speed calculation for a displacement hull
max
calculating the maximum speed (and relative efficiencies at lower
speeds) for a displacement hull.
Could someone tell me please.
Thanks in advance Max
P.S. to reply, remove the "remove this"
Dana Seero
>
> I seem to recall that there is a relatively simple equation for
> calculating the maximum speed (and relative efficiencies at lower
> speeds) for a displacement hull.
>
> Could someone tell me please.
To general rule - to determine a boat's theoretical hull speed in knots,
multiply the square root of the waterline length, in feet, by 1.4.
Use this answer to justify to your wife why a larger boat is safer.
Steve Danaher
1.4, but can vary from 1.2 to 1.5.
Steve Danaher
sdanaher...@2xtreme.net
Paul Kamen
>To general rule - to determine a boat's theoretical hull speed in
>knots, multiply the square root of the waterline length, in feet,
>by 1.4.
If you're calling it "theoretical" than you should probably use the more
precise coefficient of 1.34 knots/ft^1/2.
This is the speed of a deep-water wave having a wavelength equal to the
boat's waterline length, which in turn is a reasonable approximation of
the maximum speed of a medium or heavy displacement hull under normal
reaching and running conditions. Speed will be slower upwind, and
sometimes faster downwind with a spinnaker and surfable waves.
>Use this answer to justify to your wife why a larger boat is safer.
That's going to be tough. With speed being proportional to the square root
of size, and cost being roughly proportional to the fourth power of size,
we have dollars/knot being proportional to size to the 3.5 power. A very
strong argument for the smallest boat being the best buy.... ;-)
--
fish...@netcom.com
http://www.well.com/~pk/fishmeal.html
-"Call me Fishmeal"-
Dana Seero
>
> Dana Seero <dse...@capinc.com> writes:
>
> >To general rule - to determine a boat's theoretical hull speed in
> >knots, multiply the square root of the waterline length, in feet,
> >by 1.4.
>
> If you're calling it "theoretical" than you should probably use the more
> precise coefficient of 1.34 knots/ft^1/2.
>
precision!
Everyone I know is too poor to own a boat big enough to make a
difference.
> >Use this answer to justify to your wife why a larger boat is safer.
>
> That's going to be tough. With speed being proportional to the square root
> of size, and cost being roughly proportional to the fourth power of size,
> we have dollars/knot being proportional to size to the 3.5 power. A very
> strong argument for the smallest boat being the best buy.... ;-)
>
> --
> fish...@netcom.com
> http://www.well.com/~pk/fishmeal.html
I've tried:
"It's a good family boat."
"It's safer."
Dana Seero
>
> I seem to recall that there is a relatively simple equation for
> calculating the maximum speed (and relative efficiencies at lower
> speeds) for a displacement hull.
>
> Could someone tell me please.
So, Max, you now have the relatively simple version, in fact you have
every version on
up to the fully derived.
A more immediately useful question would be, "what is the highest
percentage of hull speed
that one can cruise at, with good economy, in:
- a small runabout / center console of 17-21' LOA
- a mid-sized cabin cruiser of 23-27'
- a BMF cruiser of over 40'
?
Because if we got a consensus answer on that, we could make sure we set
our cruise speed
optimally, saving so much money we could buy all our helpers on the NG
beers.
Jim Lea
Charles T. Low
<37045ce4...@news.demon.co.uk>...
>I seem to recall that there is a relatively simple equation for
>calculating the maximum speed (and relative efficiencies at lower
>speeds) for a displacement hull.
Max,
Yes, 1.34 times the square root of the waterline length in feet (2.43 if in
metres) gives a wavelength. This ratio is sometimes called the
"speed/length" ratio. Optimal cruising speed is around 1.1 or 1.2, presuming
a hull built for pure displacement. The physics behind it all is
interesting, and explained slightly differently by different authorities.
Notice how slowly the speed increases with length, because of the square
root thing.
Charles
****
Charles T. Low, author of "Boat Docking"
<mailto:ct...@boatdocking.com>
<http://www.boatdocking.com/>
Russell Eberhardt
<ct...@boatdocking.com> wrote:
>Optimal cruising speed is around 1.1 or 1.2, presuming
>a hull built for pure displacement.
when the bar opens?
Regards,
Russell
GYFFT
Shotley
TO REPLY BY EMAIL:
Change nospam in reply address to iee
Charles T. Low
>On Thu, 01 Apr 1999 01:47:49 GMT, "Charles T. Low"
><ct...@boatdocking.com> wrote:
>>Optimal cruising speed is around 1.1 or 1.2, presuming a hull built for
pure displacement.
>Ah, but how do you define 'Optimal'? Economy, comfort, getting home when
the bar opens?
>
>Regards,
>Russell
Good question. I'm using the long-distance range optimum. I think that on a
displacement hull boat, the bar had better be on the boat (only not in
Canada when the boat is underway!).
M Perrett
<sdanahe...@2xtreme.net> writes
>The formula is square root of WLW times a factor. The usual factor is
>1.4, but can vary from 1.2 to 1.5.
>
>Steve Danaher
>sdanaher...@2xtreme.net
--displacement do not go any more then 8 knots no matter what hp engine
you have
M Perrett
Steven Shelikoff
I know some people on the USS Enterprise that would disagree with you.
Steve
--
/ / /
\ \ \ mailto:shel...@averstar.com
/ / /
Martinp
Steven Shelikoff <shel...@averstar.com> wrote in message
news:37051FE5...@averstar.com...
> M Perrett wrote:
> >
> > In article <37026d88...@newsreader.2xtreme.net>, Steve Danaher
> > <sdanahe...@2xtreme.net> writes
> > >The formula is square root of WLW times a factor. The usual factor is
> > >1.4, but can vary from 1.2 to 1.5.
Knots.
> > >
> > >Steve Danaher
> > >sdanaher...@2xtreme.net
> >
> > --displacement do not go any more then 8 knots no matter what hp engine
> > you have
>
> I know some people on the USS Enterprise that would disagree with you.
not to mention Nuclear subs.
even the Hull R'dam Ferry does a steady 22-24 knots.
Steven Shelikoff
Nuclear subs are an interesting case because, when submerged, they don't operate
at an interface of mediums. So they don't have the "hull speed" hump in their
power/speed curve.
Patrick Lorrain
http://fmyers.com/HullSpeed.html
Best regards.
Patrick
M/V Ursule III
IG 44 MC
France
"max"@\"remove this\"carfax.demon.co.uk wrote:
> I seem to recall that there is a relatively simple equation for
> calculating the maximum speed (and relative efficiencies at lower
> speeds) for a displacement hull.
>
> Could someone tell me please.
>