Jib question

The early discussion was about a 10 degree change in the angle of the boat to the wind and I do not believe that is possible with the mainsail in the same position. Eric said in his post that the main would need trimming.
Taking your theory to its logical conclusion if you moved the jib far enough to leeward you could sail directly into the wind.
Force diagrams can be useful however if they only have letters on them then not in this case. As Daquiri says its vmg that is important and you need numbers for that.
Moving the jib to leeward affects the interaction of the sails and the slot effect. There are threads running to 500 posts on that subject with no agreed conclusion, indeed how a wing or sail works is still debatable.
I do not think this can be resolved by discussion, there are too many variables.

 

Dig it Daq.

 

Like I said earlier I'm no expert in this subject, and like to add for me this is only exercise on my spare time no further intrest. Therefore I won't taken any opinion about the amount of angles or what's the difference in Vmg. What I explaine is how I think it works, what are the changes in some of the forces and drags involved. I don't have any desire to start excessive tank and wind tunnel testings nor calculation's with coeffiencies so I won't bring any numbers. If someone want's to prove true or false you are welcome to show your thoughts, but do it with some sense and preferably with something more rigid than my quick drafts.

latestarter, the logical conclusion is the same angle as with a single sail..

Mikko, atleast make your own drawing instead of scan copy paste (no fence)

Slavi, I mentioned in some earlier posts what I think happens to aerodrag and vertical aeroforce hence better Vmg might be possible outcome but dunno, just pictured it in my head

BR Teddy

ps. I know there a couple of drawbacks I didn't mention which might cancel the benefit's but who can point them?

 

the last line is not for you Slavi

 

Anybody tried this and measured performance?
Very little else will convince anyone.
The performance measurements will be questioned, so take some care.
I don't have a boat to use right now, but I would like to know.

 

go back and read the posts on the first page of this thread by Eric Spongburg. No question about his legitimacy, he tried it in a race and proved it works. Problem is most rules prevent it from being used.

 

Petros,

Thanks for reminding me and us.
Why all the debate then?

I guess that is why I quit racing a long time ago. Too many rules. (I also wasn't very good).

You would think all the EC "designers" would pick this up since they don't need to follow such stinkin rules.

 

Overnight I have been thinking why it might work and as Petros points out it has worked.
If you imagine a long bowsprit the angle of the jib is reduced without back winding the main.
The proposal to move the the jib to leeward is equivalent to having the rear part of the imaginary sail that is attached to the bowsprit. See thumbnail

Main one would be tacking and how to shift the jib tack from side to side.

 

Petros, Marc, +1...and Erik, of course.

I did not forget, and figured it all makes sense, as long as it functions...even using a sort of socketed sprit...

I love this stuff.

 

this idea, and many others, that can be explored inexpensively with a hardware store developmental class. I agree about too many rules, and it has become too costly to be competitive, one of my motivations for creating the hardware store class.

 

+1 to that.

 

I am a bit late and certainly outgunned reputationaly, but I do have a gift for logical proof and I aspire to design sailboats so lets see if I can save everyone some time and money by proving this assertion false.

The assertion is that swinging the fore-stay to leeward by X will allow an improvement in pointing angle of the same X degrees and thus likely be worth the extra cost and complexity.

Lets start with the boat sailing to windward with the sails on center (like normal) and the sails trimmed optimally for tacking to windward -call this condition 'A'. This is a new boat (newer than new, its still conceptual at this point) and it has no design flaws to be fixed (sheeting angles are optimal, the rig is capable, and the sails are cut correctly). This is the boat and condition that must be improved for this concept to have merit, agreed?

Now lets take condition 'A' and try to improve it, per the assertion, by swinging the jib to leeward by X degrees and pointing X degrees higher to windward. Lots of angles change (all of them in fact) so I think everyone is getting lost in the calculation. Can anyone tell me what all these new angles are? I think I can.

