With respect to the comments being made regarding light wind
performance there may be some relevance of those to the mundane production, etc cruising boats that I think perhaps many are familiar with and which are generally all slow (even the "faster" ones), but a fast cruising boat will be close to or at hull speed
even in around 7-8 knots of wind
so light wind performance is always high.
Two points -
SA/D ratio only predominates when residuary resistance from wave making is small (ie at very low Froude numbers when resistance from wetted surface predominates) but as Froude number increases ie as one increases speed towards hull speed
, then residuary resistance from wave making predominates. Wavemaking resistance for a hull
is roughly proportional to displacement, so for two hulls, identical apart from displacement, the lighter hull
will have a higher speed.
Also the displacement speed of the lighter boat will be faster than the heavier displacement boat because the heavier boat's wavemaking resistance increases at a much faster rate than that for a lighter hull as displacement speed is approached (the lighter boat can reach higher Froude numbers). This higher resistance resulting in a lower displacement speed cannot be beaten by adding sail area.
Putting those two matters together the light fast cruising sail boats that I am familiar with will reach close to their hull speed even in light winds and so wavemaking is the most important as far as resistance is concerned. The same but heavier boat will still be at speeds much lower than hull speed in those same light winds. Even if the heavier boat then puts on more sail than the lighter boat so that it too reaches displacement speed (but it is difficult to get a heavy boat to hull speed in light airs regardless) then it will still be slower by nature of it being burdened with a slower displacement speed.
Wavemaking resistance is typically of order of 35% of total resistance so any increase in it through higher displacement is significant eg two boats of same lines but one 25% heavier than the other, the heavier one will have nearly 10% extra TOTAL resistance to overcome. So in other than flat calms the lighter displacement boat will be faster but will never be slower even in calms all else being equal.
It is therefore for good reason that the modern fast cruising sail boats are built both strong and light, typically using foam sandwich, kevlar and carbon but I know of older but strong light displacement timber/glass composite cruising boats that will pretty much be on displacement speed in even 7-8 knots of wind. I don't know any designers with a reputation for designing these fast cruising boats that work
on the principle that one makes a fast (as opposed to mundane speed) cruising boat by going for other than light displacement and then making up for heavy displacement by cramming on sail area.
I know Hiracer will vehemently disagree with me again on all that as he has often claimed in other threads that displacement can be made up for by piling on more sail area. But as he has told me in another thread that he does not think Froude numbers are important in sail boat design (which is the same thing as saying that wavemaking resistance is not important
) I personally would not take much notice.
Comment has been made that these light fast boats are of necessity uncomfortable and indeed in small boats they can be due to higher accelerations. But those accelerations can be controlled by good modern design and also by increasing waterline length as length most always adds comfort for a given sea state. So many of these fast boats are in the 50 foot plus WL length range if intended for independant ocean cruising (and WL length for these boats is typically within inches of being the same as the length over deck
, and LOA
being dependant on the length of the prod for carrying line furling
I think one issue is that many are likely not familiar with the light, comfortable fast cruising yachts that are increasingly around. There have certainly been many built here, most now for export and those often custom design/builds for very experienced owners.
Of course, not everyone wants speed.