[MHml] horsepower & sleds

[brian eiland]

 hello brian
       Just what is the difference between the durajet and the rim drive?
besides the durajet having 5 blades and what looks like a simpler drive
compared to the rim drive.E-tec are supposed to get the same economy as a four
stroke and you don't mix the oil. I am interested because I bought a 60 hp
durajet but have yet to try it out.
 Mike

Brian replied:
The single biggest difference is the attachment of the propulser blades to the
outer rim of the unit as opposed to providing for a close tolerance fit
between the rotating blade tips and the rim of the unit. The 'close tolerance
fit' between blade tips and rim shell usually results in lots of efficency
loses plus noise and erosion.

As stated in there initial objective, "In the 1990's, the U. S. Marine Corps
recognized the need to develop an outboard motor which permitted training
exercises to be safely conducted at both night and day with men in the water.
Attempts to adapt cages and guards to open propellers to permit in-the-water
access to the boat resulted in unacceptable performance losses with decreased
engine reliability" .Their primary objective was safety from the open prop.

It's thought to beat the good old std open prop for efficency at anything
under 20 knts. Start surrounding it with a variety of other devices such as
nozzels etc and you usually end up with more drag. Kort nozzeled props are
probably the most famous of the shrouded prop arrangements, but they are
intended for slow speed power thrusting as in tugs etc. High speed they lose
out.

Then think of the drag factor as a whole. Every piece of the underwater gear
comprising the propulsion system acts to create drag....the prop itself, the
hub, the shaft, the shaft supports, etc. Then look at the rim drive unit...no
hub, no shaft, no close tolerance blade tips to rim fits. And the blade tips
do not have to taper off at their outer portions as in the open prop
configuration. There is a lot to recommend it, but it does not lend itself to
mechanical drive as well as the newer permanent magnet drive mechanisms. Here
is another posting I made on the subject:
<http://boatdesign.net/forums/showthread.php?p=65512#post65512>
...and one that depicts several older mechanical drive unit patents

     


Brian Eiland

beiland at usa.net
www.RunningTideYachts.com
distinctive expedition yachts