Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15January 2014) This issue will take another whack at the stabiltiy/width study. The 1 February issue will be about hiking on sailboats.
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.
Chuck and Sandra Leinweber did a newyears boat camp in their his and hers Imresboats. Chuck says they make very good freighters as long as the trim is correct.
OK, This is the second whack at this subject and I hope to have better luck than on the first. The idea here is to get a general rule of thumb about how the width of a hull affects its static stability in roll. I am going to say up front that I didn't find a hard and fast result about this. But here goes the big picture...
...the boat rolls to the right a bit. The downward force is from its weight acting through a point call the "center of gravity" which doesn't move (unless some weight on the boat shifts). The upward force is from the buoyancy of the water the boat is displacing. This force almost always moves as the boat rolls. Normally as the boat rolls like this the buoyancy force moves to the side as shown above such that it acts through a place that is off to the side of the weight center. The result of this shift of buoyancy is normally to cause a torque, or "moment", that tries to return the boat to upright. The distance between the two forces is called the "righting arm". The torque, or "righting moment" is the weight times the length of the righting arm.
I am going to use the free Hullform program to study a few boat shapes that I had already on file to see what effect the width of a hull will have on the stability. Hullform can calculate the righting arm, which it calls Gz, in a flash. But it works with computer models that are based on the real hull dimensions plus guesses at the boat's weight and center of gravity "CG" location. I looked at four models which had been used in the past and already existed. For each one I widened the hull to make a wide version of the same model, then ran the righting arm analysis with Hullform to see the effect. Let's get going.
Here is a real Sportdory by John Bell, in use. It is based on the Bolger Light Dory...
The "normal hull model looks like this (the stern is slightly different because I never figured out how to do a raked stem or stern with Hullform)...
And its righting arm curve, with the weight at 410 pouns and a cg centered 20" above the bottom looks like this...
WHAT'S WRONG WITH THIS PICTURE...
The dory has almost no positive stabiltiy. This is what blew my sails out two weeks ago when I first looked at it. In fact if the cg is raised from 20" to 24", as I first tried it, the hull has NO positive stability at all. And yet the photo of John Bell shows him right side up. Well, for one thing I am not sure how high the cg is in real life since most of the weight will be in the form of seated people. But I believe the Hull results (after thinking on it for two weeks). Boats like dories aren't for everyone. You can't stand up in one for sure. But if you sit low and keep your hips swiveling with each wave they are one of the most seaworthy things you can be in. If you are old and stiff and like to sit up high get another boat. It has occurred to me that designs like this and maybe sea kayaks and such are the least stable normal designs and yet the most seaworthy in the right hands. I think if you roll with punches you can control the boat's motiion with quick and small shifts in your body. Anyway, let's look at the same thing widened from 4' to 6' by proportional scaling of the lateral hull dimensions:
And the resulting righting arm curve...
Now we have some stability to work with. This sort of shape might be used for running rivers as a "drift boat". I was hoping to report something like "making this hull 50% wider increased its stability by X percent", but even my newest computer won't divide by zero! So I moved on to the RB42...
This is another rowing design, 18' long instead of the dory's 15', and with a multichine hull shape. It is actually 6" narrower at the top than the dory. The stock design model looks like this...
And its righting moment arm curve, with 600 pounds weight centered 20" above the bottom, looks like this...
It appears a lot more civilized than the dory. I've used this shape in lots of boats. Then I widened it by 50% like this...
Looks a bit like a sailboard hull. The resulting widened hull righting arm curve looks like this...
A big increase in righting arm from .15' to .86', about 5.7 times the righting moment! It went from a rowing hull to a true sailing shape. Now let's take a sailboat shape and make it wider...
This is another multichine, not too unlike the RB42 in general, but wider and with a deeper hull. Here is the basic model...
And here is its righting arm curve with 800 pounds total centered 2' above the bottom...
Next I widened it by 50%...
And with the same weight and cg info as before we get this for the widened Frolic righting arm curve...
So here the maximum righting arm went from .60' for the standard Frolic2 to 1.64' by widening the hull 50%, a factor of 2.7. I'm thinking here that by starting with a sailing design in the first place the increase is less dramatic than with a narrow rowing hull widened to be a sailing hull. So let's look at a sailing box hull...
JBJR was designed for sail with a simple scow shape, a real Bolger box. Several have been built and they go well and are really self righting. So the model of the standard JBJR looks like this...
And with the weight at 800 pounds centered 23" above the bottom, the moment arm curve looks like this...
Next, I widened this one by 20%, the bottom going from 4' wide max to 5', to look like this...
Which gives a righting arm curve that looks like this (same weight info)...
Notice that increasing the width by 20% gave about 100% increase in righting arm!
Well, I think my idea of a rule of thumb about the width/stability relationship is shot down. In these examples it varied so much from boat to boat that I think it takes an individual study in each case to scope out. Clearly though it is the main driver in the equation - that if you really are out to increase stability you need to think about your beam first. I suppose the opposite side of the coin is that if you intend to make a boat narrower you might quickly arrive at something which is surprisingly tippy!
