Can you ever have too much draft?

Stu Jackson · May 26, 2015

Dave, I'm unclear on your post.

Can you ever have too much draft seems to imply you have a lot.

Then you said you took some out.

Which is it?

The short summary:

Light winds: flat sails min draft

Med winds: deeper draft

Heavy winds: flat sails again

shemandr · May 27, 2015

I think the answer to the question of if you can ever have too much draft, is yes. As Stu's synopsis indicates, in very light air you want a very flat sail draft so that the flow can remain attached to the sail. Too much draft and the flow with not be able to bend with the sail and the flow will detach and become the dreaded drag.

Jackdaw · May 27, 2015

Andrew is correct. In very light air you need to made sure the the flow stays attaches to the LEEWARD side of the sail as well. If it detaches it creates huge drag, and commonly your mainsail telltales will get sucked backwards behind the sail. They should all flow freely off the back of the sail. Flatten it until flow stays attached.

Scott T-Bird · May 27, 2015

If you were out during a holiday weekend, I can imagine the washing machine affect on Pistakee Lake. It has a significant affect on boat speed. Your boat, being lighter weight, can easily be throttled by the constant chop.

I'm always amazed, when sailing at night or on quiet days, how easily the boat moves along at hull speed, even with a seemingly light breeze. Then there are those weekend days when the lake is like a washing machine and hull speed is much more difficult to attain, even though it feels like the heavy keel does its part to maintain momentum and the wind should be enough.

For that reason, it is always difficult to correlate the effect of sail shape on different days. However, when the water is rough, a deeper draft does help power through the chop.

Bill Roosa · May 27, 2015

The tell-tails tell the tell. Probably the best investment in sail instrumentation is tell-tails. They tell you exactly what is going on. Then all you have to do is figure out how to optimize the shape to "go fast"
Tell-tails can assist with angle of attack, draft, and twist. what else is there?
A log entry on wind speed and apparent wind direction, traveler position, outhaul tension, and main sheet tension (marks on the main sheet and outhaul are helpfull here) jib sheet lead car position, jib sheet tension (again marks on the sheets help, use the winch as the "0" point for measuring. The halyards should be set for the "standard" tension of course.
You can do the same thing for reefing too.

Kermit · May 27, 2015

Not when it comes to beer. Well, actually, if you do have too much you should drink it as quickly as possible.

justsomeguy · May 27, 2015

Don Guillette · May 28, 2015

To second shermandr answer to your question "can you have too much draft?" The answer is yes. After about 25% draft depth, the more belly you add the more counter productive it becomes. Applying too much draft is a common problem with loose footed mains because it's easy to do as opposed to a bolt rope main.

I personally don't know how mates trim their mainsail & jib without telltales. I can't and I know what I'm looking for. When skippers asked me to join them for a sail so that I could help them as much as possible with their sail trim, as I stepped onto the boat the first thing I looked at was the fairleads (especially if they were the pin type). I wanted to see if they were frozen in place, which they generally were. The second thing I looked for were telltales. 75% of the boats didn't have them but I carried extra sets in my bag, just in case. The 3rd thing I checked was the outhaul, especially on older catalina's -- it was generally stuck and therefor like the broken clock. It was right for one point of sail and wind condition and wrong for all others.

So, within 20 seconds of stepping onto the boat I had a good idea of where to start. Additionally, I had with me a very small model sailboat which I used to explain draft depth, draft position, twist and angle of attack and could demonstrate how the sail trim controls for the main & jib effected those 4 elements. That took about 15 minutes. In the short space of maybe a 1.5 hour sail the skipper was getting more out of his/her boat than they ever had -- gals were the best listeners!! Dennis Conner wasn't calling them for sail trim advise but they were better sailors then they were 1.5 hours before.

I never charged a dime because the the joy I saw when the sail trim lights went on was payment enough. A beer and a sandwich were a nice extra plus the joy of being on the water with mostly (a few turned out to be jerks) GREAT people.

Scott T-Bird · May 28, 2015

Daveinet said:
Scott, we avoided Pistakee for that very reason. I went up to Green Lake, WI.

Nice choice! It's hard to say why you were slower. By putting more bend in the mast, could you have altered the rake and significantly affected CE? Just a thought. It sounds like it may be touchy to dial that boat in. I would get in touch with somebody from North Sails (that would be my go-to place for knowledge). You would probably get some really good advise and they may sell you a sail!

