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With 170s ... one has to ask the questions:
If the 170s is so efficient, then even with the ratings penalty do you not see more flown? ... intuitively obvious answer: inefficient overall
170s are from a bygone day when it was erroneously assumed that DDW was somehow faster and pointing ability was somehow enhanced by more sail area, etc., especially in 'light winds' ----
1. Wind strength at most 170% conditions does not usually have sufficient energy for the airstream boundary layers to remain 'attached' ... for the full cord of the sail ... and with such a sail being soooo sensitive to such boundary layer conditions, easily develops separation stalls aft of mid-cord and towards the leech sections. A full set of telltales and a row of 'gentry tufts' will usually validate this stability/instability.
2. A 170 designed/plotted for typical 'light wind sailing' will have lots of 'shoulder' - fullness in the upper sections of the sail for more 'power' but that comes with less pointing ability. Most 170s cut 'back then' were cut with the mistaken idea that you needed a larger draft (powered up) sail for 'light condition' --- entirely wrong due to the propensity of separation stalls. "powered up" sail are not FAST sails.
3. Its easy to be overpowered using a 170 and that will create more leeway/skid that drastically reduces VMG .... "going like hell but in the wrong direction (for pointing)". Plus, a (especially light weight) 170 can easily have its luff distorted by energetic/aggressive winch pressure / windloading ... and the luff hollow (entry portion of the luff) no longer matches the normal sag in the forestay/headstay .... the result is more heel, skid to leeward, ... drastically falling VMG.
4. Unless your class rules allow a reaching strut (most dont), a BIG LP sail wont have very good shape when reaching due to the undue twist that develops because .... you simply cant move the fairleads out beyond the rail of the boat.
Every boat is different and every trimmers' perceptions are different so I would suggest
Suggest that you perform some comparative 'plots' of VMG vs. 'normal windspeeds when the 170 is flown ... between the smaller genoas and the 170. Careful 'profiling' (based on a quantity of collected data points under varying conditons) of the performance of each (vs. VMG) may validate that the 170 ... just isnt worth it (ratings penalty) vs. VMG. Use FULL set of tell tales with 'gentry tufts' to set/shape each sail to perfection, then do the comparisons.
Look to 'nature' (who seemed to have this figured out millions of years ago) that the best 'gliders' are the birds that have VERY high aspect ratio wings and look to the sails being used on the modern high end boats ... high aspect ratio to the hilt. Increasing the LP% reduces AR.
BTW - The comment about mainsails being 'just steering components' ... sorry that is entirely wrong wrong wrong as the mainsail is definitely part of the 'system' and helps create the 'upwash' etc. in which the jib/'genoa operate. If the mainsail isnt trimmed to perfection you will NEVER be able to perfectly optimize/trim/set a genoa on an aerodynamic basis.
Last edited by RichH; 06-24-2009 at 08:20 PM.