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SASSAFRASS

Rudders

405 posts in this topic

Thinking about replacing the plate steel barn door on our boat this winter. Can anyone recommend a book that covers rudder shape and theory? It's for a full displacement hull.

 

Thanks

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Sassy: I cover rudders in my book. Not much you can do with a rudder on a full keel. I'd make sure whatever shape you chose that you kept it symmetrical with not too fat a trailing edge. Avoid a big radius on the trailing edge. Maybe you can get some ideas from these pics. Would help if we could see the boat.

W%209_zpstsw68rsc.jpg

P%203_zpsp2ddy2ew.jpg

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Not trolling for free design work, just a point in the right direction :D The rudder tube is on its last leg and needs to get replaced. The rudder is pretty much a chunk of plate steel and it loads up the stearing on any kind of heal. Not trying to re-invent the wheel but I think it can be improved a fair bit. And yes someday I hope the propellar fairy brings me a max prop.

 

DSC_0020.JPG

 

 

DSC_0063.JPG

 

 

DSC_0071.JPG

 

 

Under sail

 

PT%2B2015.jpg

 

 

Current sail plan

 

3.jpg

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Yuck, I do not like the way those "horizontal" ribs are not parallel to the flow of the water.

 

Steve

 

DSC_0071.JPG

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Yes the yuck factor is Strong in this one, not only are they not parralell to flow they are not even parallel to each other? Very limited travel as well, I don't know the exact degree's but it a pretty small arc. When Martha tacking makes you look like a bulk carrier rounding Flattery you know you have your work cut out. It sucks in light air as you have to get a mile of steam to come around in the same tidal exchange.

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Yuck, I do not like the way those "horizontal" ribs are not parallel to the flow of the water.

 

Steve

 

DSC_0071.JPG

Yeah, they look like air brakes (err, I mean water brakes....)

 

BTW, I like your vessel, smart looking craft!

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Very nice boat! Ya, the ribs would cause a lot of detached flow and make it very heavy. a smooth 3D airfoil shape would help. maybe some rudder forward of the rudder shaft although it would increase the draft a bit.

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Here is how I modified my barn door rudder. I added some rudder forward of the shaft (to create some 'balance') in the propeller aperture area. I did remove some 'useless' keel to make room.

 

I appears that Sassafrass could remove some of his dead wood above the propeller and do the same.

 

I could not be happier with my changes.

 

Steve

 

5c66b55c-1d19-43c2-bdba-168baf758fad_zps

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Here is how I modified my barn door rudder. I added some rudder forward of the shaft (to create some 'balance') in the propeller aperture area. I did remove some 'useless' keel to make room.

 

I appears that Sassafrass could remove some of his dead wood above the propeller and do the same.

 

I could not be happier with my changes.

 

Steve

 

5c66b55c-1d19-43c2-bdba-168baf758fad_zps

 

 

That very interesting Steve, I had contemplated the same thing but thought I would be diving into the abyss of ideas that sounded good at the time. I kinda think the arpeture is too big and the upper aft section could get filled with something like you did, I can also move the post aft some on the top. The NA we have been using Tad, has already told me to get out the planer on our next haulout and un-square the entry and exit. Did you add the plate piece at the top as well?

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Sass, I did add the plate to the top, along with a vertical rod welded to the aft end of the keel. Both of these were added to fill the gaps. All three of these changes were made simultaneously so we really cannot determine which one(s) made the biggest difference. My gut feeling is that the 'rudder forward of the shaft' addition produced the greatest change (lighter, more effective helm).

 

Steve

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Thanks Steve

and thanks Ras and overboard for the comp

 

Some idea sketches

 

rdr2.jpg

 

 

rdr3.jpg

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Seems like a TON of work moving the shaft, Although I'm sure it would work fine.

 

I like the top sketch better. Can you chop away that little bit of "usless" keel and therefore make your rudder "balance" a little bigger?

 

Steve

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Sassy:

I never thought you were "trolling for free design work". What would make you think that? I'm a professional and that's the way I do it. But I'm happy to chime in with ideas when the mood strikes.

I'll chime out now.

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Sorry Bob, I have a some what twisted sense of humor, which tends to fall flat at times, IE I was hoping you would chime in. I think your contributions are awsome here, no dissrespect intended.

 

Steve, moving the shaft is not such a big deal, it's running through the last 2 sq ft of original deck which I need to replace. You are probably right to get any benifit the fwd section should go to the hull.

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While you are doing grinder surgery in that area, check the required access dimensions for a max prop. You might need a little more space, may as well sort it out now.

That little triangle of material at the top of the prop aperture looks ok when the rudder is straight, but think of the profile presented to the water when you are trying to turn the boat.

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West system (gougeon) has a rudder design PDF, as well as their general construction PDF. There are a few other really nice magazine articles on basic foil design, explaining why you use certain shapes, I can't find them from my phone but searching for keel and rudder foil design should get you started.

