Propulsion question

[Lululund84]

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

[GordMay]

Greetings and welcome aboard the CF, Lululund.

[a64pilot]

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

[Lululund84]

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

[StuM]

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.

[Lululund84]

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

[StuM]

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