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  #11  
Old 02-21-2011
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Can you get some shots of the sail trim (shot straight up the sails), car placement, traveler position, etc.?

Out of curiosity, how are you trimming the main? Do you have a table for draft position and depth for various winds speeds? Higher winds speeds=flatter sail; harden the outhaul, add some cunningham and bend the mast. (Are you fractional or masthead rig? If fractional harden the backstay, if masthead harden the babystay / midstay)

My main trim steps - closed hauled. (after the genoa is trimmed)

Bring the boom to the centerline.

Lay under the boom on the coach roof and trim the top batten so that it is parallel to the boom (works best with partial battens, full batten might be cocked in slightly).

Ease the main or harden until along centerline. Ease until slight backwinding.

Watch the telltales on the battens, the bottom two should be straight back, the top should fly at least 60% of the time.

Discuss helm with helmsman, do they like the feel? Watch them; if they start to react to weather helm, ease the main down the traveler.

If all else fails, "If in doubt, let it out."
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  #12  
Old 02-21-2011
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Backwind (an erroneous term) is caused by the following:

Check the GENOA shaping first:
• Insufficient forestay tension - causes the genoa to go 'draft aft' and simultaneoously closes the leech (hooks it to weather) and brings the leech closer to the mainsail thus closing the so-called 'slot'.
Set up the backstay (reacts with forestay) for 15+% initial tension for 15kts. windstrengh and look to see if the leech exit shape of the Genoa(s) at the upper and middle panels are parallel to the boats centerline. If exceeding normal backstay tension ....
• increase genoa halyard tension to relocate the postion of max. draft more forward which will also simultaneously open the leech - to make the leech exit PARALLEL with the boats centerline and to open the 'slot'.
Caution - increased halyard tension can also make the luff shape 'too rounded' so beware of 'helm pressure changes' or lowering of boatspeed when you do this --- the aim should be FLAT luff entry shape on both (racing cut) sails.
• Too much jibsheet tension - does the same as Insufficient forestay tension. On a beat, the jibsheets should be tensioned only to the point of where the leech exit is PARALLEL to the boat centerline. If the jib leech exit is too far away from the main (check by slowly dropping the main traveller as you watch for the development of 'backwinding') then BARBERHAUL the clew of the genoa INBOARD until the main 'begins' to backwind and/or the boatspeed 'just begins' to drop ... then very slightly ease the barberhauler. Too much jibsheet tension causes shape problems of draft-aft and hooked-up / 'closed' leech. Most winch grinders typically put too much load into jibsheets and that results in closed leech and draft aft shape - restrain the 'winch gorillas'.
(add 2/22 - another way to do is to have parallel 'stripes' on the outboard undersurface of the spreaders and while watching the approx. distance between the stripes and the genoa leech move in/out (via jibsheet tension or barberhauler) to attain max. boat speed ... sometimes allowing the leech to open an inch or two is all it takes to increase speed or eliminate so-called 'back-wind' ... the max. speed as read on the speedo will tell you how far in or out to set the leech distance or amount of barberhauling. When barberhauling in expect the boat to point higher and with no loss in speed ... but dont get carried away or youll 'choke the slot', etc.
My sport boat has athwart ship tracks for the jib leads ... every day and every condition requires a different slot opening distance and clew/leech distance from the spreaders. Boats that sheet to the rail and without barber-haulers cant do this 'vital' adjustment.)
Aim - Get ALL the 'full set' of tell tales (including 'gentry tufts') flying perfectly in power-pointing mode (all straight back)

You simply cant set/shape the main until the genoa is flying/shaped 'perfectly'; once SHAPED and trimmed to perfection, THEN ....

