propellers and rudders...what's first?

[D&D]

Went on board a new Lagoon 400 today and noticed the propellers peering ominously up at me from JUST under the aft-most point of the stern, obviously aft of the rudders. I recalled seeing the same setup on the FP Mahe awhile back and thinking, "That's odd...I wonder why they're doing that. I assume FP must have a reason..." Now it seems Lagoon are going that way too.

My instincts (and some training as well) tell me that the rudders should perform better underway aft of the propellers with the propellers accellerating the movement of the water past the rudders...and it's hard to imagine the rudders being at all effective under sternway with the propeller aft and driving water into the back of the rudders? I suppose the idea is to keep the rudders straight when making sternway and steer with the engines, but we all know engines can fail...and those rudders can be useful! Also from the immediate proximity of the 400's propellers today it was easy to imagine entanglement with just about anything that happened to hang off the stern.

Can anyone tell me why we're seeing propellers aft of the rudders? ...or perhaps share some experiences with this 'new' (in my experience anyway) configuration?

Cheers,
Don :)

[FSMike]

Don -
I assume it has something to do with engine location as regards use of space, weight distribution, etc. As far as function goes, I'm with you - I think it's nuts.

[mikereed100]

It would not be a deal breaker for me. From a sailing perspective the rudders will see less turbulent flow and will likely perform better. On a mono it would make maneuvering under power difficult, but much less so in a cat where the engines are so widely spaced.

I would be more concerned with all that weight in the stern, but I assume the hulls have been widened to accomodate this.

Mike

[LoneStar]

I test sailed the 400 in Annapolis this year and found it to surprisingly responsive compared to both the 380 and the 410. I also assume it is for space utilization, but I don't think it would be a deal breaker.

[D&D]

From a sailing perspective the rudders will see less turbulent flow and will likely perform better. On a mono it would make maneuvering under power difficult, but much less so in a cat where the engines are so widely spaced.

Thanks everyone

Yes Mike, you'd be right about the rudders' efficiencies under sail, although of course folding props are there to minimise the propellers' turbulence.

Sternway is where I would see a very big maneuvering disadvantage, offset to a degree by the widely-spaced twin screws but with the added concern to ensure those rudders are very straight before making sternway. As I said ,however, I have no first-hand experience to support that view. Anyone...?

The wieght so far aft and the exposed position of the props (although I guess this is no worse than many large commercial vessels?) remain as potential concerns, along with sternway maneuvering in my view....and these concerns are offset by the added internal space gained by moving the engines further aft and some (?) better maneuvering under sail. A fair summary so far...?

Any marine engineers or surveyors out there care to comment? I can't help but feel we're missing something...

:confused:

[paulrack]

My concern would be mooring stern too against a wall like you do in the Med. I noticed that in Croatia a number of the walls angled down and were not completely vertical. In that scenario you could risk srtiking the prop, also if you drop the stern line there is a much bigger chance of it getting caught in the prop. I guess it depends on where you moor. I would love to hear your impressions of the L400.

[D&D]

you could risk srtiking the prop, also if you drop the stern line there is a much bigger chance of it getting caught in the prop.

Just so Paul. Of course you could say the same about the risk of striking a rudder well aft, but I would prefer bumping a rudder rather than a propeller. An out-of-line propeller on a cat means one engine maneuvering...NOT a good thing on a cat...whereas maneuvering is still quite possible with two engines and a damaged rudder.

Catching the stern line (or virtually any line that happens off the boat, or even a line passing the boat, either underway or from another vessel underway...it shouldn't happen of course, but it does!) seems almost inevitable with the props so immediately under the aftmost point of the stern.

Maybe not a deal breaker, but on my instincts and from what I've heard so far, I'll very much prefer my props forward of the rudders.

Don:)

[3Psuite]

I spoke with a guy who's cat had a similar arrangement props behind the rudder (can not remember how far to the rear), He thought it was great as he had never had his props foul as the rudders pushed any line out of the way.

It does look counter intuitive but a lot of the French boats are made that way so I guess there are not any big or very common problems arising