- Jun 4, 2002In a message dated 6/4/02 7:46:17 AM Central Daylight Time,

nickerb@... writes:

> These AIAs

Oh 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

cyber-humor.)

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

(U.S.) miles

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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