efficency of partially reefed head sail and direction of furl

smeaks

smeaks

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When sailing with a partially reefed headsail on roller reefing gear the airflow across the sail must get disturbed from the increased mass of the furled portion of the sail. The tack sailed will determine if the "clean" edge entry of the sail is either the front or the back of the sail. Does this make a difference on performance and if so what would be the desired situation? cleaner flow over front or back? I think a furling system (sailspar?) that allows a sail to be reefed either way could prove advantageous in this respect.
 
I'm no expert but my understanding is that the introduction of a foam luff to the furling headsail makes a considerable difference to its efficiency when furled. My new sail has a foam luff, but it's hard to compare since the old was baggier than my gran's old knickers.
 
My understanding (and I may well be wrong) is that the foam luff is there to make sure the sail rolls evenly. Without it the rolls will be fatter at the top and bottom of the luff as the sailcloth is reinforced more heavily at the corners and this means the middle of the exposed sail would get even more baggy with a few rolls on the forestay.

I can see what your saying about having he rolls on the forward (outside) of the sail luff or on the aft (inside) side of the luff making a difference, and I would expect having the rolls on the inside is better. (Having smooth flow over the outside is more important than smooth flow over the inside of the sail.) I might be wrong though and it might depend on how much power is coming from the foresail as opposed to the main. (I have an IOR rig so it's all about the foresail!)
 
a furled headsail is always going to be less efficient than a dedicated jib. you'll find lots of advice about trimming the sail - moving cars etc - but the convenience of roller furling is worth the penalty IMHO. And let's face it, when you're cruising, if you have to reef the sail, you'll probably be going fast enough anyway.
 
As always, you should be looking for the cleanest flow on the lee side of the sail. I guess a bi-directional reefing set-up would do that provided you un-reefed and re-reefed each time you tack. The foam "sausage" commonly fitted along the luff helps by reducing the extent to which the sail's fullness increases as you reef, so helping to keep a flatter sail.
 
An airofoil produces most of its lift on the upper surface, so the step caused by the furled sail will have the largest (negitive) effect when it is on the windward side, a boat with clockwise reefing will point better on a port tack (IMHO)
 
rgarside said:
As always, you should be looking for the cleanest flow on the lee side of the sail. I guess a bi-directional reefing set-up would do that provided you un-reefed and re-reefed each time you tack.
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I imagine the time and distance lost doing this would (in most circumstances) far outweigh the marginal gain in efficiency
 
Windsurfing sails used to be designed as "rotating assymetric foils", a concept introduced (I think) by Neil Pryde. These gave the cleaner flow to the lee side of the sail and the battens flipped over automatically when tacking or gybing.
Later sails were fitted with "camber inducers" at the front wnd of each batten and a wider mast sleeve - these also give the cleaner flow to the leeward side.
Did they help? I don't know for certain but every modern sail uses one or other technique.
 
europe172 said:
An aerofoil produces most of its lift on the upper surface, so the step caused by the furled sail will have the largest (negative) effect when it is on the windward side,
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Would that be the other way round? The leeward side of the sail being the equivalent of the upper surface of the aerofoil.
Also, the leading edge of an aerofoil is not a knife-edge . Perhaps a (smoothly) rolled bit of headsail more nearly simulates the aerofoil. Would it possibly even encourage the airflow on the windward side to take more of a straight line path from luff to leech rather than follow the curve of the sail ?
 
smeaks said:
When sailing with a partially reefed headsail on roller reefing gear the airflow across the sail must get disturbed from the increased mass of the furled portion of the sail. The tack sailed will determine if the "clean" edge entry of the sail is either the front or the back of the sail. Does this make a difference on performance and if so what would be the desired situation? cleaner flow over front or back? I think a furling system (sailspar?) that allows a sail to be reefed either way could prove advantageous in this respect.
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Hi Smeats

Yes I know where your coming from as we lack a few things on the North East coast but wind is not one of them.!

