Paddlewheel speed

[Les Toombs]

I am planning on building a sternwheel tow boat, but I have no idea
what speed (RPM) range the paddlewheel should turn???

Can anyone help me with this.

Also, would the speed range be the same for a sidewheeler??

Thanks


Les Toombs

[Joe E Brown]

My sternwheeler uses and geared motor with a max of 100 rpm. Main
thing to remember is crosswinds are havoc on tall shallow draft
models and you need extra power to work those winds. If you run in
moving water the current can give you trouble too. I find 100 rpm to
be comfortable when I need the power. Most of the time I think I'm
running around 60 rpm
Happy Modeling
Joe E Brown
http://www.dragg.net/jbrown

[Paulrjordan]

WELCOME TO PADDLEDUCKS LES, Good Question!!!

Almost the first one you ask yourself (and others) when you start
building a paddlewheeler.

I'm not going to comment on sternwheelers. Joe says he runs his at
around 60 - 100 rpm and that's good enough for me..coz he's built a
few of them.

What follows is a lengthy (as always)account of how I tackled this
whole issue with a sidewheeler.

David Powell and I went through this last Winter in Victoria when
setting up our side paddlers. Dave originally installed motors
and gearboxes for "Talisman" that would deliver the same 60 rpm to
the paddles as the original vessel. I felt 60 was too slow in a model
and geared my "Director" side paddle Tug to a top speed of 120 rpm.
Any faster would have looked a little silly and I was concerned about
cavitation.

The following is taken from notes I made at the time.

Test 1: No waterline on the model when I got her so ballasted till top
of her lowest blade was just below surface. (Had no idea where her
true waterline should be). Noted 120 rpm rotation was about maximum
I wanted "visually" but boat speed too slow in relation to her paddle
rotation (Must say, the boat looked a little silly and toylike in the
water!)

Workshop: Didn't want to increase rotation so increased area of each
paddle blade by 25%. (This now approximated the prototype far more
closely, as the original builder of the model had understated the size
of the paddles by almost 50%. Each blade was still 25% less area than
the original plans.)

Test 2: Good increase in speed but still not QUITE enough to
correspond to paddle rotation. Holding boat still, noticed alot of
water "escaping" over tops of blades. With both engines at full speed
pushed boat deeper into water and noticed immediate change in paddle
wash pattern with noticeable extra thrust "felt" against hands.
Added extra ballast to "new" waterline giving noticeable increase in
hull speed. Boat travelling really well with good wash pattern
breaking approxiametly 18" astern of the wheels but now looked too low
in the water.

Workshop: I decided at this point to get some accurate prototype
pictures of the boat to see what the correct waterline was . When
they arrived I was shocked to see how low these tugs rode in the water
with their sponsons barely two feet above the water. Take a look at
prototype photos and see how close to the surface the sponsons on
many sidewheelers appear to be. This would suggest the plunge is quite
deep on the paddles. In fact the height appeared almost exactly where
I had ended up in the previous "sea trial". I decided to now ompletely
rebuild the wheels and blades to match the prototype dimensions and to
copy the concave shape of the blades. The "Director" Class tugs were
the last side wheel paddle tugs ever built and, with their twin Paxman
diesels and huge blades (60' beam), were incredibly powerful for their
size. (just take a look at the photos in our files area). The only
thing I didn't do was replicate the feathering blade set up.

Test 3: Boat performed beautifully with hull speed close to
prototypical "full ahead both". Boat speed and paddle rotation looked
"balanced" at 120 rpm. At "prototype rotation" 60 rpm she was
definitely "under speed". Even if the paddles were rotating a little
too fast for "scale" it wasn't really noticeable between the sponsons
and water.

I was happy enough now with the set up to start building her unique
steering activated dual engine command system that I have called the
"Director" system, the drawings for which can be found in our files
area. (more on this at another time)

The only thing left to do is to actually increase her TOP paddle
speed to approx 180 rpm, so I am not constantly at "full throttle" on
the TX stick. It's far too fast for operating speeds and I'm not sure
how much it may cavitate but I'd like a bit of extra reserve in the
event of wind or current and also because batteries have a habit of
running down!!.

