Confessional: admit to boat-related things you've never quite understood...

[Buck Turgidson]

Surely the problem is that the flow regime sails work in is very different from those an aircraft wing operates in? After all, flow velocities round a sail will be tens of MPH at most. If it gets to 100 MPH, must of us would be under bare poles! Even low performance aircraft are operating at higher flow-rates than that, and that's what your average aerodynamicist will be used to.

FWIW, my understanding was that a rig with two sails will get closer to the wind because the slot effect between the sails accelerates the air-flow across the back of the after sail., thus increasing the lift experienced by the after sail. So, I tend to follow the "lift" way of thinking of how sails operate. I also suspect that flow separation at the leading edge is important in defining the different path-length - I suspect the flow on the windward side of the sail is detached from the sail, and that on the leeward side is attached. But I'm NOT an aerodynamicist!

Bernoulli still applies even at low velocities. delta P may be small but the area is big.

 

[Buck Turgidson]

No, Boeing's and Airbusses. They all use flaps and leading edge slots.

Krueger flaps? Not a slot in sight!

 

[JumbleDuck]

No, Boeing's and Airbusses. They all use flaps and leading edge slots.

Erm, point of information. In my limited experience, Boeings and Airbses have leading edge slats, not slots. Leading edge slats are the equivalent of flaps on the training edge: they principally increase the camber (curvature) of the wing to increase the angle of attack and therefore reduce the stall speed. Leading edge slots are generally fixed features of small specialist STOL aircraft, from the Fieseler Storch onwards. Slats do create slots, but the effect of these seems to be secondary to the camber change.

The "slot effect" for yachts is generally claimed to be an increase in the efficiency of the main sail as a result of flow acceleration by the jib. I don;t think there is any theoretical or experimental justification to support that notion. The aviation world discovered long, long ago that the way to get performance out of wings is to make long, thin single ones, not stack short fat ones together. I would expect that the aerodynamic disadvantages of biplanes - the upper wing pushes the lower wing down, basically - would also apply to yacht rigs. Just look at the AC boats this year ...

 

[Buck Turgidson]

Erm, point of information. In my limited experience, Boeings and Airbses have leading edge slats, not slots. Leading edge slats are the equivalent of flaps on the training edge: they principally increase the camber (curvature) of the wing to increase the angle of attack and therefore reduce the stall speed. Leading edge slots are generally fixed features of small specialist STOL aircraft, from the Fieseler Storch onwards. Slats do create slots, but the effect of these seems to be secondary to the camber change.

The "slot effect" for yachts is generally claimed to be an increase in the efficiency of the main sail as a result of flow acceleration by the jib. I don;t think there is any theoretical or experimental justification to support that notion. The aviation world discovered long, long ago that the way to get performance out of wings is to make long, thin single ones, not stack short fat ones together. I would expect that the aerodynamic disadvantages of biplanes - the upper wing pushes the lower wing down, basically - would also apply to yacht rigs. Just look at the AC boats this year ...

The slats, when used, are generally slotted except in the inboard region where they would like the stall to occur first. Here Boeing used to favour Krueger flaps and airbus now have a leading edge droop on the 380.

And a slat in isolation will reduce incidence therefore reducing AOA. Camber is indeed increased but generally it's the slot energising the the upper boundary layer which subsequently can support a greater trailing edge flap that is why slotted slats are used.

 

[Hydrozoan]

I taught fluid dynamics at university for 20+ years. Not sure if that counts, but I really would like to write something on it one day.

Well it does for me - and I hope you do! But I appreciate that writing a technical book is not to be undertaken lightly, especially if one has other commitments. The first (and worst) I did was a labour of love - but on top of a job it was a bit of a nightmare, too. Pick your time carefully so you can enjoy the process.

In the meantime, as a scientist with but modest maths, I'd still like someone to point me towards a summary of modern sail theory, going beyond the simple (and often wrong, from what Gentry says) and/or tell me if Gentry's is about the best I'll get. Or even just if it's still broadly correct, so far as it goes.

 

[JumbleDuck]

In the meantime, as a scientist with but modest maths, I'd still like someone to point me towards a summary of modern sail theory, going beyond the simple (and often wrong, from what Gentry says) and/or tell me if Gentry's is about the best I'll get. Or even just if it's still broadly correct, so far as it goes.

It's a while since I looked at the Gentry stuff, and I'm afraid I can't remember much about it. My vague recollection was that it was OK, but he was going a bit over-the-top trying to prove something which didn't really matter.

Most of the the books I used are a bit dry and undergraduate-engineer-y, and not great for the casual interested reader. However, I can recommend Anderson's "A History of Aerodynamics" which very nicely covers the development of aerodynamic knowledge and applications in parallel. It's not a very light read, but it's a good and engaging one.

There are some good books in the gliding world, which is pretty close to the sailing world in the flow regimes used. There are lots of copies of Stafford Allen's "Theory of Flight for Glider Pilots", and it has some nice discussions of stability of aerofoils, which is an aspect often overlooked in sails. The grand-daddy of them all is Thomas' "Fundamentals of Sailplane Design" which has very good chapters on lift, drag and aerofoils. Originally in German, there's an English translation available, a snip at forty quid odd.

