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I'm going to jump in with a few non-expert opinions/observations.

The stock rudder is a blade of even thickness, rounded at the leading and trailing edge.

The "airfoil" rudder tapers toward the trailing edge.

It is easy for me to imagine that water would flow with less turbulence off the back edge of a tapered blade. In fact, I have seen noticeably less turbulence coming off the "airfoil" rudder than the stock rudder.

It makes sense to me that less turbulence translates into more responsiveness and better performance, which is also what I have observed.

When a rudder is turned at an angle to the flow of water, it creates lift (albeit laterally). I can see how a rudder that tapers toward the trailing edge would create lift more cleanly and efficiently than one that does not.

Of course, the "airfoil" rudder cannot have as aggressive a profile as an airplane wing because it has to function in two directions. However, it can still function as an "airfoil" (or hydrofoil, if you prefer) even with a symmetrical cross-section.

This rudder did not turn my 26S into a J-boat, but it has improved performance noticeably.

kurz wrote: ↑Mon Jul 26, 2021 6:22 am But in fact it makes no sense for me. Airfoil or in our sense hydrofoil are wings that have different shapes on both sides. If they are same like daggerboard the do not produce lift. So as long as both rudders are same - it makes no sense to talk about foils - thats my opinion.

This is the misunderstanding... A foil is not defined as having different shapes on either side. There are many foils used on airplanes that are symmetric... especially jets and acrobatic planes. Many foils used on boats are symmetric... especially monohulls. However, there are some that have camber (non-symmetry) when used to actually lift the boat out of the water. And in some cases there are even vertical foils with camber used on dual rudders and/or dual daggerboard boats.

Foils used on boats can be called hydrofoils, however, most of the ones used were airfoils originally designed back in the 1920's through 1950's for airplanes. Although values used in the analysis are different for water and air, the principles still hold.

Stickinthemud57 is correct on what the advantage of a foil over a non-foil rudder. It simply provides less drag when centered AND when angled. Typically, that means it is more rounded on the front and more tapered leading aft. Someone just winging it (pun intended) with a tear drop shape will improve over the stock rudders and it could loosely be called a "foil". However, real foils are those having been designed to maximize lift, while minimizing drag. The only difference in our case... is lift doesn't mean lifting vertically, but "lifting" horizontally making the boat turn.



Here are two cross sectional "shapes" for a rudder. The left one is similar to a stock M or X rudder. The one on the right is a NACA 0012 foil designed back in the 1920's and is still commonly used... especially on boats. The graphs can easily be summarized buy these two comparisons.

With the rudder pointed strait, the foil has 25% less drag.
With the rudder angled 8°, the foil has 38% less drag.

My stock M rudders are pretty close to NACA 12 profile if I recall...

Jimmyt wrote: ↑Wed Aug 04, 2021 4:08 pm My stock M rudders are pretty close to NACA 12 profile if I recall...

Think you're right. Just re-looked at mine.
I been watching too much Captain'Q's and must be remembering one of those old barn-door rudders looking like a board with rounded edges.

If that's the worst mistake you've made today, you're way ahead of me.

If you ever have to route a really wide swath, using multiple shallow passes, start on one side and go to the other. Don't work from the perimeter toward the middle. Just throwing that out...

Back to rudders...
I checked one of mine awhile back, compared to a NACA 12 calc.



Love Captain Q.

.025" on the trailing edge!!! That's not a rudder, that's a razor.

Be Free wrote: ↑Thu Aug 05, 2021 9:08 am .025" on the trailing edge!!! That's not a rudder, that's a razor.

You are so right! Sometimes it's hard to build what the numbers want.

Hi folks,

I don't get notifications via email on replies. Life goes on and I forgot to check the forum.

So, just a follow up on speed. Now that I have been lake sailing my Tattoo 26 now since 2015 I have experienced a range of "top speeds".

