Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15 November 2015) This issue will complete the AF2 computer work. The 1 December issue will be about sail area math.
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.
Raoul Adamchak's beautiful customized Rio Grande. I gotta tell you to check over a tiny cockpit idea before building one because I have found some so small that I could not sit inside the boat. If the deck is low and the entry small you might not be able to get your legs inside.
118 E Randall,
Lebanon, IL 62254
Send $1 for info on 20 boats.
Hullforms Model Results
First I used this sketch of the AF2 to guess at the weight and center of gravity of the AF2 under four different load conditions.
The loading conditions were as follows:
Condition weight cgx cgy cgz Empty 600 117 0 27.6 With skipper 780 130 4.2 25.3 With two men 960 132 7.3 24.0 With two men and ballast 1360 122 5.2 17.8
Then I used the free Hullform6s program to generate this model of the AF2:
Also I modified that model to make a second model that would represent the AF2 with its cockpit flooded, as in after a knockdown. Here is that model:
Now the problem is to use the Hullforms program with the loading conditions and work up a stability curve for the AF2 and speculate on how it would sail. This is actually pretty easy because the hard work is already done. I might say that in my normal work mode it might take one or two hours to get to this point assuming the boat has already been designed.
Crank up Hullforms again and select Files, then Open, and then your model. View it to make sure you've really selected your boat. Then select "Stat" and then "Settings". Here is what you see:
Here I've already input some numbers for the first load case of a solo skipper (I see no need to analyze the empty boat). I've input 10 degrees of heel, displacement (weight of the boat and everything in it) as 780 pounds, X cg as 10.83' (it's 130" in the first calculations but is converted to feet for Hullforms, although I think Hullforms is easily reconfigured to work in other units), and -2.1' for the Z cg (converting 25.3" to feet and making it negative since Hullforms is thinking down is positive in this case, again I think you can reconfigure the program so that up is positive).
Next select "Balance" and the program does its figuring and gives you the following answers:
Hullforms figures an awful lot of stuff for you but I'm only going to comment on a few of the items. First, always check the heel angle and pitch angle to make sure you have input your numbers correctly. Here the output shows the correct heel of 10 degrees and a pitch angle of .2 degrees which sounds reasonable. Next check the displacement to see that it is indeed what you wanted. In the example it is.
The Righting Moment, in this case 499 ft-lb, is really the number I'm looking for so I write that down.
But there are other numbers I'm checking. The draft, or draught, is .718' in this case and that is of the heeled hull. You can find the draft of the upright hull simply by doing the balance with a heel angle of 0.0. The Midsection Freeboard is of interest because for some hulls that will tell you when the hull starts to flood, that is when the freeboard reaches zero it floods. But I'm not sure if that is always the case since, as in this case, the freeboard aft of the midsection is a lot lower and I would assume the AF2 cockpit will flood before the midsection freeboard becomes zero.
One of Hullforms weak points is that the View function doesn't show a heeled waterline, as do some of the other free programs. In this case an end view of the hull showing the heeled waterline would be nice but is unavailable. The side view will show a waterline but it only makes sense if the heeling value is zero.
Anyway, to put these calculations into perspective, I'd say if done by hand they might take hours for each angle of heel and load conditon. So Hullforms can do a week's hand work in a few minutes, depending mostly on how fast you can write down the answers.
The next step would be to go back to Settings and change just the angle of heel and hit Balance again, writing down the Righting Moment and taking note of the other values, until you've gone through a full meaningful range of angles.
Once you've completed the series for one load condition, it's a simple matter to go back into Settings and run a series for the next load condition by changing the displacement and the cg location. Then go through the full angle of heel series for that load condition.
You might wonder what happens when the deck goes under in Hullforms. As far as I can tell the model "decks the boat over" at the topmost line and assumes that nothing floods. If you want to study the effect of flooding you need to make a special "flooded" model as I did with AF2 and run the numbers again. Simple to do. In the case of the AF2 I think the cockpit will start to flood at maybe 50 degrees of heel so I started the calculations there.
For AF2, I can plot the following results:
As long as the Righting Moment is positive, the boat will tend to return upright. When the Righting Moment becomes negative as some angle of heel, then the boat will not return upright but instead will capsize. I ran the numbers for the flooded cockpit version and for all practical purposes the results are the same as shown above, only the way the boat floated was different.
These charts are always interesting. Below about 20 degrees of heel the righting moment increases with heel angle so the boat will be able to take more and more wind and more sail up to that point.
At the peak value of righting moment the boat will probably be at its fastest speed, at least when close hauled.
