Jim Michalak's Boat Designs

118 E Randall, Lebanon, IL 62254

A page of boat designs and essays.

(15Jul03) This issue will talk about "what is horsepower". The 1Aug issue will take a look at some real life sail area math.



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.


Looks like this Larsboat is getting a Trilars rig.




Contact info:


Jim Michalak
118 E Randall,
Lebanon, IL 62254

Send $1 for info on 20 boats.



Horse Talk


...that you lived in Olde England. Your name is Wilbur. The Duke is starting up a coal mine and wants to hire you and your horse to lift coal out of the pit. You have a horse name Mr. Edward. He is a totally ordinary work horse except he can talk.

The Duke's operation is quite small at this stage and you rig up Mr. Edward like this with a simple rope with one well greased pulley that offers no drag. The barrel is loaded up my the miners and then lifted up by walking Edward at a fast walking pace of 3.75 mph.

All goes very well until the Duke's miners pick up the pace and start working Edward harder than he can on a day to day basis. Edward calls everyone together for a meeting and tells them," Look, I'm a totally average work horse. When I walk at 3.75 mph (which is 330 feet per minute) I can tell that the most weight I can pull is 100 pounds. Let's multiply that speed by the load I pull. So 330 fpm times 100 pounds is 33,000 pounds times feet per minute. Somebody write that down and call it 'one horsepower'!"

So horsepower is a force being pulled at a speed. One horsepower is what you can expect from an average work horse over an average work day. He can come back the next day and do it again."


I used to do a fair amount of bicycle riding. (I quit because of dogs and motorists.) The books all said at the time that the standard rec rider with medium training would put out about 1/10th horsepower. With that you could ride say 100 miles in say 8 hours. Unlike Mr Edward I would never get up and come back the next day and do it again but there were plenty of riders around who could. There were plenty who could drop you off after your 100 mile loop and go off for another 100 mile loop. Really well trained riders who had the right genes could ride 300 or 400 miles a day more or less everyday!

Old timers may recall Paul MacCready's pedal powered airplane the Gossamer Condor. It was a wispy thing, about 70 pounds of tube and tape and mylar with a wing 100 feet long. They say it took 1/3 hp to make it fly at 10mph. Bicycle racer Bryan Allen once pedaled it across the English Channel in a flight that lasted over 2-1/2 hours.

But Allen wasn't in the same class as riders like Eddy Merx who I recall once sat on a dynamometer (it measures horsepower) and cranked out 6/10 hp for an hour. You would think all the great cyclist are up in that category. Most can ride a 30 mile solo time trial on a real road loop in an hour. I recall the record at the time I was riding for distance in an hour was set by a Russian at 36 miles!

And there is the idea that really big and powerful men won't cut it as bike racers. The great speed skater Eric Heyden found his limit when he went bike racing. I guess power/weight figures into it. Still for pure power you would think a big man would have the edge.

Here's a thought. Let's say you are a weight lifter. You lift a 300lb weight set from the floor to over your head in a two second motion. (I'm guessing at this.) So that might be 6 feet in 2 seconds which is180 feet per minute. That times 300 pounds is 54000 pounds-fpm. That is 1.6 horsepower for that short time.

So one might think that the difference between an ordinary man and a superman is a factor of ten. Then maybe a trained and gifted horse can put out 10hp for a short while.


Remember when Mr. Edward told the group to "write it down"? Trouble is the only guy in town who could write was the professor and he didn't quite get it right to my way of thinking. He wrote down "33000 FOOT-POUNDS PER MINUTE" instead of "33000 POUNDS-FEET PER MINUTE" like Mr. Edward said.

They mean the same thing but where I can understand the idea of pulling a load at a speed, what is "ft-lbs/min"? Sounds like a torque divided by a time and how does that explain the power of the weight lifter? The Professor realized his goof but quickly found he could get paid as a consultant to figure horsepower needs, something the Duke didn't need before.


