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pulsion, we can compare four different boats. In fact, there

are four designs from my office: the long and very slender

67-foot

, the medium-slender 57-foot

, the

47-foot ultra-shoal (27-in. draft.), medium-slender, beach-

able tunnel-drive

, and the rather

solid chunk of a tug yacht,

at 45 feet.

Though we could make the comparison using the non-

dimensional transport efficiency alone, it makes it easier to

follow if all the boats are the same size. The important con-

sideration here is that size is not length but displacement

(weight), which is the same as volume. Accordingly, I’ve nor-

malized the

,

, and

designs to the

same 45,000-pound displacement as

. I’ll refer to

the normalized example boats using the designation (n).

Thus, our four comparison boats are in Table 1 on the previ-

ous page.

Obvious Differences and “Normal” Boats

The differences between these normalized boats are

obvious and they are primarily in how long and slen-

der (or wide and beamy) they are. The DL ratio

(displacement-length ratio) is the clearest indicator

and the lower the DL ratio the longer a boat is for its

weight. You could, however, get a low DL ratio with a

wide hull that was very flat and shallow. The other

indicator is simply DWL divided by BWL (datum water-

line length divided by beam waterline), also called

“length-to-beam ratio.”

Interestingly, the seemingly rather chunky tug yacht

has a DL ratio of 334 and a DWL/BWL ratio of 3.2:1

Though heavy and beamy compared to the other boats in

our sample group, both a DL of 334 and a length-to-bea

ratio of 3.2:1 are not at all unusual these days. Many a s

called trawler yacht is in this range. Indeed, there’s nothi

specifically “wrong” with a heavy, beamy boat, but—what

we’re discussing here is efficiency. It’s the fact that boats

such proportions are not uncommon which indicates that

took that “wrong fork” in design so long ago.

Comparing Speeds and Efficiency at Hull

Speed

The common, and as we’ll see in a bit, inc

rect belief is so-called displacement hulls

limited to a fixed hull speed. This is a spee

length ratio (SL ratio) of 1.34. Assuming w

drive all three boats to an SL ratio of 1.34,

get the results in Table 2.

You can see that at nearly the same powe

the longer slender boats go faster. More i

portant, transport efficiency (E

T

) grows higher as the boat

becomes longer and more slender. This is reflected in hig

nautical miles per gallon—in improved mileage.

Slender Hulls Mean Higher “Hull Speeds”

It gets better still for slender hulls. The fact is that the rul

-thumb “hull speed” is not accurate. Maximum hull speed

not a constant 1.34 times the square root of the waterlin

feet. Instead

the constant

1.34 is a var

able and tha

variable is pr

portional to

ratio. The for

mula I’ve de

oped that de

fines this rel

tionship is:

Table 2 - Performance at Speed-Length Ratio 1.34

Boat Name Knots

HP @ SL

1.34

E

T

@ SL

1.34 gal/hr mpg

Iron Kyle (n)

8.4

99 11.86

5.4 1.56

Imagine (n)

9.1 106 12.00

5.7 1.58

Peregrine (n)

9.5 109 12.31

5.9 1.62

Ironheart

10.7 113 13.29

6.1 1.75

Table 3 - Max Speed and Hull Length

Boat Name

Max

Knots

Max SL

Ratio

Power

For Max

Knots

High

Cruise

Speed

Cruise SL

Ratio

Cruise

HP Cruise E

T

Cruise

mpg

Iron Kyle (n)

8.5 1.36 117

7.5 1.20

81 13.1 1.7

Imagine (n)

10.6 1.56 220

9.5 1.41 130 10.3 1.4

Peregrine (n)

12.3 1.72 458 10.0 1.40 156

9.0 1.2

Ironheart

16.9 2.12 419 14.0 1.76 256

7.7 1.0

Displacement-Length Ratio

Displacement-length ratio is a non-dimensional measure of ho

light a boat is for it’s length. The heavier a boat for its length th

higher it’s DL ratio and the lighter the boat the lower its DL rati

Where:

DL ratio = Displacement-length ratio

Disp.T = Displacement in long tons of 2,240 lb.

WL = Waterline, ft.

Disp.T

DL ratio =

3

0.01 x WL, ft.

Continued next p