Spry rails
History lesson first to help understand why and what spray rails are used for on a boat hull
Back in the early thirties when pioneers where designing boats for water speed records, they designed hulls that had steps built into them across the width of the hull. The purpose of the steps is to lift the hull out of the water as the boat gains speed, therefore reducing the wetted area, also reducing drag on the hull. However there are several down sides to this step design, namely that a vacuum can be created between the inner face of the vertical step and the water creating drag also the vacuum tries to pull the hull back into the water - to overcome this the designers introduced air pipes which ran from the deck through the hull to the inner face of the step. This allowed free flowing air under the hull which eliminated the vacuum look at the MISS ENGLAND II hull design. The next problem with the step design is to do with water conditions i.e. if the waves are too big, the hull will start to pound into the oncoming waves and drastically reduce the effect of the step; this also has to do with these types of hull having very little “V” shape built into the bottom of them, they are almost flat in cross-section and is the wrong shape for driving into oncoming waves obviously no good for offshore sea conditions, as the early designers of the MOTOR TORPEDO BOATS discovered and so they dispensed with the step in the hull. They found that at high speed - water was driven up the side of the hull from the chine and this increased the wetted area of the hull on the MTB. It was found, that by placing a square strip of wood at the outer edge of the chine - for the full length of the hull - it deflected the water spray away from the hull, therefore reducing the wet area at speed. These hulls still had a shallow “V” bottom and still tended to pound into the oncoming sea and give a rough ride - have a look at VOSPERS 72 FT MTB designs for the war years.
So, the next problem for the hull designers to overcome was how to give a smoother and faster ride into oncoming seas and this was done by the Fairmile and their designs. They designed a long thin hull which had a very narrow bow with a very high chine line at the bow. This gave a ‘knife-edge’ type of entry into the oncoming waves and therefore smoothing out the rough ride. These though, weren’t a true plaining hull. We have to move forwards in time; to the early 1950s when Vosper and their head designer, Commander Peter Du Cane cobbled together hull several designs and came up with several hull designs. Notably the Sabre Class or ( if you would like to call it the Percassa Hull). This hull had what is known as an ‘S’ shaped chine, giving a very fine entry at the bow, but it does flatten out to a shallow ‘V’ which incorporates the best parts of the Fairmile bow design and the Vosper design. This hull design though was still no good for the true offshore power boat racing hulls – as this design still tends to pound a bit and is still classed as a fairly wet hull.
The next stage then is to design a hull which has a very steep ‘V’ in the bottom of it; the enables the hull to be driven at speed fiercely into oncoming waves/seas which are encountered. However, if it this was just a plain ‘V’ it would be an extremely wet hull and unstable; the method of reducing this was to add more than one spray rail to the bottom of the hull; we have a fairly large deflecting spray rail at the chine then – which is a greater area than the spray rails on the hull. Also, the shape of the spray rails have now changed from a square shape to a triangular shape – and thus deflecting the water down over rather than out over – as a result converting the spray into downward thrust to lift the hull out of the water. Therefore, the more spray rails we apply to the bottom with a triangular shape the more thrust will be applied to lifting as the hull travels forwards. The other great benefit of the longitudinal spray rail is that it is not affected by size of waves or a vacuum being created as in the step design.
If you have a look at say, the Surfury design, also some of the offshore power boats of the 1960s, this will give you an idea of how many spray rails to fit to the bottom of the hull – normally it is between 3-4 per side plus the one which runs along the chine.
Aye
John e
Bluebird
