Motor gearbox for USS Susquehanna

[Mu17]

During the lockdown I have tidied my loft !  and dragged out the Imai USS Susquehanna.
This is a simple but vey nicely moulded kit even by todays standards.

I would like to sail it as an rc model but whilst I have plenty of experience with rc gliders but I am all at sea  when it comes to knowing what size motor and gearing is required for a paddler.  The model is about 500mm long with an 85mm beam.
Draught is pretty shallow at around 22mm.

As I am not much of an engineer, I wondered about using on of the Tamiya motor gearbox units.  These all seem to have the final drive via hexagonal 3mm shafts which ae too shot to each the paddles. I can't find any hexagonal steel of this size and wondered if brass would be adequate to un paddle to paddle. 

Steering is intended to be by rudder rather than differential paddles, due to the small displacement. I realise that a substantially enlarged rudder will be needed.

Any helpful comments gratefully received.
Paul

[derekwarner_decoy]

Welcome Paul

This vessel size sounds for the need of caution

If the hull is complete, could I suggest you place the hull in the bath, then gently add weight/mass to bring the hull down to the intended waterline

A simple method could be say an empty 675 ml plastic drink bottle.......then add water to the bottle as the needed weight etc

Take this bottle & weigh on the kitchen digital scales ...so the weight of the plastic bottle + weight of the water volume total will confirm to you the total weight of batteries+motor/gearbox+RC gear+ battery that you must not exceed 


These days, Google will provide you with a detailed confirmation of the individual mass etc

The image here is an array of M3 brass electrical standoffs that could be used as paddle shaft extensions [can also be contoured in an electric drill]......the weight here appears as 20gms.......

Let us know how you progress with your weight/mass displacement tests

Derek

 

[Mu17]

Hi Derek,
             I scavenged a pile of weights, sellotaped the hull halves together and used electrical tap to mark the waterline.
The ballast to get it to the waterline was  0.68 kg   which I managed before the Sellotape let the wter in
I weighed the sprue carrying the main deck and the large display base, which weighed 0.20 kg  and this is a more than generous amount for deck, funnel, masts, spars, rigging and paint.

That leaves 0.48kg or a shade over 1 pound in imperial measurement., for battery, motor/gearbox, rudder servo and a smoke unit.

A brief article on motorising Susquehanna appeared in the November 1981 issue of Model Boats magazine but it doesn't provide any useful information about the motor/gearbox. Th only interesting comment was that the builder omitted each alternate paddle blade and felt that sailing trials suggested that was a good decision.

So back to moto/gearboxes and ratios   any thoughts ?

Thanks for responding by the way.
Regards
Paul

[derekwarner_decoy]

No sorry Paul.......these light weight values are way out of my league/knowledge......... Derek

[Mu17]

Derek,  can you perhaps help with the sort of speed I want for the paddlewheel ?
Paul

[derekwarner_decoy]

Paul......we have many comments on paddle shaft speeds for models...many factors to consider

1. prototypical speed....then apply rotational scale
2. maintaining original scaled dimensions & surface area & number of blades
3. scaled immersion of blades
4. we cannot scale surface tension or characteristics of water 

So, many successful model builds use a shaft speed of approx 100 RPM +/- [with the +/-  being an electric speed controller]


Weights will be absolutely critical for you
Maybe check a few Robotics WEB sites to check the Tamiya motor gearbox weights as the Tamiya pages seem ellusive

Derek

[Walter Snowdon]

Hi paul.        forget the smoke unit, I have seen many of them and they are a waste of space!, It can be done, An old friend of mine motorised Susquehanna over twenty years ago when radio gear was larger and heavy and it sailed well. Just use gear carefully. micro servo, smallest speed sontroller you can find, Small battery that will give you half an hours sailing, belt drive the shaft using a large and small pully with rubber band drive.  About thirty years ago I fitter radio into the Airfix Great Western and it worked perfectly in calm conditions and it has less than half the displacement of Susquehanna, As an indication as to how small you can go with modern gear, Phil  one of our club members fitted out a 700th scale                   battle ship and sailed it in a fairly stiff breaze and it never shipped a drop. Main thing is. all that rigging causes a lot of windage which will cause her to heal badly. The early steam warships used to strike down most of their spars when going into action, so you could do the same. GO FOR IT. forget all the gimmicks and add ons= they are just unnecesary weight. Wile I remember, I think she had a lot of paddle blades In small scale they just interfere with each other. Reduce their number by half and they will be more efficient. Regards, Walter

[Steven S]

Tamiya supply a 3mm shaft set which include hexagon and round shafts which includes 150 and 100 long shafts.

 https://www.pololu.com/product/78]

[Mu17]

Hi Walter, thanks for the encouragement. Halving the number of paddle blades was the only solid piece of information in the 1981 article but its really good to have that confirmed.
Indeed modern radio means that there shouldn't be a serious weight problem.
I'm not a fan of elastic bands as they seem to perish rather quickly these days - and how do replace one on a on piece shaft any way   I'm sure there is an obvious answer that I don't know.

Windage on the rigging - now that is a really helpful tip and I can see that as scale reduces (in this case 1/150) its can start to be a problem. That does make me wonder if a false keel with some weight at the bottom would beneficial - assuming that there is any weight to spare for that !
 
I gather that 100-150 rpm is around the correct target - any recommendations for small motors ?
Just spotted the post about the shafts - thanks Steven, that's really useful
Many thanks
Paul

[Steven S]

I can't seem to edit my above post.
I meant to say 150mm long and the link is https://www.pololu.com/product/78

[Steven S]

Cytron and others make a line of gearmotors.  I have just ordered this one for a sternwheel towboat that I am building.  I just searched some online robot shots to find it.

https://www.robotshop.com/ca/en/cytron-12v-150rpm-18oz-in-spur-gearmotor.html