Two props on one shaft?
 

--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,

--
"Steamboat Ed" Haas : Proud to be the
Hacking the Trailing Edge! : family crackpot!
www.nmpproducts.com
---Decks a-wash in a sea of words---

Two props on one shaft?
 

This is done but I think the props are meant to be contra-rotating.
Duo Drive in UK are developing them for displacement vessels.

Two props on one shaft?
 

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,

Two props on one shaft?
 

I speculate that as the second prop us working on a column of water that's
already been accelereted somewhat, it would need greater pitch to absorb
equal torque.

I recollect that the pitch of jet turbine blades is not constant from front
to back.

"Matt Colie" wrote in message
...
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,

Two props on one shaft?
 

Matt Colie wrote:
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.


IIRC the screws on Turbinia looked rather odd.


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.


Yes and the aft prop will also be operating in turbulent &
aerated flow. Nontheless it can be made to work and I think
the idea of using a higher pitch for the aft prop is a good one.



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


Wouldn't it be the other way around? Governor less than full
advance with the engine at top RPMs and full throttle?


Fresh Breezes- Doug King

Two props on one shaft?
 

Doug,
Some good points required clarification - responses inline.

DSK wrote:
Matt Colie wrote:

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.


IIRC the screws on Turbinia looked rather odd.
Remember, the Wright bros. had to rewrite a lot of those books right
about then because the base assumptions and mathematics were not
correct. (This is the cause of the animosity with Langley.)


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.


Yes and the aft prop will also be operating in turbulent & aerated flow.
Nontheless it can be made to work and I think the idea of using a higher
pitch for the aft prop is a good one.

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

Wouldn't it be the other way around? Governor less than full advance
with the engine at top RPMs and full throttle?

I have made the assumption here that Ed (steamboat Ed) is running either
a steam engine or a diesel (no offense intended Ed). As such, if it can
use more propellor load, the engine will be speed limited (shut down) -
(either by the governor controlled throttle, an admission link-down (or
if diesel, but the rack travel stop)) by the speed limiting function of
the governor when at maximum shaft speed.

This may sound backwards, but it is how things other than SI engines do
business. SI engines can limit on carburater air limit and do not have
the flat torque curve around rated speed.


Fresh Breezes- Doug King

Matt Colie