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

"Nav" wrote in message
| 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.

Thank You Nav....... this is exactly what I was trying to tell Doug!!!

Your boat may not be all that quick... but you certainly are! ;-)

CM

Nav wrote:
Froude found that the wave making resistance rises as:

L^3 aW(v^2/Lg)

Thank you, that was what I was thinking of but couldn't remember the
formula.


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.

Overall resistance rises as a higher power because it is the sum of
friction, which rises somewhat less that the square of velocity, and
wave making resisitance, which rises as somewhat more than the square of
resistance. The usual hull speed formulas make no use of anything as
complex as the Froud number, just waterline length and an approximate
speed length ratio.

Now let's see you explain prismatic coefficient and all it's ramifications!

Fresh Breezes- Doug King

DSK wrote:
Nav wrote:

Froude found that the wave making resistance rises as:

L^3 aW(v^2/Lg)


Thank you, that was what I was thinking of but couldn't remember the
formula.


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.


Overall resistance rises as a higher power because it is the sum of
friction, which rises somewhat less that the square of velocity, and
wave making resisitance, which rises as somewhat more than the square of
resistance.

Yes, but usually the skin friction is low compared to wave making until
you start to plane...


The usual hull speed formulas make no use of anything as
complex as the Froud number, just waterline length and an approximate
speed length ratio.


Yes, and there's a lot of design in controlling wave making scale
factors (until you get to a very long thin hull).


Now let's see you explain prismatic coefficient and all it's ramifications!


The prismatic coefficient (Cp) is the ratio of the immersed volume to to
the area of the midsection times LWL. A big Cp means full ends and a
small Cp fine ends. The coefficient should exclude appendages, bulges
and deep keels. The ramifications would require a book!

Cheers

Overall resistance rises as a higher power because it is the sum of
friction, which rises somewhat less that the square of velocity, and
wave making resisitance, which rises as somewhat more than the square
of resistance.


Nav wrote:
Yes, but usually the skin friction is low compared to wave making until
you start to plane...

Sure, it's the nature of exponential functions. Skin frictiongets pretty
big but wave-making resistance is almost vertical around "hull speed."

Have you seen Frank Bethwaite's graphs of hull drag? He worked for a
long time to smooth out the hump in the drag curve when the hull starts
planing.


Yes, and there's a lot of design in controlling wave making scale
factors (until you get to a very long thin hull).

It's all fairly well understood by now. As I understand it, the issue is
to decide on a speed-length ratio you want (basing that desicion on the
characteristics that come with it) and draw a hull with a prismatic that
matches the desired S/L R. Oddly enough (or it least it seemed
counterintuitive to me) higher prismatics go with higher S/L Rs.



Now let's see you explain prismatic coefficient and all it's
ramifications!


The prismatic coefficient (Cp) is the ratio of the immersed volume to to
the area of the midsection times LWL. A big Cp means full ends and a
small Cp fine ends. The coefficient should exclude appendages, bulges
and deep keels.

OK, is that clear to everybody?
Maybe it's just me, but Cp seems like one of those things that just
cannot be understood without a good drawing.

... The ramifications would require a book!

Several books... big ones... and they're inventing new wrinkles all the
time. It's part of what makes naval architecture fun.

DSK

Youse Guys Don't Know Squat about Hull speed and wave producing.

I suggest you consult the Kiwi's. Their False Bottom changed the
location of the stern wave and put it back under the hull.

Now if you design the hull properly you will flare the stern just
sightly aft of the wave. This will bring the wave on board.

With this design you'll have no squat, You won't worry about skin
friction, The only thing you'll have to worry about isssss-----
BAIL DAMN IT!!!! GET A BUCKET AND BAIL!!!!

Froudue's Law is used back in the Club. With pieces of bread, skewers
and melted cheese-----NOW BAIL!!!!

Ole Thom