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.

Changes yet again

The Look Astern and Other Changes

The stern on the original design was also quite complicated. It had a motor well for the outboard which could be sealed off when the motor was not fitted by a plate which was held in place by a bungee cord attached top a removable curved piece that fit into two slots in the side of the well. The space on either side of the well was for storage and flotation.

So in the new version the transom is flat across and the well is eliminated. And an electric trolling motor is fitted through a PVC tube right aft of the dead wood. The original also had a leeboard which in the new version I've traded for a centreboard.

Because all the spars had to fit into the hull for rowing they were quite short which constricted the sail area. It was about 49 sqft, a boomless standing lug sail, with sheets led to each quarter. There were no permanent stays or shrouds.

This is the new design with the old rig.

The previous design was more row, motor, sail. In the new design I wanted more emphasis on the sail and less on the motor so I increased the sail area to 80 sqft keeping the the same foot and yard as in the old sail which means that the mast is now some 17 ft tall which might require stays and shrouds. So here we have the new rig. You'll notice that the centreboard is located in exactly the same place as with the old rig. But, because of the shifted centre of effort of the sail, it is deeper.

The sheets also had to be beefed up a bit given the increase in sail area.

We'll look at the interior arrangement next time and that will determine how we can lay out the shrouds and stays or even if we need to use them.

It's all about profiling

Outboard profile

The out board profile is what anyone looking at your vessel from the side will see. One of the perils of drawing outboard profiles by hand is the perspective effect caused by you looking at the profile with its top edge away from you. What appears to be a neat boat on the drawing board suddenly doesn't look all that swell when pasted up on the wall. The advantage of computer drafting is that you are already looking at the boat as it would appear on the wall.

When you are drawing the outboard profile you should include everything that you, as the designer, planned to have fitted on the boat, the builder/owner may have other ideas. So be it.

So we start with the outline of the hull as we drew it in the construction drawing. And remove any extraneous lines.

So we need to add a house. If you recall the original design called for a place to get out of the wet and an open cockpit. The design was also for up to three adults so two can be out of the wet and the other can steer. So the house should be at least half of the waterline length. And a way for those inside to see outside, a port perhaps and or a hatch. Now a comfortable seat needs to be about 15 or 16 inches high and with the same depth and minimum head room is 38 inches so the cuddy top should be some 4'6“ above the midship frame. A hatch needs to be a minimum of 2 feet square. The port can be any size or shape but should fit with the overall design. I always like a small foredeck, not that you would want to stand on it, but it gives a place for a substantial samson post forward and a small storage locker below.

So all that would look like this.

The cuddy would be open at the back for air in the cuddy and also to provide air for the engine which is air cooled. We could also decide upon a rudder at this stage.

The rudder could be transom hung or be under the boat on a rudder shaft. Because this is a small slow boat I think that a transom hung rudder would be better and fits with the type of boat.

OK you say but what about the propeller and engine. Well in this profile you can't see the engine but you're right about the propeller. The problem is you can't show the propeller until you decide the angle of the shaft and to do that you need to decide where the engine is going to go. Which brings us to the inside profile.

I lied when I said in the last post that we would do the deck plan, we really need to establish where the engine will go and what the shaft angle will be because then we can finish the outboard profile and the construction drawings for the keel.

Inboard Profile

We start with the almost completed construction profile, see why this is easier on a computer.

Then we add the cuddy that we drew on the outboard profile.The port, samson post, fore deck and some seating at the transom and in the cuddy.

Now for the engine. We have the specifications for our engine. It is 15.4 inches long, 12.6 inches wide and 13.6 inches high, the shaft height above base is 4.17 inches to the centre of the shaft. The propeller that we have is 8 inches in diameter so once we draw that in with the shaft exiting from the back edge of the keel and the propeller tip 2 inches away from the bottom of the boat, to stop thumping, we can establish a shaft angle of 5 degrees which will not interfere with the lubrication of the engine.

There is a little bit of a problem, the prop spins just a little below the keel which, if the boat takes ground, will cause damage to the prop so we'll put a skeg on the keel to protect the prop.

Now that we have established the shaft angle we can extend that back into the boat and determine where the engine will sit and draw in the engine bearers. But we have a problem, the optimal RPM for the prop is 600 RPM the engine runs optimally at 3600 RPM so we need reduction gearing. There are two ways to accomplish this, an actual reduction gear set up or v-pulleys. Given that this project is driven by cost, V-pulleys it is. A 2” steel pulley on the engine output shaft and an 8” pulley on the propellor shaft.

All of which means that the engine must be several inches above the shaft centreline probably, about a foot. There are calculators for this at http://www.gizmology.net/pulleysbelts.htm. For the purposes of this exercise we're going to place the two shafts 12” apart. Given that distance we get this,

The square box is the engine, the area under it is the engine mount, the beds should cover at least three frames to reduce vibration and strain on the hull. Covering the shaft coming into the boat is the shaft log. The concentric circles on the engine are the exhaust. We can extend the exhaust to show on the outboard profile and draw it in.

And there is the almost complete outboard profile and the almost complete inboard profile.

Outboard Profile

Inboard profile

Next time we'll finish both these drawings and move on.