Jib question

It would be interesting to get the input of John Harris of clcboats.com since he has the jibs to leeward on his proa Madness. I suppose there is no way for him to put the jib back on centerline to make an equal test.

 

X or not X -that is the question

Eric, I am glad you are still around to discuss your concept. I am disappointed that you deny one very reasonable premise that allows us to prove the inevitable conclusion -that swinging the fore stay to leeward by X does NOT improve pointing direction by X.

I am not saying that the concept is not interesting or has no merit. I am just saying that the claim that it will improve pointing by X can be disproved without leaving the drawing board. I personally hope there is some benefit, because I have the perfect design for it.

So, what is this big X that you think is enough to require a major sail trimming despite the fact that the fore stay and mast are at the same angle to true wind? If X is small then Condition 'A' trim is valid. If X is big there is no rational amount of magic that could make the boat sail up at that angle.

As far as physical testing goes, I had a great idea. The more I thought about it the more I liked it and eventually it got too good to disclose here. I am sending you an email.

 

Show us the drawing please.
BR Teddy

 

As you move the jib tack to leeward, you are effectively flattening the "sails". The pair work as a single unit, so it is depowered.

When you get to hull speed, how do you sail faster? you sail higher. To do that you flatten the sails, depowering them, so the question now becomes do you still have enough power to maintain hull speed?

How high a boat can ultimately sail is a function of its power to drag ratio. --- idkfa

In high wind, you can sail higher, in rough waters you sail lower, in light wind you sail lower.

So the question really is how much said boat was already optimised? How much is she over canvas? Does the sail shape (flatness) match the sailing conditions? Compare to similar boats, speak to brains at sail making shop. Get a carbon mast, move the weight down but don't add any.

 

What is the value of X =the angle that you drop the jib down and turn up?

 

IDKFA, obviously you can reframe the question as an unsolvable mess. And I would even suggest that is the normal state in blogs, but I think there is a lot of talent here and if we frame the question right and work logically we can generate some quality designs right here. To asses the value of this concept we need to isolate it as a variable the best we can to parameterize its performance, then we can consider all the conditions that would or would not benefit from the concept.

You suggest that this concept intrinsically flattens the sail plan somehow -if it is enabled by the tacking forestay (not possible on the centered forestay) we should consider it part of the concept.

About power/drag ratio -I prefer the terms thrust and drag -implies they are both vectors, as they are.

 

I don't turn up but that's not the point.. Can you make your point with a drawing or not?

 

Draw a line from the jib tack to main clew, then the perpendicular to the mast. Now move the tack to leeward, draw lines again. The distance to the mast is less, ie less draft, flatter sail set.

 

I'm agreeing with you Teddy. How is that lots of these fellows think that you will automatically be slowed down by this 'retacking' to leeward of the jib? WHY??

I might suggest that some of these 'experiments' be carried out on multihull craft so we eliminate the extra complicating heeling factors,....plus we get a nice wide sheeting base for our experiments.

I'm going to bring up two multihull examples that I currently know of in each of two different postings to follow this one.

 

Afterburner Racing Cat

Here is a recent posting over on Multihulls@steamradio forum that I ahave been trying to find time to answer. I think it fits in nicely with the dicussions that are occuring here, so I will ask Bill to join in the conversation.



On Sun, Aug 19, 2012
Gentlemen,

Several races ago a titanium link plate on our aft bob stay broke allowing the screacher to sky the prod ripping the bob stay strut and fittings off the forebeam. What a mess! Hind sight review suggests the rigger undersized it, but it did last 5 years or so. Repairs are proceeding, but we have races to do. I got the idea of rigging a big headsail from our bow tip and went digging in the old sail pile looking for a small 900 sq. ft screacher that had come with AB originally. Not cut for AB, it was someone else’s used sail. A jib top I think. Never did much for us and could not go very high set from the prod. We replaced it with a 1400 sq ft screacher 10 years ago. With our best efforts we are unable to carry the big screacher very high. Maybe 100 TWA in 10+ knots. We can go a little higher as the wind gets lighter. The bob stay loads are incredible as we sail high with the big screacher, I estimate 14T+. We have had 10 load related failures to different elements over the years. We use a bigger 2000 sq. ft maxi-screacher downwind, which also creates some big loads when sailed hot.

Saturday we rigged the old small screacher from the leeward bow with spinnaker sheets to the stern. Wind was 5-7 knots TWS, first leg a beam reach. It was short enough for a full hoist. We unfurled it and were surprised by the power. It looked great. I had been sure the sheeting angle would be too small. We were able to sheet in the main (1400 sq ft) a lot and to keep the jib (600 sq ft) trimmed and pulling. We were sailing higher and faster in light wind than ever before. 1.4-1.5x TWS? A later leg had 10-12 knots (flat water no whitecaps) TWS on a beam reach, where we flew a hull doing 14-16, hitting 18 in a couple puffs. Again higher and faster than before.



Our opinion is that we are getting a lot more luff tension, less tack flex, as well as less luff sag from the smaller sail. That the small screacher is acting like a big flap accelerating wind across the back of the main. It and the jib do not seem to bother each other or the main. (We don’t double slot the big jib with headsails on the prod as they do seem to interfere. Better with our small jib on inner forestay for a bigger slot.)

Question for the experts is does all this make sense?

The combined sail area of the jib and small screacher is more than the big screacher.

It seems to fill a weakness in our light wind-hot offwind arsenal.

Though it is not an easy thing to tack.

Bill Gibbs

Afterburner catamaran

 

Moveable Tack(s) Sailing Rig on a Cruising Cat

Have a look at this gentleman's experiments with adjustable tack positions on his rig.

"Catbird Suite has 4 sails, so far, all furling, including 2 genoas (one off each bow), a staysail and a mainsail. The staysail and mainsail are on soft furlers (have no luff spar and are thus easily moved) and each has three positions, i.e., the tack of the sail can be attached on either side deck or on the centerline of the boat."

Go to this home page, then click on A-frame rig:
http://www.damsl.com/
Catbird Suite

also here:
http://www.boatdesign.net/forums/boat-design/wishbone-sailing-rig-1999-11.html#post484935

http://www.boatdesign.net/forums/boat-design/wishbone-sailing-rig-1999-12.html#post485560



Think outside the box.

 

What box though

the results are summarized as

" The rig was completed in December, 2006. It works very well, is quite efficient upwind, is especially good reaching and running, and is easy to handle because all sails furl and there are other advantages as well.........""

not something like ...

"This has produced phenomenal results that are 15% more efficient than setting the jib amidships. Documented results over 12 months show ...."

A catamaran would be the ideal platform to demonstrate the concept.

Are Tornadoes restricted to centrally place jibs I wonder ?

 

Agree..
Thou there is also the factor of increased weather helm which cannot be eliminated (this is one thing I was trying to fish out from the other participants earlier in this thread) This affect can have a positive or negative impacts depending on the lead, sail balance and rudder confiquration..
BR Teddy

ps Ooops.. Sorry corrected the lee to weather..

 

Another case for using multihull platforms, as without heeling there is not the neccessity to provide 'lead'....almost anyway.

 

In this case non heeling platform doesn't help much becouse part of the sailplan is of the centerline so therefore weather helm. Thou not more than reaching..