Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1jan03) In this issue we'll look at things that affect sail rig balance. In the 15jan03 issue we will continue the topic.
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...
... 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.
A rare Tween by John Boonzaayer shown here at Richard Spelling's Messabout in Oklahoma.
Sail Rig Trim 1
I've been putting this essay off for a long time. Sometimes I think other writers have put it off for ever.
I get letters regularly about how a boat won't sail right even if it is built to plans. The problem is almost always one of balance although it might not appear that way at first glance. The usual comment is that "it won't go to windward", or "it won't tack".
Designing a boat is something of a fuzzy experience. It would be nice if you could really pin down things like weight and sail shape and then build from experience to improve and trim the boat to perfection. Almost all builders do a decent job of building to the plans but in a do-it-yourself hobby there are lots of variations. I don't see anything wrong with that and in a way the variations are what make it interesting and in the long run result in improvement. Like evolution. So even if you designed the perfect boat, which I will never do, it might never get built just so and you would never know it. Conversly, you might design a sow's ear and the builder makes some critical changes that turn it into a silk purse. But he doesn't tell you what they are and you can't figure out why all the later boats were dogs.
I'm not the only one who has this problem and I'll present a personal experience showing how the fuzz works. Back in 1989 when I built my Bolger Birdwatcher I was working to the original plans, the very same plans that Ron Meuller use to built the first prototype. We also made sails using Sailrite materials shaped to the same chapter in Sailrites book. As I was nearly complete, I wrote to Bolger about one thing or another and he replied that something was "fixed in the revisions sheet". Revision sheet?! I had no revision sheet (and still have never seen it). Anyway, Ron's prototype had experienced weather helm so strong that the rudder force had bent his rudder fittings. The rake and location of the mast was changed to fix the situation. But I had missed out on that knowledge and decided to go ahead and launch my Birdwatcher unchanged. I should also have excessive weather helm, right? If anything, I had lee helm! I never figured it out. Birdwatcher was maybe four years past on Bolger's thoughts and he let the subject pass. (Never forget that even if you are building a prototype, the designer may have drawn dozens of boats since he did yours and may not have it on his mind at all.) Sometimes all you can say is "stuff happens".
Here is the basic picture of my normal first cut at "sail balance" where the center of the sail area is right above the center of the leeboard area. See the essay about "sail area math" to refresh your memory. There are lots of variations that designers use when making this balance but this is the one I prefer because it is the simplest, and all of them are approximations. Might as well stick with a simple approximation.
Here is what we would like it to look like from straight above a boat that is not heeling for a boat with neutral balance:
The thrust and side forces of the sail are perfectly aligned and in balance with the side and drag forces on the hull. Here I've shown the boat sailing to windward such that the side force is about three times the thrust force which can happen quite easily in a real boat. So there is no rotational force on the boat. You would not need a rudder in this case, or if you had one it would be centered. (By the way, if you had a rudder in this case and left it down it is not true that you could let the tiller go and expect the boat to stay on course because usually every little rock and roll of the boat will cause the tiller and rudder to twitch and the boat will respond. So you would have to lash it centered or lift it out of the water.)
Now let us look at the picture same as above but with the sail forces moved slightly forward, for example if you hoisted a jib on a rig that was balanced without it.
Now there is a rotational force trying to turn this boat to the right, away from the wind. You can hold the boat straight by applying a rudder force to the left (tiller right) to balance it. That rudder force must be balanced by the leeboard force adding to the board's side force. This situation is called "lee helm".
But if the sail forces were moved aft of the leeboard then you would have this situation.
Now there is a rotational force trying to turn the boat to the left, into the wind. You can hold the boat straight by applying a rudder force to the right (tiller left) to balance it. In order to keep it all in balance that rudder force will subtract from the force on the leeboard. This situation is called "weather helm".
Lee helm is considered to be bad because it generates more total force (and thus drag) on the leeboard. And it is considered to be dangerous because if you fall off a boat with lee helm it will take off downwind without you.
Weather helm is considered to be good because it reduces the total force on the leeboard. And it is considered to be a safety factor because if you fall off a boat with weather helm it might swing into the wind, stall out and stop sailing and wait for you (but don't count on it).
(You flyers out there might take note that this "stability" of the sail rig is the opposite of an airplane's. Here we want the sail to stall if unattended where on an airplane the unattended wing is suppose to glide downward (downwind if you will).)
There is a handling issue involved here too in that a boat with lee helm will be more reluctant to steer into the wind and tack through the wind. Thus if you have a boat that tacks poorly the first thing I would check for is the helm, weather or lee?
Here is a simple way to check the helm. You need a boat with a tiller that swings free on the hinges and has nothing (such as sheet lines) connected to it. Get your boat sailing smoothly on a course and let go of the tiller. If the boat swings into the wind you have weather helm. If it takes off downwind you have lee helm. If it stays on course you have neutral helm. If you have a boat with a steering wheel you may not be able to do this test since many wheel systems have no feedback.
Sounds pretty simple, right? But the helm forces can change all the time as the boat balance changes.
THINGS THAT CHANGE THE BALANCE...
Let's say the captain builds that boat in the first figure perfectly. He goes for a sail sitting in the stern along with his gal and their beer and the stereo and maybe a 20hp motor. The boat squats down in the stern because of the weight there. The bow is way up in the air. She still floats fine but with that stern way down and the bow way up, when she turns into the wind, the the bow is trying to get blown backward. The skipper corrects by adding rudder to turn it into the wind, giving him lee helm under the situation. The opposite situation would be to have all the weight in the bow to give the boat more weather helm but that hardly ever happens because the tiller is in the stern. But there have been boats that were steered by changing trim this way. John Gardner in one of his books describes racing with fleets of skiffs (in New York, I think) that had spritsails and no rudders. All steering was done by skippers running fore and aft in their boats.
