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[Peter Binns]

Hi
This is my first post on MBM, having enjoyed a day at Wicksteed Park last weekend.

I have recently began a scratch-build of a 1/25 scale model of a 1938 100' ocean-going motor yacht, originally designed and built for world speed record holder Sir Malcolm Campbell, but still afloat and now fully refitted for the Getty family and on charter in the Mediterranean under the name 'Bluebird of 1938'. It is 47" long with 9" beam, 8" from bulwark to keel and of traditional shape with rounded stern. For more info see my Build Log on RC Groups http://www.rcgroups.com/forums/showthread.php?t=2049803.

It is mainly built of balsa so will not be particularly heavy, although it has a fairly deep draught. Photos of sanded hull attached. It will have twin 4-blade props, the dia of which to scale will be 1 1/2" - 1 3/4", and a single large central rudder. I am not looking for more than scale speed equivalent to 13 knots.

I therefore think I need the following:

2 suitable motors (500 min?)
1 ESC with BEC (min 25A?)
2 prop shafts min length 16"
2 waterproof stuffing boxes min length 10"
2 4-blade propellers 1/1/2" - 1 3/4" di (actual boat now has submarine-type multi-blade anti-cavitation props.
2 suitable P-struts with bearings for exposed shafts

Can anyone suggest a suitable combination of motors, ESC, shafts and props that will suit this boat?

Many thanks.
 

[Jon]

I'm currently working on a smaller motor launch, mainly trying to work out the motors at present.
A post that was made to me may be of interest to you, as the motors may be just right.
Have a look at the following:

http://www.rc-point.nl/index.php?item=buhler-6-18-vdc-motor--12v-3030-toeren-1_7a--pakket&action=article&group_id=20000271&aid=69112&lang=nl#.U4m9CyhC1dw

They may be suitable.

On a side note I used to live by Falmouth where BluebirdII was berthed while I was there, and know Bob Harvey-George quite well.
Jon

[Peter Binns]

Hi Jon
Thanks for the link. I need to brush up my Dutch!

There are any number of possibly suitable motors available so I was hoping someone might be able to say what voltage, size, RPM at nominal volts, amp draw etc would be most suitable. Having recently come by a good 6v lead acid battery, I am now wondering what spec of 6v motors might give me scale speed and longer battery duration.

Interesting that you knew the boat, and Bob Harvey-George who I think sailed it back from the US, when it was still Bluebird II and berthed in Falmouth. Did he sell it to the Getty's or did it go to Antwerp first? Either way, it looks fabulous now. There was a great Superyacht Media video of it in the Caribbean on YouTube, but it appears to have been withdrawn by the owners since I linked it to my build log on RC Groups. I should have saved it!

Peter
 

[Jon]

Hi Peter,

Yes I understood your original post, but it is incredibly difficult to advise on a full propulsion system and get it right.
I've been playing with motors batteries and ESCs on my tarpon for a while, and it took two attempts on motors and two on props on my tug.

The basics are going to be low revving motors, ideally direct drive, 6V should be OK, but running 12 and throttling back may be an option too.

If I was going to make a stab at it you going to need a low rev high torque motor running at say 3K rpm
A motor Ive heard good reports about is the MMB 755, capable of 6V or 12V
http://marksmodelbits.ieasysite.com/ecommerce_018.htm
You could start with it on 6V, and if you need more umph move to 12V
or the T12 from MMM, again 6-12V, but that is apparently a beast of a motor (don't know it's RPM though, may be 3500 at 12V from a post I read)
http://www.mobilemarinemodels.com/acatalog/Motors.html

On the ESC front, I used to like Mtronics, but as I've managed to blow 2 Marine 20's in quick succession on 12V, I would not recommend any more.
Electronize is still a good make if you don't need compact, so a pair of Electronize Type FR30 HX, would do you.
http://www.westbourne-model.co.uk/acatalog/Model_Boat_Electronic_Speed_controllers.html
There are many no-name ESCs too, but be carefull not to get one with a brake, as it's a right pain to go quickly astern.

Prop shafts, I've had good experience with MMM (mobilemarinemodels) they have all you should need, diameter will depend on your hull, but I'd say 5mm ought to be OK for you.
http://www.mobilemarinemodels.com/acatalog/Sterntubes_Propshafts.html
MMM do custom tubes too, if you need extra long, or a specific length.
No need for stuffing boxes with a modern prop shaft, but talk to MMM about it.

My last props came from Cornwall Model Boats
Again they have lots of choice, go with something that looks right from the prop size on the plans, as mentioned before, it will have to be a best guess to start with.
So from what you say your after a 40-45mm prop, 4 blade, so possibly the top four here may be of choice (LH and RH pitch for counter rotating too)
http://www.cornwallmodelboats.co.uk/acatalog/4bladem5.html

P struts, I'd likely make them, never bought them, don't really know where to get them.

Hope this helps.
Rgds
Jon