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Author Topic: twin prop'd boats - which way should prop's turn?  (Read 11865 times)

Peter Fitness

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Re: twin prop'd boats - which way should prop's turn?
« Reply #25 on: October 20, 2007, 11:56:13 PM »

The instructions for Model Slipway's "Sentinel" clearly state that the props should rotate inwards from the top, when viewed from the stern.
Peter.
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meridian

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Re: twin prop'd boats - which way should prop's turn?
« Reply #26 on: October 21, 2007, 08:45:14 AM »

Hi cdsc123,

Thank you for the explanation, which was both helpful and informative.  However, in view of your and Mr Whoppit's remarks about Coastal Forces craft, I now have a decision to make regarding my current build of an HDML. Do I fit a handed pair of props or a pair of props or doesn't it really matter?

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cdsc123

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Re: twin prop'd boats - which way should prop's turn?
« Reply #27 on: October 21, 2007, 09:05:42 AM »

Hi Meridian

The slower diesel powered HDMLs were fitted with outboard turning handed props.

What Mr Whoppit and I stated above only applies to the petrol powered faster craft e.g. MTBs and MGBs.

How's the HDML coming along?

Cheers,
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Mr Whoppit

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Re: twin prop'd boats - which way should prop's turn?
« Reply #28 on: October 21, 2007, 10:56:46 AM »

Christian Esquire and fellow Forum Members

It seems that I may have caused us to take this into the realms of High Speed Record breaking and Experimental craft of pre-hostility years.

Mr Christian is quite correct as far as his statements are concerned.   Comments regarding the harbour defence launch – if my memory does serve me rightly, these had a variety of diesel engines within, from a 300 bhp Gardner to a 300 bhp BUDA LEANOVA.  Their top speed was something in the region of 11-12 knots I believe.  Their propellers were of opposite hand.

Now may I come to the subject which is close to my heart.  Mr Christian mentions propellers turning in the same direction; I believe he is of the mind of early experimental MTBs built by Vosper.  These were built between 1936 and 1939, under the command of Commander (E) P. du Cane, R.N. (head designer for Vosper) - he designed the majority of Vosper’s MTBs.  One batch in particular, if my fading memory is correct, were MTBs 101 to 103.  These had a step in their hull and they originally had three Italian petrol engines (per craft) fitted which were of Isotta-Fraschini, of 1200 h.p. each.   In their early lives, these were experimental craft.  They were indeed fitted with a variety of propellers.  At one time they were fitted with twin-bladed propellers, which did significantly improve their performance and speed. However, they could not cure the vibration because technology was not available then as we have today.

Fruits were gained from these experiments, after the war years, for the record breaking craft.    Slo-mo-shun IV was a craft which was a world speed record holder on water, she used the technology of surface piercing propellers of the 2 blade design, where half of the propeller actually runs out of the water.  The actual propeller size for those who are interested had a pitch of 25 inches and a diameter of 13 and 5/8 inches.


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meridian

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Re: twin prop'd boats - which way should prop's turn?
« Reply #29 on: October 21, 2007, 12:10:51 PM »

Thank you cdsc123 and Mr Whoppit, a little more knowledge re HDMLs for me to digest and add to the memory banks.

Slow progress on the HDML I'm afraid as I have been unable to spend much time on it of late. However, I am just about to complete the installation of the running gear and the portholes are in place but not yet fixed in position. I hope to have the hull completed by the end of next week.
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Shipmate60

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Re: twin prop'd boats - which way should prop's turn?
« Reply #30 on: October 21, 2007, 12:17:14 PM »

meridian
I have an HDML and with twin screw, twin rudder there isnt a lot of difference, I have mine turnimg outwards, but left the rudders over scale.
The real advantage to inward turning props is on twin screw SINGLE rudder.

Bob
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meridian

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Re: twin prop'd boats - which way should prop's turn?
« Reply #31 on: October 21, 2007, 06:53:01 PM »

shipmate60,

Thanks for that. My decision is to fit outward turning handed props. Photographs of the build in due course.

