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Max Hull SL Ratio = 8.26/(DL ratio)
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
Driving all Boats to the Same Maximum 16.9 Knots
Indeed, the wider shorter boats
Iron Kyle (n)
Imagine (n)
can’t be driven anywhere close to the 16.9 knots of the very
, though you’ll note that the medium slen-
(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
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
g re-
gime. Long, slender hulls can operate in this speed
range without actually planing.
SL ratio = Speed-length ratio
Knots = Boat speed, knots
WL = Waterline length, ft.
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
knot SL
HP for
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
2.12 419 5.67 0.7