Hi both Erics,
Eric of Meander is correct in why I am thinking of this approach. I understand the extra loss WHEN both motors are engaged. BUT for the vast majority of our expected use (based on 18 months of cruising with ICE)will be at 3 knts or under, thus only using ONE motor. The idea is to maximize the efficiency of that system, with having a second motor to boost power for those rare times when we need it.
From all the data I can find - admittedly a hard task to find good data as a lot of motor efficiency curves start at around 1kw, rather than the 1/3 to 3/4 kw I am focused on - it seems like most are less than 70% efficient at that power level, even the ones most employed on elec boats on this list. Having 80 - 90% motor efficiency (even if that is lab efficiency) would beat the loss of efficiency assumed by the use of the second motor (with second drive system) on those occasions when it would be used.
OR, this is my thinking, and I am clearly open to the naysayers!!
The redundancy may be a blessing or curse, depending on if it is redundant performance or maintenance!!
Thanks all
- Dan
--- In electricboats@yahoogroups.com, "cire" <cirejay@...> wrote:
>
> Eric,
>
> I'm onboard with your point about unnecessary complication, however I think you confuse the efficiency of a hull at 50% hull speed with the efficiency of an electric more at low power.
>
> I admit to not being up on the absolute latest in electric motors but have observed in the past that they seem to operate most efficiently at 75% power or so and least at 25% or below.
>
> What I am saying is that I see the point that Dan is making; the question that you ask about it being worth the complications and loss of efficiency created by two drives is a good one.
>
> BTW, as to the use of one large battery bank, I have a 32' sailboat – Allied Seawind II – with a single 630amphr battery bank and when I explain the Peukert effect as part of my reason for a large single bank to my sailor friends, their eyes roll back into their heads:-).
>
> eric SV Meander
>
>
>
> --- In electricboats@yahoogroups.com, "Eric" <ewdysar@> wrote:
> >
> > Hi Dan,
> >
> > To me, it sounds like your plan is unnecessarily complicated. Electric motors are fairly efficient at low power (see my measured performance at 3kts, i.e. 50% hull speed). So a single motor, driving through an efficient reduction will do just about as well as what you are suggesting. I don't know the efficiency specs for a v-drive, but you may notice that on page 4 of Julian's posted pics, the author states that gear boxes should be avoided because they are not efficient. I believe that he is referring to typical marine gear boxes, and if so, I would agree. However, my Browning gear box is 98% efficient, which is difficult to match with belts (Goodyear NRG drives are one of the few belt drives that can consistently boast a 98% efficiency), regular v-belts range from 90-98%, with most operating around 95%. Chains in an oil bath may also reach 98%, but exposed chains often degrade quickly.
> >
> > Some people here that run 2 equal sized battery banks for "redundancy", they give up range due to Peukert's effect, a single bank has greater range that the same bank split in two. But if one believes that the loss to Peukert's is justified by the protection against catastrophic failure of one of their battery banks, then it might be worth it, but not to me. Likewise, your two motors will lose more through two reductions than a single properly sized motor running through an efficient drive, even before throwing in he v-drive. At least, that's the way it looks to me. That said, your plan will work, just not as efficiently as other, simpler systems.
> >
> > Fair winds,
> > Eric
> > Marina del Rey, CA
> >
> > --- In electricboats@yahoogroups.com, "dan.randolph" <danlrandolph@> wrote:
> > >
> > > Hi Eric,
> > > Thanks for all your excellent information and advice in this group.
> > >
> > > In this post you commented that V-drives may have a loss of efficiency.
> > >
> > > I am curious, as I am considering a V-drive, with two small motors (it seems that for most of my use I only need less than 1/2 the power available, with only those "bad" times when full power would be good to have). One way I have considered is a V-drive with one fixed belt to one motor, and one motor on a belt that can be engaged with a gear for when needed. The smaller motors are more efficient at very low amps, for moving at 1/2 hull speed or less.
> > >
> > > Thoughts and critiques of this plan appreciated.
> > >
> > > - Dan
> > >
> > > ---
> >
>
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