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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
, the medium-slender 57-foot
, the
47-foot ultra-shoal (27-in. draft.), medium-slender, beach-
able tunnel-drive
Peregrine/Nancy Lakin
, and the rather
solid chunk of a tug yacht,
Iron Kyle
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
Iron Kyle
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
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
) 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
@ SL
1.34 gal/hr mpg
Iron Kyle (n)
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
10.7 113 13.29
6.1 1.75
Table 3 - Max Speed and Hull Length
Boat Name
Max SL
For Max
Cruise SL
HP Cruise E
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
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
DL ratio = Displacement-length ratio
Disp.T = Displacement in long tons of 2,240 lb.
WL = Waterline, ft.
DL ratio =
0.01 x WL, ft.
Continued next p