A question for the brain trust-a new rudder

[chuckpierce]

I am building a new rudder for my P19 because, well, the old one stinks. It kicks up when I don't want it to, doesn't kick up when I do want it to, isn't well balanced, and doesn't have enough authority at high angles of heel. I have built a jig for my router that will allow me to shape a NACA 0012 foil onto the lower part of the rudder. My question is this-How far up does the foil shape need to go? Does it just need to be on the section that is lower than the skeg? Or should I take it up to where the lower (kickup) blade enters the housing that attaches it to the upper piece? There is a 8-9 inch area on the lower section that is in the water but above the bottom of the hull even with the tiller all the way over. What do you guys think?

Thanks

Chuck

[PAR]

I think the NACA shapes can help to a degree, but you can go crazy with this sort of thing and get very little fundamental change in the handling of your boat.

For example, you could incorporate "surface piercing" foil shapes in the upper 1/4" of the blade's length, for less resistance where it exits the water. You could also use a fence attached to the cheeks (assuming typical rudder construction) to help keep the blade "engaged". You could use different foil sections on the lower portion of the blade to help reduce tip eddies and induced drag, but again, you have to approach these things systematically.

If you do all mentioned above and it's better, but not what you want, where to you look to make more changes. This is because you've taken a big swing and don't know where to start looking.

The best approach is take little chips. An example would be install some fences and see how this affects the rudder. Good, do you want to keep it or no good, it's got to go. Maybe try more balance, is it good, do you want to keep it or no good, it's got to go. The whole blade NASA 0012 will be an improvement, but will it be the feel you want. Try it, is it good, do you want to keep it or no good, it's got to go. Small swings of the "I'll fix it" hammer and you'll be able to tell when you've hit the right solution or combination of changes that get you want.

The kickup feature can be fixed easily with a bungee or a down haul and a "break away" cleat. This will keep the board down, yet let it come up when it hits something.

The "bite" thing, you'll have to describe better. What is the tiller angle (blade incidence) to the centerline of the boat, when on a close hauled course, in 10 to 12 knot winds? Do you feel vibrations, hear noises at certain times (take your medication?), etc.

[chuckpierce]

Sorry for the delay replying to this it has been BUSY here-

The tiller is 4-5 degrees to weather close hauled. No vibrations. One big problem is that in a quartering sea from astern keeping the boat on course will wear you out. 7-8 hours of that and you're exhausted. Will more surface area help with that or am I barking up the wrong tree here? I have been looking at the breakaway cleats that Duckworks sells...

You make some excellent points about making changes in steps.

The lower blade shape and size are actually based on a one piece "high performance" rudder that the manufacturer used to sell and that got good reviews in terms of performance. The only negative I could see about that rudder (besides the high cost) was the lack of kickupability (don't know if that's even a word). We sail in shallow water some and the boat has to be beachable. I have never personally sailed with one of these rudders although I know folks in other parts of the country who have them and like them.

So you think the foil shape may be a waste of time and just fairing the blade fore and aft would be sufficient? Maybe as you suggest a better course of action would be to shape it less radically, try it, then I can always take off more material if needed...

I am building it initially from Douglas Fir so that I can experiment cheaply (the epoxy to glue it up cost more than the wood). When it is the way I want it there are a couple of pieces of African mahogany on the woodrack waiting...

Thanks

[ChrisObee]

The tiller is 4-5 degrees to weather close hauled. No vibrations. One big problem is that in a quartering sea from astern keeping the boat on course will wear you out. 7-8 hours of that and you're exhausted. Will more surface area help with that or am I barking up the wrong tree here?

When you say the tiller is 4-5 degrees to weather do you mean the boat tends to turn up into the wind or the opposite? 

[Hirilonde]

It would seem that he means the boat has some weather helm.  In other words it wants to head up.  If it isn't extreme I think that is a good thing, I wouldn't want to fix it.  I always measure weather helm by how much force it takes on the helm to hold course, not degrees, so it is hard for me to tell how much he has.

[chuckpierce]

Yes, she has some weatherhelm. PAR asked how many degrees off centerline the tiller is when we are sailing close hauled, so that is an answer for him . The weatherhelm gets extreme at high angles of heel in gusty high (20+ knot) winds. On this years' Texas 200 we rounded up uncontrollably at one point - we like to sail aggressively  :smile:. This WAS the first time we almost hit someone- :shock:. One of my goals here is to have more control in those conditions...

[PAR]

4 to 5 degrees is just about right for the wind strengths I mentioned previously, 3 to 4 would be perfect.

When you have a quartering or following sea, many boats, particularly boats with some level of performance will get tossed around. The stern lifts, you lose blade immersion and the resulting lose of bite causes you to have to "chase" her around, which can wear out a skipper pretty quickly. I have an early IOR boat that will drive you insane off the wind, with steering being nearly imposable under certain conditions. Balanced rudders seem more prone to this then non-balanced.

