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post #4 of Old 12-26-2010
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All depends on the boat and the shape of the hull sides, and how much 'shoulder' was inbuilt to the mainsail, especially for most planing hull lightweight large racing 'dinghies'. By 'shoulder' I mean how much 'fullness' is inbuilt to the upper panels of the mainsail ... the heavier the crew the more 'fullness' (power aloft) is usually selected.

On many of these lightweight planing lightweight racing boats such as the FD, 420, M20 & I20 Scows, the 'rake' is accomplished by an on-the-fly system usually controlled by a robust harken 'magic box' in the cockpit which lengthens or shortens the forestay to accomplish mast rake (on the fly). These boats generally also have extreme 'bendy' tapered masts which are controlled by extreme mechanical advantage block systems attached to the backstay.

Generally the lighter the winds or for less than beam reach sailing, the more forward the mast rake. Also too these boats will 'optimize' in about 15-18kts of apparent wind so that the mast is bent/raked more on 'either side' of 15-18Kts.; heavy wind (with 'flat water') and very light winds require relatively FLAT mainsail shape .... the flatness arrived at principally by the degree of mast bend (besides cunningham + outhaul pressure). On going near downwind will such boat ever employ/use a 'straight up or non-bent mast' for max. sail draft. All this 'shaping' is arrived at by careful attention to a FULL set of telltales + steering telltales AND the 'speedo'. ( ---> magazine articles ---> series of 4 articles beginning with "Checking trim on the wind, etc."

To consistently get the optimum out of any boat, you need to develop a program of laboriously recorded data of what works best on any sailing angle and wind strength, etc. .... and set up the boat to these parameters for the expected conditions of the particular day, all based on previous performance history. Optimized performance sailing requires various positions of mastrake, usually accomplished by combinations of forestay length and backstay tension. (Big boats due their complexity, size, etc. usually cannot employ such on-the-fly mast rake systems; extreme performance planing 'sportboats' usually have such raking systems.)

In 'any' boat you cant have a fast boat that is dragging its rudder through the water to any degree 'sideways' (weather helm). Rudder angle (weather helm) is one of the 'benchmarks' when setting up to race and applies to boats with mast raking systems or non-raking mast systems. Typically the 'base' tune-up requires sailing close hauled and noting the amount of rudder angle (weather helm) and on most boats should not be more than ~3-4 degrees. Method: Set up the sail SHAPE for the day's expected conditions with mast in normal raked position. Setting the proper mainsail draft position for the day: - Go onto a full speed beat to weather, (neither power-pinching nor bearing off) but with ALL tell tales streaming directly AFT on both sides and let go of the tiller/wheel (usually with the second from the top batten parallel to the boat's centerline - very common to most mainsail optimized design). If the boat 'heads up', increase mainsail halyard tension (and cunningham tension) until the tiller/wheel becomes NEUTRAL (no weather, no lee helm) AND the point/position of maximum draft (watch the draft stripes at the head and mid section) in the mainsail is at approximately 30-40% of cord length of the sail back from the luff (or at the position of max draft from the sailmakers data sheet that came with the sail). Adjust the location/position of max. draft (POMB) by halyard/cunning tension until there is 'very slight' weather helm. THEN if there is any significant 'helm', then vary the mast rake so that the boat very SLOWLY heads up when you let go of the tiller/wheel. You can adjust (cancel out) 'weather helm' by either mainsail SHAPE (halyard/cunningham) + mast rake position. You WANT 'some' side forces on the rudder to help balance but no more than that accomplished by holding the rudder than about 3-4 degress on a 'big'boat or about 2 degrees on a planing boat with a 'flat plate' rudder. Watch the wake coming from the stern to make sure the boat is NOT skidding to leeward!!! ... you want that wake coming almost dead straight (2-3 degrees) off the stern.

Summary: 'Weather helm' is most often a result of failure to apply correct halyard/cunningham tension, not 'mast-rake'. Use 'mast rake' when you are unable to properly SHAPE a mainsail (via halyard, etc. tension) to adequately correct 'weather helm' ... mast raking is a 'last resort' to gain 'perfect helm pressure'.

If in 'heavy' weather and heavy chop you can aid 'weather-cocking' when planing at super-high-speed by increasing mast rake when going 'upwind'.... the reverse when going 'downwind' mast almost straight up or a 'bit forward'.
Of course you'd NEVER want to reef such a boat, as that wouldnt be very much 'fun'; you flatten and over-rake instead. Of course if the conditions are so 'huge' that requires you to begin to rotate the board(s) up into the trunk (allowing the boat to 'skid' a bit when hit by very strong gusts, instead of 'going over'), then you must rake the mast to compensate to insure to keep a 'light' pressure on the helm and the boat going straight without noticeable helm pressure (- on the tunnel-hull scows you can sail one completely over/inverted and never feel any difference in the 'helm' if the mast rake and POMD are correct).

Hope this helps.

General set/shape/rake instructions for the M20 / I20 Scow but would apply to most 'bendy rig' with on-the-fly mast raking:
North Sails One Design Note: there are no fairlead cars on most scows with athwartship radial jib lead tracks, the fairlead angle (jib fairlead SHAPE) is set by changing the jib sheet attachment on the CLEW IRON.

Last edited by RichH; 12-26-2010 at 01:17 PM.
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