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post #1 of 7 Old 01-06-2007 Thread Starter
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Steering weak point.

When reading about experiences at sea, it seems most of the time in a rescue situation, the steering (among other things) is knocked out after a storm. Now, I understand that there are massive forces involved, but is there a common weakness? What's the most common failure when steering is disabled??
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post #2 of 7 Old 01-06-2007
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Such failure is usually from the boat being forced in reverse at speed .... a rudder and its steering structure simply can't handle such forces when going violently astern ... its a mathematical/structural problem.

Similarly, Airplanes dont 'hang-together' too well going backwards and if forced to do so ... usually 'break'.

To build for such anomalies, the structure would be 'massive' that would require a LOT of additional weight.
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post #3 of 7 Old 01-06-2007
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Giving the fact that forces against a sailboat's steering gear will be higher in reverse, I presume that due to a lack of bottom support, there is a greater probability of a spade rudder failing before a skeg hung rudder.

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post #4 of 7 Old 01-06-2007
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Trigonometry ....

In reverse the forces generated by water flowing across a rudder will generate MORE forces on a pintle hung or skeg/keel hung rudder than on a spade. Spade rudders are usually more hydrodynamically 'balanced', although you can apply many more 'hinges' to a skeg/keel, etc. rudder. Still its the 'trigonometry' of the forces applied to a rudder working in reverse that breaks the system.

If a rudder is 'locked down' when going at speed in reverse its probably safer, as its probably the 'impact' of the rudder hitting the stops that breaks the stock, the internal flanges of the rudder, the quadrant, etc.
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post #5 of 7 Old 01-06-2007
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TB...yes but also because there is no skeg to protect the rudder from impact or catching on something and getting jerked. Rudders are usually just foam covered by fiberglass with a metal grid from the rudder stock and are much more prone to damage than say a keel from impacts so many cruisers prefer a skeg to a spade even though the spade is the "speedier" of the two.

On a little different note...another common failure in steering is the wheel cables and pulleys and slack to the quadrant. On a prior boat (with a skeg rudder) we had a bump grounding backing out of the slip and the rudder was thrown over to its stoppers. Everything looked OK externally...but the eye bolt attaching the steering cables to the quadrant was bent slightly from the force and a day later, we lost steerage as the cable jumped off the quarant. We learned 2 things that day:
1. Check and lubricate tension and connections on steering cables annually and after any "events"
2. An underdeck autopilot will steer your boat if the wheel system fails. We ran around looking to get at the emergency tiller when I could have simply pushed a button quickly! (That's called seamanship...lessons from experience!!)
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post #6 of 7 Old 01-06-2007
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Rich,
I don't proclaim to be an expert on the dynamics of rudder design. But from a layman's perspective, it seems that regardless of rudder size or shape, the bottom pintle of a skeg-hung rudder MUST have a greater resistance against diametrically opposed water pressures, over a spade.

Spade rudders need to be more hydrodynamically balanced than skeg hung rudders since the only physical connection to the boat is the rudder post and is still the weakest link, thus having a higher probability of breakage.

Edit - Note to self : have morning coffee before posting.

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Last edited by TrueBlue; 01-06-2007 at 10:11 AM.
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post #7 of 7 Old 01-07-2007
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Trueblue,
I believe that Rich's point was that the spade rudder is constructed in one of two ways; semi-balanced or balanced. In either of those constructions the rudder post is not at the leading edge of the rudder as it is in most skeg mounted rudders. The objective in balancing the rudder is to get relatively equal masses forward and aft of the rudder post. This eases the strain on all components of the steering system immensely. A rudder constructed in this manner turns almost as easily astern as ahead. The skeg hung rudder, when going astern, acts just like the door that got caught by the wind while open.

Pintles and gudgeons, regardless of mounting location are weaker than the rudder stock itself. With a balanced, or semi-balanced rudder the strain is transmitted via the rudder stock upwards to the bearing. The bearing and quadrant are mounted in such a secure way that no gudg/pint can be. As previously mentioned, the damage to the skeg hung rudder probably occurs more from being slammed over against the stops than the actual force of the water.

A balanced, but more commonly a semi-balanced rudder can be mounted to a skeg or the keel but it does take a little engineering.
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