End Plate effect of lazy jacks

[sarabande]

I've been fiddling around with tension on the lazy jacks and the boom cover.

Seems that if I slacken off the lazy jacks, the boom cover adopts an endplate effect, with the cover at right angles to the mainsail. When this end plate is in place, a little tell tale tied to the end of the boom zips out strongly towards the stern. If the boom cover is tight to the mainsail, the tell tale drops off to leewards quite dramatically.

It set me wondering if a series of end plates on the mainsail would generate a lot more beneficial forward thrust ?


I might pick up one of those cheapy anemometers, stick it on a pole and see where the flow goes and how fast.

 

[Javelin]

Hmm, yes it would help maintain the pressure differential however at a cost of more wind resistance in an area of sail that possibly generates around 10% of speed or less upwind, maybe 18 to 20% on a reach and on a run the endplate effect has no relevance.

Rule of thumb is top third of the sails do two thirds of the work.

 

[PuffTheMagicDragon]

When this end plate is in place, a little tell tale tied to the end of the boom zips out strongly towards the stern. If the boom cover is tight to the mainsail, the tell tale drops off to leewards quite dramatically.

That could well be because with the 'end plate' open there would be a smoother flow of air along the bottom part of the sail whereas with it closed there would be turbulence.

 

[lpdsn]

There was an attempt in the 20s or 30s to design a boom as an end plate for the America's Cup. I think it was nicknamed Park Avenue.

 

[zoidberg]

It set me wondering if a series of end plates on the mainsail would generate a lot more beneficial forward thrust ?

You ought to join the Amateur Yacht Research Society. Their members have done a lot of work on this sort of thing, and you'd fit right in!

Then there's Marchaj's 'AeroHydroDynamics of Sailing', which would keep you awake for many, many nights. Also, many relevant papers in the archives of the Royal Aeronautical Society on 'end plate effect' and 'wing fences'. You could with advantage mount a couple of those small Smoke Test canisters ( sold in plumbers' trade outlets ) near the rear end of your boom to see and photograph the vortices which come off. That should give you a visual perspective on relative efficiency of varying 'angles of dangle' of your cover-flaps in different wind speeds.

Then you could attach multiple tufts ( mini-recording tape is better than wool string - doesn't absorb water ) as 'tell-tales' every 30cm up luff and leech, and horizontally across the span of the sail as well. That could help you determine where the laminar flow detaches ( not easy to guess in old, baggy sails ) and the effects of spanwise flow, and then glue on rows of small 'vortex generators' to help keep the flow attached and inhibit stall.

 

[zoidberg]

Rule of thumb is top third of the sails do two thirds of the work.

It was empirically established that the lower quarter-to-third of a normal mainsail is detrimentally affected by disturbed, turbulent airflow downwind of multiple dangling/coiled halyards, gear fastened to the lower mast, guardrails, stanchions, even toerails. The turbulent airflow downstream from such projections is not, as is assumed, like the tapering tail of a tadpole but spreads out like a half-open fan.

This was communicated to racing designer Nigel Irens, who checked out the idea via BAe Bristol aerodynamicists some 30-odd years ago. He then 'cleaned up' and curved the surfaces of his race boats, followed by his competitors.

Those who fly gliders are well aware of where, on their wings, 'parasite drag' creates turbulences and localised stall. They wouldn't fly with a bundle of rope tied around their wing-root, or bits of chunky metal sticking up ( or down ) thereabouts, either. We can't eliminate such on yachts, but we can be aware of the issue and minimise sources of substantial 'parasite drag'.

 

[Robert Wilson]

I came across this thread by accident when seeking basic information regarding lazyjacks, but it set me thinking.

If I understand correctly what zoidberg is saying about lower-area turbulence, if I add lazyjacks to make dropping my mainsail a neater and easier stowing exercise then I may well be adding even more turbulence to the sail when set?

I currently have three reefs (twin-line reefing for each), as well as all the usual paraphernalia on the mast, shrouds, cabin-top etc etc.

Would adding lazyjacks be worth the convenience exercise? Or am I reducing the sails efficiency quite dramatically.

