Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1Jun04) This issue will discuss putting motors on rowing and sailing boats. The 15Jun issue repeats the all important sail area math essay.
THE BOOK IS OUT!
BOATBUILDING FOR BEGINNERS (AND BEYOND)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....
ON LINE CATALOG OF MY PLANS...
...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.
THE 15TH MIDWEST HOMEBUILT BOAT MESSABOUT will take place at Rend Lake in Southern Illinois on the weekend of June 12 and 13. Remember that lots of folks come on Friday and leave early Sunday. Take I-57 to exit 77W and then follow the signs to the Gun Creek Recreation Area. Camping at this Corps of Engineers facility is $10/night and that includes the ramp fee. There is no schedule of events except a pot luck dinner in the campground on Saturday evening - we wing it. This year some of us have reserved the campsites on the water of the two middle camping loops so look for us there if we aren't at the boat ramp.
Al Myers and his new Piccup.
I get letters all the time asking how to put a motor on rowing and sailing boats. In general the answer is simply that is not a good idea at all. A very poor and often dangerous power boat is bound to be the result. I hope to give a few examples of why.
Any good rowing or sailing boat is modeled for efficiency at low speed. Low speed here would mean something like the traditional idea of "hull speed" which is often thought of as a practical speed of a displacement hull. Hull speed, in knots, is usually about 1.3 times the length of the hull, in feet, squared. So a 16' boat would have a hull speed of about 1.3 times 4 equals about 5.2 knots, with emphasis on the word "about" since not all boats or skippers are created equal. A displacement hull goes through the water and not over it as a fast modern powerboat will do. Because the displacement hull is going through the water it should be well shaped at both bow and stern, the bow efficiently spreading the water apart and the stern allows the water to smoothly return to its normal level. (The pressure of the water on this type of stern actually pushes the boat forward. A squared off deep transom below the waterline quickly separates itself from this effect and gets no beneficial push and has a lot more drag at slow speeds.) Even if the slow speed boat has a squared off transom above the water line, below the water line its lines will taper smoothly to zero and is effectively a "double ender". Because of this taper the slow speed boat will not be able to support much weight in the stern - there is little hull volume back there to provide buoyancy even if the hull there is deeply pushed down into the water.
So if you have a good paddle or rowing or sailing boat it is almost guaranteed that it will never support much weight in the stern. (If it could support a lot of weight in the stern the argument would be that it could be a much better low speed boat if the stern were made more streamlined.) And there lies the rub in almost any scheme to put the usual outboard motor on a low speed hull. Here is an example that uses the program Hullforms (get a free sample by clicking on its link way down at the end of this page). The example here is the rowing boat Oracle. You might recall Oracle was used in the "rowboat setup" essay from a few weeks past where Max shuttled himself fore and aft in it to find the best seat location to trim the boat level. In this example let's say the boat weighs 100 pounds and Max weighs 150 pounds. By centering Max's weight 100" from the front the boat trims level and looks like this:
Now let's say the plan is to put a small motor on Oracle. Let's put 30 pounds directly on the stern (few motors are lighter than that) and Max's 150 pounds 30" in front of the motor so he can use the steering tiller. Now the total weight is 280 pounds centered 134" from the bow. Hullforms predicts a new trim picture like this:
The blue line is the predicted waterline for this weight and cg location. The bow is WAY out of the water and the transom is half way under. A stern wave will swamp the boat. The boat might swamp if the skipper has to go even further aft, for example to pull the starting rope. To me the idea of mounting a motor on a boat like Oracle is downright dangerous. The fact is that I get questions about mounting outboards on even small boats like Toto. You can't talk people out of it, it seems. All you can do is warn them it is not going to work and apply for a future "I told you so" credit. Seeing a motor plate mounted on the stern of a boat like Oracle is all too common. It is easily done and doesn't change the boat as long as the motor is never mounted. So I figure man mounts motor in spite of warning, man launches boat, man mounts motor, man is unable to start motor from inside the boat without swamping it, so all and all it is a safe outcome. Although seeing the motor plate is very common, no one has ever sent a photo of such a boat actually under power. It is one of those things you won't try more than once.
