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Author Topic: A probably stupid motor question.  (Read 2772 times)

rmaddock

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A probably stupid motor question.
« on: August 25, 2011, 05:04:43 PM »

This is probably a question for Action man  :-)) but anybody can feel free to comment.

According to a history of Nobbies (top shelf of the book shop, in case you wondered  :embarrassed:) the one I'm modelling was fitted, when built in 1912, with a "bulb" type engine.



You can see the off-centre prop in the picture above.  As a powered model novice, I've a few questions.

  • Given that the model is 44" (possible more like 66" with bowsprit), what size of motor and prop would I need. Enough power to run against the wind and get out of trouble but I'm not looking to get her up on the plane.  %%
  • Unless I use a very long propshaft, I'll end up with the motor below water level.  I don't really want the shaft very long as I'd like the whole installation tucked away in a corner of the hull, not getting in the way of everything else. So, what do I do? I think there are water-tight shafts; is that possible?
  • The chances are that one of Action Man's power distribution boards would suit me well. I'm looking at running one motor (as per) a rudder servo and three (3) sail winches as well as the Rx. What do people suggest?

Cheers!

R.
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MikeA

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Re: A probably stupid motor question.
« Reply #1 on: August 25, 2011, 05:36:18 PM »

that picture link. is the model in the slide show your boat?
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malcolmfrary

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Re: A probably stupid motor question.
« Reply #2 on: August 25, 2011, 06:09:53 PM »

How about
http://www.cornwallmodelboats.co.uk/acatalog/graupner_multispeed380.html
probably best run in the lower of its voltages.  The shaft can be rendered water resistant by the application of water resisting grease.
Probably enough to get out of trouble, but again, that depends on just how much trouble, and what kind.
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rmaddock

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Re: A probably stupid motor question.
« Reply #3 on: August 25, 2011, 06:34:59 PM »

that picture link. is the model in the slide show your boat?

Yes it is, Mr Mad.
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rmaddock

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Re: A probably stupid motor question.
« Reply #4 on: August 25, 2011, 06:36:20 PM »

How about
http://www.cornwallmodelboats.co.uk/acatalog/graupner_multispeed380.html
probably best run in the lower of its voltages.  The shaft can be rendered water resistant by the application of water resisting grease.
Probably enough to get out of trouble, but again, that depends on just how much trouble, and what kind.

Okay. Can anybody explain to me the benefits of the 2:1 reduction? Is it simply to turn a larger prop from a smaller motor? More scale?
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MikeA

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Re: A probably stupid motor question.
« Reply #5 on: August 25, 2011, 06:39:23 PM »

yes 2:1 allows for driving larger props at the cost of rpm.
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rmaddock

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Re: A probably stupid motor question.
« Reply #6 on: August 25, 2011, 06:56:22 PM »

Okay, having looked at the boat, what if I went for the following:

Raboesch  45mm M4 Threaded 2 Blade L/H Brass A-Type Propeller (HERE)
Raboesch Waterproof Propshaft 186mm M4 (HERE)

What could I use to drive that setup?  Similar sort of setup to the above suggestion? 2:1 reduced 380?
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dreadnought72

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Re: A probably stupid motor question.
« Reply #7 on: August 25, 2011, 10:32:10 PM »

My boat's 44" too - but far beamier. The displacement is ~13kg. I've got a four blade 45mm prop on a tube which is below the waterline at the inboard end.

I'm using a 12V Buhler which pulls two amps at a couple of thousand revs. Should be enough for when the wind stops.

Andy
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ACTion

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Re: A probably stupid motor question.
« Reply #8 on: August 26, 2011, 08:39:21 AM »

If t'were mine I'd start with a 2.5:1 geared MFA motor on 6v and a four-blade prop of maybe 45-50mm diameter. For a more compact installation then use a 555 motor with a Graupner 2:1 gearbox G1114.20 http://www.cornwallmodelboats.co.uk/acatalog/gearboxes.html. The motor then lies above the prop tube with the gearbox at the front.
2-blade props need to be run at a much higher speed in order to obtain the same thrust (that's why tugs don't have 'em). A P92 D/B would be a good idea if you're after running three independent winches - have a look here at one such set-up http://www.action-electronics.co.uk/pdfs/HMS%20Surprise%20-%20Brian%20Wilson.pdf
The pump is used to "blast" water across the rudder to improve turns (not my idea BTW).
DM
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rmaddock

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Re: A probably stupid motor question.
« Reply #9 on: August 26, 2011, 09:12:57 AM »

If t'were mine I'd start with a 2.5:1 geared MFA motor on 6v and a four-blade prop of maybe 45-50mm diameter......2-blade props need to be run at a much higher speed in order to obtain the same thrust (that's why tugs don't have 'em). A P92 D/B would be a good idea if you're after running three independent winches

Thanks DM :-))

I was looking at 2-bladed props simply because that's what's on the full-size boat...presumably to reduce drag...but I'm more than willing to take advice.