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Yes getting the prop removal into it is the plan, right now I have to release the split coupling and drift the shaft fwd which is a PITA. Looking at the fwd rudder section idea sort of think I'm off the road in the weeds, looking at Panopes pics it looks like the roll of the stern and rudder shaft angle would have to be about perfect for it to follow the hull and be benificial.

 

I'll have to check the west system PDF out I was looking at some of them in the other sub forum yesterday. I'll probably pick up Bob's book and do some head scratching. I still like hardback books, even though they take up a ton of space.

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It would seem there has to be a proportional size relationship to the center of effort or pivot point of the hull and the blade size and angle for a foil pivoting at one end? An angle over "X" degrees will be ineffective regardless of rudder width/length and vice versa. Anything moving towards balanced would seem to reduce the arm and loading of the rudder and the overall size nessessary, but I wonder on a full keel boat how effective that would be in a practical sense sailing outside of hard turns? since there is not alot of magic going on trying to keep flow over the hull, with a traditional forward pivot point, as clean as possible be a bigger benifit, maybe?

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Making that plate into a foil would be a huge improvement. Study up on foil sections - than grind the re-inforcing ribs to that profile & skin the blade using the ribs as formers. Adding area forward of the post will reduce the amount of force needed to turn the rudder, but will not improve it's efficiency.

Raking the post aft (considered by itself) will add drag and loading. Can the piece of deadwood in front of the rudder flange joint be removed? That would allow for some balance area to be added.

A feathering prop will need more foe/aft room in the aperture, but you can shorten the shaft - you seem to have 2" extra.

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I don;t think the specific foil section matters at all with a rudder like this. Any rudder attached to the deadwood will screw up your foil section control. It would be a mistake to treat this like a spade rudder with a real leading edge.

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Longy, the "budget" plan did include putting lighting holes in the existing stock and building a foil over it. I have sort of learned my lesson on that though by having to re-do way too much stuff in the refit of the boat by trying to salvage existing vs. just doing it right. The fabrication side of it is not a problem as I have lots of access via work to built a new structure, and well my time is sort of free. One of the previous owners told me the rudder is made out of WWII tank armor? We were told boat was built for a US naval officer in Vietnam in the early 60's, lots of od lore with it. Not sure if its true but it certainly looks heavy. One of the interesting stories we verified was the guy we bought it from built the boat Hoot Mon which won the 1954 Havana cup. I found a newspaper artical online after he told me but have not been able to find it since.

 

I have been kicking the rudder and steering arrangment around in my head for a long time. Since it's a wood boat its hard to get some of the people of that flavor to take the Dolifinite goggles off and talk about anything non-traditional.

 

I really like what Steve did, try a little and see what happens.

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Getting rid of the turbulence generators is the biggest gain. Since they are structural, hiding them under a foil profile is the easiest way to do that. Getting cleaner flow across the blade will help a lot. The only experience I've had with a rudder behind keel was a Westsail 43. Small movements of the wheel did nothing - further turning would cause the boat to turn quickly. We decided that the rudder was living in a wide stream of turbulent flow from the keel, and had no force until the trailing edge got outside that flow May be why you've been told to shave down the back of the keel.

Adding balance area will reduce effort, but given my experience above, I don't know if you can get much result from adding just 2 -3" in front of your post.

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I don;t think the specific foil section matters at all with a rudder like this. Any rudder attached to the deadwood will screw up your foil section control. It would be a mistake to treat this like a spade rudder with a real leading edge.

 

Bob,

 

Is the above more about flow disturbance hull to rudder or is the only real gain just minimizing the disturbace/ drag the rudder itself will cause? I remember one of the books I have at home talking about minimum radius on the trailing edge, sort of the opposite effect of a boulbous bow?

 

In your 1st post above when you said it's important to keep things symetrical are you refering to the crosssectional shape top to bottom? IE no big radius or fans on the back, fwd rake matches aft rake?

 

Thanks

 

 

Longy, you beat me to posting the questions above. Thanks

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Sassy:

It takes a book to answer that question. Let's try this:

 

You draw a profile of the rudder you want, to scale. Then fit a rudder foil to it. That should be fun. Where is your leading edge? Where is your 30% chord? Are you certain?

The leading edge can't possibly be the stock. It's way too big a radius.

 

All this is a piece of cake on a spade but not so cut and dried on a rudder like yours. You may have to get ,,,,,creative!

 

Now, while you are having all that fun, don't forget you will need a constant thickness at the stock to avoid a really ridiculously hard build. Now you you doin' with that foil?

 

People will BS you here and try to make it complicated. It's not. You are retro fitting a rudder to an existing boat. You have minimal options without getting silly. I'd tell you how to do it but that would reduce your satisfaction when the job is done.

 

I'll give you a hint:

Think flaps on a 747.

Do they have a leading edge?

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Crying out for tubercules if ya ask me. What a shame they didn't weld those plates on parallel at least, you could have had some nice little whale speed bumps streamlining along there.