"But," you say, " I see such photos all the time in catalogs and such, often for very short boats like dinghies." True, but the dinks usually have wide square sterns to give some stern buoyancy which is one reason they row so terribly. And I'll bet almost all those photos show a second person in the bow, required ballast to keep the bow down somewhat.
THE PROBELM GETS WORSE..
...if you ever manage to start the motor. (Remember that many smaller motors have no gear shift, so you will be off to the races as soon as it fires.) Not only will your boat be close to sinking by the stern simply due to your weight, but as the boat speeds up it will try to climb its bow wave and the stern will sink a lot deeper yet. Even on a small boat you often will not be able to see over the bow.
Now that I think of it, this is something I have tried more than once. First time came with my old Bolger Jinni which had a very traditional sailing sharpie shape. Poiny bow and a small squared stern with lots of rocker on the bottom so both bow and stern were well out of the water. Unlike Oracle the Jinni had some size and weight to it and it was indeed possible to sit in the stern and start the motor without swamping. But as soon as the motor fired she went quickly to hull speed (about 5 mph I suppose), the bow rose so high that it was impossible for anyone seated in the stern to see over the bow and steering would have to be totally blind. This with a 3.5hp aircooled Clinton. The only time I was able to run it flat out (was never on plane) was to have the wife stand in the very tip of the cabin, maybe 30" from the tip of the bow.
Then I tried it again. This time with Twixt. Twixt was designed to take a bit more power as the stern was pretty wide for its side and at design load the stern transom was supposed to just touch the water. She was 11' long and 3' wide on the bottom, a flat bottomed scow for the most part, sort of a jonboat for rowing and sailing. But even with that deeper transom I was not able to go full throttle on the Clinton without it rearing up to the point where there was vision over the bow, this with an 11' boat. One day I put everything I owned up in the bow locker, rigged an extension to the steering tiller so I could sit in the middle of the boat, and in smooth water was able to get full throttle and still see over the bow which was only about 6' in front of me in this case. I suppose it was this experience that convinced me that small rowing and sailing boats should never have motors.
SO YOU STILL WON'T GIVE UP...
With a small boat like Oracle or Twixt you must keep your weight in the middle of the boat to keep it trimmed level. With Twixt I was able to get really good results with a small trolling motor, say 30 pounds thrust, by steering it with a remote push/pull stick with the weighty battery mounted forward also so the whole deal trimmed level with me in the seat. Unlike a gas motor, running the electric was a breeze by simply connecting/disconnecting the battery clips with the tiller switch left on the desired power setting. I never had to go aft. The small motor ran the rig up to 4mph which is about as fast as an 11 footer is ever going to go. I have used this same rig on my Toto and all goes fine. No, I don't use it all the time. Trouble with small electrics is that range is pretty limited and you must carry oars or paddle to feel secure about getting home. And if you are going to do that you might as well leave the heavy motor and battery at home and revel in the simplicity. I wll admit though that the electric is the quietest and smoothest thing out there, nothing like it for that.
...usually have the weight and size to support your weight in the stern even if they are modeled for efficiency at slower speeds. But by my experience they won't absorb large amounts of power efficiently and you might be surprized at how small a motor will push them.
Max's AF3 has an antique Neptune on it, 2 hp. AF3 is a direct result of my Jinni experience, basically the same size and shape. Goes 4mph nicely at half throttle with no future in more, Max says, except to generate more noise, smoke and waves.
I've had a small motor on my Birdwatcher for the past few years. Birdwatcher is 24' long, about 1000 pounds total, but narrow and a true double ender sharpie with a pointy stern in addition to the pointy bow, like a canoe. The motor mounts off the side about 3' forward of the stern point. Originally I used that 3.5hp Clinton and it ran 6 mph flat out. I think a much quieter half throttle gave maybe 4mph. This past year I mounted a quiet and smooth Johnson 5.5 on it. Cruises at about 5 mph at a very quiet third throttle. More power pushes the stern very deep with little increase in speed but lots more waves. One day I had it out with Max and a gps. As I recall it ran up to 6mph at about half throttle and would go no faster, just dug its stern in deeper and deeper. So it would seem that about two hp is enough for this boat as far as speed in good conditions goes. I would argue that maybe 4 would be ideal with the idea that the extra will come in handy in bad conditions. And you could cruise very quiet and conservatively at about half throttle.