So would the dynamically named "P79 CONDOR" e.s.c., as shown in the Wilson link, be the preferred choice for the MFA motor?

Also, I was planning to use something like Hitec winch servos. The Surprise installation is using the MFA motors and escs.  Would that still be the power board of choice if I were supplying such servos?
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ACTion

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Re: A probably stupid motor question.
« Reply #10 on: August 26, 2011, 10:12:40 AM »

P79 would be fine.
You would need a fine-pitch 2-blade prop. I think that Graupner actually do a folding one for the purpose, but I imagine it's at a Graupner price, too!
If you're using winch servos then they generally connect directly to the receiver and not via a power D/B. However some require a separate power supply e.g. a 6v battery; in that event you could use a P92 to supply 6v power to all three.
The HMS Surprise schematic was drawn to use geared motors and ESCs as that is what the customer had already installed.
Dave M
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Brooks

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Re: A probably stupid motor question.
« Reply #11 on: August 26, 2011, 10:50:44 PM »

Motoring home in a calm does not require much power, thus a small prop would work (and would reduce your prop-generated drag while you are sailing). A geared, large prop will decrease your battery draw, making the motor's battery last longer...but presumably you are not motoring for a long time, so battery life is not limiting. I'd say skip the complexity for this situation: go for a small prop and preserve your sailing performance.

Sailing home against a wind strong enough to get you into trouble, on the other hand, will require LOTS of power. The drag of the mast, sails, and rigging goes up as the square of the windspeed. A large enough prop/motor/gear combo to accomplish this will definitely degrade the sailing performance of your boat. Better, I think, to handle this scenario with seamanship - keep out of trouble by employing your brain before and during your cruise. The main seamanship tip I'd offer is to reef your sails before setting out - match your sails to the wind. The only way a sloop will get into trouble in high wind is if the sail area is too great, leading to knockdowns. Unfortunately, if the wind is this strong, even a propeller may not save you (the boat pounds, prop and rudder  are periodically exposed, progress is very slow....been there done that :-). About the best you can do is use the motor to assist your zig-zag course home  (ie beating with tacking or wears); progress directly to windward, against the waves, is going to be iffy.

If you are constrained to sail in an area where self-rescue is mandatory (along the seashore, for instance), perhaps you should devise another method of rescuing your boat, eg. rowboat or kayak.
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rmaddock

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Re: A probably stupid motor question.
« Reply #12 on: August 27, 2011, 09:46:31 AM »

Motoring home in a calm does not require much power, thus a small prop would work (and would reduce your prop-generated drag while you are sailing). A geared, large prop will decrease your battery draw, making the motor's battery last longer...but presumably you are not motoring for a long time, so battery life is not limiting. I'd say skip the complexity for this situation: go for a small prop and preserve your sailing performance.

Sailing home against a wind strong enough to get you into trouble, on the other hand, will require LOTS of power. The drag of the mast, sails, and rigging goes up as the square of the windspeed. A large enough prop/motor/gear combo to accomplish this will definitely degrade the sailing performance of your boat. Better, I think, to handle this scenario with seamanship - keep out of trouble by employing your brain before and during your cruise. The main seamanship tip I'd offer is to reef your sails before setting out - match your sails to the wind. The only way a sloop will get into trouble in high wind is if the sail area is too great, leading to knockdowns. Unfortunately, if the wind is this strong, even a propeller may not save you (the boat pounds, prop and rudder  are periodically exposed, progress is very slow....been there done that :-). About the best you can do is use the motor to assist your zig-zag course home  (ie beating with tacking or wears); progress directly to windward, against the waves, is going to be iffy.

If you are constrained to sail in an area where self-rescue is mandatory (along the seashore, for instance), perhaps you should devise another method of rescuing your boat, eg. rowboat or kayak.

Sounds advice there Brooks, thank you.  I'm used to reefing full-sized boats so I was planning on including that feature on this model....although hopefully not the traveller wire that snaps when your on a nice broad reach. It was lucky none of us ended up in Coniston Water  O0

About motors, I'm now torn. Hmm.
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MikeA

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Re: A probably stupid motor question.
« Reply #13 on: August 27, 2011, 12:12:11 PM »

well maybe you could use a powerfu one then. put a 540 brushed on it with a 30-32mm prop, 20 amp esc, 7.2 nimh. you go can go slowly for scale and if you get into the a sticky spot floor it!! :-))

that probably looks stupid hang on, the small 30mm prop will not look scale ill admit but it will create less drag. secondly with ilimination of the gear box itl will require less torgue to idle, a 800 1000 mah battery would be fine to save weight, even a lipo if funds allow.
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triumphjon

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Re: A probably stupid motor question.
« Reply #14 on: August 28, 2011, 07:30:32 PM »

graupner market a folding yacht prop , will only drive the model forwards , and when under sail power it folds flush with the shaft , ive just ordered one for my next scale sail costs around 17
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