If the bottom is fouled I wont see past 4.2 Kts. Clean bottom, high 4's, I put a product called Aurora V721 about once every 5 months. When that product is applied to the hull, the boat is very slippery. 3kts wind 2 kts speed (calm waters). 5.4kts is about as fast as I have seen since my one downwind "go fast" as I reported a while ago here using a spinnaker. The big difference I have noticed with V721 is the improved close haul performance.

My boat seems to have a sweet spot for wind speed conditions into wind. As mine is rigged with roller furling jib, over 15kts wind gusting to 20 is just overwhelming for this setup with a single reef and a partly furled jib. 8kts gusting to 15 is fun, beyond that, I can't seem to get a sail combo to provide thrust vs 40 deg heal. My standing rigging is to slack for strong wind conditions. Any tighter and I can't connect the forestay when raising the mast.

Anyway, that's my experience.

From a couple of weeks ago. Garmin shows 6.3kt, heel angle was significant. Full main and Genoa in 12-15kt wind, gusting higher. Fairly close reach.



Still getting my depth transducer location dialed in. It was not 2 ft deep...

There no way I would consider a Genoa at that wind speed on my tattoo tight to the wind.

I was sailing with my two buddies that race keel boats. They aren't happy unless there's a rail in the water, and the crew is begging for mercy. It was the first time I had them both on the boat at the same time. I've learned a lot about sail trimming from them. One of them is a hard core sail guru. The other is no slouch, either. They are constantly gettting out little bits of line and pulling this corner down, or that corner out trying to get every last iota of performance out of it. They don't care much for my boom Furler. Actually, they don't like the headsail Furler either. But, I get a lot of ideas watching them fiddle with the boat.

Typically I would have been reefed over 15kts. Not sure when they reef, but don't know that I want to find out.

We went again today. Started out blowing light. I had checked the wind predictions and knew it was going to get in the 15-18 kt range around 3-4 pm. The wind was still reasonable after lunch, so we got out the asymmetrical. As soon as we got it rigged (2-2:30), the breeze started building. It came up almost exponentially. So, after a few minutes of seeing who wanted to say what we were all thinking (this isn't a good idea anymore), someone finally said it and we shoved it back down the hatch. Within a few minutes it was blowing pretty good, so we left the Genoa Furled and came the rest of the way under full main only - still making over 5.5 kts, and surfing occasionally into the upper 6 kt zone. The wind built to 17-18kt, gusting higher, as we headed for the dog river channel to go under the bridge and back to the river shack ramp. We had pretty large following seas at that point, so i was pretty busy at the helm. They said, "sail it through the bridge, we don't want to try and drop the main out here in this wind". So we got into the river and found a semi-sheltered spot to nose up into the wind.

After we got the boat back on the trailer and ready to take home, they both said, "your boat sails good. We had some pretty good conditions to test it today, and it did alright."

Now, they didn't say it was the best boat they had ever sailed (it wasn't by a long shot). But, they had a very enjoyable day on a water-ballasted Mac power sailer. I think it surprised them.

And, yes. If they thought it totally sucked, they'd say so.

Hi JimmyT,

That is a great report.

I am keen to tighten the standing rigging, though I won't be able to connect the roller furling forestay if I do tighten it any more. I get quite a bow in the jib leading edge profile above 12 kts.

Haven't had furlers on my X or D for years and learned how to both finish the mast raise and tighten enough to pin the forestay. I run a tight rig so I understand your dilemma. I use jib halyard to the top hole of forestay tang, then through the winch for tightening. You could add a jib halyard either at regular attachment point, that resides against the mast when off-duty or attach it higher up for spinnaker use.

I see furlers with loose rigs quite often. A M beat my X once in a race. I would like a rematch now. Hanked vs furler.

Idh63 wrote: ↑Sat Mar 26, 2022 6:09 am Hi JimmyT,

That is a great report.

I am keen to tighten the standing rigging, though I won't be able to connect the roller furling forestay if I do tighten it any more. I get quite a bow in the jib leading edge profile above 12 kts.

Ah just put up more sail


J