Paddle tugs - power and speed

[Derek Warner]

Evening all

I noted a tug (1 of 3) recently on her way to intercept a 120 thousand tonne (ore carrier) - but the thing that struck me was that she must have been travelling in excess of best guess 3 times walking speed or say 15 miles per hour - although the audible engine noise (speed) was low as was the diesel exhaust plume

When pushing the ore carrier into her berth the engine audible engine noise (speed) was much higher as was again the diesel exhaust plume - naturally there is an engine overspeed limiter but this is for engine safety - not a speed limiter etc

So if tugboats by today's standards have a very high power to size ratio (so they can push massive displacement loads) do they have a high speed capability unloaded?

So if this is true, did our older paddlewheel tugs have the similar reserves of power that theoretically could propel them at higher speeds?

Not that I want to turn my 1880's PS Decoy into a race boat - the real reason I ask this is that within a week or so I must nominate the paddle blade height - from the three old photographs available I can scale paddle blade width and paddle wheel diameter but not the height

Any help or comment greatly appreciated

regards Derek

[Paulrjordan]

I have been meaning to reply to Derek's thought provoking email.
(Actually I wrote what I thought was a quite brilliant effort but
Yahoo didn't think so and "ate it") So although it's a week late..here
goes again.)

--- In Paddleducks@y..., "derekwarner" <derekwarner@b...> wrote:
> Evening all
>
> I noted a tug (1 of 3) recently on her way to intercept a 120
> thousand tonne (ore carrier) - but the thing that struck me was that
> she must have been travelling in excess of best guess 3 times
> walking speed or say 15 miles per hour - although the audible engine
> noise (speed) was low as was the diesel exhaust plume. When pushing
> the ore carrier into her berth the engine audible engine noise
> (speed) was much higher as was again the diesel exhaust plume - >
naturally there is an engine overspeed limiter but this is for
> engine safety - not a speed limiter etc
>
> So if tugboats by today's standards have a very high power to size
> ratio (so they can push massive displacement loads) do they have a
> high speed capability unloaded?

I raised this question at a gathering last Friday with one of our two
large Tug Skippers here in Victoria (I mean the TUGS are large..not
the skippers!). He said that you can be sure if a tug is running
light it's usually going to be running FAST and probably at max speed
because for them especially... TIME is MONEY! The enormous power and
torque needed to push heavy barges barges allows the tug to run light
at it's absolute maximum hull speed

As you know, all displacement hulls (as opposed to planing hulls which
"lift" at speed) have a maximum hull speed...a speed at which no
amount of power will push them faster. You can usually tell a
displacement vessel has reached its max hull speed by its wake...a
massive rolling bow wave which breaks beyond the stern uninterrupted
by an intermediate wake along the hull. This can be seen clearly in
model R/C displacement hulls which are often seen moving well beyond
scale speed and right up towards max. hull speed. For some reason
this does not look unrealistic in tugboat and some warship models, but
certainly does so in most others.

My impression from talking with Tug boat Captains is they seem to
"rule" the harbour and have the "blind eye" of authorities when it
comes to excessive speed and wake..They're generally in a hurry and
seem to be "overworked." It's an unpopular Harbour Patrol Officer
who gives a "speeding ticket" to a Tug Captain. This is generally the
way you'll see tugs when they're running light and I well recall being
regularly bounced mercilessly by tugboat wakes when I had a houseboat
moored on the River Thames in London.

Captain Dan also explained to me the various (incredible) propulsion
systems on modern tugs, from Kort, Tractor and Schottle drives right
up to variable pitch props which are controlled from the bridge and
give tremendous increases in efficiency. This might explain the noise
and smoke configurations Derek noticed on his tugs.

> So if this is true, did our older paddlewheel tugs have the similar
> reserves of power that theoretically could propel them at higher
> speeds?

Good question and this is ALL theory here, so if anyone has any issues
with what I write, please don't hesitate to correct me. First, I think
we're talking sidewheelers here. (You'll recall I posted some info I'd
found on American Riverboats and it was estimated these vessels could
REACH speeds of around 20 KNOTS!!! (now THAT's going some for a
displacement hull).

However if you look at old photos of most sidewheelers (especially the
sleek British excursion paddlers like "Waverley", "Talisman" et al)
you'll notice their bow entry is very clean and without the tell-tale
rolling bow wave and wake pattern indicating close to max hull speed.
My suspicion is that these "express" paddlers were capable of far
greater hull speeds than their power plants would provide. The very
configuration of a paddlewheel drive must have a high resistance
factor and there is definitely a maximum rotation beyond which the
wheel loses it's efficiency. Once again, I suspect the maximum
efficient rotation on a sidewheeler is far less than hull speed and
there was insufficient power available to push them up to their
maximum hull speed. However, I think it may well be that sternwheeler
hull shapes and resistance resulted in lower maximum hull speeds and
that in these cases there might have been sufficient power to attain
hull speed. (purely dangerous speculation here)

> Not that I want to turn my 1880's PS Decoy into a race boat - the
> real reason I ask this is that within a week or so I must nominate
> the paddle blade height - from the three old photographs available I
> can scale paddle blade width and paddle wheel diameter but not the >
height

No..you don't want to turn her into a raceboat, Derek, but you DO want
her to operate efficiently and to make maximum use of battery capacity
while she's running at scale speed, and still having a little extra
power in reserve. First of all, let's get the feathering blade thingie
out of the way. Looks lovely but the concensus seems to be it doesn't
increase the efficiency of a WORKING model. (no matter what Graupner
says!!)

