Wednesday, March 24, 2010

Re: [Electric Boats] Re: Torqeedo speed prop

 

My $0.02 on noise:  Anything moving makes noise, spinning things make noise, especially if they are a little bit out of balance (i.e. real world spinning things).  Prop blades create a pressure wave that can make a little vibration in the hull as the tip goes past.  The brushes scraping over the commutator gaps makes noise in a DC motor.  Gears especially make noise.  Lets keep electric drive noise in perspective - exploding fuel REALLY makes a lot of noise.
 
The smaller Torqeedo's have a double planetary transmission needed to reduce the high speed of a small brushless outrunner motor low enough to drive a big efficient prop.  (that's the secret of their high efficiency).  You can hear the gear whine on youTube videos.  I understand the new larger 2.0 and 4.0 models are significantly quieter but have not heard one run myself.  Maybe Cap'n Todd has a comment?
 
The quietest drive I know of is an old fashioned trolling motor.  All the moving parts are under water, the motor is small and there are no gears as they are direct drive.
 
Denny
 
 
 
----- Original Message -----
Sent: Wednesday, March 24, 2010 10:29 AM
Subject: Re: [Electric Boats] Re: Torqeedo speed prop

 

Scott,
I use a Torqeedo 801L on my 2500 lb. Cape Dory Typhoon and while it is not dead silent, it is 10 or 20 db quieter than any ICE. At 400 watts, half power, you can hear the waves  against the hull. I don't have any current to speak of on Maryland's West River but it gets me back fine when the wind dies in the Chesapeake. I do not experience any "loud whining" sound.
Ned

On Mar 23, 2010, at 9:41 PM, scott weber wrote:

 

I tried the Torqeedo attached to the stern of my Griffin and it couldn't push my 3,000lb boat up the CT. river against a mild current. The surprising results was the amount of noise it made. It had a horrible loud winning sound that may resulted from the attachment to the stern but I don't think so. Any one else experience the same sound? West Marine took the motor back without question. I was satisfied to be able to at least try it.

From: Chris Baker <chris@currentsunshine.com>
To: electricboats@yahoogroups.com
Sent: Tue, March 23, 2010 3:29:01 PM
Subject: Re: [Electric Boats] Re: Torqeedo speed prop

 

Hi Larry,


Its taken me a week to get a chance to reply to this and your message about comparing the stiffness of the two props.

To begin with, the props from Torqeedo are apparently a highly efficient, modern design, making using of the flexible properties of the material.  Here is a small extract from the Torqeedo website:

On the other hand, the most modern large ships have been equipped for some years now with propellers that are the result of multi-dimensional optimization calculations. In contrast to standard propellers, the pitch and camber of the propeller are not kept (almost) constant across all segments of the propeller.  Instead, the pitch and camber are optimized based on a vortex grid calculation for each single segment of the propeller in a stepwise optimization over many thousand iterations. The  additional scope for design resulting from this allows the  additional speed to be induced by the propeller at the highest rate of efficiency. Due to these characteristics, the  corresponding propeller is designated as a Variable-Pitch- Variable-Camber (VPVC) Propeller.

I thought I had seen mention of them making use of the flexible properties of the plastic to advantage but I can't find reference to it just now.

The overall package seems quite efficient compared to the setup I had before (its about double the efficiency of the brushed motor and off-the-shelf prop) and so I'm inclined to believe their technology claims. If anything they seem understated.

As to comparing the two props...

I pressed them both at the tip tying to compare the stiffness.  I can't notice enough difference between them to draw any conclusions about this.  I guess the material is the same for both props, and that the colour difference is just to differentiate easily between the standard prop and the speed prop.

The shape of the blades of the two props is markedly different.  The speed prop has much more blade area towards the tips.  Which seems odd to me as I would have intuitively expected that a prop for higher speed would have finer blades.

As yet I have had no opportunity to test the speed again of both props under perfectly calm conditions.

Cheers

Chris


On 14/03/2010, at 3:28 AM, gramplarry wrote:

 

One possible source of the reaction "But what I really notice is its power in reverse." is simply the material and thickness of the propeller. I know that a plastic prop of the correct material and thickness will actually change effective pitch in reverse. The water pressure bends the tip of the prop such that 2 and even 3 extra pitches may be added to the prop. We have been designing props to do this for years. see http://mysite. verizon.net/ resqp86c/ performanceprope llers/id18. html
Most prop people are not experienced in the plastic material capabilities and flex of a plastic prop is not mentioned because it would be thought of as a flaw when is reality it can be an advantage if properly designed used.
This change in pitch would explain the increase in speed. This is not usually noticed in the sailing world with geared transmission simply because the forward gear ratio is specified and the reverse ratio which is different is ignored. The a common transmission ratio in a geared transmission 2.2 forward and 3.2 reverse is. this is also one source of reverse handeling problems in sailboats.
larry

--- In electricboats@ yahoogroups. com, Chris Baker <chris@...> wrote:
>
> I now have one of the so called 'speed' props from Torqeedo, and if
> you are considering a Cruise 4, this prop is worthwhile.
>
> But not for the reason you might expect...
>
> First some background. The Australian Torqeedo distributor, Claude
> Desjardins, has provided me a 'speed prop' for testing on Current
> Sunshine. I understand it was designed for use on light planing hulls
> with a view to getting more top speed for them than the standard prop.
>
> Oddly enough it seems to make little difference to speed - and if
> anything is perhaps a little slower for top speed. Since I've had it
> I have not had the opportunity to test it under still conditions and
> so my tests so far have all been effected by wind. And even just a
> slight breeze can effect the results by .1 or .2 of a knot either
> way. Anyway, the top speed is still around about 6.5 knots.
>
> But what I really notice is its power in reverse. It way better than
> the standard prop. Many times better. And this big improvement in
> reverse is also confirmed by a friend who has two Cruise 4 on his Oram
> 45ft cat. He also notices very little change in speed, but a big
> improvement in reverse.
>
> The standard prop is orange and has very fine blades - the speed prop
> is white and its blades have more area especially towards the tips.
> So this is perhaps why it shows such an improvement in reverse. But I
> expect there are alos other more subtle shape changes as well that are
> helping here.
>
> Reverse on the standard prop can be really unsettling on any boat with
> substantial mass - and this would apply particularly to the sailboats
> of this group. For the original design target for these outboards,
> the small dinghy, reverse is not so important.
>
> But for sailboats manoeuvring in close quarters it can be critical.
>
> If you are thinking of getting a Torqeedo be sure to ask about the
> speed prop.
>
> Cheers
>
> Chris
>




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