thanks rick. i still can't say i understand everything, but at least i understand a bit more.
thanks also for keeping the discussion with gardner public. it's been quite educational following along.
one question regarding ventilating foils: on boats whose daggerboard sticks up through a trunk, do you think they ventilate? usually there's not that tight of a seal. air could get sucked down i suppose.
when my weta gets going 10+ knots, i can hear the daggerboard humming. the pitch jumps discretely with speed, like gears on a car, it is not continuous. third gear is pretty exciting, and i usually try to tone things down at that point.
i've been told the von karman vortices shedding off the trailing edge excite different modes of vibration. makes sense. but maybe the driving force is related to ventilation instead? should i try to seal it up?
ben arthur
weta #358
ithaca, new york
> The majority of information you read regarding "hull speed" is based
> on beamier hulls than ever contemplated for catamarans, tris and
> proas. Around the time Froude was doing his thing his frame of
> reference was probably British Navy sailing vessels and sailing cargo
> carriers. I expect he had not seen the fast light weight sailing
> boats on the other side of the world.
>
> The wave drag for a slender hull is a relatively low proportion of
> the overall drag for the speeds you have indicated and anything
> higher. Key word is slender. Hence the viscous drag dominates so
> wetted surface area is critically important.
>
> This is why the charts are useful in getting you in the ball park.
> Longer is not necessarily faster for a given power and displacement.
> These charts only consider displacement mode however at some point
> there is actually less drag to plane and the hull shape gets short
> and wide.
>
> When you are looking at the basic dimensions don't just look at how
> they are changing, also think of the actual values. A 14.5m long
> hull is quite long. A 600mm wide hull is not very wide and draft to
> beam ratio of about 40% is still quite deep. Note the increasing
> block coefficient as the speed increases.
>
> Typically when you get into these high speed displacement hulls the
> block coefficient goes up. The ends get fuller with wider entry
> angle and less rocker.
>
> A factor that might encourage a longer hull is the bow down trim
> resulting from a tall rig. So being able to pre-determine trim is a
> useful part of the analysis.