Slightly undersized jib

[Dutch01527]

I have acquired a nearly new, hank on, self tacking jib for my 28 foot Dehler. My origional one is old and has seen better days. The only issue is that the luff is about 2 foot shorter than the one it replaces (c.26 foot vs 28 foot)

- is it better to connect the tack to the forestay base at deck level or should I add a 2 foot strop to the tack and connect that to the deck. In other words fly it high or low for best performance?

- will a slightly smaller but nearly new, good quality, battened jib perform significantly worse than the slightly taller, baggy, unbattened origional?

I know that the best choice is to buy a new one at the right size and that will be the eventual solution. However I am a cruiser not a racer and the money saved can be usefully used elsewhere on the boat in the short term.

 

[lpdsn]

I guess the deciding factor will be the effect on trim from the clew position. In my (reasonably limited) experience of self-tackers there's not much you can do to affect the sheet lead by much, so if the clew is in the wrong place trim is going to be awful.

Other than that keeping the sail low to the deck is likely to be more efficient (because of the end plate effect).

 

[Dutch01527]

Good point re the clew, I will experiment when I get down to the boat. The clew has a plate about 10 inches long with holes every couple if inches so there is a fair amount of adjustment possible.

I am not sure that the end plate gap is relevant to me if I have understood it correctly, which I may well not have done. The sail needs to be at least 8 inches off the deck to clear the windlass and I think that negates any end point advantage. The racing boats that have used the end point gap have sails sweeping the deck I believe.

My assumption was that wind speeds are higher further up the sail because of wind sheer and therefore there would be a performance advantage in flying the sail higher by using the strop but that is just a guess.

 

[bbg]

But of you fly it higher the centre of effort will be higher, which will increase the heeling.

I think post #2 hit the nail on the head. The most important thing is the lead from the clew. If it were me and I had any choice, I would prefer to fly it lower rather than higher due to the effect on heeling.

 

[AndrewB]

While there is a good argument for keeping it low, sometimes flying it a little higher will give better visibility, and avoid the need to 'skirt' the jib when tacking or hardening up, advantages for cruising.

One thing I am sure about though, a new, decently shaped smallish jib is going to give you better performance than a larger old baggy one, particularly close-hauled.

 

[TSB240]

In addition to the other posters . Flying the ST jib higher could have the effect of closing the slot or back winding the main. I have found that our old baggy self tacker has to be very carefully sheeted to avoid this. I am looking forward to a new overlapping Genoa that has been designed to be reduced down and used on the self tacking track with a shortened padded luff up wind and rolled out fully for off wind. It can only be better than the 12 year old sail that was original supply by East Sails.

 

[anoccasionalyachtsman]

With a self-tacking jib the correct foot length is far more important than the luff length. Normally it's best to have the sail as low as possible to reduce heel and the gap at the bottom, but getting the right sheet position is more important here.

Make sure that you've got at least three pairs of telltales on the luff and get out there and put it up at the bottom with the sheets attached to a mid-position on the clew plate and see how it looks close-hauled. Play with sheet position, but you may have to resort to a strop if the foot is substantially shorter. Don't worry about anything other than upwind - as you already know, self-tackers need extra sheets off the wind.

 

[dom]

I guess the deciding factor will be the effect on trim from the clew position. In my (reasonably limited) experience of self-tackers there's not much you can do to affect the sheet lead by much, so if the clew is in the wrong place trim is going to be awful.

Well, there’s sometimes a little flexibility. In terms of the horizontal plane (slot/angle of attack) one can sometimes adjust track travel; in terms of the vertical plane many self-tacking jobs are fitted with a cutback clews to allow for multiple cringles.

Notwithstanding this, I think your ‘not much you can do’ comment still stands if performance is in the mix.

 

[Dutch01527]

Thanks for the reply’s. I think that I have a cunning plan.

I have fitted a down haul and the thought occurs to me that gives the option to fit it to the clew and experiment with a variable jib height easily to try and work out the best height in various condition. Combined with moving the clew positioning I should be able to work out what works best in different wind strengths and directions. If there is an appreciable difference I could keep the downhaul as the point of clew connection and vary the height at any time if that gives advantage. The downhaul runs back to a location convenient to the genoa self tacking winch which will not be being used when using the self tacker. Obviously I would loose the downhaul function but I can live with that.

 

[lw395]

Sheeting angle will matter.
Start by aiming for the line of the sheet, projected up from the clew, to hit the luff about 55% up.

It's easy enough to try tack strops of different length.
Fit some tell-tales 1/3 back from the luff and also along the leech.
Be prepared to adjust main settings to get the best out of the two sails together.

 

[bedouin]

A major part of the function of the jib, is to manage the airflow over the Main. In that respect it may be better set higher up rather than lower down, and the sail itself will perform a bit better for being higher. However as others have said the sheeting angle is likely to be the deciding factor - and that will almost certainly mean you set it at the bottom

 

[anoccasionalyachtsman]

Sheeting angle will matter.
Start by aiming for the line of the sheet, projected up from the clew, to hit the luff about 55% up.

It's a self-tacker, so the sheet is almost vertical - which is why I said that foot length is so important.

 

[dom]

It's a self-tacker, so the sheet is almost vertical - which is why I said that foot length is so important.

How can it be vertical given that the load-lines are nowhere near?

 

[anoccasionalyachtsman]

How can it be vertical given that the load-lines are nowhere near?

That's the problem with self-tacking jibs. The track has to be in front of the mast and on top of the coachroof so there's no way you can arrange the sheet by the classic 'aim half way up the luff ' method.

