Easy progress


Practical results of limitations of boat speed - Sail Closehauled sailing


When a yacht is closehauled, so little of the residual effort of her rig is transferred into forward drive that in all but the most extreme performance craft, hull speed is never attained. A typical cruiser sailing in moderately advantageous conditions will manage little better than the unmodified square root of her waterline length. Thus, a boat of LWL 25ft can expect 5 knots hard on the wind. She may do better, but a skipper who expects no more will rarely be disappointed.


Hard driving


The energy to drive a yacht does not increase in direct proportion to velocity. A reaching sailing craft moves very easily up to what would be her best speed to windward. The final quarter of her theoretical maximum is achieved by applying a rapidly rising amount of sail power. A boat capable of 8 knots will get up to six with little effort. Given the right wind, seven can be sustained by a crew with sufficient enthusiasm, but to drive her at her full 8 knots for more than short bursts will be a dramatic business, and very demanding on both gear and crew.


Hard driving Net effects of hull-speed factors on sailing boats


Life aboard a boat sailing free in a strong or even moderate breeze is generally easier if her rate of knots is ten or fifteen per cent below the theoretical maximum. On the other hand, driving her at near-displacement speeds can be a race-winner, and most cruisers love a fast passage time. The same paradox must be addressed at lower boat speeds when sailing closehauled. The last half-knot may involve a substantial trade-off in comfort and load on the yacht’s gear, but once the sea gets up nothing from a dinghy to a square-rigger will sail properly to windward unless driven reasonably hard. This is the dilemma. The seamanlike answer is to understand the issues, then make a sensible decision appropriate to the circumstances.


Sailing craft under auxiliary power


We have seen that a sailing hull will readily work up to a velocity equivalent to the simple square root of her waterline length. She is then well short of her full capacity for making waves and is pulling dramatically less wash than she would at full displacement speed. She is thus burning far less fuel. Many of today’s cruisers have engines which can push them up to hull speed, and it can be illuminating to compare engine revolutions and fuel flow at the ‘root waterline length’ speed with the flat- out equivalent. Similarly enlightening is the wash, with a powerful engine, one glance astern will tell more than a book full of words. Large auxiliaries come into their own when the yacht must smash her way into a head sea under power in a strong wind.


14 | MANUAL OF SEAMANSHIP


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