End Plate effect of lazy jacks

lpdsn said:
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.
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Thanks. I'll have to track down a copy. The concentration on the first third of foils may (also) be because - roughly speaking - the centre of lift is generally about 1/3 of the way back, so the first 1/3 of the foil generates ~2 times as much lift per unit length as the back 2/3 and is therefore ~2 times as much worth getting right.
 
reeac said:
I thought that the Park Avenue boom had lateral tracks for the foot of the sail so that the foot could be set into a prescribed curve..
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It did have lateral tracks. Loose footed mains weren't in vogue then (possibly the materials weren't really up to it) so the alternative was hoops on the boom, or at least something like a foot groove. The main advantage to be gained was in the end plate effect.
 
JumbleDuck said:
Thanks. I'll have to track down a copy. The concentration on the first third of foils may (also) be because - roughly speaking - the centre of lift is generally about 1/3 of the way back, so the first 1/3 of the foil generates ~2 times as much lift per unit length as the back 2/3 and is therefore ~2 times as much worth getting right.
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I'm also trying to read through Bethwaite at the moment. He's less from an academic background ( a Meteorologist originally I believe) but is making great play of the difference between aircraft aerodynamics and sail aerodynamics, in particular the separation bubble caused by the mast. I too am down with a bug and housebound so not looking at it full time, but will pipe up if anything siginificant comes up.
 
Surely the first thing that is going to mess up flow over the sail is the mast itself. It must break the laminar flow right from the start. In addition the stitching & batten pockets must spoil the flow as well. That assumes that the sail is not a one piece moulded sheet as developed by North years ago ( not sure if they went far with it though.)
Re foils below the water line it is quite common to see air rush downward from the waterline of a rudder when it stalls. Dinghies do it, I have seen mine kick a jet of water 2 ft into the air. Cruisers do it in a broaching stall when the rudder lifts & stalls.
I am sure that there were some rudders that had horizontal winglets near the top to prevent air moving down the blade,but have not seen them for years, so i assume they did not work
 
Daydream believer said:
Surely the first thing that is going to mess up flow over the sail is the mast itself. It must break the laminar flow right from the start.
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The effect of the mast is separation bubbles on the windward and leeward side and when the flow re-attaches it is turbulent. With a main it's not so much the difference between laminar and turbulent flow as between attached and separated flow and trimming to maximise the former. The headsail is closer to 'typical' aerofoil.
 
Daydream believer said:
Surely the first thing that is going to mess up flow over the sail is the mast itself. It must break the laminar flow right from the start. In addition the stitching & batten pockets must spoil the flow as well. That assumes that the sail is not a one piece moulded sheet as developed by North years ago ( not sure if they went far with it though.)
Re foils below the water line it is quite common to see air rush downward from the waterline of a rudder when it stalls. Dinghies do it, I have seen mine kick a jet of water 2 ft into the air. Cruisers do it in a broaching stall when the rudder lifts & stalls.
I am sure that there were some rudders that had horizontal winglets near the top to prevent air moving down the blade,but have not seen them for years, so i assume they did not work
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They are in use on a few fast boats, but only to compensate for slight design errors. The problems typically occur reaching with the chute up at over 15 knots sustained. Several Cosairs use them.
 
Only a slight divergence, now that the relative merits of lazyjacks and StakPaks have been aired.

You might consider the question of whether several fat, full-length battens ( or 'full-fat' ) with their long pockets make a useful contribution to maintaining non-turbulent airflow and thus decreasing the total drag penalty for the foil. After all, there are those - and I'm one - who perceive measurable benefits from the use of fully-battened mainsails.

There are numerous fast racing sailboats' classes using full length battens. Surely there is a body of empirical - and possibly theoretic - evidence?
 
Having read posts #16 - #28 late last night, and again this morning, all I can say is that I am overwhelmed by the sciences involved. I thought I'd asked a simple question which would induce a concurrence or rejection, in simple terms, of the merits or otherwise of fitting lazyjacks.

Silly me!

But it has been interesting to read about what goes on around my sail and hull. I shall never again sail away thinking life on a boat is a simple pleasure!!!!:confused:

Thank you one and all :encouragement:
 
thinwater said:
Which means when we reef we lose 2/3 of the drive, which is obviously not true.

I think you will need a little more support for your position. This is not a rule of thumb.
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Not really. The top third is still there, it's just a bit smaller and a bit lower down.
 
Robert Wilson said:
Having read posts #16 - #28 late last night, and again this morning, all I can say is that I am overwhelmed by the sciences involved. I thought I'd asked a simple question which would induce a concurrence or rejection, in simple terms, of the merits or otherwise of fitting lazyjacks.

Silly me!

But it has been interesting to read about what goes on around my sail and hull. I shall never again sail away thinking life on a boat is a simple pleasure!!!!:confused:

Thank you one and all :encouragement:
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You are quite right. It is a simple question with a simple answer. If you value what a stackpack and lazyjacks do for you - the convenience (and safety) of controlling the mainsail when you raise and lower it, then fit them. It will make zero difference to your passage times and reduce your heart rate and blood pressure at the start and end of your sail. If you spend sleepless nights worrying about how you can beat your opponents to the windward mark on Sunday morning racing round the cans, then maybe you will leave them off.