I don't race Khamsin, but I do like to sail as quickly as the conditions allow. I sail almost 100% single-handed, so the idea of "easy dropping and stowing" is appealing.

I will need a new main-cover in the near future, which ever way I decide about lazyjacks.

Thanks in anticipation

 

[Bobc]

I came across this thread by accident when seeking basic information regarding lazyjacks, but it set me thinking.

If I understand correctly what zoidberg is saying about lower-area turbulence, if I add lazyjacks to make dropping my mainsail a neater and easier stowing exercise then I may well be adding even more turbulence to the sail when set?

I currently have three reefs (twin-line reefing for each), as well as all the usual paraphernalia on the mast, shrouds, cabin-top etc etc.

Would adding lazyjacks be worth the convenience exercise? Or am I reducing the sails efficiency quite dramatically.

I don't race Khamsin, but I do like to sail as quickly as the conditions allow. I sail almost 100% single-handed, so the idea of "easy dropping and stowing" is appealing.

I will need a new main-cover in the near future, which ever way I decide about lazyjacks.

Thanks in anticipation

It will make 9/10 of bugger-all difference. Go with what makes life easier.

 

[PuffTheMagicDragon]

...

Would adding lazyjacks be worth the convenience exercise? ...

...I don't race Khamsin, but I do like to sail as quickly as the conditions allow. I sail almost 100% single-handed, so the idea of "easy dropping and stowing" is appealing.

I will need a new main-cover in the near future, which ever way I decide about lazyjacks.

Thanks in anticipation

I too sail "Follia Pura" (Centurion 32) mostly solo and I would sorely miss my home-made lazy-jacks-cum-sailcover otherwise known by the trade name "Stack Pack".

What I would miss would be the convenience of dropping the main when entering a harbour where it is not permitted to sail and when going round an anchorage trying to find a quiet spot where I can anchor in relative peace.
Knowing that, while setting the anchor and checking that all is OK or berthing, the sail is safely contained in its pack patiently waiting for me to put it away neatly without blowing all over the place is more than enough to convince me that this is the way to go for a solo-sailor.

 

[JumbleDuck]

It will make 9/10 of bugger-all difference. Go with what makes life easier.

+1. The worst it will do is trigger a laminar-turbulent transition in the boundary layer a little earlier, and that's no bad thing - it's what the dimples on golfballs do.

 

[Robert Wilson]

Thank you both, I appreciate your candour and encouragements! I well appreciate the benefits of having the main quickly out of the way when in a tight-spot.

Looks like I'll be adding more "string"
It'll be a DIY job, but I have a very good, and very reasonably priced, one-man-band who will make the new cover.
Superb new Spray canopy for £500.
Only trouble is that he is so good that it's almost impossible to get onto his schedule

 

[Robert Wilson]

+1. The worst it will do is trigger a laminar-turbulent transition in the boundary layer a little earlier, and that's no bad thing - it's what the dimples on golfballs do.

Well I knew that, of course

Sounds orf'ly painful.........

 

[lpdsn]

+1. The worst it will do is trigger a laminar-turbulent transition in the boundary layer a little earlier, and that's no bad thing - it's what the dimples on golfballs do.

That's interesting. I realise it's your day job, but the discussions I've had whilst racing with a couple of young yacht designers (the sort who do half the work but don't get their name on the boat, so it is their day job too) is that the drag is quite significant. I understood that to be the drag from the line itself less than the effect on the flow on the sail, but when sailing close hauled the balance between lift and drag is rather fine so small factors do make a significant difference.

If you add up all the various things that some cruisers think make bugger all difference you sometimes wonder why they bother putting the sails up at all.

That said, I do have lazyjacks. It's a cruising boat and the compromise is something I'm prepared to live with. I just don't tell myself that it's having no effect. A common way of mitigating it is to take the lazyjacks to the gooseneck - where it is likely to reduce drag but affect the flow over the sail a bit.

 

[Javelin]

Just keep in mind the top third of the sail does two thirds of the work.

 

[lpdsn]

Just keep in mind the top third of the sail does two thirds of the work.

Except for all the bit at the top you effectively lose through tip vortices.

 

[JumbleDuck]

Well I knew that, of course

Sounds orf'ly painful.........