Normally I/we reef a Genoa because of the poor weather, what I find is that there is a huge differance depending on how much the Genoa is furled, 'personally' I sail with about 50% out in poor weather (no main sail) in a (say force 5/6) and it is very surprising just how fast you can go, on my CW I usualy keep her about 35 degees off the nose.

However I also find with the Genoa just 25% out and the main with one reef also makes more comfortable heavy weather sailing close hauled in say a force 5 especially holding a good course, where as just reefed on the main sail only holding course is more difficult.

I have found it quit surprising experimenting with differant amounts of genoa only out and the results to knots and headway, but notice few sail with the Genoa reefed only, on the other hand not many sail in F5/F6 as there appears to be a lot of so called 'fair weather sailors' !!.

Mike
 
Several of the answers seem to have missed the point of the question. As has been said, in ordinary coastal sailing to windward, with each tack you will go from the more efficient arrangement to the less efficient or vice versa. Of course, using a Sailspar system with continuous reefing line it would be possible to reverse the direction of the furled roll, but to get a neat roll wouldn't be possible whilst gong about - there'd be creases and folds rolled in as the sail flogged whilst head-to-wind. It would only be of miniscule benefit if you were offshore and likely to be close hauled on one tack for hours.

The effect is minimal as the protrusion , when on the leeward side, is so small when compared with the path across the sail. I wonder if anyone can tell us how to calculate the span of the wind shadow? It is this disruption of the laminar flow which destroys the drive in the affected portion of the sail.

Rob.
 
Certainly a proper heavy weather jib is far better than a reefed genoa for performance. I would not have a furling reefing jib but for my kind of sailing I rely on my and other's weather forecasting. Most of my sailing is short 2 hr races so windward performance is vital. Only a handful of boats at our club have roller reefing and it becomes clear why when racing. The difference seems to be huge. At least one friend has gone back to separate jibs from roller reefing with huge improvement. But then i don't expect to convert folks on this forum olewill
 
rob2 said:
Several of the answers seem to have missed the point of the question. As has been said, in ordinary coastal sailing to windward, with each tack you will go from the more efficient arrangement to the less efficient or vice versa. Of course, using a Sailspar system with continuous reefing line it would be possible to reverse the direction of the furled roll, but to get a neat roll wouldn't be possible whilst gong about - there'd be creases and folds rolled in as the sail flogged whilst head-to-wind. It would only be of miniscule benefit if you were offshore and likely to be close hauled on one tack for hours.

The effect is minimal as the protrusion , when on the leeward side, is so small when compared with the path across the sail. I wonder if anyone can tell us how to calculate the span of the wind shadow? It is this disruption of the laminar flow which destroys the drive in the affected portion of the sail.

Rob.
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Yes , correct, when the step caused by the furl is on the upper side of the sail there can be no laminar flow over that surface, when on that tack the foresail will produce less power than on the other tack, more importantly pointing will be reduced as flow separation will occur eailer
 
Caer Urfa said:
Hi Smeats

Yes I know where your coming from as we lack a few things on the North East coast but wind is not one of them.!

Normally I/we reef a Genoa because of the poor weather, what I find is that there is a huge differance depending on how much the Genoa is furled, 'personally' I sail with about 50% out in poor weather (no main sail) in a (say force 5/6) and it is very surprising just how fast you can go, on my CW I usualy keep her about 35 degees off the nose.

However I also find with the Genoa just 25% out and the main with one reef also makes more comfortable heavy weather sailing close hauled in say a force 5 especially holding a good course, where as just reefed on the main sail only holding course is more difficult.

I have found it quit surprising experimenting with differant amounts of genoa only out and the results to knots and headway, but notice few sail with the Genoa reefed only, on the other hand not many sail in F5/F6 as there appears to be a lot of so called 'fair weather sailors' !!.

Mike
Click to expand...

Thanks Mike i am looking to see if i can gain an advantage with a sailspar i went from tyne to blyth four weeks ago in the a big westerly and managed to put a 270degree twist into the bottom foil! so its change the forestay and furler time!

regards

steve
 
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