Why do some RIBs still give a spine shattering ride in the rough?
Some thoughts afloat and at speed by regular contributor Tony Lee-Elliott of Flatacraft.
In my view, not all RIBs have hulls that have been designed by people who knew what they were doing! A constant Vee or Deadrise is an excellent tool in the calm, but can become a slammer in the rough unless it has the reach and speed to use just the wave tops. Flared bows, stepped hulls and reversed chines are all ways of improving the hulls’ performance and the boat’s ride. However since no two sea conditions are ever the same, you begin to understand just why every builder has a different hull shape. These design variations can run into thousands; just imagine how many different hull designs you would like for just an afternoon’s RIB trip of some 30nm or so.
You will already know that tides ebb and flow, stand on the full and lie at low, without us even bringing the moon into the equation. Seabeds rise, drop, ledge and narrow. Winds gust, fall, back, and veer with twelve combinations of force. What is that lot in combinations? Well, we have four depths, times four of movements, times twelve of speeds equals 768 different conditions – Even if the wind were to average Force 3 over your route… your RIB would very likely pass through conditions of Forces 1, 2 and 3 if not 4!
Different depths and seabeds would be encountered along the way but perhaps you would be fortunate and the tide would be at neaps rather than the fast flows of springs. This would give a minimum of 24 sea conditions that you are going to meet on your gentle afternoons RIBing. In a sail or displacement boat there would be little change in movement. However, in a RIB, where the usual speed is 30 knots, the change in movement can be quite dramatic due to wave length, height and shape.
A 6m RIB (over the tubes) is probably based on a 5m hull with a 4m water line if you’re lucky. You may think that you have a large RIB of 6m when in fact you are performing in tight conditions on a pretty small hull. A hull that has to cater for all types of sea conditions without changing its shape, (hopefully!). The only two things that may change their shape are small waves taken slowly by a displacement boat and a man when the waves are taken fast in a light planing RIB! Think about it, who would sensibly venture on such a trip in a conventional hard hull 5m runabout/sports boat with no reserve buoyancy…. safe stability at rest, massive fendering or the ability to self bail when on the plane? The answer is few. Yet the Ribster does it in safety and reasonable comfort, yet he does it on the same small sports boat hull.
With a stepped fast planing catamaran hull, we can skip over the waves and with a VSL, (very slim vessel), we can go through the waves. Although a Ribster needs his all important inflatable collar, this is not the best thing to stick on either of these two types of craft, but compromise is the name of the game. Racing RIBs are now using stepped hulls to give more speed and a softer ride, but such craft have little of the tube touching the water when at rest, so they have little stability when stationary, no good for divers!
Compromise, compromise, compromise, is the answer, not just in RIBs but in all craft. So, if you are getting a hard ride and everything around you is breaking up, falling to bits and going overboard, you may have the wrong shaped hull for that bit of water, at that speed and in those conditions. It is as simple as that, chum! Hovercraft and hydrofoils… now we are talking.