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Power requirements?

For a 4 foot hull...

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Rowan Muir17/11/2008 16:20:00
18 forum posts

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!



Len Ochiltree17/11/2008 16:33:00
525 forum posts
64 photos

Hi Rowan,

How come it weighs 10k? you must be able to cut that down,are you using 1panel or lots of those small lightweight ones,if you have a plan please put it up on the Forum for perusal.There have been several articules on Solar challenges in boat mags.

This thread will get a lot of response from members.

Watch this space.



Rowan Muir17/11/2008 17:20:00
18 forum posts

Hi there Len,

 Yes we have to carry the PV panel on board which is around 7kg although we are going to have a look at hacking into the frame to remove some weight! We also have the weight of two drive systems; one primary being run directly from the solar panel, and one secondary which will be powered by energy 'stored' in the spring (one of the aims of the project is to not use battery for power storage!) and will kick in when PV panel cannot generate and will only aim to store enough power to get back to shore... although we don't anticipate going far from the shore...

 The real unknown at this stage is the weight and constraints posed by the spring... but we wont know that until we know what we need to get out of the spring! Typical. I'll be happy to give more information if anyone thinks they may be able to advise?

I guess I'd be keen to know what kind of power similar sizes of boats require to move so that I can get some idea of the requirements of the spring as it's design is going to be tricky!

Thanks for your input though!


ashley needham17/11/2008 19:16:00
7323 forum posts
156 photos
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 needham18/11/2008 07:07:00
7323 forum posts
156 photos
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 Muir20/11/2008 10:41:00
18 forum posts

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,


Len Ochiltree20/11/2008 17:16:00
525 forum posts
64 photos

Hi Rowan,

Any chance you can put some photos on the forum ?


Len Ochiltree20/11/2008 17:19:00
525 forum posts
64 photos

Hi Vinnie,

It looks like Rowan has started 2 x identical threads can you put them together so that we can see all the Q,s and A,s together.

Thankyou you are so kind.


Vinnie Branigan20/11/2008 18:19:00
443 forum posts
294 photos
8 articles


No, sorry but it can't be done.


Paul T20/11/2008 18:51:00
7334 forum posts
1229 photos
2 articles

Hello Vinnie

Welcome to the model boat site, its always nice to see a new face, what kind of models do you build?


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