Jim Michalak's Boat Designs

118 E Randall, Lebanon, IL 62254

A page of boat designs and essays.

(1dec06) This issue will speculate on power requirements. The 15 December issue will continue the topic.



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.


Garth Battista who publishes our books at breakawaybooks.com is starting a Vole dink. He skipped the wire ties and just used duct tape sticking it together before glass taping the inside.




Contact info:


Jim Michalak
118 E Randall,
Lebanon, IL 62254

Send $1 for info on 20 boats.



Figuring Power


I got an email a few weeks back that quickly disappeared into outer space in the usual computer "accident". So I never got to answer it but I recall he had a 40 hp motor and wanted to put it on something like Dorado. By the usual rules Dorado would be limited to 30 hp. How much larger could Dorado be and take that 40hp.

The above photo shows the only Dorado I know of built and used by Ashely Cook in Australia. He uses a 30hp two cycle and that was fine. So we have a good data point to start with.


I am going to use Hullforms to enlarge the Dorado hull on paper and then run the "drag vs speed" feature that was presented in the recent forms of that great program. You should be able to click on the link at the bottom of this website to get to a free download of a version of Hullforms. It is not an "instant" program in that you must essentially define your hull shape will offsets. Once that is done it will give you hydrostatic info that would otherwise take days to figure (normally you wouldn't take the time to do them they are so tedious to do by hand).

Some of you may recall that in September 2003 I made up this gadget:

WHAT THE HAY IS THAT??? you ask. Here we have mounted on the back of my AF4 my "Thrustometer". We have a motor mounted on a bracket that would pivot freely except that its top is trapped by the big frame and there is a bathroom scale pinched in between to read the forces between. The scale reading is directly proportional to the net thrust on the transom and that should be the same as the net drag on the hull. Or so I still think. Anyway, here is the a photo of my AF4 (in its early days):

And here is an image of the hullforms model I made for the AF4:

After running the thrustometer tests I did the Hullforms drag calculations and compared the measured with the forcast, like this:

In general I guess the measured is about 15% below the predicted once above plane and that is OK I think. (I still don't understand why the 12 hp drag results were higher than the 15hp since both motors were almost identical otherwise, but such is life.) With nothing else out there to make such preditions that I am aware of I think we can use the Hullforms predictions to scope out new designs.


I made up this model of Dorado by taking my Frolic2 model and giving it a straight run in the stern, which is exactly how Dorado was designed.

You have to give it a weight to run the Hullforms calculations and I used 800 pounds. The hull is of course much lighter than that but once you hang a motor on it and a skipper and some gear I would say 800 pounds is still a pretty light all up weight. Hullforms predicts the following drag for the 18.5' standard Dorado:

What have we here? It's interesting that it predicts drag less across the board compared to AF4 which is the same size and weight as Dorado but has a real flat bottom while Dorado has a multichine. One might think that the AF4 might thus "fly" a little quicker off the water but the Hullforms professors didn't think so. (On the other hand AF4 measured drag was across the board less than the Hullforms predictions.) Hullforms predicts that the drag on the multichine Dorado is half that of AF4 at 6 knots. Could be. Certainly the Frolic2 that gave birth to Dorado is a very fast sailor.

What does this mean for horsepower? This always had me confused because after the AF4 tests it appeared that only about a third of the rated hp is being used to push the boat. I suspect the drag of the lower unit itself must be as much as the drag of the hull! Anyway, a hp is defined as 33000 ft-lb/min. Ashley reported cruising at 20 mph with the 30 hp motor. 20 mph is 1760 ft/min. Drag at 20 mph (about 18 knots) appears to be 120 pounds. Multiplying the two we get 209,000 ft-lb/min. Divide that by 33000 and you get 6.3hp driving the boat. I'm assuming Ashley is not running full power at cruise, let's say he is at 1/2 power or 15 hp. 40% of that is driving the boat, the rest lost?

What if the modern motor could deliver 40% of its full power, or 12 hp, to pushing the boat. What would top speed be for the 800 pound 18' Dorado? 12 x 33000 = 396000 ft-lb/min. Well, Hullforms predicts that at 24 knots drag would be 180 pounds. That equals 432000 ft-lb/min. So top speed would be just under 24 knots (about 28mph).


All I did was extend the transom aft another 3.5' like this:

I also increased the weight by 20% to 960 pounds. Results like this:

Another increase in drag across the board. It looks to be about 40% once the boat is on plane. So once you plane the "longer is faster" idea is no good. Bolger mentioned this in one of his essays a long time ago - the shorter boat is lighter and since only the aft fraction of the boat is in the water when the boat is on plane, extra hull length is of no advantage. A 40% increase in drag at a given speed would imply needing 40% more power to get there compared to the shorter and lighter 18' Dorado. So the 40 hp motor on the 22' Dorado would allow it to stay with the shorter boat with 30hp. Maybe.

