Riding anchor sail

[ariette]

Having suffered some 'sailing at anchor' last year in high winds I wonder if forumites have experience (good or bad) about the use of riding sails to calm things down and if good is there an off the shelf maker of such things in the UK.

 

[Goldie]

Not directly, but we use a reefed mizzen for the same purpose and it works a treat.

 

[KellysEye]

We also had a ketch but to save wear on the mizzen sail in strong winds we used a riding sail. It cuts down the swinging from up to a 90 degree angle to around 35 degrees thus significantly reducing snatch loads. I'd strongly recommend it and I'm very surprised how few boats use one given the benefits. We bought a kit to make it and you can buy it here: http://www.sailrite.com/Anchor-Riding-Sail-Kit-12-5-Sq-Feet

 

[AliM]

Piota, who sadly died recently, designed a riding sail. We used his design and made it up using an old main. It worked brilliantly. Do a search for Piota, and you will find the design and accounts of how it performed.

 

[NormanS]

I too, have a ketch, and have tried a reefed mizzen, and while it worked, I found it noisy. I have now a twin riding sail, which uses the mizzen mast track for the luff, with the two clews sheeted to the corners of the taffrail. The clews are held apart with a short "whiskerpole". This system has the advantage that the sail is full all the time and doesn't slat. I haven't had it long enough to decide whether it's a complete success, and the boat is now out of the water till the Spring.

 

[CaptainBob]

We anchor our ketch a lot on the Dart. We only get the 'anchor dance', as I call it, in wind over tide conditions. Can wind alone cause it too? When too bad to ignore I put the steering over hard, and launch a small drogue on the down-current side of the bow, which keeps the bow over nicely when there's enough current.

Would a riding sail be useful in the same situation? Would it also reduce rolling when there's some residual swell?

I think it's something we should probably investigate.

 

[NormanS]

We practically never have any significant current when anchored, but most boats will sheer about at anchor, just due to the wind. In strong winds this can lead to the rode trying to pull the anchor, first one way and then the other. Preventing this, is my main reason for a riding sail.

 

[Tranona]

Most sailmakers should be able to make one for you. Flat cut sail either flying or yanked to the backstay with clew forward and 2 sheets down to a point midships either side. I had a quote from Kemps for one about 50 sq ft for a 37 ft boat.

 

[Deleted member 36384]

Piota's sail was a flat triangle, hoisted up the backstay with the two clews hauled tight down to the quarter. The clews were held apart with a pole. It was this shape, and the deflected air flow over each quarter, which kept the stern dead down wind and stopped the bow slewing about. My description is from memory, so best look up Piota's posts where he gives an exact description. I don't believe the shape/area was particularly big either.

 

[AB1707]

This is a riding sail that you can have made up - not sure of the cost

http://www.fbyachting.it/EN/findelta_eng.html

 

[Sidedrum]

I have a riding sail made to Piota's (aka Krusty's) specification. It makes a very significant difference to the amount of dancing around at anchor, thus reducing the strain on your ground tackle and on your nerves! I wouldn't be without it now - Piota persuaded me that it is an essential piece of equipment for any boat that reckons to rely on an anchor more than on a marina berth.

 

[Ravi]

I never realised that there were these specialisy designed riding sails. A friend of mine just uses a (small) dinghy jib, on the topping lift, for the purpose (bought cheap on eBay). It seems to work.

 

[geem]

I have seen a number of yachts use their storm jib flown on the backstay and sheeted down hard. Seemed to work

 

[AliM]

As I said earlier, we made one using Piota's design. In the Baltic (no tides), we used it to great effect.

We were sheering about through an angle of 100 degrees (ie +/-50 degs to the wind) as were a few other boats, in a wind of about 15 kn. With the riding sail, that angle went down to about 20 degrees (+/-10) and became a gentle wander rather than swing-snatch-swing...

In this country, we are usually anchored in reasonably strong tides, so rarely need to use it.

 

[steve350]

Here's a great video of Piota sailing. The riding sail is shown from about 7:30 minutes onwards.



http://www.youtube.com/watch?v=KVpal17ZnYc

 

[Sybarite]

Having suffered some 'sailing at anchor' last year in high winds I wonder if forumites have experience (good or bad) about the use of riding sails to calm things down and if good is there an off the shelf maker of such things in the UK.

Try also hanging a heavy duty builders rubber bucket over the bow and immersed.

 

[GHA]

Here's a great video of Piota sailing. The riding sail is shown from about 7:30 minutes onwards.



http://www.youtube.com/watch?v=KVpal17ZnYc

Ta for the link, doesn't that setup increase the load on the anchor?

 

[jdc]

Ta for the link, doesn't that setup increase the load on the anchor?

Why don't we work it out? Approximate dimensions of the sail is 1m across the bottom x 3m long. Two triangular panels at about 45 degrees from the wind. So effective area is 1 x 3 x 1/2 x 2 x cos(45) = 2.1 m^2

Force is 1/2 . rho . C . A . v^2.

rho = 1.2 kg/m^3
C = 0.5 say (brick = 1, super slippery car = 0.3)
A = 2.1 m^2 (estimated above - may be out, but no more than a factor of 2 I guess)
v = 15m/s (30 kts: a lot for low down, probably F9 at the masthead)

Resulting force = 113 Newtons - ie about 12kg. No big deal!

At that wind force my boat exerts about 300kg on its anchor. If it sheers about there's an additional force, F = m x a.
m = 13,000 kg
a is harder to estimate, say 0.4kts to zero knots in one second? That would be 0.2m/s/s or 1/50th of a 'G'.
So F (= m x a) = 0.2 * 13,000 = 2600 Newtons, or about 260kg.

It seems to me that the static force is hardly increased by the riding sail, and saves many times the snatch pull by slowing down or reducing the severity of the yaws. What's surprising is that such a small pull right at the stern makes such a difference to the yawing about. I think the reason is that it's right at the stern. We have to consider its leverage compared to the windage of the boat without it.

Boat alone has windage of 300 kg x (distance between CoE and CLR). That distance may be only 25 cm or so, whereas the riding sail may be 6m behind the centre of lateral resistance (CLR).

So leverage causing oscillation is 300 x 0.25 = 75 kg-m.
Leverage damping oscillation is 12 x 6 = 72 kg-m.

Pretty back-of-fag-packet calcs, but nonetheless illustrate why even a small riding sail helps a lot.

 

[CaptainBob]

Nice maths!

Your calculation of drag based on half rho c a v squared... do you not also need to consider lift? The sail will adopt a non-flat shape and start acting like a sail won't it? Could this generate a force higher than the sail perpendicular to the wind as your calc assumes?