Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1March 2014) This issue will capsize a Scram Pram. The 15 March issue will rerun the bulkhead bevels essay.
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.
Ken's deluxe AF3 getting launched down in Australia.
Scram Pram Capsize Tests
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 The lee board was retracted during all of the tests.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.
SCRAM PRAM, SAILING BEACHBOAT, 16' X 6.5', 500 POUNDS EMPTY
Scram Pram is an interesting boat that I designed for Wil Gordon of Houston a few years ago. He built the prototype shown above, including making that multicolored sail. Wil had owned some production pocket cruisers and had sailed in Karl James' Jewelbox. He wanted a huge rig all in one sail for moving in very light winds. We started the design process by enlarging IMB and it turns out that at least two Scrams have been built while the prototype IMB is still abuilding.
The lines show that Scram has a multichined cross section, like my Piccup Pram, and I've found that makes for a very fast and seaworthy boat. The top is built up like a Birdwatcher. Everyone sits inside, including the skipper. The boat can also be steered by sitting in the open top slot as shown in the photo. The top of the cabin has a slot that runs full length. If you sit to the side you are inside under the deck, but you can stand up in the middle. In bad weather the slot is covered with a snap-on tarp. If you have children this is a good boat type because they will never be on deck. If you use tinted plastic for the windows and a white hull, the boat will be cool in hot weather.
But the main advantage for the Birdwatcher cabin, invented by Phil Bolger, is that the crew sits low and the crew's weight acts as ballast, instead of making a boat top heavy as with a normal cruiser where you sit on a raised deck. Sometimes the effect is so strong that the Birdwatcher boat will be quite self righting with no ballast other than the crew weight. So you can get a nice light simple self righting cruiser. With Scram it didn't quite make it. In tests of the prototype, the boat was just barely self righting with a crew of one against the floor when the boat was rolled over 90 degrees by a rope attached to the masthead. But the design has water ballast tanks on the floor to take 300 pounds of water. With the tanks full the boat is so stable that the test crew had trouble rolling the boat 90 degrees with the mast rope. When released, Scram popped right up. Tim felt that Scram would be OK with 100 pounds of ballast and that there was little need for more than 200 pounds.
A second Scram was built to prototype plans by UT Roberts of Savannah. His boat is shown above. UT's boat had two significant changes. He reduced sail area to 150 square feet from the original 170+. I made the sail shown in the photo and I feel it is quite enough, especially since the boat has a motor well built in and only 2 or 4 horsepower will push it fine on very light days. Very few people have the patience or time to sail in light flukey winds. Almost any boat this size will end up with a motor which will get used a lot. Then a huge sail rig is not the best thing to have. UT's boat also did not have water ballast. I think he has sand bags lashed below seating benches he has built in, a good option probably if the bags are very low down and very well secured. The idea of bench seating is nice but one must scope out the bench height very carefully to get enough headroom. As is, the water ballast tanks have a foot well. If I were building a Scram, and I would do so if I didn't already have 7 boats in the shed, I would build in the ballast tanks and use the smaller sail.
There have been several other Scrams built since I wrote the above article, mostly in western Canada! Everybody seems to be happy. You can watch Greg Flemming's Scram in action on YouTube at
Scram uses taped seam construction with five sheets of 1/4" plywood, nine sheets of 3/8" plywood, and one sheet of 3/16" dark Plexiglass.
Scram plans are $35.
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
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
15jan14, Width/Stability, HC Skiff
1feb14, Hiking, Shanteuse
15feb14, Dory Stability, IMB
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