Short Wave Sportfishing wrote:
On Sat, 17 Jan 2004 09:45:54 GMT,
(Lou Dempster) wrote:
Could someone please tell me what a knotical mile is in comparison to
the standard mile.
A nautical mile is 6,080 feet (1853.18 meters) and is equal to 1/60th
of a degree of latitude. A statute mile is 5,280 feet.
Also what does D/L limited speed: 10.08 kt. and Disp. Hull Speed: 5.90 kt.
mean.
Just guessing here, but the D/L should be Displacement to Length Ratio
and don't aske me how you figure it because it has been a long time
and I'd have to look it up.
I looked it up:-) It's the displacement in tons, (not tonnes) divided
by, the cube of WL length in feet divided by 100.
D div (.01L)3
However, it's basic explanation is that
the longer the boat at the waterline, the faster it is given it's
displacement (weight). For example, a boat weighing 1,000 pounds will
act like a boat weighing 2,000 pounds if you shorten the length at the
water line and vice versa - a 2,000 pound boat will act like a 1,000
pound boat if you lengthen the waterline. There are a couple of other
factors involved, but that's the basic gist of the term.
Yes as Tom says, it's a predictor of surface area resistance.
The Displacement Hull Speed is the speed at which the designed hull
will efficiently travel.
The hull creates a bow wave & a stern wave, the speed of waves is
limited by the distance between crests, if the waves try to go too fast,
the crests get too close then the wave gets too steep & the crest
breaks. The generally accepted speed of waves before the crest breaks is
1.34 times the square root of the distance between crests (in ft).
(Admiral Taylor??) It's often expressed as 1.2 to 1.3 times when using
the sq rt of the boat's waterline length.
If you try to push a non planing boat faster than that, the nose lifts
up the back of the steep breaking bow wave & the stern falls into the
trough ahead of the steep breaking stern wave; save you have huge
amounts of power you'll be trapped, dragging ever steeper & bigger bow &
stern waves along, wasting power & fuel.
There is an exception, there's always a bloody exception:-) If the WL
length to WL beam ratio exceeds 11 then it seems to span or separate the
crests enough that the rule sorta doesn't apply (the bow & stern waves
are completely unrelated waves). e.g. long very skinny multihull hulls
seem to exceed their theoretical hull speed, so do things like
destroyers, some fast skinny ferries, without needing to actually get up
& plane.
A planing type hull can lift the stern, so with enough speed available,
allowing the bow to climb up & over the bow wave then break free, but
this takes some power initially to get the boat up out of the water &
skimming across the top.
K
I am just fooling around with a hull designer and would appreciate any
info.
Have fun.
Later,
Tom
S. Woodstock, CT
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"My rod and my reel - they comfort me."
St. Pete, 12 Lb. Test