Next, let us say that you don't have a symmetric boat, as is the case of a boat with a single leeboard (as with almost all my sailboats). Now the view of forces might be like this:
Remember that the side force on the leeboard can be considerable and with that side force goes drag. I would think that when sailing hard to windward the drag of the leeboard might be a good portion of the total drag. So imagine dipping a paddle down one side of a canoe - the boat will turn in that direction. And so it is with leeboards - the boat will try to curl towards the side of the boat with the leeboard. It should have weather helm all the time but there might be situations where it has weather helm on one tack and lee helm on the other tack. How critical is it? Not very, at least not in a boat of normal proportions where the beam is no more than one fourth of the length. The first time I noticed this was on my old Bolger Jinni which had many asymmetries. The leeboard was on one side, the mizzen sail was off to one side, and both sails used sprit booms which cannot be symmetric. I could tell after a season that it was sailing with slightly different helm on each tack but thought it was due to the offset mizzen mast. That might be true to some degree, but after a few seasons I was convinced that the majority of it was due to the leeboard drag always being on one side.
Next there is sail trim. The more the sail is boomed out, the more weather helm you will have since the now the sail's forces are swinging off to one side. The extreme will be when you have it boomed out 90 degrees and all of the force is trying to rotate the hull to windward. On boats with long booms and shallow rudders, as with traditional catboats, this can be a limiting factor on your control in that the rudder may not be able to counteract the sail force and she will swing into the wind no matter what. You may have to tack downwind.
But another subtle effect comes when going to windward. As you haul the sheet inward, you will get less and less weather helm. This was very obvious on the Jinni and it would not be unusual to have lee helm when the sail was pinched all the way inward. You can in some situations steer the boat quite well by pulling the sail in and out, changing the helm from lee to weather. But if you pinch the sail in so much that you have lee helm you are most likely in a situation where you won't be getting anywhere fast.
However, I think the most important cause in variations of helm is in the heeling of the boat. Let us look at the top view of the boat as before but now she is heeled over in the wind:
Looking straight down from above, now the force of the sail is way off to one side. The sail's thrust is trying to rotate the hull into the wind on the heeled boat. The effect is usually quite powerful, sometimes to the point where the skipper or the rudder can't hold the boat on course. The more she heels, the greater the rotational force and the greater the rudder forces needed to hold it.
This would always be weather helm since the effect of sail thrust on a heeled boat is always to turn the boat into the wind.
So there is the builder's dilemma. Design you boat for weather helm in light winds (when she will sail upright) and you might get excessive weather helm in strong winds. Unless the helm of a boat is totally out of whack, I try to not make any judgement until I've sailed the boat over a wide range of conditions. Lee helm in light winds may not be a problem. Strong weather helm in strong winds might be OK. After a few times out you may decide you aren't happy with it. You think it could be better in an all around way. What to do?
Next time... we'll look at what to do.
TOON2, CUDDY SAILBOAT, 15' X 5.5', 300 POUNDS EMPTY
Toon2 is in almost every way a multichine version of the AF3. I think these multichine hulls are more like round bilge hulls in that I keep the upper chine above any normal waterline so only the very easy lower chine flows through the water. Other boats shaped this way such and Piccup Pram and Woobo have proven to be very fast and well behaved. Yet they still have a flat center bottom plank that allows you to beach the boat upright.
So one might think that a multichine like this is in every way better than a real flat bottomed boat, but that is not totally true. For one thing multichines have a few more pieces to make and must be assembled with taped seams. Taped seams are light and probably more rot resistant than chine log construction but they can be a bit harder to understand at first glance. And someone building a boat with his children might have second thoughts about the kids messing around with large pots of epoxy. Also the bottom plank of a multichine hull is a lot narrower than that of the equal flattie so floor space is reduced, a real factor if you hope to sleep two on the cabin floor, which you can't do with Toon2.
Toon2 has about the same layout as AF3 but it is about 12" wider and I gave it some bench seats. I'm thinking this will be an excellent solo boat for a short cruise with someone used to tent camping, and yet be a good day sailer for two or three adults, or maybe two adults and two kids. This size boat has always been popular and always will be because it has that sort of capacity and yet can be manhandled by a solo skipper in almost any condition.
The sail rig is the same 96 square foot sharpie sprit that I used on Pencilbox and AF3. It started as the main on my old Bolger Jinni from 20 years ago. Very easy to make and use and efficient if you can live with a 21' mast on a 15' boat.
Toon2 uses taped seam construction with no lofting or ladder building jig required. She needs seven sheets of 1/4" plywood and two sheets of 1/2" plywood.
Prototype plans for Toon2 are $20.
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:
The Australian Dorado is completed and awaiting motor and trailer. Here is an older photo....
The Australian Twang builder is done and is testing:
The Oracle builder in St. Louis has his hull basically done except for finish, hard to do in the winter.
I never did hear back from the first Skat builder - he had the project about done a couple of years ago. But another is started in Alabama.
AN INDEX OF PAST ISSUES
BACK ISSUES LISTED BY DATE
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
JB Builds AF4
JB Builds Sportdory
Hullforms Download (archived copy)
Plyboats Demo Download (archived copy)
Brokeboats (archived copy)
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)
Barry Builds Toto
Table of Contents