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Scratch build - RNXS Exploit 1/16 scale

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paul (lost in france)04/09/2012 07:11:48
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7 forum posts
45 photos

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?

Bob Abell04/09/2012 07:32:16
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5264 forum posts
1585 photos

What have we forgotten?

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

Bob

Paul T04/09/2012 07:50:45
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4127 forum posts
781 photos
2 articles

Bob

Its one of these

mar_2321_detail_250x169 (small).jpg

Bob Abell04/09/2012 08:17:33
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5264 forum posts
1585 photos

Groan!................Another blinking' lifeboat!.................lol

Bob
neil howard-pritchard04/09/2012 14:59:44
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1005 forum posts
78 photos

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.

shipwright04/09/2012 20:39:12
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161 forum posts
51 photos

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

paul (lost in france)05/09/2012 08:11:17
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7 forum posts
45 photos

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!

paul (lost in france)12/09/2012 07:29:01
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7 forum posts
45 photos

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

shipwright12/09/2012 17:12:49
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161 forum posts
51 photos

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

paul (lost in france)12/09/2012 18:45:16
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7 forum posts
45 photos

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?

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