If you continue to heel beyond the maximum righting moment value, things are going to go downhill for you pretty quickly. If you are sailing at 20 degrees heel and the wind gusts up you will have to release the sheet or do something to relieve the boat because she will be overpowered. In this region more wind means more heel which means less stability which means still more heel which means still less stability, etc., until the boat capsizes. But if you relieve the pressure on the sail before reaching a heeling angle where the righting moment is zero, about 50 degrees for the unballasted boat, she should return upright. Beyond that capsize heeling angle, she will capsize with no force on the sail.
This comes as no surprize to me because my old Jinni, which I capsized twice, behaved exactly this way. It's best sailing was at about 20 degrees of heel. When it capsized, it did so at about 50 degrees of heel, before it shipped any water, and there was nothing to be done to bring it back. Lucklily it floated high on its side and was self rescuing.
The hypothetical ballasted version of AF2 is a bit different, of course. Here the maximum righting moment is a lot higher than for the unballasted version and the peak comes at about 30 degrees. You could carry a lot more sail for a given wind, or you can stay out in maybe 40% more wind than with the unballasted boat. You might sail this one at with 5 or 10 degrees more heel. The numbers show it would self right up to 90 degrees but you must remember that the cockpit will still flood at about 50 degrees.
At any rate you should save your Hullforms model. As you build your real boat you can weigh the actual parts and update the cg data, then update the computer stability model.
With the stablility data you can get a guess at the maximum wind the boat can handle for each load condition. The hull righting moment will balance the moment formed by the equal and opposite lateral forces of the sail and the leeboard (or centerboard or keel). In AF2 the center of sail area is 12' from the center of the immersed leeboard area. As an example, the solo condition has a maximum righting moment of 540 ft-lbs. That divided by 12 gives a maximum sail force of 45 pounds. Sail area is 115 square feet, so pressure on the sail would be 45/115=.4 psf at maximum righting moment. A normal sail will develop a pressure of about .005 times the square of the wind speed (in knots). For this example the .4 psf would mean a wind speed of 9 knots.
For the ballasted version the maximum righting moment is 960 ft-lbs. That would mean a maximum sail force of 80 pounds, or .7 psf for the 115 square foot sail. That corresponds to a wind speed of about 12 knots.
ADJUSTMENTS FOR HIKING...
Actually the maximum righting moments when the boat is sailed "flat" can be higher than what is shown because the crew might be sitting to windward. Remember that Hullforms always assumes the cg is on centerline so this correction must be made by hand.
As a rough approximation of the extra righting moment there you can take the case of the solo skipper where I figured the cg is actually 4.2" off center when he sits to windward. 4.2" x 780# is another 273 ft-lbs of righting moment. It's a bit less at 20 degrees of heel but the total would be around 540 + 270 = 810 ft-lbs. Going back to the wind formula, that would amount to 68 pounds on the sail, which is .6 psf, which corresponds to about 11 knots of wind. A fair increase.
So these righting moment curves are a bit nebulous and a good skipper can get more out of his boat than a bad one - but you don't need me to tell you that. "Hiking" can fail for lots of reasons. For one thing its value becomes less and less as the boat heels more and more. It's worthless at 90 degrees of heel. Second, you or your crew can without thinking reach to the leeside of the boat and there goes your extra righting moment. I suppose the worse case comes when you sail by yourself, hiking with everything maxed out and the wind gusting up. You want to ease the sheet and release some pressure on the sail to prevent capsize. But the sheet somehow rests over yonder on the leeside of the boat! If you don't ease the sheet the boat capsizes. If you reach for the sheet the boat capsizes. (With a bit of luck you can head up into the wind with the rudder and ease the force on the sail and then grab for the sheet.)
CABIN SAILBOAT, 25' X 6', 900 POUNDS EMPTY
About 15 years ago I built a Payson Canoe and used it for several years before selling it. I replaced it about 10 years ago with my Toto double paddle canoe. Toto has the same multichine cross section as the Payson Canoe but I tried for a long lean bow which would be better in rough water and more foregiving of bow down trim. I still have that Toto, unchanged in any way since new, and still use it all summer. Amanda Johnson demonstrates:
The Toto shape worked so well that I used it in other designs like Roar2 and RB42. I tried it also in a sailing boat, the 20' Frolic2 (the original Frolic was narrower, more of a rowboat than a sailboat). Frolic2 was unballasted with a small cuddy and I hoped it would be a good daysailer and one man camping boat.
Bill Moffitt had built my Woobo design and funded a 20% enlargement of Frolic2 that would have a cabin, water ballast, and a yawl rig for cruising near the Gulf Coast.
The 20% enlargement idea went very well except that I had to deepen the hull more than that to give some serious headroom in the cabin, but it doesn't have standing headroom. Great empasis was placed on ease of use and rigging. The main mast is short and stepped in a tabernacle. There is a draining anchor well in the bow, a small storage segment under the front deck. The mast tabernacle is bolted to the bulkhead that forms the front of the sleeping cabin. There is a utility room aft of the sleeping room. Water ballast tanks are under the bunks and in the sides of the utility room floor, about 600 pounds of ballast as I recall. Aft of the cabin is the self draining raised cockpit with storage volume under the deck. Finally there is a self draining motor well across the stern. Construction is taped seam plywood.
