How to know if you need more power or less drag?

[danstanford]

Just got this result in a search for other trim items and it reminded me to reply with the results of our experimentation. In the end we found we had too much forestay sag causing a too deep jib with the aft end of it rounding up into the main. Once I got that under control the boat woke up going to windward.

I have a new boat now but am still wrestling with the topic of the original post. We can get to our polar targets once the breeze gets up over 9 kts but below that we struggle to get to them. We are planning to get the whole team involved in the difference between needing power when we are slow on the starting line or after a tack to escape bad airs and then lowering drag as we come up to speed. Playing the backstay is the one tool we tend to ignore in this transition and so it will be a focus.

 

[shemandr]

I’ve never sailed a J99 so take that into consideration of my reply.
It is a fractionally rigged boat with a bendy mast, I presume. In that case I would employ the backstay to add power or subtract drag quite a bit. I think tell tales on the main a few feet back from the mast would be helpful. If they show stall then there is too much camber in the main and it should be flattened. If the are streaming there is the minimum drag. They will stream even if you are under powered. So you want them on the edge of breaking. When it’s windier they will probably not be useful.
I like adjusting the backstay because it can be done much faster than adjusting the outhaul and it affects the middle of the sail and not just the foot.
And, of course, avoid bad air at almost all costs!

 

[danstanford]

I’ve never sailed a J99 so take that into consideration of my reply.
It is a fractionally rigged boat with a bendy mast, I presume. In that case I would employ the backstay to add power or subtract drag quite a bit. I think tell tales on the main a few feet back from the mast would be helpful. If they show stall then there is too much camber in the main and it should be flattened. If the are streaming there is the minimum drag. They will stream even if you are under powered. So you want them on the edge of breaking. When it’s windier they will probably not be useful.
I like adjusting the backstay because it can be done much faster than adjusting the outhaul and it affects the middle of the sail and not just the foot.
And, of course, avoid bad air at almost all costs!

We have several tell-tales in various places on both main and jib to insure we are not suffering from stall and they are great to show just that as you say. The fraction is quite high and the mast is aluminum so bending the mast is not a piece of cake but for sure the backstay does work in this manner but perhaps less than desired.
This photo has the backstay on full and I will post another photo with the backstay off.

 

[danstanford]

As you can see, the various tell-tales are flying where we need them to be (you have to trust me that the leeward side look the same) even though the leech telltales are diverted a bit to leeward in this photo we do know to keep them in the air.

 

[shemandr]

For the above two photos how was the boat performing relative to the polars and what was the windspeed?
To my eye for each the draft is pretty far forward (Good for a wide steering groove) and a bit deep. Although the tell tales show no hint of stalling t that depth. How well were you pointing?

 

[danstanford]

For the above two photos how was the boat performing relative to the polars and what was the windspeed?
To my eye for each the draft is pretty far forward (Good for a wide steering groove) and a bit deep. Although the tell tales show no hint of stalling t that depth. How well were you pointing?

These were taken before the race as we were poking around making sure we had good trim for the conditions. The boat points quite well though if you look at the photos for the windex you can see we were a bit below our normal beat angles.

I would say we are generally happy with our performance but looking to eke out that last half a knot. The outstanding question for me is how to know when you should be lowering drag and the best current answer I see is when we need to manage heel. The answer may be difficult to ascertain because I want it to be not too nuanced.

 

[colemj]

Here are analyses of those sails if this helps. Stripes are numbered from bottom (1) to top (3).

 

[colemj]

BTW, is that 3di raw? Carbon/spectra or spectra/kevlar? The sails don't show a great difference between hard backstay and not, which is one feature of high modulus material sails built on a mold. The lack of much change in the camber , while slightly moving the draft between the two also suggests this.

It does seem a bit draft-forward with the backstay on, as well as maybe hooked a bit at the top, but looks like it might be the expected designed shape with the backstay off.

 

[danstanford]

Thanks for the analysis Cole! We took these photos to get an idea what these are and I had not managed to find software to do it with.

These are 3Di Raw and we have been surprised we could not get them flatter but that is probably a backstay throw issue. The mast is aluminum and not all that bendy it seems though the rig tensions may also be preventing more bend. We have added all the rake we can and count on the backstay to add forestay tension which it does well.

What are your thoughts as to the camber measurements?

Honestly the draft forward element is mostly a result of our guys hating wrinkles and hauling on the cunningham till they like the look of the sail. This is a work in progress as we try to position the draft in both sails more consistently. The clutches slip a bit which also leads to us putting more halyard tension on initially.

 

[colemj]

I don't really have any thoughts on that, as I am used to catamaran sails which have different draft and camber characteristics. I don't have any experience with monohull sails. To me, the draft is more forward than a catamaran's would be, and the camber in the ballpark for a cruising sail, but probably a bit much for a racing sail.

North should be able to tell you the designed camber and draft, as that is a part of their design software. Anyone there should be able to pull up your file.

With 3di, the draft is pretty much locked in place, so halyard and cunningham isn't going to do much but distort this. Maybe open up the upper leech at most. Likewise, the outhaul will be responsive, but limited in how much it does - think of it as an on/off or 3 position thing instead of continual adjustment. 3di does flatten well with mast bend, so perhaps you need a bit more backstay to see that better.

We also have 3di, and the above is only my opinion on my experimentation with sail controls. I could be wrong about it. You would get better answers from your North rep.

Mark

 

[shemandr]

You would most certainly get better answers from your sail maker than me.
One problem with molded sails is as Mark states. The shape is pretty much as is. If that shape is fast - great. If not you’re more or less stuck with it.
I’m still thinking the sail can be more flat. By loosening the lower aft shrouds you may allow the mid- mast to move forward more when the backstay is tensioned. That normally will flatten the middle of the sail. You have a double spreader rig so it’s more complicated. But that is the concept.
I don’t think that the mast being aluminum governs how much bend can be put in the mast. It is the mast wall thickness, mast diameter and taper that govern bendiness. My Ranger 29, a masthead boat, had a tree trunk for a mast. Backstay added forestay tension but little bend. My Mark 25 had a spaghetti mast which was all about bend.
Both were aluminum.