Prop RPM Help Requested

I am attempting to convert a Revell Type IX U Boat to RC.

I have a 385 brushed motor and a (handed) pair of Raboesch 25mm diameter 3 blade props.

What rpm range should I be looking for for the props? I assume i will need a reduction gearbox (from max 11,000 according to the motor box). So what reduction ratio do I need?

Thanks for any advice.

I always like to have 2:1 to 3:1 reduction in motors. (It limits current draw and is generally best if your props have the (usual) coarse pitch.

Others would disagree with this particularly when brushless motors are used. (i.e they use direct drive.)

Have you had a look at MFA como motors and gearboxes? A very good range and good service and they save a lot of grief doing gearboxes:

https://www.theassociationofmodelsubmariners.com/t1400-mfa-como-drillls-motorsgearboxes-and-belt-drives?highlight=motors

You also ask about gearboxes out in the water. I have never tried this but I think that David Merriman and Norbert Bruggen offer them. Seems like a good idea.

Thanks very much for that. I think it will be the MFA 2.5:1 geared 385 motor in the WTC and just the 1:1 drive to twin shafts from the single motor output in the wet area.

That should be fine. Without having done it, I would think that the wet 1:1 drive will be OK. (Slower running so not too much water turbulence & friction)

Using MFA gearboxes means that you can change ratios fairly easily too. Nigel has a post on here also about incorporating a shaft seal into the gearbox. (I haven't tried it.)

About a 3D printed O ring fit - I don't know either. I'm old school (lathe!). Tim G is exploring this technology.

There are some very good tips here on sliding O ring fit (one from John Redearth particularly.):

https://www.theassociationofmodelsubmariners.com/t537p125-piston-ballast-systems?highlight=piston+tanks

Andy L has a video on lathe machining WTC caps somewhere. Post on the Dive-In Facebook site and he will let you know, I'm sure.

David

I have achieved a 'close sliding fit' for a 3d printed end cap, after some trial and error, and now have a version with an O ring fitted which looks as though it is compressing uniformly all round. I have come close to the 85% groove depth by blind chance.

I haven't tested for actual watertightness yet as my 'trial fit' end caps were printed as hollow, to save filament until a reasonable fit looks OK. Of course, now I have run out of the filament I used so need to order some more.

Don't have a lathe or the skills to use one, so 3D printing it is.

BuxtonRob wrote:I have achieved a 'close sliding fit' for a 3d printed end cap, after some trial and error, and now have a version with an O ring fitted which looks as though it is compressing uniformly all round. I have come close to the 85% groove depth by blind chance.

I haven't tested for actual watertightness yet as my 'trial fit' end caps were printed as hollow, to save filament until a reasonable fit looks OK. Of course, now I have run out of the filament I used so need to order some more.

Don't have a lathe or the skills to use one, so 3D printing it is.

one suggestion i would make is to print using a higher temp filament than PLA... you don't say which type of filament you are using, but PLA's glass temp is too low for my comfort...

https://www.youtube.com/watch?v=TTswoDnxYHs

i have switched to PETG as the filament i use for anything that is going to be used directly. for items that will become masters for molds, i will use PLA.

Using ASA for this. Can be acetone smoothed (which should give a watertight skin) and can be glued with polystyrene cement, so can be glued to the kit parts as necessary.