Mainsail shape
• Aim for perfect set/shape ... all telltales streaming parallel / straight back, especially the leech tales. Set MAX draft via the outhaull while watching the speedo while on a 'power-beat' (all tails straight back for 'power pointing').
* Adjust for neutral helm pressure, with traveller at center, (added 2/22 - and second batten from the top - aft end parallel to the CL) by adjusting main halyard and cunningham tension, then ease a wee-bit until the boat s-l-o-w-l-y heads up when the helm is released. Add as little additional halyard strain to keep the luff entry area of the main FLATISH - too much halyard strain will cause the position of max. draft to go forward and the forward position may be too close to the jib/genoa (shows as 'backwinding') and if you cant adjust out the backwinding (by fore/aft draft position), then ease the genoa barber hauler or ease the jibsheets. (added 2/22 - if you cant achieve neutral helm by the previous; then and only then, consider to change the mast rake .. and start all over again with the 'set-up' and 'shape-up').
Its OK to have a 'happy bubble' of backwinding ... just a small section in the mid luff to show backwind = but verify with the speedo!
• Mast 'prebend' - allows a flatter mainsail luff entry shape - less prone to 'backwinding'. The C&C 34 is a double spreader rig so you should have ~ 1-1/2" prebend in the mast, even with rod rigging. A sailmaker EXPECTS the proper prebend when he cuts the main and if you dont have it, the mainsail will be too full and with a ROUNDED (backwind vulnerable) luff entry.
• If this is a woven DACRON mainsail - "How to properly RAISE a Woven Dacron Mainsail" - How to properly RAISE a woven dacron mainsail - SailboatOwners.com

THEN once the main is shaped to perfection ..... go back and reset all the shape/trim/ of Genoa !!!!!!!!! and if needed come back and readjust / reshape the main, again. One sail influences the aerodynamic flow over the other ... requires many adjustments and checks !!!!!!!

Forestay to Luff Shape Matching.
I have an article posted on "matching forestay tension to luff shape" at: MatchingLuffHollow.gif picture by svAquila - Photobucket .... as if this 'match' between forestay tension and luff curve shape (luff hollow) isnt set up perfect, its probably one of the prime causes of 'lousy pointing ability' ... and sometimes too much 'backwinding', closed leech shapes, etc.

Backwinding as most sailors assume is NOT what is happening when the luff of the main 'seems to collapse due to too much wind in the 'slot'', in actuality its the 'timing' of the interaction between the genoa and the main (aerodynamically called 'bootstrapping') is off because the CIRCULATION flow around both main and genoa are not correct and are canceling one another in the region of the 'slot'. (Sorry folks, your high school 'science teachers' have been 'dead wring' ... for almost 100 years!! no big surprise there, huh.).

Suggest you also go to: ArvelGentry.com ---> Magazine articles
---> then the 'sequence' of articles:
Checking Trim on the Wind,
Achieving Proper Balance,
Sailing to Windward,
Are You at Optimum Trim?,

Gentry is the sailor/aerodynamicist who debunked the 'slot effect', backwinding, and wrote the 'sailing world shaking' article - "How Sails Really Work", etc. etc. ... all based on firm aerodynamic principles. His Mag. articles plus watching the speedo as you 'set up for the day' will Im sure correct ALL of your 'backwind' problems.
... hopefully also my article on 'matching forestay tension to luff shape' will also help solve your 'backwind' problem (of incorrect aerodynamic 'bootstrapping').

Also, suggest you go back review all the elemental basics of trim, etc. A good 'reference' for the 'basics' would be: DonGuilette's "Sail Trim Users Guide" Sail Trim Users Guide (US delivery)

;-)

Last edited by RichH; 02-24-2011 at 01:36 PM.
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  #13  
Old 02-22-2011
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Great stuff, Rich, as always.... just one minor correction, I believe..

The 78 C&C 34 is a single spreader rig with a 3rd set of shrouds terminating on the mast above the spreaders.
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Last edited by Faster; 02-22-2011 at 08:58 AM.
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  #14  
Old 02-22-2011
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If single spreader rig, then 3/4" (1"max) prebend.
I crewed on a 'hot' C+C 32, with double spreaders and assumed the 34 also had 2 sets.
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  #15  
Old 02-22-2011
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How old is the main?
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  #16  
Old 02-24-2011
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Whow - Thanks to RichH

