Monday, December 14, 2009

Re: [Electric Boats] Re: Electric Catamaran

 

Jason,


In your message of a few days ago you asked:

 


Chris, you quoted : 1500 watts = 4.5knots = 6 hrs
I would imagine my cat would be similar and 5 Knots would possibly be < 2 KW?


I don't have much depth theory or practise about these things and all I know comes from tinkering with my boat and some reading over the past year or so.

For what its worth, I can make a few observations that could help you form an opinion...

From the experience of Bryan of School's Out he found that one motor is always more efficient than two at any speed.  Even though when using one motor the thrust is asymmetrical it still uses less total watts than when using two motors.  When using one there would be a slight additional drag because the rudders are slightly angled to compensate for the "skid steer" effect of just using one motor.  But seems that once under way this effect is minimal.  And in relation to your earlier contemplation about having the ability to shift a motor inboard for this kind of thing, you won't need to worry about it.

My boat has a waterline of 43 ft, and has bulbous bow, which I understand helps with lower drag (and hey, that's what they use on the big oil tankers so it must be a good thing).  So I expect its particularly easy to push through the water. If you take a look at this graph on my blog at http://currentsunshine.com/?p=175  all three curves show greater than 2kw for 5knots boatspeed.

Bryan's cat has a waterline of 45ft and at 5 knots boatspeed against a 10 knot apparent wind he has recorded around 3000 watts power requirement for two motors. It displaces between 5 and 6 tonnnes.

For what we're doing we need to scrounge every % of efficiency we can get our hands on.  And, as mentioned by others, using outboard legs and adding a pancake motor makes your task that much harder.  I'm convinced that the reason the Torqeedo units are so efficient is because they have control of the whole drive train and have matched motor and prop superbly.  It gonna be much harder for you to get the right prop to match whatever motor you choose.  You'll be limited in prop choice by the spline configuration of the outboard leg you choose. I've been down this path with an Epod motor which came with an appallingly bad choice of prop from the factory and I had to experiment with buying off-the-shelf props that would match the splines. I even had one repitched to experiment with efficiency but in the end abandoned the chase and bought a Torqeedo.  The Torqeedo has proven to be around twice as efficient as the Epod.

I hope this helps.

Cheers

Chris

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Re: [Electric Boats] Re: Electric Catamaran

 

Hi Jason,


I mentioned previously a friend who has Torqeedos on his cat School's Out and they are on arms that fold up under the bridge deck.  You can see a youtube video about it by searching for "schools out new torqeedo" and here's a link to it it http://www.youtube.com/watch?v=mJv-lGPXxHc

He has built the arms himself and has used actuators to raise and lower them easily.

Take a look - I think you'll find it matches your wishes pretty closely.

Cheers

Chris

On 14/12/2009, at 11:21 PM, jason_d_s_uk wrote:

 

The other option I have instead of outboards is to mount the motors in each hull. The only problem is that I would want some kind of drive leg that I can lift out of the water as I beach my cat and also don't want any drag when actually sailing.

Anyone know if this is possible and how to go about it in a DIY capacity ?

It's slightly off topic, but then is part of the design I would require for having in-hull motors and I'm sure would apply to other electric converters out there!

Regards,

Jason.


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[Electric Boats] Pitch of Kipawa Propeller

 

I had a bizarre exchange with the Kipawa propeller customer service number today. I emailed the customer service yesterday to ask the diameter and pitch of a Kipawa prop for a MK Endura 30/34 and received a response email today with the diameter, which apparently is not a secret, and the statement "We do not give out the pitch of the Kipawa Propeller."

When I called and asked them why in the world they would not tell me the pitch on the prop, since it is what it is and since it is not a trade secret, I was told that I could order one and measure it and return it if I wanted but they would not give it out.

I am in still interested in knowing the pitch of the prop (without ordering, measuring and returning one if it has a pitch I don't want). I have variously seen the pitch online as being listed as 6" or 9". Does anyone know the pitch of the Kipawa props? If so, and assuming Kipara did not have you sign a nondisclosure agreement as a condition of the purchase, would you let me know the pitch? Thank you.

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Posted by at No comments:

[Electric Boats] Re: Electric Catamaran

 

The other option I have instead of outboards is to mount the motors in each hull. The only problem is that I would want some kind of drive leg that I can lift out of the water as I beach my cat and also don't want any drag when actually sailing.

