Yer all correct matey's... The "maximum efficient hull speed for a heavy
displacement hull is 1.34 times square root of the waterline length (WLL)".
The key here is "heavy displacement". This is a simple formulae keyed to
"heavy displacement" and a range extending all the way to 1.6 times square
root of waterline length and above is possible for medium and light
displacement vessels (Royce's Sailing Illustrated page 140). Basically, the
Potter is limited by it's displacement to length ratio (DL). Being a light
displacement vessel with an almost flat bottom should theoretically allow it
to achieve and exceed high end of this range (1.6x WLL) for extended periods
of time, while traditional heavy displacement wine glass hulls (Alberg
Typhoon, Island Packet, Pacific Seacraft, etc.) would wallow in their
troughs...
With its biaxial glass, a displacement of 1300lbs and a waterline length of
16'9" the P19 has a DL ratio of approximately 80 making it an Ultra light
displacement sailboat (ULDB). The first ULDB, Windward Passage (DL=100) was
built in 68'. Windward Passage beat every other sailboat of it's time for
almost 15 years.
If you take out the 1,200 lbs of extra "can't live without" gear we stick into
our Potter19s, throw that Putt Putt in the drink, get rid of that huge
battery, move the crew forward and put some nice racing sails (large spinnaker
anyone?) and a masthead rig on, it should be a fast performer capable of
exceeding that 1.6 for long and extended periods...
Notwithstanding the safety record of ULDB's (Transpac, Around the world) we
have hundreds of sailors that swear that heavy and slow a la "Island Packet"
is better than light and fast a la "Windward Passage"... Since many of these
"sailor's" do most of the writing, in sailing mags, they continue to expound
their "heavy and slow" theories while ridiculing all the speed and safety
records established by ULDB's. Their gospel remains that the only boat worth
having is a heavy ponderous crab crusher limited to less that 1.34 x WL Length
with a DL of 300 or more... To fit their ends they publicly claim no sailboat
can exceed this limit. They have done this for so long and so loudly that
almost everyone believes them... Woe to anyone that does not agree...
Now, Off the pulpit, back to the Stock Market and the soothsayers of doom at
CNBC...
Solar Fry
P19 #1028, "Elusive"
In a message dated 98-10-06 13:01:09 EDT, eric@theftnet.net (Eric Johnson)
writes:
To: wwpotter@tscnet.com
Various mathemeticians noted:
> I believe the formula is 1.2 x the square root of the LWL, in feet and
> knots.
> P19 LWL is 16, so sq root is 4 x 1.2 = 4.8 Knots.
<snip>
>I've had a couple of wild rides when my P-19 hit 7.4 mph. Quite a thrill.
>Is that something like 6 knots? I don't have the conversion formula close
by.
<snip>
>I haven't actually 'calculated' the hull speed, but under sail w/ main and
>lapper in a moderate wind, my GPSIII said 6mph.
<snip>
>sqrt(16) * 1.5 = 6 knots.
<snip>
>The maximum hull speed of a 19 foot (at the waterline) boat with a
>frictionless, displacement hull is 5.3 or 5.4 knots. However, the P-19
with
>its flat bottom is neither frctionless nor a pure displacement hull. (Its
LOA
>isshorter than 19 feet as well) It can get up on a plane and excape its bow
>wave. So, it can go faster than that in a good breeze.
I'm a little surprised there's so much disagreement, on everything from the
formula to to LWL of a P19. I don't believe friction has much to do with
hull speed (just on the power required to achieve it), and while the P19
'looks' like a planing hull, we operate it at displacement speeds.
Every text I know says the formula is
Hull speed in knots = 1.34 * sqrt(LWL)
The 'official' LWL for a P19 is 16'9" so that works out to 5.48 knots, or
about 6.3 mph. This is consistent with my findings, measured by GPS. My
fishfinder paddlewheel transducer isn't very consistent at all with the GPS.
Lots of things can affect speed. I doubt any P19 has ever actually planed
for more than a few seconds, but depending on wind and wave conditions,
certainly can exceed this limit by a small amount for extended periods (e.g.
surfing down large waves, getting towed down the interstate :).
But I doubt we'll ever see a P19 play 'flying dutchman' and do the 10+ knot
thing under sail, especially with the keel down because of its resistance.
It would be insane to try to with the keel up, because it would require an
obscene amount of wind dead astern, and a broach would always be imminent
with that much wind, so it would be terribly unsafe to try with the keel up.
If the transom and hull could handle it (doubtful) a 25hp motor might get it
planing. There are some books (Dave Gerr's "The Nature of Boats" come to
mind) that have formulas that tell you how much horsepower you need for a
given displacement to plane. I never ran them all the way through with the
P19, but it was a large amount, and would require more sail that we can
carry or more wind than any of us would dare to sail in! And with that much
wind, the waves would be so intense that planing would still be unlikely.
In short, I don't think you can get there from here.
It can get into semi-planing state (I once had a GPS report an average speed
over 10 minutes of 6 knots under sail), but as far as I can tell, can't
maintain it very long.
>>