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Thanks all!

 

Lots to think about, been up for about 40hrs dealing with a angry auxillary generator that hates me, so very nice to have something resembling fun to figure out.

 

Will return later with some tangible tangents... and maybe a dwg

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Feathering strips from the keel to the front of the rudder?

 

That way you can keep the stock at the leading edge.

 

The rudder is still going to be heavyish as you don't have a balancing component forward of the stock, but that looks like a lot of extra work and then you can't fair it into the aft edge of the keel.

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That Taylor article is very good. Note how he treats the rudder like an extension of the shape forward of it. It's not a NACA foil. It's the end of a long foil like shape.

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Bob,

 

Yep, thats exactly how Colvin drew my rudder. The original part of the rudder has no camber whatsoever - just flat panels with the thickest part being the post. Works great when the keel is left 100% intact.

 

The problems arise when hacks like me, chop away large sections of the keel (Colvin is rolling in his grave) to mount ever larger propellers. In my case, the lower third of the rudder is still acting, as you say, like "the end of a long foil". The middle third is fucked because the post (thickest part) is now the leading edge. I reckon the top third of the rudder is not too bad now, because that triangle shaped 'balance' piece that I added has a small(ish) diameter leading edge with curved plates that sort of make a decent foil shape.

 

I'm sure that if we put my rudder in a wind tunnel the flow would look hideous with all these transitions, edges, welding lumps, and gaps.

 

I like your Carbon Cutter rudder with it's 'chastity strut' best, for a full keel.

 

Steve

 

5c66b55c-1d19-43c2-bdba-168baf758fad_zps

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I'll give you a hint:

Think flaps on a 747.

Do they have a leading edge?

 

 

Actually, they are Fowler flaps and they most definitely do have a very carefully designed leading edge. Now the rudder, maybe not really.

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So what is the purpose of those two diamond-shaped cutouts on the rudder? I once saw a rudder with similar, but larger, cutouts and it looked like a horrible idea -- turning a perfectly good rudder into some kind of drogue. Or am I missing something?

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The rudder is more like the ailerons on a wing . A movable section cut into the trailing edge of an airfoil. the fowler flaps are more like the sails on the boat several airfoil sections positioned to help increase the lift. how about a multi element rudder like the AC wing sails, or a movable trim tab on the rudder to lighten the load on the helm

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DDW:

I think you are taking my comment too literally. Overboard figured it out. My point was that they are just faired extensions of the wing foil. Of course they have a leading edge but is it the same radius leading edge radius as you would have on a spade rudder? Is max thickness of the flap at 30% chord?.

 

Valis:

I think the big Chinese junks had diamonds cut in their rudder blades. I'll have to think about that.

 

If the root problem here is helm pressure that relates to a poorly balanced boat and incremental rudder changes will not be much help. It's just a bandage on a far bigger problem. In most cases it's the keel that is in the wrong place that is causing the helm pressure. Adding balance to the rudder like Pano did would help. You are going to have to find a way to add some balance if you want to relieve helm pressure.

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Is this what your thinking about, Bob?

 

A fenestrated rudder?

 

post-13551-0-60536200-1469630118.gif

 

post-13551-0-18172500-1469630130.jpeg

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Balder: Exacty. Maybe, with those big, low aspect ratio barn door rudders with center of pressure well aft of the stock they were trying to relive some of the pressure by letttig the high pressure side "leak" through to the low pressure side.

I'd type more but my cat has decided its time to "attack Bob's fingers" time. Vert cute but annoying when you are trying to type.

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Is this what your thinking about, Bob?

 

A fenestrated rudder?

 

attachicon.gifimage.gif

 

attachicon.gifimage.jpeg

holy shit, that's a candidate for the society if i've ever seen one.

sorry for the OT

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Fenestrated rudders

 

l2691-003.jpg

 

Bob, maybe your cat is trying to fenestrate your fingers.

 

Rudder.jpg

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Looks like the Chinese may have invented the spade rudder!

 

Fenestrated? Cool, I have a new word.

I'll get my hair cut today. "Please try to avoid the fenestrated look."

 

Maybe those holes are all little vortex generators to help keep the flow attached.

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DDW:

I think you are taking my comment too literally. Overboard figured it out. My point was that they are just faired extensions of the wing foil. Of course they have a leading edge but is it the same radius leading edge radius as you would have on a spade rudder? Is max thickness of the flap at 30% chord?.

 

 

 

The flaps and ailerons on a 747 are two different things and work different aerodynamically. The flaps do have about the same leading edge radius as you would have on a spade rudder, and do have a max thickness around 30% chord, and are not a faired extension of the wing foil. If you meant "ailerons" but typed "flaps" then what you said makes more sense. Slotted Fowler flaps extend and separate from the wing as they are deployed, becoming a separate airfoil - kind of like the mainsail on an IOR rig.

