Shaft or Outdrive...this will interest Victor

Nautorius

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Hi All,

As the weather so so bad over the weekend I spent most of my time trawling boat sites. I got a sudden urge to try and track down all the manufacturers of Powered Catamarans and realised that different manufacturers have different ideas on the best propulsion. I was focusing on small cruising planning Catamarans 24-30ft and found some with Outboards (Motorcat 30), some with Shaft (Excitecat 8.08) and some with Outdrives (Leisurecat).

So my question is this. If you were specing a 28ft x 13ft Planning Catamaran for cruising, Weight 3500KG what propulsion would you stick on the back.

Outboard 2x150hp 4 stroke, Inboard Volvo D3's 160hp on shafts (or similar) or twin Volvo D3 160hp outdrive with SX or DP Drive.

Boat is to be used in our kind of climate, would aim to cruise at 18-22kts and with all engines top speed would be about 28/30kts.

So come on and help me out, as it is bugging me!

Cheers

Paul /forums/images/graemlins/smile.gif
 

Nautorius

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Nope,

Non of that nonsense! (It would be too expensive in this case)

So I think it is between Props and Shafts or Outdrive with SX or DP. The point is what are the major differences for a Catamaran (8m by 4m) and the boat is cruising not all out sports!

Cheers

Paul /forums/images/graemlins/smile.gif
 

hobiecat

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I am no naval architect and based on my experience. First issue is whether you really want a planing catamaran or a displacement catamaran. The displacment catamaran is not slowed by its own bow-wave (as there is none to speak of) - it is mainly slowed by frictional resistance. It means you can have small engines as it is easy to push a long slim sharp hull through the water rather than a fat blunt one. Outboards are great - but everyone seems to want huge outboard engines. They do not understand that they may do nothing and can even be counter productive. Displacement cats best cruising speed seems to be around 16-18 knots and top speed say around 25 no matter what the size of cat. If you go displacement then weight is not so much of an issue so diesel engine boys are happy. Outdrives are best - if you want to debate this in relation to cats I am happy to do so.

If you go for a planing cat then you will need massive engines to get on the plane (Steve Curtis style) as there is not much planing surface area. Power to weight ratio is therefore important to get the boat to get out of the water and fly. Best get the big petrol outboards or inboards out and drink fuel by the gallon until you fly - then planing cats can be absolute screamers and actually very efficient at travelling very fast. May sometimes suit fisherman to get to fishing grounds and fish on a stable surface.

Of course you still have everything in between on a cat just like a mono ie. the spectrum between displacement and planing (semi displacement or semi planing)

Hope this helps and not too contentious. Can you help me with my dog question which I will post in a minute???
 

Bajansailor

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Following on re displacement powercats, I designed (and helped to build) an aluminium power cat here to be used as a water taxi - this is the cat in my avatar photo, and I have posted some photos of her previously.

She is 15m long (49'), a tad under 5m (16') wide, has very skinny displacement hulls (L/B ratio on the water line of about 15), has 2 x 115 hp 4 stroke Yamaha O/B motors, and she can manage a cruising speed of about 22 knots light and 13 knots when loaded with 40 passengers. Even loaded she can achieve just under 2 mpg.
Which is not too bad in comparison to monohulls, but is crap when compared to eg the Motorcat 30 - have a look at the figures for mpg for the Motorcat 30 on Victor's website - they are very impressive!
 

Nautorius

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Great Victor,

Just what I was looking for. So if the hulls were 8m long by 4ft wide (full) and at water level 7.5m long by 2m with a piercing bow would these be efficient? Also are Outdrives a better option than shaft on these things (weight 3500kg loaded)

Cheers

Paul /forums/images/graemlins/smile.gif
 

hobiecat

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length:beam ratio of around 12:1 on each hull seems popular on an efficient cat and gives a bit of accommodation. Your cat hull seems pretty fat to me so probably not very efficient. Some cats are shocking and worse than monos!!!!!! as you are trying to push two fat monos through the water - imagine the wash!!. For catamaran purchase the rules turn upside down compared to a mono. Look for small engines and light weight/composite construction for best cats. Outdrives are more efficient, better steering, go backwards better, and simpler to set up (rudders on each side can be difficult) and they can be raised if you are drying out.
 

duncan

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also add that DFI 2 stroke OBs may be better than 4 strokes as they (tend to be) lighter and with higher torque through the rev range - hence better suited to displacement cats.

I believe.............. /forums/images/graemlins/cool.gif
 

hobiecat

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Ok - so I would go outdrive or outboards - forget shafts. To decide whether inboards is better than outboards is a matter of weight differential between the two. Are hulls designed to carry the extra weight? If it really is a planing cat you are talking about then the added weight will make it more difficult to fly. If the cat is light composite it will help. 3500 kg sounds fairly reasonable
 

Nautorius

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Ok So if Cat was Light Composite and weighed bugger all then Outdrives with diesel inboards would work a treat! Originally planned for TAMD 120 (shaft) so D3's on an outdrive would be a lot lighter!

Thanks, I now understand what I am reading better!

Cheers

Paul /forums/images/graemlins/smile.gif
 
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