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Old 20-03-2008, 16:25   #136
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Alan, my point is that 44'cruisingcat has cited no basis for his statement that it doesn't matter whether or not props are in front of the rudders. It has been tried in the past, and I have read many warnings not to do it. I would agree that it wouldn't be as bad to do this in a boat with two widely separated propellers, because you can to some extent maneuver with your engines, but I don't see any basis for 44'cruisingcat's assertion other that his love of taking argumentative positions and escalating them to the point of rudeness and even profanity. Articles I have read show the use of engines and rudder positions simultaneously in maneuvering catamarans next to docks, including an interesting trick involving moving straight sideways by using your rudders and engines simultaneously. This clearly wouldn't work as well if the rudders weren't interacting with the props' stream.
Most of the newer designs from Lagoon and Fountaine Pajot actually now have the engines aft of the rudders, to optimise cabin space,so probably a few hundred of those sailing around.

Seen from a balance/pitching/hobbyhorsing point of view, then this far aft engine position is not optimal, but the transoms on most of those boats are probaly over 120 cms on a 44 footer, so probably not the issue it would be on a performance type hull.

With the wide stance of twin engined cats, I don't think that the position of the props is as critical as on a monohull, as they use the "prop-walk" to their advantage,more so than on a cat.

Moving a cat sideways on to a dock can be done.The reason you probably would need to do this is a wind blowing you off. On a normal cruising cat, the wind forces will be very difficult to overcome unless you have a decent current flowing. (at least 2-3 knots) At least I haven't been able to do it with my cat in over 10 knots of wind. (where the props are forward of the rudders.) I use a spring line and use the engines to get me in.

Regards

Alan
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Old 20-03-2008, 16:53   #137
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Alan, my point is that 44'cruisingcat has cited no basis for his statement that it doesn't matter whether or not props are in front of the rudders.
I'd like you to show me where I made that "statement".(hint: you're going to struggle.) You have to put words in other people's mouths in order to win an argument. And even then you have failed, because you are wrong.

BTW I'm not saying there would be no advantage to having rudders directly behind the props - just that it isn't absolutely essential, and that boats can motor effectively on one engine and maneouver very well using two engines without that "essential".

As for it not working on big boats, at least a dozen times every day at our local marina, large passenger caramarans (from 80 up to 250 passenger capacity) are docked without using their rudders. Most of them have external "steering" stations on the bridge wings, used for docking. The only controls there are throttle/shift controls. No rudder controls at all. They pirrouette through 180 degrees and reverse into their berths just on their propellors. They do this in a wide range of wind conditions, up to at least 35 knots. And before you ask, many of them, including the biggest, don't have bow or stern thrusters.
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Old 20-03-2008, 17:12   #138
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"The only controls there are throttle/shift controls. No rudder controls at all. They pirrouette (sic) through 180 degrees and reverse into their berths just on their propellors." The stations I have seen like that have joysticks for controlling the rudders. Also, you can get autopilot controls that aren't even permanently installed. A lot of these commercial power cats have jets rather than rudders and props, and are steered by directing the jets from side to side. See: http://www.allamericanmarine.com/ima...m_exterior.jpg
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Old 20-03-2008, 19:26   #139
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This is true. But these boats don't. Not all of them anyway. The biggest and oldest of them doesn't - there are simply a pair of throttle/shift levers on the bridge wings.

Regardless of that, the facts remain - you CAN motor on one engine even if your rudders aren't directly behind your props, and you CAN maneouver into a berth.

Oh, and what about that "statement" I supposedly made?
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Old 20-03-2008, 22:58   #140
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I am not discussing outboard power cats, as outboards can be turned, and therefore do not have the same issues as inboards.
My last 2 cats had single UNSTEERABLE outboards.

I had no real issues with manouverability.

I certainly dont plan on using the rudders to park the new boat, centralise rudder and use engines me thinks.

Dave
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Old 20-03-2008, 23:15   #141
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Most of the newer designs from Lagoon and Fountaine Pajot actually now have the engines aft of the rudders, to optimise cabin space,so probably a few hundred of those sailing around.


Alan
Please tell me they face in reverse with the shaft going forward to another box shooting the shaft back.
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Old 20-03-2008, 23:45   #142
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Hi, Alan - Are you getting your info online about the new French cats? If so, where? Do they have 'v' drives?
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Old 21-03-2008, 00:22   #143
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Please tell me they face in reverse with the shaft going forward to another box shooting the shaft back.
Nuh
They have saildrives mounted aft of the rudder. There was a picture here but cant remember where. I was surprised to say the least when I saw it
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Old 21-03-2008, 04:15   #144
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Please tell me they face in reverse with the shaft going forward to another box shooting the shaft back.