Not to be undone, Mr. Edward invented an engine powered by explosive burning of his own manure. He brought it out to the Duke's mine and rigged it like this:

So when the engine ran it spun a drum with a 1' radius (2' diameter) which wound up a rope and pulled the coal out of the mine. Let's see..... the weight of the load times that 1' radius would give us "foot-lbs". Now how to figure the time into the equation. Here is how it is done. For the "speed" part of Mr. Edward's method of figuring horsepower you need to calculate the speed of the rope as it is wound up on the drum. The circumference of the drum is 6.28 times its radius. So the surface speed in feet per minute is 6.28 times rpm. So if the motor has a 1' drum radius and spins at rpm and pulls up a weight of T pounds (the torque is also T because of the 1' radius) then the horsepower is rpm times T times 6.28 divided by 33000 which all comes to Torque times RPM/5254.


Your 10 hp motor is turning 4500 rpm. What is the torque? Answer: You can rewrite the hp= T x rpm/5254 as T=5254 x hp/rpm so torque for the 10hp motor turning 4500 rpm is T=5254 x 10/4500 = 11.7 foot-pounds. It doesn't sound like much. On Mr. Edward's drum you could easily stall out the motor by grabbing the rope. But the speed of the rope when you grab it would be 4500x6.28=28260 feet per minute which is 320 mph! Best to gear it down.

Some modern dynamometers work more or less as above except they might use a pump like a water pump or electric device with the engine mount reactions having a scale that will read the torque directly. I think the "load" on the pump is controlled by a valve that regulates how much water is entering the pump. So the dyno operator might rev up the motor with no load and then open the water valve to allow it to start pumping water and load up the motor. The rpms will drop until the motor's effort matches the demand of the pump. The operator takes note of the rpm and the torque (which is the reading on the spring scale times the arm length). He will add a bit more load and the rev's drop a bit more and takes his readings. And so forth. If he plots out the torque vs rpm he might get something like this:

Of course since horsepower is just torque times rpm times a constant you can quickly figure the horsepower and plot it. For example from the chart above we might read at 5000 rpm that the torque is 4.05 ft-lbs. So the hp at 5000 rpm is 4.05 x 5000 / 5254 = 3.9. The hp chart below is from a real old magazine where a West Bend 580 gokart motor was tested. Motors like this were also used in smaller outboards like the Elgins. Here is the horsepower chart for that motor:

Note that peak horsepower does not usually occur at the same rpm as peak torque. If you are gearing your motor or prop for say maximum boat speed you will want to arrange it so the motor is turning at the rpm for peak hp with throttle wide open.

For most of us I think the idea of force times speed is a lot more useful than that of torque divided by time.

POP QUIZ #2!!!

You are fishing quietly by the NO WAKE sign when a cruiser boat goes by 20' away at 30 mph. You see his motor cover says 225 hp. How hard is his big outboard pushing on that transom? Of course power at the motor head is not going to be the same as power delivered at the prop but let's take a look at it for fun. 225 hp is 225x33000=7425000pounds-ft/min. 30 mph is 2640 fpm. So the load figures to be 7425000/2640=2800 pounds at the prop.

POP QUIZ #3!!!

You are building a reproduction of the clipper ship Lightning (David McKay design in Boston) for the movies. The rig will be put in digitally but the hull is supposed to move as the real thing. How much horsepower do you need? My book says, "On march1, 1854, the Old Man wrote in the logbook, "Wind South, strong gales .... 18 to 18.5 knots, lee rail under. ..distance run in 24 hours, 436 miles." The ship had a 226ft waterline and 13000 square YARDS of sail. Her backstays were more than 3-1/2" in diameter of Russian hemp. Well, 18 knots is 1800 ft/min. Not sure what a 'gale' might mean but let's say 40 knot winds. Usually I figure sail pressure as 1 psf in a 14knot wind and the force increases with the square of the speed. So in 40 knots the pressure would be (40/14)^2=8psf. Now, the boat sets 13000 x 9 = 117,000 sqft of sail. So in 40 knots (assuming full sail which is not likely) we have 117,000 x 8 = about 1 million pounds of drive on the hull. So the force times the speed is 1800 million pounds-ft/min. Divide by 33000 gives an equal to 54000 hp! It probably isn't really that much but it has to be a really big number and I've heard 25000 sail hp quoted for the sailing clippers. My book points out the Lightning's fast passage still stands to this day (at least until 1952 when the book was written) and that it took 30 years before a steamship beat the Lightning's record.

