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[Neil]

My friend's son who is studying for a degree in electrical and mechanical engineering and in his first term at university has been set a task and to be ready in two weeks..........

a model boat for a tug of war contest.....maximum outlay to be £20 quid, but that, I think, can include things borrowed..................and he has called upon me for help in all things......first mistake, lol

This was sprung upon me whilst I was just about to get into the bath about an hour ago, so I have had plenty of time to think about it whilst wallowing.

because of time and cost and the fact that it will probably be only used for the one time............ a  simple springer tug type shaped hull, with a high torque buhler motor of 24-36volt output running on 3 pp9 batteries soldered in series to give 27 volts, throwing one of my 92mm props that I had for the Clyde lifeboat with a simple on off switch, no radio and a set rudder fin........totally simple layout.............

but how would this stack up against what others have been talking about in his "class" of small motors throwing small props at 15K revs on their own designed hulls ( probably the usual pointy bit at the front and flat bit at the back).............your thoughts gents, please, and quickly.

ta....he has to start building this saturday.

neil.

[boatmadman]

I favour high torque, low revs and big prop.

Small prop + high revs can lead to cavitation and loss of efficiency. Can he use a nozzle? There are big gains to be had with a nozzle at low speeds.

Ian

PS, good project, keep us informed

Modify: if its a straight tug o war, there will be no advantage of a sharp pointy hull over a springer type as its all about power through the prop, not speed through the water. This supports the thought of a large prop as it can shift more water

[Shipmate60]

The larger, slower turning prop should win if powers are about equal.
As the prop is turning slower it should carry more pitch and retain a considerable advantage in efficiency over a smaller faster running prop.
As practical application tugs have large slow running props which allows the prop(s) to "bite" the water.

Bob

[Neil]

I thought that guys...................but would it be the same in a model??

as for nozzles, Ian...............I was hoping to keep it simple for him as he's never dabbled in anything except real cars and motorcycles..............and as the prop will be so big, I don't see that a nozzle on a small springer would make a great deal of differenct.........but will tell him about them.
cheers, guys.
neil.

[boatmadman]

Neil, my opinion for what it is worth, is that even on a model a nozzle will give a useful gain in thrust as there is a significant reduction in water being thrown off the tips of the prop blades and diverted into the direction required.

Ian

[Neil]

cheers ian.............will pit that to him.
neil.

[dreadnought72]

^ What Ian says. Confining the high pressure, post-prop water with a nozzle - however crude - has got to be a winner. Got to equal more thrust. Go here for some ideas.

Andy

[boatmadman]

^ What Ian says. Confining the high pressure, post-prop water with a nozzle - however crude - has got to be a winner. Got to equal more thrust. Go here for some ideas.

Andy

Nice to have your gut instinct confirmed isn't it? Shame it doesn't happen more often!

[Neil]

I think we are getting far too technical now, andy..............he just wants it basic and a very simple quick build.........

will low revs high torque and big prop for under 20 quid including prop shaft, wood and glue, beat a fast high revving motor and small prop ..........and I think the first two answers from Ian and Bob, answered that very well...........many thanks guys.................we'll go down that route.
cheers, neil.

It's not that I'm dissing those ideas and links, but this is a quick build so that the lecturer can assess the research and conclusions that he has come to in a short space of time.
sorry didn't mean to offend.
neil.

[ardarossan]

Neil, As it's so basic, why not do a 'Mythbusters' type build in a universal motor mount. Then try both types of motor, both types of prop and nozzles if you have one, and pull against some a Fishing scales (No pun intended).

In fact a test-rig could be set up in a existing model/hull and the prop/motors combinations tested almost immediately, i.e. before the 'project boat' is even started, enabling you (him) to evaluate hard data and install the most powerful combination without compromsing build time.

Andy

 

[dreadnought72]

...sorry didn't mean to offend.

None taken Neil! This is, after all, only the Model Boat Mayhem corner of the internet. 

But your plans so far seem admirable & acheivable ... and therefore it might be worth a couple of bonus points to shroud the prop with a short tube (doesn't need to be fancy) in order to maximise your torquey thrust. (And/or convince the lecturer that your friend's son is thinking/aware about the deeper issues here.)

Andy

[boatmadman]

Yeah, think outside the box, or in the tube!!

[Neil]

yeh, thanks guys..........I don't suppose the prop has to fit too closely to the tube, and the last thing I want is to file the ends of the blades of the prop I am lending him...........it cost me 70 quid and is one of a pair....................I suppose a piece of rainwater drain pipe would do for the nozzle..........what I don't want to do is to actually make it for him, as it is his project..........just want to point him in the right direction.......the rudder I suppose as it will be a pure straight tug of war will be a fixed fin behind the prop ( and  Kortz nozzle tube if he so desires to fit one)

cheers, guys............will let you know how he went.
neil.

[Shipmate60]

dreadnought72,
I agree that a Kort Nozzle can increase efficiency but the ones quoted were specialized prop/Kort systems.
Just putting a straight pipe over a non-Kort 3 blade prop will not make any appreciable power increase.
It will however increase steering response.
To get the full effect of a Kort nozzle you need a prop matched to the diameter of the Kort ans a matched aerofoil section within the Kort.
If it was that simple with shipowners being so cost conscious you would see lots of pipe welded around props on full size vessels.

