Jim Michalak's Boat Designs

1024 Merrill St, Lebanon, IL 62254

A page of boat designs and essays.

(15 September 2020) We look at bit at hull coefficients. The 1 October issue will continue the topic.


... the world is currently out of Diazo method blueprint paper that I have used for 30 years now. Yes, it is considered obsolete but the machines are much cheaper and more reliable than newer copiers. In 30 years I have had three of them, the most expensive was $250, none required factory maintainance, two are still working. What can I say? The paper is normal paper coated with light sensative stuff. Anyway, they say in a few weeks they should be ready with more. Until then I am out. Any orders will be returned, I guess. Sorry about that.



... is out now, written by me and edited by Garth Battista of Breakaway Books. You might find it at your bookstore. If not check it out at the....


...which can now be found at Duckworks Magazine. You order with a shopping cart set up and pay with credit cards or by Paypal. Then Duckworks sends me an email about the order and then I send the plans right from me to you.


Gene Berry looks at his Mikesboat rig on its maiden voyage. Note that he has swept the leeboard aft to trim away some weather helm. Lots of weight in the stern looks bad trimwise but doesn't seem to hurt the sailing.



Contact info:


Jim Michalak
1024 Merrill St,
Lebanon, IL 62254

Send $1 for info on 20 boats.





Ship design got serious and scientific over 100 years ago when men like Froude started studying the elements of hull resistance through the water. Ships then were strictly "displacement" boats, with no "planing" involved. Resistance was broken into four main catagories. 1) Wave-making. 2) Frictional. 3) Pressure or Form. 4) Air resistance. I'll only discuss 1 and 3 now. Frictional resistance is from the water rubbing past the skin. Air resistance is of the wind against the above water boat.

Waves come mostly from the bow and again from the stern and from any abrupt changes in the hull along its length. All the waves combine in one way or another to make humps and hollows in a resistance curve as the various waves reinforce and cancel each other. Attempts to hang pedictable numbers on the event lead to the "Froude Number". Usually we see the calculation of the Froude Number more in the form of the "speed-length ratio" which is equal to speed (in knots) divided by the square root of the waterline length (in feet). Usually a reasonable maximum speed-length ratio of 1 would be expected from a normal displacement hull although it can be a bit higher. Thus a hull with a 16' waterline length might peak at about 4 knots. A 25' waterline hull would peak at about 5 knots, etc.. The longer the waterline the faster the boat.

It's just a rule of thumb with lots of exceptions. Those of us who row a lot know that there is a lot more involved. For one thing the above rule of thumb makes no allowance for hull shape. For example a fine 16' rowboat will row two or three time faster than a 16' jonboat designed for a gasoline engine.

The Pressure or Form resistance effect is supposed to allow for some of those other factors that separate the faster ones from the slow ones. The form of the hull affects the turbulence and eddying of the hull as the water moves aft.

The wave-making and form resistance elements are often joined together into the "residual resistance".

The idea behind all this was to design a ship, make a small model of the hull and test it in a tank, measuring drag. This to be done at the same speed-length ratio as intended for the full ship, so the model has the same wave pattern as the full sized ship. The frictional drag element of the model can be calculated and subtracted from the whole. Eventually the entire business can be scaled up to the full sized ship and performance or powering requirements determined from the small model tests. That was the whole idea behind the head scratching.


As I said before, some shapes are faster than others. For big ships, where the studies originated, it was found that a rough measure of shape could be the "block coefficient". Figures 1 and 2 show the idea behind the block coefficient.

block coefficient

block area

Here is how it's done. Let's say your boat has a waterline length of L, a maximum waterline beam of W and draft of D. Then you might imagine it fitting neatly into a rectangular block with length L, waterline width W and depth D. That block will have a volume of LxWxD. The block coefficient is calculated by dividing the actual volume of your below-the-water hull by the volume of the imaginary rectangular block.

So there are two parts to the puzzle here. The underwater hull volume is determined by the weight of the boat. Once you've defined the underwater hull the L,W, and D, you need to measure the block are easily found.

First you need to know the total weight of your boat and everything in it. Not always an easy number to come by.

The boat's total weight is equal to the amount of water it "displaces" or pushes aside. Fresh water weighs about 62 pounds per cubic foot and salt water at maybe 64 pounds per cubic foot. So if your boat's total weight were 620 pounds it would push aside, "displace", about 10 cubic feet of fresh water.

Next you must push your imaginary design down into the imaginary water until the volume of hull under the waterline equals the volume of water to be displaced at the given weight. This is usually a trial and error calculation that will be explained next issue. Once you find a suitable draft to balance your weight, you measure the waterline length and maximum waterline beam at that draft. Those are the L, W, and D used to size the "block" of the block coefficient. Multiply L times W times D to get the volume of the imaginary "block"

Lastly, you divide the actual displacement by the volume of that imaginary block. That is the Block Coeffiecient.

