How do you determine motor size, prop pitch, etc.?

[sgtmac1]

I was wondering if there was a way to make an educated guess on the size of motor needed, size of prop and its pitch, and other variables for my 1/48th scale SS-306 fleet boat, which is 6'6" long. I've seen a pic somewhere that showed a boat that had a clear plastic extension added to the rudder to aid in turning and though that maybe there was a discussion on how to take these things into account. Or is it mostly trial and error?



This is pic is of the standard props for a fleet boat-do you think the model should be bigger or scale like and what difference will it make in the water?
Thanks,

[Guest]

Re: How do you determine motor size, prop pitch, etc.?

Prop diameter and pitch and how it affects performance can be quite complex, however in model form it's not too difficult for the average sort of performance we're aiming for in our models.

With a scale model, if we are aiming to be authentic, generally the diameter and style of prop are decided for us by the original 1:1 version.

An area we can tinker with is the pitch, although this may ot be required, because propeller pitch often tends to correlate in model form. generally the optimum ratio is between 0.8 to 1.5 times the prop diameter, with the sweet spot usually somewhere between 1-1.3. Single screw boats pose an additional hazard compared with twin screw, especially higher performance models- torque induced roll. This has more of a pronounced effect on submarines than surface boats, so you'll often find that clued up builders of single propped subs will be conservative with the pitch, often designing in less than optimum value in order to minimise the torque band and pushing the power further up the RPM band.

As your boat is twin screw, you don't have to worry too much about this, and you can go for higher pitch and benefit from slower shaft speeds, which work more efficiently in subs and help the life the watertight glands.

Predicting how much speed you will get out of your boat with a specific prop is rather difficult, as many factors affect this. Hull shape, blade area, the way the props are located, propeller slip etc.

You can get a rough estimate, but ultimately there is still an element of trial and error. A very accurate way is to try and find someone with a similar or identical model, and pick their brain.

When it comes to deciding motor and gear train (if required), personal preference is a major factor, but a fairly good way is to decide roughly what speed you hope the model can do on a specific prop and then work back from their.

An electric motor should be able to turn that prop at no lower than 80% of its unloaded RPM, if the motor is bogging down, then you either need a gearbox or a different motor.