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Old 13-02-2016, 07:15   #361
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Re: Oceanis 485 steering failure

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Originally Posted by Dockhead View Post
Hard to say -- without some background on what is shown here, and what happened to it, you can't really tell anything.

If (I'm guessing?) that's a Dashew boat, whose rudder someone (I'm guessing?) put hard onto some rocks, then I'd say that looks pretty good. Broke off half way down, and if (I'm guessing again) the shaft wasn't bent, water integrity was not compromised, and the boat could still be steered, I would say that's just about an optimum result . But again -- just guessing at what is being depicted.


You can't make a rudder which is totally impervious to being broken if run onto rocks. Or at least, it would be very difficult, expensive, and with much compromise to other values. But you can make a rudder which will survive being run onto rocks without (a) compromising watertight integrity of the vessel; and (b) without completely losing the ability to steer. I will insist on it, on my next boat.
My boat... pre me... Grounded hard...

Rebuilding the rudder, I left the bottom section sacrificial as it was... Sturdy as hell... 3 layers of 1708 to make the foot... 24 lb foam to interface... 4 layers 1708 to wrap the whole thing... plus extra on leading and trailing edges...

Shaft and watertight integrity remained as you expected.... If the blade ever pounds I expect the same sacrificing... Well HOPE anyway!

Wait??? I hope not to find out???
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Old 13-02-2016, 07:29   #362
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Re: Oceanis 485 steering failure

This has been really informative. Next time I am near my rudder assembly i will take pictures and post them here for comparison as I think it's pretty well built and designed. 3.5 inch diameter stainless rudderstock, lower roller bearing inserted from below in hugely reinforced recess in hull, upper roller bearing bolted to underside of cockpit floor via heavy metal structure.
I don't know how far my rudder post goes down the rudder blade. If I were to build a new spade rudder, I would essentially specify a 2/3 length rudder as a complete unit, glassed across bottom. Then we could attach the last 1/3 as a breakaway in a hard grounding that could fall off and one could still steer boat to get repairs. Or if the bottom of rudder is simply damaged and breached in a moderate grounding, the water ingress would be limited to bottom 1/3.
Is this what Dashew does?


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Old 13-02-2016, 07:38   #363
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Re: Oceanis 485 steering failure

Quote:
Originally Posted by HappyMdRSailor View Post
My boat... pre me... Grounded hard...

Rebuilding the rudder, I left the bottom section sacrificial as it was... Sturdy as hell... 3 layers of 1708 to make the foot... 24 lb foam to interface... 4 layers 1708 to wrap the whole thing... plus extra on leading and trailing edges...

Shaft and watertight integrity remained as you expected.... If the blade ever pounds I expect the same sacrificing... Well HOPE anyway!

Wait??? I hope not to find out???
Good rudder design! Glad to see that it's not an unknown art.
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Old 13-02-2016, 08:10   #364
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Re: Oceanis 485 steering failure

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Originally Posted by Dockhead View Post
....

Another point which is relevant to the original discussion, and which I'm surprised hasn't been mentioned, is that there is great tension between the value of strength and safety in the rudder design, and hydrodynamic performance.

For hydrodynamic performance, there is nothing like a deep, high aspect spade rudder, not too thick. Obviously that is hardest rudder to make strong....
But concerning the basic configuration of the rudder -- choice of a thin, deep, high aspect rudder -- for the kind of sailing done by 99% of Bene owners, this is the right choice. It makes a huge difference in how well the boat sails, especially in light weather fun day sailing and light coastal cruising situations.
...
Hydrodynamics are fundamental to how a boat sails and performs. If we didn't care about them, then we would all be sailing full keel crab crushers with barn door rudders. I think maybe too little attention has been paid to that in relation to the Beneteau design.
A deep rudder is also a safety feature while sailing. On my previous boat and other similar mass production main market boats I was used to relatively deep rudders and to the control they can give over the boat and also to know about their limits in what regards controlling the boat on really strong gusts sailing fast.

That means that in almost all boats that I have sailed for a considerable period of time, I was used to lose momentarily control over the rudder on those situations (strong winds with strong gusts) not frequently but on muscular conditions going fast with the boat rounding up to the wind, once or twice during a passage. Yes you can prevent that going slower with the boat reefed but I like to sail fast.

My actual boat has a really deep rudder, almost as deep as the 2.25m keel and the control the rudder allows really surprised me. On the conditions where I was used to broach, now the rudder just becomes harder and I have time to let go the sail or the traveler taking the strain off the rudder.