Andrew
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funtimefrankie

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Re: twin prop'd boats - which way should prop's turn?
« Reply #32 on: October 29, 2007, 01:20:57 PM »

Any thoughts on when the (twin) props are in Korts or ASDs?
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catengineman

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Re: twin prop'd boats - which way should prop's turn?
« Reply #33 on: October 29, 2007, 05:37:49 PM »

I am at the moment on a DAMEN tug where BOTH screws turn the same direction and they are in Cort's YES there is a noticed difference to the operation of a same size tug from the same maker but with opposed turning screws.

Outward on opposed clock on same.

R,
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Bryan Young

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Re: twin prop'd boats - which way should prop's turn?
« Reply #34 on: October 29, 2007, 07:44:31 PM »

I think....
They do both go the same way,'cept it's to the right not to the left.
So you're both kinda right.
Just like the props  :)
No it is'nt. I have been waiting for ages for someone else to reply to this! Look very carefully at an enlarged bit of the pic. and you will see that they screw outwards. Presumably for ahead movement. I would absolutely HATE to have to drive a boat/ship with the props turning the same way....although I believe WW2 Lancaster bombers all had screws turning in the same direction. (?).
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catengineman

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Re: twin prop'd boats - which way should prop's turn?
« Reply #35 on: October 30, 2007, 06:17:47 PM »

Hi having had the opportunity to 'drive' operate a small craft with twin screws which went the same way I can tell you that other than docking (which if you know what you are doing the 'walk' can be used to assist) there is not that much difference in performance etc, but you do have to remember that when stopping from full ahead the vessel WILL turn even though the rudders are at midships. the vessel had open screws and balanced rudders. In the tug I an on at pressent the astern movement 'walk' is not so noticeable but fine control is no where as good as when the screws are opposed.
Oh and when going ahead the amount of transverse trust is almost lost therefore negligible and corrected via a slight rudder movement just as with a single screw vessel.

Just my findings
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Bryan Young

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Re: twin prop'd boats - which way should prop's turn?
« Reply #36 on: October 30, 2007, 09:29:48 PM »

Christian Esquire and fellow Forum Members

It seems that I may have caused us to take this into the realms of High Speed Record breaking and Experimental craft of pre-hostility years.

Mr Christian is quite correct as far as his statements are concerned.   Comments regarding the harbour defence launch – if my memory does serve me rightly, these had a variety of diesel engines within, from a 300 bhp Gardner to a 300 bhp BUDA LEANOVA.  Their top speed was something in the region of 11-12 knots I believe.  Their propellers were of opposite hand.

Now may I come to the subject which is close to my heart.  Mr Christian mentions propellers turning in the same direction; I believe he is of the mind of early experimental MTBs built by Vosper.  These were built between 1936 and 1939, under the command of Commander (E) P. du Cane, R.N. (head designer for Vosper) - he designed the majority of Vosper’s MTBs.  One batch in particular, if my fading memory is correct, were MTBs 101 to 103.  These had a step in their hull and they originally had three Italian petrol engines (per craft) fitted which were of Isotta-Fraschini, of 1200 h.p. each.   In their early lives, these were experimental craft.  They were indeed fitted with a variety of propellers.  At one time they were fitted with twin-bladed propellers, which did significantly improve their performance and speed. However, they could not cure the vibration because technology was not available then as we have today.

Fruits were gained from these experiments, after the war years, for the record breaking craft.    Slo-mo-shun IV was a craft which was a world speed record holder on water, she used the technology of surface piercing propellers of the 2 blade design, where half of the propeller actually runs out of the water.  The actual propeller size for those who are interested had a pitch of 25 inches and a diameter of 13 and 5/8 inches.



You mention surface-piercing props. Can you tell me (us) why this should be? If it was done "on purpose", then could it be to do with a reduction in the effects of transverse thrust? Interesting concept. Cheers. BY.
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