Power requirements?

[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

[Len OchiltreeParticipant @lenochiltree67043]

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.

Regards.

Len.

[Rowan MuirParticipant @rowanmuir31282]

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!

Rowan

[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

[Len OchiltreeParticipant @lenochiltree67043]

Hi Rowan,

Any chance you can put some photos on the forum ?

Len.

[Len OchiltreeParticipant @lenochiltree67043]

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.

Len.

[Vinnie BraniganParticipant @vinniebranigan92297]

Len,

No, sorry but it can’t be done.

Vinnie

[Paul TParticipant @pault84577]

Hello Vinnie

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

Paul 

[ashley needhamParticipant @ashleyneedham69188]

Rowan. I would have thought that having a cat would be better than a hull and two outriggers, from a drag point of view, as the outriggers do not contribute to the floatation of the overall mass, they just add drag…unless they are so large that they DO add floatation as an integral part. What comes first..the motor or the prop…good question. i think that in model boat building the prop comes with the hull and style of vessel…for instance if anyone was building a modern warship of so and so dimensions, then a prop of approx so big and of such a style for that vessel/navy would be chosen, in proportion to that normally used on a full sized vessel. Undoubtably a large prop turning slowly is the most efficient, and so you would want to go with that basis. How big ?? it might be an  idea to talk to the man at the "prop shop" –Simon is it?(help me out here chaps)  as he designs props. No doubt he will want to know what power there is available. The prop for the electric motor on a known gear ratio would be easier, as the engine power is known and the overall RPM could be calculated and so a prop could be chosen on that basis.

I was going to suggest using a bulbous bow, as this gives an improvement in hull efficiency…up to 15% I have read at cruising speeds, or even trying a semi submersible twin hull arrangement, however you may actually be going TOO slow for these devices to be effective.

Perhaps a Kort nozzle might help? no experience of them myself, but they aid low speed push apparently. Having two props near each other might not be a good idea, as one might interfere with the other (and you dont have much power to play with) so siting of these items needs to be considered…again easier on a cat, not so easy on a monohull. Is this craft required to be radio controlled or a simple free runner???

Ashley

[Rowan MuirParticipant @rowanmuir31282]

Hi there Ashley,

Thanks so much for your input! I have found an image of the hull (well, if you ignore the rest of the model) to give you an idea of the shape. It’s 4 foot long…

http://h1071118.hobbyshopnow.com/products/description.asp?prod=PRB3600

It’s the PT 109 model, basic light fibreglass hull, now with some reinforcements.

 The man from the prop shop sounds like he might be an interesting chap to talk to? I’m hoping to get a bit of prop research done today. Hopefully by the end of the day I’ll feel a little less like I’m going round in circles! Re the two props; I realise that it’s not an efficient set up however if we try to integrate the primary and secondary drive mechanisms we seem to get even more problems in terms of switching the ‘drive’ from the direct panel shaft to the spring shaft. Admittedly, transmissions are not my strong point but having one primary drive centered and the secondary drive offset, just to get the boat to shore seemed a great deal simpler (less likely to fail!) than trying to create a method of switching between two input shafts? Feel free to correct me there though! : )

 The boat is to be RC, and we’ve several servos to control different aspects. We aim to be able to have controllable speed and to complete a slalom course on the wee loch nearby…

Here’s hoping!

Thanks,

Rowan

[profParticipant @prof]

why dont you try putting some plastic bottles around the edges as that will help with bouyancy problems

[Rowan MuirParticipant @rowanmuir31282]

Might anyone know of some simple method to calculate the power required to move the boat?

[Len OchiltreeParticipant @lenochiltree67043]

Yes,

you jam in the biggest motor you can! 

Len  

[Steve ArendseParticipant @stevearendse23393]

i built a mining ship,1,4m in length,weighing 2.7 kg-powered it with a 9 v battery pack,placed small bilge pump and inlet was from forward mid hull ,reduced the outlet nozzle and placed it between waterline and plymsol marking…great speed ,around 4 knots.it sucked in water andf pushed out at stern,,when it moved faster it worked betterneed rudder though..

[Rowan MuirParticipant @rowanmuir31282]

Unfortunately jamming the biggest motor that we can on is not particularaly justifiable in a report! : ) We are not carrying out the relavent calculations… Thanks though! And interesting post on the mining ship!

[ashley needhamParticipant @ashleyneedham69188]

Rowan.We are advising you blind here. most of the advice so far given is not applicable to your project or otherwise defunct (for instance you already have a hull ). If we could know some parameters or object of the excercise this would assist anyone giving relavent information. Ashley

[KiwimodellerParticipant @kiwimodeller]

Two things that might help the research in to the alternative propulsion – first go back in time and see if anybody has copies of magazines pre WW2. It was common in the early 1900’s to have boats powered by clockwork springs and any photos you could find might give some idea of prop size. The second idea is why limit yourself to a water prop? If I was wanting a lightweight reliable propulsion system I would be trying an air prop driven by strands of rubber as some of the old model aircraft were. You would need some sort of lock that was radio controlled so you could release it when required but my memory of aircraft with multiple long strands of rubber wound up with an old eggbeater hand drill was that they were powerful and ran for a long time relatively speaking. It never hurts to think outside the square. Cheers, Ian.

[Len OchiltreeParticipant @lenochiltree67043]

Hi Rowan,

Its like walking through Treacle getting answers from you, so could we see photos of where you are up to hull wise/solar panel etc. 

What exatly did the person who set you this challenge ask for?, was it something dreamt up during a session in the local hostalry? 

If it was a written challenge can we see it? 

The more we know the more we can help mate, 

Regards.

Len 

[Rowan MuirParticipant @rowanmuir31282]

For a 4 foot hull…

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