Scorpion Ribs


The Story… “Article from David Marsh”

The illusion

‘So, just how hard can it be to design a RIB’ I mused. There’s no accommodation to worry about, no champagne and oyster bar to squeeze onto the flybridge and no Lawrence Llewelyn-Bowen type worries about what colour of combination microwave the builder will fit. All you’ve got is just lots of inflated rubber, something I’d always wanted to, err, get into – speaking naval architecturally of course! Chatting with Graham Jelley at the 1994 Workboat show in Port Solent, it seemed as though I’d already achieved the most difficult part of the process – persuading one of Britain’s leading RIB builders to let a complete RIB novice design a new range of RIBs from the ground up. But I knew it wouldn’t be difficult; whiz off a new hull on the computer, stick on the tubes, don’t forget the deck, and most important of all, bang the invoice in quickly. A few days work at most, even for a designer with a complete lack of RIB experience. Or so I thought.

The reality

Just like any design, the starting point for the Scorpion RIBs was a detailed design brief. How fast? What size? How heavy? What engines? It was at this early stage that the difficult realities of RIB design began to sink in. Normally, a naval architect would not consider extending or shortening any hull more than about +/- 5%; much more and you need to start again if you want a shape which is not compromised. However, Graham’s brief was for a single hull mould that could turn out everything from a 7.5 metre day boat to a 9.5 metre cabin boat almost 27% longer and, of course, still handle and perform better than anything else. But this dilemma paled compared with the other interlinked problem – loading variation. On a big flybridge boat say, although there’s a lot more to consider; generators, galley equipment, bow thrusters and the like, the overall variation in weight due to different engines is minimal. Not on Scorpion’s RIBs though, where I was expected to produce the consummate form that could cope with everything from a single 100hp outboard weighing just 140kgs, to a pair of Yanmar’s 300hp inboard diesels weighing in at a stern-sinking 1,052kgs with their Mercruiser outdrive legs. On a boat weighing no more than 1,000kgs without its engines, that was going to be a real challenge. So where did we start? We began by taking a look at everything else on the market. In some ways this was very useful and encouraging. What struck me was that despite being so small compared to the boats I was used to designing, so many RIBs exhibited no real attention to design and engineering detail. Decks were frequently no more than crude plywood structures, and the tubes often had little support, so there seemed to be lots of scope for improvement. The disturbing thing for me was finding that there was absolutely no consensus on what constituted a good hull shape. Gazing down the graceful, very shallow forefoot of a Revenger hull, and then switching my attention to the deep, ultra-sharp bow sections of an Osprey hull, it quickly became clear that there was no real generic optimum we could use as a starting point. We would have to start from scratch and develop a new shape unlike any other.

The design process

You cannot design a hull in isolation. The structure, its weight and even the quality objectives need to be considered at the outset. Long before the hull took on its final form, we started debating the pros and cons of plywood decks against a moulded glass fibre structure. The design process was very much a team effort with four members; Graham Jelly, Chris Peterson, and Jonathan from Scorpion, and myself. Fortunately, the voting went in favour of the moulded deck option. I drew a large dish on the outboard edge of the upstand at the edges of the deck, exactly matching the 23cm radius of the hypalon tubes, and it ran the length of the boat, so the whole run of the tube was supported. It was an expensive process fabricating an accurate deck moulding with complex curves, but in my opinion, it was a no-brainer; Scorpion’s moulded decks not only provided a high class finish which plywood decks cannot hope to compete with, but also give unmatched support for the tubes. Computer Aided Design (CAD) provided the solution to the two main problems – changing length and hugely varying loads. The advantages of CAD are well documented; accuracy beyond your wildest dreams, scorching speed, and the near-instant calculation of hydrostatic and stability data. For us, however, the principal advantage was the ‘What If’ abilities of CAD. What happens if we change that single 200hp outboard for a pair of 300hp diesels? What if we lengthen the hull from 7.5 to 9.5 metres? How deep will the boat sink? How will it trim? And so on and so on, ad infinitum, often at the cost of no more than a few key-strokes. This ‘what if’ potential was indispensable because the Scorpion team were obsessed with getting the height of the tubes spot on, irrespective of loading. Too low and they would drag in the water at speed, too high and the boat would feel unstable when you stepped aboard. CAD was instrumental in ensuring that the Scorpion performed well in so many different disguises.

The hull shape

However useful CAD is though, it does not generate even the most basic shape for you, let alone one as successful as the Scorpion. When I started, my CAD screen was just as empty as a blank sheet of paper. Under normal circumstances this would not be the case. My hard disc is full of generic shapes – the typical 45 foot 30 knot flybridge boat, the generic 30 foot long keeled sailing boat, and many more besides. But no RIBs. So what did we use in these virgin-like circumstances? That’s right, gut feeling. I drew a basic deep-Vee shape that ‘felt’ right. Then we all stared at it, pondering and discussing. I tweaked a little here and a little there, modified the chines in places, refined the spray rails, lifted the tubes a few millimetres, and then tweaked a little more. Then I looked at the hydrostatics again. But, ultimately, we put aside the prismatic coefficient, ignored the transverse metacentric height, and shunned the longitudinal moment of inertia – because it was when we all ‘felt happy’ that we lit the blue touch paper and started building. One of the joys of working with Graham and his Scorpion team was that they were all pedants. They were prepared to pay heavily, with time and money, to achieve the long term benefits of the tiniest design refinements. For instance, the area above the chine rolls gently round until it meets the tubes, so that when the boat lands heavily, water peels smoothly off the hull rather than slamming into the tubes. And as this water streams across the bottom of the tubes, it doesn’t try and rip off the strip connecting the tubes to the hull because the bonding strip has been recessed flush into the shallowest of rebates in the hull mould. The constant tweaking and refinement continued throughout the design and building process. And even though the basic 7.5 to 9.5 metre models remain, the development continues today with new moulds, improved engineering and better detailing in evidence. Scorpion used the existing models as the basis for the development of their new 10m RIB, widening the original by 12% and extending it to 10 metres.

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