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Author Topic: which propeller?  (Read 1969 times)

sunnybob

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which propeller?
« on: April 19, 2008, 12:17:38 am »

After several attempts to repair the drive shafts on my cabin cruiser, I'm having no luck at all, so I now have to seriously consider new props and shafts.

All of the running gear I have is 50 years old, imperial and obsolete, so how do I work out what size and shape propellers I should buy?

The boat is a metre long, 300mm wide. At the moment it has two brass triple bladed props of approx 45mm diameter. the full size boat only has a top speed of 10 knots, so I'm not after any racing designs.


thanks,
Bob
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Shipmate60

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Re: which propeller?
« Reply #1 on: April 19, 2008, 12:32:47 am »

If the old props were ok, just replace with similar.

Bob
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FullLeatherJacket

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Re: which propeller?
« Reply #2 on: April 19, 2008, 08:24:31 am »

Bob
Can you re-tap the threads to fit a modern metric shaft? I know Simon "Prop Shop" needs the money, but if the props aren't damaged then why buy new ones?
FLJ
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barriew

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Re: which propeller?
« Reply #3 on: April 19, 2008, 11:07:14 am »

Bob,

If the props are tapped 4BA, then they will fit M4 new shafts - just add some threadlock ;)

Barrie
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Big Ada

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Re: which propeller?
« Reply #4 on: April 19, 2008, 12:45:44 pm »

Or just drill right through and put a nut either side.
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sunnybob

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Re: which propeller?
« Reply #5 on: April 19, 2008, 02:04:45 pm »

Oh I'm doing some learning on this boat. if my dad were still alive, I think I'd give him a slap!

The problem arose because it was a hand made mechanical diff gearbox driving two shafts, with a bloody great i/2 hp motor that nearly sank the boat! (remember, this was built in the late 50's with valve r/c gear)

All of that had to be dumped because of its age, so I installed two motors and one esc to control the twin shafts.

I've now found that the threads on both ends of both shafts are not tapped square! so theres a huge vibration in the system.Add to that the shafts are a tight fit in the tubes, and the motors have been struggling to turn the props.

I've now made the shafts slimmer for most of their length, just keeping an inch at the motor end the same thickness, to keep water out of the engine compartment.

This worked well on a dry run, so Ive just had the thing in the bath working out speeds and rudder adjustments.
BUT.... it seemed as though whatever I did, it either went backwards, or turned hard right.??????

After several minutes head scratching and cursing, I lifted the back end out to see which prop was going slow, when (have you guessed it?) I saw the props were PAIRED :o :o :o

Because of that old mechanical gearbox, these props have to contra rotate!! >>:-( >>:-( >>:-( >>:-(

So, another question now...

now that I've worked out which way each one is pushing forwards, should the left, or the right prop be going clockwise?

While you give that some thought, I'm off to rewire one of the motors O0

I WILL get this working!!!!!!!!!!!!!
Bob
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Peterm

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Re: which propeller?
« Reply #6 on: April 19, 2008, 03:12:52 pm »

On two of my boats the left hand prop (from the rear) goes clockwise, the right hand one goes anticlockwise.   All I can say is I have no problems with this set-up.   Pete M
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barriew

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Re: which propeller?
« Reply #7 on: April 19, 2008, 03:44:56 pm »

As you said earlier I think, that speed is not important in this model, then it won't matter a great deal as long as the both turn in opposite directions.

Barrie
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FullLeatherJacket

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Re: which propeller?
« Reply #8 on: April 19, 2008, 05:35:49 pm »

As you said earlier I think, that speed is not important in this model, then it won't matter a great deal as long as the both turn in opposite directions.
Barrie

BW What happens if only one  turns in an opposite direction...............?  ???

FLJ

(Pay no attention - I've been on this damned PC all week writing an instruction manual and I'm getting hallucinations...........and it's not the G&T!!)
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Shipmate60

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Re: which propeller?
« Reply #9 on: April 19, 2008, 05:51:53 pm »

sunnybob,
How many rudders does she have.
If a single rudder steering is enhanced by the props rotating inwards.
From the stern the LH prop turning clockwise when going ahead and the RH prop turning anticlockwise when going ahead.
If she has 2 rudders this isn't so important for tighter turning.

Bob
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sunnybob

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Re: which propeller?
« Reply #10 on: April 19, 2008, 09:13:42 pm »

BW What happens if only one  turns in an opposite direction...............? 

we all slip into a parallel universe, and have to keep shouting "42"

The boat has two props and two rudders, so i suppose it doesnt make much difference which prop goes widdershins.

Now I've rewired the r.h. motor, I am impressed by the speed and water displacement. O0 O0
For the first time since I started this, I'm smiling ;D ;D ;D ;D

If it would only stop raining, I could take it on some big water tomorrow :P
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