Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1May 2014) This issue will capsize again only on paper. The the 15 May issue will look at some hull coefficients.
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.
Our old buddie Wojtek Baginski in Warsaw, Poland, has fittied his Robote with sails! Being careful, tippy open boat in the cold river water. He has some youtube videos of it at https://www.youtube.com/watch?v=Ubm-lSI2TAU&list=UU3pPpZeNqGuAaJal9fUb5Hw and https://www.youtube.com/watch?v=AAadGUxdNx0&list=UU3pPpZeNqGuAaJal9fUb5Hw.
I recently got a report of an AF3 capsize in Australia under non sailing conditions. Actually I know of three other similar knockdowns/swampings of AF3 and Frolic2 which have similar slot top cabins. What they all had in common was that someone (or more than one) person was standing in (or on top of) the cabin slot at the time. I decided to take a Hullform look at what happens to stability when someone stands up in a light sailboat and also the effect of that weight being near the bow, as when standing beside the mast as you fuss with the sail. As usual I am starting to write this without really knowing what the results will be. So here goes...
I AM GOING TO START...
... with my Hullform model of Frolic2. Three of the accidents were with AF3 but my AF3 Hullform model was lost many years ago in nameless hard drive crashes. Frolic is a bit larger and heavier and especially wider than AF3 but is multichined and has the same style of cabin and cabin depth.
Next, I had to figure at least a guess at the weight and cg locations, both in height and also in longitudinal cg as I wanted to see the effect of weight moving towards the bow. All this is pretty much my usual guess. As for the empty hull weight, I simply did my usual of adding up the weight of all the plywood sheets used to make the hull. That usually seems to be a very good estimate. The ply sheets are all diagrammed in the plans so the total is quickly done. Thus for Frolic2 we have 11 sheets of 1/4" plywood at 25 pounds per sheet and 2 sheets of 1/2" ply at 50 pounds per sheet. That totals 375 pounds and I put down 400 in the calculations. Here they are...
I am showing 5 "cases". The first is just the empty boat. Case2 is with a normally seated solo crewman. Case 3 is solo man standing in the cockpit. Case 4 is solo man standing towards the bow. Case 5 is solo man standing on the deck at the mast.
This is pretty much the base line, minimum sailing weight, solo crew properly seated. But remember he is on the boat's centerline and not sitting to the windward side. So in real life the righting moment could be larger than Hullform calculates. As it is Hullform says the "righting arm" is a maximum of .83' at about 20 degrees of heel. So the "righting moment" at that point would be .83 x the total weight, or .83x639=530 ft pounds. If the skipper were to move his 200 pounds to windward by 1' that would add another 200 ft pounds to the righting moment so it is quite significant. Of course the opposite is true too, if he were to reach over to the leeward side of the boat the same amount he would reduce the righting moment by 200 ft pounds. So a small sailing boat is a balancing act.
In this case the solo crewman stands up in the cockpit. The righting arm reduces as he does this from .83' to .65', a loss of stabiltiy of 24%. That's not as much as I expected but it is still significant.
Here the skipper moves forward and is standing next to the mast, as he might to fuss with the sail, with his feet on the boat's bottom and his top sticking through the cabin slot. The stability takes another hit as this happens. I think what happens is the bow sinks and the stern rises as he does this and the bow is much narrower that the stern. Now the maximum stability is about half the usual. This is I think the situation with three of the four capsizes. As I recall one AF3 skipper was simply up forward and trying to haul in his anchor. Off hand I don't see why that would be a problem unless he already had his sail up and got caught up in a series of misadventures of the sort that happens to all of us at one time or another. The second capsize like this was while sailing, helper at the tiller while the skipper went forward to fuss with the sail (sailing on a run as I recall) on Frolic2. The last case like this, and most recent, was an AF3 on a windless day being towed by a large powerboat driven by someone who's boating experiences were limited to beer and gasoline. The towing boat would not slow down and the tow line was tied solidly to both. It got out of control yawing and over she went. Well, I have been there too but luckily in my case the powerboat slowed down. The real answer here is NEVER TIE SOLID TO THE TOW BOAT. Wrap the tow line around something and hold the loose end in your hand. Then you can let go of it at any time. Do the same if you are on the other end the tow line in the towing boat.
I suppose another thing to notice is that these boats usually can recover from about 45 degrees of heel. But note also that the maximum righting arm occurs at about 20 degrees of heel. So I see almost no reason to sail at over 20 degrees of heel.
This is the case shown in the AF3 capsize article. Standing ON THE DECK and fussing with the sail. Righting arm is now just .14'. So if the skipper's weight is less than 6" off centerline over she goes! Don't do this.
BLOBSTER, SAILBOAT, 16' X 6.5', 750 POUNDS EMPTY
Blobster has a lot of features I like in a boat. Lots of volume for its size, sort of like Micro or Scram Pram. The multichine shape is almost exactly like Scram's but this one does not have a Birdwatcher cabin. It has the more traditional cabin with a raised watertight deck behind. Also it has one feature I would love to have in my personal boats - a step-through bow so that when you beach you can go forward through the cabin and out the front without going into the water or climbing over the bow. The cabin also has a slot top roof.
This shape of boat with multichines has proven good in rough water and with fair speed in spite of its blobular proportions. Blobster has about 600 pounds of water ballast in its belly and should be OK to 90 degrees heel although such depends mostly on weight distribution of the crew, something the designer has little control over. On the other hand, if the crew jumps overboard the boat will be almost assured of righting without their help. Then the problem becomes reboarding. Be prepared!
Sail rig is a large but simple 139 square foot balanced lug on an 18' mast. Mast is stepped off center to allow you to walk upright down the slot top and out the front. Should be rigged in an instant with no one going on deck ever. All very low tech built with common materials but effective.
The prototype Blobster was built by Miles Bore in Australia. He had built several boats prior, including a Micro, and did a great job of it using very nice materials.
Outside and on its trailer for the first time, the likeness to Micro is clear. But you can see it was meant to be much easier to trailer and launch.
Then he used it for a while as a low power motor cruiser while he finished the sail rig. You can see how easy the boat was meant to use from the beach with its step through bow transom.
Then for the rig and off for a sail, shown here with ballast tanks full. Miles reports it sails with no vices so far. No, he hasn't capsized it yet.
Miles got this photo of the inside. The living platform you see is 7' long and 6' wide max and about 42" at its deepest from platform to slot top. With the slot top cabin and step through transom it is a pretty airy home. Maybe not the little cruiser that Micro is but clearly much easier to use in general and a good choice for a daysail when you have but a couple of hours at the lake.
Great job, Miles!
Blobster uses taped seam construction. Five sheets of 1/4" plywood, eleven sheets of 3/8" plywood and one sheet of 1/2" plywood.
Plans for Blobster are $45.
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
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
15jan14, Width/Stability, HC Skiff
1feb14, Hiking, Shanteuse
15feb14, Dory Stability, IMB
1mar14, Scram Capsize, Scrampram
15mar14, Bulkhead Bevels, Frolic2
1apr14, Capsize Lessons, RiverRunner
15apr14, AF3 Capsize, Sneakerbox
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