Attachments

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Attachments - Designing for strength

The following are a series of stress calculations for a typical attachment (skeg/keel) on a metal boat.  The loads represent light loading conditions, and are not meant to represent any specific skeg/keel.  Rather, these examples are meant as a generic guide to what happens under various attachment strategies.

Each analysis is comprised of two pictures and a scale.  The picture on the left is an exterior view, the picture on the right an interior view.  The angle of each view is chosen to show the area of maximum stress.  The scale in the middle, as well as the colors are relative to each set of pictures.  You cannot compare the colors from one drawing to the colors of the next.  You must compare the scale.

The initial load is 1000 PSI in all cases.  These drawings serve to show the stress risers that result, and how keel and skeg attachments can be engineered to minimize and eliminate stress risers.

The above picture represents a keel/skeg attached with no transverse support.  This method multiplies the stress in the attachment by a factor of about 10, and maximizes it the hull.  This can be expected to lead to failures in the skin of the boat, and a loss of water-tight integrity.

The above picture represents a keel/skeg attached with a transverse support welded to the hull inside.  This method multiplies the stress in the attachment by a factor of about 3.3, and maximizes it the hull and skeg/keel at the point of attachment.  This can be expected to lead to failures in the skin of the boat, and a loss of water-tight integrity.

The above picture represents a keel/skeg attached with a transverse support welded to the skeg/keel and the hull inside.  This method multiplies the stress in the attachment by a factor of about 1.7, and maximizes it the hull and skeg/keel at the point of attachment.  This can be expected to lead to failures in the skin of the boat, and a loss of water-tight integrity.

The above picture represents a keel/skeg attached with a transverse support welded to skeg/keel and the stringers but not to the hull.  This method multiplies the stress in the attachment by a factor of about 2.7, and maximizes it the transverse support.  This avoids failures in the skin of the boat, and a loss of water-tight integrity, but can lead to internal structural failures.

The above picture represents a keel/skeg attached with a 2x transverse support welded to skeg/keel and the stringers but not to the hull.  This method multiplies the stress in the attachment by a factor of about 1.5, and maximizes it outside the hull.  This avoids failures in the skin of the boat, and a loss of water-tight integrity, because the likely point of failure is outside the hull.

Deep Floors

The next series of pictures explores the effects of adding deep floors to strengthen the skeg/keel.

Above.  Not bad, but requires transverse welding to the hull.

Above.  Not much better, and still requires transverse welding to the hull.

Above.  Good.  Some transverse weakness and requires welding to the hull.

Above.  Very good, but requires transverse welding to the hull.

Deep Floors - no transverse welding

The following series eliminates transverse welding.

Above.  No transverse welding, but transverse weakness.

Above.  No transverse welding, but still some transverse weakness.

Above.  Good, and no transverse welding to the hull.

Above.  Very good, and no transverse welding to the hull.