MadSci Network: Engineering

Re: Why don't boats have transmissions?

Date: Fri Mar 23 21:01:33 2001
Posted By: John Metcalfe, Staff, Computing and Information Services, Texas A&M University at Galveston
Area of science: Engineering
ID: 983826967.Eg

Hi John,
Good question... And I have two parts to the answer that are inter-
related.  The first answer is that boats do in fact have transmissions, 
however it's not quite like what we have in cars... you'll notice that 
boats have a neutral, forward, and reverse.  This is in fact a simple 
transmission.  As for why there aren't more gears than just one, is a 
little more complicated.  
   Unlike a car's tires, which are fairly efficient at any speed, a boat's 
(or even a fixed bladed airplane) propeller (whether it's exposed or 
internally housed as an impeller on a jet boat)(and engine) is really 
designed and set to be truly efficient at one speed.  This is called the 
pitch of a propeller.  If it spins too fast, it "slips" (and "cavitates") 
in its rotation through the water, if it spins too slowly, you save gas, 
but take forever to get there and thus waste gas in the time you take to 
get there.  Usually, the engines are set for their peak performance at 
the "ideal" speed or RPMs, and thus they really don't need more gears like 
a car.  A car on the other hand, has a lot of friction to overcome and the 
tires aren't supposed to slip at all.  Thus, you want to start with a lot 
of mechanical advantage (in first gear) and work your way up to lots of 
speed (in say 5th or sth gear).  So basically a boat engine can do what a 
car engine can't, go from a stand-still to maximum efficient RPM's and let 
the boat catch up as the propeller first slips and then bites into the 
water and accelerates to speed... this would be like throwing your car 
into it's top gear and dropping the clutch at a stop light.  Instant stall.
   As for higher speed, if you look in a motorboating magazine, you'll see 
dozens of ads for "high performance" propellers... These are simply 
propellers that take a bigger bite out of the water with each rotation and 
thus give you higher speed, at the cost of fuel economy.

Hope I answered your questions well enough for you, if you have more, 
write back to us and we'll get back to you as quick as we can with the 

John Metcalfe

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