Fairmile D – help needed

[David Hepworth 1Participant @davidhepworth1]

Hi all

I'm building a 1/24 scale Fairmile D.

Everything going fine so far but i've hit a snag which, hopefully, the more experienced can advise me on.

The Fairmile D superstructure is quite a modest affair and therefore the hole in the deck it conceals is quite small and gives limited access when removed. There's enough access to change batteries and get at the electronics but, if I needed to replace a motor, i'd be snookered.

Question is…..do you folk ever fit the deck and hope nothing fails, or is there a more cunning way of retaining access. Is there a practical way of making the whole deck removable and, if so, how do you make the 'seam' invisible?

Thanks in advance

Dave.

[David Hepworth 1Participant @davidhepworth1]

[Ray Wood 3Participant @raywood3]

Hi David,

It rather depends how far you have progressed with the build ? Large areas of the deck can be built in situ with sub frames at an early stage, more difficult if the deck has camber on it, I use this method with cling film so the deck/hatch can be removed after construction.

I never had a motor fail so far, but normally test the drive set up early on to make sure all is well 😄

Good luck with the build, and welcome to the forum 👍

Any pictures of your build will be welcome.

Regards Ray

[ashley needhamParticipant @ashleyneedham69188]

Hi David and welcome to the forum.

Like Ray, i find that the motors tend to last, its access to the motor/prop connection (u/j or whatever) is more likely the trouble. Also if you need to change the prop you need to hold the other end of the shaft.

Similarly the steering gear, access thereof.

No chance of hiding ahatch under a ….something…raft/gun and so=on?

Ashley

Edited By ashley needham on 15/06/2020 07:58:32

[Malcolm FraryParticipant @malcolmfrary95515]

There is quite a lot of gear on the aft deck, which should be very amenable to hiding the join of a removable panel. Also good for hiding the screw heads holding it down on its gasket. Anything mechanical is, at some time, going to need access for attention.

[Ray Wood 3Participant @raywood3]

Hi David,

This is the access for Mtb 71, there is a screwed down panel at the stern for the deck mounted rudder servo.

The centre hatch is where the engines are on the fullsize, my planking joints are so bad you couldn't see the join 😄

Regards Ray

[Dave Cooper 6Participant @davecooper6]

On my little RAF Rescue launch (1/48 scale). I made the main superstructure removable. Space is quite tight inside…

This gives access to Rx, motor, coupling, top end of shaft, speed controller and LiPo (battery pack). Rudder access and servo linkage was more difficult. I fitted a separate screw down hatch at the stern for this at a later date.

It was easier for me as, being an own design, I made a lot of it up a I went along….I wouldn't worry too much about deck joins etc showing as once it's out on the water you won't be able to see them !

Have fun,

Dave C

[harry smith 1Participant @harrysmith1]

Hi David

A flush deck setup like I made for my Corona Cabin Cruiser works very well.

Checkout my Albums on lift out decks.

I first did this setup on my Eileen fishing boat which has limited space as per your boat.

Harry Smith

AKA Canabus

[David Hepworth 1Participant @davidhepworth1]

I'm thinking an 'aft' deck hatch is the way to go.

Thanks for the advice

-Ray Wood "…my planking joints are so bad you couldn't see the join"

-Dave Cooper "I wouldn't worry too much about deck joins etc showing as once it's out on the water you won't be able to see them !" wise words.

[David Hepworth 1Participant @davidhepworth1]

Next thing…

I'm running with 4 props & 4 motors (brushed Graupner 600 Eco).

I currently have them all on a single ESC. This works fine on the bench but i'm reading about folk that split port & starboard motors off different ESCs, and using differential throttle to help with slow speed steering.

Would I see much benefit from this or is it over the top?

If so, would you recommend just doing it off two joysticks or using a 'rudder mixer'?

Thanks

Dave

[Ray Wood 3Participant @raywood3]

Hi Dave,

This must be quite a large model with 4 X 600 motors ?? 60" ?

I can never see the benefit of separate throttles for normal boating, the real thing would have used the rudders I guess most of the time 😄

Keep us posted it sounds like an interesting build.

Regards Ray

 

Edited By Ray Wood 2 on 16/06/2020 09:28:06

[ashley needhamParticipant @ashleyneedham69188]

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 1Participant @davidhepworth1]

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 6Participant @davecooper6]

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 MilbournParticipant @davemilbourn48782]

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 – 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 1Participant @davidhepworth1]

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 EParticipant @johnwe]

 

 

 

 

 

 

 

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 EParticipant @johnwe]

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 1Participant @davidhepworth1]

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

[Dave MilbournParticipant @davemilbourn48782]

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 1Participant @davidhepworth1]

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?

[John W EParticipant @johnwe]

hi there

can you tell us how many speed controllers you are going to use and are you going to use scale props? Scale props will need to be near enough 40mm diameter with 3 blades.

If you are driving a 600 motor on 7.2 volts from a NiCad or a Lipo – you will be drawing a fair few amps – and if you are using 2 motors per speed controller you will be very close (if not over the top) of the 25 amp limit to these speed controllers and no matter how much lap top power/programme power you have IT WILL STILL FRY THE SPEED controllers – I KNOW as I have a few tee shirts

John

[David Hepworth 1Participant @davidhepworth1]

Hi John

Motors are Graupner 600 ECO, with 30mm 3 blade brass props.

I'm driving at 12v (2 x 6v 12Ah AGM) and nowhere near max power as the scale speed would be stupid fast.

Yup – I'm expecting to have to buy a second ESC but…

As well as current limiting, my ESC has thermal limiting too (they're more usually used for robotics applications).

The batteries i'm fitting arrived today so I should be able to get the hull in the bath.

Static water is worst case. If it works in static water, it'll be fine on the pond.

If it goes into current limit, or thermal limit, i'll have my answer.

[Dave MilbournParticipant @davemilbourn48782]

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

Yeah – whatever…..

DM

[David Hepworth 1Participant @davidhepworth1]

Posted by Dave Milbourn on 17/06/2020 11:09:37:

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

Yeah – whatever…..

DM

Dave

Have I upset you in some way?

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