Anyone know if this is possible and how to go about it in a DIY capacity ?

It's slightly off topic, but then is part of the design I would require for having in-hull motors and I'm sure would apply to other electric converters out there!

Regards,

Jason.

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Saturday, December 12, 2009

[Electric Boats] Re:New Online magazine

At the risk of being told that I am off subject,I would like to bring
members attention to a new online EV magazine that I found which gives
an insight into the current EV R&D which will eventually bleed down to
our level.

http://viewer.zmags.com/publication/7d322b63#/7d322b63/1

Peter


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[Electric Boats] Re: Electric Catamaran

 

That's generally true if the hull shape is the same and the displacement doesn't change very much.

--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Jason,
>
> Shorter hulls generally take more power to drive a similar speed than longer hulls due to the given speed being a larger percentage of hull speed. I know that multihulls can be different, but the general guideline usually holds true. My personal experience with trimarans indicates that an F-31 (31') will motor slightly faster than an F-27 (27') with identical engines. Your catamaran may take more power than Chris' to attain the same speed.
>
> In a traditional outboard, the gearbox does three things, reduces shaft speed through the gear ratio, allows for selecting forward and reverse and allows the vertical drive shaft to turn the horizontal prop shaft through a 90 deg gearset, all in an integrated package. The gearbox is located at the bottom of the drive leg and contributes to a loss of efficiency. Purpose built electric outboards like the Torqeedo and trolling motors can eliminate this efficiency loss by mounting the drive motor horizontally at the bottom of the drive leg and driving the prop directly.
>
> One of the difficulties in converting traditional outboards is that prop sizes are usually limited by the anti-ventilation plate part of the drive leg. There are also very few choices in pitch when you are dealing with the smaller (<30hp) outboards.
>
> Fair winds,
> Eric
> Marina del Rey
>
> --- In electricboats@yahoogroups.com, "jason_d_s_uk" <jason_d_s_uk@> wrote:
> >
> > Thanks for all of the comments.
> >
> > Chris, you quoted : 1500 watts = 4.5knots = 6 hrs
> > I would imagine my cat would be similar and 5 Knots would possibly be < 2 KW?
> >
> >
> > Has anyone taken apart an old outboard? Is the gearing in the top section? Would electric benefit from the gearing or better without?
> >
> > My main lack of understanding is with prop sizes. With an electric motor, the best torque is in the low revs, so surely a large prop would be best? ...
> >
> > Thanks for all of the responses, (in adavance)
> >
> > Regards,
> >
> > Jason.
> >
>

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Friday, December 11, 2009

[Electric Boats] Re: Electric Catamaran

 

Jason,

Shorter hulls generally take more power to drive a similar speed than longer hulls due to the given speed being a larger percentage of hull speed. I know that multihulls can be different, but the general guideline usually holds true. My personal experience with trimarans indicates that an F-31 (31') will motor slightly faster than an F-27 (27') with identical engines. Your catamaran may take more power than Chris' to attain the same speed.

In a traditional outboard, the gearbox does three things, reduces shaft speed through the gear ratio, allows for selecting forward and reverse and allows the vertical drive shaft to turn the horizontal prop shaft through a 90 deg gearset, all in an integrated package. The gearbox is located at the bottom of the drive leg and contributes to a loss of efficiency. Purpose built electric outboards like the Torqeedo and trolling motors can eliminate this efficiency loss by mounting the drive motor horizontally at the bottom of the drive leg and driving the prop directly.

One of the difficulties in converting traditional outboards is that prop sizes are usually limited by the anti-ventilation plate part of the drive leg. There are also very few choices in pitch when you are dealing with the smaller (<30hp) outboards.

Fair winds,
Eric
Marina del Rey

--- In electricboats@yahoogroups.com, "jason_d_s_uk" <jason_d_s_uk@...> wrote:
>
> Thanks for all of the comments.
>
> Chris, you quoted : 1500 watts = 4.5knots = 6 hrs
> I would imagine my cat would be similar and 5 Knots would possibly be < 2 KW?
>
>
> Has anyone taken apart an old outboard? Is the gearing in the top section? Would electric benefit from the gearing or better without?
>
> My main lack of understanding is with prop sizes. With an electric motor, the best torque is in the low revs, so surely a large prop would be best? ...
>
> Thanks for all of the responses, (in adavance)
>
> Regards,
>
> Jason.
>

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