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New "Hydrodynamic" foil shaped rudder.


Barry Pyeatt

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After seeing many discussions on rudder efficiency in the Weekenders and Vacationers, I decided to make a more "Hydrodynamic" foil shaped rudder in addition to the stock rudder.

I had promised to post some photos of the resulting rudder but have not been able to do so until now. So this is a bit of a revisit to an old topic.

I had finished a stock rudder fabricated from 3/4" stock and shaped it according to the Stevenson's Video. I've used minor variations of that rudder on several boats at this point and have a feel for how they work in somewhat varied conditions.

I also have a friend that is a Marine Architect & "small boat" designer and an avid sailboat "racing nut" in the strongest sense of the word. :roll:

He is familiar with the Stevenson's designs and the Weekender and suggested that I do something like this to make a comparison and then do just that; give them both an honest workout over a period of time in all sorts of conditions and with various weight and sail configurations. Then decide if it is worth the difference. He suspects that it will help the light air performance substantially.

I was looking for a suitable chunk of wood to do this and I came upon an old sign that I have kept since the mid 60's or 70's. The reason I kept it was that it was 1 3/4" thick Vertical grain Doug Fir with extremely tight grain. Impossible to find today on the market and the price would be prohibitive if you could find a piece. At least to just try out a different variation of a rudder design.

I sent him a number of cross section foil shapes that I worked up from an article on centerboard/rudder designs and he suggested the one that I have here.

The board required a little resurfacing before I could use it so the net thickness worked out to just over 1 9/16" thick.

I had to allow for the 3/4" clearance on the rudder box so I had to lay out a radiused relief on the top to bring it down to the 3/4" thickness. Using a router on a pivot base I routed out both sides of the board before shaping the "foil" shape to the rest of the rudder.

I cut the taperd shape to the blank next freehand on my tablesaw.

I next used the table saw to rough out the edges by ripping them against a tall fence attachment. That brought the rudder to roughly the foil profile that I was looking for.

Then I took a jack plane and very quickly brought the blank down to very close to the finished foil shape. This could have been done with a Sureform as well but I chose the plane for the longer truer stroking ability giving it a very even, smooth surface.

I then cut the bottom edge rounded profile of the rudder on my band saw. and using my router and a round over bit zipped off both edges. That only left a little hand shaping with a cabinet scraper and a sanding block and it was ready to finish.

The total actual fabrication time for the rudder was approximately 4 1/2 hours from layout to first coat of finish. There are probably 5-6 coats of Varathane Classic Outdoor Gloss Varnish on the rudder in these photos. I had applied two coats of primer and one coat of Rustoleum gloss white to the top edge. I will receive one finish coat of white and probably 3-4 more coats of varnish prior to use. The dimensions are stock Weekender except for the top edge radius and of course the thickness.

I'll give both a fair workout and post some opinions on my findings after I get some sailing time in on both of them.

I do have one minor concern about this rudder. It will likely want to float up a lot more than the stock rudder. If necessary I am prepared to bore out the bottom edge and put in Lead plugs to help weight it down.

One other major point of discussion between my friend and myself has to do with turning points on the hull design. He thinks that it would be more effective with a centerboard design but certainly helpful with the stock keel design as well on the Weekender and Vacationer. So...nothing ventured, nothing gained. :lol:

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Barry,

First, I like fat rudders because they are more resistant to stall than thin ones. Yours looks great although the experts call for a squared off trailing edge rather than a rounded one.

I do have a couple of other comments. Such a rudder seems out of place on a boat like the Weekender. A highly efficient rudder to steer a boat with an inefficient lateral resistance spread out along the keel. Unless the stock rudder is very bad, I doubt that you will notice a great improvement. Also, the maximum stress on the rudder is just at the point where you have it necked down in width and thickness with a couple of large holes to further weaken it. Since it probably doesn't have much lateral resistance or sail area to work against, there should be no breakage though.

Not meant to be negative but perhaps useful.

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Thanks Tom!

The points are well taken and were considered as we thought it out. Stalling at low speeds was the primary concern in trying this out. I don't have any great expectations about this other than trying to see if it makes any significant difference with stalling.

The inefficiencys in the keel were probably the primary point of question when we first started to think about this. The bottom trailing edge being squared off was missed. I can see where that could make a difference. Fortunately that is easy enough to change.

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Barry

Would it be asking too much for you to give us (me) a 'cross section' view of the new rudder? and the reason it will perform better?