RichH · May 29, 2015

Some hints on setting up the amount of draft in a mainsail:

1. Sailmakers generally target/design a mainsail so that the designed amount of draft is typically at optimum for sailing in 12-15 kts. of wind.
A. At much less than 12-15kts. such a sail will need to be flattened to lessen the risk of a leeside separation stall - an invisible separation of airflow from the leeside due to the airflow not having enough 'energy' to stay 'attached' to the leeside. Low energy / low speed airflow will generally separate (stall) if the airflow is forced to 'turn corners'. Most 'cruising' design sails will have a 'rounded' shape to the luff section that produces a 'forgiving' shape for the inattentive (cruising) helmsman; the rounded shape luff leading edge (from the edge of the luff to about 5-10% back) being 'rounded' is where the separation stall will begin to occur.
A sail (for non-planing/displacement sailing) that is cut/designed for racing has a greatly reduced 'roundness' at the luff area, is less subject to separation stalls, has a very small range of angle of attack ... the helmsman needs to be constantly 'on' to keep the racing sail within its designed 'un-forgiving' angle of attack.
B. 12-15kts of wind usually implies a wave height of approx. 2 ft. (for coastal or inshore/lake sailing; larger waves for sailing 'offshore'). Unless specified otherwise by the purchaser, the 'plain vanilla' sail will be designed with normal amount of sail draft for this 2ft. of 'chop'.

There is a relational 'balance' between sailing in waves OR in 'flat' water.
Waves require that the sail develop POWER to 'punch through' waves .... hence the design amount of draft inbuilt to the sail (12-15kts w/2ft. waves).

Rx: The balance of these two sailing conditions ... sailing in (2 ft.) waves OR sailing in 'flat' water requires differing amounts of draft: Full draft for POWER for 'punching' through or into waves; OR, flattened (less) draft for SPEED sailing in 'flat' water.
Flat sails = HIGH gear (speed); Well drafted sail = LOW gear (power) ......

• How to set the correct amount of draft for EACH day's sailing conditions:
1. Go onto a beat and adjust sails for 'perfect' tale tell flow (all tales perfectly streaming straight back, but IGNORE slight so-called 'backwinding' !!!!!).
2. ADJUST the amount of OUTHAUL tension while watching the speedo !!!!!!!! The correct amount of mainsail draft will occur when the speedo is at its 'maximum'. Adjust outhaul tension to arrive at maximum boat speed. Readjust shape and trim to keep the tell tales properly flowing, etc. and repeat with outhaul adjustment, etc.
2a. For light winds and little to NO waves, start with a fully tensioned outhaul and FLATTENED mainsail. The flattened shape will help prevent 'separation stalls' (watch your tell tales for separation stalls on the leeside).
2a Alternate. On a boat with a 'bendy rig' - a fractional rig whose (tapered) mast can be severely bent by extreme backstay tension, begin by applying maximum backstay tension (to reduce draft/flattening) and slowly release that backstay tension while watching the speedo for max. speed .... plus while or in addition to adjusting the outhaul.

Backwinding: is a condition when the 'recirculating air flow' AROUND each sail is mathematically canceling each other IN the 'slot', (the combined and sum total of opposing) recirculating airflow speeds IN the slot is approaching zero and the sails are operating at near MAXIMUM aerodynamic efficiency (you may even see that some of the midcord windward side tell tales are (correctly) pointing 'forward' in 'perfect' circulation flow conditions !!!!!) .... OR the shape of the luff front edge of the main is 'too rounded', .... or the leech of the jib is operating too close to the mainsail, etc.
Rx: 'slightly' release jibsheet tension to 'open' the slot distance; 'slightly' increase the mainsail outhaul tension (flatten the main); release some tension in the mainsail halyard (relocates the point at which the max. draft occurs towards aft - but not so much as you induce noticeable 'weather helm') ...... BUT BUT- while watching the speedometer; all adjustment for so-called backwinding is to gain MAXIMUM boat speed (VMG).
Its entirely OK to carry a 'happy bubble' of slight/occasional 'backwinding' .... if the boat is sailing at MAXIMUM boatspeed when 'pointing'. If the backwinding is 'detractive', consider using slightly 'smaller' jib or a jib with more concave ('hollow') shape to its leech, or open the slot an inch or so - whatever is FASTER.

For sailing is SUPER LIGHT winds and flat water: consider to install a 'row of steering tell tales' on the jib. A horizontal row of 4-5 tell tales on each side of the jib at 'eyeball height' of the helmsman. When the aft-most steering tell tale on the leeside begins to droop down - separation stall !!!!, so 'head up' a degree or two and/or 'reshape'/flatten the sails .... all done while watching the speedo AND the leeside tell tales !!!!!!!!

For sailing in super-light winds, most boats will sail well/fast if the sails are set-up/shaped the exact same as when sailing in 'above' 15 kts. of wind. ...... flattened!!!!!!!! Watch the SPEEDO when 'shaping' the sails, always aim/adjust for MAXIMUM boat speed and watch the tales for 'separation stalls' on the leeside.

