Tuesday, July 19, 2011

Re: [Electric Boats] Intro

 

Suggested reading (most of the calculations are done using an 18 foot canoe hull but the logic will hold for you):
 
ELECTRIC  OUTBOARD  DRIVE for SMALL  BOATS  A  Do-It-Yourself Handbook  July 15, 2010 by Todd   Wells
 
        http://www.scribd.com/fullscreen/36750318
 
Run that speed calc for one third and two thirds of hull speed to see why I used those numbers.
 
But I am going to posit a real rough calc based on needing 4 kw to run at 90% of hull speed at 6 knots.
  For 4 knots less than 1 kw and at 2 knots less than 0.25 kw.
  Stated another way: very rough figuring,
     Compared to the amount needed to get to 90 percent of hull speed. 
     you need 1/6th the energy to get around 6-tenths of hull speed
     and 1/18th the energy to get around 3-tenths of hull speed.
  Give or take 20 percent error on the energy part.....  Winking smile
 
    
 
As for regen, a prop that is efficient driving forward cannot be nearly as efficient for regen. Slippage, shaft, gearing, cable and charger losses all add up. It's simplistic for me to translate all of those losses into drag but expecting a whole lot of benefit compared to the drag seems unreasonable.
 
 
Sent: Monday, July 18, 2011 9:48 PM
Subject: Re: [Electric Boats] Intro
 
 

Yes, I understand the Torqeedo should not be left in the water. It wouldn't; neither would the pod if I went that way, because it would be retracted when not in use.

 
Regarding regen... effectively 1 hp astern for every 0.1 hp recovered? Wow, is it really that bad? So I'd lose around 0.5 to 1.0 knot for a couple hundred watts charge? I guess it might still be worthwhile on longer trips.
 
I'm looking for around 3-4 hours use at 2/3 hull speed. The boat should be very easily driven, and according to this calculator http://www.psychosnail.com/BoatSpeedCalculator.aspxI , based on my LWL and displacement I should need around 4kW to achieve 6 knots, theoretically of course.
 
But what I'm really looking for is real world experience of pods. The Torqeedo route would be an easy one to follow, but I really would like a more permanent inboard installation, without resorting to a shaft drive.
 
 
 
 
On 18 Jul 2011, at 19:24, Ed Bachmann edbz wrote:

 
 
Torqeedo does not recommend leaving their motors under water all the time.
 
But the smaller ones are very light and can be easily taken out of a motor well though you might consider some sort of jack if your design does not have the space to tilt the motor out of the water. Note Torqeedo also has remote control options - nice.
 
As for regen, do not consider it unless you have a dedicated tow-behind generator unit. The losses of power are never going to balance out against the drag. Think about regen as running a motor in reverse - generating one horsepower of reverse (drag really) for every one-tenth-or-less horsepower of recovered power.
 
Questions to ask yourself: How fast do you need to go and how far do you need to get? Do you need more than one-third hull speed? Would two-thirds hull-speed be satisfactory if you could only run that fast for 30 minutes?
 
As you know, everything on a boat is a tradeoff.
 
 
 
Sent: Monday, July 18, 2011 6:43 PM
Subject: Re: [Electric Boats] Intro
 
 

Hi Capt. Mike,

 
Thanks for the input. My boat has an extended hull over the original design, with a traditional transom not suited to an outboard. Also, the aesthetics are important to me and I don't want an ugly outboard bracket hanging off the back. I could possibly have an outboard in a cockpit well, and I hear what you're saying about pods and motors running underwater, but unless I'm mistaken the Torqeedo actually uses a motor in an underwater pod relying on seals too. I am actually considering a Torqeedo, but I would prefer a more 'permanent' installation, and the option of regen under sail.
 
Cheers,
 
Mike
 
 
 
 
On 12 Jul 2011, at 11:18, Capt. Mike wrote:

 

Mike:

Sounds like a great design for EP. I do wonder why you are so interested in a retractable pod if your boats design was based on using an outboard. Seems like you might accomplish the same thing with an electric outboard like a Torqeedo. Personally I never liked the idea of a pod. I don't like the idea of an electric motor operating under water and depending of seals to keep the water out.

Capt. Mike

Sent from on board BIANKA
http://biankablog.blogspot.com

From: Sarah Shepherd <shepsinorbit@yahoo.co.uk>
Date: Mon, 11 Jul 2011 19:34:51 -0400
Subject: [Electric Boats] Intro
 
 

Hello Everyone,
I'm new to this forum so here's a quick hello to tell you all what I'm up to (or hope to be!) I'm planning the build of a 29 foot light displacement performance gaffer which I will be starting to build in wood-epoxy early next year. I'm an experienced sailor but very new to boatbuilding and just as new to electric drives. Since it will be used primarily as a day sailer and plugged into shore power at night, with the occasional weekend at anchor, I intend going the electric route. This also fits with the boat's environmentally friendly approach - sustainable cork will feature in the floors, spars will be bamboo/carbon composite and I hope to use environmentally friendly epoxy. Not to mention wanting to keep a smelly, oily diesel out of my clean new boat! I have been doing a lot of research on the web and am really excited about all the electric drive possibilities out there. I had no idea there were already so many manufacturers! One option I'm looking into is the pod type drive unit rather than a saildrive, such as the Podmaster from Mastervolt. I want to avoid shaft drive for reasons of simplicity and drag. I am thinking about mounting the pod on a retractable frame with a hull-shaped plate fixed underneath that will close off the aperture when the pod is retracted, similar to a retractable bow thruster in concept. The reason for this is to further reduce drag (my boat is based on another design that used a conventional outboard motor) and to keep the pod out of the water when not in use. I'd be interested to hear from anyone with any experience with these pods, or have any comments on their performance/efficiency etc versus an electric saildrive, or indeed any links or info at all that I may find useful? I'm
Thanks in advance for the help.
Kind regards,
Mike

 
 
 
 

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