Model Boat Mayhem

Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: RipSlider on October 17, 2007, 03:36:19 pm

Title: twin prop'd boats - which way should prop's turn?
Post by: RipSlider on October 17, 2007, 03:36:19 pm: Hello all.

Just a really quick question.

Currently I'm building a twin proped boat.

If I look at from the rear, so that the prop's are facing me, should the prop's turn in to each other (left prop clock wise), or away from each other ( left prop anti clock wise) ?

I have fitted L/H and R/H prop's on the respective sides.

Many thanks

Steve
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: DickyD on October 17, 2007, 03:41:46 pm: Hi RipSlider
You will get many different replies on thi probably.
On my boats the twin props turn in as we found it improves the steering.
If you are only concerned with speed have them turning out.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Shipmate60 on October 17, 2007, 03:59:11 pm: RipSlider,
How many rudders are fitted?

Bob
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: DickyD on October 17, 2007, 04:21:04 pm: Thought you were supposed to be working Bob :-\
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Martin (Admin) on October 17, 2007, 05:23:06 pm: Also, what type of boat is it?
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: cdsc123 on October 17, 2007, 05:40:20 pm: Type "prop rotation" up where it says "search", you'll be surprised at the wealth on knowledgeable info which has already been offered on the subject.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: RipSlider on October 17, 2007, 09:01:40 pm: Thanks for the answers so far.

Boat is a fairly shallow draft PCF a riverine craft. Twin, fairly large rudders, located direction stern of each prop.

Sadly I don't have a working search facility on this site, so posted. Mods should free free to take down if required.

Steve
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: gribeauval on October 17, 2007, 10:04:55 pm: Have a look at this thread,

http://www.modelboatmayhem.co.uk/forum/index.php?topic=263.0 (http://www.modelboatmayhem.co.uk/forum/index.php?topic=263.0)

regards Mike ;)
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Martin (Admin) on October 17, 2007, 11:50:41 pm: Found this if it's any use.... looks like they both turn anti-clock! :o

(http://swiftboats.org/reunion/2007/nab/104tour/touroftheboat.jpg)

You could always email them and ask...... http://pcf45.com/dedication/vumdirections.html (http://pcf45.com/dedication/vumdirections.html)
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Bryan Young on October 18, 2007, 07:38:25 pm: Quote from: Forum Admin - Martin on October 17, 2007, 11:50:41 pm
Found this if it's any use.... looks like they both turn anti-clock! :o

(http://swiftboats.org/reunion/2007/nab/104tour/touroftheboat.jpg)

You could always email them and ask...... http://pcf45.com/dedication/vumdirections.html (http://pcf45.com/dedication/vumdirections.html)
Looking at the picture I would very humbly suggest that you look at again. The leading edge of the stbd. prop. is "screwing in" and so is then an "outward" turning item. Sorry to rain on your parade! BY.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: cdsc123 on October 19, 2007, 08:34:04 am: 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 :)
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: portside II on October 19, 2007, 09:28:55 am: depends from where your looking {-).
i supose there is a propper way of reading the direction ,either from the front or back .
daz
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Peterm on October 19, 2007, 10:28:25 am: Normally read from the rear. Pete M
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Bryan Young on October 19, 2007, 07:01:58 pm: Quote from: portside II on October 19, 2007, 09:28:55 am
depends from where your looking {-).
i supose there is a propper way of reading the direction ,either from the front or back .
daz
We have been down this route before. In the UK we look at the props from aft. In many European countries they look from the front. That is why when you order a "right handed" prop from your local model shop it comes back as a "left"....'cos they are all made in some faraway country if which we know little. If you want a "right handed" prop then order a "left-handed" one!
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: cdsc123 on October 19, 2007, 09:40:53 pm: We'll have to continue to agree to disagree about that, I've yet to come across the country that views from in front.

Standing to be corrected... ;D
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Bryan Young on October 19, 2007, 10:59:20 pm: Quote from: cdsc123 on October 19, 2007, 09:40:53 pm
We'll have to continue to agree to disagree about that, I've yet to come across the country that views from in front.

Standing to be corrected... ;D
Just about all of them. Especially the Netherlands.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: funtimefrankie on October 20, 2007, 09:29:48 am: Does it matter if you call them, left or right hand, if you are talking twin props, after all you want one of each, you may as well call them Bob and George. ;D
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: portside II on October 20, 2007, 09:40:28 am: here here well said
if you order a pair and the are both the same then they aint a pair whatever country your from
apart from the ones in Martins picture O0
daz
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Mr Whoppit on October 20, 2007, 11:15:26 am: Ladies and Gentlemen, may I bring it to your kind attention that the rotation of propellers and the way they are viewed indeed varies.

