Top left: typical keel attachment methods. Above: a generic Ker hollow keel installed with the mast step and jack visible on top of the internal keel attachment structure. Left: a direct comparison using Finite Element Analysis of the stresses generated in Ker’s hollow steel keel method (far left) with another popular method referred to in the text. Note the high stress indicated in red at the fillet weld in this alternative welded keel, while a similar high stress can also be seen on the inside of the corner of the welded assembly, potentially leading to the growth of cracks that will not be found during a visual inspection. Below left: external view of the salami slice arrangement employed in the Ker hollow keel and (lower right) a view of the section taken through the same keel fin
Ultimate strength in excess of 900mPa can be expected. Forging – Over-sized billets of steel,
such as 4340 steel, are forged to approxi- mate shape before machining to final shape. The forging process improves the reliability and toughness of the material, aligning its microstructure with the general shape of the part, but it’s hard to put a reliable number on the advantage versus the same part built from the same metal in a non-forged state. Welding – Offers the weight advantages
of hollow construction, but comes with its own challenges, particularly for T-shape constructions. Welding relies on human skill more than most other forms of keel construction, which is an important vari- able. There are a very few highly experi- enced and well-reputed companies. Composites – Usually carbon fibre,
taking advantage of the material’s excel- lent uni-directional strength. Potentially the lightest method of keel construction. Only a deep socket attachment is viable, to keep local compression and shear loads to manageable levels. Lack of torsional stiff- ness and resulting flutter are a problem if insufficient material is applied to resist it; ISO doesn’t offer loadcases for that but DNVGL has suitable rules.
50 SEAHORSE
Forms of keel attachment Direct bolt-on – Perhaps the oldest form of keel attachment originated when much of a ‘long keel’ shape was formed by the hull and the lead shoe was bolted onto the bottom of that shape. The approach evolved towards complete keels being bolted on that way, either to a shallow sump or to the flat of the bottom of the hull. It works fine as long as the width of the keel attachment is sufficient for its depth, and has certain pro- duction advantages in that no significant fairing work is required with a new boat. When the keel is short-chord and deep the method is no longer applicable and some accidents have occurred to this type of keel when suitability has been marginal. Socket method – The keel is posted into
the boat so that the load is taken in a simple lever manner as is common for a rudder or aeroplane wing. To reduce the loads the socket is best kept long, but to reduce the weight and deflection of the bulb the socket is best kept short. A short socket leads to high local loads on the boat’s internal struc- ture and high shearing loads in the part of the keel that is inserted in the socket. To avoid an unclear loadpath the keel
bolts are ideally limited to a simple vertical loadcase. All our earliest raceboat designs had socket keels and we used two
approaches for the bolting: one was putting a pair of small vertical bolts on Teflon washers to ensure they didn’t take any hor- izontal loads, requiring the head of the keel to be a tight fit in the socket; the other was to insert an oversized bolt from above that actually carried the ‘upper bearing’ load of the keel. The purpose of that second approach was to remove the need to make the socket a perfect fit to the keel. We have observed very shallow socket
solutions by others, which have the conve- nience of less intrusion into the interior of the yacht, but we generally steer clear of solutions that cause very high local loads on composites and require perfect con- struction to function as intended. Socketed connections are suitable for welded keel spars and if sufficiently tall are also suitable for composite keel spars. T in a recess – An alternative to the
socket method for high-performance keels, it can be connected to lower-profile inte- rior structures and is potentially stiffer, due to the advantage of transitioning to a wider structure earlier. But structurally it is impossible for composite keels and also presents significant challenges for welded construction types. Most modern yachts that are not pure
raceboats use this method for practical accommodation reasons and most of the last generation of monohull America’s Cup boats did so for stiffness advantages. As an aside, one of our competitors in
the 32nd AC ended up building five keels out of titanium then losing precious sailing time because when they designed their hull plug they didn’t allow sufficient depth in the recess to enable them to create a large enough radius in the T-junction.
q
KER YACHT DESIGN
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122
