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Old 23-08-2006, 12:37   #1
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Inboard Propellers?

Whenever I mention "Inboard Propellers" to friends with catamarans, their eyes seem to glaze over and they start looking off toward the horizon!

Boats with inboard propellers - the stbd prop turns left and port turns right when going ahead - have some big advantages over the more common "outboard propeller" set-ups and I believe would provide a great advantage on catamarans

The last big boat I drove (95 ft) was set-up this way. After the innitial learning curve, I came to appreciate the superior handling characteristics the inboard configuration gave the vessel.

Essentially - I could "walk" the vessel sideways which enabled me to park the boat in places I'd previously steer away from.

Tight quarters? No Problem.
Wind beam on or off the dock? No Problem.
Adverse current? No Problem.

The trick was to lock the rudder(s) hard over to push the stern in the direction you wanted to go - and use the prop walk of the other (reversing) prop to control the angle of approach. Primary control was in the throttles with no need to turn the rudder(s) at all.

It provides a similar effect of a bow thruster and I could have driven the boat sideways all the way across the harbour, if need be.

Offshore oil supply & crew boats are commonly set-up this way because it affords far better control at slow speeds and more thrust at cruising speeds.

The only draw-back I found was that the vessel will no longer rotate on her axis, as expected, with one engine in forward and the other in reverse... which, IMHO, is a small loss compaired to the major gain in safe docking control.

I'm just curious why catamaran owners / builders never seem to set their propulsion systems in this beneficial configuration.

Kirk
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Old 23-08-2006, 14:46   #2
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Correct me if I'm wrong, but I do believe that twin OB and sail drives do twist opposite directions. And yes, twin screws do have a great advantage, especially on Cats.

I think the problem with Cats and inboard props is space for machinery. It takes a large cat (say over 50') to have enough space to get around the engine and equipment. Where as, a sail drive or outboard is like a lawn mower setup and EZ to work on from above. And doesn't take up extra cabin space.

Another thing with Cats is they are sometimes designed to get real close in the shallows and even beach them at times. A inboard prop would get damaged unless it had a skeg of some kind IMHO...................._/)
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Old 23-08-2006, 19:12   #3
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Maybe I'm not explaining clearly...

I'm not talking about inboard or outboard or inboard / outboard engine installations. This is specifically about which direction counter-rotating propellers turn in relation to each-other when going forward.

If you walk around any boatyard you'll notice that about 99.9% of all twin screw, counter-rotating propulsion systems are set-up to turn "outboard" when going forward.

As viewed from behind the boat, when going forward, the stbd prop turns clockwise (right) and the port prop turns counter-clockwise (left) and is referred to as "outboard turning props".

"Inboard turning props" turn, as viewed from behind the boat, when going forward, the stbd prop turns counter-clockwise (left) and the port prop turns clockwise (right).

In other words... the tops of the props are turning toward each-other (inboard) instead of away from each-other, when going forward.

I don't own a sailing catamaran but I am a working relief captain on three different ones (52 - 60 ft) doing day trips & sunsets around St Thomas & St John... all if which have outboard turning props. They turn on a dime but will not "walk" when docking. I've worked on only two vessels in my career which were set-up with inboard turning props (one was a power cat) and I remain amaized at the superior handling characteristics they had simply because their propulsion systems were set-up a little differently.

All I'm trying to say is this is something my twin-hulled brothers & sisters should consider. It would require very little cost or effort to simply change the props to the opposite engines and fiddle with the shifter linkage. Once you've gotten the feel of it you'll certainly be impressed with the difference in ease of handling.

And that's all I have to say about that.

Kirk
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Old 23-08-2006, 21:51   #4
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Lightbulb Never mind

Please excuse my ignorance. I guess I should read stuff over better before posting.

But to get back to the point! From what I understand the reason behind having the props turn inboard is because it gives the vessel more forward thrust.

I could see having the outboard direction on barg tenders but when you want to gofast, inboard creates more of a jet stream. Especially while hydroplaning IMHO.........................._/)
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