Upwind sail trim

[Jimski]

This is a shot of us sailing upwind close hauled, boat speed 5.5 knots. We were pointing pretty well. Wind was probably about 12-15 gusting 18. Can anyone tell me why there is a bubble in the luff of the main? Should I trim to remove it and if so how? I thought we were trimmed pretty well.

This picture was taken in a race this past January. Yes, we were wearing shorts - this is Texas. We were about 20 yards from the finish line.

 

[kappykaplan]

The airflow coming off the jib is causing the bubble. Trim the main sheet as tightly as possible, and use the traveler to bring the boom to the middle of the boat.

 

[Jackdaw]

In that breeze its fine. It called a 'speed bubble'

Its cause by the genoa backwinding into the main. If you were happy with boat speed and angle of heel, than you were good. Looks like you were, or close. If you had a few degrees of heel to give before your optimum angle, some traveler UP would have lessened the bubble, helped pointing, given better speed. But with more heel.

 

[shemandr]

On the Ranger we called it a Ranger Bubble. Not a bad thing. It's the 70's style sail plan. The main is acting as the steering aileron. It's not so much about drive. The jib is doing that. The main is steering the sailplan. To get rid of the bubble you would have to raise the traveler up so high as to stall, and create drag with the main. If the leech tell tails are flying you're good. The sail plan works fine with the bubble.
If you've sailed a Nonsuch, with the wishbone boom, or similar rigs, you know that the turbulence created by the fat mast is somewhat mitigated by the flat exit of the sail plan. After watching this rig for 24 hours or so, I came to understand that it was what happened after the bubble that drove the boat. From the bubble looking aft, the sail had a pretty good shape. Not classic sail theory but no one's ever been able to 'splain classic sail theory to me.

 

[Jimski]

Thanks for the replies. I am apparently over analyzing it. I can't complain about performance - we did finish in 1st place in cruising club class in that race. I am still learning and this bubble didn't make sense to me, thought I might be able to improve.

On a related note, what upwind boat speed do people use? I try to keep boat speed at 5.5 knots upwind. At that speed we seem to point as well as most cruising boats and better than some.

 

[Bill Roosa]

The entire top 1/2 of the main is "in the bubble" I'd recommend that you reef the main and power it up with more chamber (less clew out haul tension). The bubble is backwinding from the jib, the jib is providing most of the drive so I'm thinking you are getting a lot of lee helm. You want weather helm for fastest speed.
In ALL cases the 3 degrees of weather helm tells you everything you need to know about sail balance. less than 3 degrees of weather helm (3 to 0 to lee helm) means the jib is over powered or the main is underpowered. more than 3 degrees of weather helm means the main overpowered or th jib is underpowered. Lean all the ways to power up/down the sails and most of this sail "theory" gets really easy. It is all about balancing the sail plan!

 

[weinie]

Crack the jib off a hair and see if your speed improves. The jib will be slightly under trimmed but the main might be more efficient without the large bubble letting you point higher. Experiment and see. When I used to sail a masthead boat with a 150 genoa, I would often have to do this.

 

[jeepbluetj]

I'm about as far from a sail trim expert as you can get. Jackdaw knows more about sail trim when he's asleep than I do paying real attention. I also don't race my boat, but I'll bring beer to anybody that needs crew for a race....

That said, on my C-30, I'd have the boom a little more up towards the center. Maybe a reef (as the C-30 is a fat fat fat boat and heel slows it down). I'll see 6.0 to 6.3's close hauled in 18+ kts of wind. But I'll bet I'm not pointing as high. (New full 4 batten cruising main, blown out 130 and 150 headsails). I also find with the boom pretty much in the center I can control heel quite well with adding/removing twist on the main. I think that has a lot to do with the full batten sail - twist really changes the performance of my boat. My C-30 has the jib tracks outboard, and that limits pointing. I really should try a barberhaul sometime to move the sheet more inboard to improve pointing.

Speed I aim for? Whatever we can get Went to Catalina once just cracked off a bit from close hauled and never dropped below 6 - rocket ride with some south wind (big swell though). That is not usual for my area. Typical light air around here I'm happy with anything over 4kts at wind speed around 10ish. Makes a pretty big difference as to when the bottom was cleaned too.

 

[Jackdaw]

The point about target upwind speed is a very important one.

For any given true wind speed, knowing how fast you should be going when sailing in upwind mode is key to good racing. And while the best place to get this info is from your boats polars , you can often create your own chart based on observed performance. We keep easy-to-read versions taped all over the boat for crew to see.

It really helps because while reading a true wind angle of 38 vs 36 is hard to do, speed is easy to read. And if you are somehow going faster then your target in a particular wind speed, it can only mean ONE THING. You're not pointing. You're footing.

 

[svquest]

I don't really see much wrong with what you've got. Speed bubbles are no big deal. The basic on a sail plan is the luff of the jib/genoa and the leech of the main do the most work. From a distance standpoint how close were you to Ben (Flyer)? Since y'all both have tall rigs, dig thru the Lacy's photos and look at his boat. You'll rarely find him with bad trim.

As some of the folks have mentioned you could use the traveler to pull the main up to center line but flat is usually faster and you're line looks pretty good.

K