C&C 27 Association Forum

dmetchick

Member

Mast Rake - Mark V

We've got a 1985 Mark V and feel we should be getting better upwind point while racing in moderate conditions (8-12 kts wind) than we are. To help out, we just put an adjustable backstay on the boat.
I'm wondering how much bend I can really put into the mast. Any tips or insight would be helpful. Thanks.
Doug M

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Doug M

Guest

Re: Mast Rake - Mark V

There is, of course, a big difference between mast rake and mast bend.  Mast rake refers to the angle that a (presumably straight) mast makes with the deck of the boat.  This is usually measured by suspending a weight from the main halyard and measuring the horizontal distance between the halyard and the gooseneck.  Mast bend refers to the distance at mid-mast between a taut line from the masthead to the mast base or gooseneck.  Mast rake adjustments are used to move the sail plan center of effort fore and aft to control weather helm or lee helm.  Mast bend is used to flatten the mainsail by pulling fabric forward in the middle of the sail.  Mast rake is primarily adjusted by changing the length of the forestay (via turnbuckle adjustments), assuming that you have the standard adjustable backstay rig.  Mast bend is controlled mainly by downward pressure on the top of the mast, usually achieved with adjustable backstay tension.  In the case of the Mark V, the babystay may also contribute slightly to mast bend.  To get an idea of the rake and bend measurements recommended by C&C in 1985 after extensive testing of the Mark V prototype, Smoke, you can refer to the information posted on this site under the Sea Change page, under Tuning Guide.  The indicated data is probably mostly relevant to a boat raced with 4-5 experienced crew and with easily adjusted standing rig to accomodate different weather conditions.  My experience with my 1985 Mark V, hull #75, differs.  I originally had 12" of mast rake, whereas the C&C tuning guide recommends 15".  With this setting, I found excessive weather helm.  I sailed for a whole season that way, and in spite of every effort to depower the mainsail, the weather helm was excessive in heavy air conditions.  However, the boat pointed upwind very well in light air.  Since I usually sail single handed and therefore can't put much weight on the rail, and since I have roller furling on the jib and cannot easily change the forestay adjustment, I decided that this was not the ideal configuration for me.  So I put 6" stay extenders on both sides of the adjustable backstay and screwed the forestay turnbuckle all the way in.  This moved the mast head forward and resulted in 6" of mast rake.  In this configuration, I occasionally feel a slight lee helm in light air, but still have significant but less weather helm in extreme conditions.  This seems like a better compromise for me.  I don't like ragging my mainsail too much in heavy air since it is an older Kevlar sail and can't stand too much abuse.  C&C notes that the Mark V mast is easily bent, and advises moderate shroud tension.  Shroud tension mainly controls "pre-bend", which is a "permanent" static bend.  I use 1000 lbs. in each of the main (top) and lower shrouds, and 500 lbs. in the small mid-shrouds.  These seem to be the minimum tensions that prevent shroud sag on the leeward side in heavy air.  The backstay tension, mainsheet tension, and vang tension all contribute to adding adjustable dynamic bend to the mast.  Mainsheet and vang tend to push the boom into the gooseneck and bend the mast.  In light air, you do not want too much mast bend, and in heavy air you usually want to flatten the mainsail as much as possible.  The optimum amount of prebend depends on the cut and age of your mainsail.  But I don't want to get into sail trim issues here.  The best way to sort out these matters is to experiment with your own boat, but unfortunately that could take several seasons depending on how much time you can devote.  Maybe some other owners will contribute their findings.
Bob.

Guest

Re: Mast Rake - Mark V

Hi
In 8 knots we don't use much bend at all - about 6".  In 12 knots we have about 8".  In 15 K about 10 to 12".  More backstay will tighten up the forestay and take some sag out of it, helping you to point.  But, more backstay also flattens the main and opens the leach, leading to less power. 
Other light air ideas - make sure not no overtighten the main sheet.
Jim