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[Patrick Henry]

My USS Dragonfish(a modified Trumpeter USS Seawolf)is powered by a 385 motor from a 6volt 4600a/h nicad pack, driving a 35mm seven blade scimitar prop(if I can get one made)

Question...what's the favourite prop shaft diameter to use? I'm wondering if I can away with these http://www.cornwallmodelboats.co.uk/acatalog/m2_propshafts.html, they should be easier to cut and alter to the lengths I need. I have to make two shafts, one from the motor through the wtc end cap, and one from there onwards to the prop.

Advice/comments anyone?



Rich

[Bunkerbarge]

I've got two, 2mm shafts in the Revel U-Boat.  The motors are quite a bit smaller, being only servo motors and the props are only 25mm but a 2mm shaft is way over the strength requirement anyway.  It goes through the water at a reasonably fast pace and the model does displace about 1.2 kg just to give you an idea.

You of course only have one shaft and you have a bigger motor and a bigger prop although I would guess the model displacement is in the same ball park.  If you try to imagine how much force at a radius of 17mm from a 2mm shaft would be required to twist it, then try to imagine how much force is going to be required to turn your prop then I'm sure that a 2mm shaft will be fine.

I'm sure there are members out there who can provide pages of calculations to prove one way or another but I'd be happy with gut feeling.

[Turbulent]

Rich,

I made mine from 3mm Brass, as to the prop itself, I cheated, left the fwd bit of the kit propulsor in place & found a 3 blade prop' that fitted behind that, it was hidden by the cowling.

I'll try & take some pic's for you tomorrow & post them.

385 motor- - bit big dont you think? Did that bulkhead arrive ok?

[Patrick Henry]

BB..thanks for that, much appreciated. I don't do reams of calculus...don't understand it!

Paul...now that's a damn good idea, I like that. Why didn't I think of that? (Don't even think of answering...) Bulkhead arrived mate, thank you kindly...left you a message on f/book.
The 385 motor...I didn't have a smaller one, that's why I used it. I can always throttle back if it roars off....mind you, the full size Seawolf is pretty quick!


Rich

[Turbulent]

Does this help?

[Patrick Henry]

Helps tremendously chap, many thanks for those...this afternoon's little project is now under way.

Thanks mate...most grateful to you.


Rich

[Subculture]

The main difficulty with smaller prop shafts is sealing them (for subs). Simmerrings go down to 3mm, beyond that you will need to make a shaft seal using an o-ring or silicone tubing, or revert to a greased stern tube.

[Bunkerbarge]

Just a word of caution there.  I initially had a 12" 2mm shaft in a 10" 4mm tube and was told to fill it with grease.  The motor wouldn't even turn it!  I was only using a small servo motor but it sort of defeats the object if you need a bigger motor just to turn the shafts.

I went back to a light oil which doesn't let water in and allows the shafts to spin freely.

[Patrick Henry]

Belt and braces here Andy....I have a silicone tube seal on the part of the shaft that exits the wtc, plus the shaft will be filled with a waterproof grease courtesy of our fitters at work. Just about to drill the shaft tube and fit the grease fill pipe now.



Rich

[Patrick Henry]

Just a word of caution there.  I initially had a 12" 2mm shaft in a 10" 4mm tube and was told to fill it with grease.  The motor wouldn't even turn it!  I was only using a small servo motor but it sort of defeats the object if you need a bigger motor just to turn the shafts.

I went back to a light oil which doesn't let water in and allows the shafts to spin freely.

The longest part of the two shafts on the Dragonfish is barely five inches long, BB...the internal section is just on two inches and the external section four point nine nine nine nine nine inches. Ish...


Rich