For those that don't know ...

The transition from having a fluid velocity to stationary at a surface happens over a (generally) thin layer called the "boundary layer". The boundary layer can either be "laminar", with nice smooth flow parallel to the surface, or "turbulent" which means the flow is all over the place. Boundary layers start laminar, but if the surface is long enough - which depends on the fluid properties and the velocity outside the boundary layer) - will turn turbulent at some point. Turbulent boundary layers are draggier than laminar ones, but the longer you wait before transition the draggier they are, so if you can't guarantee a laminar boundary layer all the way it often makes sense to trigger a turbulent one earlier.

Aircraft, and most modern gliders, use turbulator tape for boundary layer control:

However, that's for incredibly smooth surfaces. I would have thought that on a woven sail the transition would happen almost at once. I could be wrong, though, and would welcome correction from anyone who knows for sure.

By the way, the very much higher viscosity of water compared to air means, I think, that it's relatively easy to keep laminar flow all the way across keels and rudders, which is why you don't see turbulators on them.

Apologies for the lecture. I'm laid up with flu and bored.

 

[JumbleDuck]

Except for all the bit at the top you effectively lose through tip vortices.

If your sail is vaguely semi-elliptical you should have a vortex sheet behind it, rather than one honkin' great tip vortex.

 

[lpdsn]

If your sail is vaguely semi-elliptical you should have a vortex sheet behind it, rather than one honkin' great tip vortex.

Possible to do but there are very few mainsails like that though on cruising boats. Hard to combine excessive roach with a backstay, even using a wand. It's not even common on everyday racers as IRC heavily penalises it, although I did see a DB 36 advertised a few years ago where the owner had clearly just decided to go for it and had a huge roach on the main.

 

[JumbleDuck]

Possible to do but there are very few mainsails like that though on cruising boats. Hard to combine excessive roach with a backstay, even using a wand. It's not even common on everyday racers as IRC heavily penalises it, although I did see a DB 36 advertised a few years ago where the owner had clearly just decided to go for it and had a huge roach on the main.

Even the triangular shape of a normal Bermudan, especially with roach, should shed a reasonable vortex sheet. It's only aerofoils with constant chord which have vicious tip vortices.

 

[lpdsn]

For those that don't know ...

The transition from having a fluid velocity to stationary at a surface happens over a (generally) thin layer called the "boundary layer". The boundary layer can either be "laminar", with nice smooth flow parallel to the surface, or "turbulent" which means the flow is all over the place. Boundary layers start laminar, but if the surface is long enough - which depends on the fluid properties and the velocity outside the boundary layer) - will turn turbulent at some point. Turbulent boundary layers are draggier than laminar ones, but the longer you wait before transition the draggier they are, so if you can't guarantee a laminar boundary layer all the way it often makes sense to trigger a turbulent one earlier.

Aircraft, and most modern gliders, use turbulator tape for boundary layer control:



However, that's for incredibly smooth surfaces. I would have thought that on a woven sail the transition would happen almost at once. I could be wrong, though, and would welcome correction from anyone who knows for sure.

By the way, the very much higher viscosity of water compared to air means, I think, that it's relatively easy to keep laminar flow all the way across keels and rudders, which is why you don't see turbulators on them.

Apologies for the lecture. I'm laid up with flu and bored.

That's interesting. I've a copy of CA Marchaj's Sail Performance. At one point when discussing laminar and turbulent flow in the boundary layer of sails and the effect of surface roughness he goes on to discuss the effects of localised roughness. In that he concludes that there is a larger detrimental effect from roughness near the leading edge than near the trailing edge. If you can beg, borrow or steal a copy it is on page 101 (Chpt 6) with the accompanying Fig 83. I've the 2003 edition.

In terms of keels and rudders on racers the usual practice when fairing is to put (even) more effort into the foils than the hull and the put even more effort than that into the leading edge and first third of the foil as in most conditions laminar flow beyond that is unlikely. Also to make the trailing edge cut off of the keel assymetric (although I've heard various explanations of that last point).

PS I realise CA Marchaj's work is quite old - I have more recent books but his is the easiest to skim read to find the bit I was looking for.