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Old 22-05-2019, 01:17   #1
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Boat: Moody 45 DS
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Monohull sailboat twin rudder alignment?

Before launching this spring I noticed my rudders were not parallel. There was significant “toe out”, duck footed stance if you will. I researched this topic and could not find any clear answers. High speed powerboats with twin rudders are recommended to have some toe in or toe out apparently to reduce vibration. What’s recommended for a cruising sailboat?

I assume the answer is that the cruising sailboat rudders should be parallel to the midline of the boat or aligned to minimize drag. I decided to adjust the linkage between the rudders while underway. On a flat calm day my wife steered a straight course at six knots from the starboard helm while I took my tools down into the port lazarette. I disconnected the linkage between the rudders and let the port rudder trail freely while I adjusted the linkage to this point of least resistance. In the process we earned a couple of tenths of boat speed and the helm feels normal.

So did I do the right thing, or do cruising monohulls with twin rudders need some toe in or toe out?
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Old 22-05-2019, 06:47   #2
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Re: Monohull sailboat twin rudder alignment?

You did the correct thing.

That said, the rudder toe-in or toe-out angle will vary boat to boat, and what is correct for one hull shape won't necessarily be correct for another.

For example, in the twin engine sterndrive world where the vessel may be running at 30+ knots, the basic setting for twin sterndrives will have the trailing end of the drive perhaps 3/4" closer together than the leading/front end of the drive. This "toe out" varies, mostly with the dead rise angle of the hull because the water isn't necessarily flowing straight aft off of the hull surface.

In the twin inboard engine powerboat world the same thing applies...water flow doesn't necessarily follow the straight fore and aft lines of the hull. It's dependent on dead rise angle, lifting strakes, position of the rudder relative to the propshaft centerline, prop pockets, etc. Running the prototype hull with the tiebar disconnected is more or less standard procedure to find the best rudder position for a particular model, and then that position can be set on the production line for follow on hulls.

So, the remaining question is: What did you find out for your hull? Are the rudders straight fore and aft, or is there some toe-in or toe-out?

DougR
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Old 23-05-2019, 02:21   #3
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Re: Monohull sailboat twin rudder alignment?

DougR,
Thanks for the reply!
I suspect the misalignment occurred last year when I had my rudder bearings replaced, one side in Portugal and the other side in Greece. The boat sailed well, but I suspect the toe out caused significant drag. Now I hope to be sailing faster and using less diesel under power.
The adjustment of the linkage between rudders was simple to do. I suspect there are many twin rudder monohulls than could benefit from a similar tune up. I encourage other owners to get intimate with their rudders.
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