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Old 11-05-2013
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On Design: Hull

Quote:
Originally Posted by Faster View Post
Interesting how the Ker 40 and 37 Bolt depart so radically from the current drop of wide transoms/chines on offer everywhere else.. the boat must have an extremely effective keel/RM.

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Yes, ker designs are a bit different specially in what regards hull transom design but then it is because he designs mostly top crewed racing boats. When he designs solo boats he does similar to the other designers:

Here you can see some other Ker designs. The last one is a solo boat the others are designed to be sailed by a crew.








Thae Bolt 37 hull can provide the best of two worlds: Great hull form stability, maximization of keel RM, little drag downwind or upwind even on light winds.

The hull is designed to sit on the side of the transom when heeled with not much drag. It is designed to sail with considerable heel upwind developing Max RM when "sited" on its lateral part of the hull. On that position the boat maximizes the big righting moment from the keel (big B/D ratio) and adds it to the big righting moment of the hull provided by that lateral part of the hull that than is almost a flat surface.

The solo boats and cruising boats influenced by them work other way: They are designed to sail with relatively little heel and the hull is designed to provide hull form at low angles of heel (like almost all boats) and to offer a big resistance to big heel angles, stopping the heel at about 20º. This provides a boat easier to sail and will stop rolling downwind.

the Ker hulls need a good crew to stop rolling downwind. The hull does not much to prevent that but offers very little drag in that position, providing the boat is perfectly balanced and that implies a constant trimming/wheel work.

If you look at modern fast designed cruiser-racers you will see that most are trying to achieve boats that can mix the two situations: a boat good for solo racing/sailing and also good for crewed racing/sailing, a compromise that as all compromises is not the perfect solution for any of the cases.

These boats even if, like Ker boats, have almost all beam pulled back, are not beamy boats, not if compared with solo racers or cruisers based on solo racers, but have on the hull design and particularly on the transom some of the characteristics that are typical of solo boats. You can see this clearly on JPK and Archambault cruiser-racers, boats that perform well on both worlds, solo and crewed:







As I have said on a previous post, while the modern crop of mass production tend to have hulls based on solo racers hull design, the performance cruisers tend to be a mix between the two worlds, with a more or less influence of solo racers.

Here you have some modern mass production performance cruisers that are a good example of that compromise, the Dehler 38 and the Elan 320:





and some modern mass production main market cruisers very influenced by solo racer hulls, the Dufour 500 and the Oceanis 41:





Other performance cruisers, or other main market cruisers, have a lesser influence of solo racers hulls and are more "in tradition", meaning that they are more influenced by traditional crewed racers and boats developed by ratting formulas. The truth is that racing boats were always the main influence in what regards design of sailboats, including cruising boats and the different ratting formulas had always a major influence on design.

Today solo racing boats, that have a much more open formula, have introduced a new parallel design direction. Its bigger influence on cruisers has to do with the fact that cruisers are generally solo sailed or have a short crew, like the solo racers and their hulls are more adapted to this situation.

Some modern performance cruisers on the "tradition", Xp 38, Arcona 410:





Some modern main market cruisers "on tradition, the Jeanneau 409 and the XC 50"





All great boats with different compromises, meaning different advantages and disadvantages What is important to each sailor is to understand the advantages and disadvantages of each type of design to be able to chose the right compromise for what he wants from a sailboat. Not all like to sail the same way and in the same conditions.

Regards

Paulo
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Last edited by PCP; 11-05-2013 at 08:32 AM.
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