Jib sheeting angle

[Torchy]

I've been doing some research on this...well, I googled it a few times.

12 degrees seems to be a sort of norm for dinghies ie best compromise for a range of conditions.

Loco has 2 positions around 12 degrees (actually a little under) and 9.5 degrees.

Angles in excess of 12 degrees might be beneficial in very windy stuff...but how much does it depend on the style/cut of jib?

Any thoughts?

[phil_kirk]

I think the horizontal sheeting angle is only part of the equation.  The vertical angle of the sheet to the clew of the sail is also important to control the sail twist and air flow through the slot.

Example the ent rules restrict the sheeting angle mean means it is a much higher angle than the 12 degrees. This restriction is overcome by flat cut sails being sheeted tight to reduce twist allowing the boats to point as high as many other hiking classes.

With the closer sheeting angle in the Cherub it is important to allow some twist in the jib and get that twist right to get the rig working well.  So the vertical sheeting angle is very important.  This is often controlled by adjusting the height of the sail on the luff wire, different clew attachement points and the jib luff tension. With the generally short distances between the sail clew and the jib track any adjustment makes a big difference to the twist in the sail.

If you run a narrower horizontal angle you may be able to set the sail with a bit more twist.

[Torchy]

Thanks Phil...clearly Cherubs are set up to have some leech twist. Keeping control of that as the sail is eased (to get the boat planing upwind) is pretty key I think.

Photo session this weekend hopefully with boat on the side and various positions of sheet attachment, sheeting angle and sheet tension. That way I might be able to interpret what I see and feel on the water.

My suspicion is that an intermediate angle of sheeting might be useful in some conditions...it's easily adjusted using fast pins.

[phil_kirk]

You can cotrol the twist with the sheet tension but it is a very coarse adjustment.  I have occasionally had it too tight and photos of the slot have shown it too closed at the top.   It is difficult to get repeatability if you rely of the jib sheet tension.

[Torchy]

Yes...agreed coarse adjustment - I'm thinking of light weather when we can look directly at the slot and see what it is doing.

Sheet attachment point and sheeting angle are the things to get right.

[Andrew Whapshott]

Hmm.. tried fitting Mirrors?       Could work...

[Torchy]

Took me a while Andy...but I got it! btw the only mirrors I plan to install are rear view to watch the rest of the fleet (in my dreams) 

This little tuning nerd is going to make do with photos taken tomorrow at Bartley...and accept that it may make no difference at all compared to basic sailing skills...and next I need to learn to tame the little puppy

[Phil Alderson]

I have been told that telltails on the mainsail at about 2/3 jib height and about a foot from the mast gives good feedback for the slot air flow.
I have been meaning to get around to putting some on to test this but have not yet.

[kokopelli]

good idea on the tell tales i am going to fit them on my new main ASAP.  As mentioned about Ents. . .  I remember if you were less than 5mm off the setting you could say goodbuy to the rest of the fleet!   (i def dont need mirrors but have been thinking for Pwtheli about a binoclar fixing, as they had on MTBs in the war, so that i can see where you fast boys have gone)! 

[roland_trim]

Hmm.. tried fitting Mirrors? 

Now we are talking - one either side and then we have the world's weirdest trimaran :-)

[Ben Howett]

If you look around any racing lead-mine (and even quite a few cruiser racers now) with non-overlapping headsails you will either see athwartships jib tracks or in-haulers to control the slot. A jib trimmer who knows what they are doing will be using this almost as much as the sheet upwind.

Im not sure the effects are quite as important in small boats though, maybe because the range of movement you need is so fine its pretty hard to get right. Perhaps because dingies are relatively light they don't respond as well to narrow sheeting angles so the range of worthwhile movement gets smaller and the weight and complexity for most boats makes it not worthwhile?

Either way there really isn't any magic numbers for angles of attack or slot width... it will vary from boat to boat and sailmaker to sailmaker.  You should be able to feel the difference if you start playing with it though - Too narrow and your boat speed will plummet - The right amount is generally just before that happens.

[Torchy]

Thanks Ben...and others.

Mainsail telltales - definitely a gem

I looked at the slot on Loco very carefully and documented it all yesterday.

The initial impression is that easing the jib sheet even slightly has a MASSIVE effect on the slot, with the leech opening up by a major amount with the smallest easing of the sheet and with the leech becoming highly curved.

For the photos I 'standardised' sheet position to:
* 'Pinned' = jib sheeted to end of splice (this from experience is approximately where we sheet it in real life when prioritising 'pointing'...ie on a smallish reservoir)
* Eased = The smallest increment I felt I could easily achieve on the water - this was about 2cm and was done by eye (in practice one would ease it by this amount, then pull it in slightly to adjust by eye)

Just in case I'm over-reacting I will post photos but I need to use the other computer

...to control the leech effectively in a boat which deliberately sails off the wind to find speed may need wide sheeting angles - discuss

btw It appeared to me that both jibs I looked at had been cut in such a way as to 'compensate' for this effect ie despite the size of the effect it was remarkable how much the leech appeared to still be controlled despite the clew (board) adopting some 'gymnastic' angles. A sail with a simple eye and no board would have been 'horrible'.

[Torchy]

btw (to Ben) - I think the reason keelboats have finer sheeting angles is to do with 'foil efficiency'

Dinghies sail just as high due to their very efficient foils by pointing lower but get less leeway. An efficient dinghy can point lower (and go faster) but still make as much ground upwind. The ideal angle off the wind is therefore different for each. Keelboats tend to have finer sheeting angles.

[Ben Howett]

Off topic so I don't want to get into a discussion here - But I think assuming all dinghy foils are more efficient than all keelboat foils is a bit of a red herring...

[Torchy]

Agreed Ben...some have very efficient foils but that tends to be the more recent designs

There may of course be other reasons for the differing sheeting angles but from what I can see the 'rule' tends to be keelboat = fine sheeting angle = less than 12 degrees

All pretty major generalisations but unless you have lifting foils in a keelboat they can't be (relatively speaking) high aspect. Look at the foils on an RC 1 metre yacht. Full size it would never get into any harbour in the world.

Interestingly 1 metre yachts tend to have a relatively wide sheeting angle on the jib