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.

A Rose is a Rose or maybe not

The Eye of the Beholder

Every boat is beautiful to its owner, you won't find anyone who will admit that their boat is a dog but what is beautiful to one eye is not to another.

If you're designing for yourself this little problem isn't a problem but if you're designing for someone else then it can be.

PhilBolger designed some very square boats, I happen to think that they are beautiful because they meet the design purpose. Similarly JimMichalak designs squareish boats and I like the look of them too. Other people don't. So at the outset you need to find out what the client wants the boat for and what he or she thinks makes a good looking boat.

You can do this using cartoons or sketches which you can discuss with the client.

TadRoberts has a website that shows his designs and a Facebook page where he discusses various designs of his and others.

I recently entered a design contest which asked for a design for a houseboat without really setting out the parameters. What they did was ask for a design and your reasons for selecting that particular design and how you would use it.

Here is the profile that I submitted,

I like the sheer to be at its lowest about 2/3rds of the way from the bow. I think that that is very pretty even on a squareish boat.

The interior layout should also suit the client, in order to get the design started I imagined a young, somewhat penniless, couple with a small child who wanted a weekend getaway. The idea was to use the vessel as a cottage for two on a large wilderness lake and it shouldn't cost a lot to build nor be too difficult. They also wanted it to be trailerable so they could use it as a travel trailer much along the lines of Roy Schreyer's Dianes Rose.

The boat is essentially a trimaran with a large center hull for storage under the cabin and two smaller hulls for flotation with a tiny house on top. It's not meant to go fast but it is meant to be beachable. It's 24 feet by 8 foot 6 inches, which is the maximum trailerable width.

I like George Buehler's idea that a boat that's meant to be lived on should do drinks for 6, feed 4, and sleep two. So the two chairs and the sofa bed seat six for drinks, the dinette table plus one chair is room for four for lunch or dinner and the sofa bed sleeps two. Because of the small child I made the dinette table able to lower to make a small bed. There is ample play space on the foredeck and high bulwarks to keep the child on the boat and not in the water.

My wife says that if you're going to do dishes by hand then you should be able to look out the window and since part of the reason for having this boat was to take a gander at nature in the wild we have lots of windows where they count.

The toilet is a composting one to keep the non-potable tankage to grey water only and eliminate the need to do major pump outs.

Now you may think that this little square houseboat is downright ugly and you're entitled to think that but my imaginary couple don't because it meets their needs and budget.

Next time we'll look at Kuai Lei, my Chinese junk, and why I designed it the way I did.

Stability at last

The End of Stability

Work, work, work! No time for blogging until now.

Let's look at a really round bottom boat first.

So first we'll join the chines which leaves us with a trapezoid, and finding the centre of that is described here (centre already marked). If we join the centre of that line with the centre of the curve we essentially have two almost triangles and finding the centre of those is described here.

Once we have the centre of each almost triangle

We can join those centres with a line.

Where that line crosses the line join the center of the curve and the chine line is the centre of the area as both almost triangles are the same size.

We then join the two centres of area and do the math set out here and that gives us the centre of buoyancy

Again if you overlay this hull with the other two the centres of buoyancy are almost the same.

Well what about a boat with no parallel sides. That's even easier than our square boat.

You can divide the underwater area into two triangles and we already know how to find the centers there so I won't go into detail. What is most amazing is that an overlay of this dory type hull onto the other three puts the CB in almost the same place as all the others.

The only thing that is the same about the four hulls is their waterline at the beam. The dory shape is very much smaller displacement although the other three are about the same. I'm going to investigate this further and will comment on it at some later date.

Next time aesthetics or does it look good?

Stability

Next to last kick at the stability cat

Well it turns out that it does work for small round bottom boats and boats without two parallel sides. I wouldn't say that it is one hundred per cent accurate but for the boats we're dealing with it's close enough. Here's how we go about it .

This is the hull we dealt with last time,

So I took that hull shape and rounded off the corners, gives a sort of canoe shape,

We'll keep the centre of gravity the same but the waterline needs adjusting to account for the change in underwater shape. Once that is done then we're going to create a trapezoid, first by cutting off the round parts, those volumes are so small, and equal in area, 50 sq inches, that it makes little difference to the calculation at this juncture.

Now we're going to join the upper ends of those lines that cut off the round part to make a trapezoid and a triangle and then find the centre of those bodies as we did previously.

Once we have both centres then we need to do some calculation, first we'll join both centres with a line. Our centre of buoyancy is somewhere on that line. But where.

In my last post I sent you to Jim Michalak's page here, http://www.jimsboats.com/1dec15.htm where he sets out how to calculate the center of effort of sails. We can do the same thing here. So lets name the triangle area small or AS and the trapezoid AL. We'll call the line joining the two centres LS-LL. Now we measure the area of AS and AL and the length of LS-LL

AS is 131.45 sqin and AL is 270.75 sqin, LS-LL is 12”. Divide AS into AL and add 1, that come out to 3.06. Divide LS-LL by 3.06, which is 3.9. The center of buoyancy is 3.9 inches from LL.

I can hear the mathematicians out there saying that's not right! But here's an example using a lever and fulcrum.

How much weight do we have to hang off the end of the nine foot arm to balance the lever assuming the lever weighs nothing? (which is what we're actually doing when we calculate the relative distance of the centres of our areas)

10x3/9 = 3.33 lbs

So now lets turn that around and say where do we put the fulcrum to balance a 10 lb weight and a 3.33 lb weight on a 12 foot lever?

((10/3.33)+1) divided into 12' = 3' from the heavy end.

Getting back to our vessel, at 5 degrees of heel the righting arm is 2.5” and GM is 2' 4.8”

How does that compare with our square bottom vessel? At the same degree of heel the square boat had a righting arm of 1” and a GM of a foot however if you overlay the two at the waterline,the centres of buoyancy are almost identical.

Next time a quick look at a greater degree of heel and a rounder bottom vessel and one without parallel sides.