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Max Hull SL Ratio = 8.26/(DL ratio)

0.311

But never less than 1.34.

This gives the maximum speed-length ratio a hull can be

driven without planing. Applying this to our example boats,

we get the results in Table 3, previous page.

This shows just how much faster long, slender hulls can be

driven without planing. Of course, going faster uses more

power on any hull form (long and slender or short and wide)

and this shows up as lower transport efficiencies and miles

per gallon at the higher speeds. The important thing here is

that the longer boats can be driven at these speeds, where

the short wide boats can't—at least without modifying their

hulls.

Driving all Boats to the Same Maximum 16.9 Knots

Indeed, the wider shorter boats

and

can’t be driven anywhere close to the 16.9 knots of the very

slender

, though you’ll note that the medium slen-

der

(n) can achieve a max SL ratio of 1.56. You can

take hulls of these displacements, and overall hull propor-

tions and modify them—by giving them planing-hull charac-

teristics—to allow them achieve 16.9 knots. They

would then be true semi-planing hulls. The resulting

power required is in Table 4.

Once again—if we drive to the same high speed the

slenderest hull is capable off—we see just how much

more efficient more slender hull forms are. The trans-

port efficiency and miles per gallon delineate this

clearly.

Next issue, we’ll conclude our investigation of power-

boat efficiency by looking at the effect of propulsion

efficiency, at the effect of overall size, and we’ll examine

considerations in slender hull forms with regard to seake

ing and accommodations.

Speed-Length Ratio (SL Ratio)

Speed-length ratio (SL ratio) is the non-dimensional

method of assessing how fast a boat is going relative t

its length. Boats operating at SL ratios under 1.34 are

considerd displacment-speed boats and boats running

SL ratios over 3 are considerd fully planing. In between

1.34 and 3 is the semi-displacment or semi-plani

n

g re-

gime. Long, slender hulls can operate in this speed

range without actually planing.

Where:

SL ratio = Speed-length ratio

Knots = Boat speed, knots

WL = Waterline length, ft.

Knots

SL ratio =

WL, ft.

Fuel-economy being as important as it is these days, there’s a natural feeling that hybrid diesel/electric or gasoline/

electric vessels may offer improved mileage (better efficiency). It seems obvious: It works on cars so it should work for

boats. Unfortunately, hybrid propulsion is not the solution for boats.

All ground vehicles (cars, truck, buses, etc.) spend a significant portion of their time either braking, coasting, going dow

hill, or creeping along in bumper-to-bumper traffic. In all of these situations, the internal combustion engine needs to d

liver little or no power, yet—in conventional vehicles—it must continue to run inefficiently nevertheless. Hybrid electric ca

take advantage of this by effectively shutting down or electronically nearly shutting down the internal combustion engin

and using stored electric power during these specific periods. If you add in capturing regenerated power during braking

and going downhill, the fuel savings are significant.

So, why doesn’t this work on boats? Simple. Boats never do any of these things. They don’t brake, coast, roll downhill, o

spend hours creeping in slow traffic. Marine propulsion engines are always producing the continuous power needed to

erate the constant thrust required to overcome the resistance of the water at any speed the boat is operating at. There’

simply no gain to be had from the hybrid approach.

Worse still, every time you transfer energy from one form to another, there’s a loss. So, going from, say, a diesel genera

to storage batteries, looses power, and then going from the batteries to the electric propulsion motor looses more powe

Hybrids Are Not The Answer

Continued on page

Table 4 - Driving all Boats to the Same Maximum 16.9 Knot

Boat Name

16.9

knot SL

Ratio

HP for

16.9

knots

E

T

16

.9

knots mpg

Iron Kyle (n)

2.70 635 3.74 0.4

Imagine (n)

2.50 558 4.26 0.5

Peregrine (n)

2.37 511 4.65 0.6

Ironheart

2.12 419 5.67 0.7