As I continue to build Kingsfold my teammate is doing a great job of studying all he can find about the Race to Alaska. We will be very well prepared as a result, come next June.
One important source of information is the many reports from the Teams racing north. Several of them have hit logs and whatnot and received damage to varying degrees. For example, Team Pear Shaped Racing, on a Multi 23 trimaran hit a submerged large piece of wood with the dagger board and then the rudder. They were able to stop and get the rudder fully repaired (The team later withdrew, but because of health reasons.). But before they did get it repaired, they said that the rudder developed considerable weather helm.
Knowing this story, my teammate expressed his concerns about Kingsfold's rudder configuration (Paul's barndoor with skeg design) and having a log hit the skeg &/or rudder and take us out of the Race. He was wondering if a third gudgeon/pintle higher up on the skeg would be a good idea.
The appendages on a Multi 23 trimaran are pretty delicate compaired to the barndoor setup. Make a good, solid attachment of the skeg and it'll absorb most of the impact damage it might see.
This is kinda timely. Yesterday I took out my weekender with the new experimental rudder I tossed together for a test run. Basically, what I had done was to add 3/4 inch to the leading edge of a standard weekender rudder to see if I could reduce weather helm. The new rudder is not as shapely as I would make if I were intending to keep it long term, but otherwise it is according to plan.
I didn't get a good test of it because I had casually tied my jib halyard shackle with two half hitches instead of a bowline, and when I hoisted the jib all I got was the halyard. I sure wasn't going to go back and fix that to the tune of a half hour. Instead, I decided to practice sailing under main alone. Because I didn't have the jib, weather helm was enhanced, but not unbearable. In the gusts, it got to be a handful, but that is nothing new. Helm deflection was identical to the standard rudder I have been using. The biggest thing I noticed was that in a tack that rudder showed a lot of authority. I usually use the jib to push me through a tack, but the bigger rudder brought me around and through the eye of the wind just fine. I will try to get out again this week and this time I will use the jib to give it a real test. So far, I like the bigger rudder and if this works out I will make a nice one out of lumber.
I know that all this isn't real relevant to Kingsfold, because of the different keel configuration, but on a standard weekender, I believe that the rudder plays a greater roll than on other boats, so it should be carefully considered and constructed.
Al
Rudder balance is hard to get right, though if it's under 15% of the chord, it usually makes a noticable, but not detrimental difference in deflection angles and general feel.
Test in 8 - 10 knots of steady wind, on a close hauled course and you should have less than 5 degrees of deflection. 3 - 4 degrees is a well balanced helm, 4 - 5 degrees is getting into the too much balance in the blade, for these wind strengths and over 5 degrees is too much. When I do these tests, I start with about 20% and cut it back until it gets right on. Right on is 3 degrees in 10 knots of wind. You'll have modest helm pressure, when the boat's "in the groove". Helm pressure will proportionally increase with wind strength and this is normal. The bigger the "balance" in the blade, the harder backing up (reversing) under power will become. It'll still backs up, but the helm balance will get caught by prop wash and flow from the trailing edge, which makes handling in reverse sketchy.
I just got back a little while ago from testing the experimental rudder. The wind was around 6 to 8 mph with gusts to around 15 to just under 20 mph: the normal wind for that lake. I guess I didn't notice a whole lot of difference between my regular rudder and the new one when it came to holding a close hauled course. Maybe it was a bit better, but not enough to be certain about. On a reach it may have been better, but maybe not. I may cut the stop notch on the front of the rudder to allow it to twist the tip a bit more under the boat to see if that has an effect. I know that has a huge effect on the weather helm on my CP 16. I also wonder if I cut off 3/4 inch from the trailing edge if that would have an effect. It sure is nice to have a piece of equipment that I can butcher with no worries. Speaking of which, I am going to call the guy with the weekender on craigslist tomorrow, and if he still has it, I am going to go get it and put it in storage until I can get to it after the yawl is done.
Al
When I'm trying to "dial" something in like this I'll just hot glue some rigid foam to the leading (or whatever) edge and take it for a sail. You'll get enough information without a bunch of expensive, so you can make a decision about the finalized appendage adjustments. I've also used plywood for this approuch, simply rounding over the leading edge with a belt sander to get a rough idea.
