John Elwood's Roar2 (with a motor mount which I don't advise)
Contents:
Contact info:
Jim Michalak
118 E Randall,
Lebanon, IL 62254Send $1 for info on 20 boats.
Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1Dec99) This issue finishes the series by using the cg and weights numbers and the Hullforms model from the previous two issues to get stability data for the boat. Next issue, 15Dec99, will give an update on my AF4 design
WEBSITE ALERT...
If you are interested in free analytical boat design software go to Hullforms Download (archived copy) and download some. I'm using the oldest version in this issue, Hullforms 6S, if you care to follow along.
|
|
Left:
John Elwood's Roar2 (with a motor mount which I don't advise) |
|
|
Hullforms Model Results
TO RECAP...
In the 1nov issue 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 in the 15nov issue 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.
TO START...
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.
THE RESULTS...
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.
MAXIMUM WINDS...
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.)
NEXT TIME...
I'll give an update on my AF4.
Roar2
< WIDTH="262" HEIGHT="262">
ROAR2, ROWBOAT, 14' X 42", 75 POUNDS EMPTY
Roar2 is a modification of the original Roar which had a plumb stem. After I had designed, built, and paddled Toto with its V entry bow I went back and cut the lower plumb bow off my Roar and converted it to a Toto-like bow. So Roar2 has a deep V entry which is carried well aft. About two feet of the sharp bow is immersed and provides a skeg action forward. As a result Roar2 behaves well in all aspects of wind and waves and is more capable in rough going.
< WIDTH="548" HEIGHT="109">
The new shape makes a different sound - a "swish, gurgle" as it cleaves the water, where the original Roar has a "tap, tap" sound typical of boats that go over the water instead of through it. I suspect the original shape is slightly faster in smooth water but the new bow has the edge everywhere else. She'll row at 4-1/2 mph with medium effort using the 7 foot oars detailed in the plans. Adding a passenger to either version will hardly slow her, although acceleration and deceleration are affected.
Walter Kahlhammer built a clipper version of Roar2 without the bracing shown across the wales and reports his boat was still rigid. But the aft cross brace is almost mandatory for use as a passenger's back rest. Without it the passenger will soon tire and lean to one side or slide aft to rest against the transom, throwing off the trim in a way that will drive the oarsman crazy. (Walter uses a removable passenger seat.)
These are excellent camping boats, light enough to solo cartop, large enough for much gear and with a flat bottom plank long enough to sleep on while the whole rig sits upright. Kevin Garber took a Roar2 on a three day row of the Big Bend region of the Rio Grand, seeing no humans from put-in to take-out. He brought a folding chair, a barbacue, and a tent fly with poles. In camp he set up the fly over the hull and slept in the boat.
Plans for Roar2 are $15. Taped seam construction from four sheets of 1/4" plywood. No lofting or building jigs.
Prototype News
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.
Here are the prototypes abuilding that I know of:
Jonsboat: I suspect a few of these have been built but I've never gotten a report. Here are photos of one being built by Chuck Leinweber of Duckworks Magazine I'm told this boat is now complete except for painting. (As I'm writing this I know of two other Jonsboats in the final stages.)
< WIDTH="246" HEIGHT="168">
Mayfly12: A Mayfly12 is going together up in Minnesota. The decks are on and he's into the sailing bits. By the way, the sailing bits on almost any sailboat large or small consume about half of the effort in labor and materials. Just when you thought you were about finished! (Just talked to the builder and the project is on hold until next spring while he moves into his new house.)
AF4B: A builder in Virginia is building AF4Breve, a 15.5' version of the 18' AF4. I tried to talk him into building the 18' version but he had two very good reasons to go shorter - a short trailer on hand and insufficient building space for the larger boat. The AF4B is essentially a "scrunched" version of AF4 but comes from a whole new set of drawings. The prototype is in the final building stages where the sanding and filling and painting seem endless. (Just heard anew from the builder. Seems his property and priorities have been temporarily rearranged by Hurricane Floyd. So progress on AF4B is now on hold.)
AF2: An AF2 has been started in Oklahoma that should be close to the plans. Bottom (double layer of plywood) is on and finished and the hull has been flipped upright by brute strength, no ropes or jacks used. See the flip at Rich's site. Now I'm told the cabin roof is on and the seats are in so events are moving rapidly. This boat is far enough south that it could be launched during the winter.
Hullforms Download (archived copy)