> 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.