Rich
I have been sailing since I was 13. At various times I have raced in just about every kind of boat. I thought I knew something about sail trim. Your post was mind opening. Thank you for taking the time to write it.
One question on terms - bootstrapping - don't know the term. Are you saying the turbulence vortices from the two airfoils are creating a low presure region between the sails?
(By the way, Mrs Dermondy was my high school physics instructor - lo those many years ago - just on the off chance she is out there reading - Rich wasn't referring to you. I know you were always right. )
Alan
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  #17  
Old 02-24-2011
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Quote:
Originally Posted by RichH View Post
If single spreader rig, then 3/4" (1"max) prebend.
I crewed on a 'hot' C+C 32, with double spreaders and assumed the 34 also had 2 sets.
Yes - single spreader rig in our 34.
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  #18  
Old 02-24-2011
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Reply re age of main

Quote:
Originally Posted by Stu Jackson View Post
How old is the main?
I am not certain of the age, but it appears to be in good shape and is well cared for. It has been lightly sailed and does not have any obvious signs of being blown out, though it wouldn't be anyones choice in a highly competitive fleet. I feel fairly confident that the main's age isn't the primary cause of this problem. Its something in the boat set up and sail trim.
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Old 02-24-2011
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Thanks Also to JackDale

I just was reading my print outs and wanted to say thanks to jackdale too. I have a lot to think about for Saturday... If only the forecast would switch from current 6 kn from the East to about 15 kn from the North.
Alan
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  #20  
Old 02-25-2011
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Quote:
Originally Posted by midfleet View Post

1. Prebend. Let's call it 0-1/2" We'll measure on Saturday. The mast butt is locked in position on the step, but I have about 1" of blocks in the partners on the aft edge of the mast to try and induce a little bit of prebend, but with this particular section, bend isn't as easy to come by as if we had a smaller section 2 spreader rig.
On your boat you dont have the 'traditional' lower forwards nor lower afts ... but all the shrouds are in the same plane .... only remedy to establish prebend will be is to employ a baby stay from the fore deck to the mast.

Quote:
Originally Posted by midfleet View Post
2. Mainsail age. The main is 5 years old (North NorLam), with 3 seasons of use (boat was not sailed for 2 seasons and main was stored rolled in the attic). After the 1st season, it was recut for 4 full battens as the previous owners were planning on cruising. While the main isn't nearing it's life yet, in our conditions I would estimate it has ~2 more seasons on it.
A Norlam sail will be stable, until the leech is so stretched out ... not a dacron sail so the shape in-built is what your going to get.

Quote:
Originally Posted by midfleet View Post
3. Genoa trim. We have adjustable cars and adjust so the tell-tales break evenly top to bottom. In air ~8+ we try to have the draft in the forward 1/3 of the sail. With the 34 being such a tall aspect rig with a relatively long "J", my thoughts are that the boat is mostly driven by the genoa, hence the thought process that we get he boat moving with the genoa first and the main is secondary. This is what drove the J/35 well. Sort of sounds like I'm off base on this thought process.
"Mostly driven by the Genoa" .... a widespread misconception. The jib influences the main and the main influences the jib ... see comments on 'bootstrapping'. If the slot open distance is off and the leech shape of the genoa is off - you now have TWO sails with bad aerodynamic flow.

