PocketShip.net

I am thinking of putting small roundovers on the leading and trailing edges of the rudder to smooth out the water flow. It seem obvious to me that this would help on the trailing edge. Not so much on the leading edge when the rudder is centered because it is directly behind the keel. But it seems to me it would help smooth out the water flow when the rudder is turnws to either port or starboard, which is almost all the time.

I have the rudder assembled and roughed out except for the bottom plate. If I decide to do the roundovers I would want to do them before the bottom plate is installed so I can get them a little closer to the bottom plate. I won't go quite all the way to it so as to leave wood area for attachment and for the fillet.

Please let me know what you think about this idea.

I suggested this a while ago on this forum, but as I recall nobody was very excited about it. Not sure why. But i went with my gut and did a 1/4 inch roundover on the leading edge. As far as I know, the trailing edge of a foil (like a rudder) is supposed to be sharp and thin, not rounded. So the trailing edge should be as sharp and narrow as possible. In my opinion 3/4 inch is still pretty thick, and I may eventually add an extension strip to narrow that 3/4 to maybe 1/4 inch thickness. But no roundover there.

I also rounded over the rudder end plate edges as well, by the way.

Craig

Thanks Craig. I may still round the trailing edge slightly--just seems to me that the water would flow better, fewer eddies. I also thought about small roundovers on the plate. I'll give that some thought.

http://www.clcboats.com/forum/5/thread/27285.html

I couldn't find this earlier but eventually got the link for this discussion on the clc forum. Some of the discussion is confusing I think but lots of good details on rudder design. I had the rudder design from John Harris saved as a file on my computer for future reference

Thanks for the info Craig. After much thought I decided to use 1/2" roundover on the leading edges and 1/4" on the trailing edges. I feathered out the ends with sandpaper, so the rounding reaches pretty close to the bottom plate,though there is very little rounding by the time it gets there. I think it looks nice, and I always round corners on wood anyway. Square corners are fragile. I will slightly round all the edges with sandpaper.

I have pretty much decided to round the edges on the bottom plate also, although I will do this by hand with sandpaper.

When I was doing rudder research for my other boat, I learned a lot about rudders. The gist of it is PocketShip's rudder is a bit of a slug -- by design. The boat's shallow draft plus relatively slow hull speed means the end plate will work about as well as a high aspect (longer, thinner) rudder without compromising the shallow draft. Rounding off the edges will help a little but even more so will be ensuring that the turbulence coming off the keel is minimized and that the water is directed over the foil of the rudder and the end plate. In other words, the keel should be absolutely straight with the rudder right behind it, with the end plate aligned with the bottom of the keel. I know that seems obvious, but this will help more than any amount of rounding to maximize performance. (And for the record, my rudder has some nice rounding to it.)

Hi. I rounded off the trailing edge of the rudder, about 1/4", not enough to weaken the plywood sheet-to-interior rudder frame bond. I also rounded the edges of the rudder plate. Seemed to me that rounding the leading edge of the rudder would confuse the flow of water at the keel-rudder gap. Having done all that, I doubt that rounding rudder edges contributed anything to the speed or handling of the boat....just looks nice. Having sailed Carlyn J a bit, I'm happy with the rudder. I remind myself that she is not a race boat, but is a great little pocket cruiser by design. Good luck. Jer (aka mtsailor), s/v Carlyn J (which is gunkholing in the garage for the winter). http://www.flickr.com/photos/mtsailor95/

Thanks to everyone for their thoughtful replies--they helped a lot. I did some rounding--not to make the boat faster though. My hope is that it might perform a little better in light air, which we get a lot of here, even though it is Oklahoma. But when you think about it, getting the boat moving in very light air would be helped very little if at all by smoothing water flow at the rudder--that slow it would probably make no measurable difference at all. But, I did it anyway--can't hurt and it looks nice.

Uh oh...my emergency aerodynamics intervention may have come too late.

A generous round over on the leading edge of the rudder is a good idea. It will enable the flow at the leading edge (which is coming off the keel) to attach cleanly to the rudder.

The trailing edge is a different story. You want those corners to be as sharp as possible. This may be counter-intuitive to armchair aerodynamicists, but it is true. With a sharp corner, the flow coming off the trailing edge will always (assuming the rudder isn't stallled) separate at that sharp corner, even when the rudder is deflected. Yeah there is a small amount of drag due to the little bubble of separation due to the blunt trailing edge, though it isn't as bad as you might think...but I won't go into details on that.

With rounded corners, there isn't a fixed separation point, so the flow may stay attached a little longer in absolutely perfect conditions, but instead it will tend to wander around and the flow will basically end up unzipping way forward of where the sharp corner would have been. So now there's a bigger bubble of seperation behind the rudder, and thus more drag. And it gets worse when you deflect the rudder, because that seperation point is now going to be really happy to move forward, and all that area behind the seperation point isn't going to contribute a wooden nickel to your rudder effectiveness. So, now you've got more drag and less rudder effectiveness. Bummer.

That being said, the magnitude of the degradation that we're talking about isn't such that it would make the rudder useless or anything. Even sailing a sharp corner rudder and a 1/2" round-over rudder back-to-back probably won't yield a perceptible difference. But it'll be there just the same.

Jon, question -- if going to a point is 'best' on the trailing edge, would it make sense to add such a point to PocketShip's rudder? It would not be difficult to add maybe a couple inches back there to get to a point. I doubt I have the time or energy to do so but curious nonetheless. I have my leading edge slightly rounded but not the trailing edge, based on prior comments.

On my other rudder, I used a NACA 0012 profile, which comes back to a sharp point at the trailing edge. I've wondered why PocketShip's doesn't do that. It basically cuts itself off around station 8 (from left to right, including station zero).