Bob

[Tug Fanatic]

......... and then there is the question of which make/design of prop is best at transferring the power of the motor into thrust.

I don't think that a straight tube will add much - in fact you might loose thrust. For a kort nozzle to work well you need a prop that is designed for a nozzle (different blade shape) as well as an airfoil section in the nozzle.

What are the design limits?

One of the best answers would be a borrowed/second hand trolling motor bottom unit on the bottom of a model hull. 

New £20 isn't going to buy much motor/coupling/propshaft/prop.

 

[Neil]

as budding electrical and mechanical engineers........those are the problems they have to overcome themselves................all he asked me was hi or low reving engine and what shape for a quick hull..........I think those questions have now been answered without going into great design deapths.

cheers to all that contributed.
neil.

[grendel]

for what its worth does a quick hull matter, as from the start the models will be stationary, a flat back to the hull will resist the backwards motion imparted by the other ship, so wide and flat at the back, pointy at the front would work, the wider and flatter at the back the better (make the back of the boat a good sea anchor) you dont need to be able to reverse it, also the flatness at the back will allow the flow of water from the opponents prop to push your ship. (for even further out of the box  thinking)
imagine a big fold up scoop out of the bottom of the boat, that if you move forwards will fold out of the way, but resists backwards movement.
Grendel

[Netleyned]

Quick as in quick to make methinks.   

Ned

[Neil]

Quick as in quick to make methinks.   

Ned

yes you are right Ned......they have two weeks to make and have the contest from this coming sunday.
neil.

[Tug Fanatic]

The biggest problem is the best answer for £20. I spend more than that on the prop & shaft alone.

I know that this has gone beyond the original question but that is what happens in forums.

What is the best tug power system for £20, £30, £40, £50 etc is an interesting question.

[malcolmfrary]

The person specifying the problem didn't mention korts, but did mention a budget.  Did he suggest any particular power level?  Motors come in many varieties, and need matching to the props, so adross' suggestion to enable easy swapping has lots of merit.  A small high revving one might well take the same electrical power as a larger slow revving one and provide more confusion than information.  Within the cost limit, there is little chance of funding a rev counter, so the only available comparison is likely to be current drawn, presumably they will be using the same voltage battery.
Keeping costs down, the late towboatjoe had some very good ideas on home made props - I don't know how long his site will persist, but it's still at http://towboatjoe.tripod.com/shafts.htm  
Thinking about comparative power transfer, why 2 boats pulling away from each other?  Why not a double ended springer?*  2 sterns, so just seeing which pushes hardest.  Both props could be driven off the same motor with gearing/pulleys to arrange for the different speeds.
*Think back to the Simpsons episode where they borrowed the plot from "The Fly" and Bart produced a dog with two tail ends.

[grendel]

plank of wood motor, battery and paddle wheels - sorted, nice big paddles on the wheel, nice reduction on the pulleys, get it turning at a reasonable rate and you may even be able to fling up enough water to sink the other boat. advantage - no need for a prop shaft or prop (reduced costs). paddle can be made from a length of round wood (1"-2" diameter) with sawcuts to slot blades into, fixed across the back of the plank on two extended bars, small notch carved round the shaft of the paddle at one end for the rubber drive belt, and round a smaller pulley on the motor. small wood block as superstructure and thick dowel as funnel for cosmetics.

Grendel

[malcolmfrary]

Just made the mistake of re-reading the original question.  Is this a classfull of students each making their own boat?  Has there been any size specified or is it just who can produce the most pull for £20 regardless?

[grendel]

more thoughts, although when the british Navy matched 2 equal boats paddle vs prop the following resulted -

The Alecto reached full power quicker, and pulled the Rattler backwards for 5 minutes.  However, at five minutes the Rattler reached full steam, and brought the Alecto to a stop.  With the Alecto's paddlewheels turning furiously, the Rattler then towed the Alecto backwards at a rate of 2.8 knots.  From then on all new ships of the British Navy were propeller driven.

so basically if you have paddles and can get the power in first you might stand a chance.
another thought for power source - old battery drill (or very cheap brand new one for about a tenner), full charge in the battery on fastest setting, either to a big prop or paddle wheel.
Grendel

[grendel]

Hi torque slow revving motor/battery unit http://www.diy.com/nav/fix/power-tools/drilling-screwdriving/drilling/cordless/12V-Cordless-Drill-Driver-10957593 £13
bring the propshaft above water level and you dont need an outer casing so could just use a threaded rod direct into the drill chuck (file 3 flats for better grip) prop on end of threaded shaft.
http://www.diy.com/nav/fix/hardware/nuts-bolts/threaded_rods/FFA-Concept-Zinced-Steel-Threaded-Rod-L-1000mm-x-W-8mm-9283910?skuId=9293610 6mm threaded rod - £2.50
there we go most of the parts (barring the prop) £15.50
Grendel