20835Re: [bolger] now hull speed, was Re: mast options for Ruben's Nym ph
- Jun 4, 2002In a message dated 6/4/02 7:46:17 AM Central Daylight Time,
> These AIAsOh Dear Me! I see I've inspired our humorists to new levels of cyber-speak,
> (acronyms, initialisms and abbreviations) are not only confusing to me, but
> I sometimes cannot understand if they are technical terms from the topic
> under discussion, or cyber-speak (like BTW).
and I can't get the jokes because I don't understand the "AIA's" ("AIA" being
an new "initialism" for me.) (Thanks for including the definition in
parantheses, BTW. :-)) (The AIA "BTW" and "emoticon" are my attempt at
I'll try to provide a de-coder for my AIA's, in the off-chance that you are
actually interested and not just setting me up as a figure of fun:
S/L = "speed to length ratio"; mathematically this is a boat's speed in
nautical miles per hour ("knots" or "Kts.") divided by the square root of its
waterline length (LWL) in feet. The significance of this ratio is that the
energy expended in creating the surface waves generated by boats that
progress through the water at "displacement" speeds is what limits their
speeds. A 16' boat at 4 Kts. will produce the same wavelength relative to its
length as a 36' boat at 6 Kts. Both boats would be moving at an S/L of 1.0.
At that S/L ratio the wavelength of the wave and the length of the LWL
coincide. Above that S/l the boat has to start climbing up its own bow wave
to go faster. Above an S/L ratio of 1.0, the amount of power required to
increase speed increases at an accelerating rate as speed increases.
D/L = "displacement to length ratio"; in the form used by Gerr, this is the
displacement of the boat in "long tons" (2240 pounds per "long ton") divided
by the cube of one percent of the LWL in feet. The dividend is a
dimensionless number, which is perfectly arbitrary, as far as I can tell, but
commonly used to compare boat designs. The higher the ratio, the heavier the
boat is for its length and the more power it takes to drive it to a given
speed. Unless the boat is designed to "plane", there is an S/L ratio where
adding additional power results in rapidly diminishing returns.
O/B = outboard motor
Kts. = "nautical miles per hour"; a nautical mile is about 1.15 statute
C ("Crouch Formula") = this is just a constant; George Crouch was an eminent
designer of high performance planing power boats in the '30's and this is
derived from his work. Gerr gives values of "C" ranging from 150 ("average
runabout") to 230 ("racing power catamarans").
Kts. = "nautical miles per hour" a.k.a. "knots"; a nautical mile is
approximately 1.15 statute miles; an hour is an hour, I think.
SHP = "shaft horsepower"; this is the power available to turn the propeller.
With inboard engines deductions from crankshaft horsepower have to be made
for power losses to accessories, transmission, bearings and etc. O/B motors
are now rated for SHP, but I have recently purchased a 1983 vintage Mercury
O/B "40" that was subsequently marketed as a 35 SHP model when Mercury
changed its ratings from "crankshaft" HP to SHP.
LWL = I believe this is an "intialism" for "Load Water Line"; for purposes
like calculating S/L and D/L, it is the distance between the stern of the
boat and that portion that contacts the water at its most forward point.
Overhangs, bowsprits, pulpits, etc. do not interact with the air/water
interface and so "LWL" is used as a reference datum, rather that "LOA"
(Length Over All), a different dimension, with its own valuable uses. (e.g.
docking fees, trailer length, storage facilities, etc.)
Ciao for Niao,
Bill in MN
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