Distribution of rocker.

David Raison's "philosophy" is very interesting regarding race hull shape and rocker: he is the flexible, open minded sailor/designer of the scow bowed 747 that just stomped the whole fleet in the latest Mini-Transat, plus setting a new record. When this full bowed boat appeared, skeptics just shook their heads - but now they are scrambling to catch up. Anyway Raison's "raison d'etre" (pas mal, eh, une humour petit) is that the very full bow is a return to the ancient “cods head, (but flat and full) mackerel tail” – meaning the greatest curve/rocker is in the bow section area and then it goes almost completely flat, in profile, to the full width stern. Raison was after the most powerful boat he could get in the rule and one that had balanced, low wetted surface area when heeled. You can’t really tell from the photograph but the hull’s after sections, which really begin just behind the curved bow, from then on looks completely flat, or almost so, in profile; but in cross section every thing is softtly curved. This is a new direction and Raison is the bright guy who introduced it. In future monohull design. there will be more scows appearing.

 

Correction: below waterline the 747 curves are soft but near flat while the turn in the bilge, is HARD which changes to no-flare topsides. For those who require measurements, sorry, haven't got them.
The latest KJ 60 here shows how slight the rocker is plus the change from flat after sections to chine at waterline. Normally slight to flat rocker is the fastest design. NZ designer Jim Young is an advocate for near flat rocker on his designs and has even designed boats with none at all, completely flat rocker and flat in cross section from near bow to stern too.

 

You can employ these really flat buttock hull forms in ocean racing craft, as they don't have to do much maneuvering, but bring them into a situation where they might have to "tack it out" with someone and a boat with more rocker will kick their butt. As has been mentioned, everything is a trade off of some sort. The KJ-60 above is fine for it's very narrow preformance environment. It's fairly well suited for long distance ocean racing, but change it's environment a little and it'll get hammered, by what would on paper, seem to be inferior craft.

 

Correct. Actually there is more rocker on the multihulls than the fast monos, especially on the latest MOD 70's; a slow tacking multi is somewhat of a problem anyway, a result because of their wide overall beam and lightness.

 

I see straight lines not curved-sexy ones on the MOD70 vaka. The shapes including the amas and the trapezoidal transoms, to me, has all to do with minimizing pitching in waves, at the expense of increased hull drag too. Don't think they're too bothered about tacking duels.

I think the rocker develops from the other parameters, like volume distribution, heeled shape, LCB etc. Don't think one bends these to obtain a desired rocker shape.... Unless you have a good reason for doing so, like a planing hull or big fast open water tri or 4k shtbx.

All we can say is wide hulls generally have flatter rockers than narrow hulls, and both are fast.

IMHO idkfa.

 

Rocker on a high performance trimaran with too little beam can be a problem because it increases the angle of heel of the boat at the instant of main hull takeoff. There is another thread about this but among boats that appeared to be high performance the range of this angle was between 13 degees and
26 degrees, if I remember correctly.
So if you want a high L/B main hull and the rocker that usually results from that you have to have enough beam to try to keep the takeoff angle as low as possible.

 

This is really interesting stuff.

When I was designing an outrigger sailing canoe, I put the maximum point of rocker well back. I noticed the rocker is distributed the same in Gary Dierking's Ulua design.



I reasoned it would help the canoe in tacking. And surprisingly, the thing tacked easily. When I tried a quick and cheap PVC ama with no rocker, it was a bear.

The more banana, the quicker tacking. You distribute rocker forward in the ama to counteract the diagonal press of sail, but the main hull of the canoe is a less pronounced opposite.

 

Yes rocker helps tacking, no questions there.

Your main hull, is a hull, it moves through the water. On a MOD70 it is reserved buoyancy for the amas, to be used when required, a pitch limiter.

There is a lot of rocker on the MOD70, so they are not designing for min angle of lift-off.

 

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It's always a compromise. The Mod 70's use curved lifting foils in the ama which then doesn't sink as much as a non-foiled ama allowing the takeoff angle to be lower than it would otherwise.
Looks like the take off angle of a Mod 70 is around 13 degrees which is the same as USA 17-which is good.

 

Traditional Grand banks dory have from 2 to 4 inches of rocker, Traditional St. Pierre ocean dory has much more as do the dorys used for river running. The 2 to 4 inches of the grand banks has evolved to better rowing characteristics. The St. Pierre and river dorys allow faster directional changes (turning). As with many characteristics of boat design alot is the trial and error of the years which of course translates into basic design rules. If you do this the result will be a little more of this in her handling until enough info is gathered to develop formulae. Basic design rules and a hammer is my comforth level, used up all my math years ago--

 

BMW Oracle used a curved rocker that would have satisfied the requirements of tacking and min lift-off. The MOD70 uses straight lines cause they are primarily concerned with limiting pitching/burying of the amas in waves. The intention is that the vaka is always flying until it slams on to the surface of the water limiting pitching motion, the BO rocker would not be so sudden/forceful. BO would have been less concerned with waves and more with tacking and drag while it was in the water.

 

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idkfa, The Mod70 main hull has fairly substantial rocker as does USA 17-I don't see straight lines-except a substantially smaller rocker on the amas of both boats even though they carry the same weight. Explanation: the ama foils lift a high percentage of the boats weight and because of that facillitate quicker tacking than a boat with an ama that supports the weight of the boat primarily by buoyancy.

http://www.multionedesign.com/en/ good pictures showing main hull rocker

Thread in multihulls-showing that MOD 70 is designed to have the curved foil support 70% of the boats weight when flying the main hull-see interview. Therefore the ama 30% is primarily for pitch control resulting in straighter lines and very much less rocker than the main hull, as you said idkfa.
http://www.boatdesign.net/forums/multihulls/multihull-one-design-orma-70-a-32514.html

 

I usually put the rocker clear of the wall and lampstand and close enough to the fire to keep warm but not singe my socks...

 

Well, guess it's all a matter of personal preferences, but I definitely wouldn't let a rocker dwell in my house, try to keep me warm and mess with my socks...

 

What ever you do keep the double barrel scatter gun and the corn cob within arms reach--that sip of white lightning doesn't allow easy egress from a seat that has barrel staves nailed to a kitchen chair--