If you want empirical evidence of their value - just walk around any marina or sail through moorings.
 
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Bobc said:
Not really. The top third is still there, it's just a bit smaller and a bit lower down.
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Have you considered a career in politics?;)

As for Mr. Wilson, yes, they are darn handy. No, the effect on speed is not devastating. We know this because if you make them adjustable you can retract them tightly in the the dead air right behind the mast where they can have no effect. The real reason this is sometimes done, how ever, is not speed but ease of hoisting (so they don't snag on battens).

One note on full battens and lazy jacks; the lazy jacks should not extend more than about 65% of the length of the boom or rise above the spreaders. This eliminates most of the snagging, and with full battens, it is all you need.
 
thinwater said:
Have you considered a career in politics?;)

As for Mr. Wilson, yes, they are darn handy. No, the effect on speed is not devastating. We know this because if you make them adjustable you can retract them tightly in the the dead air right behind the mast where they can have no effect. The real reason this is sometimes done, how ever, is not speed but ease of hoisting (so they don't snag on battens).

One note on full battens and lazy jacks; the lazy jacks should not extend more than about 65% of the length of the boom or rise above the spreaders. This eliminates most of the snagging, and with full battens, it is all you need.
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Those comments I can understand and appreciate! :encouragement:
 
I'm showing my age here.... I can remember a YM article (One man and His Boat or similar?) probably in the late 1980s, whereby the late Sir John Oakeley recounted how he'd stopped removing or rolling his stackpack sides as the boat (his Dehler 36CWS) was faster with them and I'm pretty sure he cited the endplate effect. As Sir John was an ex Flying Dutchman National, European and World Champion and skipper of the British America's Cup entry on Lionheart amongst other sailing achievements, I'd suggest an element of credibility to the theory! Mind you, I've never seen an America's cup boat with a stackpack - yet....... :)


Edit: it was the August 1992 edition 'Getting the best of your: Dehler 36 CWS' I think - I searched on Google.
 
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Goldie said:
I'm showing my age here.... I can remember a YM article (One man and His Boat or similar?) probably in the late 1980s, whereby the late Sir John Oakeley recounted how he'd stopped removing or rolling his stackpack sides as the boat (his Dehler 36CWS) was faster with them and I'm pretty sure he cited the endplate effect. As Sir John was an ex Flying Dutchman National, European and World Champion and skipper of the British America's Cup entry on Lionheart amongst other sailing achievements, I'd suggest an element of credibility to the theory! Mind you, I've never seen an America's cup boat with a stackpack - yet....... :)


Edit: it was the August 1992 edition 'Getting the best of your: Dehler 36 CWS' I think - I searched on Google.
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Of course he could've just been psych'ing the opposition. Has anybody else verified his claims in the intervening 25 years?
 
lpdsn said:
Of course he could've just been psych'ing the opposition. Has anybody else verified his claims in the intervening 25 years?
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Doubt it as it is a bit of a non issue. Pretty sure that if those who fit such things found that it had a significant negative affect on their performance of their boat they would take them. Not difficult to work out the benefits and decide whether you value them or not and very difficult to measure any negatives.
 
What does persist, despite the chatter about the benefits of loose footed mainsails, are shelf foot and attached foot sails, even on very high performance boats (MOD 70 etc.). Out haul takes more force and they are more cumbersome to remove at the end of the day, but they do add some end plate effect without significant downsides.
 
Tranona said:
Doubt it as it is a bit of a non issue. Pretty sure that if those who fit such things found that it had a significant negative affect on their performance of their boat they would take them. Not difficult to work out the benefits and decide whether you value them or not and very difficult to measure any negatives.
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They're fitted for convenience. Nobody fits them to get a performance gain. There's just too much extra drag to imagine that any marginal end plate effect will give a net benefit. Lumping on extra drag when reaching doesn't really kill performance but the same drag when close hauled does.

Fair enough if people want to have them. I do myself on my own cruising boat, but I don't try to kid myself that there's not a performance penalty.
 
lpdsn said:
They're fitted for convenience. Nobody fits them to get a performance gain. There's just too much extra drag to imagine that any marginal end plate effect will give a net benefit. Lumping on extra drag when reaching doesn't really kill performance but the same drag when close hauled does.

Fair enough if people want to have them. I do myself on my own cruising boat, but I don't try to kid myself that there's not a performance penalty.
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Have you ever quantified the performance loss? If so, how?

The point I made earlier is that you may be able to notice it against another boat in a one to one situation (assuming you can control all the other variables) but on a passage would not make any difference that you could tie down specifically to that.

As to Oakley's comment, from what I know of him when I was involved on the margins of Lionheart that sounds typical of his gamesmanship approach to life!
 
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