I am building new sails for an older boat and I am trying to improve the boats pointing ability while beating.
With the old jib I often had not enought tension at the topping lift line, especially in gusts the clew of the jib started to rise and then the twist at the leech was too high, causing luffing at the top part of the sail. The jib is mounted at the top of the mast, so I was able to increase the tension at the topping lift with more backstay tension and without some additional mast bending. But I think I need too much tension at the backstay to get the jib under control.
The pivot point at the jib club is now at 14% which might be at the lower end of the scale. I have read about pivot points up to 25% at modern yachts. So I want to move the pivot point at my jib club aft to get a better ratio between forestay and topping-lift-line tension (hope, the forestay doesn’t loose too much tension). Additionally I want to move the jib’s attachement point at the deck aft to get a similar slot between jib and main.
So with a jib slightly backwards to the main/mast and slightly moved to the weather, what effect would this have to the pointing ability (and performance at the jib/main unit)?
Might be marginal, but every bit counts.
The naval architect Eric Sponberg ran tests that indicated the conventional model yacht jib pivot (“teeter totter” fixture with the pivot aft of the jib tack) is exactly wrong for pointing ability. I use a rigid jib stay on my RG65, just a piece of carbon fiber tube with a wire running through it. This attaches to a jib radial at the tack which has a simple vang to control twist. The arrangement points very, very well.
thanks for sharing your pictures. With your configuration I guess in a gust some marginal bending and flexing would point into the right direction to get a high wind condition sail shape at your jib. Well, I have to deal with a 4.4 kg displacement with a classical hull shape. So adapting your design, I might run into some structural problems.
Yes, I had something like this in mind, so with 14% offset it tried the lower end of the scale. But I’ve never seen some numbers, figures which would describe the effect. So I might run into the wrong direction with a 25% offset, but isn’t this offset usual at IOM etc?
I have no experience with IOMs, so I can’t say. Scaling a rigid jib stay up to that size would be tricky. Jib radials, on the other hand, have been used to good effect on Marbleheads.
I don’t think we have much choice in the IOM but to use a jib pivot point as per the rules but putting the rules aside for now and focussing on the question here. I personally found the pointing to come more from the sail twist, mast rake and jib slot. I usually use the jib pivot point to help balancing the boat so the jib and the main work together with the luff breaking together. just my 2 cents.
one thing though I find balance key to enjoying sailing a boat, rc or full size. a gentle weather helm is fine to find optimum pointing but strapping a boat for maximum pointing usually results in the opposite as the speed reduces and the waterflow slows around the finkeel which certainly doesn’t help pointing.
thanks for your reply! I mentioned the IOM because they are so popular and with the usual 25% jib pivot offset the tack is still windward at a close hauled. I often heard, that the windward tack (and forestay) should reduce the pointing ability but I did not understand why.
And with a swing rig in other boat classes … even they reach the windward mark (BTW here around, most new RG65’S are equiped with swing-rigs and an additional attachment for a conventional c-rig for heavy air).
Earl is using a radial jib attachement with his RG65, so the tack remains at the centerline.
I have no doubt this will work in the RG65-weight region.
So I talked to some friends with larger boats and a radial jib attachement. They often had some problems with the jib’s leech tension. With too much twist at the leech and a luffing top section at the jib they had to bear away slightly.
So the same problem as with my 14% jib pivot offset.
When you try to answer this question you have to think of the effect that the jib tack position has on everything else. Let us suppose that 1) the rest of the rig will follow the trim of the jib. That is that the extra 4 degrees of jib angle will result in an extra 4 degrees of main angle, i.e. they will be trimmed together. And 2) the rig will be adjusted or trimmed to the same angle with respect to the wind. Given that , the position of the jib tack is then observed in relation to the hull, more specifically, the hull center line. So the jib tack at 4 degrees above center line in effect trims the hull at 4 degrees below center line. Since the keel is on the center line the keel is now 4 degrees below the center line of the rig. So the jib tack at 4 degrees above center line in effect trims the keel to 4 degrees below center line. In order for the keel to have a high enough angle of attack generate enough lift to counteract the side forces of the rig, the course sailed must be now be 4 more degrees lower. I have tried to diagram it below. Think of it as adjusting the AOA of the keel. Tack to weather decreases AOA!, to leeward increases it.
Some good thought already went into this thread.
But if i am not mistaken Gregg28 you have missed the point of the “offset jib” a bit.
The idea is to set the jib tack to leeward, opposite of what your sketch suggests.
The result is that the angle of attack of the jib to the wind is reduced, meaning that you can correct the boat into the wind to achieve the max possibble angle of attack. Here is a crappy sketch i made
We have two imaginary boats, both are sailing at the same angle relative to the wind.
Boat on the left has the jib offset to leeward, the right boat has the jib in the centerline(red line).
The angle of attack experienced by the jib on the left is smaller than that on the right, if the boats are sailing the same direction.
Now lets assume that both boats are beating as close to the wind as possible(as close to the wind as the right boat can go), both jibs are the same. From here we can see that the skipper of the right boat cant go steeper into the wind, or his jib would become a flag.(meaning that alpha is the max angle of attack for this sail)
The skipper of the left boat can max-out his angle of attack on the jib, by correcting the course more to windward, effectively sailing higher.
The difference between beta and beta’ represents the angle to windward gained by the offset jib.
There are some other things to look at here,
1st is how to trim the main: the angle between the main and the jib should always remain the same. And the main should be trimmed to the centerline or even to windward if needed(hardly possible in the real world)
2nd leeway increase: the resultant side force will grow, leeway is the angle of attack on the boats finns and hull, so wave making and induced resistance will grow, and for those who run very thin finns a chance of stalling your keel will be greater.
The above is the result of thought, i have not tested this yet, but will do on a 750mm national class sailboat(a class unique to slovenia),this class barely any limits so such a radical system can be tested in races.
I don’t think I missed the point. I am saying basically the same thing that you are, but from the other direction. The typical attachment on model yachts, particularly IOMs has the jib attachment at 20 to 25% back from the tack. This puts the tack to weather of the centerline. I was trying to explain why that is a bad thing. The breakback fittings on the old Elmaleh swing rigs allowed you to drop the tack back down to the centerline, an obviously benificial adjustment.
So my starting point was ‘am I moving to the wrong direction’ with a jib pivot offset moving from 14% to 25%.
So Gregg28, your post hit the nail and regarding the right boat in your sketch I should loose some pointing ability.
Well, my boat is known as ‘heeling Regina’ and with the third servo I usually trimm the main to controll the weather helm and heeling angle (or use it to depower the rig). The new jib has 7% draft at 48% chord lenght, so the jib’s luffing should be delayed, compared to the old jib. Had the first test last sunday, but because of the storms these days it was all about ‘how to depower a rig’ :bag:
Your regina look beutiful!, and it is nice to see a full keel boat beeing pushed hard
It is definetly promising to move the pivot point of the jib towards 0%(no real need to go to -20% like the footy above), I am not sure how long the luff on Regina is, but i would go for an “empty” forestay(like on the footy above), it is simple. But you do need more tension in the rigg(becouse the two forestays now share the total tension and the jib luff has to stay tight!
If the luff gets loose and you can’t get more tension from the rigg, i would use a 3mm carbon tube to keep the jib in control.
:reyes: :idea:
