Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(15February 2018) We discuss spar deflections. The 1 March issue will be about sailing trim.
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....
REND LAKE 2018...
...will take place on June 8 and 9, always on the weekend before Father's Day weekend. WE HAVE ALREADY NAILED SITES 25 THROUGH 29 SO THE END OF THE LOOP IS OURS. THANKS TO ALL WHO HELPED NAIL THEM DOWN.
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.
Another new Toto out by San Francisco by Dave Bink.
1024 Merrill St,
Lebanon, IL 62254
Send $1 for info on 20 boats.
Sail Rig Spars
I'll contiune the "sail thoughts" that I started a while back. This time will discuss the actions of spars a bit, especially with a look at deflections of spars used in simple homebuilt rigs and how they affect some things the sailmaker might do
Figure 1 shows an idealized version of the spars of a balanced lug rig. Here I've assumed that all of the halyard load is transmitted through the luff and leach of the sail. That's not totally true but in a well cut sail only the luff and leach will be tight if the wind isn't blowing because the sail is cut to have a loose pocket of draft in its center. But when the wind starts blowing the entire sail developes tension and the yard and boom are loaded by that cloth along their length. Anyway, I've shown the halyard with a nice round 100 pound load. What are the deflections of the yard and boom?
Figure 2 shows how to figure the deflection of a beam loaded as in Figure 1. The source of the equation is a book called Roark's Formulas For Stress and Strain. That book is a super cookbook for the stress analyst, the stressman's bible.
I think most of the elements of the formula are plain except for the "EI". E in an equation that predicts deflection is alway the modulus of elasticity, pretty much a spring rate for the material and it has units of "pounds per square inch" and has nothing to do with the strength of the material, only its stiffness.. It's a very large number. For aluminum E is 10 million. For wood E is about 1.5 to 2 million The length dimension "a" is always from the load "W" to the nearest end of the beam.
The "I" in the equation is the moment of inertia of the cross section of the spar. It has units of "inches to the fourth". Figure 3 shows how to figure the moment of inertia for square cross sections and round cross sections. As you see the I of both the square and round sections are related to the fourth power of the thickness of the section so a little increase in thickness gives a big increase in moment of inertia and thus in stiffness. For example a 1" diameter boom has an I of .05. A 1.5" diameter boom will have an I of .24. And we have increased the bending stiffness by a factor of 5! By the way, if you have a hollow spar, the I can be figured simply by taking the I of the OD of the tube and subtracting the I of the ID of the tube. For example, a round tube of 1.5" OD and 1" ID would have an I of .24 - .05 = .19. So the tube is almost 4 times as stiff as the 1" solid spar but only 26% heavier. So tubes are popular with those who can afford them.
Now, if you crank through all the equations for the yard assuming the yard is of wood and has a cross section of 1-1/2" square you will find the yard will flex over 3" under the 100 pound halyard load. (Actually the work was done with an applied load W = 70 pounds because the yard is tilted 45 degrees and only a 70 pound component of the 100 pounds is acting perpendicular to the beam.) So what?? It's actually a big deal to the sail maker. If you recall the essay on sailcloth you might remember that a sail 10' wide with a 1' draft will actually have an arc length of about 10' 2-1/2" across its middle. So the sailmaker will sew in the extra 2-1/2" to supply the draft. That might appear as a bulge of 1-1/4" along both the foot and head of the sail. So if the yard and boom are straight, the proper amount of draft will appear in the center of the sail. But if the yard flexes 3" under the sail load, it will pull all the draft out of the sail! So one of the problems the sailmaker has to face is to guess at the flexing of the rig. In this case he might supply the head with 3" + 1-1/4" = 4-1/4" of curve at the head of the sail.
But then again he may not supply that much and here is why. One nice thing about a lug rig with a flexing yard is that as shown above the flexing will pull draft out of the sail when a large sail load is applied. In many cases that might be considered to be good because it will flatten the sail in high winds. On the other hand, if you feel your lug sail does not have enough draft in high winds, one thing you could try to cure the situation might be to increase the size of the yard.
If you run the numbers for the boom, assuming the same 1-1/2" square section, you will find the flexing is small compared to that of the yard, about 1/2". That's mostly because the boom is shorter and because it is supported closer to its ends, near where the loads are being introduced. In fact it is possible that the boom will flex towards the sail under a large load and add draft to the sail in high winds.
Figure 4 shows a free standing sharpie sprit rig. The sail of a sharpie sprit can be pretensioned by tightening up the snotter which is the tackle on the mast end of the sprit boom. How much will the mast flex?
Let's say the mast here has a 2" square cross section. And let's say the snotter tackle is a 2 to 1 thing so that the skipper pulls on the rope with a 50 pound tug and it results in a 100 pound preload on the sprit boom. The boom is in compression. You can imagine the mast deflecting under the load like a bow and arrow. Crank through the equations and it turns out the mast will flex over 3", assuming the mast is 2" square from top to bottom.
But few masts are made that way. Almost all are tapered and the taper greatly affects the deflection. As above, the sailmaker needs to know the deflection so he can allow extra material in the form of a bulge on the luff that anticipates the flex of the mast. Jim Grant of Sailrite kits recommended that the sailmaker predict the flex allowance of an unstayed mast by placing the mast on sawhorses and place a weight equal in pounds to 1/3 the sail area in square feet at the center of the mast. Then you stretch a line along the mast and measure to it a various stations along the mast and make a plot of the mast flex. Then that flex is added to the draft allowance of the sail at the luff. Usually the flex allowance is a lot more than the draft allowance!
WHAT I THINK IT ALL MEANS....
