Scratch build – RNXS Exploit 1/16 scale

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4 September 2012 at 07:11 #36292
paul (lost in france)
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@paullostinfrance
I’ve spent the past 6 months deciding on what to build; I wanted to build something big because for my 3 year old son a boat that is over 1m long is practically real size in his eyes!

I gave up on the idea of ‘upscaling’ a tug plan that I liked and went for something completely different, the RNXS Exploit. It’s 1/16 scale, 1.3m long so ticks that box and I liked the look of it, also when we return back to the UK I could potentially show my sons the real boat in the harbour (may be!).

So the plans came, it looks bigger than I expected!. My wife’s first question was where it is going to live when I’ve finished it (valid point I guess). There is no construction detail so not a great choice for a novice like me, so in at the deep end!

I thought it a good idea to trace the frame lines onto file separators so when I fold them out I have both sides symmetrical. The idea worked but I noticed after that the left side of the frame lines on the plans are wider than the right side! So I scanned in the frame lines and using a drawing application corrected the scaling issue, then started again!,. Is it normal for plans to have errors, should I be looking out for more ahead?
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4 September 2012 at 07:11 #6821
paul (lost in france)
Participant
@paullostinfrance
4 September 2012 at 07:32 #36293
Bob Abell 2
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@bobabell2
What have we forgotten?

Never heard of an RNXS Exploit…….Picture please?

Bob
4 September 2012 at 07:50 #36295
Paul T
Participant
@pault84577
Bob

Its one of these
4 September 2012 at 08:17 #36297
Bob Abell 2
Participant
@bobabell2
Groan!…………….Another blinking' lifeboat!……………..lol

Bob
4 September 2012 at 14:59 #36303
neil hp
Participant
@neilhp
that's not a lifeboat, Bob……..they're all orange and blue……..it's a small RN patrol boat that is used for training young officers and crew on, if not mistaken.
4 September 2012 at 20:39 #36308
shipwright
Participant
@shipwright
Hello Paul,

Firstly you might want to look at **LINK** for various views of your fast patrol vessel (now used as training vessel for Birmingham University RN Training Unit).

You asked about people's experience of constructing from a plan (ie scratch build) – there are many experienced modellers on this website who only build from scratch (I cannot claim to be experienced as I have only just finished my first scratch build). I built a pilot launch using a Kingston Mouldings grp hull and the outline plan provided by them – I had to work out the position and shape of the frames for myself – as it was grp hull not many frames needed. I assume that the plans provided by MMI (Marine Models International) are comprehensive and with instructions. I found no errors in the outline plans provided by Kingston Mouldings.

It took me about 5 months to complete my pilot launch (though obviously I had other things to do during that period) – I found scratch building a lot more difficult than building a model from a kit (I constructed HMS Biter from Nautical Marine Models kit – took about 5 weeks mostly spent on that construction). Personally I would start with a kit model if I had no previous experience of constructing model boats.

Ian
5 September 2012 at 08:11 #36315
paul (lost in france)
Participant
@paullostinfrance
Hi Ian,

Thanks for the link, the photos are great, I could not figure out from the plan the curvature of the deck from the stern but this is clear in the photos.

Yes perhaps a scratch build for my first big project was a bit ambitious! I built a laser cut kit ‘sea scout’ with my sons earlier in the year so it’s not my first experience of model boats but I am a complete novice to it all!

I move back to the UK next august so ideally want to complete it before then!
12 September 2012 at 07:29 #36464
paul (lost in france)
Participant
@paullostinfrance
Can anyone help!, How do I work out what size motors I need? I am getting near to sheeting the hull but just in case I need to trim the frames down to accommodate the motors I thought I better decide now on the motor choice.

The boat is 1.3m long, 40cm wide, approx 20kg when finished. It has dual props, the drawing shows 45mm props (but maybe that’s just for illustration).

Any advice would be appreciated
12 September 2012 at 17:12 #36472
shipwright
Participant
@shipwright
Hello Paul,

Pleased to hear that your model boat construction is proceeding well.

I am somewhat taken aback at the figure of 20 kg displacement for a model that is 1.3 metres overall length. On the assumption that 20kg is the correct overall displacement I am sure that you will need a lot of power. For instance my scale model fast patrol boat HMS Biter is about 5 kg displacement and 3 ft overall length. In order to get that model going at 6 knots I have 2 x Graupner 600 Speed ECO operating off 7.2v 6.6 Ah NiMH batteries with 35 mm 3 bladed naval style Raboesch brass props (shaft diameter = 4mm). The real HMS Biter could manage at least 40 knots which translates to 8 knots at 1:16 scale. If you want a scale speed of about 3 to 4 knots for your naval boat you will need more power than I get from the running gear in HMS Biter. I know from measurements that when running flat out the combined current flowing through the 2 motors is about 18 amps at a voltage of 7.2 volts which equates to a power consumption of 130 watts. Assuming an efficiency of 70% then the power into the 2 propshafts would be about 90 watts (ie 45 watts per shaft).

I believe that you will need about 70 watts per propshaft = 140 watts total and then assuming efficiency of 70% the power from the battery would be about 200 watts.

Given that you need a lot power I suggest that you install brushless motors because they are more efficient (90 % is easily achievable). However all of my experience has been with brushed motors. Hopefully another member of this forum can provide advice on a suitable brushless motor/ESC/battery/prop combination.

A final thought. I would strongly advise that you do a flotation test as soon as the hull is watertight. Use weights inside the hull to see where the waterline lies. Is the weight of the superstructure likely to be a significant proportion of the 20kg ? The reason for this question is that a heavy superstructure can make a boat unstable (this depends upon the relative positions of the centre of buoyancy and the centre of gravity.

Ian
12 September 2012 at 18:45 #36475
paul (lost in france)
Participant
@paullostinfrance
Thanks, I made a mistake the drawing states 15kg, not sure if that includes a typical weight for the battery/motor etc. I have made the frames out of 6mm ply and sheeting with 2mm ply, I’ll fibreglass it afterwards.

I have a 40amp Viper ESC and two 12v 10amp sealed lead acid batteries going spare from another project (Chicken coup light dimming system – long story!) so ideally would like to stick with the brushed motor, although these batteries are about 4kg in total.

I was tempted to install two ‘MFA Torpedo 800’ however I was applying no science behind sizing them, only that there max current looked reasonable and could wire them up in parallel with the single ESC.

Good point on the floatation test, the super structure does sit high against the fairly swallow hull so I may need to consider lighter ply than I was going to use – I’m guessing I can do the floatation test once fibre glassed before painting etc?
12 September 2012 at 19:26 #36476
shipwright
Participant
@shipwright
Paul,

Are your chickens now in the dark ?!

On a more serious note the stall current of the MFA Torpedo 800 is about 30 A at 12 volts – so you will need to protect the motors and ESC by using a suitable fuse. At max efficiency (only 58%) the input power is about 60 watts (5.3 A at 12 V ) and output power 36 watts. The question is – will 36 watts be sufficient power to get a speed of 3 knots ? I can't calculate the answer but my guess is that you will need more power.

I assume that you have looked at the data provided on the MFACOMO website : **LINK**

Yes – do the flotation before painting – all you need is a watertight hull – if you have completed the superstructure mount that on to the vessel – then you can test stability as well as seeing where the waterline is.

Ian
5 November 2014 at 13:59 #53375
shipwright
Participant
@shipwright
Hello Paul ("Lost in France&quot,

If you are still in contact with this website it would be nice to see a photo or two of your completed ship. Your last album posting was in October 2013 showing the model on test in a swimming pool (but clearly not finished).

Regards.

Ian
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