Go with a "slab" style foil shape. This is a rounded off leading edge, flat sides and a tapered trailing edge. This requires little effort and is nearly as good as the fancy NACA shapes. Leave the trailing edge square (don't round it) and make it about an 1/8" wide, so it's not easy damaged.

A rudder doesn't have to "kickup" to be shoal or bleachable. You could use a cassette style, which is like a dagger board case on pintals or an Aussie style, which is very similar, except the back edge of the case is open ("U" shaped) and bungee cords hold the blade in place until it bangs into something.

Helm pressure in naturally supposed to get heavier as the winds build up. This is the boats' way of telling you to reef or "I'll dump 'ya, for not paying attention". As a boat heels the rudder becomes less and less effective and eventually you lose steering control. This means a broach or hard rounding, depending on what you're doing and conditions. When you're on the edge of control, you've done something wrong, which most of the time is waiting too long to reef. All boats will have a "groove" that they like to sail in. This is their "happy place" and they'll make their best speeds in the groove. If you let the boat get out of it's groove, it's not sailing as fast as it can and you're probably working more then you need to. Let the boat tell you what it want and pay attention, because like any woman, it can change it's mind and you'll need to adjust.

[Palolo Hawaii]

As a boat heels the rudder becomes less and less effective and eventually you lose steering control. This means a broach or hard rounding, depending on what you're doing and conditions.

To add to PAR's great info; also as the boat heels, the leeward side of the bow (may) exert more force-to-windward on the hull: compounded with the rudder's reduced efficiency and you've got the ingredients for a round-up, even when not off the wind.

Caveat: I am not sure what a P19 looks like. I encountered it most on the Int'l 110 with its box-section hull. Heel beyond a certain point and this force from the leeward bow wave  was quite noticeable.

Sorry if I don't have the technical terms for it, but I have experienced the effect. PAR might be able to explain it properly.

Good luck,

pat.

[chuckpierce]

OK, thanks for the advice. I think I am going to leave the spur on the front for balancing with the idea that I can always cut it off later if I don't like it, and do the shape that you suggested leaving the center thick enough that I can shave it down to a foil shape if I need to later. I should be able to sail with it in February and I will let you guys know what happens with it. I really appreciate you all taking the time to help me out with this!

Chuck

[chuckpierce]

OK-here is the result of my efforts and your advice-

I sailed with it several times in March, the tiller was extremely light and in 10 kts of wind still had very little weatherhelm. I changed the angle of the top piece so that the bottom of the blade moved back a couple of inches and the blade was not as far under the skeg. Sailed with it in April a couple of times, and liked it.

Tiller pressures are still much lighter, but not so light you can't feel what is going on. As the boat heels, tiller gets heavier (although never as heavy as the original) and weatherhelm increases. The boat is more responsive at slow speeds as well, and according to the Navman we get steerage at .2 knots.

The boat will no longer round up on her own with the rudder all the way over. She WILL however round up if you keep the tiller where it is when the roundup starts. If a  short gust starts the boat on its way into a roundup you can wait until it starts then pull the tiller to weather and she heads right back down without completing the roundup. Before, in gusty winds she would round up when heeled just a little over 20 degrees for a second or two. Now I can let her roundup if the gust continues or if the gust just lasts a couple of seconds I can turn back down without it completing. I realize that this calls for quick and correct decisions about what to let it do, but I like being able to decide for myself rather than being at the mercy of the boat.

Following seas are easy to deal with now-smaller tiller movements and much lower pressures. The advice to not worry about a NACA foil was dead on. Week before last we completed the Texas 200 with the new rudder and it performed very well in contrast to the rudder problems we had in last year's event.

Much thanks to all of you for your advice and opinions, you saved me a lot of time and trouble and experimentation!

Chuck

[PAR]

It was a bit of a wait, but the results are the thing that's important.

Tiller pressure decreases are probably the balance working for you. If you aren't getting enough "information" from the blade, through the tiller, then shave a little off the leading edge and try it. Just a small sliver here can make a substantial difference and can fine tune the blade for your average sailing conditions.

Everything you describe is normal and clearly a positive step forward with this new setup (ain't yacht design fun). Do you need as much helm deflection (rudder angle) to hold a close hauled course as you use to?

It's difficult to tell, but the images of the new blade look like it's a fairly fat section. What are the basic dimensions (cord, span, thickness, etc.)? How about installing some small 90 degree fences? I'd place them parallel to the LWL, one on each side of the blade just above the tip "rounding". Some 1/16" x 3/4" angle aluminum stock (let into the face), screwed to the blade should do. It doesn't make a huge impact on the project and it can be easily remedied if it's the wrong direction.

I would suspect, given the changes and general shape (that I can see in the photos), slow speed and moderate wind strength preformance should be improved. In the same vain, high speed and in higher wind strengths, you could have stalling issues. This would be common of a fairly low aspect (2.5:1?), thick and slab sided foil section. Higher speeds like thinner foils and better shapes of course, but it depends on your sailing style and the usual wind strengths you sail in.

Way to go Chuck and thanks for the update.