Re: [Electric Boats] introduction

 

Vendor alert
My thinking comes from selling motors and batteries to about 100 clients and personally installing about 20 systems. From the experience of these installations and likely another 200 people I have helped but did not sell a system to, this the order that I use to size and select a system:

First, get the right propeller.  If the boat was a diesel an electric will likely be able to use the prop if properly geared.  They are not all 2 to 1 but that is a good place to start.

Second, get the right motor.  The marine engineer who designed your boat likely used a motor that would get the boat to hull speed in calm conditions and then add 50% to 100% more horsepower.  That is to say for your 9000 pound boat a 25hp diesel would have been chosen or the 30hp A4 of the time.  With electric you do not need to go to the level as I believe 120% to 150% of the power to push the boat to hull speed is adequate to address adverse sea conditions and emergencies.  The instant power and the flat power curve is what helps here.

Third, select the right battery for your sailing and boat's needs.  I would recommend this order of evaluation, minimum power storage needed based on the budget of the project.  Flooded are the most economical but the least effective.  AGMs are better and safer and economically attractive.  LiFePO4 are the best but worst economically.  If I had the money for liFePO4, I would make sure to spend it on sails first then on batteries.

This is how I know the real power of electric systems work as a traction marine system.  The Maximum power rating of the controller and motor is the limiting factor with about 30% of its rating being continuous.  At about a 20 min rating it is about 40% of the maximum (this is close to all controllers we have evaluated and worked with).  Therefore, if the controller is a 450amp max think in terms of 180 for a 20 minute draw, less for continuous.  Motor heat is less of an issue than the controller.  With about an 85% conversion we see built into the mechanical and electric systems that is about 12hp ((450*.4)*.85)/746=11.6hp.  This is close to what the marine engineer saw when he selected the systems for the boat.  No engineered system on the market does the conversion of electric power to the prop better, they are all within about 10% of these numbers.  Electric conversion is the same and mechanical conversions might differ slightly.

Your boat is a well-built cruiser at 9000 pounds and with a full keel and a prop in the aperture.  A 10kw motor will push the boat with authority and the 5kw motor will not get you to hull speed.  A 200AH AGM battery bank will give you about 25NM at 3kts and about 15NM at 4kts.  You are not built for speed but safety and the boat can take weight.  If you had a fast boat and were in a weight sensitive sailing pattern (racing) then I would consider liFePO4 batteries. 

Speed & Power
Kts	Amps	Watts
2.5	10	480
3.9	40	1920
4.9	80	3840
5.7	120	5760
6.2	160	7680
6.7	190	9120
Hull speed	6.23

Final thoughts.  About 90% of our clients went with the hull speed plus 20% or more power and are very satisfied.  The other 10% went with less and they are also satisfied.  The making a decision with all the facts and to match to your sailing pattern is the key.  Very personal decision

Mike

Electric Yacht of Southern California

---In electricboats@{{emailDomain}}, <oak_box@...> wrote:

(To the rest of the group - PLEASE challenge this if you disagree with the logic!!)

 

Matt,

 

What is your situation in terms of the location you're in, how far you need to motor, current, tides, and exposure to potentially bad weather?

 

For your conversion, you may find that one of the most expensive components, and most variable in cost - is the battery bank.

 

Assuming you go with a 48V system....

 

The cheapest and easiest option is to get 4 deep cycle 12V batteries, and the cheapest trickle charger you can find.  This will set you back about $500-600.  Thundersky has an inexpensive 48V charger that works, and charges at low current, so you will need to be patient.   :)

 

HOWEVER - the reality is that deep cycle batteries really won't work very well at all with an electric motor system.  They don't hold up well at all to being drawn down heavily, and don't have much capacity.

 

AGM batteries are better.

 

Lithium is probably what you should really go for - but also can be a VERY expensive solution.

 

An option here - is to go Lithium, but just start with a very small (low amp hour) lithium bank.   As you have more $$ available, you can later add more Li banks in parallel, or save your "small" initial bank as a "backup".

 

This assumes:

1)  That your motoring is MOSTLY (at least at first) just to get you in and out of a marina - that you will mostly SAIL the boat.

2)  That you will (at least at first) stay in reasonably protected waters where you won't NEED to have a dependence on a long motoring cycle at high power (to overcome strong waves, wind, or current).

 

To suppliment, if you spend a little more on the charging system, and the charing system is able to put out reasonably high power, you can possibly find a cheap old generator to power the charger if you do get stuck.

 

In other words - be careful, don't go out if conditions aren't great, and this works much better on protected lakes where the marina is "right there", than on "big water" where you have to motor a lot or deal with currents / tides / wind / etc...

 

If this isn't your situation, you may want to reconsider things.  An electric solution that has plenty of margin for longer motoring simply isn't cheap.  No two ways about it. 

 

Good luck!

John

From: matt strickland <matthew_strickland@...>
To: electricboats@yahoogroups.com
Sent: Thursday, December 19, 2013 11:03 PM
Subject: [Electric Boats] introduction

 

Hello,

I'm new to the group and joined because I've recently acquired an Alberg 30 as my first boat bigger than a 420, which I will ultimately convert it to electric.  As cost is an issue (when isn't it?), I may have to patch up the existing Atomic 4 first.  Does anyone have advice on the best ways to get the conversion done at lowest cost, even if it means coming back and upgrading components later on as funds become available?  I'd really like to convert now if possible.

Thanks,

Matt

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