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Discussion on Motors and Props

Updating data for a magazine article

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ashley needham18/09/2020 18:05:30
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Before this gets too detailed, I think the gist is....you have a model, it works well enough with the motor and prop/battery combo you have chosen....write it down.

Endless theorising is NOT what`s required for the purposes of the article as I understand it, I believe it is aimed at knowing what works with various types of hull and that`s that. (plse correct if wrong)

Are we including airboat prop or EDF combinations to bring the list fully up to date?? devil

Ashley

Very few of my boats are of any use for such an article as most of them are specials and thus the combinations I used are of no practical use for anyone.

Colin Bishop18/09/2020 18:34:39
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Interesting Chris, but rather outside the scope of the intended article which is aimed at scale models.

I am presently building a 1:150 scale of the Aberdeen line passenger liner ss Miltiades of 1903 to 1:150 scale. I have some 1:1200 waterline drawings, some photos of the orignal plans taken in the National Maritime Museum last year plus a lot of photos taken of the builder's model which used to be displayed in the London Science Museum.

From the builder's model, the draught marks on the hull indicate that the three blade merchant style props are 15 feet in diameter. At 1:150 scale that would be pretty much 30mm on the model. Raboesch sell this type of propellor in this size so that is the back end of the model taken care of. (it's a scale model so it has to look right, no black plastic two bladers)

I am using a conventional brushed motor/NiMH battery setup as it is convenient, relatively inexpensive and will do the job. I know from experience that 280 size motors would struggle a bit with this size of prop and an M4 13 inch shaft. A 540/545 type would be overkill and too heavy so the sweet spot is a 385 low drain type which is fitted in many other models of this overall size and weight. I also know that a 7.2v battery would probably be OK but an 8.4v pack would give a bit of power in hand. After that the speed will be dependent on how far I push the throttle stick.So, for me, job done using empirical practical experience withput any need to enter into tests or calculations although I will check the current draw to confirm it is within reasonable limits.

Obviously the less experienced modeller, or one coming into the hobby (who is the target of the article) won't have that experience to draw on but he may be building one of the Deans Marine traditional merchant ships which have siimilar dimensions and could therefore fit a similar setup to mine in the reasonably sure knowledge that his model will perform similarly - and that is really all he wants to know.

A lot of the points made above are in fact made in the original article which explains pitch, blade area etc. etc. so that readers have a basic idea of the factors involved without having to reach for a calculator.

Colin

Edited By Colin Bishop on 18/09/2020 18:36:44

Chris E18/09/2020 19:53:05
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Posted by ashley needham on 18/09/2020 18:05:30:

.......................................Ashley

Very few of my boats are of any use for such an article as most of them are specials and thus the combinations I used are of no practical use for anyone.

Ashley

I would have thought that you Landers, Mintanic & Sans Pareil (+possibly others) were good candidates. No example will ever be identical to all models of the same size.

Edited By Chris E on 18/09/2020 19:53:23

Colin Bishop18/09/2020 21:12:02
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One other thing not mentioned above but which was covered in the original article is the beneficial effects of gearing (gears or pulleys) n helping to reduce the disparity between optimum motor and prop speeds. Something that is frequently overlooked. Several of my models feature pulley drive which considerably improves efficiency.

My big 48 inch Fishery Cruiser has pulley drive and cruises at 2.5 amps.

Colin

brenda (2).jpg

ashley needham24/09/2020 08:59:29
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I used the wattmeter on my medium lander, twin 385 can motors! S35 two blade plastic props and on 9.6V on the day, and it gave me a 5A reading.

Looking in the shed, I have measured up a few boats propelled by the humble385 motor,

largest first. Props for all mostly s35 or brass 30-40mm. All dimension in mm

LST. 1600 x 300. 3x 385 Flat bottomed so not as heavy as it looks

Titanic. 1200 x 130. 3x 385

Argus a/c carrier 900x 140 2x 385

LCT 750x130 2x385

LCT 500x130 1x 385

In addition, the Devastation (not measured),

The motor copes with 40mm props, and the motors barely get warm. Landers normally run on 6 cell Nimh, the others on 7 or 8 cell for a bit more oomph. Neither the LST or Titanic are underpowered on 9.6v. Hasto be a go-to motor if ever there was one!

 

Ashley

 

Edited By ashley needham on 24/09/2020 09:02:29

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