To date, propellers are the most efficient way of moving a vessel through the water, by converting the shaft horse power into thrust.

Please see the pictures I have included below.

One of these photographs (see picture A) is from the early 1900’s, this shows an early Cruiser. The propellers here are rotating inboard. The blades of the propeller are long and narrow.

You will see from the photograph (see picture B) from around 1937 shows a Cruiser with ‘quad’ propellers and the forward propeller is turning outboard. This propeller shape has changed as you will see. It is what is classed as a wide tipped blade. (Elliptical)

The next photograph, (see picture C) shows a modern ship, propellers are in ‘pods’. If you note that the propellers on this vessel turn inboard, and, also the propellers’ face forward to the bow of the ship, this in turn gives more efficiency from the electric drive motors in the pods.

The last photograph I have included is that of The Turbinia. (See picture D). Close examination of this, will show you that there is not one propeller on the shaft, but, there are several – of all varying designs which are again, turning outboard.

All the above are viewed from the stern, looking towards the bow.

From this we can deduct that the direction to which a propeller rotates depends upon which vessel it is fitted to and what the vessel is designed to do.

During the period of the war years a lot of our British multi-propellered small inshore vessels such as MTBs (motor torpedo boats) the propellers rotated the same way on a lot of them. This was because there was quite a shortage of reverse gear boxes.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: DickyD on October 20, 2007, 04:15:46 pm: Nicely illustrated explanation Mr Whoppit O0
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: meridian on October 20, 2007, 07:37:41 pm: Earlier this year I completed my first build, which was a Model Slipway kit of a 'Range Safety Craft' which I adapted it as an MOD Police Launch. The kit was supplied with two, two-bladed 'same hand' props which I used as supplied. To be honest, I have not noticed that there is any major trim required to compensate for this. To all intents and purposes the blades of two rudders lie parallel to the centreline of the boat.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: John W E on October 20, 2007, 08:05:30 pm: Hi there Meridian

I know it may make very little difference, but I feel that the props should have been handed left and right....I think it may be an oversight by the model suppliers. But, then again, in this world who knows.

Sometimes a model can steer to one side when you have both same handed props running in the same direction. :o

aye
John e
Bluebird
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: meechingman on October 20, 2007, 08:22:54 pm: Quote from: Bluebird on October 20, 2007, 08:05:30 pm

Sometimes a model can steer to one side when you have both same handed props running in the same direction. :o

And the swing when going astern might be even worse.

Andy
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: meridian on October 20, 2007, 10:34:11 pm: Hello Bluebird,

No it wasn't an oversight on the part of the model suppliers. The instructions clearly state, " The propellers (which are not handed) are simply screwed onto the shafts, locking them in place with the nuts provided".

However, this would not appear to be a problem as I note that PCF-104 is fitted with props of the same hand.

In view of contradicting opinions etc I intend to fit a LH and RH prop to see if I can detect any real difference. At the same time I will also try and compare the effects of inwards rotating and outwards rotating. Will report my findings in due course.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: cdsc123 on October 20, 2007, 11:13:50 pm: Hi Meridian

The contradicting opinions are about prop rotation always being viewed from astern as an international convention and not dependent on where you happen to be in mainland Europe (I deal with Vetus NL and they certainly view from astern).

A handed pair of props, not to be confused with a pair of props, will certainly give your model better handling, of that there is no debate here.

The kit you built, being of a relatively high performance craft, possibly supplied 2 LH props so that they wouldn't try to unscrew themselves. I believe it is also the case that 2 blade props deliver more performance and further, are not generally available in handed pairs as are 3 blade props.

As Mr woppit states, Coastal Forces craft (and RAF rescue craft) were fitted with props of the same handing, whether this was due to the cost, reliability, availability or power-robbing characteristics of handed 'boxes as opposed to simple boxes is a matter for some debate, literature I have tends to suggest the extra 2 or 3 knots achieved with same-handed props was worth the sacrifice of difficult manouvering.

Cheers,
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Peter Fitness 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.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: meridian 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?
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: cdsc123 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,
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Mr Whoppit 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.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: meridian 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.
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Shipmate60 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
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: meridian 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
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: funtimefrankie on October 29, 2007, 01:20:57 pm: Any thoughts on when the (twin) props are in Korts or ASDs?
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: catengineman 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,
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Bryan Young on October 29, 2007, 07:44:31 pm: Quote from: cdsc123 on October 19, 2007, 08:34:04 am
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. (?).
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: catengineman 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
Title: Re: twin prop'd boats - which way should prop's turn?
Post by: Bryan Young on October 30, 2007, 09:29:48 pm: Quote from: Mr Whoppit 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.

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.