Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1APR98) This issue will discuss the effect of "hiking" on stability and how to figure the force on the sail actually needed to knock down the boat. Next issue, about 15 April, I'll have at last some photos and test results of my new AF4.
TEXAS MESSABOUT NOTICE... Tim Webber is putting on a messabout near Houston on April 25 and 26. For more info see his page at http://www.infohwy.com/~tbertw/messabt.htm
FIRST A WORD ABOUT SAFETY...
I've been testing the AF4 when weather and schedule permit. Every Friday the St. Louis paper gives the fishing report and I read it because it gives the water temperatures of the local waters. Right now they are about 50 degrees F. Now, here is some valuable info that came with the life jacket I bought last year:
WATER TEMP UNCONSCIOUSNESS DEATH 32.5 under 15 min 15 to 45 min 32.5 to 40 15 to 30 min 30 to 90 min 40 to 50 30 to 60 min 1 to 3 hrs 50 to 60 1 to 2 hrs 1 to 6 hrs 60 to 70 2 to 7 hrs 2 to 40 hrs 70 to 80 2 to 12 hrs 3 hrs to indefinite Over 80 indefinite indefinite
If you wonder how the data was gathered click here, scroll down about 60 percent, and read. Clearly anyone falling into our local waters currently needs saving right away. There may a good chance that you won't save yourself unless you are very close to shore or can get right back into your boat. I've also heard the cold water can be completely incapacitating to the point where you MUST wear a life jacket to even float. It's quite sobering when you think about what can happen.
DON O'HEARN'S PIRAGUA
EFFECTS OF HIKING....
In the last issues I looked at the effect of ballast on the boat's stability but limited things to having the CG on centerline. But the effect of a crew's being off the center of the boat, "hiking", can be significant even for mid sized boats like Jukebox and Jewelbox. Let's look at a couple of examples based on the new boat Frolic2 shown down in the featured boat section.
Take a look at Figure 1 which shows why hiking works so well. If the boat's overall CG is offset from centerline, that offset is added, to a certain degree, directly to the righting arm calculated by the stability program. I say "to a degree" because the added effect is actually the offset multipiled by the cosine of the heel angle. That would mean it is quite effective at small heel angles, drecreasing as heeling increases to the point where hiking has no benefit at 90 degrees of heel. (Unlike traditional ballast which is most effective at 90 degrees of heel.)
So here is how you can do the numbers. First you need to figure the CG. But now we also need to figure the lateral CG where in previous examples we figured only the vertical CG.
Here is a first cut at the CG for Frolic2: Item Weight (W) Height(X) (Y) WX WY Hull 400 12" 0" 4800 0 Mast 20 90" 0 1800 0 Yard 8 190" 0 1500 0 Sail 5 120" 0 600 0 Boom 6 65" 0 390 0 Crew 200 20" 24" 4000 4800 Total 640 20.5" 7.5" 13100 4800
So the chart shows the Frolic2 with a 200 pound crew sitting on a side bench has a total weight of 640 pounds, it's total vertical CG is 20.5" above the inside bottom (which I used as a reference point), and 7.5" off centerline to the side the crewman is sitting on.
With this information, and knowing the lines of the boat, we can fire up Hullform6S and crank out a few stability calculations to get an overall picture of the boats stability. But there's a problem for now: Hullform6S won't figure CG's that are not on centerline. So we run the stability calculations on centerline and adjust them afterward by hand (or write a simple spreadsheet if you must).
In detail here is how I do it. Hullform gives the righting moment (in foot pounds) and I divide those numbers by the weight (in pounds) to get the righting arm (in feet). Then I take the CG offset (in feet it equals .625'), multiply by the cosine of the heel angle, and add that to the Hullform righting arm to get the new righting arm (in feet). That gets multiplied by the weight (in pounds) to get the "hiking" righting moment.
Here's the example for Frolic2 with the single off center crewman: Heel (H) H6S Righting Arm .625' X Cos H Total Arm Total M 10 .64' .615' 1.26' 800 20 .82' .59' 1.41' 900 30 .77' .54' 1.31' 840 40 .61' .48' 1.09' 700 50 .44' .40' .84' 540 60 .25' .31'. .56'. 360 70 .09' .21' .30' 190
Here is what is looks like:
Well, you can see that the effect of the skipper hiking (or in this case just sitting on the upwind side of the boat), about doubles the righting moment of the boat. I'm a bit surprised that the boat still shows a positive righting moment up to 70 degrees of heel. But the Hullform analysis indicated that the rail is going under at 60 degrees. If it really works out that way it ain't so bad. It certainly ought to give the crew a good indication that it's time to back off the sheet! Actually, if the oarports are left uncovered there will be some flooding even before then.
On the chart I've also shown the effect of a second 200 pounder on the upwind seat. Quite an effect. Righting moments are way up again, although the program showed the boat would now flood over the rail at 50 degrees of heel. I want to show this because the crew you sail with will certainly have an effect on the impression a boat leaves you. Sometimes I've admired how some folks seem totally unfazed by stiff winds only to find they are always sailing with heavy crews. (My old Jinni capsized on me twice while solo, but never with two crew even though there were some wild sailing days.) One similar problem is the case where a swimmer tries to reenter a light boat. If solo, the boat might easily capsize as he loads his weight on the rail to pull himself into the boat. He may not think too much about that if he never sails solo and the extra crew weight in always in there to stabilize things as he climbs back in over the rail.