Sometimes I just let go the rudder a bit, not because I could not maintain course but because I find it is too much force on the rudder (I have a huge wheel).

Yes, I believe my rudder is strong and well built (I have already took it out) but I believe that a two rudder set up is in what regards efforts a much better solution.

Giving the same control as a deep rudder the forces are not so multiplied by a much bigger arm and the efforts are smaller. Also advantages with the redundancy and with a better protection when going backwards med mooring. A bit worse in what regards being protected by the keel and on that particular a single one is better protected.
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Old 13-02-2016, 08:32   #365
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Re: Oceanis 485 steering failure

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Originally Posted by Polux View Post
...I believe that a two rudder set up is in what regards efforts a much better solution...
The big problem with dual rudders is that you are many times more likely to develop a steering problem (or much worse, a flooding problem), much more than twice as likely, due to the rudders being completely exposed, not protected behind the keel, as is a single rudder.
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Old 13-02-2016, 08:46   #366
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Re: Oceanis 485 steering failure

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The big problem with dual rudders is that you are many times more likely to develop a steering problem (or much worse, a flooding problem), much more than twice as likely, due to the rudders being completely exposed, not protected behind the keel, as is a single rudder.
More exposed yes, flooding problem I don't agree. Since the arm is much smaller the efforts transmitted by the rudder to the rudder post, hull and all structure are much smaller, even on the case of breakage of rudder while hitting a solid object. Much more probabilities of having a clean breakage without damaging the hull or the rudder tube.

In fact I cannot recall a single case where a breakage on a twin rudder had resulted in a boat having sunk but I can remember many case of boats with twin rudders having one broke and keep sailing to port slowly with only one rudder.
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Old 13-02-2016, 09:18   #367
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Re: Oceanis 485 steering failure

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Originally Posted by Polux View Post
More exposed yes, flooding problem I don't agree. Since the arm is much smaller the efforts transmitted by the rudder to the rudder post, hull and all structure are much smaller, even on the case of breakage of rudder while hitting a solid object. Much more probabilities of having a clean breakage without damaging the hull or the rudder tube.

In fact I cannot recall a single case where a breakage on a twin rudder had resulted in a boat having sunk but I can remember many case of boats with twin rudders having one broke and keep sailing to port slowly with only one rudder.
does the alpha 42 with twin rudders work for you?
Its nicely sunken now.
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Old 13-02-2016, 09:37   #368
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Re: Oceanis 485 steering failure

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Originally Posted by sailnow2011 View Post
does the alpha 42 with twin rudders work for you?
Its nicely sunken now.
The Alpha 42 suffered one bent rudder and one rudder blade spinning around its post (just from breezing through the article). I believe the hull and rudder tubes were OK but the boat was going in circles.

Here is the article: http://http://www.wavetrain.net/news...ng-be-good-too

Builder's response: http://http://www.wavetrain.net/news...ilder-responds
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Old 13-02-2016, 09:47   #369
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Re: Oceanis 485 steering failure

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Originally Posted by boom23 View Post
The Alpha 42 suffered one bent rudder and one rudder blade spinning around its post (just from breezing through the article). I believe the hull and rudder tubes were OK but the boat was going in circles.

Here is the article: http://http://www.wavetrain.net/news...ng-be-good-too

Builder's response: http://http://www.wavetrain.net/news...ilder-responds
Indeed. And that's a catamaran too. Completely irrelevant to the question of whether twin rudders might be a better compromise in terms of control versus strength and safety, which is what Polux was talking about.

Polux's argument on that point sounds pretty good to me. However, twin rudders don't get prop wash so have a big disadvantage for harbor maneuvers in astern. I guess nothing a retractable stern thruster wouldn't solve, in a large cruiser.
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Old 13-02-2016, 09:54   #370
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Re: Oceanis 485 steering failure

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Originally Posted by Polux View Post
More exposed yes, flooding problem I don't agree. Since the arm is much smaller the efforts transmitted by the rudder to the rudder post, hull and all structure are much smaller, even on the case of breakage of rudder while hitting a solid object. Much more probabilities of having a clean breakage without damaging the hull or the rudder tube.

In fact I cannot recall a single case where a breakage on a twin rudder had resulted in a boat having sunk but I can remember many case of boats with twin rudders having one broke and keep sailing to port slowly with only one rudder.
Actually you could design a twin rudder to break off completely, rather than just half way as with a single rudder. Since you have another rudder to keep steering with.