Bill couldn't start his Caprice right away but Chuck Leinweber of Duckworksmagazine gave it a try. He has the room and tools and smarts to tackle a project like this with no hesitation. There weren't many changes from the plans that I know of, the main one being adding a conventional footwell to the aft deck which is designed to have a hatch type of foot well as with the Bolger Micro.
Chuck trailered his Caprice up from Texas to our Midwest Messabout this June and I had a chance to go over it, sail it for a couple of hours, and watch it sail from other boats. Wonderful!
Chuck tells me it takes less than 15 minutes to rig the boat to as you see here. As shown the boat has its ballast so you see it beaches very well indeed. I asked about the ballast. He can flood the tanks without power, just open the access plate, reach in and pull the fill plug and let the water rush in. Since the tops of the tanks are about even with the normal waterline he has to move his weight around to keep the tank depressed long enough to completely fill. Then you reach into the filled tank, replace the plug in the bottom, and then replace the access plate in the top. There are two tanks to fill.
Are the ballast tanks worth the building effort? On a multichine hull like this the tops of the tanks form flats that give places for bunks and storage so that is good. When full the boat should have a very good range of stabiltiy. Chuck's boat has never been in rough going as I'm writing this so the effect of the ballast remains to be proven. It has been capsized in a practice but the ballast tanks were empty and the boat was empty with no crew, etc.. But the ballast is a success from the standpoint that Chuck is able to tow his Caprice behind a four cylinder pickup truck. My idea was to pull the boat up the ramp and drain the tanks afterward by simply pulling the drains plugs. But Chuck has found it best by far to empty the tanks before recovering the boat at the ramp. So he uses a bilge pump in each tank to pump them empty. I'd be worried about water ballast tanks on a wooden boat from a rot standpoint and would be careful to open all the plugs and access panels when the boat is stored.
(I should add that I think an empty Caprice will weigh about 900 pounds based on the plywood sheet count (eight sheets of 1/4", nine sheets of 3/8" , five sheets of 1/2" and one sheet of 3/4"). But any boat like this can hold an awful lot of gear and junk.)
Caprice has the tabernacle setup that I first saw on Karl James' sharpie. The mainsail is 190 square feet, balanced lug. The mast is fairly short, stowing within the length of the boat when lowered. Chuck demonstrated putting up the mast, maybe a 15 second operation. I've been drawing these for a while on different boats but this is the first one I know of to get built and used. I'm greatly relieved that is all works so well. Before you decide to tack a tabernacle like this onto your boat, be advised that the tabernacle posts go clear to the hull bottom with big bolts all around a beefy bulkhead.
I thought Caprice sailed very well in the light winds we had that weekend. Tacked very smoothly through 90 to 100 degrees which is all you can ever get with a low tech rig. Very smooth and quiet compared to the sharpies I'm used to. It didn't seem at all sensitive to fore-aft trim. In the light winds it went 5 knots on the GPS which is certainly fast for the conditions.
Well, all in all I thought Caprice was everything I was hoping for.. Plans 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.
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:
And the first D'arcy Bryn is to the point the builder can sit and relax in it and imagine boating. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....
The first Jukebox3 is on the (cold) water. The mast is a bit too short - always make your mast too long. A bit more testing will be nice...
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.) He is painting it now...
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
1dec14, Sail Area Math, Ladybug
15dec14, Poly Laminates, Sportdory
1jan15, Sharpie Spritsail, OliveOyl
15jan15, Knockdown Recovery, Dockbox
1feb15, Mike Monies, Laguna
15feb15, Cartopping, IMB
1mar15, WeeVee Lessons, Vole
15mar15, Bulkhead Bevels, Frolic2
1apr15, Capsize Lessons, Riverrunner
15apr15, Hollow Spars, Slam Dink
1may15, Boat Costs, Blobster
15may15, Small Boat Rudders, Roar2
1jun15, Emergency Flotation, RB42
15jun15, Thailand Mixer Cruise, Mixer
1jul15, Rend Lake 2015, Musicbox3
15jul15, Box Boat Stability, Mikesboat
1aug15, Taped Joints, Cormorant
15aug15, Plywood Butt Joints, Paulsboat
1sep15, Navigator Cabins, Vireo
15sep15, Boxboat Stability 2, Philsboat
1oct15, Center of Gravity, Larsboat
15oct15, Hullforms Model, Jonsboat
1nov15, Port Aransas2015, Piccup Pram
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