 

Here is a 727, 747 is similar:

 

post-4075-0-25856800-1469633754_thumb.jpg

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DDW::

Nice but I'm not sure that a discussion of airplane design terminology here is going to be much help to this barn door rudder problem. There is a minor technology gap.

 

I don't have a very good grasp of those specific terms. I realize my use of the term "flaps" was technically incorrect. I have watched those slotted Fowler flaps be deployed many times and they always amaze me.

Thanks for the new term.

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Well it may have a little relevance. There is a pretty big prop aperture on the OPs boat. If he cut away more of it below the prop down to near the lower bearing, now it begins to look a little like your carbon cutter rudder with most of the rudder leading edge exposed. There will be flow through the aperture and over both sides of about 90% of the rudder's span. Just like a slotted flap. The post would need to be stronger because of the larger span between bearings, but it could be made balanced, and would generate a LOT more lift, probably steer about 100% better and easier. A feathering prop wouldn't hurt the rudder flow either.

 

There are a few airplanes with the ailerons separated and hung off the trailing edge as a separate airfoil with shape and all that. It makes them much more effective for the same area, also they can be balanced for less control force. It isn't done often because it is overall more drag compared to a single airfoil - but drag on this boat is probably not a big concern.

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DDW:

I think you are taking my comment too literally. Overboard figured it out. My point was that they are just faired extensions of the wing foil.

"too literally"? DDW and Overbored understand aircraft terminology better than Perry. Both object to Perry's comparison to flaps vs. ailerons in trying to convey his point. Perry scolds DDW and praises Overbored (who "figured it out"). Just another CA day...

 

 

image-208674-galleryV9-hmxz.jpg

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DDW:

I like that. I agree with trying to get more leading edge. Kind of an upside down version of what Pano did. Below the shaft appears to be the only option for that. In a bang for the buck situation that might be the best approach depending on how thick the trailing edge of the cut away keel would be. Maybe they could do a quick taper there to clean it up. I'd hate to see the OP go to great trouble and expense only to say, "Yeah, I think I can tell the difference." Might as well do it right.

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Soooooo... I did my homework. :)

 

Following the airplane crumbs and Steves succesful changes I read a bunch about wing foils and the related flow and pressure, alot of it is way over my head, but it is getting clearer. I Found this paper by Bernd Kohler on a lecture he gave at a multihull symposium in 1988. This helped alot in breaking things down to a understandable level. Unfortunately my internet connection is too slow to read the aticle Fleetwood posted. So, this is what I have.

 

Step one, identifying what I have.

 

I have a full keel boat and the rudder is the trailing edge of the foil formed by the hull shape and rudder in its entirety. This is the basline for anything to do with the rudder on my boat. They are one animal and cannot be separated.

 

Step two, indeintifying what factors can be improved

 

The leading edge of the foil is the stem and as Tad already told me get out the planer and un-square it.

Parasitic drag is something I'm sort of getting my head around, actually showing why a fair hull works

Other than the stem and entry, the hull is fair which brings me to the back end of the foil, the rudder, as the only other factor that can be improved

 

Step three, Designing the rudder

 

I have spent lots of time hawse pipe R&Ding things to make them work or work better, I've also spent lots of time aruging with vendors and design engineers who tell me X should do this than that, when it doesn't. Probably 99% of the time this is atributed to other unknowns or unaccounted factors that are not easily seen or understood. From a practical application and operational stand point its is important to understand how and why things work but also important to accept things that work even if you don't know why. If some process is proven to do the the same thing reliably, it doens't always make sense to try and change it to conform to how you think it should work.

A very long winded way of saying if it's not broke don't fix it. With that in mind, I think I can use some of the existing rudder features as starting point. The stock size has proven adequate for 50 years, the overall area of the blade has also proven to not be in excess of the stock and mechanical steering so using that footprint as a appriximate max dimmension seems reasonable.This of course can all change, but it is a reasonable basline to start from.

 

 

Rudder Design considerations

 

These seem to be the factors to try and incoorperate.

 

Minimal change in shape from the deadwood to the leading edge of the stock

 

Deawood can be faired some to reduce volume at the end provided the curve can be carried aft to the trailing edge of the rudder

 

Symetrical shape balancing contruction requirments and providing a fair exit on the trailing edge

 

Elimination where practial of any pressure variations via separation or gaps between rudder, hull and deadwood

 

As high a aspect ratio as practical in the given space. (Now the rudder pic Bob posted finally makes sense)

Involentary nocturnal tackle relocation eliminates the rudder extending beyond its current lowest point

 

I will work on a sketch of vs.1.0, For starters I will pretend the prop arpeture does not exist.

 

The next vs, will incorperate the prop arpeture and the contrusion and design changes it requires

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Bob,

 

Yep, thats exactly how Colvin drew my rudder. The original part of the rudder has no camber whatsoever - just flat panels with the thickest part being the post. Works great when the keel is left 100% intact.

