Jib question

Eric,

I don't know... Intuitively there's something wrong with that reasoning, but if you say that it has been measured and confirmed, I have no arguments to oppose. Some analysis (or better, real-world measurements) should be done anyways before jumping into conclusions, imo.

Let's take it to the extremes - move the jib tack by, say, 30°-40° leewards. What happens, the boat can improve it's close hauling angle by 30°-40°? I don't think so. For some boats it could mean going straight into the wind. Or perhaps I have again understood your idea wrongly (which is perfectly possible)...

There's the underwater part (hull and keel) to consider too, and the equilibrium of forces and moments. I believe that the final result of the operation you have described would be a net decrease of boat forward speed and of the angle into the wind. But it has yet to be seen whether the combination of the two, which makes the Vmg, would be any better.

Cheers

 

me either , I was actually referring to your comments :-

I was merely clarifying your assessment of Robs (and your) 'genius' level. More like 'keen observer' and 'clever innovator' perhaps ?

 

My possibly poor reading of that diagram does indeed show an improvement in heading, but at the expense of a big increase in sideways drag and reduced speed.

Instinctively, this is what I would have expected.

Is that how others interpret it ?

 

Headsail tack track

Ross Turners Jarcats are all fitted with headsail tack tracks as part of the forebeam.

 

Ok, I think I've got it now. Makes sense for small angles of tack swing, but is not intuitive at first glance. It is important to correctly visualize what happens with the keel/hull and how does it's path relative to the true wind change after the sails have been trimmed to a new position. However, I would like to see the real-life data about gains.

Attached is a pdf file with a raw sketch of the speed vectors involved. The numbers are rough and serve more as a qualitative indication. The important thing to note is that the AoA of both the keel and the sails can remain essentially constant in the process, as well as the true wind speed. The hull speed relative to the water will decrease as the angle of swing is increased, which is the limiting factor for the usefulness of this system. The hauling angle to the wind is indeed decreased by the amount equal to the leeward swing of the jib tack, but the increase in the Vmg is uncertain. The gain might be just apparent, due to a decrease of the angle to the true wind, but not supported by an improvement in the Vmg. In the attached drawing, the Vmg has actually decreased by 10%, but again - this is just a geometrical approach to the problem. A more in-depth analysis by someone who has time to do it would be welcome.

Any comments? Paul B?

 

There's another advantage too. The tack moves closer to vertical gives more drive..

 

Daiquiri, thanks for that; I think that is a pretty good explanation. It will be interesting to see what others can do with boat tests of their own.

Eric

 

Have you forgotten that the pointing angle is the sum of the aerodynamic and hydrodynamic drag angles? To really improve pointing, moving the jib tack would have to reduce the aero drag angle with as many degrees... I don't think it will.

 

I am aware of that. The aero drag angle, once the boat is turned and the line of the rig (jib to main) is back in position, should be pretty close to what it was before. But the boat itself and its keel is pointed a little higher, and so the hydro drag angle will likely be less. With less hydro drag angle, there is less angle of attach on the keel, so the keel lift is going to be less, but the demand to balance loads is also going to be less.

As we have said, we haven't done the full calculation work up on this yet, so it's a bit of guess work, we have to admit. That's why we would like to see people experiment with this idea to help document how well it works. I have seen this effect on other boats, but I don't have my own test boat and so have not had an opportunity to document it in any kind of a scientific way.

Eric

 

Imho, the water speed will be lower because the drive component of the aerodynamic force will be smaller. At the same time the sideways force component will still be large. So the keel will have to work at a somewhat higher AoA to give the required counterbalancing sideways force, giving more induced drag. So you have less drag from the hull because of the smaller water speed, but, at the same time, a higher induced drag from the keel. Which one wins? I don't know.
It is possible that at the end you get, yes, a higher pointing angle, but also a smaller Vmg. And the Vmg is the only judge when close reaching.

 

Well, at the risk of being reamed by Paul (I usually deserve it), Ill give it a go-

One objection I have to the movable headstay idea is that it increases the shroud tension. Lets say we increase the shroud angle to return the shroud tension to its jib on centerline value. We don't gain as much sheeting angle as we might think. If you run 3,4,5 spreaders, then the spreaders don't grow as much, but how many craft running rigs that refined are performance limited by sheeting angle issues in the first place?

If a 20 degree headstay was tacked 5 degrees downwind with 4000 pounds tension, the windward cap shroud would see nearly 700 pounds more tension at a 10 degree shroud angle. That suggests an entirely different panel layout for the mast in addition to longer spreaders.

The result is that the bottom of the jib could be sheeted tighter, but the top would end up with worse shroud clearance problems than with the centerline arrangement. Details depend on the specific shroud geometry and on the performance potential of the craft. Hull speed/ wind speed is probably the most sensitive measure for performance potential regarding this problem.

(edit) Okay, first goof. The windward cap shroud would see half that increase. The leeward one would see a decrease. The difference would be about 700 pounds.

 

They really work? ]

 

A smart guy like you with more points than me should be able to easily explain this. So why ask me?


So the backpedalling has begun.

We have gone from this:

To this:

Often times when working a problem or evaluating data I find it beneficial to change the scale. So, as a couple of you have noted, let’s change the scale and rotate the rig to 45 degrees from the CL. Let’s keep the sailplan in the original placement at 45 degrees from the true wind (assuming a boat that tacks through 90 degrees).

The bow is now pointed directly into the wind. Does anyone believe the boat will travel in that direction?

Where is the vector for the drive from the sails? What direction is it pointing in? What is it trying to do to the hull and keel? What do you think the net leeway angle is going to be?

Does anyone believe:

Again, I suggest you think about the reasoning behind the gybing boards in the I14 and 505 classes.

 

In theory.

 

what happens to the sheeting angle i.e the in hauler when you drop the tack to leward?