Electric wheels?

[spark]

"...if wind blows harder do you regain that half knot or just end up heeling more with no increase in speed?"

You regain the half knot.

"...still unclear how a fixed prop is able to cause less drag than one spinning in generator mode. Shouldn't drag be same or less?.. "

I have two contradictory sources on prop drag.
Gordon Trower (Yacht and Small Craft Design) states that tests show that a locked propeller creates less drag then a freely rotating one.
Larsson & Eliasson (Principles of Yacht Design) state the converse.
Not sure who to believe but adding the load of the generator (motor) will not increase the drag of the prop beyond the worst case (locked or free-spinning) which is what most yachties live with at the moment. It will, certainly, slow the rotation of the prop and Robin2 is correct that a large diameter propeller is required for regen, as it is for an electric drive.

 

[chippie]

Re: Prop Drag

Some time ago this subject came up and the person arguing the case for a rotating propellor having more drag used an autogyro as an example.
The point was made that with a stationary blade/wing the thing would plummet to earth. With the blade gently rotating there was enough lift(drag) for it to descend safely.
Comments anyone?

 

[Robin2]

Re: Prop Drag

I will deal with this next week - no time now

 

[spark]

Re: Prop Drag

I've been thinking about this and I reckon we might be trying to apply a general principle where it isn't appropriate.

Maybe both my sources are correct.

Imagine a 3 or 4 bladed propeller with high ratio of blade area to swept area. I could see this creating more drag when locked than when spinning.

Compare this to a two-bladed prop with slender blades, i.e. a low ratio of blade area to swept area (the autogyro being an extreme example). Would this one create less drag when locked than when spinning?

 

[Trevethan]

Re: Prop Drag

Been thinking about this too.. course I isn't too bright.. but.. in the case of the autogyro the lift rotor is in the vertical axis and lift/drag is generated above the prop, caused by the aircraft's horizontally movement.

If forward movement ceases, i.e the drive prop shuts down tha aircraft, stalls until sufficient forward motion - movement in the plane of the rotor brings the gyro make up to speed.. correct?

a spinning, or otherwise boat prop isn't able to move at 90 degrees to its axis unlike the autogyro. To develop an autogyro effect wouldn't the vessel have be moving sideways?

I have just re-read this and think its a load of balearics so will someone smarter than me help me out?

 

[chippie]

Re: spark

You make a good point on the relative drag between each type of propellor. I suppose a slender prop will always have less drag than one with more cross sectional area.
I had always thought that the prop would take the line of least resistance ie it wouldnt start turning until it was easier for it to do so than staying stationary, which in fact is what happens. Whether this affects the speed of the boat I am unsure.
I dont know how to define the problem in terms of physics, but in terms of cross sectional area presented to the water at any given point in time they (fixed or spinning) would be the same,but the spinning prop may generate more turbulence.
Upon further thought , the autogyro 'prop' is more a rotary wing than a boat propellor is.
I am open to all arguments and am still undecided.

 

[chippie]

Re: Trevethan

You may well be right, the autogyro prop is more a rotary wing and a boat prop is a screw or auger.
I think the jury is still out.

I must go , the All Blacks are about to play the Springboks.

 

[Robin2]

Re: Prop Drag

There are two separate issues here. (1) does a propellor create more drag when stationary than when revolving and (2) does one type of propellor create more drag than another. The answer to both is yes.

I suspect we can profitably leave aside the debate on autogyros although they are interesting in their own right.

Going back to the question of generating power from a propellor I reckon people may have been overlooking the fact that the propellor attached to, say, a 20hp engine, would not be anywhere near big enough to generate 20hp if it was being used in generating mode. I suspect it would generate very little power - perhaps less than 1/2 hp - which is why we are not conscious of them creating much drag.

 

[Twister_Ken]

B4 the thread disappears...

...can I suggest to 'the management' that this topic* would be worth an article or series of articles in one of the publications?

* electric propulsion, regenertaion, drag penalties, weight savings (?) hybrid systems, etc<P ID="edit"><FONT SIZE=-1>Edited by Twister_Ken on 23/07/2002 11:34 (server time).</FONT></P>

 

[Trevethan]

Re: B4 the thread disappears...

I'd be very interested in reading about.

 

[Robin2]

Re: B4 the thread disappears...

These last two comments sound a bit like you don't believe me

 

[John_Hampton]

I have just installed a Briggs Stratton (Lynch motor under license) on my 29 ft River Thames boat. does approx. 4knts @ 45A 24 V. This motor will give upto 8hp i believe at much higher voltage/current.

If you want to experiment with electric power I have 4 2.5Kw 24 volt motors for sale, they are surpus but un-used. The same power as the Vetus unit but they will need a) a conroller and b) gearing down. cost £200 each.

Other points, for electric power do not use sealed batteries as the large charging currents will shorten their life as you will not be able to replenish water. ( I have read up a lot on this subject before spending 1000's on it so just believe this, you MUST have traction batteries. see www.onboardpower.com for Trojan batteries.

