BOATING BASICS The chine and strakes are clearly visible on this monohull. Strakes should provide lift and spray deflection in the bow to mid-section areas where it is needed, and a reduction in water-drag on the aft planing area.
Part 2: The right angles and height make all the difference
By Rob Naysmith Down South Marine
a planing hull, let’s discuss other contributing factors ...
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STRAKES Strakes — those ridges running parallel to the keel, lengthwise along the underside of a ski-boat hull — are designed to serve two purposes. Firstly, they aid lift as the water pressure on the flat surfaces forces them upwards, and secondly, they help deflect the spray when the boat is planing. The problem with strakes is that they create drag due to their increased surface area, and if they’re not perfectly in line with the water flow, their drag effect increases still further. With this in mind, many designers make use of the “stagnation line� — the
N the last edition of SKI-BOAT we discussed hull types and the effectiveness of the chines on a planing hull. It’s all food for thought as we wait in anticipation of a time when we can all go fishing again. In this issue we will look at other important features so that the next time you are scolded for gazing lovingly at your boat, you’ll be able to say you’re studying the chines, strakes or deadrise. And believe me, just using those words will stop any further questioning. We finished the last article looking at features that give a planing hull lift, and as lift is the fundamental feature of 34 • SKI-BOAT July/August 2020
theoretical waterline under the hull when the boat is planing — for determining the ideal number and lengths of the strakes, all in an attempt to minimise this drag. This is evident on many hulls where the strakes end a third of the way before the transom, usually in a V formation due to the hull shape and stagnation line. In practice what is achieved is lift and spray deflection in the bow to midsection areas where it is needed, and a reduction in water-drag on the aft planing area. One final aspect to check on the strakes is that they do not come to an abrupt, square type end before the transom, and if they do then the strake should not be in line with the engine.