Edit. Which is why the sailmaker will measure the boat himself, and have to fit a huge clew board in case his guess at foot length is a bit out.

 

[Dutch01527]

Sheeting angle will matter.
Start by aiming for the line of the sheet, projected up from the clew, to hit the luff about 55% up.

[/I had read about 40-45% up i.e: just below half way up in this excellent article. https://www.sailmagazine.com/racing/headsail-sheeting. The article also indicates a foot length of 70-80% which mine is.

Given that my self tacking track is on the coach house roof that would mean flying the sail high, maybe 3 foot above the deck which has some attractions in improved visibility and higher wind speeds as long as the sheet angle is right and heeling is reasonable. I am taking the sail over to my local sailmaker to have it laundered and a batten replaced so I will ask his advice as well.

 

[dom]

That's the problem with self-tacking jibs. The track has to be in front of the mast and on top of the coachroof so there's no way you can arrange the sheet by the classic 'aim half way up the luff ' method.Edit. Which is why the sailmaker will measure the boat himself, and have to fit a huge clew board in case his guess at foot length is a bit out.

The sheet may be very short for sure, but once the jib loads up it will definitionally have to align itself with the sail load lines. The large clew board with multiple cringles is really there to try and get these load lines right in the first place.

 

[lw395]

I did mean 55% up, it's a number I've used. But I'm not that confident in it for a 'blade' jib wth a short foot.
As the sheet is quite short, small movements will change the angle quite quickly.
I don't believe there will be one magic number, for instance in RS800's they alter the angle according to conditions. (As an aside they mostly do this with several sheeting positions in the clew board.)

You have to have sheet tension, angle, pointing and mainsheeting all working together, and if you change one, e.g. to pinch or foot or power through chop, the best settings for all the others will change.
So I was just suggesting a ball park to start in. Then refine from there.
If you start at 55% up, then you'd be fairly sure downwards was the direction to move in.
Maybe work out a couple of settings, 45 and 55%, then try each with a range of sheet tensions and track adjustments. Then try small changes.

What can be misleading is to start with things really grossly wrong, because then small changes sometimes don't give the expected results.

When the sail is set, do look at the leech from behind and see if it's consistent with the main shape. Not too close low down and far away high up or v/v.

Some tell tales on the main say 40% back may help too.
I just use a bit on knitting wool stuck on with a square of spinnaker tape.

 

[anoccasionalyachtsman]

The sheet may be very short for sure, but once the jib loads up it will definitionally have to align itself with the sail load lines. The large clew board with multiple cringles is really there to try and get these load lines right in the first place.

On a boat with a self-tacking jib, and where the object is to maximise sail area in the foretriangle the designer extends the foot as far aft as he dares, and places the track as near the mast foot as possible. By very careful measuring it's possible to have the clew almost over the track, and by putting a proportionally larger tension in an almost vertical sheet the leech is pulled straight down, leaving the curvature of the foot controlled only by the skillful cut, and with almost no tension in it from the sheet.

On a boat with a lower priority on performance the sailmaker's job is a lot easier - but because the track has to be in front of the mast there will almost always be more tension in the leech than you'd get with a conventional sheet. Of course, a taller coachroof or taller supports for the track will reduce that.

I knows a bit about this because I did the sail trials for a new model and was stuck twixt designer and sailmaker.

 

[lw395]

On a boat with a self-tacking jib, and where the object is to maximise sail area in the foretriangle the designer extends the foot as far aft as he dares, and places the track as near the mast foot as possible. By very careful measuring it's possible to have the clew almost over the track, and by putting a proportionally larger tension in an almost vertical sheet the leech is pulled straight down, leaving the curvature of the foot controlled only by the skillful cut, and with almost no tension in it from the sheet.

On a boat with a lower priority on performance the sailmaker's job is a lot easier - but because the track has to be in front of the mast there will almost always be more tension in the leech than you'd get with a conventional sheet. Of course, a taller coachroof or taller supports for the track will reduce that.

I knows a bit about this because I did the sail trials for a new model and was stuck twixt designer and sailmaker.

That sounds like a specially constructed sail, built to carry a great deal of tension in the leech.
But even then, as soon as you crack the sheet, the clew will move forwards and the angle will change.
Surely also pressure from the wind will make the sail fuller, which isn't going to be helpful?

Was thiswith a straight track, radius tack or some other cunning shape?

 

[anoccasionalyachtsman]

That sounds like a specially constructed sail, built to carry a great deal of tension in the leech.But even then, as soon as you crack the sheet, the clew will move forwards and the angle will change.Surely also pressure from the wind will make the sail fuller, which isn't going to be helpful?Was thiswith a straight track, radius tack or some other cunning shape?

Not a particularly special sail, but this is one of the inherent problems with them. And yes, the whole thing is setup for that beautiful 12kt beat, and as soon as you ease the sheet you lose leech tension and they twist too much. The few owners that care about it rig a pair of conventional sheets for offwind. Yes, I know it obviates the point of the things but if that 12kt beat is up a lovely river, there's nothing better than a s-t jib.

Oh, and you didn't mention roller reefing - that sucks too! A ton of foot tension and nothing on the leech. If an owner also bought an overlapping genoa from us it did come with a mark to which it could be rolled and also used as an s-t, so that helped them a bit.

The track shape doesn't make any difference to the sail's set, but it's critical to the ability to actually 'self' tack. we had curved on our boats - choosing the radius of the curve and the angle from vertical of the plane it lay in was indeed cunning! (But not always convincingly right).