What power motor?!?

[Rowan MuirParticipant @rowanmuir31282]

Hi there,

A group of us are designing and building a solar powered boat for a (very important!) university project. We are currently at the stage of sizing a motor for the boat, however we’ll only be able to get 12V and 4A max from the solar panel. We’re also having to provide our back up supply from a power spring (don’t ask) so need to get an idea of the power required to move the boat.

 Does anybody have an thoughts on how much power it migt take to propel a 4 foot hull carrying around 10kg?? We’re not hoping to move very fast… a slow crawl would be quite enough! Even a rough estimate would be really appreciated untill we can complete some drag testing… Does anyone have a similar size boat for comparrisson?

 Any comments really appreciated!

Thanks,

Rowan

[Paul TParticipant @pault84577]

Hi Rowan

The guy to ask would be 60watt he is without a doubt our leading expert on motors and power draw.

Paul

[Ian KennedyParticipant @iankennedy31889]

Hello Rowan,

 Are you using a storage cell for the power from the solar panel ie; a 12v battery, or tapping the power direct from the pannel?

if you are running direct from the pannel, i would consider a 5 pole geared motor from athe MFA/COMO range of motor/gearbox drives.

Possibly select a 919D series 385 12v with a reduction ratio of 10-1, this will reduce the amount of amps gobbled up from your panel and still give you decent performance, at 12 volts this will give you 1436 rpm on the drve shaft.

Maplin electronics sell these at about £19.00 or you can buy direct from http://www.modelflightaccessories.com

Also hull  design will be important in the efficiency of the model, blunt bow and wide hull  =  more drag = less speed = more power used

A  fine lined pointed bow and narrow width hull = less drag = more speed = less power to propel the model.

Good luck !! and i hope this helps you in some way.

Ian

[ashley needhamParticipant @ashleyneedham69188]

Rowan. Hull design comes in here. A properly designed displacement hull designed for a low top speed would take a lot less power to propel than, say, a speedboat type planing hull. You may get some benefit from a catamarn hull..and of course using light weight materials, For instance you might use a carbon fibre space frame (kite rods for instance) and polystyrene foam hulls (for a cat). 48 watts that you have should be enough to move the hull at a slow pace, especially if you choose the right motor and/or consider a geared drive to make use of an efficient large propeller. Without knowing what sort of hull you envisage its difficult to say how much power may be required. I suspect you may be best off making several prototypes, noit using the solar panel perhaps, but ballasting for same. Do we assume a clockwork type spring is being contemplated?? Ashley

[ashley needhamParticipant @ashleyneedham69188]

Thinking about it a cat is the way to go, as the solar panels are very heavy and need to be flat for max efficiency. A cat would give you the stability you need. unless there is a need for the project to incorporate a spring, it may be best left out..the drive from it would only cause drag unless retracted. A space frame could easily be made from kite rods and sockets made from aluminium tube bits welded together, and epoxied to the rods, megga stiff.   Ashley

[Rowan MuirParticipant @rowanmuir31282]

Hi there, thanks so much for your help!

We are actually doing both in terms of the drive. One prop will be run directly from the PV panel so thanks, we will investigate the MFA/COMO range! Any thoughts on a suitable ‘efficient large propellor’ to go with this? We’ll have a look.

 The real problem is the ‘back up’ drive which unfortunately has to be a spring. It seems that we can design a constant force spring to give us a certain amount of power for a certain amount of time, but we seem to have too many unknowns to design the system! We need to know info about the prop before we can design the spring, and also info about the motor… Would it make sense to choose the prop first and then design around that? That’s an interesting point about the 48 Watts being enough to drive the boat at a slow pace….

In terms of the boat/hull design, we have one hull but two outriggers for stability. The hull isn’t blunt, but it’s not that streamline either…

Thanks for your help so far though… please continue to fire any thoughts or questions to me if you have any! I’d really appreciate it if anyone had suggestions on good props too.

 Thanks again,

Rowan

[Ian KennedyParticipant @iankennedy31889]

Hi Rowan,

Glad to be of some help……..Regarding choice of prop, this can be a minefield as prop design is quite complex but i would go for a 4 bladed brass prop.

The reason for this is that you can twist more or less pitch into the blades, to give an increase in speed  with more pitch or better efficiency with less pich.

The Spring system could be very similar to the K.E.R.S system (Kinetic energy recovery system) that next years F1 cars will be using.

I would consider a clock type spring driven off a clutch mechanism taking power through gearing off the prop shaft and then when the maximum tension of the spring is achieved this could be released back to the shaft through gearing to give the correct rotation and increased power/duration.

This does sound a very interesting idea and i wish you good luck !

Ian

[Rowan MuirParticipant @rowanmuir31282]

Hi there,

This thread continues http://www.modelboats.co.uk/forum/forummessages.asp?dt=4&UTN=1328&last=1&V=4&SP= if you’re interested  : )

[Rowan MuirParticipant @rowanmuir31282]

Some general advice very much appreciated!

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