 

The problems arise when hacks like me, chop away large sections of the keel (Colvin is rolling in his grave) to mount ever larger propellers. In my case, the lower third of the rudder is still acting, as you say, like "the end of a long foil". The middle third is fucked because the post (thickest part) is now the leading edge. I reckon the top third of the rudder is not too bad now, because that triangle shaped 'balance' piece that I added has a small(ish) diameter leading edge with curved plates that sort of make a decent foil shape.

 

I'm sure that if we put my rudder in a wind tunnel the flow would look hideous with all these transitions, edges, welding lumps, and gaps.

 

I like your Carbon Cutter rudder with it's 'chastity strut' best, for a full keel.

 

Steve

 

5c66b55c-1d19-43c2-bdba-168baf758fad_zps

That looks lijke a huge improvement. Let us know how it works out sailing ,particularly how that balance helps.

I use PVC grey plastic hose barbs for bearings. They have done circumnavigations and lasted decades with no sign of wear.;

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Bob,

 

Yep, thats exactly how Colvin drew my rudder. The original part of the rudder has no camber whatsoever - just flat panels with the thickest part being the post. Works great when the keel is left 100% intact.

 

The problems arise when hacks like me, chop away large sections of the keel (Colvin is rolling in his grave) to mount ever larger propellers. In my case, the lower third of the rudder is still acting, as you say, like "the end of a long foil". The middle third is fucked because the post (thickest part) is now the leading edge. I reckon the top third of the rudder is not too bad now, because that triangle shaped 'balance' piece that I added has a small(ish) diameter leading edge with curved plates that sort of make a decent foil shape.

 

I'm sure that if we put my rudder in a wind tunnel the flow would look hideous with all these transitions, edges, welding lumps, and gaps.

 

I like your Carbon Cutter rudder with it's 'chastity strut' best, for a full keel.

 

Steve

 

5c66b55c-1d19-43c2-bdba-168baf758fad_zps

That looks lijke a huge improvement. Let us know how it works out sailing ,particularly how that balance helps.

I use PVC grey plastic hose barbs for bearings. They have done circumnavigations and lasted decades with no sign of wear.;

 

Of course you do and why do we care? OH thats right we don't.

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That looks lijke a huge improvement. Let us know how it works out sailing ,particularly how that balance helps.

 

 

I use PVC grey plastic hose barbs for bearings. They have done circumnavigations and lasted decades with no sign of wear.;

 

I am very satisfied with the results of those rudder modifications. Helm is quite a bit lighter and less rudder deflection is needed. Prior to the mods, a gurgling sound could be heard (rudder ventilating?) when sailing fully powered. Now, all is quiet.

 

My rudder bearings are aluminum on aluminum. They have lasted decades with no sign of wear.

 

Steve

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Lasting decades sitting aground in Comox is a great feat!

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Troll food deleted...

 

 

On the leading edge of the rudder topic, is the benifit of potential lift out weighed by the loss in continuity of form in the transition from hull to rudder. Or I guess under sail properly trimmed you would hope to have small movments per seastate etc and have a minimual angle of attack on the rudder over a mean time. So overall hull movement and the turbulance generated via the water leaving the rudder would be minimal vs trying to open up more of the leading edge of the rudder for lift but at the same time making a bad transition point off of the deawood. Without having a modified keel or cut away of some sort with adequate distance and exit shape so the water leaving the hull does not have big effect onthe rudder or what ever gain in lift you would get for manuevering. The arpeture is there and I guess there will always be a compramise of some sort to make that work but ??

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If it was me: what diameter half round would enclose the existing leading edge? I'd use that to set rudder thickness, than taper back to trailing edge as you want after gazing at profile sections. Where there is room forward of the post I'd extend into that and reduce the width of the leading edge. Unless you can add 3-4" or more of balance, I'd strive to not have the leading edge stick out past the keel width. Keep the flow smoother past the joint.

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It's always a compromise. Having a keel hung rudder, when you deflect it what you end up with is a cambered foil, starting from the stem and ending in the rudder. A cambered foil has turning force but not as much. If you could open up the aperture so that the rudder acts like a proper airfoil, with flow on both sides, it should be much more effective. It becomes a proper lifting surface. For best effect get as much of the leading edge clear as you can. Then add a little area ahead of the post - 17% or so is classic for a spade, I might go a little conservative as I have heard stories about too much balance with the prop that close.

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Common on aerobatic aircraft are aileron spades or "shovels". They do the same thing as adding area ahead of the pivot line of a control surface - they lighten the control force.

 

A friend of mine built a set of these spades and on the first test flight he found that they were too large/powerful. Soon after take-off, he applied some aileron and the stick was yanked from his hand! The dude had 9 lives and he used one of them on that flight.

 

Steve

 

(random picture from the internet)

DSC02074.jpg

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Fenestrated rudders

 

l2691-003.jpg

 

Bob, maybe your cat is trying to fenestrate your fingers.

 

Rudder.jpg

I think those rudders need to be defenestrated, Czech style.

