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Fairmile D - help needed

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ashley needham16/06/2020 09:32:25
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6572 forum posts
144 photos

Personally I don’t think it would make much difference having the motors split. The only thing I would say is that you need to check how much current they all take when in the water and have an esc to handle that.

I have also found that having a (say) 40A esc is all well and good but when used at anything like their max or even say 2/3 the capacity, they get warm/hot and need cooling. Nothing over complicated, but air needs to be vented in the hull or you have just made a floating oven....

Ashley

David Hepworth 116/06/2020 10:30:14
10 forum posts

Hi Ray, Yes, it's about 60". 1/24 scale.

It has to have its own shelf in my workshop and gets put away before I can do anything else.

I bought the hull (fibreglass) years ago from a pal, and have only just got around to doing anything with it.

The motors, shafts, props and A frames came with it.

I chickened out of doing a scratch build superstructure. I'm using Russel Wilson's kit. So far it's coming together quite well.

It's upside down at the moment getting a coat of paint on the hull. I tried acrylic paint...and its rubbish. Just not binding with the filbreglass. So, thats coming off. Must remember... do a test patch first... do a test patch first.

Dave Cooper 616/06/2020 10:44:02
163 forum posts
16 photos

David, my Dad was a skipper on 70' recue launches in the war. They mainly used the two 'outer' throttles for manoeuvring in harbour (3 shaft boat). I imagine at speed it would just be helm steering...

For the cooling on my boat I made some air extraction vents. You can just see these on the album "Dave's RAF Launch", (currently on page 5 of Albums) - between the main mast and the gun turret.

I use an old-fashioned "Bob's Board" - resistive mat - type of speed controller. Very 'old-tech' now and was concerned about heat build-up in the hull. So far it has worked well.

The air vents work on a simple aerodynamic principle, any water spills over the top decking run down to the stern and there is an internal lip in the vents to prevent any 'backwash' (probably overkill !).

You may be able to fit /hide something similar on your boat - as long as they have a supply of reasonably undisturbed air they should work...

Let us know how you get on

Dave Milbourn16/06/2020 11:11:47
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3998 forum posts
282 photos

David

Speed 600 ECOs draw around 12A at maximum efficiency. You're intending to put nearly 50A through just one ESC? At that current the output MOSFETs of the ESC will get VERY HOT unless it's genuinely capable of carrying a lot of current continuously e.g. >60A. There must be such a beast somewhere but I don't know of one off-hand and it's bound to be pretty expensive. On that score beware of Far Eastern-manufactured units which claIm 320A - it's baloney.

I would use 2 x ACTion P94 Dual ESC/mixers myself - which is expensive unless you're the guy who makes them wink - but certainly not more than two motors of that power on one speed controller. You could then either use tank steering on two sticks or parallel the two ESCs on the throttle channel with a Y-lead from the Rx.

In my book if you need any sort of motor cooling then the motor is being overloaded. The same goes for the speed controller.

Dave M

David Hepworth 116/06/2020 11:23:01
10 forum posts

I'm using one of these ESCs. Has anyone used one of these before?

**LINK**

Its more usually used for robotics but accepts RC servo inputs so why not?.

Good for 25A continuous without heatsink and runs at 22khz so no ESC 'squeal'.

I've been playing about with it on the bench and It's a cracking piece of kit. Fully programmable for max speed, acceleration, current, temperature etc.

I'll set the current limit at max (25A) and see how the boat behaves in the pond. If it's too sluggish, i'll add another ESC and split the motors port/starboard.

John W E16/06/2020 16:24:12
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263 forum posts
257 photos

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Edited By John W E on 16/06/2020 16:24:49

Edited By John W E on 16/06/2020 16:26:42

Edited By John W E on 16/06/2020 16:27:16

John W E16/06/2020 16:25:03
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263 forum posts
257 photos

Hi David

I would suggest that you go with independent drives - port and starboard for this style of model - as you will find using scale propellers and rudders, this model will turn quite easily and manoeuvre.

When I built HMS Exeter, I incorporated a P94, which I split between port and starboard. HMS Exeter has 4 motors and they were Speed 600 driving 30 mm props - it was overscale speed though with these motors  - but, the speed controller handled it quite easily. I dropped the motors down to 500s to give a more realistic speed.

As for access into the hull on the dog boat, I had a similar problem when I built the Fairmile B Class. What I managed to do there was under the area of aft gun - underneath the gun bandstand - is an area which can be opened up in the deck to allow access to the rudder linkage and also propeller shaft couplings. I found that the bridge area, which was the main access, is quite adequate to give you good access inside the hull - because of the hull you are building. Here are a couple of photos - and there are more photos in my albums of both hulls.

John

Edited By John W E on 16/06/2020 16:29:00

David Hepworth 116/06/2020 17:10:38
10 forum posts

Thanks John, very useful.

I agree, I think 4 x 600s will be over powered but that's what came with the hull so they're getting used.

As mentioned above, I can program my ESC for whatever max power I like to give a reasonable max scale speed.

That'll be me at the pond with my laptop plugged into the boat wink

Dave Milbourn16/06/2020 18:44:03
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3998 forum posts
282 photos

DC electricity isn't very tricky stuff, whatever connotations and complications the digital revolution might have brought us. However 25A is still only 25A, while those particular motors are best running at 12A each. Restricting the current to 50% is like selecting the wrong gear in a car or on a bike. No doubt your laptop knows better, so good luck.

Dave M

David Hepworth 117/06/2020 08:39:58
10 forum posts
Posted by Dave Milbourn on 16/06/2020 18:44:03:

DC electricity isn't very tricky stuff, whatever connotations and complications the digital revolution might have brought us. However 25A is still only 25A, while those particular motors are best running at 12A each. Restricting the current to 50% is like selecting the wrong gear in a car or on a bike. No doubt your laptop knows better, so good luck.

Dave M

Hi Dave

As ESCs use PWM, is 25A at 50% duty is the same as constant 25A?

What is the difference between a purely resistive load and a high frequency reactive load?

My laptop doesn't know better, it's just a tool.

I'm not saying you're wrong - you're probably right but no harm in having a bit play about, is there?

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