rudder post sizing of spade rudder

Discussion in 'Boat Design' started by melamphyrum, Feb 7, 2014.

  1. melamphyrum
    Joined: Jan 2014
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    melamphyrum Junior Member

    I have recently bought a 1975 custom IOR 3/4 Tonner sailboat. The boat was designed by Ben Lexcen (aka Bob Miller) and built by Custom Yacth Services, Australia using c-flex and vinylester.
    All of my previous boats had long keels with attached rudders. My new boat has a fin keel and an unsupported (no skeg) balanced spade rudder. I am fitting the boat out for offshore cruising, but the strength of the rudder post concerns me. It seems like a strong arrangement, but the forces in a real seaway can be enormous on the rudder. The rudder post is 1 3/4" solid stainless steel. It is well supported inside the boat by a long rudder tube which is laminated to a substantial bulkhead, so no worries here. In a real life situation the rudder post would fail where it enters the hull.
    I quickly did a finite element simulation of the deformation of the rudder post and to see how much force it can survive. The model is a very simple one. Please look at the first picture attached. The rudder post inside the boat is fixed (blues area). A normal force of 5000 N is acting on the rudder blade. This force roughly corresponds to the drag of a 0.5 sqm plate moving across the water with 10 knots (a quite realistic situation when a boat is pushed sideways by a breaking wave). The force on the rudder can be much higher if it hith something.
    The result is shown on the second picture. The safety factor (ratio of failure stress to equivalent stress) is plotted. Anything below 1 (red area) fails.
    The result makes me wonder if it is a good isea to go offshore in this boat...
    I would like to hear your opinion. Is this rudder post undersized?
    Is there a rule of thumb to size rudder posts?
    Is this kind of failure common?
    Thank you in advance.

    Attached Files:

  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi Den,

    Welcome to the forum. There are engineering standards in place these days that cover rudder construction, and the one most applicable to you would be the ISO-12215-8 standard, which you can purchase here:

    IOR boats were designed primarily for racing around the buoys and limited offshore areas. They were notoriously unsafe offshore because of stability as well as rudder issues. That is a very general statement, and individual boats may contravene that. However, you have focused in on a potential weak point and are right to question it.

    Your boat was built long before this ISO rudder standard came into place, which was only in the last year or so. Therefore, do not be surprised if the rudder stock diameter and strength does not meet the standard. Chances are, if you want the rudder to meet the standard, the rudder stock would have to be made bigger in diameter, and that means not only redesigning and rebuilding the rudder, but also removing the existing rudder stock tube and bearings and replacing them with new equipment.

    I also recommend that you read C.A. (Tony) Marchaj's (pronounced MAR-ki) book, "Seaworthiness, The Forgotten Factor" which describes the issues with IOR boats that were brought to light by the 1979 Fastnet Race disaster. A large number of IOR type boats capsized while others broke their rudders.

    I hope that helps.

  3. dougfrolich
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    dougfrolich Senior Member

  4. melamphyrum
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    melamphyrum Junior Member

    Thank you very much for your informative replies. I especially enjoyed the papers on Eric's webpage about keel and rudder design and engineering. I learnt a lot from these clearly written, practical papers.

    The ABS Guide is very useful to get a quick estimate for the minimum sizing of scantlings. For rudder stock sizing it apparently uses the maximum lift force acting on the rudder at hull speed. In case of my rudder, this force is 6270 N. This force is higher than what I used in my original simulations. For my rudder the formulae in the Guide suggest a solid stainles steel (grade 316) rudder post with a 2” diameter. Therefore, even the recommended minimum is higher than the rudder stock in my boat (which is 1 ¾”). Not good. At this point I am not ready to make major modifications to the boat, but at least these calculations show that I will have to make absolutely sure to carry some kind of emergency rudder.
    I re-run my simulation with a 2” stock and 6270 N. The stress still exceeds the yield strength by 10% which makes me wonder. The keel and rudder are two essential parts of the boat that –I feel- should be over engineered with at least a safety factor of 2. Especially since our design load is the load that acts in a quite normal sailing situation, not an extrem load from grounding or surfing etc.

