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Old 07-03-2009, 14:56   #1
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Engine sizes

Does anyone know how many HP would be adequate for each engine on a twin engine, straight propeller shaft cat, around 50", suitable for ocean crossing? Should the engines be of different sizes?
I can understand that the hull shape etc also can be a factor in this, but I would just like an approximation.
Thanks in advance,
Rolf
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Old 07-03-2009, 15:01   #2
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tolly,
why not look at advertisemants for cats around 50'? Long time manufacturers have the experience for engine size. Look at the cats in the charter business. Those engines might be oversized for the business.
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Old 07-03-2009, 16:11   #3
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Both engines should be the same, and making the assumption you're talking diesel - you'll be good to go in the 50-75hp range (ea) dependent upon the fully laden capacity, and windage, of the boat. Below 50hp will generally be underpowered and over 75hp will generally be overkill. If it's an exceptionally light 50 foot performance oriented craft - then as little as 45 hp 'could' be reasonable - but for the majority of standard cruising 50 footers - the above range should apply.
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Old 08-03-2009, 14:16   #4
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Total power of 5 hp/1 ton should be enough.
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Old 08-03-2009, 14:23   #5
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At least 40hp per hull
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Old 08-03-2009, 14:56   #6
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This owner designed and owner built 44 footer shown below has a 9.9 HP Yamaha outboard. It has also singlehanded 3 times from Mexico to BC Cananda....:-)

From Chris White's Altantic 48'

Ample diesel horsepower is provided by twin Yanmar 40 HP saildrives located in separate engine compartments aft. Not only does this isolate the engine noise and smell it insures the props are deeply immersed so that they will not ventilate while motoring in rough conditions.
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Old 08-03-2009, 15:18   #7
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I know my boat, and wish I had double the hp. When the winds get up to 60mph on my nose, I know I'm not going anywhere.

The guy with 9.9 on 44' is either lucky or very..very cautious.
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Old 08-03-2009, 15:23   #8
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engine size

A rough guide line for most boats and catamarans is 1hp for every foot loa
or 4hp per ton all up
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Old 08-03-2009, 16:22   #9
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From a mono guy .... don't err on the side of too small. If you need your engines in a blow you will be glad of every extra horsey neighing under the hatches

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Old 08-03-2009, 16:46   #10
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I've gone 65hp/side on my 50 footer

BUT they are 3.3 litre slow revving with heaps of torque

Quote:
This rugged engine package is designed with the serious boater in mind.
The max 65 hp is achieved at just 2600 RPM.
At an amazingly low 1800 RPM this engine delivers almost 55 HP. Ideal for those long cruises.

http://www.tadiesels.com/cu-B3.3M.html

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Old 08-03-2009, 19:54   #11
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50hp a side is plenty. Key is swinging the biggest possible prop ie having a decent reduction gear around 3:1 - this improves the prop thrust. A 40hp engine with a 3:1 reduction willproduce similar thrust to a 50hp engine with a 2:1 reduction. The other point to remember is virtually all yacht engines suffer from being run under loaded, which long term is not good for them.
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Old 08-03-2009, 19:54   #12
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A: You think you've gone deaf.
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Old 08-03-2009, 21:39   #13
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Quote:
Originally Posted by Tropic Cat View Post
I know my boat, and wish I had double the hp. When the winds get up to 60mph on my nose, I know I'm not going anywhere.

The guy with 9.9 on 44' is either lucky or very..very cautious.


I look at his boat as sailors sail boat......you handle it much like the days of sail only. Except it is much better sailing boat and can go to weather etc. With such small aux. power, you treat the boat as if you have no engine. So you try not to be anywhere where you have to go against a 60 kt. wind. Just good seamanship, if you are looking to go against a 60 kt blow, you better hope those engines don't have any crud in the bottom of the furl tanks to get stirred up.
The idea of big chunk of weight in the sterns, and a big wheel in the water is great for a motor boat. But it ruins a true sailboat to the point that your light air performance suffers so bad, you use the engine a lot....mmmm...kind of a downward spiral. But to each his own. I also am happy with a 9.9 hp out board. It is a sailing boat....

I don't want to get thread drift as I know this guy ask for reco's on engine size. My preference is as little as possible. In early cruising books, Hiscocks recommended 1/2 HP for each ton. Now I see it is up to 4 HP per ton or 1 HP per foot. Egads man......
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Old 11-03-2009, 02:53   #14
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I was on a 40" cat with 27hp each

and just one engine would push it to crusing speed without too much effort.

too big uses too much fuel.

It is all about HP vs Hull design and intended load weight
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Old 11-03-2009, 07:41   #15
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I agree that this is very dependent upon the boat. Here's another data point:

I have a Catana 48 and just replaced a pair of 28 HP Volvo's with a pair of 35 HP Beta diesels because one of the volvos died. I have gone from feeling a little underpowered to feeling like I have plenty of power. I changed the standard 2 blade props to feathering props at the same time, so it's hard to tell how much difference was due to changing the engines and how much the props. I think the 35 hp was well worth it, but have not so far felt any need for any extra power. I have not yet had to power in to really strong winds, but I have more than enough power to get up to "hull speed" and push beyond (with rapidly diminishing increases in speed for increasing power).

The Catana 48 is relatively light at around 22,000 lbs for a 48' boat, with relatively narrow hulls, but is a cruising boat not a racing boat.

Mark.
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