Rudder & Steering

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Overview

The original Improbable rudder was a giant transom-hung rudder, just like a giant dinghy, that was hung on a skeg that extended below the hull.  Due to the rating penalty from the just implemented IOR handicapping rules (remember, this was in 1970) the rudder was changed to an under-transom setup, although still on a skeg.  Unfortunately, the whole configuration was very heavy, and also high in drag.  Although the notable “roostertail” in the wake at surfing speeds was exciting to read about, it was no doubt a result of the turbulence caused by the high drag of the skeg & rudder.

Philosophy

Although Gary Mull was a great designer, he somehow missed it on the first two iterations of the Improbable’s rudders.  The original transom-hung version was appealing from the “big dinghy” concept point of view, yet was heavy and inefficient.  The 2nd gen rudder which was tucked under the transom (largely for improving the IOR rating) could steer the boat adequately however it took an enormously long tiller to do it.  The high steering load was largely due to the low-aspect ratio profile with the center of effort quite far away from the rotational axis of the rudder shaft. The overall weight was very high, and the hull shape tapering into the skeg in front of the rudder was effectively a protuberance interrupting the natural flow of water around the (otherwise very efficient) hull shape at the back of the boat. Rudder design and the materials to build them have come a long way since 1970, effectively allowing a new design that can match the efficiency of the rest of the hull, and structure that is much lighter.

The Plan

The new rudder design initial conception is by myself with help from hydrodynamicist Paul Bogataj and designer Neil Racicot, which will be refined and structurally engineered by Larry Tuttle of Waterat (based in Santa Cruz, CA).  Larry happens to be the builder of the rudder on Tom Wylie designed “Ocean Planet” (the IMOCA 60 that I raced twice around the world).  That rudder was fine-tuned by myself for more balance (more area in front of the rotational axis, to reduce the force needed to turn the rudder) also with help on the shaping by Paul Bogataj who in addition to the rudder did the keel foil shape and bulb design on Ocean Planet. The rudder shaft improbable will remain vertical, for maximum efficiency while the boat is heeled over.  The profile will be moderately high aspect ratio, with approximately 19% balance ratio for light steering load.

Challenges

There are numerous challenges.  For instance, keeping the shaft vertical requires tricky massaging the shape of the hull at the top of the rudder.  This is because there are compromises to made to allow turning the rudder without squeezing into the hull, while not having too much space between the two.  Also, the angle of the hull relative to the shaft and rudder means that the rudder bearing must be recessed somewhat into the hull, which put the bearing a bit further from the top of the rudder blade.  Getting everything sorted out has required coordination between all the players by me, a lot of modeling by Neil Racicot, considerable time on quotes/etc. by Phil Quartararo at PYI (supplier of the rudder bearings/sleeves/etc.), machining and advice by Jim Betts and Steve Kaestner at James Betts Enterprises, and of course the engineering and build by Larry Tuttle at Waterat.

Progress

My part in the Improbable rudder project started in the mid-80’s, when my dad first allowed me to start modifying it to reduce the steering load:

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This did provide some improvement!  However, there was/is a long way to go from there.  

I haven’t been able to find many pictures of the rudder after my initial efforts back then.  And only one of the further evolution…my dad later added some area to the nose of the rudder below the skeg.  Which added some balance, as the area on the nose was in front of the rotational axis of the rudder shaft.  You can just the rudder in the first shot below, and then there are some pics of the rudder, skeg, the tiller head & autopilot system (huge!), etc.

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Then in 2018...finally my chance:

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Progress in 2019:

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In 2020 I couldn’t get to the boat, however we got design work done, with help from hydro ace Paul Bogataj, and modeling by Neil Racicot:

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