Panel Discussion

A panel discussion

Many, if not most, small boat designs are made with stitch and glue building in mind. For that to succeed panels must be developed. There is software to do that however some of these programs are expensive, others are complicated or will not work with certain operating systems. Hulls and Windows 10 come to mind.

However there is a way to develop panels using simple geometry. To describe this method I will recreate my Sampram dinghy design.

Here are the lines,

The first panel we'll develop is the bottom. You can find the length of the panel by measuring the curve of the bottom and the widths come from the section half breadths. So we start by drawing a baseline and then transferring the distance between section lines to the baseline. I've used my computer program but all of it can be done by hand with compasses, rulers and squares.

Repeating the operation until we have all the section distances transferred. You'll notice that not all of the circles are the same size, this is how we determine the true length of the bottom.

The half breadth measurements are taken from the sections drawing and transferred to the section distance marks on the baseline. 

Draw a vertical line through each section distance mark and where that vertical line cuts the half breadth circle is an intersection on the edge of the bottom. Join all the intersections  in a smooth curve and eliminate the circles and vertical lines and you have a half panel developed for the bottom. 

Divide the length of the baseline by ten and mark off section lines for dimensioning.

You can infer that the same can be done for the sides however it is slightly more difficult. First thing to do is to develop the bow and stern transoms and from those developed panels you can measure the true length of the side of each transom. To develop the panels draw two lines perpendicular to the transom at the gunwale and at the chine.

From the lines plan get the half breadth of the top and bottom of the transom 

and apply those lengths to the perpendiculars. 

Join the intersections, eliminate all the unnecessary lines and you have a half panel of the transom. 

 

Now we're ready to start developing the side panels! Next Time!

Tiny Tug

The Little Tug

The first iteration of this design looked like this,

It was obvious to me that there was too little bearing aft to make this a successful little tug. Working from the original concept I redrew the lines to give the boat more bearing aft, to place the center of the deeper box keel under the steering position and give a little more room for the propulsion system.

The original lines lines looked like this,

The redrawn lines like this,

As you can see the stern is wider as is the box keel, there is more bulk in the box keel closer to the fore and aft centre of the boat giving more space for the propulsion system.

We discussed the propulsion system here  and here . The little tug with the propulsion system fitted looks like this,

The two wheels at the helm station control the direction of travel, the smaller wheel controls the forward and aft motion of the Kitchen rudder clamshells given reverse, neutral and forward and the larger wheel moves the rudder arm from port to starboard. Here is the rudder layout.

You can see from this drawing that the bulk of the box keel falls right under the helmsman, the towing point and the motor.

Here's the tug in profile and plan. I'm pretty happy with it.

In the Kitchen

In the Kitchen

Rather than go one at great length about how the Kitchen rudder actually works you can actually see the operation of it here and here.

Why I am so enamored of the Kitchen rudder? Because it gives you twin engine manoeuverability with a single engine, watch here. The vessel you see in the video is single engine and the inner wheel in the wheel house controls the angle of the Kitchen rudder and the outer wheel is the helm or steering wheel. You can only get that sort of manoeuverability normally in a twin screw vessel with opposed outward turning wheels.

For that reason the Kitchen rudder eminently suited to our small tug not only can you run the engine a constant level of revolutions but you can vary the speed, stop within a boat length and quickly manoeuver to avoid girding or girting. (depending upon where you are in the world)

The controls for the Kitchen rudder could be hydraulic, Bowden or Morse cable or line on a drum. For one of my designs, much larger that the little tug, I designed an hydraulic system. However for the little tug a simple drum and rope system is more than adequate.

More on the little tug to come.

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.

Conceptually speaking

Concepts

You are not going to find the ideal boat.
You are not even going to have it if you design it from scratch.
-Carl Lane

This is boat from the board of Tad Roberts, it was built by Barrett Faneuf, it's beautiful.

Years ago hundreds of these little launches plyed the waters of the BC coast, they were the main means of transportation mainly because there were no roads.

The reason I put this up is because in 2012 I saw a version puttering around Ladysmith harbour(it was nowhere near as good looking). So at that time  I thought that as a concept a low powered 15' plywood launch would be a great starting point.