If you want a small generator 12/24 DC at 100 Amps try www.adverc.co.uk they sell one for £1493+VAT

Hope this helps, my installation with 350 Watts of solar cells to charge 900aH @ 24V of batteries will be complete in the next 10 days so will be able to give practical advice after it is commisioned at john@kaboot.co.uk

 

[Robin2]

This is very interesting. I wish I could find a use for one of your motors but I definitely can NOT (message to self )

Other readers might bear in mind that electric motors are not constrained to a particular voltage except that very high voltages might cause breakdown of insulation. The important limiting factor is the current which will be lower, for a given supply voltage, at higher motor speeds. This means, if the motor can be geared to run faster you can apply a higher voltage to bring it back to the rated current at the high speed. Generally, then, running a motor faster will allow it to put out more power. You can easily over-cook a motor by putting too much load
on it so that it cannot run fast enough and as a result the current will be too high.

Of course, none of this contradicts my earlier views that a motor driven propellor will not be suitable to generate enough power while the boat is under sail.

Also, people may be interested to know that many electric model aircraft consume 45A at 24v from NiCd batteries - albeit for 5 to 10 minutes only.

 

[Sammy]

Just to put in my thoughts. I run a small fleet of electric boats (hire) 21' long with 1.5 Hp 48 volt motors these have a top speed of about 5kts and they will last for about 9hrs flat out, the hull has a low resistance factor. The problem I see is that to achieve this sort of capacity they need about an 8hr charge with a 3KW 240 volt charger . with these sort of figures in mind why bother? why not fit a reliable diesel with a good drive chain and leave it at that. Fuel cells are not quite with us yet.

www.BoatsThames.com

 

[Guest]

Poster: Sammy
>Just to put in my thoughts. I run a small fleet of electric >boats (hire) 21' long with 1.5 Hp 48 volt motors these have a >top speed of about 5kts and they will last for about 9hrs flat >out, the hull has a low resistance factor. The problem I see >is that to achieve this sort of capacity they need about an >8hr charge with a 3KW 240 volt charger . with these sort of >figures in mind why bother?

Why are these figures so off-putting Sammy? Don't the boats recharge nightly? Have your clients been showing up with water skiis, and been disappointed?

You need to rewrite a bit of your web site then:
<blockquote><font size=1>In reply to:</font><hr>

They run almost silently and are simple to operate. Perfect for a quiet and relaxing day out.

<hr></blockquote>


Cheers

Lock
http://groups.yahoo.com/group/QCYCTender/

 

[Sammy]

I was refering to having to charge up the batteries with a diesel generator on a sailing boat. For the use I put them to on the river they are fantastic really quiet very little trouble (if the staff remember to plug them in!) and I cant hear the charger running because they charge when I'm not there. But to run a genny to put the power in the batteries to give you silent running?

All the best.

www.BoatsThames.com

 

[Trevethan]

Its the same principle used on hybrid cars... batteries are used much of the time. But when assistance is needed a small efficient fossil fuel power plant kicks in.

The design gives you nice long legs. quiet operation mich of the time, allows for a smaller quieter diesel genny, operating at optimum speed, compared to a regular engine. Gives you a number of alternative recharge options - solarpanels , wind, and hydro turbines. as well as the diesel and shore power.

Heck.. put one on a boat on a swinging mooring and the tide will charge it 20 hours a day!

 

[Paulka]

I really don't see the point of wasting 50% of the power a diesel engine provides you with, just to have the propeller driven by an electric motor.
Let alone the additional weight, price, difficulty of maintenance, sensitivity of the unavoidably sophisticated equipment to salty athmosphere, etc.

 

[Guest]

<blockquote><font size=1>In reply to:</font><hr>

Poster: Sammy
Subject: Re: Electric wheels?
I was refering to having to charge up the batteries with a diesel generator on a sailing boat.

<hr></blockquote>

ah! ok...

<blockquote><font size=1>In reply to:</font><hr>

For the use I put them to on the river they are fantastic really quiet very little trouble (if the staff remember to plug them in!)

<hr></blockquote>

hehe... good point. I've seen alarm systems that report low charge/water conditions (flashing lights, beeping), but nothing "time based", now that you mention it... I'm trying to work up plans for a 40-persons electric ferryboat, so will "file away" something about some sort of alarm system if the boat is off and not charging!

<blockquote><font size=1>In reply to:</font><hr>

and I cant hear the charger running because they charge when I'm not there.

<hr></blockquote>

Sammy - cooling fans? transformer hum? not generators?! Please tell me what it is you're hearing while charging?

Cheers

Lock

ps... my site for the project:
http://groups.yahoo.com/group/QCYCTender/

 

[Julian]

An idea I had on this thread is to replace some or all of the ballast on a sailing boat with the batteries need to hold the charge. This would obviously depend upon hull and keel shape but should be practical on some boats.