I haven't started the rudder yet and if this will be a big improvement I might just skip the regular shape.

Thanx

Andrew

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This is approximately the foil shape at the top of the rudder section. It gets shorter in length as it nears the bottom.

In a nutshell, sort of, anyway.

A flat board has no flow over the leading edge or off the trailing edge. It won't make an efficient rudder when trying to turn because the leading edge blocks the flow of water and creates turbulence which causes the rudder to stall and be completely ineffective.

If you round over the leading edge and taper the trailing edge the rudder has more of an "airfoil" type of cross section and the water flows easily along the length of the rudder. When you turn the rudder, the water again will flow more easily along the shaped cross section and not stall as quickly, if at all. Therefor you end up with the rudder moving the transom and the boat pivots on the keel at some point and the bow moves in a new direction.

If you increase the chord of the cross section (width at the thickest point) and ideally have an infinitely sharp trailing edge, the rudder begins to act like an airfoil under water. It begins to create lift off of the sides of the rudder. When going straight the lift is equal on both sides so the rudder and boat track straight. (There are some other factors involved but this is a simplified version.)

Then when you move the tiller or wheel and begin the turn, the water has further to go in its flow around the longer side of the rudder than along the shorter side and it has to flow faster to get to the trailing edge of the rudder so lift is generated in the direction opposite the direction you are turning the bow. At slow speeds this should (in theory anyway) be considerably more efficient than a narrower chord design or one with flat sides. It should help turning and help to prevent the rudder from stalling at slow speeds. This should help when tacking in light wind conditions where there isn't much forward momentum.

Now then, on a Weekender, Vacationer, etc. where the keel is long and wants to keep the boat tracking in a straight line as opposed to a traditional designed hull with either a centerboard or a fixed keel which allows the boat to pivot on its keel or centerboard and turn, this may help.

In higher momentum conditions, turning and tacking isn't so much a difficult thing to accomplish with a normally shaped rudder per the designs. The "stock" rudder as designed, will still stall if turned too far and actually becomes a brake, slowing the boat down and trying to bring it to a stop. This makes tacking at slow to moderate speeds more difficult and in really light conditions almost impossible. There has to be sufficient flow across the rudder to bring the bow around and maintain forward momentum in order to make the tack.

That is one of the main reasons you really don't want your rudder to move more than about 30-35 degrees either side of center. (It becomes a "water brake" instead of a rudder very quickly past that point and especially so at slow speeds.)

By having a more efficient "hydrodynamic" (airfoil) shape, the rudder will hopefully generate lift and not have a tendency to want to stall at low speeds. Thereby allowing the boat to maintain whatever momentum it has and help bring the bow around so the tack can be completed without stalling.

At higher speeds I don't expect it to be much different than the stock design.

Tom's input about stress on the rudder is absolutely a consideration, especially in higher efficiency rudder designs, especially if you have more sail area and the resulting stress on the rudder can either pull apart the rudder box or break the rudder/tiller, or some point that is the weakest part of the system.

I've taken a "thick" rudder and stepped it down to effectively less than half as thick as the actual rudder itself and then drilled two holes in it near the transition so that the hold down/lift rope can be attached. (Not an ideal situation.)

That weakens the rudder at that point even more. On a Weekender with its sail area the stresses on the rudder under normal conditions are well withing the strength factors of the rudder as designed. Depending upon how the grain is oriented on the actual board that the rudder is fabricated from, the point where the rudder enters the rudder box is where the greatest stress occurs on the rudder itself. That stress is transferred to the tiller and if its grain orientation and type of wood is laid out in certain combinations, and then we drill some holes in the tiller to attach the steering as well as the rudder box, we've created (unknowingly) a combination which has a series of weak points. Any one of which can break under stress.

Drill two holes near the end of a narrow board, along a line following the grain of a piece of wood and thru bolt them. Then apply pressure to the opposite end. The breaking point will be at the point where the bolts go thru the wood. The grain has been interrupted by the bolt holes and it splits along the grain at that point.

Then do the same thing but stagger the bolt holes into different portions of the grain and the result is stronger but it will still split along the grain where the bolt holes go thru it.

Do it thru the grain and stagger them and it is a much stronger situation and can handle the stress better.

So this got a bit long winded, but hopefully it helps to explain a bit about what I'm setting out to compare. :roll: It may not make any difference on this hull design or it may help a bit. That is what I wish to find out.

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Thanx for the informative and quick reply Barry.

