Stuart M Roy
Naval Architect - Yacht Designer
YACHT & POWERCRAFT DESIGN SERVICES
The 20-knot Drascombe - A Design Study
I have always admired the Drascombe boats designed by the late John Watkinson and currently supplied by Churchouse Boats Ltd. To me they have the perfect blend of aesthetic appeal, practicality and affordability to provide a family with the maximum amount of enjoyment while afloat. But in eleven years of Drascombe ownership I did find myself, on sailing back late in the day against a foul tide, wondering whether it would be possible to create a faster one that still looked as pretty.
Back in 1983 I sketched a proposal for a 20-knot Drascombe-style boat, but this drawing remained unpublished. Recently, having seen a number of Drascombe tributes being produced, including the fine “Caboteur” by Jean-Louis Grenier, it is clear that there is growing interest in a wider range of Drascombe-style designs, with an attractive sheer line and distinctive planking features. Also with the success of the Macgregor planing motor-sailers and other “get home quickly” boats, that are hardly pretty, I decided it was time to do a more detailed design study for a fast Drascombe-style boat - to see if it is possible to provide a fast boat with good aesthetic appeal.
To go as fast as 20 knots a small boat will have to be capable of planing - skimming over the water rather than displacing it and moving through it. The essence of the proposal is to provide the conditions for planing performance. In the proposed design this is achieved by installing plenty of power, keeping the boat light by using a foam sandwich centreboard instead of a heavy steel one and making the underwater shape substantially different, particularly at the aft end. Above the waterline an attempt has been made to capture the aesthetic appeal of a Drascombe boat, as shown in the cartoon above and in the Study Plans.
The proposed boat is 6.2m long and is approximately Longboat sized, but may look more like a scaled up Lugger due to the higher freeboard to avoid spray being a problem at speed. The stern has also been widened by 30% to accommodate a 50hp engine and provide the necessary buoyancy in the hull to support its weight, as well as to encourage planing.
The rig is close to that of the Longboat with 15.3 sq.m of sail area, but to allow the larger engine to tilt sufficiently, bringing the propeller clear of the water for sailing, the mizzen mast has been offset to port, as in the Dabber.
Under power the proposed design should achieve the target of 20 knots. The established performance prediction methods used by naval architects would indicate that power of 24.4 kW is required for 20 knots. A 50 hp engine can provide 36.8 kW, giving a theoretical maximum speed of 23.5 knots, but these calculations assume perfectly flat-water conditions, no air resistance, and an optimal propeller match.
Sailing performance is undoubtedly going to be adversely affected by the additional wetted surface area, deeper sections aft and large blunt transom. This loss of performance would be most noticeable when beating to windward, although the aerofoil section shape for the centreboard instead of a flat plate would be an advantage. Downwind in a blow the broad deep transom may well promote skimming, giving the conventional hulls a run for their money.
But would anyone want one anyway? I have already mentioned the growing number of tributes and developments of Drascombes and the increased sales of get home quickly boats. A safety boat for Drascombe Association rallies is another possibility. But the trouble with a 20-knot Drascombe is that it just isn’t part of the Drascombe ethos “The sail that becomes a way of life”. If people want to go fast they should get a powerboat. And anyway, if it is not designed by John Watkinson and built under licence, it would not be a Drascombe at all! But having said that, who can say what prototypes the innovative original team of Watkinson, Westell and Churchouse would be trying out in some quiet Devon estuary if they were still together today? I would suggest that a high speed Drascombe would be just one of those prototypes.
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By clicking on these links, the Full Text, describing and justifying the features of the 20-knot Drascombe in detail and the Study Plans can be viewed and down-loaded in PDF form.
and would anyone want one anyway?