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As for balance, "classic" was 15% for years with many "balanced" rudders closer to 10%. I have done some at 17% but I would no go greater. Getting the balance wrong either way may require rebuilding your rudder like some folks have had to do.

 

Pano:

Are those"shovels" just flat plate?

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Pano:

Are those"shovels" just flat plate?

Yep. They most often have a bend along the sides for stiffness.

 

I know, they look like aerodynamic dog shit but that is the way it is done.

 

Steve

 

(edit: Oops, Longy beat me to it.)

 

20081119_134257_spade.JPG

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Common on aerobatic aircraft are aileron spades or "shovels". They do the same thing as adding area ahead of the pivot line of a control surface - they lighten the control force.

 

A friend of mine built a set of these spades and on the first test flight he found that they were too large/powerful. Soon after take-off, he applied some aileron and the stick was yanked from his hand! The dude had 9 lives and he used one of them on that flight.

 

Steve

 

(random picture from the internet)

DSC02074.jpg

 

"like power steering in a car" for pushing the ailerons either way:

 

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Here is a top down view of a shape that looks ok. This is something I can build that might not weigh a ton. Solid wood edges glued and fastened to a SS frame, 3 layers of ply for the panels and covered in epoxy and cloth. Will work on a side view now and options for the arpeture

 

rdr%2Bvs%2B1.0.jpeg

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Sassy:

You need to tighten up that trailing edge. You DO NOT want a big radius. Ideally you want a crisp flat about .25" wide.

You only need 40 degs each side.

 

Nice sketch.

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Make a sacrificial knock out piece for shaft removal where the circle thing is now

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Thanks Bob will adjust

 

rdr%2Bvs%2B1.1.jpeg

 

 

Longy without the circle it won't be nearly impossible to change the zinc, and I won't drop tools on the bottom and hamburger my hands :blink:

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It's not needed to pull the shaft out? If so delete that thing!

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Sassy:

That looks much better. You might check out the specs for NACA 0012. That's a very popular rudder foil.

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Sas,

 

Sorry to be so cryptic with my first response. I've been watching this whole thread to see how it pans out and was pretty sure that Bob's favorite rudder section the venerable NACA 0012 would get mentioned in detail. I can give you a template or even extrude that profile for you to get a blank foil that will contain your present rudder post and can even try and get some balance area into the aperture in a similar manner the Panope has shared. I noticed that you have some good CAD skills yourself so if you can give me shaft angle (rudder and drive shaft) and a profile of the aperture (I can perhaps get that from the photo you have already posted) I will model something up and we can look at how we can use the 0012 and get some balanced area fwd of the pivot and take it from there. I agree with Bob that 15% would be the max for the balance area but DDW and his 17% might be valid too. It would be hard to achieve that with the limited span across the aperture. I once built rudders with a 25% lead (under protest) and they were a disaster! I foresee an approach similar to Panope's setup which looks very good to my eye. He is happy with the results and lets see what we can do.

 

BTW Love your boat and the work involved to 'fix' the sheer is impressive. The rudder would pale in comparison.

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Didn't we determine that making a true foil shape on the rudder is irrelevant since the leading edge is the stem? It's more important to shape the rudder to taper to the trailing edge. Did I miss something?

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This is a NACA 0012 section, ~12" chord, ~36" tall, extruded or "swept" along a line that is angled sixteen degrees aft at the top (to match the trailing edge of your keel?). A Rhino Grasshopper script so easily scaled to exact dimensions.

 

Three gudgeon mount points is complicated though, eh? Could drop one or the other of the bottom two gudgeons?

 

foil_rudder_2106Jul27a.jpg

 

Or drop the keel gudgeons and go with a spade rudder that gives you much more freedom in shape and distance from the prop to work with. Getting a modern, balanced, more effective, vertical axis spade rudder looks possible, depending on cost of top bearings and installation work.

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You're talking about replacing an attached steel rudder with a fig-leaf rudder? Brent would have your knackers for that.

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Wow you guys are fast. Ras liek a idiot I did not take hard dimm last fall when I hauled out, the being able to sail part has bee nvery counter productive the last year. i need to get in the water when I get home next week and get dim. Proa, the gudgeon arrangment may change, I would liek to put a pilot in with a below deck quadrant so blocking and a gland/bearing arrangment would be added at the hull, right now the span is from the middle gudgeon to a tapered roller bearing on deck. The lower point could probably be re-done to strap the deawood and be ebough maybe.

 

Below is 1200mm naca 0012 with the fwd section lopped off, due to it skinnying up so much I nixed the solid wood leading edge in favor of carrying the center plate out and the final layer of ply, lots of shipwright in a bottle on the end.