A hull like a barge which is totally squared off will have a block coefficient of 1, the maximum you could have. If you refined the ends of that square barge and make them pointy and smooth and round you reduce the block coefficient for the design maybe down to .5 and it would go through the water with a lot less waves than the totally rectangular boat, even though it's main cross section is still rectangular.

But let's say you left the ends squared off and got a .5 block coefficient by using a V cross section instead of refined ends?

To avoid that confusion, the concept was refined from the "block" coefficient into the "prismatic" coefficient.


This is a refinement of the block coefficient. Imagine you have a V or round sectioned hull, or any section which deviates from a rectangular cross section common on big ships. Then you use the "Prismatic Coefficient". Figure 3 shows how that one is figured.

prismatic coefficient

Essentially the prismatic coefficient is figured the same way as the block coefficient except the rectangular block that you figured in the block coefficient is replaced by a similar constant section prism which has the same length as the boat's waterline and a cross section identical to the actual boat cross section.

You can see that the prismatic coefficient sort of takes away the cross sectional element that might confuse the block coefficient calculation. For example you might have two boats with a block coefficient of .5, one with very fine ends and a square cross section, and another with totally square off ends and a V bottom. But if the two were figured using the prismatic coefficient, the first boat would still have a prismatic coefficient of .5 and the second would have a prismatic coefficient of 1!

Anyway, that's all it is.


The prismatic coefficient is a rough measure of the fineness of a hull. A square barge will have a prismatic coefficient of 1. If you streamline one end and leave the other end blunt, as with a modern planing powerboat, you might have a prismatic coefficient of .75. If you streamline both ends you might get down to a prismatic coefficient of .5.

But the experimenters tell us the "optimum" prismatic coefficient is about .6. I assume they are talking displacement hulls in fairly smooth water. A check of my old Mechanical Engineer's Handbook shows almost all modern big ships do indeed have a prismatic coefficient of about .6. Some boats like crude oil carriers are closer to .75, apparently the extra volume in the blunter hull being more important than the speed that might be gained from finer ends. The Edmund Fitzgerald came in above .85! (The edition of my book was written before the wreck.)

We see small boat designers pushing the lines of their designs around to get the optimum. Me, I don't think it is anywhere near that simple or worth chasing after for two reasons. First is that our little boats often operate in conditions that are comparativly rougher than those of a large ship. So I'm pretty sure ends finer than the usual optimum might be better for most of us. And the other reason is that if you design almost any normal looking displacement boat, you will almost always end up with a prismatic coefficient between .5 and .6. The .5 value might be for a fine lined rowing boat or sailing boat with nice pointy ends. The .6 is pretty typical of a sailing scow. Yes, the scow with the "ideal" prismatic coefficient should outsail the other boat in smooth water, but the fine ended boat might keep going in rough stuff long after the scow had to go home.




Philsboat is essentially an IMB with the nose extended to a pointy bow. The width and multichine configuration are the same as IMB's. The cabin is 3" deeper because Phil is at least 3" taller than most of us. In a boat with a Birdwatcher cabin like this one added depth to the cabin makes it safer in that the righting forces in a knockdown are greater. That would be true of any boat if the center of gravity did not move with the cabin roof but with the normal cruiser adding depth to the cabin also means raising the crew deck up so the folks can see over the raised cabin. And that means the CG is elevated too, and then all bets are off concerning self righting. But with a boat like Philsboat eveyone rides down low inside looking out through the windows.

Here is a photo of Bob Williams' IMB:

In addition to the pointy bow and added headroom I added what I hope is a serious motor mount. Probably 3hp will drive it as fast as it will ever go and that at part throttle. But the motor well gets to looking pretty large even for such a small motor. For one thing it must be deep to put a short shaft motor on a deep stern like this so I ran it straight down to the boat's bottom. Working on the motor down it its well will be about impossible and you might need to keep an eye on your knuckles when you pull the starting rope. And the well must be surprisingly wide to allow the motor to swivel in steering although the usual case here will be to keep the motor locked straight ahead and steer with the tiller. The wide well pushed the rudder off center and you need a crooked tiller or rudder linkage to make it all work. I opted for a simple but crooked tiller.

I also added low seats like those I saw added to the two IMB's that came to the Lake Conroe Messabout. Pretty much the same as what I have in Scram Pram where the seats do double duty as water ballast tanks. Philsboat seats could easily be converted into water ballast tanks also but the IMB capsize tests imply the ballast isn't needed.

The sail rig uses a balanced lug, 113 square feet and the same as that of a Bolger Windsprint.

Philsboat uses taped seam construction. Five sheets of 1/4" plywood, five sheets of 3/8" and three sheets of 1/2" plywood.

Actually the size and material list for Philsboat are about the same as that of Scram Pram. So which boat would be better? Take Philsboat if you are a pointy bow guy. It should be better in really rough water. On the other hand Scram is wider and roomier. It has a flat step through bow that will splash and spit in rough water but makes beaching a very nice experience.