Yeah, this rudder is made from an old off cut piece of 3/4 plywood. When I am done monkeying around, I will either toss it or use it as a template if I am successful in making changes that please me.
Al
Making grandiose changes to a rudder can be difficult. There's so many things that can affect its performance envelop, some quite minor, others dramatic. I first look at plan form to get a weed shedding leading edge and cut down on tip vortices, as much as practical. The tip shapes are usually governed by sectional shape and aspect ratio choices. In other words, what's the Froude number the blade will operate in and are the sectional choices, maximizing lift at these speeds. With the section defined, tip shapes and plan form tend to align with needs. The balance portion of the equation, often is a crutch for the boat and/or rig, more so, than making the blade more efficient. By this I mean you put in what you need to get what you want, as opposed to having design factors determine its need.
As a set of rules on this particular boat, the "00" series of sectional shapes are going to be more friendly and forgiving, offering higher stall speeds and flow, at the Froude number it will operate within. The stock aspect ratio, maybe just a tad longer will do fine without balance, assuming the boat was well balanced before the changes. If addressing a tendency to a heavy helm, I'd be inclined to look at raking the rig a bit first and checking actual rudder deflection angles, before adding any balance. If rig tuning doesn't fix the issue, small amounts of balance, "sneaking" up on the actual amount needed is the usual route I take. More often than not, I start with way more balance than I need and end up trimming much of it off, as I "dial" it in. Lastly, parallel edges makes building the blade a lot easier to maintain accurate foil sections. You can setup a foil cutting jig and just plow along, getting dead bang foil shapes the full length of the blade. Also the tip will need some washout and you can usually just eyeball this with a belt sander, tapering it down, while maintaining the approximate sectional shapes.
For these types of boats you can way over think this thing. The low Froude numbers mean just about anything from a hunk of plywood, without even rounded edges will do pretty well. Just rounding over the leading edge will make a big improvement, as would substituting a plate of aluminum or steel for the blade, again with blunt edges. A lot of sailors are built just like this will little harm. This is mostly because they can't go fast enough to take advantage of better foil shapes. I like fat foils on boats of this general performance. This lets the blade be naturally strong (it's thick) and maintain good flow properties across a wide range of incidence. It'll never go fast enough for a skinny 5 digit foil section to help very much. I do think carrying some of the lateral load on the rudder will make these designs perform a bit better, which means a reasonably efficient rudder (foil shapes).
One blade I made last summer was about as simple it could be. The leading and trailing edges were parallel and just a length of Douglas fir. The leading edges was rounded over to a 2:1 ellipse shape and a 1/8" deep rabbet cut into it. 2 sectional shape formers were installed perpendicular to the edges and 1/8" plywood convinced to bend around, landing into the rabbets. The 3 spaces between the formers were filled with foam and the whole shooting match given 2 layers of 6 ounce cloth. The blade was light, strong, well shaped and easy to make. A lot more trouble than what these boats need, but it was also for a 22' sloop over 2,500 pounds too.
I built my mast step with a good bit of play both forward and aft, and side to side. I didn't do this on purpose, just a mistake in measuring, but I later found that it was quite useful. I made up a bunch of wedges that hold the mast vertical, with others maintaining the rake that I like. Nowadays, I let the forestay determine the rake of the mast when I tension it. On my boat, I found that allowing the mast to tllt a bit forward of the angle of the mast box, gives me better balance. I use a cam lever to attach the forestay now which really sped up set up time, but I haven't fine tuned it yet. Later this summer I will probably take a day and go through all that when I don't have anything better to do.
On the CP forum, one of the hot topics is replacing the aluminum flat rudder with a foiled one. It is universally agreed that the foiled rudder is superior in every way. I guess I take their word for it because I use the foiled rudder instead of the flat aluminum rudder without questioning it. When someone complains about weather helm on that board, the first thing that they recommend is to make sure that the rudder is pulled completely down and forward. That must be universal, because on my weekender if the rudder gets pulled back from the fully forward position the weather helm gets to be a real workout. If I allow the rudder to swing forward a bit more by re-shaping the stop, maybe I can reduce weather helm a bit. It is an easy thing to try with a sacrificial board.
