Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15mar07) This issue will return to estimating power. The 1 April issue will probably rerun an old rowing essay.
MESSABOUT NOTICE:
THE REND LAKE MESSABOUT WILL TAKE PLACE ON JUNE 8 and 9 AT THE GUN CREEK RECREATION AREA AT REND LAKE IN SOUTHERN ILLINOIS. MORE DETAILS WILL FOLLOW BUT IT MIGHT BE PRUDENT TO MAKE CAMPING RESERVATIONS IF YOU INTEND TO ATTEND.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.
Figuring Power 5
LET'S DO A SMALL SKIFF THIS TIME....
We've been looking a power skiffs in previous episodes of Figuring Power where we use the Hullform program to conjure up things like estimates of top and cruising speeds. I cast about my collection of Hullform models and noticed one for the Mayfly12. I have included the writeup for Mayfly12 later in this webpage but here is a photo of one by Mike Cerio in New York (no wind):
Mayfly12 is a 12' plywood skiff for rowing and sailing. In past episodes we focused on planing speeds in power skiffs but you may have noted that the Hullform drag predictions include two curves by different professors. We were using the planing curve but now we will only use the low "displacement speed" curve. No, Mayfly12 won't plane in any sane way. If you gave it enough power to plane it would flip over backwards.
Here is my Mayfly12 Hullform model:
Now, Mayfly12 will probably weigh about 120 pounds, could be as light as say 80 pounds if it were made for rowing only. Add two adults to bring it up to 470 pounds total and place them such that she trims level (cg 6.5' from the bow) and Hullform says she will float like this:
By the way, I loaded the Hullform model to 600 pounds total and got this, a figure I would accept to be the max you should load this boat:
Now to run the Hullform drag predictions. Using the 6.5' cg location it made this graph showing drag at different weights up to 600 pounds total:
A bit of a surprise here - weight has almost no effect on speed vs drag. Above 3 knots you might see a little but the fastest boat is but 1/4 knot better than the slowest, and the weight varies by a factor of 2. And it looks to me like the heaviest boat is the fastest! I should say right here that I have no idea how the curves are calculated but add that for the planing curves there was reason to take them seriously.
I would expect Mayfly12 to row at about 3knots in a normal mild way. You might sprint it up to 4 knots for a short time. I guess that means the oars are producing say 4 pounds of thrust at "cruise" and 10 pounds at "full throttle". Not very much. A horsepower is defined as 33000 ft-lbs/min. One knot is about 6000 ft/hr or 100ft/min. So if we go 3knots with 4 pounds thrust we are talking 3x100x4=1200 ft-lbs/min or .04hp. Not very much. Full arm throttle would be say 4knots at 10 pounds thrust or 4 x 100 x 10 = 4000 ft-lbs/min or .12 hp. Well, what do you expect? Even Popeye's arms are no where near as big as a horse's butt.
CHANGING TRIM...
I ran the Hullform model with variations on the cg location. Remember that the "level" trim came when the cg was placed 6.5' aft of the bow. Here are the results of the cg variations:
I'm shocked! The level trim is not the fastest. And the trim has real effect on the speed, much more than the weight! Weight forward is a lot faster than weight aft. Out of curiousity I ran a picture of the model with the cg at 5.0' aft of the bow, about the fastest cg location predicted in Hullform:
It says bow down all the way until it just touches the water. Stern way up. The program recognizes that at cg = 5 the bow is starting to push serious water and note that drag goes way up again at a cg =4. Do I really believe this? Haven't caught Hullform lying yet. And I do have something of a data point. I have been sailing my Birdwatcher for almost two decades now and it was clear from the start that you could have a grand time with it by getting into the big open lake, tying off the tiller so it would self steer well, then walk forward and sit in the forward most position of the top slot. That is way in the bow. The bow goes well down from your weight, the stern comes up, and the boat does not slow down.
BEYOND HUMAN .....
Let's say you want to know what happens beyond 4 knots. After all, a cheap trolling motor is rated at 30 pounds thrust (static, don't know what it is at speed). So ran the program again at 450 pounds with varying cg locations , up to a whopping 6 knots at which point it is thinking about planing:
Looks like I would say "about 5 knots" with a real 30 pound thrust motor. Hey! Let's do the hp thing again for grins: 30 x 5 x 100 = 15000 ft-lbs/min. About .45hp which is believable.
WELL....
Do I believe this - I guess I should. With modern gps it would be easy to check. With a good electric setup, which would avoid the scatter involved with an oarsman trying to maintain a given "thrust", one might simply get it all running steady and shift weight forward and aft and check the speed as you go.
But there is another message I see here. Remember I thought a boat like Mayfly12 would be rowed almost all the time at about 3knots. At that speed the graphs show almost nothing matters, not weight, not trim. A sweet spot?
NEXT TIME...
...we take another look at rowing geometry.
MAYFLY12
MAYFLY12, ROW/SAIL SKIFF, 12' X 4', 120 POUNDS EMPTY
Mayfly12 is a small general purpose row/sail skiff based on the lines of the traditional flat iron skiffs of 100 years ago. By that I mean she has a nearly vertical stem, flaring sides from the midsection to the stern, and a raking stern. And she has quite a bit of rocker as did the originals. Mayfly12 is made from two sheets of 1/4" plywood and two sheets of 3/8" plywood. The heavier plywood is used to make a stiff bottom although one might save 20 pounds by using 1/4" plywood all over. I think the popularity of 12' skiffs is that they are usually all that one can cartop solo. Beyond that size and you should use a trailer. Once you have decided to get a trailer you might as well go to a 16' boat which is usually the most boat one can manhandle solo in the water. Sailboats larger than 16' are getting out of the "impulse" category and you should do some serious thinking before building one. Lightweight and simple boats like canoes and rowboats can be cartopped successfully at almost any length provided they weigh less than 100 pounds.
Here is the prototype boat by Mike Cerio in New York. There were at least two other Mayfly12's built but I did not get this short of photo so kept carrying the boat as a prototype. One of those boats was built with a daggerboard in lieu of the leeboard and it sailed quite well. This might have been Mike's first boat, certainly not his last:
I've kept the sharpie sprit sail that might have been found on a traditional skiff 100 years ago. It requires no store bought fittings to work well and its simplicity means it will work well most of the time. Its only real drawback is the 16' mast it requires. Unlike the traditional boats, I have used my favorite pivoting leeboard and kickup rudder so you can sail in shallows with no fear. And this boat has buoyancy/storage boxes fore and aft to allow self rescue of a capsized boat. By the way, to reboard a boat like this after capsize you usally need to scoop some water out of the cockpit before trying to climb back into the boat, and you usally must reboard over the stern. These boats are often too light to accept the weight of a man trying to climb over the side without capsizing again. The bailing is needed to rid the boat of water sloshing from side to side causing an uncontolled roll that might lead to another capsize.
Plans for Mayfly12 are $25. No jigs or lofting required. Uses old style glue and nail construction for the most part, but has taped seams on the side/bottom joint.
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:
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
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)
