Airflow over me genny.

[Silkie]

I've recently started to notice (when sailing in a fresh breeze) that a significant flow leaves my genoa in a downward direction. This can be seen from the little ripples it makes on the sea off the leeward bow.

I know that some racing sails can sweep the deck to channel this air so is this just an unavoidable effect of having a high-cut genoa or could Silkie's performance be improved by trimming the sail more efficiently?

[claymore]

How close is the tack of your genny to the foredeck?

[ash]

Could Silkie's performance be improved by trimming the sail more efficiently?

I believe that the deck sweepers are to maximise sail area, rather than to channel the air. My larger genoa sits close to the deck, but the smaller one is on a strop so that the foot clears the pulpit.

Airflow likes to stay straight, and take the line of least resistance so the flow over your leech must be restricted.

Do you use telltales to set the position of your sheet block? Might be worth fitting some extra on the main body of the sail to monitor the airflow.

Is your leech line too tight? causing a hook

You mention fresh breeze - Is the genny party furled? Does it go baggy when reefed?

I would tend to set the sheet block further aft in strong wind, to tighten the foot and allow the top of the leech to open up.

I've got a couple of good books on sail trim but they're up on the boat!

I won't ask if the genoa is old and baggy!

Good luck

Ash

[little boy blue]

jenny and no 3 jib for hurley 22 on ybw sale forum.

[Silkie]

A couple of pics, the first fairly close-hauled in a gentle breeze and the second reaching in very light air:





The tack is as low as possible without modifying the straps under the furler (about 15"-18" off the deck I suppose) but with the cut of the sail and Silkie's sheer it must be over 3' at the clew.

I do have tell-tales but they could probably be more usefully positioned. I recently tightened the leech line because it was too loose but it's not too tight.

The sail is probably fairly old and baggy. I only notice this effect when there's plenty of wind so she'll also be well heeled (reefed or not) with the foot that much closer to the water as a result.

I've also got some good books on sail trim but seem to have difficulty putting the advice into practice. I could really do with a few hours sailing (in the right conditions) with a guru who could put me on the right track.

Looks like a decent breeze this weekend so I'll put some serious effort into the trimming and see if I can't do any better by the old gal.

[ash]

Silkie's furling drum does appear to be higher than usual, but I don't think that's relevant, anyway seeing to leeward can be handy! The furling system should probably have had another part section of foil fitted. Looking at your top photo, the head of the sail seems to be well short of the diverter, again not relevent at this time but there could be a danger of the halyard wrapping around the foil. These two things only mean that you're selling Silkie short on luff length and hence sail area, but maybe she doesn't need more.

Having the clew high means more sheet between the clew and the block which makes the block position less critical, but means that you need to use the aft part of the track.

If you take your second picture, and draw a line along the sheet and extend it to the forestay, it should be halfway up the luff. In very light airs it should point higher.

The UV strip may be taking all the pressure from the sheet, and prevent the sail material being tightened as it should.

How much do you trust your autopilot? Enough to allow you time to lie on your back on the foredeck, and take a photo looking up the inside of the sail?

Ash

[Silkie]

The drum is mounted above a couple of SS strips to accommodate a bottle screw I think though most rigs don't seem to bother with such adjustment.

The sail was higher up the foil originally (with a 10" strop between drum and tack) but I always felt that I couldn't get the sheet lead far enough back and so removed the strop. Looking at the uncropped version of the first pic here I think the lead is too far back with the car at the end of the track. I've only had the halyard wrap up once and that was because I tried to roll the sail away without any tension on the halyard!

The UV strip is now pretty much sacrificed and regularly acquires a new split when the wind gets up so I don't think it's restricting the genoa too much!

I rarely feel that Silkie is under-canvassed so can probably live without a maximum luff sail. I'll try to get some pics this weekend.

Dave

Thanks for the detailed replies BTW. Are you sailing this weekend?

[ash]

Are you sailing this weekend?

Yes, Joan and I are planning to go up on Fri evening and return on Sun evening. Looks like a nice weekend, weather wise - about time.

I would agree that keeping the drum clear of the bottle screw is worth while.

My drawing on your picture puts the extension of the sheet onto the luff just where it crosses the skyline.

Ash

[ash]

BTW - Chased your link to Photobucket, but didn't get to see your album - assume it's not set up for public display.

I've only just started my album on photo bucket - search on

ashley-smith-technical-leisure

or follow links on website - see profile

Ash

[Silkie]

BTW - Chased your link to Photobucket, but didn't get to see your album - assume it's not set up for public display.

No - it's not really a proper album. I just use it as a host for pics I want to show elsewhere - forums, the blog etc.

Off up the road now. Have a good weekend.

Dave

[Swagman]

I was always taught that all winds come at you in a angled but downward direction. Out of the sky if you like - as opposed to flowing along the water level.

I was told this is why an rear inclined foresail is so efficient going to windward - and why it makes sense to let your mast go lean foreward when scooting off downwind.

I've never doubted this advice as I've seen plenty of gusts hit still water and spread out in differing directions.

So whilst a lower foot headsail may capture more air and perhaps direct more of it aft - I suspect you'll always see an element of wind passing the boat in a downward direction.

Cheers
JOHN

[Silkie]

Thanks for the advice John and good timing too since I did some experimenting last weekend and was just about to post the results.

Spent many hours tacking into a gentle breeze over flat water and did a lot of fiddling with the sheet lead while watching the tell-tales. The first picture above was confusing me since an extension of the sheet hits the luff about 40% of the way up it indicating that the lead is too far aft and this ran counter to what I thought was wrong.

However Silkie is not close-hauled in the picture and the situation changes significantly with the genoa sheeted in hard. The clew comes much further aft, the lead is very much steeper and as a result the leach is too tight and the foot too loose. This is a DEFINITE conclusion!

But what to do about it without spending any money, that is the question?

I feel a new thread coming on.

See DIY stand-up blocks

[Swagman]

If you fit a line so you can haul the lead blocks forward when you need themt there, and ease off to allow them to move aft when you don't - then you can trim the sail properly on each angle of sailing.
That's surely the cheapest solution to the trimming issue?
Cheers
JOHN

[Aja]

Your problem is not uncommon. If your sheet block is at its farthest travel aft all you can do is lower the height of the clew above deck.

This will tighten the foot and open the leech slightly. This is what you want when hard on the wind in a breeze. This will reduce the draught (in conjunction with tension on the halyard) and allow better pointing and more power. The opposite is tru in lighter winds. Move the car forward and release tension on the halyard which will give more shape to the sail.

One way to lower the clew is to move the whole sail down i.e. lower the fixing point at the tack probably by using a smaller chain plate/bottlescrew set up. This will allow you to use more travel aft on your sheet block.

You may have the height to allow you to get your anchor over the bow roller without snagging.

Hope this helps

Donald

[Silkie]

Thanks for that Donald. All advice (even from that smartrrse Claymore) is gratefully received.

..all you can do is lower the height of the clew above deck.

In the end I achieved the desired result through raising the turning point of the jib sheet by 4 or 5 inches by mating a single block to the old bullseye slides.

See DIY stand-up blocks for more detail.