French term (edited): 2 lignes d’arbres supra-divergentes

a pair of propellers on a boat will always be counter-rotating, to do otherwise is simply not an option (or more precisely an extremely bad option).

The additional meaning given by "supra-divergentes" is about which way these two propellers are rotating. They are rotating in opposite directions - one clockwise the other one anti-clockwise - sure, but that still leaves two options.
"supra-divergentes" means the top of each propeller is rotating away from the ship.
Couldn't find the term in EN for that, thus CL only 3 (but CL5 regarding what "lignes d’arbres supra-divergentes" means)

"Hélices multiples

Pour diverses raisons et, notamment, pour tenir compte des limites de résistance des matériaux, il est nécessaire de ne pas imprimer à une seule ligne d’arbres une trop forte poussée. Dans le cas de navires rapides dotés d’un puissant appareil propulsif (paquebots par exemple), on est donc conduit à répartir la puissance entre deux ou quatre lignes d’arbres (exceptionnellement trois). Malgré une perte de rendement, cette formule donne certaines facilités : possibilité d’une allure réduite avec deux lignes d’arbres au lieu de quatre, pour un rendement optimal de l’appareil moteur, poursuite du voyage avec un ou deux appareils propulsifs en panne, souplesse de manœuvre en faisant battre en avant les hélices d’un bord et en arrière celles de l’autre, etc.

En marche normale, la rotation en sens inverse de deux hélices jumelles évite la dissymétrie de l’écoulement des filets d’eau qui, dans le cas de l’hélice unique, oblige à donner un certain angle à la barre pour compenser leur action transversale.

Si deux hélices jumelles tournent vers l’intérieur du navire, elles sont supra-convergentes, et,

si elles tournent vers l’extérieur, elles sont supra-divergentes.

Cette dernière disposition assure une meilleure maniabilité et a, généralement, la faveur des constructeurs."

[http://www.larousse.fr/archives/grande-encyclopedie/page/653...]

http://en.wikipedia.org/wiki/Counter-rotating_propellers

--------------------------------------------------
Note added at 36 mins (2013-05-01 10:08:44 GMT)
--------------------------------------------------

According to this Wiki article, the convention for counter-roatating aircraft propellers is supra-convergent, but it notes that the reverse configuration has also been used:

"Counter-rotating propellers generally spin clockwise on the left engine and counter-clockwise on the right. The advantage of such designs is that counter-rotating propellers balance the effects of torque and p-factor, eliminating the problem of the critical engine.

In designing the Lockheed P-38 Lightning, the decision was made to reverse the counter-rotation such that the "tops" of the propeller arcs move outwards, away from each other. Tests on the initial XP-38 prototype demonstrated greater accuracy in gunnery with the unusual configuration. The counter-rotating powerplants of the German World War II Henschel Hs 129 ground attack aircraft, Heinkel He 177 heavy bomber and Messerschmitt Me 323 transport used the same rotational "sense" as the production P-38 did."

--------------------------------------------------
Note added at 7 hrs (2013-05-01 16:51:48 GMT)
--------------------------------------------------

counter-rotating, such that the "tops" of the propeller arcs move outwards, away from each other

seems to be about as succinct as you can make it to me, if we have to get into explanations.

Alternatively, one might refer to 'inverse counter-rotating'; but the danger with that is that if the reader doesn't know which direction is 'normal', then 'inverse' won't mean anything anyway.

--------------------------------------------------
Note added at 8 hrs (2013-05-01 17:32:59 GMT)
--------------------------------------------------

Right, Michael's 'counter-rotating outwards' certainly seems concise — though is not actually unambiguous, as it presupposes we are all agreed that it is the TOP of the propellers that is moving outwards.

See all my discussion points and references below.
"Counter-rotating" can be in either of 2 directions (as Daryo has pointed out, and been shot down for his trouble), and "supra-divergent" means "counter-rotating outwards".

--------------------------------------------------
Note added at 8 hrs (2013-05-01 18:13:31 GMT)
--------------------------------------------------

For those who have not read - or don't have access to - the Discussion above, here is a definition, which seems clear enough to me, from http://www.modelflying.co.uk/forums/postings.asp?th=79157

"both rotating "inwards" - ie the top of the blade arc swings towards the fuselage"
Accordingly, "rotating outwards" means the top of the blade arc swings away from the fuselage - which complies with the FR definition of "supra-divergent".
I don't believe that leaves any room for ambiguity.

Ce sont des arbres moteurs qui transmettent le mouvement de rotation à d'autres pieces tels que des hélices . Dans ce cas elle sont divergentes , c'est a dire qu'elles tournent vers l'exterieur.