The problem with the fluid dynamics of sailing is that you are balancing effects on aerofoils (the sails) and hydrofoils (keel and rudder) with all sorts of interesting interactions. Marchaj is definitive on this, but his books are not for the faint hearted!

OK, quick summary. Find a book about how wings work which you get on with, and rotate everything in it through 90 degrees!

 

[JumbleDuck]

The slats, when used, are generally slotted except in the inboard region where they would like the stall to occur first. Here Boeing used to favour Krueger flaps and airbus now have a leading edge droop on the 380.

And a slat in isolation will reduce incidence therefore reducing AOA. Camber is indeed increased but generally it's the slot energising the the upper boundary layer which subsequently can support a greater trailing edge flap that is why slotted slats are used.

Thanks for that. I'd have expected the uper flow to separate long before the trailing edge flaps ... one lives and learns! My practical aerodynamic interest was always in gliders, which have lots of flaps and boundary layer trickery (blowers, turbulators and so on) but nothing much on leading edges, though I think one of the Akaflieg research groups in Germany tried slats on something once.

 

[Hydrozoan]

... OK, quick summary. Find a book about how wings work which you get on with, and rotate everything in it through 90 degrees!

Many thanks for those - but one last question if I may, in relation to the above. As a previous poster remarked, sails are thin and wings are not - does that not make any important difference?

 

[awol]

Thank you for this. I have never found a yacht club in the Solent either a) worth joining or b) open to me joining. As for my brother, he does not exist unless you know different Dad.

I think the message is that finding someone to demonstrate is worth a thousand words of explanation. There are umpteen pages on the web, Youtube clips etc. that cover it but they tend not to show all the many snafus that spinnakers encourage. Getting it right has a satisfaction of its own and if you do it playing with others (commonly referred to as "racing") it can be even more fun.
Is the Solent really so anti-social that there are no clubs that welcome enthusiastic sailors? What a sad place!

 

[JumbleDuck]

Many thanks for those - but one last question if I may, in relation to the above. As a previous poster remarked, sails are thin and wings are not - does that not make any important difference?

Not really. Just think of a wing as two sails with the bit in between filled in.

 

[Uricanejack]

At some point last spring. Someone recommended Norths Book on sail trim. I bought one and found it helped.

 

[onesea]

For those with spinnakers/ cruising chutes with no knowledge of them. I would suggest go racing a couple of times as crew then take some one out with you and go step by step with lots of sea room.

If you are considering doing it with SWMBO only be careful a spinnaker can be an easy way to scare them.

 

[alant]

I'm sure, I've seen a paper which states that the slot effect is a myth.

PS
Uno/windsurfer rigs, seem to manage quite well, without any slot, myth or otherwise.

Found it.
I have a book entitled "The Best of Sail Trim", edited by Charles Mason & published by Adlard Coles (ISBN 0-229-11843-7).
One of the authors, is Arvel Gentry, a research aerodynamicist at Douglas Aircraft.

Some of his papers, include
"How a Sail Gives Lift", "Another Look at Slot Effect", "More on the Slot Effect".
In the latter paper, he has segregated the "major jib-mainsail interaction effects", into " the effect of the jib on the mainsail & the effect of the mainsail on the jib".

"The major effects of a jib on the mainsail are:

1) The jib causes the stagnation point on the mainsail to shift around toward the leading edge of the mast (the header effect).
2) As a result, the peak suction velocities on the forward lee-side of the main are greatly reduced. Since the peak suction velocities are reduced, the recovery adverse pressure gradients are also reduced.
3) Because of reduced pressure gradients on the mainsail, the possibility of the boundary layer separating and the airfoil stalling is reduced.
4) A mainsail can be operated efficiently at higher angles of attack without flow separation and stalling would then be the case with just the mainsail alone. This is caused by a reduction in velocities over the forward-lee part of the mainsail rather than by a speed-up in the flow which is the popular theory.
5) Much less air goes between the headstay and the mast when the jib is placed in the flow with the main. The circulations of main and jib tend to oppose and cancel each other in the area between the two sails, and more air is therefore forced over the lee side of the jib.
6) As the jib is sheeted in closer to the main, there is a continuing decreas in suction pressure on the lee side of the main. When pressures both to the windward and leeward side of the mainsail become equal, there no longer is the pressure difference across the sail necessary to maintain the airfoil shape, and the sail begins to luff.

The major effects of the mainsail on the jib are:

1) The upwash flow ahead of the mainsail causes the stagnation point on the jib to be shifted around toward the windward side of the sail, and the boat can be pointed closer to the wind without the jib stalling or luffing.
2) The leech of the jib is in a high-speed flow region created by the mainsail. The leech velocity on the jib is therefore, higher than if the jib alone were used.
3) Because of the higher leech velocity, velocities along the entire lee surface of the jib are greatly increased when both the jib and the main are used, and this contributes to the high practical efficiency of the jib.
4) The higher lee-surface velocities on the jib mean the jib can be operated at higher angles of attack before the jib lee-side flow will separate and stall.
5) Because of all this, proper trim and shape of the mainsail significally affect the efficiency of the overlapping jib. Anything that causes a velocity reduction in the region of the leech of the jib (such as some separation on the aft part of the main) results in a lower driving force contributed by the jib.
6) The trim of the main significally affects the pointing ability of the boat, for it directly influences the upwash that approaches the luff of the jib.
7) The mast in front of the main-sail always has been blamed for making the main less efficient than a jib.
From my studies, I believe this is only part of the answer. Another, and equally important factor, is the increased velocity on the jib and the fact that its Kutta condition must be satisfied in a local high speed flow region that is created by the mainsail."

