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Old 14-02-2015, 00:23   #1
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Twin rudder windvane?

Hello cruisers,

Next on my boat finish list is the windvane steering system. Following a very helpful thread a while back, I have come to the conclusion that a trim tab auxiliary rudder windvane will be the best compromise for our boat, given a few limitations around stern clutter, boat behaviour and performance, hull shape etc.

Progress has been good and early prototypes have been promising, leading me to about an 20% to 80% area distribution for a very balanced rudder that works well at low speed with a naca0015 profile.

So yesterday I was measuring for the mounting system which will be a pair of pillow block bearing flush mounted to the stern and I had a thought. Being a canoe stern hull I will be offsetting the mounting to the port side, not by much, but around ten inches from the centreline. I had chosen the starboard side ( Viking style) because of a permanent boarding ladder I had planned for the port side. But now I have finished and fitted the ladder I discovered I have enough space on the port side even with the boarding ladder. So my thought ( probably inspired by the Volvo 60s) was, "what about twin auxiliary rudders?"

Ok, lots of extra labour, but aside from that, what are the advantages or disadvantages of twin rudders that might apply to this situation? Redundancy would be nice, increased directional stability, smaller or shorter rudders might be possible?

Thoughts?

Matt


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Old 14-02-2015, 05:46   #2
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Re: Twin rudder windvane?

Twin rudders tend to catch weed, scultch and ice, much more than a centerline one, but an interesting Idea. I take it you have seen Bill belchers and John letchers books on self steering?

My gut feeling is that overall it wouldn't be quite s efficient as a single bigger centerline rudder due to end tip losses, ventilation and interference. Presumably static friction would also be higher? But the redundancy is nice.

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Old 14-02-2015, 12:11   #3
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Re: Twin rudder windvane?

Hi Ben,

Did not know of either of those books. The Bill Belcher sounds very good, but selling for nearly $200 online. Ouch. Hopefully I have enough data now without it, but if I run into trouble I will consider it for remedial action. The John Letcher book is harder to get a grasp of, as in what the content includes. Sounded more like a what's available rather than how you can build it from the summary?

I have slept on this twin rudder idea, and it still feels interesting, and I don't really need to change anything at this stage, just keep building as I am, and if it works well, and I have the energy, build another.

As you note, centreline would be better, but it just is not possible on our boat without messing up a lot of stuff that currently works well. I do hope that the small offset will not prove problematic, only time will tell, and if it does, then I will probably be forced to the twin rudder system. At least the fabrication costs are not prohibitive, it's just the time that adds up.

Matt
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Old 14-02-2015, 13:15   #4
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Re: Twin rudder windvane?

If you are serious about this do some research on twin rudder boats. They are typically designed to only use the leeward rudder with the windward either lifting out of the way, or the heel of the boat lifting it out of the water.

Twin rudders in the water is a lot of extra drag that doesn't help stearing underway very much. Since the windward rudder lifts so much it tends to ventilate.
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Old 14-02-2015, 14:55   #5
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Re: Twin rudder windvane?

G'Day Matt,

I agree with Greg's post above. I doubt if one can extrapolate between dual rudders as on modern very beamy, shallow stern racers and your rather different vessel. For sure give it a try with the original single rudder design before going to the additional effort of constructing the dual setup. IMO if the single is large enough it is likely to work better than a closely spaced dual... but I've not tried it myself!

Jim

PS I have a copy of LEtcher's book and have read Belcher's. The former is somewhat more theoretical, the latter more how to build specific designs. I could likely send you the Letcher for a read if you feel so inclined.

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Old 14-02-2015, 16:41   #6
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Re: Twin rudder windvane?

Ha, Jim I've got belchers, but not letchers. Sv Sarana has what looks like some good info about trim tab design and construction under resources on their site.

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Old 14-02-2015, 17:51   #7
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Re: Twin rudder windvane?

Hmmm, picking a distinctly negative gut feel here. That's ok, thanks for the honest feedback, that's why I posted the idea for comments.

Seems for now the smart thing is to plug on with the original design and revisit the dual rudder idea later if I find problems with the offset. Maybe the leeward rudder in the water, or something like that. Or maybe stop going off on tangents...

Anyway, started the today on the boat pretty poorly. Swapping the headsail I lost the furler halyard, now I have to climb the mast to retrieve it. About to go trawl through the threads on climbing masts. Sigh.

Matt



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Old 14-02-2015, 17:54   #8
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Re: Twin rudder windvane?

Quote:
Originally Posted by Jim Cate View Post

PS I have a copy of LEtcher's book and have read Belcher's. The former is somewhat more theoretical, the latter more how to build specific designs. I could likely send you the Letcher for a read if you feel so inclined.

Jim


Thank you for the offer Jim, better still, when I am moored in the same sheltered bay as I2 I'll row over with a bottle or two of the good South Ozzie wines that we don't let the East Coast folk know about and trade them for a flip through the pages.

Off to climb the mast now... Gawd I am an idiot.



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Old 14-02-2015, 18:02   #9
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Re: Twin rudder windvane?

Quote:
Originally Posted by Snowpetrel View Post
Ha, Jim I've got belchers, but not letchers. Sv Sarana has what looks like some good info about trim tab design and construction under resources on their site.

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Ben, thanks for the heads up on that one. And at the price I'd be an idiot not to buy the second part of the document... Oh, hang on, I just lost the halyard up the mast... I AM an idiot. Still, I'll not compound the issue. I'll report back on the quality of the document.

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Old 14-02-2015, 18:44   #10
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Re: Twin rudder windvane?

Ah well, Matt...

If you have never lost a halyard up a mast, you likely haven't sailed much. Bet it won't happen again (soon anyway).

Jim
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Old 15-02-2015, 00:50   #11
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Re: Twin rudder windvane?

Matt,

One time we ran aground on a bommie in Hawaii. Just happened it was the highest tide of the year, but we didn't know that, yet. And I (sigh) failed to secure the snap shackle for the main halyard that Jim wanted to use to raise the main so as to heel the boat. That boat had only one main halyard. A long time, and an adventure later, we retrieved it. I had not done well.

However, FWIW, do get the wind vane as close as possible to amidships. I am sure it will be much easier to use then. IMO, the other huge deal about windvanes is to reduce friction wherever possible.

Ann
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