Bill,
8 hours at 5 kt is asking a lot for a boat that size. It's doable, but you're going to need a pretty hefty battery bank. Also, you'll probably need over 2:1 reduction, maybe closer to 3:1. The motor will have a "natural" RPM at which it wants to turn for any given voltage. Put a load on it and it will try and pull as many amps as it needs to maintain that RPM. Therefore if you're looking for 1200 at the prop, and your motor's "natural" RPM at 48V is say 3600, you'll need something like 3:1 reduction. If you tried to run direct drive on that big of a boat, it would draw some serious amps and likely burn something up.
There are a lot of variables (mainly displacement, prop, motor selection, and battery size) that drive what reduction ratio you select, and there can be a certain amount of a trial-and-error, mostly due to variations in prop efficiency.
If you're considering using a "store bought" package, I would second Donald's suggestion to talk to the proposed vendor. Scott at Electric Yacht has done many conversions and would be able to give you some realistic numbers. Lest I seem partial (Scott is my brother) I would recommend talking to one or two others as well for comparison. If you decide to go the DIY or "kit" route, there are some knowledgeable folks here that can help you work out the details. I suggest reading a bunch of old posts and look at the photos and data sections of this group, and ask lot's of questions.
Regards,
Jim
--- In electricboats@yahoogroups.com, Donald M Swanson <postal6@...> wrote:
>
> Bill
>
> I used a two to one reduction. I have simple Interstate marine
> batteries, 12 volt, SRM 27, CCA 675. I have run two hours straight on my
> system, which dragged me down to a 50 percent charge which is a no no.
> I try to run at 3 knots or less, and not to consume below 60 percent of
> available power.
>
> The setup you have described , two 48 volt systems I can not give you any
> advice on. You are looking at a lot of additional weight, and charging
> requirements. I woul best defer you to Scott McMillan of electric yatch
> who I bought my system from, or any of the other knowledgeable
> professionals here within the group.
>
> On Mon, 07 Mar 2011 16:48:20 -0000 "Bill Tucker"
> <bill@...> writes:
>
> Donald,
>
> I am also thinking of the system for an Ericson 35-2 11600 lbs. a couple
> of questions; What reduction did you use 2 to 1? What batteries do you
> have? and how long can you run on a charge?
> I am thinking of a 1 to 1.5 or direct drive system to get 5 knts. Do you
> have any problem with tork?
>
> My goal is to run for about 8 hours at 5 knts. (1200 rpm.) I am thinking
> of using 2 sets of 48 volt batteries, and switch off and charge one with
> a generator while running on the other.
>
> What do you think?
>
> Thanks
>
> --- In electricboats@yahoogroups.com, Donald M Swanson <postal6@>
> wrote:
> >
> > Not sure who you got your kit from Carlos, but I have an Ericson 27
> that
> > has been on the system for going on two years and I have no complaints.
>
> > I might add that I am using wet cell batteries too. The only advice I
> > can give you is don't push your speed. It will get you out of a quick
> > jam with a little burst of speed, but try to keep it at one half of
> hull
> > speed, in my case 3 knots or less, which will give you a couple of
> hours
> > of cruising without any problems if you are in a bay. Depending on your
> > boat, you will need to probably go with a three blade prop like I did,
> > and with that said, you will do fine. Don
> >
> > On Sun, 06 Mar 2011 01:05:40 -0000 "Carlos Silva" <pahuac@>
> > writes:
> >
> > I am about to start my conversion with this kit and therefore would
> very
> > much appreciate comments on actual experiences and tips.
> >
> > Thanks,
> >
> >
> >
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>
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