Bob Austin had written: I have rolled a boat to 90 degrees in hurricane force winds on a North Atlantic crossing--my boat was an extremely seaworthy motorsailer--and I suspect that most power passagemakers in that size (62 feet length over all) would have also had serious problems in these types of seas. <snipped> Brian commented: I suspect that most all-powered, monohull vessels would have had even significantly more trouble than your motor sailer in those conditions. Two reasons, the inertia qualities of the sailing rig, and the hull shapes involved. The mast structure on a sailing boat, on a motorsailer, and even 'steadying rigs' on trawlers act to dampen the rolling motion in a seaway as a result of inertia. A sailing vessel that has lost her rig is in more peril than with it. The power vessel with no rig suffers accordingly. The beamy, hard chine, relatively flat bottomed, virtually keel-less shapes of many planning power craft are much more susceptible to rolling, and possess less damping qualities than most sailing shapes. Very seldom has one found the need to put anti-roll (stabilizing) fins on a motorsailer, whereas they are virtually a requirement on any power craft going off-shore. Look what happens when the electric power got interrupted to the fin stabilizer system on this salty looking trawler, <http://www.boatdesign.net/forums/showpost.php?p=40251&postcount=17> <http://www.boatdesign.net/forums/showpost.phpp=40383&postcount=21> ....almost lost the vessel, and this is not a heavy sea. Recently I wrote a discussion of catamaran hull forms as excellent candidates for the motorsailer concept, "Motor/Sailing Catamaran Concept". In that paper I quoted some fairly extensive tank testing research carried out by Lock Crowther at the Univ. of Southampton: *Note: reference source, Lock Crowther Designs "This work (tank testing at Southampton Univ) has indicated that the well designed catamaran is remarkably safe in breaking waves up to considerable height, even when beam on, we were unable to capsize a power catamaran yacht in the largest wave which could be generated. This corresponded to a 52' wave for a catamaran of 40' beam. Scaling this down to a typical 24' beam cruising cat means she should be O.K. in a 31' breaking beam sea. An equivalent size mono-hull power boat was easily capsized by a 25' breaking sea, and in tests with conventional yachts after the Fastnet disaster, it was found that a 40' mono-hull yacht could capsized in a 12' breaking sea." ______________________________________________________________ Bob Austin had written: Please understand I am being the devil's advocate--and am still interested in cats--but I think that there quite some way to go--at least in the production boats. Brian responded: I agree ______________________________________________________________ Bob Austin had written: Definitely a monohull of 34 feet is safer than a PDQ--the PDQ is light and has very quick motion. Henry Clews responded: I'm not sure I agree with you that a "34' monohull is safer than a PDQ", it seems to me there are many variables. I agree the cat will likely end up inverted if it gets rolled close to 90°, but I'd think the chances of it reaching that angle might be somewhat less than the monohull in the same conditions -- what say you? Brian replied: In the smaller sizes, and less beamy configurations the multihull form does not fair that much better than the monohull in terms of rolling motion in a seaway. You can see some of this effect if Crowther’s test figures are extrapolated down to smaller vessel sizes. Where the smaller multihull still has an advantage is making headway in a seaway. This has been demonstrated in a number of instances with a variety of smaller sport boat designs powered by twin outboards claiming a smoother ride in choppy conditions. Some of these have been more successful than others depending upon their bow, hull, beam, and tunnel configurations, both separately and in combination. Most interesting though is a general observation that might be made about most all of the power cat designs, that their smoother motion in a head-sea is only derived by increasing their speed, rather than backing down on the throttle. If one slows to great extent, the extra areas of the tunnel can become slamming points. This need to maintain a certain higher speed may not be most desirable in a really heavy sea. _________________________________________________________________ Phil Eslinger had written: I own a mono hull with a bulbous bow. When we are cruising in seas heavy enough to lift the bow out of the water, we get bulb slap which is obnoxious in its own right. I am considering moving to a mono hull with a fine entry bow Brian responded: I am on again and off again about the advantages of bulbs. But I would venture a guess that your bulbs are cylindrical in shape as I have seen on a number of displacement monohulls. This shape will slap. It might be possible to add a new bottom shape to this bulb that might reduce this slapping. Brian Eiland beiland@usa.net http://www.RunningTideYachts.com distinctive multihull expedition yachts