There is another factor to consider with beam. In this study the weight of each example was not varied from narrow to wide. But in real life the wider boat will be heavier, maybe a lot heavier. And remember that weight is the other factor in the righting moment. So, for example, if you made you boat 50% wider and found its righting arm doubled, the maximum righting moment might be tripled because the boat might now be 50% heavier.
HC SKIFF, SHARPIE SAILBOAT, 18' X 5.5', 450 POUNDS EMPTY
HC Skiff is a rework of a "typical sharpie skiff" that appears on page 101 of Howard Chapelle's great book AMERICAN SMALL SAILING CRAFT. I can almost say that if you own only one book about small sailing boats that this is the one to have. The lines of my version are straight out of Howard's offset table. By today's standards it is a long narrow hull, 18' long and about 4' wide on the bottom. The stem rides about an inch above the water, then the bottom goes straight to the deepest point of the hull at about half the length of the hull. Then the bottom sweeps way up above the waterline for a very high and small transom. There were working hulls. Heavy loads could be carried without immersing the stern transom which is always harmful to a boat with no motor. You put everything in the stern and the stern still stays clear of the water. I think a hull shaped like this is little improved upon by any modern flattie skiff thinking. Chapelle said the shape was common all along the East Coast from 1860 to 1900. The prototype shown here was built by Dan Roach of Danvers, Massachusetts, very close to where Chapelle found the original boat he surveyed. Dan wrote quite a bit to me saying it was fast and handy and easily carried his family including the dog. (I'd quote what he said but I lost his email in a computer accident. So will somebody tell me why they put the "empty file" button right next to the "send message" button??)
I used modern plywood construction on HC Skiff. The old boats were nailed together from 1" boards. Then they were left outside in the water until they died. They must have been very heavy both in their construction and in their soakage, not to mention the clams and oysters. My version uses five sheets of 1/2" plywood and five sheets of 1/4" plywood and will weigh a fraction of the original. It's all done with prefab parts and no jigs or lofting.
I had to tinker with the sail rig which was of the same style shown here but at least 50% larger. On the heavy old boats that might have been OK but I'm sure it would be way too much for the lighter plywood boat. I kept the large centerboard. I added hull air boxes bow and stern for a chance at self rescue after swamping. Also added some benches where the original boat was wide open from stem to stern with three simple thwarts.
Plans for HC Skiff 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.
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 out West.
This is Ted Arkey's Jukebox2 down in Sydney. Shown with the "ketchooner" rig, featuring his own polytarp sails, that is shown on the plans. Should have a sailing report soon.
And the Vole in New York is Garth Battista's of www.breakawaybooks.com, printer of my book and Max's old outboard book and many other fine sports books. Beautiful job! Garth is using a small lug rig for sail, not the sharpie sprit sail shown on the plans, so I will continue to carry the design as a prototype boat. But he has used it extensively on his Bahamas trip towed behind his Cormorant. Sort of like having a compact car towed behind an RV.
And a Deansbox seen in Texas:
Another prototype Twister is well along:
And the first D'arcy Bryn is taped and bottom painted. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....
AN INDEX OF PAST ISSUES
THE WAY BACK ISSUES RETURN!
MANY THANKS TO CANADIAN READER GAETAN JETTE WHO NOT ONLY SAVED THEM FROM THE 1997 BEGINNING BUT ALSO PUT TOGETHER AN EXCELLENT INDEX PAGE TO SORT THEM OUT....
THE WAY BACK ISSUES
1feb13, Drawing Boats 2, Shanteuse
15feb13, Drawing Boats 3, IMB
1mar13, Figuring Displacement, Paddleplank
15mar13, Drawing Boats 4, Frolic2
1apr13, Drawing Boats 5, RiverRunner
15apr13, Drawing Boats 6, Picara
1may13, Two Letters About Keels, Blobster
15may13, Drawing Boats 7, Roar2
1jun13, Drawing Boats 8, Polepunt
15jun13, Rend Lake 2013, Toto
1jul13, Drawing Boats 9, AF4 Grande
15jul13, Taped Seams, Mikesboat
1aug13, Plywood Butt Joints, Paulsboat
15aug13, Sink Weights, Cormorant
1sep13, Lugsail Rigging, Hapscut
15sep13, Sharpie Spritsail Rigging, Philsboat
1oct13, Modifying Boats 1, Larsboat
15oct13, Modifying Boats 2, Jonsboat
1nov13, Modifying Boats 3, Piccup Pram
15nov13, Sail Area Math, Caprice
1dec13, Stretched Stability, Ladybug
15dec13, Trailering, Sportdory
1jan14, Cartopping, OliveOyl
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
JB Builds AF4
JB Builds Sportdory
Puddle Duck Website
Brian builds Roar2
Herb builds AF3
Herb builds RB42
Barry Builds Toto
Table of Contents