Lets look at the boat from the winds perspective. In condition 'A' the boat is coming at us at an angle , sails trimmed optimally, and they look like a triangle with a certain height and base. Swing the jib out by X and the triangle gets bigger. Turn the boat upwind by X and the triangle looks to be the same dimensions as it was in 'A'. This is a critical point so lets give it more consideration. We turned the sail out, then turned the boat back, so the true wind angle to the sail plan is the same as in condition 'A', and what are the optimal sheeting angles for wind angle in condition 'A'? By definition they are the same as in condition 'A', but with one caveat, boat speed. So lets start with the condition 'A' trim and see what our boat speed is, then make adjustments like the sailors we are. The true wind on the rig is the same strength and true wind angle as 'A' so the lift and drag would be the same as 'A' and the sheeting would be the same as 'A'. The only difference is now the resulting force is angled X degrees back from the direction of the hull and keel. If you swing the force vector from the sail back by X the drive is reduced proportional to the Sin(X) and the heeling moment increases proportional to Cos(X). (Not looking good for boat speed) On the other hand we have the increase in wind speed due to pointing higher which is proportional to the Cos(X) and also it's resultant drag on the boat and rigging which increases proportionally. For small angles Sin is roughly the angle in radians and Cos is roughly 1. So for small angles the thrust will be reduced proportional to the angle and heeling will be approximately the same. So we turned our sail plan by X turned the boat -X and trimmed the sails and the boat is decelerating due to thrust being reduced by X (in Radians). The keel still needs to produce approximately the same force against leeway as in condition 'A' but the boat speed is dropping below 'A'. The relationship between speed, force and attack angle on the keel is failing -too much force, not enough speed, so angle will fall off -the keel is stalling.

So I think I have proved that dropping the forestay to leeward X degree can NOT improve pointing by X, that still doesn't mean there isn't any improvement. No, but if you follow the method I laid out above on a real design you will find that for any significant angle X the sail will lose so much thrust because of it's reduced angle to the keel that there is no way that an improved slot or sail separation could possibly counter the loss. And that is just the theoretical performance. Practical application would surely result in significant weight increase in the bow (bad for downwind) and/or reduced forestay tension (bad for upwind).

If anyone tells me that they improved their performance to windward by dropping their forestay to leeward, I would conclude that their rig was less than optimal to start with -most likely their jib sheeting angle was too wide for upwind work.

 

You are right, but if one increases the size of the sails and make them flatter. Even thing else can remain the same, then it works.

 

Skyak, I welcome your thoughts and I think your basic statement of the problem is a good summary. I disagree with you on one fundamental assumption, and that is that the sheeting angles of condition A (jib on CL)and B (jib to leeward) are the same. They are not. The sheeting angles will have to change to obtain the best sail trim of each sail in either condition. Also, what I think is happening (and I have not had a chance to go into a wind tunnel and test this myself) is that the relative positions of the jib and the main change considerably when the jib tack is shifted to leeward. Their position, shape and sheeting are all different. As a result, the circulation of the air around the sails changes, and for the better, so that the combination of the sails in configuration B produces more lift overall than it did in condition A when the jib is tacked to the centerline. So the aerodynamic vectors all change in length and direction. You cannot assume that all the forces in condition B are the same as in condition A. The vector diagrams would be totally different, and everything changes.

As for other practical factors related to extra equipment on the bow or a sagging headstay, those are minor issues. There are any number of ways to adjust the tack of the jib with minimal increase in equipment weight (could be done for much less than the weight of a human being on the bow) and such equipment could be configured to keep headsay tension tight. Even if the headstay is a bit slack, sails can be cut and shaped to take that into account so that they can still draw well.

It is interesting that in the research that I am aware of, to my knowledge no one has ever really tested this effect before, and I think this is because we as designers and sailors are so ingrained with symmetric thinking (what is set for starboard tack must be mirrored for port tack) that we are stuck with sails tacked or mounted on the hull centerline. We are also confined by measurement and handicapping rules that don't allow moving of the sails athwarships. So, I welcome others who can obtain direct experience of this effect by testing with their own boats and report back here. All this theoreticall talk gets us only to theoretical conclusions, and I would like to see more evidence in controlled trials.

Eric

 

bigger+flatter=faster

This is the universal solution to upwind sailing -no argument there. So why does it work and what are the limits/costs. By reducing the depth of the curve you are reducing the drag (and to a lesser degree overall power but you are making that up with increased area). Reduced drag effectively swings the force of the sail forward, increasing thrust in the direction you are sailing. Note this is swinging the vector in the opposite direction of the concept we are evaluating. The question is; can the swinging fore stay to leeward improve a boat that has sails that are as big and flat as can be? Or does the concept make the sails any bigger or flatter? In a word 'No'.

And what are the limitations of bigger and flatter? Well the tension on a sail per a set wind load W would be roughly T=Wl/2d so the tension is going up exponentially as depth is reduced and the sail is flattened. And obviously, having the right sail for every wind level costs more.