Hollow Sections aft on ULTIME Trimaran Amas

I curious about the rationale for the hollow "rocker" on these two trimaran amas:
CLARIFICATION: In the title I said "hollow sections" aft when what I meant was hollow rocker---sorry......


Banque Populaire


Gitana 17

 

Nothing new about this: Paul Whiting did hollow after sections in the late 1970s on Magic Bus, Newspaper Taxi and Smackwater Jack, in fact a number of New Zealand designers did the same on their planing IOR boats of that period. Okay, may have been for rule gains and other compromises but still, hollow is hollow.

 

The Multi50 trimarans Arkema and to a lesser extent the former Actual (now named Reaute Chocolat) have gone down a similar path with their ama hull forms, seems to work well from the results they have achieved.

 

While they may look hollow the sight lines and various curves of the hard turn in the topsides can be deceptive, I would suspect they are just straight rather than slightly hollow.

 

Designed to work on the wave at speed as a planing/semi planing hull form.

 

Adds hull depth in way of the main beam and foils. So makes the hull much stronger where the loads are higher. I suspect the SAC is still quite smooth as the hull becomes chined aft so the WSA increase relative o a rounder hull

Lots of designers have added bustle aft on multihulls, even I have. So too did Lock Crowther



Richard Woods

 

Hollow in after sections profile, not multihull top sides strakes and chines viewed from ahead or astern. Here is hollow stern profile on Laurie Davidson's Fun.

 

These shapes were first originated / tested as Serie 64 High speed semi displacement hulls at David Taylor Basin...........

 

Design of 16' foot beach catamaran https://www.boatdesign.net/threads/design-of-16-foot-beach-catamaran.58985/page-2#post-811762
Please: Posting 16 - 28

 

As reorted by a famous beach cat navan architect who was involved in the C-Cat program Groupama C

When asked : Why thekeel line of your C -Cat is flat at the bow and at the stern ?

This september 2013 answer was :
The flat at the bow aims to minimize the downward sucking forces of the waves train in the floating mode and create some little dynamic lift to help offsetting this downward force.

The flat / concave / hollow keel line at the stern was supposed to minimize wetted area .

Regards

Erwan

 

Not sure you can really compare the hull of a catamaran sailed in flat water inshore with the outrigger of an ocean going trimaran. Inshore boats tend to have more rounded/flatter forward sections, offshore boats tend to be more V'eed. A trimaran main hull tends to have a lot of rocker, an outrigger tends to have a very flat run these days and most now have square sections aft

The old IOR boats were partly drawn that way for rating advantage and to plane, something multihulls don't do

Richard Woods

 

What does SAC mean?
How about CSA?
Is there any online nautical abbreviation website someone can give me a link?
Should there be one in this site?

 

It would be interesting to know, in addition to "a trimaran main hull tends to have ....., an outrigger tends to have......", the why of those "tends to". Would someone be so kind to give explanations about it?. Something similar to what Erwan explains in his post # 10, which is very interesting. Thank you

 

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From Guillaume Verdier on Gitana 17:
Flat-bottomed hull and floats: As is the case on the America's Cup craft, the Maxi Edmond de Rothschild is equipped with three planing hulls with inverted bows and flat bottoms, so as to increase the ability to fly and provide excellent stability in heavy seas. The central hull and floats have a high freeboard for improved protection at sea.

 

There are still no technical reasons that explain the "why" of things. In any case, those foilers start flying very quickly so I do not think flat bottoms, high freeboards, etc. improve the conditions of their flights. The "improvements" in the hull seem more appropriate while the ship does not fly. I think that in this, in addition to showing amazement for everything that seems new (and in many cases it is not), would require some technical explanation at the elementary level.