I'm assuming there is a point at which the 'bigness' of the chord becomes 'too much'? (think of a sideways teardrop) [i'd draw it but the work firewall won't let me send pictures] and I also assume that the chord 'bigness' is dependant on the length of the foil.

Having made this plethora of assumptions, would it be fair to assume that a longer foil with a bigger chord would out perform a similarly proportioned but smaller foil? or does this just cause more trouble than it's worth??

When I think 'foils' I think plane wings, and having argued as I have above (if any of the assumptions are right) then why dont they put 747 wings on Cessnas?

What I was aiming at was if I double the foil length of the rudder and increase the chord accordingly, will I get a proportional amount of performance increase??

Thanx

Andrew

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This is one source that we used in consideration.

http://mothboat.tripod.com/CMBA/Building/foils.htm

My buddy has a bunch of marine references that have to do with foil shapes and specifications.

I would presume that as you get thicker foils, you will eventually begin to reduce their effectiveness as the flow will need to travel just too far when compared to the movement thru the water.

This is a good article on how to build a rudder that will handle the stresses imposed upon it.

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Regarding Tom's initial comments:

Would someone mind explaining to me why a long, shallow keel is not as good as a deep skinny one? I've thought of either adding chine strips to my Core Sound, or of installing the centerboard so that it protudes a couple inches even when retracted. (This to give me more control in very shallow water.)

Jeff CS20 #65

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Long shallow vs. skinny-deep is a comparison of Aspect Ratio. High Aspect Ratio lifting surfaces (racy keels, sailplane wings, modern windmill blades, etc.) are more efficient than Low Aspect Ratio ones (barndoor rudders, Praarie windmills, etc.) The Weekender has a super-low aspect keel, but the lateral resistance afforded by the keel is only part of the story. In these boats the chine and the lower forefoot are a lot of the resistance to sliding off "downhill". We had a nice high-aspect centerboard on the boat and it wasn't that noticable in use. That's why it was dropped: It just didn't add that much to the boat. The High aspect rudder is kept as it reduces loads on the box, works much better than low-aspect ones, and is very easy to make. (And we never have had ANY trouble tacking and sailing with ours, though we do have a lot of stick time with these machines.) A foil-shaped rudder will probably be an improvement at low speeds, but personally it wouldn't be worth the time to me (though I'm curious as to whether there will be some revelatory deal when this is tested.) If you're curious to make one and enjoy the process, it doesn't seem like a bad idea at all. You may find a bit more drag at higher speeds though, and maybe some handling changes too.

Mike

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I wonder if the mast angle has more to do with being able to tack effectively. That's the one big variable I see in the boats in pictures, and the reports of not being able to tack might be related to this rather than to the design. I do bear off, get up some speed, and then tack, but its very rare that I have any problem getting all the way through the tack. Yet I've heard of people having to pump the rudder, etc.

I would think a better foil design on the rudder would help with speed a bit, and perhaps keep the tiller from vibrating in puffs (I found a difference when I squared off the trailing edge of my rudder). But I don't think it would help in tacking.

But I'm a big believer in experimentation, and we'll know soon enough!

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  • 1 month later...

Mike and Ray: sorry I didn't thank you properly! I somehow didn't get notice of your posts, and only discovered Ray's when I came back here to learn about foils. (It's too cold to do much in the garage, so that means its rudder time.)

I think I'll leave the centerboard alone -- even protruding a couple of inches, it wouldn't add much to the lateral resistance provided by the keel.

My thanks.

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Thanks for bringing this thread back 'up' Jeff.

Great info.

Barry, your explanation of the foil and how it works was awsome. Now I finally get it.

I was just about to start a new rudder for my boat, so the timing of my finally seeing this thread was perfect.

I had figured on making a similar foil shape to how I did my dagger board.

daggerboardProfile.jpg

Would it be an improvement to increase the thickess of this foil (3/4") as you have done with your rudder.....or would it be improvement enough to duplicate that foil for my new rudder.

Also, I have another question. This is the only sailboat I have sailed with a lifting rudder (which I love). Is it usual to have a way to hold it down as well as a way to keep it up? I wanted to simplify mine if I could, I've gone looking at other designs...it is hard to tell from images, but none of the others I have seen seem to have a method of keeping the rudder down. )although I could just be missing out on some obvious detail)

How do the BnB lifting rudder designs work? (Beeker?)

Now that I am rattling on with questions....

In notice all manner of shapes and sizes when it comes to rudder blades (speaking of the 'side view' not the profile).

