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Old 10-04-2012, 17:51   #1
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Join Date: Oct 2008
Location: Charleston, SC
Boat: in the market for a cat
Posts: 6
Prop sizing for DE upgrade

So first allow me to apologize for being another novice fascinated by the promise of electric sailing. I understand it is more expensive. I understand it is heavier. I understand that it is untested. I also understand it is far more gentle and much quieter. Also, it's pretty much the future and that is where I am going to spend the rest of my life, so I am interested...

I am looking at a pretty big faster cat, specifically an Outremer 49 or a Dolphin 46 (if I can find one). I am not a naval architect, nor an electrical or diesel technician. There are so many ways to skin the cat on the Diesel-Electric question, but I am leaning toward a high VDC (144) solution. I am hoping to do something like one 9 KW motor for each hull, backed by a couple of compact 9 KW DC generators with around an hour of overall battery reserve.

So my question is, for a layman, how would you figure out if the size is right to allow you to do a comfortable 7 knts? And, how do you select the prop for this job?

I have read that due to their torque at low RPM, larger props are indicted with electric motors. How do you decide what size and pitch is right for a 12-14 tn cat? How do you adjust the "calculators" that are abundant on the intertubes for the greater torque capabilities of the EP?

I understand that this is complex and there are a host of variables to be addressed. Which variables matter most? What formulas do I plug them into?

I am looking for an 85% number that I can use for budgeting purposes to try to decide if I can take this on, without my wife leaving me for a more sane person. I thank you in advance and look forward to the lively debates that DE always brings into the conversation.

Your friend,
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