ubay

Hi I
am looking to build my second Rc boat from scratch and i am stuck even before i begin buying the parts, my first model failed during its maiden voyage and ended up sinking (pva was'nt waterproof and the panels split off :/)

This time i an looking to build a cargo ship (fibrefglass hull so it wont sink), but not necisarily to scale. i have found this hull on ebay (http://www.ebay.co.uk/itm/FIBREGLASS..._qi=RTM1489731)

Now for the main problem what motor/motor set up should i buy, i was thinking of low rpm and high torque, say approx 60-100RPM but this would mean i would have to buy a gearbox as most motor Rpm's are in the 1000's at full throttle, i am not expecting break neck speeds a light sprint at the most. I am expecting the ship will be heavy once completed as i will be fitting a bow thruster, lights and maybe a functioning derrick/crane depending on my funds.

What should i use for the main propulsion?

mfr02

60-100 rpm might be good for a full size boat going slowly. On a model it would be OK for holding station. Several hundred to a few thousand rpm is nearer the target. Look at a 385 motor (brushed, 5 pole, up to 15 volts), run it on a 4 or 6 cell NiMH battery, direct drive, 3 blade 35 mm prop.
It might get a bit crowded in there if you go for all the extras, and you need to be careful with weight distribution. The hull might not let water in from below, but if it falls over, it will fill and sink just as surely.
When you get your hull, weigh it, mark what you reckon the waterline will be, fill it with water to that level, weigh it again. Subtract the one from the other, you now know exactly how much your hull can carry, and what your budget is for the total of everything that you are going to fit it with.

ubay

I was be hoping to spend less than £200 but looking at the cost of parts I am guessing it would come to more than that. Thanks for clearing up my confusion i will go direct drive. Is this the motor you mean (http://www.cornwallmodelboats.co.uk/...mfa-re385.html) and could i run it off two, 12v (in parrallel, keeps same voltage) electric scooter batteries similair to this one (http://www.ebay.co.uk/itm/like/13041...ff14=83&ff19=0) as i dont fancy splashing out of high tech bateries, as they are quite heavy for there size it might do well to keep the hull at even trim (carful calculations needed, or just trial and error )

The esc therefore shouldnt need to be greater than 20A, or should i go higher for a safty margin (http://www.ebay.co.uk/itm/Turnigy-20...item1e7e880781)

mfr02

I doubt whether just one of those batteries would even fit in the hull. Equally I doubt whether the hull could support one in water, let alone two. At 12 volts, it would be over powering the boat. A NiMH pack of 4 or 6 cells (4.8 - 7.2 volts), about 3000mAH you will get appropriate performance. 20A ESC? No Way. Unless you are talking Chinese amps. An mtronics Viper 10 will be more than adequate. The Turnigy has "programmable braking". The last thing you want on a boat is brakes, or a braking sequence to go through before reversing. If it can be programmed out, a good choice.
NiMH are not "hi-tech". With the right charger, they are possibly the simplest batteries available to set up and use. It is also much cheaper to buy the right ones first go, rather than buying the correct ones as well. Blowing a quarter of your cash budget on batteries that you will be unable to use will not be a good experience.
There is no guesswork with the weight - Archimedes knew what he was talking about when he wasn't inventing the screw.
You are quoting £ - are you in the UK? If you are have you found Component_Shop yet? Good for batteries.

ubay

The battery dilemma was because i already have them so buying more would reduce the amount of cash i have and yes i am from the UK, the £ sign is correct (budget approx $310). Its a good point about the breaking and the 'mtronics Viper 10' is £22.99 ($35.93) which i quite a big proportion of my budget just for 1 Esc, where i will need 2 (one for the bow thruster as well), suddenly my budget does seem very small. and there are no good model shops around where i live the only one is modelzone and that's gone into administration, so i will be buying most if not all parts off the internet.

mfr02

Alternative brushed ESCs, made in UK, here - http://www.mr-rcworld.co.uk/index.php?ukey=home
A battery that will drive a boat that size at something like hull speed all afternoon - http://www.componentshop.co.uk/6-0v-...ack-5-x-1.html
BTW, doing some very basic arithmetic on the hull in your link, the completed model can weigh about 6-9 pounds, but no more. That's your weight budget. Hull performance is limited by submerged length, trying to increase speed above this usually only results in an unstable boat and a battery that goes flat faster.

ubay

A bit of background about me before i continue i am an marine engineer student currently in my final year, and i do believe your calculations are a little bit off for the mass that the model can hold, as i am assuming the block co-efficient is around 0.7 as the form is of a container carrier (to the best of my knowledge, i could be wrong). The draft at the bottom of the transom is 6.5cm (calculated using digitizing software) therefore this is the minimum permissible draft for full propeller immersion. Using the following formula [Displacement=L*B*T*CB*(density of fluid, ignoring this as its fresh water)], =0.85*0.16*0.065*0.7=0.00618 m3 or 6.18 kg (13.6 pounds minimum/optimum).
Sorry if i am still wrong but this is one of the fundamentals to the displacements of ships.

Just to keep you in the loop i have purchased the hull but can take up to a month to arrive and i will be doing a documented build of the next year or so. Dont expect it to be done anytime soon i still need to design the superstructure etc, probably a simple bulk carrier design

Further calculations i have done is that the maximum propeller size and therefore optimum is 40mm diameter, 4 blades (but still unsure as this may put to much strain on a 380'ish motor)

Thanks for all your help and insight for the the electronics so far, i do appreciate it

mfr02

OK, go for a 545. As regards the displacement of the finished item, that was a cautious guess from the ebay information without knowing how deep you intend it to sit. When you have the hull, the empirical method of filling it with water to the waterline depth will give you the answer for what the hull can carry. A smaller battery, or batteries will give you a lot more freedom of choice regarding where things go. I am sure that somewhere in a marine engineering course they would have mentioned weight distribution.

ubay

Meh.... thats the naval architects job :P, but yeah an even trim = a happy boat. I never actually thought of buying smaller batteries and placing it around the model that might actually work. but may bring the cost of the build up. Now to design the weather deck etc.. thats the fun part but i may have to wait for the model to arrive as i dont know the shape of the hull at the top (for scale CAD drawings).

1 thing i dont quite understand about the motors is the numberings, i am guessing the higher the number the more 'powerful' the motor is but what does the numbers actually mean? power, torque, rpm (optimum)....