Glenn,
Good reasoning on developing your budget, but I find your orignal budget a little aggressive, you will propbably go over that number by the time you get a running conversion with basic instrumentation. You can leave some stuff off, but you'll probably end up spending the money eventually. To do the project as you describe, I would brace yourself for about $5k US. Who knows, you could even come in under that...
Looking at your design parameter, you said that your basic "stretch" requirement is 20Mn @ 4kts. I find that somewhat challenging but still reasonable. Let's do a little math.. :)
Because we don't know anything else about your system yet, we'll have to go with some broad estimates. But I will go out on a limb and state that these "guesses" will be closer than you might think.
The first assumption is that your boat will take about 1500W (2hp) to power at 4kts. You might do better, you could do worse, but this is close enough for estimating. At 4kts, you'll need 5 hours of capacity. I never estimate a battery pack at more than 80% depth of discharge (DoD), even if the battery chemistry can take it. If you think about it, when was the last time that you ran your car to 100% DoD of the gas tank? I think that we all fill up before we run completely out of fuel and typically treat our boats the same.
Speaking of battery chemistry, different types of batteries act differently in storing energy, delivering energy and their lifespan measured in charging cycles. How far you regularly discharge your batteries can affect their lifespan, but this thread is not the place for that discussion, I don't want to get sidetracked...
So for your battery types, I will use 65% DoD for flooded batteries (FLA) and 80% DoD for AGM. If you want to use different DoD, let me know and I can recalculate the numbers.
So we're aiming for 5 hrs at 1500W. Here we go....
FLA batteries like T-105s or T-125s have a Peukerts exponent of 1.25 and using a 65% DoD, you need 14kWh to give you the required 9100Wh usable to run for 5 hours. That's 290Ah of FLA batteries at 48V. US Battery makes the US305EXC 290Ah 6V battery that weighs 102 pounds; you would need 8 batteries for a combined weight of 816 pounds. These batteries seem to cost about $200 a piece or $1600 for your boat
AGM batteries have a Peukerts exponent around 1.15 and using a 80% DoD, you need 11kWh to give you the required 8800Wh usable to run for 5 hours. That's 230Ah of AGM batteries at 48V. Trojan makes the 8D-AGM 230Ah 12V battery that weighs 167 pounds; you would need 4 batteries for a combined weight of 668 pounds. These batteries appear to cost around $650 online or $2600 for your boat
Just for fun, let's look at LiFePO4 batteries, they have a Peukerts exponent around 1.05 and using a 80% DoD, you need 10kWh to give you the required 8000Wh usable to run for 5 hours. That's 200Ah of LiFePO4 batteries at 48V. Thundersky makes the TS-LYP-200AHA 200Ah 3.2V battery that weighs 16.1 pounds; you would need 16 cells for a combined weight of 260 pounds. These batteries cost $260 or $4160 for your boat. Lithium batteries should have extra protection; a battery management system (BMS) will cost approximately $500 for these batteries, pushing the cost to around $4700.
Whew, that's a lot of info, but I suggest that you mull it over for a while and see how it feels. Another answer is to change your requirements, a 15 mile range @ 4kts is obviously cheaper and lighter and so is 20Nm @ 3kts.
But I like your original request, my 30' ketch conversion with LiFePO4 batteries is spec'd to run 16Nm @ 4 kts. I should be able to verify that performance within a month or two and I'll post my results here.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Glenn Dennis" <vega1184@...> wrote:
>
> Eric and all
>
> I set the budget based on what it would cost me to repower with a new/old stock diesel (Volvo MD2001 9 Hp, 1 Cyl). I currently have a non-working inboard gas with feathering prop (Albin Combi drive) that is too small to work with anything else. I use a 9.9 outboard mounted on the transom, it works for the most part but if there is over 2 or 3 ft swell the prop tends to come out of the water and you can't charge the house batteries. Most of my motoring is just out and in to the Club, less than 10 min @ 3 or 4 Kts, each way most days. Top speed is not a real concern 5 or 5.5 kts would be more than enough, but would I have to have longer range for when you have to get back. I would like 20 Nm @ 4kts. I have had the boat two years now and mostly just day sail with a couple of weekend trips every now and then, but I want to do longer trips. Also I sail solo often.
>
> With either option I will need Prop, shaft, coupling, exhaust, etc. Because of design the largest prop I could fit would be 13 inches, maybe 14 if I could get it back far enough, I was considering a 13 X 13 pitch. As for space the Vega is blessed/cursed with 23" wide X 48" long X 16" high void behind the current engine occupied by only an exhaust pipe. Also it's easy to make wider, I could fit 4 X 8d batteries no problem. Weight however would be another issue at 4 x 66 kg she may have a bow up attitude. I could try to keep them as for forward as possible may making a step cover for one. 6 volt GC batteries would be easier to distribute.
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> "As long as you approach this project with your eyes open to what it really means, then you shouldn't be disappointed."
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> That's why I'm here, trying to learn as much as possible before I comitt to the project. Oh and it would be nice to have a quiet "green" boat.
>
>
> Glenn
>
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