Summary - The outhaul is what controls the amount of draft, the tension in the mainsail halyard determines fore/aft where the point of maximum draft occurs in the main (weather helm).
Use lots of sail draft for when sailing in WAVES; FLAT sails for speed sailing in FLAT water, ..... once you have all your tell tales flying correctly.
Flat sails = HIGH gear; Well drafted sails = LOW gear ......
Adjust in/out on the outhaul according to the SPEEDO. Doing so will automatically adjust the correct amount of draft for the day's wind/wave conditions.

Other.
Setting slot distance - General rule: jib clew comes IN towards the boats centerline in super light winds (but typically not so that the jib tack to clew angle or angle of attack to the centerline is LESS than 10°), jib clew goes OUT away from the boats centerline in 'blammo' conditions. Adjust in accordance to maximum BOAT SPEED obtained.

hope this helps.

Joe · Jun 1, 2015

Daveinet said:
So while we are on the subject, I do have lower telltails on the jib. There is only one set on the luff in the lower third. Often I find the lower windward tell tail flying nearly straight up. (this is without any significant heal) Not sure what that means other than I have wind shooting up the sail instead of flowing aft.

Go spend $5.00 - $8.00 on a new set of tell tales... there are instructions on where to place them on your sail and how to read them.... absolutely no excuse not to equip your boat with this simple, inexpensive tool.

NYLON RIBBON $8.00

or WOOL YARN $5.30

shemandr · Jun 1, 2015

Daveinet said:
. Often I find the lower windward tell tail flying nearly straight up. (this is without any significant heal) Not sure what that means other than I have wind shooting up the sail instead of flowing aft.

I think it means there is a vortice on the windward side of the sail which is the result of stall there. You are either heading too high or your sail isn't trimmed in enough. If the other tell tails are flying it's a twist problem and you should move the jib lead back a notch.
To be more detailed, the stall could also be a result of the jib entry being too round too far forward for the wind condition. That could require a halyard adjustment - slacking- to allow the draft of the sail to move aft and making a more flat entry for the luff -if you are an attentive driver. Or, if this isn't vague enough, a backstay adjustment may flatten the jib entry by reducing forestay sag.
What makes sail trim endlessly interesting to me is that it is rarely trim this or slack that. As Don often says the whole system is synergistic. And it isn't the same from one boat to another. So, when you get the boat in the zone, make note of all the controls so hopefully you can get back to that trim.
Nah nah nah nah...

Jackdaw · Jun 1, 2015

Daveinet said:
So while we are on the subject, I do have lower telltails on the jib. There is only one set on the luff in the lower third. Often I find the lower windward tell tail flying nearly straight up. (this is without any significant heal) Not sure what that means other than I have wind shooting up the sail instead of flowing aft.

You CAN sail with only one set but you will not know if your twist is correct. Not ideal but sometime a good way to learn.

With one set:

UPWIND:
Trim the jib in until the leach is parallel with the boom (same angle)
Steer the boat to keep them both flying.
Turn AWAY from the one the flutters.
Driver focuses ONLY on those telltails!

OFFWIND:
Steer the boat to its course.
Adjust the sheet (in/out) until they BOTH fly.

Jackdaw · Jun 2, 2015

Daveinet said:
As I think more about it, I did loose power out of the main, because the boat often needed to be held from turning downwind. I've never had this problem before, I actually missed a tack once, which has not ever happened in the past. The only confusing aspect is that on Sunday, I was over powered much of the time, but yet could not get any speed, even in those inbetween times when the boat was not over powered. I would have thought that the flatter sail would work up until just before it was over powered. But that seemed to not be the case.

OK now not quite clear. In your first post you said mostly light. Now you say mostly overpowered?

Anyway, assuming you were overpowered.

Many sailors (in particular on fractional boats) think it is enough to flatten the sailplan when the breeze comes on. This is NOT THE CASE.

To do this, many will flatten the main (tension everything, lots of sheet etc) and then lower the traveler when a puff hits. The whole sail just luffs and shakes in the breeze generating no lift but tons of drag.

The KEY is to ADD TWIST. Bring your traveler way up. You want the boom and the bottom 1/3 of the mainsail on the centerline or slightly above. And now then ease the sheet. Vang loose. The top 1/3 of the sail will just weathervane into the stronger breeze aloft, buts that OK. You'll sail the bottom 2/3s. The boat will flatten out and speed way up. The goal is keel the boat flat. Speed will take care of itself.

Remember that when it is breezy, sailing effectively is all about dealing with excess pressure.