Nope,

Bog standard saildrive setup, with the whole sheebang AFT of the rudders. I haven't got a picture, but have seen it at the FP factory as well as at a couple of boatshows.

Sailed on a Lavezzi, and it was no problem in close quarters as long as you used a bit more throttle than normal. You just have to get used to being a bit more agressive than otherwise. Suprisingly not a major difference in handling. I wouldn't reccomend a 2 bladed folding prop for these, go for a 3 bladed, you need the extra bite in reverse.

regards

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Old 21-03-2008, 13:20   #145
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Prop location

Well, one actual fact is worth a thousand theories-
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Old 21-03-2008, 16:29   #146
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And yet you refuse to admit your theory:

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For fuel economy, catamarans sometimes motor with just one motor. I don't think you could do that if you placed the props elsewhere.
Is wrong.

Even in the face of HUNDREDS of facts - in the form of the HUNDREDS of catamarans with propellors not mounted directly in front of rudders, which CAN motor on one engine, and do so successfully every day. There are very likely boats out there at this very moment proving you wrong.

And still you refuse to admit it.
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Old 21-03-2008, 16:40   #147
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Guys,

my original question was about the underwater lines for a performance cruiser.

I haven't finalised the underwater lines, but will be targeting a Prismatic coefficient of 0.62.. After that, we will run the CFD/Michlet to see how we can improve them.

I have just about finished on the layout, so thought I would show you, and hope to get some feedback and questions.

LOA is 14.9m///49 ft.
Max beam is 8.60m/28'2"
Draught is. 0.52m/1'9"
Weight is around 7200 kgs lightship
2 x 39 hp Yanmars with saildrives

Regards

Alan
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Old 22-03-2008, 20:14   #148
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Looks good Alan! Very nice lines. Inquiring minds need to know - will the saildrives be in-line with the rudders? In front or behind?
Seriously now - is this for personal use only, or are you planning on taking over the cruising cat market?

Kevin
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Old 22-03-2008, 20:51   #149
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Is it safe to come back into the conversation, yet?

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"I don't think it is absolutely necessary to locate the props right in front of the rudders" Hi, Kevin- For fuel economy, catamarans sometimes motor with just one motor. I don't think you could do that if you placed the props elsewhere.
I disagree - being in the slipstream of a prop puts assymetric thrust on the rudder - much like it does in single-engine prop planes. This requires helm or 'trim' to counteract the influence on the rudder. Large ships have their port and stbd engines turning in opposite directions to nullify this effect. Unless your rudders are independent, you'll have both rudders 'trimmed' to counter one rudder's slipstream - the other rudder (which is slightly turned, but not in a slipstream) will just be adding drag.

Quote:
Then there is the question at maneuvering at dock. This would work better on a catamaran than on a monohull, because of the wider separation of the engines-so you can do some maneuvering by accelerating one motor more than another, or even by reversing one engine and keeping the other in forward, but you are still going to lose some maneuverability in close quarters if your props aren't fairly close in front of your rudders.
Can't say for certain with cat's as I don't have enough experience manoeuvring them with different rudder/engine arrangements, yet. But I have done a lot of shiphandling with very big ships - some had twin screws in front of twin rudders, some had twin screws with a single rudder between the slipstreams. Although they had some characteristic differences, both types were equally manoeuvrable. While I agree that the rudder can be effectively used to 'vector' the thrust off a prop, it is generally of little use when opposing the engines, and in fact can have unpredictable results. Have used some other arrangements too, but doubt that single screw, kort nozzle, z-drive or voight-schneider have much bearing on the discussion.

Kevin
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Old 23-03-2008, 00:21   #150
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Guys,

my original question was about the underwater lines for a performance cruiser.

I haven't finalised the underwater lines, but will be targeting a Prismatic coefficient of 0.62.. After that, we will run the CFD/Michlet to see how we can improve them.

I have just about finished on the layout, so thought I would show you, and hope to get some feedback and questions.

LOA is 14.9m///49 ft.
Max beam is 8.60m/28'2"
Draught is. 0.52m/1'9"
Weight is around 7200 kgs lightship
2 x 39 hp Yanmars with saildrives

Regards

Alan
Well, the placement of the rudders/props is kind of related to the underwater lines, isn't it?

The boat looks good, I was wondering why you have so much rear overhang, assuming the waterline shown is at max displacement. (Is it?)

Seems like a waste of potential waterline length, for no real gain.

What kind of materials would you build from? Are you actually thinking of building this, or is the design a goal in itself?
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