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Old 11-08-2016, 05:56   #1
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Propulsion question

Hi all, we have just bought a 38' James Wharram cat. The previous owners have installed 2 x 25hp four stroke Yamaha outboards but are unable to tell us any range of cruising speed to assist us in planning our maiden voyage home. Can anyone help with a rough estimate of what speeds we can expect? TIA
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Old 11-08-2016, 06:18   #2
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Re: Propulsion question

Greetings and welcome aboard the CF, Lululund.
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Old 11-08-2016, 06:30   #3
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Re: Propulsion question

Hull speed will be 1.34 x the square root of waterline length in feet.
My IP 38 for example has a 33 ft waterline length, so my hull speed works out to 7.7 kts. that is pretty much the max speed I could go, above that drag just goes crazy, takes huge amounts of power to exceed hull speed by much.
I have found on my boat that about a kt off of hull speed gives me a good economical cruise, that last kt increases fuel consumption by about 50% for me.
So find your hull speed, back off of that a bit and that should be cruise speed, then you have to cruise for a good long time and then re-fill your tanks to see what your fuel burn is.
I'm going to throw out a guess of about 3/4 to 1 gl an hour per engine, and especially if you can tilt an engine out of the water it's likely that running one at a time will be your best MPG.
But I am not Cat owner so I can't speak from experience, but if nothing else cutting the running hours on my engines in half would be attractive to me.


Let me edit this a bit before others chime in.
Being a Cat due to your narrow hulls, I do not believe you are as constrained to hull speed as my fat ole mono hull is, but I think it is still a viable number for you to determine an economical cruise speed when motoring
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Old 11-08-2016, 16:48   #4
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Re: Propulsion question

I'll get cracking on that furmula.i have a 200nm trip,I no @ 5500 max rpm it will use aprox 9.3lphr. Iam also able to lift one outboard out of the water.thanks again
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Old 11-08-2016, 17:11   #5
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Re: Propulsion question

Are you planning to motor all the way?

What sort of winds and currents are you anticipating on your 200nm trip: strength, direction?

Without knowing any more, I'd figure on 5.5 - 6 knots as a planning speed for a 38ft Wharram. Under the right conditions, you should get considerably better than that.
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Old 12-08-2016, 01:39   #6
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Re: Propulsion question

Thanks Stu,coming down from derby to Broome.light winds forecasted but big tides which we try use to our advantage.the boats still on the hard stand that why we haven't done a sea trial for speed ect the sails seem ok but will motor as we have a small window. I worked on 5kts but as you said it should do a bit more.cheers
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Old 12-08-2016, 03:18   #7
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Re: Propulsion question

Derby, eh? That explains your comment about the tides on the other thread you started on this subject
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