- Jun 4, 2002While I find this technical stuff interesting, I am a bit boggled by the

abbreviations, initialisms and acronyms here abounding. These AIAs

(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). Anyone else have this concern

and some suggestions about how to ease this pain?

-----Original Message-----

From: wmrpage@... [mailto:wmrpage@...]

Sent: Monday, June 03, 2002 10:25 PM

To: bolger@yahoogroups.com

Subject: Re: [bolger] now hull speed, was Re: mast options for Ruben's Nymph

In a message dated 5/31/02 9:43:14 PM Central Daylight Time,

lincolnr@... writes:

> BTW, if I got your equations right, a good single scull would have

<http://www.stillwaterdesign.com/Pics/older25c.jpg>

> "D/L" of less than 15. I have seen a "wakeless launch" which probably

> has a "D/L" of around 30, scoot around at maybe 15 knots or so with

> practically no wake. See:

> http://www.stillwaterdesign.com/Pics/older25c.jpg

>

My Lotus 1-2-3 spreadsheet tells me that a 21' scull with an all-up weight

of

311 lbs. gives a D/L of 15, so I think you got your sums right. I was

surprised and intrigued by your observation!

Gerr's formulae indicate that this hypothetical scull has a maximum S/L of

3.6, which translates into a "non-planing" top speed of 16.5 Kts. His

displacement SHP formula indicates that it would take approx. .6 SHP to go 6

Kts., possibly attainable by a highly-trained and strong rower, I suppose.

Just how reliable Gerr's formulae are for very low D/L boats at high S/L

ratios is questionable, however, as application of the formulae to the

Stillwater catamaran shows. The displacement formula seems to overstate

power

requirements at high S/L ratios, while the Crouch (planing) formula seems to

understate power requirements at lower S/L ratios.

The Stillwater catamaran is 26' LWL. It has a claimed top speed of 18 Kts.

w/15 SHP long-shaft O/B motor. I assume that the claimed top speed is

accurate, probably a bit conservative - obviously it is a quantity

relatively

easily verified by a buyer who could be expected to complain if the claim

was

not met. (Do you have any insight into why the design requires a long-shaft

motor?)

I've estimated it's gross displacement at 1300# (4 x 200# adults + 15hp O/B

motor + 5 gal. gas tank) or 900# (2 x 200# adults + 15hp O/B motor + 5 gal.

gas tank). This gives D/L ratios of 33 and 23, respectively, S/L maximums of

2.8 and 3.1, which translate to 14.3 Kts and 15.6 Kts top speed. Gerr's

displacement SHP formula gives, respectively, SHP requirements of these

speeds as 23.9 SHP and 22.4 SHP. Conversely, calculated top speeds for 15

SHP

are 12.2 Kts and something in excess of 13.8 Kts.

Crouch's formula for planing boats (I'm rather arbitrarily using C = 200)

gives speeds of something less than 16 Kts. and 19.4 Kts. respectively for

15

SHP. Interestingly, at this power level, the lighter-loaded boat approaches

the 60#/SHP ratio that is the highest #/SHP ratio that Gerr shows on his

Crouch chart.

One could reasonably infer that the performance of low D/L boats at high S/L

ratios falls somewhere in between the displacement SHP curve and the Crouch

SHP curve, perhaps approximating the first up to a certain speed and

approximating the second after that, but I've run innumerable graphs

comparing the two curves for various hypothetical boats and have not

detected

any significant relationship in where the divergence point between the two

curves occurs.

Gerr does not discuss the source of the displacement formula he uses. I

suspect it is derived from tests on full scale naval and maritime merchant

vessels, supplemented by model tests of similar types at the U.S.N. Taylor

basin. Crouch's formula seems to be empirically derived from the performance

of boats of low #/SHP ratios. If I am correct, there is a gap in the data

for

boats of low D/L ratios like Bolger's "Sneakeasy" and others of its ilk.

I believe that the University of Illinois did some wind tunnel testing of

airfoils at low-Reynold's numbers that is of considerable interest to model

airplane builders, while totally devoid of pertinence to commercial airplane

designers. One can hope that perhaps, someday, some suitably equiped

engineering faculty might turn its sights on the phenomenom of low D/L

boats,

like "Sneakeasy".

Ciao for Niao,

I've epoxy to mix,

Bill in MN

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