Tuesday, April 21, 2015

Keel and Rudder: Design vs. As-built

Jim Backus designed the 37' LNVT with 3000lbs of ballast to be located just below the waterline and as far outboard as possible.

However, the tug was built with the ballast in the keel.  Did the ballast's relocation result in changes to the keel?  To find out let's compare the tug described in the blueprints to the tug we own, i.e. the as-built tug.

The LNVT's 'Revised Hull Line Drawing', dated 19 June 1981, calls for a 4-3/4" wide keel. This thickness is to be consistent from top to bottom and from stem to stern.

Here's a shot of Thistle 37VT47's keel. Looking at the root (where the keel attaches to the hull) the light makes clear the keel's fore-aft and top-to-bottom tappers. At the aft end of the keel the root is 6" wide while at the bottom it's 5" wide.   Moving along the root towards the bow, the keel's width increases to ~8".  But the bottom of the keel remains 5" wide.

The bottom line: the as-built root is thicker than designed but tapers quickly to almost the specified thickness--5" vs. 4-3/4". There's also some taper along the keel's longitudinal direction which wasn't specified in the plans.

In the above picture the as-built measurements (in red) were made by Allan Seymour, Sally W. 37VT42, the numbers in black are takeoffs from the blue print.  The height of the keel was measured from points 3', 6' and 9' forward from the end of the shoe.  The fillet between the hull and keel makes it difficult to get an exact measurement. While there are differences between the designed and as-built keel, they appear to be minor.   Since there are known differences between the designed and as-built shoe, perhaps basing all measurements from the rudder post's center-line would make more sense.

Most glaring, in these preliminary measurements, is the difference between the designed and as-built rudder. The blueprint calls for an approximate 32"w x 32"h rudder; the as-built is 30"w x 30"h.  The rudderpost's center line was designed to be 6-5/8" aft of the rudder's leading edge; it is actually 3-3/4" aft.

Dave Howell, Nellie D. 37VT63

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