As I recall Chuck Leinweber is using a 4hp motor on his 26' Caprice which is bound to be heavier than Birdwatcher but has a smoother underwater shape.
Karl James used 4 hp originally on his Jewelbox but I think he later upped it to 6. His brother Pete built Petesboat to be more of a motorsailer. At the time I was using my Twixt but here we doubled its size to 24' while keeping the same basic sharpie scow shape with a wide transom that just touched the water in normal trim. Pete went cruising at first with just 4.5 hp saying it was fine at displacement speeds in good weather. But the idea was to plane the beast and after a year or two he got a deal on a 60hp job. Went 20 mph at full throttle as I recall with the skipper required to stand in order to see over the bow. He said that the boat, motor, and skipper were a lot happier throttled back to 15 mph. The photo below shows Petesboat in its early days with the 4.5hp motor on centerline and what looks like maybe a 2hp backup.
NEXT TIME: We'll repeat the all important sail area math essay.
CAMPJON, CUDDY JONBOAT, 15.5' X 5', 300 POUNDS EMPTY
Campjon is based on my Jonsboat design. I increased the hull depth to 24" from 18" and again to 30" in the region of the 6' long cuddy cabin. The cabin decks slope upward towards the center so the "headroom" in the middle is about 3', usually that is enough to allow sitting up while seated on the floor. There is a slotted walkway on the cabin centerline just like I've used on my AF sharpies, but this one also has a step through to the front deck. The front deck itself is fairly large and easier to use as a boarding platform than with the AF sharpies. But it is very important to NEVER let anyone ride on a front deck like this when the boat is in motion. If they slip off the deck they have no option but to slide under the boat full length and through the prop in an instant.
The prototype Campjon was built by Luke Spreadborough in Queensland Australia. Here he is using a 2.5hp motor.
He now uses an Evinrude18 as shown in this newer photo and like the guru has become something of a collector of old motors and now they show up like orphans on his doorstep.
I doubt if the increased freeboard makes the boat any safer since a flat boat like this is not good in rough water in any case. The higher sides do give a feeling of greater security since this will be a boat you are "in" instead of "on". This would be a better family boat since the kids can be kept cornered in the cuddy while underway. They can stand in the slot, or look out the front or out the windows. I suspect the cuddy will find a lot more use as a spot for the portapotti than as a camping cabin. The cockpit as drawn has no seats and is itself a big playpen. For seating I suggest using common folding chairs like I have used with satisfaction on my AF4. When the boat is not underway the chairs are folded and stowed, or relocated to another part of the boat or the shore.
As for power I would suggest a good 15 hp motor or a 10hp to keep the capability of using the smaller lakes which are wisely limited to 10 hp. With 10hp and with two normal sized adults the boat should plane, and be very lively with one aboard. But I've noticed with my own AF4 that weight gets added as time goes by and I've noticed that folks usually build their boats heavier than I do. I should add that the Coast Guard has suggestions about powering small boats and they come down hard on flat bottomed hard chine boats like jonboats because of handling concerns. I think they would advise a max of 25 hp in a hull like this. (My AF4, which is about the same weight as Campjon, went 22mph with a 40 year old Johnson 18. Greg Rinaca's Jonsboat went 26mph with 18hp.) There is a draining motor well in the stern.
There are two more Campjons around that I know of. This one is by Adam Abrego in New York and has a raised cabin:
And this one by Wojtech Baginski has its cabin both raised and extended and glassed over for river cruising in Poland:
Constuction is nail and glue, with no jigs or lofting. From four sheets of 1/4" plywood, two sheets of 3/8" plywood, and two sheets of 1/2" plywood.
Plans for Campjon are $30.
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.
The Texas Ladybug is done! Hopefully a complete story in a few weeks:
Out West the Picara picks up where it left off before the winter:
The Deep South Skat is done! Hopefully a full story in a few weeks:
Another Picara, this one with a 1' stretch in the middle, going together in Arkansas. Sailmaking done and its on to glassing the hull.
This is an AF4Grande (upside down):
AN INDEX OF PAST ISSUES
BACK ISSUES LISTED BY DATE
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
JB Builds AF4
JB Builds Sportdory
Hullforms Download (archived copy)
Plyboats Demo Download (archived copy)
Brokeboats (archived copy)
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)
Barry Builds Toto
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