 

rdr%2Bvs%2B1.2.jpeg

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Wow you guys are fast. Ras liek a idiot I did not take hard dimm last fall when I hauled out, the being able to sail part has bee nvery counter productive the last year. i need to get in the water when I get home next week and get dim. Proa, the gudgeon arrangment may change, I would liek to put a pilot in with a below deck quadrant so blocking and a gland/bearing arrangment would be added at the hull, right now the span is from the middle gudgeon to a tapered roller bearing on deck. The lower point could probably be re-done to strap the deawood and be ebough maybe.

 

Below is 1200mm naca 0012 with the fwd section lopped off, due to it skinnying up so much I nixed the solid wood leading edge in favor of carrying the center plate out and the final layer of ply, lots of shipwright in a bottle on the end.

 

rdr%2Bvs%2B1.2.jpeg

 

Do it right when you build it, there's lots of people here who can tell you how. I'm not saying that's wrong, but...

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Ish, for sure. trying to throw all my bad ideas against the wall and see what happens. Anything composite is not my specialty. Done a a bit of fab work and am a ok wood butcher so I tend to lean towards the metal backbone I know. Looked at solid wood and solid ply as well.

 

I like the train of thought going, my one hold up is messing with the deadwood at all. If I had as-build's or any plans it could probably be approached but with out knowing if there is a 3' x 3/4" bolt running though something I'm going to cut off I'm leery of messing with it. I could see adding shape or fairing but big lops are a little iffy.

 

Proa is making me think too much :blink: I'm getting the feeling that the proper foil shape, full height, length a little shorter than existing and some Ras mataz on the front upper end might be the road........this will of course mean much fairing of the deadwood but that is probably the right way to do it. time to think more

 

Ish- Is your new look Dee Snider in diapers?

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Am I reading your drawings correctly, that it will be hollow? If so then your proposed construction method worries me a bit - plywood and water are never a great mix, and since it is a rudder you can be assured water will get in there. I rebuild mine - got some sheet foam and shaped it, glassed over that. I'm sure water will get in eventually. You can seal the heck out of the exterior, but water sitting inside it will fester.

(No Ish, not referring to Uncle Fester, leave him out of this!)

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Sassy:

I warned you early on that some people will over complicate this project.

I believe the rudder blade can be flat.

I also think some camber, as you have it now drawn would be better.

I do not think adherence to a specific foil shape is required. It's just outside the overall nature of your boat. (Spoiler on a Toyota Corolla?) No offense to your lovely boat.

Would a strict foil i.e. NACA foil section be better? Yes but I'll bet my JS 200 that you would never know the difference.

You will need some type of leading edge detail where you add the rudder balance area. Try to echo the look of the 0012 nose.

 

Don't let people fool you into thinking this is complicated. I have done countless rudders like this. Rudders that were built not just talked about.

This is easy. Build it as you now have it drawn and all will be good.

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Ish- Is your new look Dee Snider in diapers?

 

I have no idea who this is.

 

BVulDts8_400x400.jpeg

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As if anyone is trying to "fool you"... Where does that animosity come from?

 

Give me these three dimensions and I'll give you a CAD file:

 

rudder_naca_0012.png

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Didn't we determine that making a true foil shape on the rudder is irrelevant since the leading edge is the stem? It's more important to shape the rudder to taper to the trailing edge. Did I miss something?

 

If you read everything Bob wrote, I think you will find he implied this a tricky bit where the usual rules don't always work. Looking at the big picture, yes the stem is the leading edge, but looking just at the leading edge of the rudder it's running into turbulent water in the aperture. So you want to hit it with something at as good for dividing the water and getting smooth flow on the rudder as there is room for.

 

I ran across an article on rudder construction and posted it on the Origami thread before I realized this thread was getting nitty-gritty about how to build. So here it is again.

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If you drop the bottom gudgeon and extend the second gudgeon aft, the rudder post could be more vertical, better balanced and ventilated, further aft from prop.

 

image-208674-galleryV9-hmxz_Jul28a.jpg

 

16.2 degrees from vertical? Post length (span) and chord dimensions?

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Yuck, I do not like the way those "horizontal" ribs are not parallel to the flow of the water.

 

Steve

 

DSC_0071.JPG

Yeah, they look like air brakes (err, I mean water brakes....)

 

BTW, I like your vessel, smart looking craft!

 

 

Great discussion, and very nice boat. While an altogether new, optimal rudder is the end goal, I agree with Pano and Kim that, currently, drag looks like a big problem.

 

I might consider fairing out the existing rudder with your wood and glass skills as close to the NACA 0021 as you can, recognizing that the solution could last years but is not permanent, just to see how much of an improvement was gained by removing as much drag as possible. This doesn't add additional balance of course, but there is no structural work required, it's a much smaller project, and the results might be huge, as they say now.

 

Best of luck!

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If it was my boat, I would pretend that the stiffeners were foils to lift the stern. I might even invent an obscure acronym to describe the system. Could this fool the editor in making the boat feature on the front page?

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Yea, that's gonna feel a whole lot better. Move the zinc to the top or bottom of the plate and maybe add a tubercle or two that protrude just a little into the upper part of the prop aperture. Whale approved.

