Marine Designs
We have boat plans (both yacht designs, catamaran designs and power designs) for all sizes. From the 12ft Mushulu Dinghy stitch & glue yacht tender to the Oceansky 57 catamaran. We design timber boats, plywood boats, strip plank boats, fibreglass composite boats, steel and aluminum boats to very modern styling for both monohull designs or multihull designs, powerboat or sailboat and in round bilge, multi-chine or radius chine hull forms.
G'day everybody,
Just thought I'd do a little thread on spray rails, their locations and proportions for you all.
In the early days, many designers ran the spray rails along buttock lines. In other words, the spray rails were dead straight if you looked at them in plan view or from directly beneath the hull. This caused the spray rails to curve up in profile and intersect the chine. The reasoning was that the waer flowed straight aft along the rail wich generated added lift, with minimul added resistance. Most designers have abondoned this arrangement, finding that the intersection of the spray rail with the chine caused local spray in that area. However, a few designers still prefer this method.
Generally, the optimal arangement or RUN of the spray rails according to most thinking, is that the rails are dead straight (follow buttocks) aft of station 4 or 5, ( divide the boats DWL by 10 equal stations) but curve in (plan view) as well as sloping gently up, rather than following the buttocks as they run forward. In this way, the rails doesn't cause an intersection at the chine.
Within reason, the more rails the better: however, more than 4 per side is generally overkill. most designs could get along acceptably with no spray rails at all. The same hull without rails will, though, be a little wetter and a little slower and will have a little less dynamic stability.
Usually I draw two to three spray rails on our designs.
Spray rails should be of constant width (unless they taper away aft) from around station 4 or 5 aft. Width should be approximately 1.5% to 2.5% of chine beam. The spray rail underside should be flat atwartships or angled down slightly outboard, but no more than 4 degrees. As the spray rails run forward, they taper away to zero width.
Spray rails should not be too effective on the forebody of the hull. If high lift is developed here, when the forebody hits a wave, large accelerations will accur, reducing the psitive effect of the V-shaped bow section. Always taper the rails in the forward sections.
It is sometimes a good idea to taper the lower spray rails aft - if they run approximately toward the running gear (propellor and rudder) - so that these spray rails end about 1.5 to 2 stations ahead of the running gear. This is because it's possible, that the spray rails traps air/foam and guides it to the running gear possibly causing ventilation.
Now you can have a bit of fun designing spray rails for your boat !
Looking forward to your comments.
Have a good one
Mark Bowdidge (MRINA)
Bowdidge Marine Designs