We're talking THRUST here...so let's look at the components to this:

1) Float area (width and height of each blade)
2) Float shape (flat, concave, feathering?)
3) Total float area (number of floats)
4) Wheel diameter
5) Rotation speed (not torque)
6) Plunge (position of blade in relation to waterline)

I'm going to take a break here..cos sometimes Yahoo mysteriously
"swallows" posts if they're too long and I also want to check out some
of my previous posts where I think I've dealt with the issue of
paddlewheel thrust before. Part 2 follows when....ever!!

Regards

PJ

[Derek Warner]

Sorry PJ - I am a little lost here - I understand a hull has a design
speed - but consider that the parameters for determining this are
typically - vessel type, intended run, distance, tonnage (displacement) and
in today's terms economical engine (cruising) speed are is relative to just
about all of the above

Whilst sure, a 100K tonner designed for 25000 shaft HP could not accept
75000 shaft HP without structural reclassification, an increase of any %
over the original 25000 shaft HP will produce a theoretical increase in
vessel speed in an ocean or large body of water (although not linear in %)

"OR"

Are you talking about the phenomena of again say a 100K tonner entering a
land locked (apart from a small mouth) port where the vessel is
progressively displacing the same 100K of water back into the ocean which
increases the speed of water under the vessel over that of the vessels speed
and hence the vessel increases it's draft?

Any thoughts or have missed something at school? - regards Derek

[Paulrjordan]

Oops, Derek, my statement was misleading and should be ammended to:

" As you know, all displacement hulls have a maximum hull speed...a
speed at which an increase in power in no longer proportionate to an
increase in speed and MAY provide the threshold of inefficiency."

Or something along these lines anyway.

Here are a few definitions of "HULL SPEED"

1) The Handbook of Sailing, Bob Bond defines it as "the maximum speed
a hull can achieve without planing" (p 342) and goes on to qualify:
"only boats which can plane, or which have very narrow hulls, can
travel faster than this maximum theoretical speed" (p 327).

2) (Steve)Colgate's Basic Sailing Theory (p 119) states: "the
theoretical speed beyond which a displacement boat cannot go, usually
1.34" times the root of waterline line length.

3). Chapman's Piloting (p 217) refers to the root of the wavelength
multiplied by 1.34 and (p 26) refers to the LWL root times 1.34. He
calls this a close approximation and describes this as the point at
which there is no reasonable increase in speed for the hull for a
reasonable increase in power.

4)MarinerKayaks.com has an interesting approach to the subject,
describing hull speed as "the maximum practical speed of a
displacement (non-planing) hull. Since a wave's length is proportional
to the square of its speed, the wave created by a moving hull will at
some speed become longer than the hull's waterline. At this speed
the stern of the craft will no longer be supported by any of the
following wave crests. You will feel the stern squat into the
trough following the bow wave. You will also notice that far
greater effort yields little increase in speed because to go
faster you must now, in addition to the other forms of resistance,
also work against gravity to climb out of the trough."

Derek, if you are interested in examining hull speed from three
different factors (frankly I think the whole subject is all a little
fuzzy and is terribly over hyped by competition boat manufacturers)
then have a look at this Website.

http://www.surfbaud.co.uk/~wave/surfprop/Formulae/Hull/hull.html

It also covers the formulae for HULL SPEED, PLANING SPEED, BLOCK
COEFFICIENT (Hull shape) and the WAKE FACTOR I was referring to in my
previous email about "rolling bow waves" etc.

However, when you've finished reading it all, let's get back to
figuring out an answer to your original question which really amounts
to "what is the most efficient "plunge" for the paddle blades
(floats)on "PS Decoy"?

I have to run right now..but I'll EXPOUND a little later in the week
on MY thought about model paddle wheel blades for what it's worth.
Just didn't want my previous post to cause confusion (in which it
succeeded eminently!)

PJ

[Derek Warner]

Thanks PJ - you have clarified points that I did not understand & will read
http://www.surfbaud.co.uk/~wave/surfprop/Formulae/Hull/hull.html tonight

But I wrote it wrong too (maybe the second glass of medicinal spirit?)

When I spoke of the 100K tonner entering a land locked port with a small
mouth, the vessel does not increase it's draft but sinks closer to the core
of the earth as there is a progressive displacement of 100K of water to
float the said vessel in the restricted port mouth when entering at an angle
and on a roll in bad weather etc

Today's vessels have the benefit of computers to optimise fuel effiency
relative to the planned date & time of arrival at the nominated port

All very interesting but not for our paddleships of yesteryear which were
simply but beautifully inefficient and consumed fuel at the rate of
technology
of the day

regards Derek

[Derek Warner]

Thanks PJ > Have read it - reminds me of a little of certain "statements!"
in marine mechanics & design @ college all those years ago - and as you
pre-empted, not necessarily related to our task at hand or should I say not
technically correct when revisited by today's computer design capacity

> figuring out an answer to your original question which really amounts
> to "what is the most efficient "plunge" for the paddle blades
> (floats) on "PS Decoy"?

All will progress by the tried & trued "Trial &Error Method " - hope to
resolve by mid August this year

regards Derek