Could the WE rudder be improved in any way in a change of shape or mass?....and what would the effect be of a wider...or deeper rudder?

Thanks in advance.

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Tim,

In reverse order (sort of, anyway).

Hold downs for the rudder. The Weekender design calls for a rope to be looped thru the rudder box and attached to the leading edge of the rudder above the bottom edge of the rudder box. This is tied off to a cleat on the stub tiller to hold the rudder down. The trailing edge is also attached to the other end of the rope and is utilized to hold the rudder up and can be similarly cleated off to hold it.

Now then....all works fine unless you hit something and then something has to give. Solutions include the use of bungee cord instead of rope to hold it down but tied and cleated off the same way. Then if the rudder hits something the bungee cord can at least give some movement upon impact and allow the rudder to swing up. Others are using a simple tension type cam cleat that releases upon impact. Reasonably inexpensive and works like a charm. Some rudders have had lead added to the bottom edge to weight it down.

I personally lean towards the releasing cam cleat.

Wider, deeper, different shape.

I opted for the same size/shape from a profile view to make a comparison in efficiency with a thicker rudder. Only the use of it will determine what, (if any), advantage it may make. The potential is for it to develop more of a lift to help make tacking quicker and maintain more forward momentum thru the tack. Thicker shaped foils do have greater efficiency in tank studies and that has been pretty well documented. As to their effectiveness on a Weekender or Vacationer is yet to be explored.

The high aspect rudder design as it exists is a well done rudder. AT 3/4" thick at the chord if shaped as recommended, it is efficient and works well.

I would anticipate that the boat will have a different feel, whether or not that is a good thing is yet to be determined. Lots of different conditions and trials and then change to the stock rudder under as close to the same conditions as possible and do the same things all over again to compare.

Changing the shape to a larger profile would add another set of variables to the comparison.

My marine architect buddy feels that the Weekender/Vacationer rudder is well designed, as is, making it larger is most likely going to decrease its efficiency and make the boat slower. Changing to a barn door rudder design is less going to be less efficient and be more difficult to handle. Making a balanced barn door type adds another degree of difficulty to it as well. In tank studies, the high aspect designs seem to consistently outperform most of the others on this type of a hull.

I would think that as you have modified your hull shape, anything shaped much differently than the stock profile would be going off into the unknown and into a lot of trial and error.

One of the primary reasons a number of builders have had rudder problems have some very sound logic to the issues involved.

First in my mind, is the overall rigging balance and following true to design. As I think Frank alluded, some builders have mast angle deviations without even knowing it. This can give the boat a heavy rudder handling issue immediately. As the boat moves faster, the helm gets heavier, (normal). Bringing the mast back to its designed angle will help to balance the helm. Although it will still have a tendency to get heavier as the wind picks up and the boat works harder. This can be balanced with sail trimming to a great extent and working with sizing of the jib to better balance the load.

There have been a number of complaints about stalling in trying to make the tack even in a reasonable wind condition. A large number of these situations may be resolved simply by limiting the amount of swing to either side of center that the rudder can swing. Moving it much beyond 30 degrees, either side of center, simply creates drag and stalling of the rudder. It is like trying to push a board sideways thru the water. The boat now isn't turning, it is slowing down very quickly because the rudder now is acting like a brake rather than a foil to move the boat in a new direction. The solution is simply to install a pair of stops on either side of the tiller that prevent it from being moved too far in either direction.

It is my hope that increasing the thickness of the chord of the foil will improve the ability of the rudder to be efficient at slow speeds and not limit it at higher speeds.

Another issue is the tendency to make the leading edge too sharp rather than rounded. This causes the rudder to loose efficiency as well. It doesn't develop the "lift" that we are looking for and instead wants to stall as it slices thru the water.

Same with not shaping the leading edge and/or tapering the trailing edge. Ideally the trailing edge will have an infinitely sharp edge, but it is impractical to even attempt to accomplish this and maintain it. So somewhere it has to be squared off or rounded off. Squaring it off is more favorable than rounding it off as long as the taper is fairly long and the squared off trailing edge is quite thin.

Ideally the leading edge will have an "airfoil" rounded leading edge, symmetrical on both sides and with a thickest point located about 30-35% back from the leading edge. Since the rudder is narrower at the bottom than at the top this means that in a perfect situation that shape will change as you go further down the length of the rudder so that the cross section is still the same ratios as at the top.