Another thing to consider is the case of the solo skipper who is seated upwind with the boat clipping along at maximum righting moment. He has to lean to the center or lee side to clear a line or something. The force on the sail is not affected by his actions, but the boats righting abililty is cut in half, or less. The boat capsizes! He needs to depower the sail by slacking the sheets before leaning over to the lee side. I suspect this may one of the most common reasons folks capsize small boats.
CONVERTING RIGHTING MOMENTS TO SAIL FORCES....
This is easy as pie! Look at Figure 3:
Once you've figured the righting moment, all you need to do to balance the boat is to divide the righting moment by the distance between the center of area of the centerboard (or daggerboard of leeboard or keel, whatever you are using to prevent leeway) and the center of area of the sail. On Frolic2 the two area centers are about 12' apart. So if the maximum righting moment is 900 foot pounds, the maximum sail force (and centerboard force, on a normal boat the sail force and centerboard forces are equal and opposite) is 900/12=75 pounds.
What does that mean? We'll return to this subject in a while and relate the righting moment curve and the resulting sail forces to the wind's speed over the sail and the water's speed over the board. Given the righting moment curve and a certain sail we can figure the wind needed to capsize the boat. Or given a wind speed figure the maximum sail we can carry.
FROLIC2, BEACHBOAT, 20' 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.
The original Frolic was in the catalog for several years. It was similar to Frolic2 but was narrower, 20' long and 4' wide. That one was intended for row/sail cruising but since then I've learned that no one wants to row a boat this size more than once. To do so requires conditions that are near perfect, no wind and no waves. Even on a perfect day, powerboat wakes stop you dead. So I widened Frolic 12" and got Frolic 2. I thinks it's all around better now. More room, more stability, bench seating for more comfort, and a serious motor well. I followed the lead of Petesboat for the engine well. I put the well in the center of the boat and offset the rudder. Why not? You could still row this boat in perfect conditions, maybe more than once.
Here's the writeup in the Prototypes Catalog:
.....Here in the Midwest we have lots of manmade lakes. The big ones are usually about two miles wide and ten miles long. They're shallow, 30 feet deep in the old river channel and with an average depth of 10 feet. Huge areas are only waist deep. The rivers and lakes run North/South as a rule and so do the prevailing winds. White caps appear in 12 knot winds, sometimes less. A 15 knot breeze might produce two foot waves, closely spaced with near vertical faces and foamy tops.
This sort of condition is common in a lot of places. Certainly the Gulf Coast can be the same, and I've heard it's true in the Chesapeake. It's not true everywhere, I guess, because I hear of folks sailing litle flatties in big waters and hard winds and you can't do that here without taking a dangerour beating. I designed Frolic2 to deal with those conditions. She's long and narrow with multichines and a long lean bow to handle the waves. She has my usual slot top cuddy cabin to sleep in, and a pivoting leeboard and rudder for shallows and beaches.
The lug rig is for quick and easy rowing, and towing and stowing. It may be too large but these rigs reef well. They can be closer winded reefed than when full, perhaps because the yard is better controlled. For that matter a sharpie sprit sail the same as Pencilbox's may be smarter in rought water operation. Swithing rigs won't be hard. The mast can be relocated almost anywhere in the slot top without altering the hull to any degree. You just need extra partner and step fittings.
Bigger boats like this can row well if there is no wind or waves. But I've put a small motor well on this one and 2 hp is probably a realistic max.
She needs eleven sheets of 1/4" plywood and two sheets of 1/2" plywood with jigless taped seam construction. Plans are $20 until a prototype is built and tested.
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.
Anyway..... Anytime 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. There are three boats in that catagory right now.
The Texas Scram Pram, a 16' multichine Birdwatcher type, has been loaned to someone who says he will test it inside and out, with GPS.
No new word on Skat . Guard Duty.
No progress on the Way Up North AF3. Temps at -27C! Not clear to me if this is to be a water boat or an ice boat.
No progress on Harmonica (ex Fusebox). Spring Break.
Here on the AF4 front, the boat really isn't a prototype any more and testing goes on when I can do it. The 10 horse Johnson is not in use although Dave Carnell found good fixit info at his library and sent it on to me. Turns out the engine is 42 years old! Dave's book is called something like "Fixing Old Outboards" but the book itself is 15 years old. Not discouraged because the motor doesn't appear worn out. Thanks to the new info I found something wrong with the carb (no fuel filter and its' opening closed with a plate) and gotten extreme suspicion that the carb float is not the correct one. Would explain why it wouldn't run more than a minute at a time. Also got the proper jet settings. Test running in a bucket of water I found submerging the water inlet is not enough to get cooling water, the water in the bucket needs to be deep enough to submerge the water pump.
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