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Thank you everyone for all the great technical advice! Bob, Proa and Rass especially for the very generous insight. In the, prepared for technical discussions vain, I definitely showed up to the party with half a can of cheese whiz and stale Doritos. Without having a template with hard dimensions any more modeling is probably not worth it, but thank you for the offers. Will hopefully be able to add some updates in a week or so. A good excuse to get the dive gear sorted out!

 

Thanks again everyone.

 

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Vertical option, pivot at ~17% of chord.

foil_rudder_2106Jul28b1.jpgfoil_rudder_2106Jul28b2.jpg

Could be spade style or the second gudgeon could be extended aft by doubling (at least) the length of the current bracket (see light blue "new bottom gudgeon"):

foil_rudder_2106Jul28b3.jpg

FYI:

naca_0012_thickness.png

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Perhaps the greatest (only?) reason to have a keel hung rudder is to avoid snagging lines and kelp. Make sure the lowest gudgeon is at the ABSOLUTE bottom of the rudder.

 

Steve

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Wanted to add that I am thouroughly excited about this project. Coming into it I had little to no knowledge of rudders and foils other than they are useful for red right return. As with most things I now know or have a clearer picture on just how much "I don't know". Although we won't be winning races "yet", I foresee a big improvment in the boat by the time its all said and done.

 

I should have brought pictures of the rudder in when negotiating our rating for the CL M regatta in PT.....

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Fenestrated? Cool, I have a new word.

I'll get my hair cut today. "Please try to avoid the fenestrated look.

Could be a cooking technique. "take the fenestrated tomatoes and add to the pan'.

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Ok, still another day before heading home.....

 

Think I will try a middle approach when I get home before I commit to a ton of materials etc. and expiramenting is always fun!

 

rdr%2B6.jpg

 

In the above I'll template the yellow lines and lay it out full scale on a sheet of door skin or somehting like that. The prop can get moved fwd some I just need to verify mininmum and max dimensions for the arpeture. Provided the bolts move I can pull the rudder in the water and demo at the blue line or something close to it. I can then laminate and bolt up solid island fir to the dim of the red outline, maybe rabbit and ply for the trailing edge glass the whole thing and see how it works.Stick with this shape but bring from radius to foil on the projected leading edge in open arpeture,

rdr%2Bvs%2B1.2.jpeg

 

Easy to shape and mess with

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If you're going to replace parts and restore existing keel gudgeons, then fairing the rudder surface (horizontal contours) to a NACA foil might be the best you can do. I doubt there is any helm balance from that small triangle.

foil_rudder_2106Jul28c.jpg


The only way I see to get a "balanced rudder" is to move to a vertical rudder post, which gives more room for leading edge area between the prop and the rudder shaft.

foil_rudder_2106Jul29b.jpg

Two ways of getting a vertical rudder post:

  • Extend bottom gudgeon(s) aft. - The engineering and construction of this "strut" is significant!
  • Forget about bottom gudgeons/strut and use a spade rudder. - Straightforward parts and solution, rudder profiles are textbook, all deadwood below prop can be carved away at will, perhaps refined over years of haul outs.

P.S. Again, dimensions shown are not to scale for this boat.

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Making that plate into a foil would be a huge improvement. Study up on foil sections - than grind the re-inforcing ribs to that profile & skin the blade using the ribs as formers. Adding area forward of the post will reduce the amount of force needed to turn the rudder, but will not improve it's efficiency.

Raking the post aft (considered by itself) will add drag and loading. Can the piece of deadwood in front of the rudder flange joint be removed? That would allow for some balance area to be added.

A feathering prop will need more foe/aft room in the aperture, but you can shorten the shaft - you seem to have 2" extra.

Raking the bottom of the rudder post aft increases the tendency for the rudder to suck air down from

the surface and stall easily. I have seen tank tests which shows a huge decrease in effectiveness

with the slightest rake aft. Read what Donald Street has to say about this. He asked many designers

why they do it, and the best answer he could get was "It looks fast and sells boats."

Vertical or slightly foreward is best.

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Sassy:

I warned you early on that some people will over complicate this project.

I believe the rudder blade can be flat.

I also think some camber, as you have it now drawn would be better.

I do not think adherence to a specific foil shape is required. It's just outside the overall nature of your boat. (Spoiler on a Toyota Corolla?) No offense to your lovely boat.

Would a strict foil i.e. NACA foil section be better? Yes but I'll bet my JS 200 that you would never know the difference.

You will need some type of leading edge detail where you add the rudder balance area. Try to echo the look of the 0012 nose.

 

Don't let people fool you into thinking this is complicated. I have done countless rudders like this. Rudders that were built not just talked about.

This is easy. Build it as you now have it drawn and all will be good.

I don't know why any designer or builder would make a rudder out of a single plate,

when it is just as easy to make a proper airfoil one, which is also lighter.

It takes me under an hour to build one out of ten gauge plate.

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