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Old 16-04-2016, 16:01   #1
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HP required to motor at 10 kts?????

Need help with decision please... I have a 48 foot aluminum sailing catamaran with daggerboards.. Max beam at water line is 5 ft, max length at water line is 45 ft. Currently have two yanmar 48hp engines with 17 in fixed 3 bladed props. I prefer to motor with one engine. I motor at 6 kts @2500 RPM on one engine and 7.5 kts with both engines. I would really like to motor at 10 kts. How would I determine the engine HP and sail drive and prop requirements to achieve this. And is it possible with one engine? I have an 8KW genny on the SB side, so was wondering if I added a larger engine on the Port side (to balance the weight) what type of speed I could expect. I do understand that any inputs would only be approximate, but I do not know where to look for this esoteric info. Any help would be GREATLY appreciated.
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Old 16-04-2016, 17:40   #2
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Re: HP required to motor at 10 kts?????

I would probably give Richard Woods a call or email Sailing Catamarans and ask him to run the numbers. My guess is that you would need something north of 100hp to get you to that speed, but that's just a WAG. Richard had designed both sailing and power cats and could provide a calculated number.
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Old 16-04-2016, 17:46   #3
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Re: HP required to motor at 10 kts?????

How fast does it motor with the current motors at full throttle?
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Old 16-04-2016, 20:04   #4
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Re: HP required to motor at 10 kts?????

The real problem with motoring that fast with one motor is that the boat is trying to turn constantly and being held on course with the rudders. It's a heck of a lot of unneeded stress for whatever your perceiving the need to motor that fast is. I'd suggest you find some other issue to focus your attention on.
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Old 17-04-2016, 06:22   #5
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Re: HP required to motor at 10 kts?????

Greetings and welcome aboard the CF, anyborg.
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Old 17-04-2016, 06:51   #6
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Re: HP required to motor at 10 kts?????

Um... hull speed for a single hull 49 feet long is about 9.5 knots...
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Old 17-04-2016, 07:04   #7
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Re: HP required to motor at 10 kts?????

The standard formula for hell speed doesn't apply very well to most catamarans. Hull shape and weight play a big part in the maximum speed achievable.

It is tough to say what power is required without a lot more information.
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Old 17-04-2016, 07:07   #8
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Re: HP required to motor at 10 kts?????

two 800 hp engines
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Old 17-04-2016, 08:36   #9
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Re: HP required to motor at 10 kts?????

Naw, you don't need 800's, a couple of these big dogs will get 'er done.

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Old 17-04-2016, 08:41   #10
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Re: HP required to motor at 10 kts?????

Buy a power boat!
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Old 17-04-2016, 13:51   #11
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Re: HP required to motor at 10 kts?????

If that is what you get with 2 48s I doubt you would make ten with one engine crabbing you along. If it ain't broke don't fix it.
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Old 17-04-2016, 13:59   #12
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Re: HP required to motor at 10 kts?????

It's a complex problem.

You may be running out of rpms before you run out of torque currently.

You should start by pitching up one of the current props and seeing what happens.

From there, you will start to get a better understanding of top end.

Realize your low speed handling (bollard pull) will suffer as you pitch up.

As a data point, a single 30hp outboard pushes my very similar boat at 7.2 knots. However, I'm pitched low and could probably eek out another knot or 2 if I had a higher pitch. Not messing with any of that until the boat is 100% done though.
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Old 17-04-2016, 16:08   #13
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Re: HP required to motor at 10 kts?????

http://www.kelsall.com/TechnicalArti...llFormulas.pdf

The figures from the above formulae are particularly accurate for making changes - provided your hull shape does not begin to dig in the transom as the power is increased.

In my experience, driving on one engine does not present a problem unless maneuvering

Happy boating,

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Old 17-04-2016, 19:24   #14
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Re: HP required to motor at 10 kts?????

Quote:
Originally Posted by Kokanee View Post
The standard formula for hell speed doesn't apply very well to most catamarans. Hull shape and weight play a big part in the maximum speed achievable.

It is tough to say what power is required without a lot more information.
Information required:
You say you beam is 5 feet. I assume that is the beam of each hull.
What is the pitch of your 17 inch diameter props?
Draft & weight of boat are also required.
And your gear ratio.


It is possible, with your long narrow hulls, that your existing engines may achieve 10 knots with the right prop, assuming the boat is light enough.
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Old 18-04-2016, 08:35   #15
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Re: HP required to motor at 10 kts?????

Put up your sails, you should be able to sail at more than 10 knots
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