HP required to motor at 10 kts?????

[anyborg]

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.

[Stumble]

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.

[44'cruisingcat]

How fast does it motor with the current motors at full throttle?

[Palarran]

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.

[GordMay]

Greetings and welcome aboard the CF, anyborg.

[jimbunyard]

Um... hull speed for a single hull 49 feet long is about 9.5 knots...

[Kokanee]

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.

[geoleo]

two 800 hp engines

[donradcliffe]

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

https://www.youtube.com/watch?v=LOP9TYfjNhQ

[philip.cat]

Buy a power boat!

[Cadence]

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.

[cruisersfarm]

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.

[DerekKelsall]

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,

Derek..

[Kokanee]

Originally Posted by Kokanee

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.

[805gregg]

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