Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[Wykenwizard]

I am a bit confused about motor and battery sizes relative to size of boat. Is there a rule of thumb that would allow me to work out what the power requirement would be for a given boat length? For instance, I am currently working on a 1/30 scale 34.5 inch hull with a 8.5 inch beam. Can anyone tell me how to assess the motor/battery combination I need?

[unbuiltnautilus]

Nothing simple really. However, it would help to know the type of vessel, fast boat, slow boat, displacement or planing hull?
There is enough knowledge in the group that can be tapped into once the parameters are narrowed down a bit.

[Wykenwizard]

Thank you for your reply. Currently I am building a 1/30 scale HS class tug. The hull is from Models by Design. I have built a few boats in the past, but have never been entirely happy with the motor/power arrangements. The biggest I built was Milford Star and the lead acid battery I used was huge. Not sure if it actually needed such a big battery, but went more for the high capacity. I just wondered if there is any particular rule of thumb for determining these things?

[unbuiltnautilus]

About the biggest practical battery size for a Milford Star would be a 6V 4A Gel type, tight fit though, especially if the battery dies on you after a year or so. I had one of these cross my bench a few years ago, with a 6V 10A battery buried below the decks, and it was dead! This pretty much rendered the model grounded. There was no way to remove the battery without removing the entire foredeck. I am becoming more of a fan of NiMh batteries of a higher capacity for this type of scale boat. 7.2V and somewhere between 4600mAh and 5300mAh packs. Easier to fit through hatches.
Motor choice is decided by prop size firstly. I guess a prop for this size of boat is about 40mm or 45mm diameter. You will either need a lightly geared motor or one of the Torquey crawler motors used in RC off roaders. I would suggest something like a 45 or 50 turn crawler motor. a 2.5:1 geared motor by MFA would be a good choice with the bigger prop sizes, but probably won't have the output RPM for the smaller props. What you don't want is one of the faster motors sometimes listed for boats, as suitable for 6V to 12V operation. seen these more than once propelling a scale boat round the pond at excessive speed. The usual response being that the owner can always use the throttle for scale speed..what the owner should do is fit the right motor for the right model, but hey, that's just me!!
You cannot go far wrong asking on here, although you may end up with a never ending list, usually ranging from good to excellent suggestions, usually

[malcolmfrary]

With a scale displacement model, if you know the horsepower of the real thing, you can scale that.  Multiply by 750 to turn HP into Watts, divide that by the cube of the scale, then multiply by 2 to make up for motor and model inefficiencies.
Knowing the watts gives the current at any voltage, but remember that the power demanded by the motor is determined by the load imposed by the prop.
Size of battery.  Generally as much as will fit (both the space it takes and the weight) to ensure that it will be able to supply the current needed for as long as it is wanted.  Different boats have different power requirements, both plain basic power and the speed of rotation (RPM) that the motor can actually do. 
Matching the motor and prop is important.  Fast boat, small prop being turned very fast, slow but powerful, large prop turning more slowly.  Much repeated rule of thumb is "Prop diameter no more that the motor can, prop should have fewer blades than the motor has poles".  Thats for direct drive brushed motors.  Gearing effectively increases both effective diameter and nmber of poles.  Going brushless is a whole different world.

[Bob K]

You said "The hull is from Models by Design".

My suggestion is you call them to ask about the ideal motor and battery for it.
Frankly, they are best to know, having sold many of them plus feedback from customers

[DaveM]

Not sure if it actually needed such a big battery, but went more for the high capacity.
Please be aware that sealed lead-acid batteries will never deliver their nominal capacity when run at the sort of currents we demand of them - even with low-drain motors. You would be lucky to get half of it, so NiMH batteries are a better option all round. If you need extra ballast then use lead flashing or steel shot.
DaveM

[john44]

I'm with Dave on this one


John