DSK wrote:

"DSK" wrote | Heck, a Soverel 33 will move at 4 or 5 knots in almost
no wind at all,
| if well sailed. Maybe it's just making a ripple IYHO? Just a few
| weekends ago I watched a Kirie Elite 30-something (looked to be
about 35
| feet) and a C&C 34+ tearing around the racecourse in winds of about 3
| knots and chop.



Capt. Mooron wrote:

The hell you say..... 4 or 5 kts in no wind!!! Okay Doug.... put
down the
Jack Daniels and back away from the bar!! ;-)


No booze involved, not even American beer. The Soverel 33 is renowned as
a light-air speedster though, probably not a fair comparison. They make
a wake you can hear from 1/2 mile off on days when the wind is barely
enough to disturb cigarette smoke.


I have no experience with the Kirie Elite but I have been on a C&C 34 in
Vancouver. There is no way a C&C 34 can be described as "tearing
around" at
3 knots... even with no chop!


It wasn't a C&C 34, it was the 34/36+ (or it might have been the 34/36
XL, not sure). Like this

http://www.yachtworld.com/core/listi...oat_id=1193723


The boat rates around 90 PHRF, in other words more than a minute per
mile faster than your boat.


| Yeah, there's that. But when the boat reaches some significant percent
| of hull speed, it's going to making waves not ripples.

Okay let's explore that point.... down wind with the wave train at let's
average it about a 2ft wave height and a 6 ft between crests. The boat is
doing lets say half an average hull speed [6kts]... on a dead downwind
run....so we'll call it 3 kts speed.


Huh??? That's not at all how it works. The wave-making resistance of a
hull increases with her speed in proportion to her Froude number, which
is a fairly complex derivative.


The Froude number (Fn) is quite simple, it's:

Fn= v/Sqrt(Lg) It's dimensionless like the Reynolds number and v=speed,
L waterline lingth and g the gravitational constant.

By the way, the ratio v/sqrt(L) is called the Taylor quotient.


But let's make it simple... a hull with a 30' waterline has a "hull
speed" of 7.3 knots... meaning that at that speed, the crests of her
wave train will be 30' apart and she will require tremendous amounts of
increased applied power to go faster. At roughly 2/3 that speed, or 4.4
knots, she will be making waves of half her waterline length.


Froude found that the wave making resistance rises as:

L^3 aW(v^2/Lg)

where a is the form coefficient, w weight which suggests that the wave
making resistance rises with the square of speed. A factor that
increases the steepness somewhat is the squat of the vesses as speed
rises changing a. The skin friction directly rises with v. So that's
resistance, but the power needed then rises with the cube of v.

Cheers