    Eric: Thank you for the recommendation of Marchaj’s book. I read it a couple of times along with other, more technical books from Marchaj. These books were one of the reasons why all of my previous boats had long keels: Folksong 25, Marieholm 32, Golden Hind 31.
    Marchaj suggests that a heavy full keel boat with a high rolling moment of inertia and high damping behaves differently in a seaway and has a lower chance to capsize than a shallow bodied IOR boat like Grimalkin. Coupled with the fact that the heavy, long keel boats are tend to be built stronger these kind of boats are safer. But the price you pay is agility and maneuverability. This is exactly the reason why I decided to try a ‘fin keel with spade rudder’ type boat after many years spent in the long-keelers. We motor very little when cruising (normally around 10 l of fuel per month), and the extremely limited maneuverability of the Golden Hind 31 (it needs a football field to turn around) makes close quarters handling very difficult. I know that our new boat is not the best compromise between seaworthiness, sailing performance, and maneuverability, but I think not the worst either. I posted some pictures of the boat in my first post:
    This particular early IOR boat has quite deep sections and a narrow waterline beam, rooting the boat into the water better and hopefully make it a bit more safer than other, more extrem IOR designs.
    Our plan is to cruise the boat from Florida to Maine along the East coast of the USA next summer. During this cruise I will see how the boat behaves and based on this experience I will decide whether to keep her for real offshore cruising or go back to a more traditional hull form.
  5. Baltic Bandit

    Baltic Bandit Previous Member

    I wouldn't be as harsh as Eric is on IOR in general. IOR was a particular design rule. And boats that were going to spend most of their life racing inshore/nearshore were often distorted to take advantage of the rule. But rudders are not the primary thing that failed in the Fastnet. overall stability was. so you have to look at your whole boat.

    the thing is that with most emergency rudders, the conditions in which your primary rudder will fail (absent a collision with a submerged object) are so rough that you will not be able to deploy the emergency rudder.

    Coastal cruising FL to Maine is basically near-shore cruising and I assume you will run for shelter at the sign of any major storm. Now as to a "more traditional hull form" - remember that fast is also safe. full-keel boats surfing down a wave are almost guaranteed to broach when they hit the bottom, whereas a more modern design like a J-120 etc, won't because they don't have the loss of flow over their foils that the full keel boat experiences.

    But hitting something and having flooding in the rudder compartment IS an issue. and that's what I would look at a lot more closely at.

  6. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    For what it worth, in about 1979 whilst sailing a Farr 727, Genie (built 74?) in a Cowes to St Malo race we came to an abrupt stop off the Minquers. The boat with kite up and full sail was doing around 6.5-7 Kn when the keel hit a submerged object. It may have been a big basking shark or submerged container, even a submerged tree, it completely stopped the boat. It was not a rock, we were quite a way off the edge of the Minquers plateau.

    The owner and myself instantly looked inside the cabin expecting water to be pouring in somewhere at the leading edge of the keel attachment. There was a considerable noise at the time, but the boat just picked herself up and resumed speed after the complete stop. We checked for weed, scoured the sea for any sign of debris, shark(s) or anything, checked echo sounder etc. Nothing at all and no leak or any damage was ever found.

    This boat would have done the Fastnet in 79 and I would have been on her, but she was classed too small. Despite her small size I believe she would have been able to survive. She is still being raced (offshore including) to this day as far as I am aware. So not all these old boats are too fragile for offshore work. She is an early planing type 1/4 tonner but very well behaved, a lot better than many other 24' boats of the time or even a bit later. Hope that gives you a bit more confidence in some of these older craft.

    Be good to get your 3/4 tonner back into action. All you can do is check the essentials, keel bolts, rudder shaft etc. Depends on the bearing mount arrangement and how easy that is to change if you want to up the diameter of the shaft. She must have already sailed a lot sea miles and she is still in one piece, so she can't be too far out. I would be as much concerned about corrosion on the shaft as appeared in another thread here, after so much time.
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