Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15may07) This issue will show me trying to measure rowing horsepower. The 1june issue will rerun the bevels essay.
THE 18TH MIDWEST HOMEBUILT BOAT MESSABOUT will take place at Rend Lake in Southern Illinois on the weekend of June 8 and 9. 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. There is no schedule of events except a pot luck dinner in the campground on Saturday evening - we wing it. 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. .
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.
Brian Dennis's Veep14 project in the gooey stage.
Rowing Power 2
REMEMBER THIS FIGURE FROM A MONTH AGO....
The idea at the time was to show how oars work to gear up the speed of your hands. Thus when you pull the oars aft at 2mph the boat goes forward at 4mph. The force your hands feel is reduced down at the blades by the same ratio. The exact ratio depends on the geometry of the oars but in general the 2:1 ratio is close.
All this was theory. I really didn't know what the forces on the oars might be although you know the force your hand feels is more than 0 and less than say 50 pounds. So,,,
I SET OUT TO MEASURE REAL OAR FORCE...
Here is my measuring arrangement, very hush hush sort of testing, something I would never show back in the days of the Cold War ....
What we have here is my well worn Roar2, built about 1990 I think. The oar is placed forward of the usual oarlock so it is free to move forward but not aft. A loop of line ("found" line, the best kind) is run aboud the oar loom in the usual place where the lock would be and run around a 50lb fisherman's scale (imported from exotic China just for me) and the scale is clamped to the wale. So the rower tugs on the oar handle, the oar blade digs into the water pushing the boat forward and both of those forces are reacted by the loop and scale and the rower reads the scale as he pulls along. It is a sloppier setup than the lock because the scale deflects a bit with each pull. With a short bit of practice one gets used to it, especially if the conditions are calm. If condtions are calm and the boat is running straight you can assume the force on the opposite oar is equal to the one being measured.
So I did that, using a gps as I pulled along to measure speed too. As a rule I pull about 30 strokes per minute. I quickly saw that I was in the groove when the scale jumped to about 25 pounds with each tug of the oars and the gps bounced between 3.5 and 4.0 mph in those calm conditions. This is pretty typical I think of rowing Roar2 in good conditions.
I measured up my 7' oars and found the forces were applied something like this, and the only way for the oar forces to balance is also shown below.
So when the force at the oarlock was measuring 25#, we had 18# at my hands and 7# at the blade. With two oars working we had 14# pushing the boat forward at maximum load.
Nothing was static, of course. The scale force was zero most of the time and the gps speed bounced around as we went along. Making an idealized graph of the situation would look something like this:
I suppose you can look at this and say the "average horsepower pushing the boat" is something like, well the speed is about 3.75 mph which is 330 ft/min, and the force is really about 14# max (we have two oars pushing 7#), so the peak hp is about (330 x 14) /33000 = .14hp. But the average might be about .05hp since the full power is on less than half the stroke. Well, this is all pretty approximate but such is life.
That is interesting, you might say. But so what!
NOW TO COMPARE MEASURED WITH THEORY
Roar2 was designed before I knew about Hullform, before I had a computer of any sort for that matter. I brewed up a Hullform model for Roar2 using the offsets on the plans.
Then I let Hullform run the drag numbers using a weight of 270# (100# for the boat and gear and 170# for me) with the weight located for level trim. Here is what Hullform thought about that:
One thing to keep in mind is that Hullform is thinking in knots and my gps thinks in mph. So 3.75 mph on the gps is 3.25 knots.
Well, which line to believe on Hullform? Prof Oortmerssen's curve looks to lose its confidence past 3 knots, going all a-squiggle. But it agrees with Holtrop and Mennen until then. Prof Gerritsma is in a world of his own and maybe he forgot to multiply by two somewhere. Prof Savitsky didn't make the chart for some reason, perhaps he is buried under someone else's line. Well, with this sort of professional disagreement I am quite certain another all expenses paid conference in Hawaii or the Riviera is in order. In the meantime I will try to verify things with my $5 Chinese scale.
Now, if you think the scale implied the average delivered hp to be .05 at 3.25 knots, then we can try to figure a force that gives us .05hp. For example, .05hp is .05 x 33000 = 1650 ft-lb/min and 3.25knots is 325fpm. So .05hp at 325fpm needs 5# of drag. That seems to be about right about in the middle of all those lines!. That settles it! We really do need that conference! I'm tired of Hawaii! Let's be really professional about this and do the Riviera this year!
Actually, there is a better way to do this...
A BETTER SIMPLE TEST...
Would be to simply tow Roar2 with someone in it to reflect real life weight and trim and measure the tug on the tow line with the Chinese scale. Let's see, we need now two boats, one a powerboat, at least two people who don't argue much, and calm weather. It doesn't look so simple anymore. Maybe someday....
We'll how you can cut the bevels that fit your bulkheads to the hull.
ROAR2, ROWBOAT, 14' X 42", 75 POUNDS EMPTY
Roar2 is a modification of the original Roar which had a plumb stem. After I had designed, built, and paddled Toto with its V entry bow I went back and cut the lower plumb bow off my Roar and converted it to a Toto-like bow. So Roar2 has a deep V entry which is carried well aft. About two feet of the sharp bow is immersed and provides a skeg action forward. As a result Roar2 behaves well in all aspects of wind and waves and is more capable in rough going.
The new shape makes a different sound - a "swish, gurgle" as it cleaves the water, where the original Roar has a "tap, tap" sound typical of boats that go over the water instead of through it. I suspect the original shape is slightly faster in smooth water but the new bow has the edge everywhere else. She'll row at 4-1/2 mph with medium effort using the 7 foot oars detailed in the plans. Adding a passenger to either version will hardly slow her, although acceleration and deceleration are affected.
Walter Kahlhammer built a clipper version of Roar2 without the bracing shown across the wales and reports his boat was still rigid. But the aft cross brace is almost mandatory for use as a passenger's back rest. Without it the passenger will soon tire and lean to one side or slide aft to rest against the transom, throwing off the trim in a way that will drive the oarsman crazy. (Walter uses a removable passenger seat.)
These are excellent camping boats, light enough to solo cartop, large enough for much gear and with a flat bottom plank long enough to sleep on while the whole rig sits upright. Kevin Garber took a Roar2 on a three day row of the Big Bend region of the Rio Grand, seeing no humans from put-in to take-out. He brought a folding chair, a barbacue, and a tent fly with poles. In camp he set up the fly over the hull and slept in the boat.
Plans for Roar2 are $15. Taped seam construction from four sheets of 1/4" plywood. No lofting or building jigs.
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:
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.
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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