Again let's take a guess at top speed now with 40hp at 40% "efficiency" or 16hp actually pushing the hull. That is 16 x 33000 = 528000 ft-lb/min. Well, Hullforms predicts that at 23 knots drag will be 240 pounds and that is 552000 ft-lb/min so top speed should be a bit less, maybe 21 or 22 knots (about 25mph).

Note that below planing speeds the 18' hull at 800 pounds and the 22' hull at 960 pounds are predicted to have about the same drag.


We'll ponder power requirements as we make Dorado wider and then both wider and longer.




Dorado is a Southern Hemisphere constellation and also this boat designed for Ashley Cook in New South Wales. The idea behind it was for a rough water power boat with a sleepable cuddy cabin. We started with my Frolic2 sailboat (which itself evolved from the Toto canoe) and straightened out the stern lines to adapt it for planing power. After mocking it up Ashley said he and family needed another 3" of headroom so Dorado B was born and built. The plans show it both ways. I should add that even though Dorado started out as a modified sailing hull, it probably would be a terrible sailer just as Frolic2 would be a terrible power boat on plane. The two worlds just don't mix and the idea of "just hoisting a simple sail" on Dorado is bound to be a sure loser both because it will sail poorly and because you will lose the simplicity of a straight power boat. Not that the idea isn't worth more study. It would take a special person to do all the tweaking involved with making the two-way boat work well.

The shape works! Ashley has been running DoradoB into the rough stuff. In the above photo he is off the east coast of Australia with nothing but ocean between him and New Zealand. He says the big curling wave to his side is a permanent fixture and fatal to boaters trying it. This sort of water is the real advantage of a bottom shape more complex that simple flat bottom, of course. A flat bottomed boat like AF4 would get beaten up badly and be clumsey in such waters. Also the more complex bottom handles better all around, especially in tight turns, than the flattie and that is why the Coast Guard will give a horse power allowable rating quite a bit higher to such a boat when compared to the flattie. The flattie has the advantage of a lot more floor space, less draft, simpler construction, and faster speed in smooth water - so flatties live on. (Ashley sent me a video of Dorado doing sharp full throttle turns, something I would never do in AF4.)

He is using a new 30 hp two stroke motor. The Coast Guard would say that is the max for this hull. With the 30 Ashley reports cruising at about 20 mph with two adults on board. As an aside I should mention that in boats like this one needs to pay attention to the prop specs to prevent the motor from going over redline at full throttle. If the usual prop is for a heavier and slower boat then I would expect the motor to rev too high at full throttle in a lighter faster boat. The idea is to select a prop with a pitch that will put the tach on the red line at full throttle.

The hull is made with taped seams needing two sheets of 1/2" plywood and eight sheets of 3/8" plywood, a total of about 400 pounds of wood. I would expect the stripped hull to weigh about 500 pounds with framing and fiberglass. I would add at least 150 pounds to that for a motor and its gear which is why I estimate the empty weight to be about 650 pounds. Add two adults and fuel and now you are around 1100 pounds total. The old rule for motors on planing powerboats is a horse for each 50 pounds so if you havea 25 horse motor you will be able to cruise this boat at 2/3 throttle. I would say 15 hp is the minimum you might use and expect to plane, even then you must stay very light and run near full throttle.

Plans for Dorado are $40 when ordered directly from me. You can also get them through www.duckworksmagazine.com.


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.

I think David Hahn's Out West Picara is the winner of the Picara race. Shown here on its first sail except there was no wind. Hopefully more later. (Not sure if a polytarp sail is suitable for a boat this heavy.

Here is a Musicbox2 I heard about through the grapevine.

We have a Philsboat going together in California. You can see the interior room in this 15' boat:

And here is another Philsboat in northern Illinois:

And here we have an LHF17 rowingboat. Tthe builder reports it to be at the limit of the taped seam method because of the wicked twist in the garboard. That would be true with any building method but perhaps the real Herreshoff builders used steamed planks. The builder also doesn't like the method of blending the ends of the planks into the stems (I copied from Bolger) using putty and tape saying he would prefer traditional rabated joint. Not a project for the first timer! Still it is nearly done now. Just lots and lots and lots of sanding.....

HOLY COW! A Jukebox2 takes shape in Minnesota. Unheated shop means no work during the winter. Check out that building rig!

And the Vole in New York. Going very quickly but most likely there will be little more done during the cold winter.





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