Originally Posted by baeckmo
Numbers, numbers, gimme numbers first!
Sorry, it is not so easy. Aside from the simple dimensions there is not much to be found. Length Overall, Hull Length etc. are here: http://rafnar.com/kerfi/wp-content/u...chure-1608.pdf
. But I would like more as for instance the lift to drag ratio of the OK-Hull.
There has been tanktesting in Maloy/Norway, in Potsdam/Germany and in Vienna/Austria but RAFNAR seems not to be interested to publish the results. A hundred 1m models have been investigated, modified and tested again - but nothing was published -- or I was not able to find it.
Together with scientists of the Iceland University Reykjavik motion responses (MR) such as vibration and slamming during navigation were investigated. MR cause discomfort to crews and passengers right from the beginning of every sea journey, and often lead to injuries, especially in rough seas.
It was found, that the RAFNAR OK-Hull design reduced hazardous levels of slamming by up to 95 percent (six instances, versus 112 on a standard rescue boat). These results were published in a short article http://rafnar.com/kerfi/wp-content/u...ort-at-sea.pdf
and this seem to be the most important arguement for RAfNAR to sell their Ribs.
"Basing his thinking upon a scalable series of ‘circle-segments’ making up the eventual formula for the unique OK keel and hull shape, Ossur Kristinsson determined after endless model testing in the towing tanks of Potsdam, Vienna, and Maloy, Norway that he had perfected his concept. With his final prototypes of open RIBs being extensively tested by the Icelandic Coast Guard and the local Search and Rescue teams in the extreme North Atlantic conditions around Iceland – and sea-trialing for some 3000 nm in the process – Ossur has pronounced a sweet-spot in what he refers to as his ‘base model’, scalable from the smallest prototype of 6 m up to 40 m and beyond. This concept formula has been patented as an invention in the United States.
An engineer originally trained in prosthetics and orthotics, and owner of a variety of boats and yachts over the years – up to 40 m in length – Ossur Kristinsson had become convinced that the traditional concepts surrounding hull-form and water-flow dynamics were nowhere near fully explored or developed. The uncomfortable slamming and instability of a planing or semi- planing hull and the inefficiency of a conventional displacement hull, drove him to re-think the shapes and radii making up the ideal underwater keel and hull shape. Understanding clearly that having as much of a vessel’s hull remaining in the water for as much of the time underway as possible promoted stability, comfort and good sea-keeping performance, Ossur’s goal was to perfect the hull and keel shape so as to pass the displacement form through the water as efficiently and effortlessly as physics would allow, thereby allowing bodies to transgress accepted limits passing through water as ‘displacement’ boats.
In his sights was the target of dramatically beating the displacement water- line limit speed rule, while at the same time sustaining comfort, stability, and in-water performance. Ossur believes that two important physical aspects of water are to be taken into account when considering hull-shape: firstly, water is incompressible, hence the violent slamming experienced when a planing or semi-planing boat bounces along atop it, and secondly, the surface tension of water is tremendous with regard to penetrating forces, again augmenting slamming effect and the counter-forces experienced when a vessel tries to punch through a moderate or large sea and waves. In Ossur’s words, ‘the water fights back’ zone whas when hitting waves at an approximately 45° angle.
The results show that Rafnar’s craft Stefnir is significantly more stable than the comparison craft. Rafnar’s innovative hull and keel design is therefore an important feature when it comes to reducing the risk crew members are exposed to on board, not least when it comes to the negative long-term effects of prolongued exposure to whole body vibration.
This means that crew safety is greatly improved, the risk of physical damage due to drasitcally fewer slams is significantly reduced, and the crew is therefore able to focus on the task at hand in a more efficient and safe manner.
The patented hull and keel design used in all Rafnar models is a key factor that contributes to better on-board safety. In fact, a trial conducted by the University of Iceland in Icelandic conditions in January shows that Whole Body Vibration (WBV), which can negatively impact the crew’s health, especially if endured long-term, is significantly reduced in Rafnar’s boats.
The trial, that compared Stefnir and a common coastguard model of a similar size and weight, factored in different angles when hitting waves, wave height, cruising speed and wind factors, and showed that Stefnir performed significantly better than the comparison craft.
The ISO 2631-1:1997 standard defines two different WBV health guidance caution zones, as 2.8m/s2
and 5.6 m/s2. Moreover, according to the standard, the VDV health guidance caution zone is defined by vibration dose values (VDV) between 8.5 m /s 1.75 and 17 m/s 1.75.
When measuring instances of 1gz z zz, the hazardous condition limit, at the bow of each craft, Stefnir only showed six instances compared to 112 instances in the comparison craft. THAT IS 95% When looking exclusively at instances at the cockpit that surpass the 0.57g health guidance caution zone, the Rafnar craft again performed much better than the comparison craft, with only 6 registered instances compared to 33 instances at the cockpit of the comparison craft." http://www.affairaction.com/Articles/RIBs.pdf