That
Tug Concept
I
haven't posted in a while primarily because that 15 foot tug took my
fancy so I decided to work it up to a complete design. I haven't
finished it yet but I wanted to talk about propulsion, which is part
of boat design, and the Kitchen Rudder.
The
Kitchen rudder is the familiar name for
"Kitchen's Patent Reversing Rudders", a combination rudder
and directional propulsion delivery system for relatively slow speed
displacement boats which was invented in the early 20th century by
John G.A.Kitchen of Lancashire, England. It turns the rudder into a
directional thruster,
and allows the engine to maintain constant revolutions and direction
of drive shaft rotation while altering thrust
by use of a control which directs thrust forward or aft. Only the
rudder pivots; the propeller
itself is on a fixed shaft and does not. (Wikipedia)
Because
we're dealing with a tug, thrust and torque are important. The Kitchen
rudder allows the engine to be run at maximum torque constantly and
the thrust to be maximized. So this design utilizes a 23hp Honda
horizontal shaft engine which develops maximum torque at 2500 RPM.
The question is - how big a wheel?
There
are a number of ways to calculate this all of which contain some
guess work, none is truly scientific.
I
propose working backwards from hull speed.
Most
people think of a propeller as screwing its way through the water and
this is a good concept for thinking about pitch but in fact a
propeller is a pump and pushes the boat forward by pushing the water
aft. And that is the principle behind the Kitchen rudder, it directs
the flow of water to not only control speed but direction.
A
tug needs thrust, a lot of it, Our hull is a displacement hull 13.8
feet on the waterline, hull speed for such a craft is the square root
of 13.8 times 1.34ish which gives about 5 kts. 5Kts is 500 feet per minute,
(6000 feet in a nautical mile x 5/60).
Westlawn
has prepared curves of speed versus lbs/hp, using those curves
determines that our proposed 23hp is about right for this little tug.
Using the curves you can determine that the tug requires 1 hp for
every 100lbs of displacement for a hull speed of 5kts. Our
displacement is approximately 2300lbs divided by 100 is 23.
The
next question is the pitch, that's the bit that pushes the water aft,
since we don't want to run the engine at top RPM but at the speed
that will maximize torque we will use 2500 RPM with a 5 to 1
reduction (500RPM at the shaft) to further maximize torque. Hull
speed is 5kts, that works out to 500 feet per minute. So we need to
move a foot per RPM so the pitch is 12”.
Now
we switch to Dave Gerr's ideas on prop diameter, see chapter 32 of
his book, The Nature of Boats. He
has prepared a handy nomograph for determining diameter, using that
nomograph we find we need a 26” diameter prop. Well that won't
work the tug isn't that deep.
So
we'll have to work backwards yet again.
The
maximum diameter that the little tug can handle is 12”, lets reduce
the reduction gearing to 2 to 1 and the RPM to 2400, that's 1200
revolutions at the shaft which gives us a recommend diameter of 16”
and a pitch of 5”. But we can only use a 12” prop. So we need to
increase the pitch. Dave Gerr says for each inch reduction in
diameter pitch must be increased by 2”. (16-12 = 4 x2=8+5 =13) look
at that we're back to nearly our 12” pitch so lets go with a square
prop 12x12, that will reduce our top speed but we don't care as a tug
works a slow speeds anyway. We might even increase the RPM reduction to 3 to 1.
Next
time further exposition on the Kitchen rudder.
