Ed,

You are correct about Turbinia. I was told that their thinking was that
this could be an alternative to a larger radius (a problem with the
higher than expected shaft speeds) but still a chance to put all the
horsepower into the water. Turbinia was not the only vessel with this
feature.

Problem: A marine screw propellor creates thrust by accelerating a
column of water. The second prop will be in the column "wash" of the
first and have much less mass to accelerate.

This is why pushers work better than tractors when the prop can be
behind the hull.

Do you have horsepower left over at flank? (Up against the govenor with
less than best admission or partial throttle...)

It might help.

Matt Colie (yes- an NA and ME and glad to offer opinions)




steamer wrote:
--Wondering about this arrangement and am thinking of taking a whack
at it just for grins. I've got a relatively flat bottomed boat with a long
length of prop shaft beneath; two struts to keep shaft in line and a
relatively gentle angle between shaft and hull. At present I'm turning a
15x21 3-bladed prop but I've got a 12x21 3-bladed prop which would fit
neatly about 18" ahead of it. I know that back in 1903 Turbinia had three
shafts with 3 props per shaft and I've heard that a character I can't get
hold of at the moment tried this on his steamboat a few years back, but I
don't know if there is any scientific evidence of improvement in speed by
doing so. Comments??
--TIA,