On my weekender I have a handy gauge for checking helm deflection. The hatch cover for the lazerette has glue joints that fall on the centerline of the boat, so that it is easy to check deflection though I have to guesstimate the exact angle of the tiller. So far, I am not unhappy with the angle that I typically see, I just want to be able to balance the boat better so that I can get her to self steer better. I use a bungee wrapped around the tiller to hold it steady when I have to let go, but if there is too much force on it, it will slip pretty quickly. I like this set up because if something happens, I can grab the tiller and move it without having to let the clutch go. I realize that on such a small boat that if I move around very much it will throw the whole thing off, but I still want to be able to go forward with some confidence that things will stay fairly steady.
This is a fun topic. As you say, there is some art to getting it right, and I am just now looking into that foggy space. It will be interesting.
Al
It's often misdiagnosed (weather helm). The usual solutions are to rake the mast forward a touch, which moves the CE forward and eases weather helm. Additionally, you can hoist a bigger headsail which will do the same thing and one of the reasons, sloops usually have a bunch of different size headsails in the locker. Another, not as common approuch is to add lateral area aft, which could be as simple as setting the rudder blade rake aft a bit, again to ease weather helm, but the real question is do you have a weather helm issue or is it something that seems like weather helm (the usual case). I had to do this when I made my sloop into a ketch. I had to add a significant skeg in front of the rudder to get the helm to balance under the new sail plan, adding a full 10% more lateral area aft of the centerboard.
The check is as I described previously, sailing a close hauled course in 8 - 10 knots of steady wind and checking the actual deflection angle of the rudder. The simple way to do this is apply some tape to the forward end of the hatch in the aft deck and mark off several degrees to each side of the centerline. In ten knots, close hauled you shouldn't have any more than 5 with 4 being prefered in this wind strength. If you have more, you have a weather helm issue. If you are 5 or less you have another issue that's making the helm feel heavy.
About sacrificial boards, I actually have a blank I made years ago, NACA 0012 section, 12" wide, parallel leading and trailing edges, with a 3:1 aspect ratio. Over the years I've drilled new pivot bolt holes to check this board in other rudderheads, but otherwise it's still the same board I made and it's my "go too" when checking these sort of issues. Essentially, I look to see if I can adjust rake and depth to get the same performance as the blade I'm looking to improve upon. Once this is established, I play with depth (aspect ratio), balance (pivot bolt location) and rake (slope of leading edge). Sometimes, I'll hot glue a length of 3/4" plywood to the leading edge to get more balance, using a simple 3/4" roundover on it. I use automotive Bondo to fair this new piece in and this is good for a few rides, before it falls off.
Self steering is a tough one on a gaff sloop. The most basic way is to bring the jib sheet aft, through a snatch block so you can cross over the cockpit, then through a turning block over to the tiller or tiller arm (wheel setup). On the other side of the tiller (leeward) you tie a bungee cord on a cam cleat, so you can adjust the tension to "fight" the pull of the jib sheet. In operation the jib pulls on the sheet, which is now to windward of the tiller, in a puff or gust, which cause the tiller to come up a bit and the boat wants to fall off, but the bungee is counteracting this pull, which pulls the helm down and she wants to round. You play with the bungee tension at first, but eventually you'll find the sweet spot and she'll self steer on most points of sail, with close and beam reaches being the easiest to setup. Underway, as you hit a lull the helm will steer up, because of the bungee cord, then the sail stalls and the boat falls off, causing the sail to fill again and the process repeats. The boat sails a scalloped like course, as she rounds and falls with wind pressures changes on the bungee, but essentially in the direction you want to go.
One method I use a lot on smaller boats is the "tiller comb". This is a piece of wood or metal that's shaped like a hair comb, except the "teeth" are the width of the tiller. There's a few different ways to do it, but I like to use a different shaped comb, which is more like short dowels sticking up through a piece of wood, that engage into a hole on the underside of the tiller. You find a point of sail the boat likes, place the tiller in the comb, which holds the tiller, then spend a minute or two fine tuning her to self steer. This doesn't really self steer so much as hold the helm for a little while so you can go forward and grab a fresh beer from the cooler. Coupled this with a bungee on the jib sheet and you can make it fully self steer, which some initial adjustments. Most all of my "combs" are mounted on a stiff piano hinge, so it can be flipped up when needed or flipped down to be out of the way of the tiller when used normally.