Your Searay is a planing hull. You will either be operating it as a planing hull, (with probably a much bigger motor than your 15kw unit) with a very short range, or else indisplacement mode. Planing hulls are not particularly efficient at displacement speeds, compared to a true displacement hull. This is one reason sailboats work well with electric propulsion. Typically, we electrocruise at about half of hull speed. For your boat, assuming 18 foot waterline length, that is 3kt. Typically (depending on the battery bank) you will have about 5 hours run time, give or take a bunch. The nice thing about electric drive in a sailboat is that the range is not a really big deal, since the motor is mostly used for getting in and out of harbor only. So, a fairly modest bank, say 105ah at 48v, is often sufficient for a small sailboat. For a motorboat, the electric drive is needed the entire time that the boat is making way. So, a big bank is needed, which still only gives you very limited range. Now if you were to install maybe 1200w of solar panels (which would probably completely cover your boat) and the appropriate charge controllers, on a sunny day you could extend your range greatly, During the sunny part of the day you would not be draining your batteries at all. Go slow enough, and you would even have a net charge. This setup would probably also want a 2kw porttable generator, like the excellent Ryobi or Honda units. So, you will still have gasoline onboard. The portable generators MUST be operated topside! They are NOT engineered to operate in a fuel rich atmosphere or without ventilation of exhaust gases! You would want your cockpit hatch closed, as well, especially during fueling. Running a generator to charge batteries to operate an electric motor to turn a prop is VERY inefficient. However, the purpose of such a generator is not for routine operation, but for emergency range extension. A "Gitter Home" power source.
Yes, you can use your current stern drive with an electric motor. Assuming that the gear ratio, prop size and pitch, etc are suitable for your motor, hull, and voltage. Most e-boaters like a larger prop, the largest you can spin with reasonable hull clearance. It's not a deal breaker but it is probably going to make a difference. Anyway, all those factors make for a big balancing act to get everything working optimally together as a system.
Another thing you could do is to add an outboard mount, and keep one of those remarkable little Tohatsu 6hp or similar modern outboard for getting home when you have discharged the batteries to minimum (50% for lead acid) and you would otherwise be calling sea tow. The new 4 stroke outboards are very reliable, and fuel efficient. Yeah, they are heavier than a 2 stroke but you don't have to mix oil into the gas, or remember which can is mixed and which is not, stuff like that. If you are going to have gasoline onboard anyway for a generator, may as well have an outboard, too.
Pay attention to your motor specs. Says 15kw, right? 15kw at what voltage? Big difference between what you will get at 48v and what you would get at say 96v or 120v. FWIW most small boats use 48v because the electrical standards are much simpler below 50v. Take the motor's rated sustained current and multiply by 48 for a more realistic power output, and remember you will seldom use that much current. A good rule of thumb is to consider slow cruise to be the battery rated amp hours divided by 20. So you get say 4 8D batteries at 245 amp hours, that would be 12.5a at 48v, or 600w into the motor controller. Call it 575w into the motor itself. If you are curious, that is 0.77hp. You would have that output available for 10 hours and still have your batteries at a safe 50% at the end, without recharging by solar or other means. Exceed that power level, and something called Peukert's Effect comes into play. At say 100a or 4800w, you might think you have almost an hour and a half range, but actually only have about 45 minutes. That would be 6.43hp, minus losses in the controller. The cheapest practical batteries currently, in terms of watt/hours per dollar, seem to be 6v golf cart batteries, typically 220ah rating. 8 of these in series makes a very nice 48v 220ah bank. So the Peukert neutral discharge current would be 11a for 10 hours. For your size boat, you probably should keep battery weight down to 600 to 1000 lbs. You can get more juice per pound by going with Lithium batteries but they have their drawbacks, namely price and safety. Safety issues are not nearly what they once were, and a good BMS can prevent or at least mitigate most catastrophes. On the plus side, not only do you get more wh per lb, you can draw the batteries down much further and they typically last a lot more charge/discharge cycles. So if you have deep pockets they are worth considering.
If you can find an old open lifeboat, it would make a much better platform for an electric motorboat. You might also consider a catamaran. Or get a sailboat, and enjoy both e-boating and sailing. If you have money to burn, burn some of it on a more suitable hull.
---In electricboats@yahoogroups.com, <greg.yarman@...> wrote :
| Reply via web post | • | Reply to sender | • | Reply to group | • | Start a New Topic | • | Messages in this topic (3) |
No comments:
Post a Comment