To put this all into perspective, I doubt if anyone except a Cup boat has the resources to do a meaningful analysis of the deflections of the spars of the boat. Even then, I would not trust the results of a paper analysis unless it was backed up by a wealth of real life experirence. There are so many variables. But for us little sailors it is worth while to get a handle on how much the rig is flexing and what effects the flexing is having on the set of the sail.
IMB, SAIL BEACHBOAT, 13.5' X 5.5', 350 POUNDS EMPTY
IMB features a "Birdwatcher" cabin, full length with panoramic windows and a center walkway slot in the roof. Everyone rides inside. This style of boat was invented by Phil Bolger in the early 1980's.
These boats can be self righting with minimal, or no, ballast because crew weight works as ballast. They sit low looking out through the windows (although standing in normal winds is quite acceptable). The cabin sides provide lots of buoyancy up high to ensure a good range of stability. IMB, which is small with a light bottom, should reliably self right from 60 or 70 degrees and in the test described above self righted from a full 90 degrees of roll.
These boats are operated from within the cabin, like an automobile. No one need ever go on deck. For boating with children I can see no equal.
These are usually cool inside. The tinted windows cut the sun's power. The crew can sit in the shade of the deck. Downdraft from the sail cascades through the walkway. (By the way, at the Conroe messabout two boaters with Lexan windows noted that mosquito spray will ruin Lexan with one application and they noted belatedly that the back of the spray can says so.)
IMB has an 8' long cabin on a multichine pram hull. The prototype was built to perfection by Gerry Scott of Cleveland, Texas. At the Conroe (Houston) messabout I got a chance to look over his boat plus the only other IMB I know of built by Bob Williams. Both boats were quite true to the plans. Both had added low inside seats which made them more pleasant to use to the point that I will show some seats on the plans. I was worried when I drew IMB that the headroom would be minimal so drew no seats thinking the crew would sit on the floor, as with the original Birdwatcher.
While I was sailing with Gerry, Bob's boat came out on the lake with four adult males and no sign of bogging down, showing that these fat pram shapes, very much like my Piccup Pram, can handle a lot of weight in the 13.5' length.
(Later they rescued a mermaid and returned to the dock with five total.)
I don't know if either boat had ever been weighed and the 350 pounds I quote as the empty weight is just a guess. One of the ideas behind the boat was that it might be towed behind a compact car and I was glad to see that Gerry tows his behind a 1500cc mini SUV.
Both men adjusted well to the lug sail/leeboard rig. Gerry's has the blueprint 104 square foot sail and Bob's uses the 114 square foot Bolger Windsprint sail available from Payson. I used to worry a bit about running a leeboard on a full cabin boat like this since handling must be done by remote control, so to speak. No problem. Both boats have the leeboard lanyard running to a cleat on the aft deck. The leeboard position is plainly in view at all times through the cabin window. In use these leeboards need only lanyards to pull them down. Once down they will usually stay down until they strike something. Then they pop up and you will need to pull them down again. I've never seen a need for a lanyard to pull the board up although I've seen several rigged that way. The Dovekie design had elaborate cam operated levers in the cabin that operated the leeboards and I thought that all very clever. But in talking to some Dovekie owners I found the internal levers are not universally loved since they can often be in the way. Anyway, my idea was not to run the down lanyard to the aft deck but rather through a small hole in the side of the boat, say 1/2" for a 1/4" lanyard, so it could be operated totally from inside the cabin.
Both Gerry's and Bob's boats used electric trolling motors. The plans show rowing ports and no provisions for a motor. A boat like this won't be a fast row boat but it might be useful in a calm. Even the 24' Birdwatcher would row about 2.5mph in a calm. But I'll admit that adding a motor to Birdwatcher makes it a much more useful thing.
IMB takes two sheets of 1/2" plywood, eight sheets of 1/4" plywood and one sheet of 3/16" Plexiglass. Taped seam construction using no jigs or lofting.
IMB plans 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.
We have a Picara finished by Ken Giles, past Mayfly16 master, and into its trials. The hull was built by Vincent Lavender in Massachusetts. There have been other Picaras finished in the past but I never got a sailing report for them...
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:
A brave soul has started a Robbsboat. He has a builder's blog at http://tomsrobbsboat.blogspot.com. (OOPS! He found a mistake in the side bevels of bulkhead5, says 20 degrees but should be 10 degrees.) This boat has been sailed and is being tested. He has found the sail area a bit much for his area and is putting in serious reef points.
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
1mar17, Normsboat Capsize, AF4Breve
15mar17, Underwater Board Shape, Harmonica
1apr17, Capsize Lesson, RiverRunner
15apr17, Measuring Leeway, Mayfly16
1may17, Scarfing Lumber, Blobster
15may17, Rigging Lugsails, QT Skiff
1jun17, Rowing1, Mayfly14
15jun17, Rend Lake 2017, Mixer
1jul17, Rowing2, Viola14
15jul17, Rowing3, Vamp
1aug17, RowingSetup, Oracle
15aug17, Taped Seams, Cormorant
1sep17, OliveOly Capsize Test, OliveOly
15sep17, Plywood Butt Joints, Philsboat
1oct17, Sailing OliveOyl, Larsboat
15oct17, Water Ballast, Jonsboat
1nov17, Water Ballast Details, Piccup Pram
15nov17, Scram Pram Capsize, Harmonica
1dec17, Sail Area Math, Ladybug
15dec17, Cartopping, Sportdory
1jan18, Trailering, Normsboat
15jan18, AF3 Capsize Test, Robote
1feb18, Bulkhead Bevels, Toto
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