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Old 23-03-2007, 17:34   #1
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Rudders, outboard or inboard

Given the flimsyness, expense and complexity of self steering and inside steering arrangements needed on boats with inboard rudders, as compared to the extremely simple arrangement of a trimtab securely attached to the trailing edge of an outboard rudder, I can't see the logic of offshore cruising boats having inboard rudders. A friend , watching a guy build a boat with an inboard rudder , calculated the cost of going for an inboard rudder with the required hydraulic steering, huge autopilot, power to run it and self steering arangement , at an extra $10,000.
I have used a tiny autopilot powering my trimtab on my rudder since 1992 with tiny power drain, for several Pacific crossings with no problems. My trimtab powered windvane will steer me downwind in anything over two knots of wind .I've been told that servo pendulum vanes need at least 8 knots of wind. Everywhere I go I meet people with broken servo units , even on boats with outboard rudders.
So where's the logic in putting the rudder inboard? Seems kinda stupid to me.
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Old 23-03-2007, 18:13   #2
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You mean something like this guy has?

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Old 23-03-2007, 18:22   #3
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Quote:
I can't see the logic of offshore cruising boats having inboard rudders.
I'm not sure your lack of vision is enough to change the world. It does not mean you are wrong. There are many routes that work and in the history of global sailing far less effective strategies than even yours do work. What is possible is not always desirable. What you have been told may not be the truth.

In situations where you seem satisfied with your own solution why is something else "stupid". I can accept that you have a solution that works for you but why should anyone else have to defend another solution just because you don't see the merit?

Please explain how your system works. At this point I find that a more interesting discussion than why something you don't understand what so ever is somehow worse from your point of view.
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Old 23-03-2007, 19:34   #4
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I carefully installed a Monitor windvane on our previous boat (a 35' mono) which would keep the boat on course, clear down to 2 knots of wind speed. One time we were going dead downwind with the cruising spinnaker and the boat was only making about a half a knot in glassy conditions, but the Monitor worked to keep the boat on course. Even my wife didn't believe me until I demonstrated it to her. The paddle reacted to the course change and the oar turned. Even though the oar didn't move the steering wheel at that slow speed, just the turning of the oar was enough to return the boat to proper course. The trick is properly balancing your sailplan and minimizing friction everywhere in ALL the steering gear as well as the windvane control lines.

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Old 23-03-2007, 22:47   #5
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I have seen what Louis mentioned a few times. It usually is a small tillerpilot type autopilot working the trim tab either directly or by some type of linkage. Seems to make alot of sense to me knowing how little force is required to move the trim tab.

A differant question I have about the trim tab is what is the effect of going in reverse. Is it important to be able to lock the trim tab so that it is not forced hard over one way or the other or is the effect negligable.
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Old 24-03-2007, 00:33   #6
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Inboard Rudders

They are part of the balance of the sail plan. If one were to remove an inboard rudder and install a tramsom hung rudder the COE (center of effort) would not be on center any longer.

With the self steering the inboard rudder acts as an extention of the keel. The self steering only has to move slightly to adjust the boat in the direction it's already going. By all rights the sails can be adjusted so the boat can sail without even moving the rudder, it's the turning and tacking that requires the major effort. Try tacking with just the trimtab.

Also an outboard rudder, if mounted on the transom adds to the lenght of the boat without adding to the LWL. Full keel boats don't have a choice. But, they are fun in the marina's

AND like mentioned earlier, going backwards maybe a challange. A spade (inboard) rudder, if balanced properly, will not exert so much force in the pivot points going backwards. If you were to slide backwards down a steep swell with an outboard rudder it might prove to change your mind.
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Old 24-03-2007, 02:56   #7
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While the outboard rudder is simpler & cheaper in construction, it’s inherently less efficient than the inboard rudder.

KEEL & RUDDER DESIGN ~ by David Vacanti
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Old 24-03-2007, 07:49   #8
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Quote:
Originally Posted by GordMay
While the outboard rudder is simpler & cheaper in construction, it’s inherently less efficient than the inboard rudder.

KEEL & RUDDER DESIGN ~ by David Vacanti
Professional BoatBuilder - June/July 2005
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The article isn't exactly clear or convincing on the whole efficiency aspect.

“For the same rudder shape and area, an inboard rudder is almost (my emphasis) always more efficient than an outboard rudder”

However given that most high tech ( ie. expensive) racing sailboats built these days have inboard rudders, I'll give a slight advantage to inboard rudders, very slight.

However he does go on to mention the simplicity of one vs the complexity and cost of the other. It would seem to me that a cruiser's first considerations should be simplicity and safety, not some “almost always more efficient” argument that really means nothing to a fully load cruising boat.
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Old 24-03-2007, 17:18   #9
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Not convinced...

I am not convinced that an external rudder is less efficient.

Almost all small sail boats have external rudders and many are dramatically fast.

I would expect that the external rudders would have a lower wetted surface (of rudder and hull sitting above) and would be more effective (greater lever arm) with a much lighter actuating and mounting mechanism (tiller with pintle and gudgeon).

The real reason for using internal rudders is that the rudder is the most important device on a boat and the external location is very vulnerable to damage to both the rudder and the actuating mechanism.