So I drew these lines,

With that as a starting point I created the basic launch

Then added to that basic launch to make a yard boat,

Then modified the lines to produce a tug with a deeper body to allow for a heavier engine and scantlings and a horizontal prop  shaft.

The tug has a covered after deck and protection for the operator.

And then I thought what about a steam launch at which point I realized that a plywood boat would not be appropriate so I modified the lines yet again still keeping the original concept.

All that from a simple concept drawn from seeing a small launch puttering about.

CONCEPTS!

The concepts the thing

Practice safe design: Use a concept.

Petrula Vrontikis

So what was the concept for Kuai Lei? What I wanted was a small boat capable of carrying me around the Gulf Islands and possibly doing an overnight. As I mentioned last time it all started with this.

If you compare the sweep of the sheer in the above picture and the sheer of Kuai Lei here you'll see that they're almost the same. Further inspiration came from Jonque de Plaisance. The JRA magazine which featured Kuai Lei erroneously said that the sheer came from Jonque de Plaisance when in fact it came from this old picture.

With the sheer determined then came the question of square or round boat. The Jonques are round but I'm a fan of Phil Bolger so square was it.

Next question - how big? She need to be trailerable so nothing much over twenty feet. In fact she's twenty feet LOA but only 12 feet on the waterline not including the deadwood. From the stern post to the leading edge of the daggerboard she's 15 feet.

I wanted a small cabin long enough to sleep in with sitting head room and a large cockpit so essentially she was to be divided into two spaces.

So we had a concept, now came the work of actually designing the boat.

First came the lines.

Then once that was done it was on to what cabin height, what sail configuration. Should the masts be canted forwards as on most junks, can we cant the mast forward even if we wanted to? All design questions that need to be answered.

First and foremost form must match function. Lets look at cabin height. I'm 5'6 and a half (I used to be 5'7” but you shrink with age), the height of the sheer at midships is just short of three feet so another three feet on top of that would give me standing headroom but the boat would look clunky and boxy. At most another foot would be suitable. But taking that height out to the gunwale would also produce a boxy look.

Here is the main bulkhead as drawn and as built,

The left side is as drawn, the right side as built (the builder always knows better than the designer). The height to the top of the hatch is just over 4 feet which means that you cannot have sitting headroom except at the centre line. Them's the breaks.

To see what I mean by boxy, here's the bow of the boat as designed,

And here it is with the cabin taken to the gunwale without any slope.

Ugly ain't it?

We'll explore this concept thing some more next time.

JOY

Kuai Lei

This is Kuai Lei, at my mooring in Ladysmith Harbour. She is my iteration of a chinese junk. Why a junk you might well ask . I like the look of them and the ease of hoisting and reefing sail. How and why did I design her the way I did?

Kuai Lei is a cross between long micro and a chinese junk. I found a picture on the Cheap Pages of a junk hull that showed a large sliding rudder balancing a large dagger board

which was fitted just ahead the mast. In another picture, on another site, the dagger board was again forward of the main mast, between that and the forward mast. So I thought why not? No keel to get in the way when grounding and no great lumps of leeboard to knock off on the dock.

This was the first go around,

but the thought of that high stern and the forward daggerboard made me think that perhaps there just might be to much weather helm or that I might have to double the size of the rudder.

The second try was better, at least I thought so.

JimMichalak's writings had turned my mind to stitch and glue for the hull. Then I realized I had no idea how to develop the panels. All my building in the past had been the old way, frames, stem, transom then stringers. Scribing was the way to develop side and bottom planking.

Much thought ensued but I finally came up with a solution and no the web was not helpful. To test it out I got some basswood, printed the developed panels and glued the print to the basswood. I cut the panels out and made a model. Ha, it worked, what do you know about that. But then there were second thoughts. Five bucks worth of basswood is one thing four hundred dollars worth of plywood entirely another.

More indecision. So I designed a small dinghy just to test everything out

It's just a small, two sheet, pram style dinghy based on the sampan but it became my test bed and yes the panel development did work full size.

More on the design process next time.