Hope this helps.

 

[Spirit of Glenans]

If it's ignorance confession time I'm going to start another topic for discussion.

In Secondary Ports calculations, the Almanac gives you the time differences usually at 1200, 1800 etc for you to interpolate between to find the time difference for the secondary port you are interested in, based on the times of high and low water for the relevant primary port. Sometimes these time differences are negligible and sometimes they are appreciable. For the occasional secondary port where the minutes to add or subtract are large they are expressed as +131 or similar. I have never found anyone who can tell me whether that means one hour and thirty one minutes or one hundred and thirty one minutes!

This is slightly embarrassing as I teach and examine this stuff! However I remember asking a Hydrographer and he didn't know either so that makes me feel slightly better. Furthermore when I did my YM theory exam twenty five years ago, I asked the instructor and he mumbled something and changed the subject. Fortunately in real life, the requirement to use the figures for the very occasional secondary ports that give me this potential problem are so few that I've never found it a problem. I'd like to know the answer though...

To check; do the calculation for a sample secondary port and then look it up on Easytide or a similar website.

 

[dancrane]

CONFESSION TIME...PAINT-STRIPPER

Only kidding. I never chucked the stuff over posh cars in affluent London streets, or used it to blind arch-villains in the style of a Sherlock melodrama...

...but I'm tempted to use it (actually I'm tempted NOT to use it) to rid the Osprey of her paint. That green really isn't my favourite shade, but the paint itself seems good quality...

...so, what's the least I'll need to do to get new paint to key on to the old? If the green is secure on the fibreglass, can't I just roughen the green to give the new stuff a grip?

 

[Thistle]

CONFESSION TIME...PAINT-STRIPPER

Only kidding. I never chucked the stuff over posh cars in affluent London streets, or used it to blind arch-villains in the style of a Sherlock melodrama...

...but I'm tempted to use it (actually I'm tempted NOT to use it) to rid the Osprey of her paint. That green really isn't my favourite shade, but the paint itself seems good quality...

...so, what's the least I'll need to do to get new paint to key on to the old? If the green is secure on the fibreglass, can't I just roughen the green to give the new stuff a grip?

Is the old paint one-pack or two-pack? And the new? New one-pack will stick pretty well to any well-keyed surface but two-pack will not stick to one-pack.

 

[dancrane]

Is the old paint one-pack or two-pack? New one-pack will stick pretty well to any well-keyed surface but two-pack will not stick to one-pack.

Thanks for that, Thistle...although your answer does have me thinking of the cough that'll lead me to the coffin they'll carry me off in.

I've no idea what type the old paint is...if I'd known the man who chose that colour, I'd have reported the offence. To be fair though, the paint isn't flaking or apparently of a poor quality - hence I hope I could just roughen it then paint over...using new one-pack, of course.

 

[Seajet]

Dan,

funny you should mention this as my chums' Osprey which looks the same make as yours has bright blue paint, which is just about acceptable.

It seems to be very tough stuff; one thing I'd be slightly concerned about overpainting it is that the aft vertical faces of the tanks are quite flexible - I had to do a repair and stiffening job due to damage from a previous ownership.

So any paint would not only have to adhere to the old stuff, but be flexible too; not a huge problem, just another consideration.

 

[dancrane]

Thanks Andy. It hadn't occurred to me that modern paint might not be flexible enough for the occasional flexing of dinghy decks, etc.

That aft vertical face of the tank you refer to...do you mean the rear cockpit bulkhead? It's been rather crudely removed from my boat, and I plan restoring it with rigid ply.

I guess Vern's boat is like this one?

View attachment 37813

I found a small unused pot of white International Interlux Super...bought it ten years back then forgot about it. Is that likely to work on the Osprey, do you think?

Assuming I don't lazily cut corners, I'm hoping I can abrade the old paint with sandpaper, clean off the dust (with acetone?) then apply one or two coats - (might need another pot) and say goodbye to the green.

I believe my decade-old paint predates current 'green' regulations, and as such it should be more effective...but if a second coat requires a pot of new paint of a different composition, may I run into difficulties?

 

[Seajet]

Dan,

well yes it's like that but more ' vivid ' blue !¬

The flexing bits I meant are at the aft ends of the side tanks, the vertical inner sections - in this case just before the stern tank.

It's a long time since I used Interlux, TBH I don't remember what it was like to use; probably worth a go unless as you say you need more then matching may well be a snag.

The modern answer being Toplac which I find good stuff but tends to be thick requiring thinning in my experience.

I can only assume the original paint, green or blue, was some sort of early dynamite epoxy !