Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15AUG98) This issue will present the results of some capsize testing Tim Webber did with the prototype Scram Pram. Next issue, about 1September, I'll go back to the sail thoughts to discuss the workings of spars.
WEB SITE ALERT...I think I mentioned this one before but I'll mention it again and this time I'll put it down with the permanent links. If you click on Hullforms Download (archived copy) you take a trip to Blue Peter Marine down in Australia, in particular a page where you can download a program called Hullforms. My favorite version is 6S which runs very well in DOS. With Hullforms you can "model" a hull and check its static hydrodynamic qualities such as desplacement/draft, righting moments, pitch changes with shifting CG, etc. It's limited in some ways but I've never caught it telling a fib. It's not a CAD program for drawing pretty pictures of your boat.
MIDWEST MESSABOUT ALERT...For the past several years we have had a messabout at the Paynetown Recreation Area at Lake Monroe south of Bloomington, Indiana, always on the third weekend of September. This year that would be on 19-20 September. I'm not in charge of this one, Bob Bringle of Indianapolis is. Haven't heard from him about it yet. But unless we hear otherwise, let's show up there on 19-20 September.
CHRIS CRANDALL'S HARMONICA
Tim Webber has been sailing Wil Gordon's prototype Scram Pram all summer. He wrote that the boat was knocked down by a gust in a gybe during a sharp downwind turn. There was no water in the ballast tanks at the time. Well, the boat was stable on its side and would not self right. His wife went overboard and "touched" the leeboard which brought the boat back upright quickly.
The behavior was not a total shock. Ulric Robert's Scram in Georgia had done the same (no ballast) when it was tied to a dock, it's sail hoisted and sheeted hard on a gusty day with a large man on deck. Similarly his boat went over and stayed on its side, the man on deck slid off and when he put his hands on the skeg, the boat righted.
The boat is designed to have 300 pounds of water ballast. But even I thought it would self right from 90 degrees without it. Not exactly so although clearly it was close. Scram was designed well before I got the Hullforms program off the internet. That's not really an excuse since it is possible to get a good handle on these things with hand calculations. More about that later. Anyway, Tim went on to do the only scientific testing of a Birdwatcher hull that I know of. Here is his letter about the test:
Finally made it to the Lake today. Went to Lake Conroe (the much larger of the two small lakes that I haunt.).
We took the boat out in 3 feet of water. We put Jeff inside (220 lbs) and popped it over on its side with the halyard. The lee board was retracted during all of the tests. Up and down relates to the side of the boat the lee board was on being up or down ( port or starboard). We put the weighted lee board up. The boat was not stable on its side and righted immediately when we released it. We repeated the test with the lee board on the bottom, the way Jean and I capsized. It was stable. Jeff moved to the very bottom and leaned and it slowly went up. We found that on either side, Jeff's weight at the very bottom would right the boat. We did not put on the sail tho. It was very close. The righting force at the end of the mast was about 3 - 5 lbs. With the board on the top, the force was about 10 lbs. We then flooded the two main tanks. The boat could not be capsized without me moving into shallower water. If I was in waist deep water, the boat would pick me up at about 30 - 40 degrees. Jeff rigged an "anti righting" line and he was able to capsize the boat. With the lee board on the top or the bottom of the capsize, it was an immediate righting. With the third tank flooded, it was even more pronounced. The boat was quite difficult to capsize and would right itself immediately! We then took it sailing. We only had the knotstick along. It would not get above 3 3/4 knots at any time sailing. With the tanks empty on other trips, we had no trouble getting 4 - 5 knots in gusts. Boat is a little more sluggish with the tanks full. I am sure that what I call sluggish, Wil would refer to as stable. I like lighter boats and Wil likes heavier boats. The tanks filled in about 5 minutes each. Wil has the 2 main tanks vented with plastic pipes and valves. We could close the deck plates and close the valves for a few seconds and then open them. We could hear a rush of air come out of the vents. Left the vents open on a capsize test and the water GUSHED out the top of the vent pipe.
I wish that time would have permitted capsize tests with the sail on. I am sure that an immersed sail would detract significantly to the righting abilities of the boat. I have been trying to figure out some way to put flotation on the mast head or upper yard to help in those conditions.
