I agree with James. I think Edward's numbers are way high, possibly by as much as 10x. Maybe a decimal point was off in the calculation?
My 5000 pound Albin 25 takes about 2KW to go 5kt. If memory serves, a 14 ft aluminum skiff will do 3 kt or so with a plain old 12V trolling motor drawing maybe 30 or 40A (360 to 480 Watts).
As for bow thrusters, it might work but they are not normally rated for continuous duty. I would consider a trolling motor, possibly modified to fit in a well and with an aftermarket prop. Don't know if they're still around, but Glen-L used to have some pretty cute designs using this concept. You could also build a trolling motor into the rudder and mount the control head remotely.
I don't have any experience with the Torqueedos, but I'll bet one of the small ones would be more than adequate.
Jim McMillan
--- In electricboats@yahoogroups.com, James Sizemore <james@...> wrote:
>
> Batteries with built in BMS are getting more common and at better prices everyday.
> The Winston cells get very reasonable with volume buys, but even with out that they are not bad:
> http://www.balqon.com/store.php with built in BMS and 5 year warranty.
>
> And I will stand by my assumption there is no way a 17 foot boat _under_ hull speed is going to need more power to move then a cal 30 that is four times the weight.
>
> But I guess we can agree to disagree.
>
>
>
>
> On Aug 20, 2012, at 9:17 PM, Edward Bachmann <edbz@...> wrote:
>
> > OK $10,000 is a tad high but not as bad as you contend. I will admit to thinking a turnkey battery system with support from the seller.
> >
> > A pile of bare batteries do not make anything safely usable. This is a small open boat and everything has to be safe when wet.
> >
> > A Cal 30 has a hull speed a bit over 7 knots this boat has one a bit over 5 knots (water line 15.75 feet)..Your 5 knots is right at 2/3 hull speed.
> >
> > I’ll stand by this boat needing close to 4,000 watts to do 5+ knots. And something like 2000 watts for 2/3 hull speed.(3.5 knots).
> >
> > How would you size and configure something with a range of 30 miles at 3.5 knots including housing, fuses, etc. Figure 15,000 watt hours.
> >
> >
> > From: James Sizemore
> > Sent: Monday, August 20, 2012 8:36 PM
> > To: electricboats@yahoogroups.com
> > Cc: mailto:electricboats@yahoogroups.com
> > Subject: Re: [Electric Boats] Lewmar bow thrusters & intro
> >
> >
> > 10,000 dollar in lithium batteries would be a 38kw bank at 48 volt and 800 AH. I doubt he would need a bank that large and that he could fit it in a 17 foot boat and still have room for himself.
> >
> > Even if by some magic he needed 100 amps (4800 watts) to push the boat to 5 knots, he would still get 8 hours of travel time. Let's be serious here, his boat will need nowhere near 100 amps to reach hull speed. Could we at least stay a little inside reality here?
> >
> > A cal 30 only needs 2.4kw to reach 5 knots. With a 3000 dollars lithium bank (9.6kw) he could travel 100% hull speed and go for 9 hours easy. At 3 knots he could probably go 24 hours strait.
> >
> >
> >
> >
> >
> >
> > On Aug 20, 2012, at 4:40 PM, Edward Bachmann <edbz@...> wrote:
> >
> >>
> >> The question is how much will you want to spend on batteries.
> >>
> >> The Penobscot 17 is a pretty boat with a hull speed around 5.3 knots.
> >>
> >> With $10,000 invested in lithium batteries, you could travel all day and a long way long way at something like 2/3 hull speed (<4 knots).
> >>
> >> For a total investment of under $3,000 you will find the useful range/speed limited to around 1/3 hull speed (<2 knots).
> >> (Figure a Torqeedo 1003 with an extra battery pack)
> >>
> >> You might get another knot or two out of a twin hull design of the same length. Something like Slider might be fun for two people. http://slidercat.com/blog/wordpress/?page_id=2
> >>
> >>
> >> From: Stan and Sandy
> >> Sent: Sunday, August 19, 2012 1:38 PM
> >> To: electricboats@yahoogroups.com
> >> Subject: [Electric Boats] Lewmar bow thrusters & intro
> >>
> >>
> >> Hi
> >>
> >> I'm a new member with aspirations to build an electrically powered boat to cruIse the coastal waters around Oahu with one passenger. I live on the North Shore of Oahu.
> >>
> >> •The boat
> >> Haven't started building yet, but am pretty committed to a Penobscot 17 from Arch Davis (archdavisdesigns.com). The theory being that an easily driven sail/row boat is a good candidate for electric construction, and the 17 is large enough to be comfortable and handle some chop.
> >>
> >> • The power
> >> Lots of questions and options here. I have read with interest the many discussions of inboard vs outboard vs trolling motor comparisons. However, I haven't seen (but may have missed, you guys have _lots_ of posts--interesting, but a bit daunting) discussion on thrusters for primary propulsion. Nothing definite yet, but would appreciate discussion of the Lewmar 185TT from WMJ Marine (wmjmarine.com). Don't want to exceed hull speed, but don't want to be "full throttle" the entire time I'm out.
> >>
> >> Any feedback?
> >>
> >> Stan
> >>
> >>
> >
> >
> >
>
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