I know ... but it might be worth a try.

Maybe even in combination with the rudders lashed hard over, and/or the bridle assymetric - rudders so any forward motion turns the boat and increases the angle of attack further stalling the foil.

What you are looking for is an assymetric setup that can't oscillate, a bit like flying a single stringed kite, wind speed only increases altitude until an equilibrium is reached.

Experiment away ... and good luck. Not a problem many of us face.

 

[bbg]

When anchoring, I would use two anchors in a V formation.

At mooring and anchor, something hung from between the transoms to increase drag. A bucket might do but might not be strong enough.

 

[fisherman]

If you have two heavy pickup chains, one from each hull to the mooring, would that dampen the movement? If they were attached well down the riser you might experiment with taking up the weight of the chain on the boat rather than the buoy, if not rigged so already.

 

[peterb]

Your problem isn't one of steady forces. An oscillation like yours implies some form of positive feedback; a small deflection causes a force such as to increase the deflection, which then increases until it is halted by a pull on the mooring bridle, which pulls the bows round so that the whole procedure is repeated in reverse. In feedback systems like this the usual cure takes the form of an energy absorbing device, so that in each cycle the energy absorbed is greater than the energy imparted by wind/tidal effects. The easiest way is probably to use some form of damping like a sea anchor at either the bow or the stern (whichever moves most). Try a builder's bucket, heavy plastic with a ring in the middle of the handle. Keep the line short; the idea is to make the bucket move through the water so that it absobs energy.

With no wind, I would expect the boat to lie to the tide. As the wind increases it will reach a point where the boat swings and lies to the wind. Under what combinations of tide and wind does the oscillation happen? If you look down over the stern during the swinging, what does the water flow look like? Is it along the line of the boat, or is it transverse, is it smooth or turbulent? If the helm is free, does it follow the swing (if so, then clamping it could increase the damping).

Sorry about the questions, but they all add to the evidence!

 

[exfinnsailor]

Simple Really

 

[bignick]

I think you forgot the smokestack!!

 

[ripvan1]

Sorted

 

[ColdFusion]

SL

in order to prevent the mast 'sailing', it must point directly into or down wind.

If you don't have the mast freely pivoting and weathercocking, then every proposal using water flow is just going to set the mast across wind (and generate unwanted drive) at some stage as wind and tide conspire to be at different angles to each other.

When she has taken ground, this is especially true as the wind will blow where it listeth, and not conveniently fore and aft.

The only guaranteed way to align the mast and wind direction is a freely pivoting mast, either driven by a 'rudder' at the outhaul end of the boom to counter the jib boom's windage, or a sophisticated drive system with sensor and power.

Or an aerial version of a trim tab self steering ? Now that could provide some non-electrical power to move the mast.

I think that's spot on - the mast has to point directly into the wind or downwind, whereas the boat may not be doing that due to tides. Any solution has to deal with that issue.

It may be worth asking the same question on multihulls4us ?