The inboard rudder also feeds loads into a three dimensional structure as oposed to a two dimensional structure with an outboard rudder.
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Old 30-03-2007, 17:01   #10
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I began cruising with an inboard rudder . I sailed a tank tested boat to New Zealand singlehanded, broaching continuously all the way.There I moved the rudder six feet further aft ,without changing the balance in any noticeable way. It became far more efficient.
Spending an extra ten thousand dollars on something which is far more complex and fragile, with no real benefits is stupid , period. An ouboard rudder is far stronger than any inboard rudder. You can make the pins as thick as you want with no disadvantage, and the rudder head can easily be made much stronger than any shaft you could fit inboard . Exposed ? To what? Is anyone suggesting that a piece of steel square tubing making up the rudder head ,6 inches by 2 inches can be dammaged by a dollop of water 6 inches by 3 ft? How much pressure does a 6 inch by 3 ft dollop of water exert? Not enough to dammage steel. The position of the rudder blade makes it no more exposed that one under the counter , unless one believes that what you can't see the sea can't see either.
Is a monitor hung off the stern less vulnerable than a steel rudder head of an outboard rudder? Hard to imagine anything flimsier than a monitor when it comes to impact resistance.
Yes I do lock my trimtab when docking. The locking mechanism lets me fine tune it for absolutly no helm.The further aft the rudder is the more efficient it, is especially when reversing. Further aft means more leverage on the hull. I believe tests which show any increased efficiency of inboard rudders may be tersting the rudder in isolation without considering the extra leverage of their being further aft.
I can easily unship my rudder for a paint job while the boat remains in the water.
The area of the trimtab going the opposite direction does reduce the effectiveness of the rudder by the equivalent of reducing the rudder area by the area of the trimtab, but that's a small price to pay for the simplicity and super reliability of trimtab steering.The reduction is negligible.
I've slid backwards down many a steep swell in 9 pacific crossings over 35 years . No problem .It only pushes the rudder post against the transom ,harmlessly.Only the bottom pin takes the load ,and that's overbuilt.
When I read in "Log of the Mahina", about the skipper having to tie a reef in the main ,almost permanently I lookd at the transom rake , perfect for an outboard rudder on a skeg and thought how much simpler that would have made the average Vega.
When I read David Lewis's book "Icebird " I couldn't help but notice how much easier his voayge would have been if he had got rid of that goofy keel attached rudder and put a skeg hung rudder over the stern, with inside steering hooked up to a trimtab.
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Old 30-03-2007, 21:38   #11
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Steel boats OK, but the rest...

To me, an external rudder is good on a steel boat where the mounting is pretty much the same strength as the rest of the boat.

On a wood or fibreglass (or even ferro ) boat the mountings are pointloads on what is the weakest part of the hull (the transom).

A double ender with large mounting straps would not be as affected.
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Old 31-03-2007, 10:00   #12
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I have to agree with you, "Keep It Simple Sailor" whenever possible, I have a transom hung rudder on my 32' double ender and will be building a trim tab this year. I too read "Ice bird" I would think that possible damage from bergi bits, may have been a real hazard in that situation. For those of us that stay out of the ice zone, the only other hazard I could see is another boat bumping or crashing into your stern, likely in a marina.
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Old 31-03-2007, 17:56   #13
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Rudders

The transom on a wooden or fibreglass boat offers the greatest opportunity to spread the load out with big backup plates. Thereabsolutely is no reason it should be the weakest. It certainly is no weaker than the point where a rudder shaft goes ,thru the hull. A freind said outboard rudders on double neders seem to have a higher failure rate , whereas they are almost zero failure rate on transom boats. I suggested that this may be ebcause they tend to leave the top pintle out , to enable the rudder to follow the curve of the sternpost( style over substance ) leaving the top three feet of the rudder post unsupported. The load on a transom mounted rudder is minimal as long as the boat is going foreward , Which it will be doing when encountering bergy bits. It only gets loaded when backing down , which simply pushes the rudder post against the transoom, not a problem.
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Old 31-03-2007, 21:19   #14
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Quote:
Originally Posted by Louis Riel
I began cruising with an inboard rudder . I sailed a tank tested boat to New Zealand singlehanded, broaching continuously all the way.There I moved the rudder six feet further aft ,without changing the balance in any noticeable way. It became far more efficient.
Perhaps if you had concentrated on moving your centre of effort forward by trimming your sails or changing your rig plan you would have achieved the same result. Outboard rudders seem OK for tiller steer but if wheel steering is the choice then I can see no compelling reason for them. Why does virtually every craft on the water carry an inboard mounted rudder?? Far be it from me to question the wisdom of the vast majority of ship designers.
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Old 31-03-2007, 22:11   #15
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Quote:
Originally Posted by Louis Riel
Spending an extra ten thousand dollars on something which is far more complex and fragile, with no real benefits is stupid , period.
And yet, inboard rudders are extremely common on boats as small as a daysailer and as large as a supertanker. If the design is a stupid as you think, people would not use it.

The only reasonable conclusion is that there are benefits that you are not aware of. A few ideas have been raised here, but there may be others. You might want to look for a book on "naval architecture" or "boat design". There would have to be a discussion of the advantages of each design.
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