Jeff decide to jump overboard to play. We had no boarding ladder with us. It took several attempts to get Jeff back in from the 90 degree water. He was a little exhausted from the "ordeal" but quickly recovered from his hypothermia. We had to fashion a rope ladder with a bowline on a bight to get him back in!!! As Jeff said, "it was an idea, but not a GOOD idea!!" Jeff is now helping design a boarding ladder with few moving parts and easy to use.
In summary, about 100 lbs is a minimum ballast. I think that 200#s is about right. The water ballast was very easy to use.
I have tried to cover everything, but if you want more info, feel free to ask.
WHAT I THINK IT ALL MEANS....
The unballasted Scram is clearly very close to being self righting but isn't quite there. With the large sail hoisted the situation will clealy be worse, especially if the sail fills with water.
The ballast clearly makes the boat self righting from 90 degrees of heel.. Perhaps the 300 pounds of ballast is overkill. No reason to doubt Tim's statement that 200 pounds is fine and 100 pounds minimum. It appears that the two side ballast tanks should remain but the little center tank could be safely eliminated. If you don't like water ballast then metal of the same weight could be well attached to the inside bottom for the same effect.
The effect of the leeboard is interesting and one I hadn't thought of. The boat is more stable on its side if the leeboard is on the side of the boat that has rolled into the water than when the leeboard is on the dry side of the knocked down boat. I hadn't thought the effect would have been so strong. (I'm wondering now if the 40 pounds of lead Bolger calls out for in the Birdwatcher centerboard is mostly for overall ballast effects rather than to sink the centerboard?)
I have a feeling that Tim's boat is about at its maximum reasonable weight with two big men and 300 pounds of ballast on board. We don't know the empty weight of the protoype Scram. But the fact that he's seen a substatial change in performance means she's settled down on her lines and may be dragging a transom and chines through the water. Hard to say without hard numbers.
The problem of getting back into a boat after going overboard is something we sailors haven't spent much time worrying about, and we should. You won't be able to find a glass power boat that doesn't have a platform and ladder on its stern as do some new sail cruisers. We need to work on ways to reboard our little sailboats. If you have a pet method, send us an email describing it and we'll include it in a future page about the subject.
Lastly, I want to add that in another letter, Tim said that the righting ability of the Scram peaked at no more that 45 degrees of heel as determined by the force he had to use on the halyard while hauling the boat over.
HIND SIGHT IS 20/20....
Given Tim's results I go my Hullforms6S file out of retirement and modeled the Scram on it for the first time. Making the model in Hullforms is not the "drag a line to shape with the mouse" affair most people want. The program will do that but I never got any accuracy that way. Instead I use the option where I can type in hull offsets at each station.
The program asks for CG location, of course, and that is always a guess unless some really fancy measuring can be done. I've never seen it done to a completed boat. In fact the way it is usually done is to roll the completed boat and measure the righting moment! Here we will guess at the major pieces, their weights and their individual CG locations.
Item Weight CG above bottom W x CG Hull 600 21" 12600 Mast 37 108" 3996 Boom 15 60" 900 Yard 11 216" 2376 Sail 6 120" 720 Jeff(A) 200 18" 3600 Gear 100 15" 750 Total(A) 969 27" 25842 Jeff(B) 200 9" 1800 Total(B) 969 24.8" 24042 Ballast 300 3" 900 Total(C) 1269 21.1" 26742
What I've done is guess at three configurations. A is with Jeff seated and no ballast (969 pounds at 27"). B is with Jeff flat against the floor and no ballast(969 pounds at 24.2"). C is with Jeff seated and 300 pounds of ballast(1269 pounds at 21.1").
Here is a chart of the Hullforms stability analysis:
I thought the results agree pretty well with Tim's tests. I was not able to model in the effects of the leeboard position although it can be done by hand. The results show the maximum righting moment occurs at about 25 degrees. There is no need that I can see to ever sail beyond that on purpose. The two capsizes were "accidental", happening in gusts. The one thing I expected to see but did not see was a flattening of the graphs in the 90 degree heel to match the "stable when knocked down" experience of both boats. I may dig into that a little more.
We'll ponder the working of spars as we return to the Sail Thoughts.
MINI SHANTY, 13' X 5', 400 POUNDS EMPTY
Someday I may get to put my full catalog on the net. For now I'll put one design in each issue.