Quote:
Originally Posted by midfleet View Post
4. Boom position. In light air, I ask ondine to run the traveler car all the way to windward (or close), so the boom is above centerline ---- NO NO NO NO!!!!!! NEVER EVER EVER ... unless you are stopping the boat by 'backwinding the main'. It takes that much to get any feel in the rudder. I can tell he doesn't like it trimmed that way, maybe I need to recalibrate the feel in my hands! The wind is typically so light that this is when we go from almost 0 heel to 15 degrees. The boat doesn't seem like it wants to go until we get it heeled some. The 34 has a cabin top mounted traveler, so even block to block, we're not talking about more than 6" above CL.
And this is probably one of the 'main' problems .... in 'ghosting' air, the boom should NEVER EVER be above the centerline, the traveller may be well to windward (to take the weight off the boom and allow it to rise to SET the proper twist) .... and the first or second batten from the top (sometimes the third from the top in 'super light' and depending on the amount or twist needed) aft section parallel to the boats centerline, but if the boom (foot of the mainsail) is ABOVE the centerline you will redirect the areodynamic flow to the WRONG SIDE of the mast and lower sections of the mainsail, esstentially shut down the circulating flow around BOTH sails and do essentially nothing more than create a lot of turbulence.
Other - for 'ghosting' conditions you also usually need FLAT shaped sails to prevent leeside flow 'separation' .... and your main is probably NOT backwinding but 'separating' !!!!!!!! with the flow redirecting from the leech of the genoa, 'TO' the aft panels and leech of the main which is keeping that section 'full'. Your angle of attack on the main is way off if the boom is on the windward side of the C-line. Suggest you begin your adjustments of the main as if you were sailing unreefed at 20 Kts but with a slack halyard tension (to keep the luff entry FLAT), apply strong outhaul pressure to flatten the mid-cord, only apply 'light' pressure o n the mainsheet to prevent the leech from 'hooking up' and control the angle of attack by the traveller.
Use a FULL set of telltales (tales at midcord or normal position of max draft or near the equivalent distance of the leech of the genoa and of course AT the leech. The leech tales are the most important (so called 'kutta condition' so that the flow on BOTH sides of the leech exit is EQUAL and the tales flowing straight back. For mid-cord tales start with traveller slightly to windward, and adjust the OUTHAUL in or out, most out, while watching the midcords as you 'slowly release' the outhaul and until the leeside midcord tales 'droop' (showing a separation stall) then tighten back until those tales are flowing correctly - straight back or bothsides at an EQUAL 'droop angle' .... this will set MAXIMUM draft for the conditions. Watch the speedo as you set max draft via outhaul; you 'verfiy' the amount of max. draft by the max attainable value on the SPEEDO - not by 'guessing'.

For 'super-light winds' .... Set up this way with FLAT sails and with the CORRECT angle of attack on the main (boom to leeward and twist set to perfection with all leech tales flowing straight back) you will have to accelerate by 'dropping down' a bit until at the max speed, once there and up to the max speed you can then 'power-pinch' (scalloping course) by occasionally overtensioning the mainsheet so that the leech 'hooks to windward' (and the shape looks like a huge jet aircraft with its 'flaps' down for a landing) .. but immediately 'bear off' when the speedo shows a drop in speed. For accelerating, youre going to have 'open up' and bear off', etc.
With the boom on the windward side, your are directing the flow onto the WRONG side of the main, destroying any 'bootstrapping', using the genoa in reduced airflow - caused by the improper trim. FLAT SAILS, especially at the luff entry (slack halyards), and 'open' leeches .... ALL tell-tales or at least all leeward side tales flowing straight back. IN 'light' winds there isnt much 'energy' in the air streams to stay 'attached', especially noted at the luff section and point of max. draft --- therefore in 'light' winds FLAT luff entry shape, FLAT midcord/ small draft' and straight as an arrow leech aligned with the airflow - again, set up as if you were sailing in 20 kts, but with a slack halyard .... and use the speedo and tell tales tell you how much draft to put into BOTH sails. The boom has to be on the leeward side of the CL and no matter how far the traveller is to windward (for allowing the boom to rise to set the twist.) For low energy winds, if you attempt to make the airstreams 'turn corners' ... you will usually develop an invisible 'separation stall' (the tell tales are the ONLY way to discern an invisible separation stall)
Other - in 'light' winds the barberhauler 'comes inboard' to close the slot distance -- use the speedo to tell you 'how much'.

Quote:
Originally Posted by midfleet View Post
5. I'll try to post some pics here. Sorry, we don't have any looking straight up yet, but this may give y'all a feel.
Forget the pics. Do some trials of the setup mentioned, get MAX SPEED out of the boat .... and post pics of the 'trophy' (stack of empty beer cans, etc.) you win.

http://www.sailnet.com/forums/images/attach/jpg.gif

http://www.sailnet.com/forums/images/attach/jpg.gif

Thanks for all the tips! I'm stoked to have such a good crew and group that is taking a huge interest and ownership in figuring out how to make the boat go fast! People can call me out any time (as long as you're right http://www.sailnet.com/forums/images/smilies/cool.gif ), it's what helps kick my ass into getting everything right on the boat.

Last edited by RichH; 02-25-2011 at 01:48 PM.
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