Update, 2007. Chris Feller completed his Philsboat, probably the first prototype completed, and brought it to the 2007 Rend Lake Messabout. Very well made and to plans except he used the 91 sq ft lugsail he had on hand for his AF3, with some mast rake changes he calculated with his "sail area math". Sailed correctly rigtht off the drawing board, so to speak. We had a chance to use it for a few really nice days. In a good sailing breeze, say 10 to 15 knots with occasional whitecaps, our gps bobbed between 5.5 and 6mph for two hours of reaching as we crossed back and forth on the big lake. When I used the Philsboat I thought it was probably just as easy to enter from a beach as the blunt bowed Scram. The front deck is about 2' high so you can sit on it and swing you legs around onto the deck and then into the cabin.

Chris is a big boy but there was plenty of room inside for the two of us and more. Rumor has it that at the campsite the night before Chris slept in the Philsboat on the trailer with an airconditioner in the front hatch plugged into the campsite power. So the boat is plenty big in the way most people would use it.

The stern layout has the motor in a small well to one side and the rudder offset to the other side to give the motor room to swing. There is no linkage to the tiller, the tiller is simply "unstraight" so that is falls on centerline at the skipper's hand. Seems to work well and is quite simple. Chris uses a 2hp Honda which has a fairly large cowling for its size and must be rotated 180 degrees to grab reverse. He said the motor well size is a bit too small for that. Karl James had the same problem with his Jewelbox a long time ago and simply cut away the side of the hull in top of the motor well region - after all it is just there for looks. Chris said he tried the boat under power and found it went 6 mph max, just like under sail. No surprize to me. My Birdwatcher also maxes out at 6mph under 3hp and under 5.5 hp. This sort of hull will only go so fast. Add more power and you just dig a deeper hole and make a bigger wake - you won't go faster. But sometimes more power is nice on a windy day.

(As an update, Chris has brought his Philsboat to several more Rend Lake meets. In 2008 the meet was very windy and Philsboat got a full windows wet knockdown with Chris and Tom Hamernik on board. She self righted with no issues and they kept right on sailing, but Chris confessed he has 100 pounds of lead shot under the seats. Also he is using a 3hp vintage Johnson which he says is quieter and smoother than the Honda.)

You might recall from the Prototypes section recently that there was another Philsboat being built in California. That one last I heard was about to be launched. Has more changes from blueprint than Chris's boat but still is pretty true to form. Seems to be known as Bumble and made by Rex Meach.

And in New Zealand Rob Kellock has been sailing his with a junk rig. He has been knocked down a couple of times and self righted as planned:

Plans for Philsboat are $45.


Prototype News

Some of you may know that in addition to the one buck catalog which now contains 20 "done" boats, I offer another catalog of 20 unbuilt prototypes. The buck catalog has on its last page a list and brief description of the boats currently in the Catalog of Prototypes. That catalog also contains some articles that I wrote for Messing About In Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3. The both together amount to 50 pages for $4, an offer you may have seen in Woodenboat ads. Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currency that I can admire but not spend.) I'm way too small for credit cards.

We have a Picara finished by Ken Giles, past Mayfly16 master, and into its trials. The hull was built by Vincent Lavender in Massachusetts. There have been other Picaras finished in the past but I never got a sailing report for them...

And the Vole in New York is Garth Battista's of www.breakawaybooks.com, printer of my book and Max's old outboard book and many other fine sports books. Beautiful job! Garth is using a small lug rig for sail, not the sharpie sprit sail shown on the plans, so I will continue to carry the design as a prototype boat. But he has used it extensively on his Bahamas trip towed behind his Cormorant. Sort of like having a compact car towed behind an RV.

And a Deansbox seen in Texas:

Another prototype Twister is well along:

A brave soul has started a Robbsboat. He has a builder's blog at http://tomsrobbsboat.blogspot.com. (OOPS! He found a mistake in the side bevels of bulkhead5, says 20 degrees but should be 10 degrees.) This boat has been sailed and is being tested. He has found the sail area a bit much for his area and is putting in serious reef points.






1oct19, Herb's OliveOyl, Larsboat

15oct19, Herb's OliveOyl 2, Jonsboat

1nov19, Herb's OliveOyl 3, Shanteuse

15nov19, Herb's OliveOyl 4, Piccup

1dec19, Taped Seams, Ladybug

15dec19, Plywood Butt Joints, Sportdory

1jan20, Sail Area Math, Normsboat

15jan20, Trailering, Robote

1feb20, Bulkhead Bevels, Toto

15feb20, Cartopping, IMB

1mar20, Small Boat Rudders, AF4Breve

15mar20, Rudder Sink Weights, Scram Pram

1apr20, Two Totos, River Runner

15apr20, Water Ballast, Mayfly16

1may20, Water Ballast Details, Blobster

15may20, Mast Tabernacles, Laguna

1jun20, Underwater Boards, QT Skiff

15jun20, Capsize Lessons, Mixer

1jul20, Scarfing Lumber, Vireo14

15jul20, Lugsail Rigging, Vamp

1aug20, Prop Slip, Oracle

15aug20, Sharpie Sail Rigging, Cormorant

1sep20, Guessing At Weight, OliveOyl


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