And a twin rudder can be much shorter, without giving up aspect ratio. Another big plus.

I don't know about the boundary layer effects though -- does that translate from aerodynamics to hydrodynamics? Does the rudder need to get out beyond the boundary layer to work well?

And you can design them with camber so that the leeward rudder is more vertical when heeling, which will make it more efficient.

I guess twin rudders have a lot of advantages.


However, having gone from a boat which would not go straight in astern (long fin keel) to one which is well controllable in astern (bulb keel), I would never, ever want to go back.
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Old 13-02-2016, 11:11   #371
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Re: Oceanis 485 steering failure

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does the alpha 42 with twin rudders work for you?
Its nicely sunken now.
That is a cat.

I was talking about monohulls that is what is being discussed on this thread.

Besides the accident with that cat had not to do with better or worse protection regarding the keel and neither about rudders being broke after hitting something since the rudders of that cat did only hit....water.
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Old 13-02-2016, 11:23   #372
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Re: Oceanis 485 steering failure

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Originally Posted by Dockhead View Post
...
However, twin rudders don't get prop wash so have a big disadvantage for harbor maneuvers in astern. I guess nothing a retractable stern thruster wouldn't solve, in a large cruiser.
Yes, that is a disadvantage and I would not say the bigger one is not the absence of the prop wash since some boats with saildrive (most of them today) have the rudder so far away from the propeller that the lateral opposite movement induced by the propeller, when it turns, is more useful than prop wash on docking maneuvers.

For me the biggest disadvantage in maneuvering is the considerable bigger turning circle of those rudders compared with a single rudder.

Anyway weighting advantages and disadvantages I would change my deep single ruder for an equally effective twin rudder in a heartbeat. Not going much to marinas anyway.
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Old 13-02-2016, 11:28   #373
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Re: Oceanis 485 steering failure

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Actually you could design a twin rudder to break off completely, rather than just half way as with a single rudder. Since you have another rudder to keep steering with.

And a twin rudder can be much shorter, without giving up aspect ratio. Another big plus.

I don't know about the boundary layer effects though -- does that translate from aerodynamics to hydrodynamics? Does the rudder need to get out beyond the boundary layer to work well?

And you can design them with camber so that the leeward rudder is more vertical when heeling, which will make it more efficient.

I guess twin rudders have a lot of advantages.

However, having gone from a boat which would not go straight in astern (long fin keel) to one which is well controllable in astern (bulb keel), I would never, ever want to go back.

No personal experience but did see twin cambered rudders on a boat once.

If one rudder breaks surely the remaining one would work normally on one tack but hardly at all on the other?

What am I missing?
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Old 13-02-2016, 11:35   #374
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Re: Oceanis 485 steering failure

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Originally Posted by Dockhead View Post
Actually you could design a twin rudder to break off completely, rather than just half way as with a single rudder. Since you have another rudder to keep steering with.

And a twin rudder can be much shorter, without giving up aspect ratio. Another big plus.

I don't know about the boundary layer effects though -- does that translate from aerodynamics to hydrodynamics? Does the rudder need to get out beyond the boundary layer to work well?

And you can design them with camber so that the leeward rudder is more vertical when heeling, which will make it more efficient.

I guess twin rudders have a lot of advantages.


However, having gone from a boat which would not go straight in astern (long fin keel) to one which is well controllable in astern (bulb keel), I would never, ever want to go back.
Another advantage on modern beamy hulls is that one of the rudders is on the center of the longitudinal wet water surface of the boat.

Those boats have a very asymmetrical water plan and a single rudder, except when sailing downwind or motoring, will always be out of the center of that longitudinal water surface and that will diminish greatly its efficiency and increase its drag.

Also that asymmetrical water surface increases the protection given to the keel to the more immersed rudder since it is like if the boat is sailing slightly sideways.
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Old 13-02-2016, 11:39   #375
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Re: Oceanis 485 steering failure

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No personal experience but did see twin cambered rudders on a boat once.

If one rudder breaks surely the remaining one would work normally on one tack but hardly at all on the other?

What am I missing?
In that (emergency?) situation, I would think that the priority would be to keep the boat level and maneuverable instead of going fast. I other words, I would adjust the sails or change sailing direction to minimize healing so that I can maneuver the boat with one rudder.
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