(Well, John D. Rockefeller squashed Mr. Edward's hopes for a world full of manure powered combustion engines. Mr. Edward moved to Hollywood and changed his name to Mr. Ed and the rest is history.)


...We'll take a look at real life sail area math as I contemplate putting a lug rig on my Birdwatcher.




Jonsboat is just a jonboat. But where I live that says a lot because most of the boats around here are jonboats and for a good reason. These things will float on dew if the motor is up. This one shows 640 pounds displacement with only 3" of draft. That should float the hull and a small motor and two men. The shape of the hull encourages fast speeds in smooth water and I'd say this one will plane with 10 hp at that weight, although "planing" is often in the eye of the beholder. But a 15 or 20 horse motor is often just a little heavier and just a little more expensive and the extra power can be useful if you start to carry extra passengers (recalling that to plane a boat you usually need 1 hp for each 50 pounds of weight.) The prototype was built by Greg Rinaca of Coldspring, Texas and his boat is shown above at a Lake Conroe messabout. But here is another one finished about the same time by Chuck Leinweber of Harper, Texas:


In the photo of Chuck's boat you can see the wide open center that I prefer in my own personal boats. To keep the wide open boat structurally stiff I boxed in the bow, used a wide wale, and braced the aft corners.

I usually study the shapes of commercial welded aluminum jonboats. It's surprising to see the little touches the builders have worked into such a simple idea. I guess they make these things by the thousands and it is worth while to study the details. Anyway, Jonsboat is a plywood copy of a livery boat I saw turned upside down for the winter. What struck me about that hull was that its bottom was constant width from stem to stern even though the sides had flare and curvature. When I got home I figured out they did it and copied it. I don't know if it gives a superior shape in any way but the bottom of this boat is planked with two constant width sheets of plywood.


Greg Rinaca put a new 18 hp Nissan two cycle engine on his boat, Here is a photo of it:


The installation presented a few interesting thoughts. First I've been telling everyone to stick with 10 hp although it's well known that I'm a big chicken about these things. Greg reported no problems and a top speed of 26 mph. I think the Coast Guard would limit a hull like this to about 25 hp, the main factors being the length, width, flat bottom, and steering location. Second, if you look closely at the transom of Greg's boat you will see that he has built up the transom in the motor mount area about 2". When I designed Jonsboat I really didn't know much about motors except that there were short and long shaft motors. I thought the short ones needed 15" of transom depth and didn't really know about the long shafts. Jonsboat has a natural depth of about 17" so I left the transom on the drawing at 17" and did some hand waving in the drawing notes about scooping out or building up the transom to match the requirements of your motor.

I think the upshot of it all is that short shaft motors need 15" from the top of the mount to the bottom of the hull and long shaft motors need 20". There was a lot of discussion about where the "cavitation" plate, which is the small flat plate right above the propellor, should fall with respect to the hull. I asked some expert mechanics at a local boat dealer and they all swore on a stack of tech manuals that a high powered boat will not steer safely if the cavitation plate is below the bottom of the hull, the correct location being about 1/2" to 1" above the bottom. But Greg had the Nissan manual and it said the correct position is about 1" BELOW the bottom. Kilburn Adams has a new Yamaha and its manual says the same thing. So I guess small motors are different from big ones in that respect.

But it seems to be not all that critical, at least for the small motors. Greg ran his Jonsboat with the 18 hp Nissan with the original 17" transom for a while and measured the top speed as 26 mph. Then he raised the transom over 2" and got the same top speed!

There is nothing to building Jonsboat. There five sheets of plywood and I'm suggesting 1/2" for the bottom and 1/4" for everything else. It's all stuck together with glue and nails using no lofting or jigs. I always suggest glassing the chines for abrasion resistance but I've never glassed more than that on my own boats and haven't regretted it. The cost, mess, and added labor of glassing the hull that is out of the water is enormous. My pocketbook and patience won't stand it. Glassing the chines and bottom is a bit different because it won't show and fussy finishing is not required.

Plans for Jonsboat are $25.


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.

Here are the prototypes abuilding that I know of:

The Alabama Skat is in the paint stage:

This Seal Cove Skiff was completed a while back, built within a few weeks of when the plans were released. There was a delay in launch until the Canadian ice melted. Write up soon.

The Texas Ladybug builder prepares for taping seams:





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