Harmonica was originally called Fusebox when it was in the catalog of prototypes. The prototype was built and renamed by Chris Crandall. Chris and his building partner David Unruh (1997/1998 Kansas Special Education Teacher of theYear) brought the Harmonica to our Midwest Messabout at Rend Lake in Illinois last June for its first showing and first launching. The prototype was built in Chris' rec room requiring the hiring of experts to remove the glass wall from the room to extricate the boat. The experts also carried the boat out of the room, one on each corner of the boat, before reinstalling the wall, confirming our guess that the boat weighed no more than 400 pounds.
The prototype boat had less glass area than I had drawn and after being in the prototype I should say that the windows you see in the photos are quite a good arrangement. In fact I was impressed with the total boat. It was clearly large enough for two people to overnight and for one person to hang out in for quite a while. It's very simple and cheap and easy to use because of its size and weight. Many will suggest to add this or that and enlarge, but the increase in the complexity will be exponential while the increase in enjoyment will not.
Here is the writeup in the current catalog:
Harmonica is a tiny shanty boat that sleeps two in its cabin. There's a porch up front suitable for lounging and a small utility room in the stern for the kitchen and water closet. I think the thing is arranged so that two people could wait out an all day soaker without feeling too pressed. For protected waters only. The prototype was built by Chris Crandall of Lawrence, Kansas.
This boat was originally called Fusebox. It was intended to be an electric boat for the wonderful little conservation lakes we have around here. But later I thought that the electric scheme was strained because few trolling motors could push this box on a windy day. And because I noticed none of the local conservation lakes have electric plugs at the docks for battery recharge. Putting a gallon of fuel on board is a lot easier than taking an 80 pound trolling battery home for recharge. Chris used a 1 horse Tanaka and that was about the minimum required, pushing the boat at 2 or 3 mph. I would say that 5 hp would be a reasonable maximum. In short, any small gas motor you might find at a yard sale will have enough power. The lakes I'm thinking of are limited to 10 horses or less. You don't need much power or speed because you never can go more than a mile in any direction. (I believe there is an experienced shanty boater in Florida building one of these with the intention of sticking with the electric idea, something he is also experienced with. Says his big shanty is a bother to launch by himself and Harmonica will fill the bill for a quiet solo overnight. In my opinion a solo boater who is not easily bored might spend a week or so on Harmonica.)
Harmonica soaks up four sheets of 3/8" plywood and six sheets of 1/4" plywood and uses very simple glue and nail jigless construction. Blueprints with keyed instructions are $25.
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. (If you order a catalog from an internet page you might state that in your letter so I can get an idea of how effective this medium is.) Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currancy that I can admire but not spend.) I'm way too small for credit cards.
Usually when a design from the Catalog of Prototypes starts getting built I pull it and replace it with another prototype. So that boat goes into limbo until the builder finishes and sends a test report and a photo.
Here are the prototypes abuilding that I know of:
Sneakerbox: Here is a boat that was in the prototypes catalog only a short while so most folks won't know about it. I'll try to feature it soon. It's an "instant" version of the 12' garvey box that appears on page 65 of Chapelle's American Small Sailing Craft, a book you should have in your collection. The builder says it is done and has taken it with him on vacation. Perhaps soon we'll get a report and a photo.
Sportdory: Word from Dave Burdecki in California was that he had his completed but no word of the launching. You can check out John Bell's Sportdory progress by clicking here .
Skat - the prototype Skat builder is Roger Palaski. Up until now he was a mystery man know to me only as "Roger", his Skat plans bought for him by someone else. Then I find out he has his own web page with the Skat construction photos on it! Skat is a small somewhat traditional 12' cat boat with a gaff rig. Even has a centerboard, the only boat I've ever designed with one! See Roger's progress by clicking here.
Nothing new on the Texas IMB. Click here to visit Tim Webber's page and see some photos of the IMB as of a few weeks ago. Then poke around Tim's web page a bit.
Fatcat2: A builder in Wisconsin has one done except for the sail rig. This is a cuddy cabin multichine, 15' long and 6' wide with bench seating and a big gaff.
BACK ISSUES LISTED BY DATE
Mother of All Boat Links
Messing About In Boats
Shantyboats (archived copy)
The Boatbuilding Community
Kilburn's Sturdee Dory
Bruce Builds Roar
Herb builds AF3 (archived copy)
Hullforms Download (archived copy)
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