If we were also looking at making the rudder have maximum efficiency, it would have a winglet plate attached to the bottom so that water didn't have a tendency to flow downward off the bottom edge. But we are not really into trying to improve racing efficiency, we are simply trying to see if the boat will perform better with a thicker, foil shape than with a thinner simpler shape. And to see if it is worth taking the time to make a modified rudder to accomplish it.

Sorry, got a bit long winded there, but perhaps others will not mind. I certainly don't want this to be construed as an Expert opinion. And I'm doing it simply because I am curious and have been reading about a number of people with issues that perhaps could be addressed here in one way or another. :roll: And besides, it is interesting, at least to me to see if what I read about rudder efficiency and performance can be applied here to this design and have it make any significant difference.

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Techie people might like this article - http://www.appliedfluids.com/UUST01.pdf - which talks about "tubercles" on the leading edge of a rudder, trying to simulate what humpback whales have on their flippers. Evidently if the cross section is thick enough, the tubercles (areas where the leading edge is thinner) increase lift even more than a NCAA foil section.

A "corregated" rudder might not look very technical though.

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Yikes. Either you type very well or you spent the whole evening answering my questions. I better be careful with these questions.

Thanks for that.

The only comments I would make are...winglets would adversely effect a very useful aspect of the stock rudder design. :) In the 'up' position...the rudder still works quite well to some degree. I let the rudder fly when heading for the beach or sailing through some thin water...happily, it still has some effect.

Duckworks sells the load release cleat. Looks like the cats whiskers that.

From what I have read and I have experienced with my boat, I personally don't think mast rake would be a large enough issue to explain the radical differences reported in either weather helm or helm balance or the ability to tack easily at any speed without falling off first....or for that matter just being able to carry through a low speed tack.

Even if a builder goofed by a long shot....and I have not personally seen any images that would suggest anyone has.....it still would not be enough from my limited perspective.

When I first traded my bike for this boat, the PO had built to plan. While trying to sort out the poor tacking issue and the weather helm issue, I moved that mast as far one way one and another as it could be moved, and it made zero difference as far as weather helm went. When I made the new smaller and much lighter mast, I was able to wedge it at deck level to wherever I wanted it. There was a lot of fore/aft movement and adjustability. I monkeyed around with different spots just to see if there was much of a difference, prior to a semi permanent location, fixed wedges and the mast boot. No difference from what I could feel. I am sure there was a difference, but pretty small I would say.

Earlier in the thread Mike said that they tried a centerboard and found no improvement, Yet I found quite a radical difference. This thing holds its course and digs in like you would expect going to windward now. Previously I was able to actually see sideslip when I had close enough visual reference. It was even more apparent when sailing with other boats. I am not suggesting Mike is wrong, but rather that I am very happy with the mods made so far.

The main reason for changes to the keel was to reduce the weather helm and to get boat to tack properly. Done and done.

Nothing was going to fix the tacking issue other than a what I did. She is the sweetest tacking boat now by gar.

On to rudder reconstruction. In my case I have actually kind of worn out the old unit. The rudder has become very sloppy in the case and bangs around too much. As well as that, the whole structure was made for the wheel steering system which I 'temporarily' modified for a tiller. I would like to make a nicer, more permanent job of the whole thing.

Again, thanks for the reply. Cheers.

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A relatively minor adjustment in mast rake can make a huge difference in the pointing ability and amount of weather helm in a boat. In bermudan rigs, production boats will have people change the rake by 2 or 5% to correct weatherhelm, and the sailors report huge differences in handling. They have the advantage of aluminum masts that can be bent easily by tightening the backstay ... and our boats, being gaffers, have neither a backstay nor a bendable aluminum mast.

I've never understood the complaint that the Weekender won't make it through a tack. I tacked all-the-way through, right from the beginning, and couldn't figure out why my sailing friends were so surprised. When they took the helm, and tacked, they were amazed. The boat is so light that if you bear off a little, pick up a bit of speed, and tack, you make it through. This is in light air, where the heavier boats are motoring because they won't move ... I'm talking under 3 knots of wind, folks.

I struggled to get the angle of the mast correct, tearing out and putting in the mast box three times before it was right. So I think the angle of the mast may have a lot to do with the differences people have in the way the boats sail. There's not much else that can be done differently, except for the rudder itself.

Its at least worth checking to see if the angle of the mast matches the plans. Shimming the mast in the mast box a few degrees forward to reduce weatherhelm might just do the trick.

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