It's actually pretty easy to get decent performance from a vehicle
because you have so much less drag to consider and you can mitigate a
lot of what's left. Air drag is pretty minimal for a car until you
get to decent speeds, and you can do an awful lot with tire pressure
and fooling with bearings, lubricants and aero aids. The best
comparison to what you get in a boat might be gravity.
I recall a Ford Aspire conversion that I helped a friend with that
got an easy 30 to 40 miles per charge as long as the trip was North
or East/West of his home, but it was all Fred could do to struggle
the 3 miles to the top of that nasty hill to his South and the pack
was pretty much done at the top.. It's like that for a boat but every
direction acts like that hill because water is so "draggy".
You can overcome some of it with lower speeds or a slicker hull, or
you can use a design that knocks down drag by planing or trapped-air
support. It takes a lot of sustained power to keep those up though.
I'll attach a Picture Fred's tiny runabout. The motor here's a G-32
jet starter from a 707, the boat's an 8 ft A-Stock Runabout, usually
powered by a 15 hp racing Evinrude.
This video shows the current world record run of 98.8 set by Mike
Bontoft in 2008:
http://www.youtube.com/watch?v=-yNu2_LlO9s
At speed, this boat rides on air trapped between the sponsons and
distributed under the boat for the length of the hull...You'll see
that better on the return pass where the skidfin's not kicking up spray.
The battery pack was a huge help as well. In the past, we ran about
600 pounds of automotive batteries. The weight forced more water
contact and the pack's limited density meant about a 5 mph loss on
the return due to battery-sag. Mike's Lithium Pack weighed only 80
pounds. and though 12v less than the allowed 144v, the lighter weight
let the hull fly as designed, and the pack's deeper density not only
eliminated sag, it allowed a step-up gearbox to bring the racing prop
up to the 9,500 to 10,000 rpm where it's most efficient. And when you
hear Mike's wife Brigitte exclaim over his 99+ pass, be aware her
remark was actually made on Mike's return pass...The first time ever
that an electric boat was faster on it's back-up pass than on it's
initial pass.
John
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