So my OWN experience dictates that it's 100 -120 rpm for my 30"
sidewheeler, but it seems so much depends on paddle shape, size
and plunge. Now you can see why I also like to keep the weight of the
boat down.

I'd like to hear from the everyone else with THEIR experiences on
this important subject.

Hope this all helps..it's a fascinating topic

Paul J

[MikeDD656]

Sounds like it may be best to set up gearing for 200rpm and only use what is
needed, knowing that there is plenty of reserve

[Paulrjordan]

Probably right, Mike..but cavitation has to be a factor over 120.
Since these are displacement hulls, hull speed/weight of course is
another part of the equation.

Dave Powell has gone for Raw Power in 'Talisman" with a rotation far
in excess of 200 rpm (remember he started out at 60 rpm which was the
prototype speed) and in the Test Tank this paddler has enough oomph to
churn butter!!! The cavitation is so intense it throws out a huge
carpet of bubbles astern!!! ...quite impressive really but...!!

She's due for "sea trials" very shortly and it will be very
interesting to see how the set up works because he has so much power
in reserve. John Gough with "Bulldog" was rotating at "around" 120
and came up with the same conclusion as I did... prototypically
acceptable rotation/hull speed but needs that extra bit of thrust in
case of wind or current.

I have looked at the wash patterns of models (and real ones) and
there's definitely a good looking "swell" that occurs approximately
two to three diameters astern of the wheel(s) on a correctly trimmed
paddler. This seems to disappear if the waterline is either too high
or too low or the rotation is too fast (or too slow). I have all
kinds of notes on this stuff and I wonder if sometimes I get a little
confused by it all!!!

My approach is to build the hull and mechanics as light as possible,
drop the boat in the water and start experimenting. (oh by the way,
youll need to build temporary paddle boxes over the wheels as the
"aquatic display" can be quite spectacular without them!) Once
again, I am very pleased with my own "Director" side paddler's
performance (especially since she was such a DOG when I got her). Not
only does she look prototypically to scale when travelling on the
water, but she is also highly competitive in steering competitions and
can do things which are virtually impossible for many conventional
prop models.

Some of you have asked me to explain her control system (rudder and
two fully INDEPENDANT forward/reverse engines in 2 channels!!!) which
I'll do next week...and which I hope will prompt a call from Trevor
(Bodiam)in England..also a "Director" Class Tug builder and one who
has designed a much more efficient system than mine.

Next question..HOW MANY OF YOU HAVE INSTALLED A SINGLE ENGINE RATHER
THAN DUAL ENGINES IN YOUR SIDE PADDLERS?

Paul J

[Paulrjordan]

I have just received an email from our member, Edmar
(Mammini) edmarmam@ig.com.br in Sao Paulo Brazil which I would like to
pass on.

Edmar wrote>
" Beside this, there is a commentary to the folks of the group. They
are discussing the correct RPM for a model and the speed.
Well, there is a formula to calculate both and are similar.

RPM is the square root of the scale that multiply the real RPM.
Speed is the square root of the scale that divide the real speed.

Example : 1/4 x 1' like 1:48 SQ 48 is 6.92 x 60real
= 475model RPM
" " " "
real knot8 divided 6.92 = knot1.15 model

I'm waiting you commentary E.Mammini"

Edmar: I should warn you, that Physics was never my strong subject..so
I hope I have this correct. I think I understand the first
formula...but in your email there were a some " " " (ditto marks) in
the second one and I'm not sure where they belong.
But from what I can see..am I correct in thinking that, according to
the first formula, the paddle speed for a 1:48 model which has a
prototype rotation of 60 RPM would be 475 RPM? (wow that seems
awfully fast..unless I'm displaying my ignorance here!!)

Maybe you can expand on this for us. The next question in relation to
this formula is SHOULD WE BE EXAGGERATING OUR PADDLE BLADES to allow
for slower rotation and greater thrust..and if so..what are the
parameters of scale we should be considering?

Looking forward to hearing from ALL you Engineers out there!

Paul J

Thank you