Model Boat Mayhem
Mess Deck: General Section => Model Boating => Topic started by: penfold on July 01, 2007, 07:57:36 pm
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if this question is answered in this forum cuold you point me in the right direction,but here goes
what is the correct way to have counter rotating props mounted in a hull( ie port, clockwise)
does it depend on the vessel? i have built several boats with two props and not put too much thought into which side to mount them,can it affect ship handling
there are some very clever people on this sight who i bet know of a link to a ship builders with technicalities ,
i know this will sound sad,but i really enjoy the engineering/ techy side off this hobby and jugding from this forum a lot of other people do!
keep up the good work you lot,cheers P
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Hi There. I am fairly new to this myself but am pretty sure it is starboard clocksise and port anti clockwise. Cheers Graham :)
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Normally outward turning looking from the stern forwards.
Paul...
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Inboard will give better steering, top of prop turns towards keel.
Outboard will give slightly more speed.
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That depends on wether you have got one or two rudders Dicky. ;)
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I got three Paul
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You should go see the doc then mate ;) :D
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This is a typical modern twin screw, twin rudder set up.
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Ver impressive, BB. Have you got a shot form the side? those rudders look very thick (low aspect ratio), and are they those clever ones where the tab on the TE turns more than the main rudder? Presumably there is some work on the aspect ratio of rudders and the design speed of the water past them. I seem to remember that Fokker did some work on slow speed aerofoils, and came up with a surprisingly thick wing for his D7. (Before my time, I might add)
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Inboard will give better steering, top of prop turns towards keel.
Outboard will give slightly more speed.
If you think about it - twin props rotating in opposing directions will either be trying to "bury" the stern deeper into the water OR trying to lift the stern out of the water.
So the answer probably depends on what type of boat you are trying to propel.
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Unless.....theyre in a kort nozzle, in which case it doesnt really matter. ;D
I dont think it matters that much in a models case anyway as the water isnt 'scaled down', of far more importance in my opinion is to get the two motors set up in their favoured rotation, they all seem to favour a certain direction or other, left & right handed props can then be fitted to suit max power...probably forward?
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Twin screws, twin props inboard definately gives better steering and outboard more speed .I've tried it ,it works.
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Inboard / outboard rather than state which is the correct it would be much better to test each of the two ideas on the particular vessel.
Why. HULL differences will make a difference to HOW the screws move the water and therefore how the vessel reacts. plus the position / angle of the props in relation to rudders the space between the said items and also the type of props, all these things are variables which in the real world are taken into calculations (far be it from me to say that you have to include all this in building a model) [which as a rule WE do not] Time honered tradition of "if it works for you then thats OK" and try it and see scenario applies IMHO.
Richard,
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If you're using a mixer you need to check whether it will function OK with outboard turning props, mine will only do what it is meant to do with inboard turning props.
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Here's a side view for you to compare.
The Rudders are a Heinz, high lift, flap rudder design, similar to a Becker but quite differrent if you talk to Mr Heinz, the creator and designer!
The aspect ratio is actually not that much different to any other rudder but they look a bit more substantial as they have a flat trailing edge.
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If you're using a mixer you need to check whether it will function OK with outboard turning props, mine will only do what it is meant to do with inboard turning props.
what make is it?
and can you not swop the motor wires over?
Peter
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Hi Peter
The make is JJC, I tried all reverse polarity permutations but could only get it to do what I wanted by having inboard turning props.
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Inboard will give better steering, top of prop turns towards keel.
Outboard will give slightly more speed.
If you think about it - twin props rotating in opposing directions will either be trying to "bury" the stern deeper into the water OR trying to lift the stern out of the water.
So the answer probably depends on what type of boat you are trying to propel.
I have thought about this idea for awhile now. And I still cannot understand what you are getting at.
The only difference between the bottom of the prop blade and the top part is water pressure, so no matter what rotation you have you will still have the same (minimal) pressure difference. Not knowing your profession I have no desire to insult your intelligence; but if you consider a ships propeller as just a slice out of a wood screw that part of the screw (when part of the whole) woill be just as effective whichever way it turns. Or perhaps you are confusing "rotation" with "tranverse thrust"? On a single screw (back to woodwork again) ship transverse thrust is an elemental fact. A single screw "real" ship will need a certain amount of permanent helm to keep it going in a straight line because the prop is always going to kick the back end out one way or another. This "anomaly" (for want of a better word) is really handy when berthing. Especially with model ships. With a clockwise rotating prop berthing "port-side-to" is a doddle. Approach the berthing point at an angle and give a gentle nudge astern, and the back end will just swing in neatly. Or it should!
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I was told this was called "paddle-wheel effect" by a retired naval comander, years and years ago. On our old canal criuser it was quite noticeable, and could push the stern against the bank with quite a bump. Just a burst of forward for one side and reverse for the other. ( I think it worked on either side, depending on the gear you were in)
This the boat in question, 26 foot of dodgey wood, with a v8 Ford engine, reputedly a Dunkirk Little Ship. Now gone to that great big boat yard in the heavens.
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In yacting it is referred to as "Propwalk" and used to assist berthing/unberthing.