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Re: [PSUBS-MAILIST] Underwater pod on surface vessel.
--- Captain Nemo <vulcania@interpac.net> wrote:
> An effect I once heard a British friend refer to as "running with a bone in
> her teeth" (describing the bow wave). In the first half of the 20th
> century, military submarines had naviform bows, and suffered from this
> phenomenon when running on the surface. That changed with the arrival of
> streamlined "teardrop shaped" hulls (ALBACORE, SKIPJACK, and successors).
Interesting! What is the behavior of "modern" subs when on the surface?
> surface waves plowed up by the bow cause a bow-up pitch once the boat
> > exceeds "hull speed" which is generally quoted as 1.34 x waterline length
> in feet = hull speed in
> > kts.
>
> For the general information of all, howsabout providing an example of this
> formula in use based on an arbitrary hull length of your choice, to
> demonstrate how works?
Sure... but first, mea culpa for a typo: it's 1.34 x square root(LWL in ft)...
So, for a 25' boat, hull speed in knots is:
1.34 x SQRT(25)= 1.34 x 5 = 6.7 kts
(Some sources quibble with the figure 1.34, so call it 1.34 +/-.04 - but this is a detail)
Here are some other handy numbers:
WL Length WL Length Hull Speed
(in feet) (in meters) (in knots)
10 3.05 4.24
20 6.10 5.99
30 9.14 7.34
40 12.19 8.47
50 15.24 9.48
60 18.29 10.38
70 21.34 11.21
80 24.38 11.99
90 27.43 12.71
100 30.48 13.40
So, from the above it's clear that a 10' pod could add about a knot to a 40' boat's max speed, and
perhaps half that to its most efficient cruising speed.
-L
=====
"Balee haih-aih ha-hutcha see,
Sanana na hutcha heah."
- Marvin Gaye
=====
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