Wheel of a deal

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.