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    twin propeller setup
    by drspock ๐Ÿ‡ฌ๐Ÿ‡ง ( Leading Seaman)
    ๐Ÿ“ฃ










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    15 Posts 14 Replies 1 Photo 25 Likes
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    drspock
    Leading Seaman
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฌ๐Ÿ‡ง United Kingdom
    Online: 3 days ago
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    Hi Kevin,and all other contributors!
    Overwhelmed by all the useful information!
    This demonstrates the fantastic value of this forum.
    Thanks to all.

    John
    3
    kevinsharpe
    Master Seaman
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฌ๐Ÿ‡ง United Kingdom
    Online: 17 days ago
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    Hi

    I guess you would have had to try the boat with one of the props contra rotating, you may well have seen an increase in speed and thrust. Also the more course the prop, the more critical the effect of loss of water pressure caused by the other and the greater effect of cavitation .

    Regards

    Kevin
    1
    redpmg
    Lieutenant Commander
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฟ๐Ÿ‡ฆ South Africa
    Online: 4 hours ago
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    Years ago my brother in law worked for Natal Parks Board on Spioenkop Reserve, which has a huge dam. They had an 18' ski boat fitted with two large outboards (80 hp?) . They both had the same clockwise rotation - yet there was no effect on the boat in terms of yaw or tilt - not sure if there was any effect on speed.
    Had a look at the Shark Cage Diving boats here mostly Semi Rigid inflatables or Catamaran hulls up to about 60'. Some of them have 4 x 300hp outboards - minimum is 2 x 250 hp. Same story - all have props turning the same way ............
    Not sure what you make of that - but I always try to use counter rotating props on my models anyway - but having seen those not sure I would bother on a full size boat equipped with outboards.
    My two pairs of model outboards are counter rotating funnily enough....
    1
    kevinsharpe
    Master Seaman
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฌ๐Ÿ‡ง United Kingdom
    Online: 17 days ago
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    Hi Doug

    Yes I get the point you make re torque cancellation. The other reason for contra rotation in twin props is the prevention of cavitation of the props. This would occur when one of the props suckswater away from the other. This creates a reduction in pressure around the starved prop resulting in water boiling around the prop blades due to pressure reduction and causing damage and pitting to the prop.

    Regards

    Kevin
    1
    RNinMunich
    Fleet Admiral
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฉ๐Ÿ‡ช Germany
    Online: 2 hours ago
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    Hi Kevin,
    "The purpose of having left hand and right hand props installed on a twin screw vessel is in order to stop one propellor sucking water away from the other, therefore making one of the props inefficient by starving it of water that it needs to develop thrust. "
    Well, they say you learn something new every day.
    But I'm not sure that the above is a learning experience!
    I've never seen that postulated in any analysis of single versus twin screw performance for high speed naval craft. Including those done by the USN Design Bureau. Which prop would under which circumstances 'suck' water away from the other?
    The main reason for using opposite prop rotation on a twin screw setup is to cancel out the torque reaction of each screw on the hull (regardless of inboard or outboard rotation for ahead). This effect is even more noticeable on models as they don't have the mass inertia of their big bruvvers๐Ÿ˜‰
    I do agree with you though that inboard rotation should be used especially if you have twin screws with a single rudder. Like most WW2 destroyers for instance.
    USN calculations and practical trials showed that for twin screw small ships; such as Fast Attack Craft, FPBs, Corvettes etc, with twin rudders that outboard turning screws gave a minimal advantage in manoeuvrability when the rudders were set slightly inboard of the shafts.
    Since USS Crocket was a Gunboat (which would now be called an FAC) capable of 50 knots and seems to have two rudders it would seem logical to make the screws outward turning on drspock's model.
    Cheers, Doug ๐Ÿ˜Ž
    1
    Young at heart - slightly older in other places ๐Ÿ˜‰ Cheers Doug
    RNinMunich
    Fleet Admiral
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฉ๐Ÿ‡ช Germany
    Online: 2 hours ago
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    "You can use tank steering as the device has two speed controllers in built, but you need to have two channels for this i.e. One channel for each motor."
    True - Mode 1. But why waste an RX channel when the built in rudder mixer gives you excellent manoeuvrability anyway
    You do normally connect two channels from the RX to the P94 board (Modes 2-4) BUT; one for the rudder and one for the two ESCs.

    To avoid any further confusion regarding the P94 board attached is the 'official' data sheet describing the various operating modes available; including reversing the inner motor on hard rudder commands. So spinning on your axis should be possible, depending somewhat on the length to beam ratio of the boat.
    But then destroyers aren't built to do that anyway ๐Ÿ˜
    Mind you, I'm gonna try it with my 1.35m 1/72 H class destroyer; 135x12cm ๐Ÿ˜‰
    An' I'll be using the P94 to do it.
    I agree with Pete (our 'Rooky๐Ÿ˜ฎ' Lieutenant) the P94 is the simplest way to get the best of both worlds - if not the cheapest!
    But add up the price of two ESC's and a Mixer Board and you won't be far off the P94 price anyway. Maybe above it๐Ÿค”
    Cheers, Doug ๐Ÿ˜Ž
    2
    Young at heart - slightly older in other places ๐Ÿ˜‰ Cheers Doug
    kevinsharpe
    Master Seaman
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฌ๐Ÿ‡ง United Kingdom
    Online: 17 days ago
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    Hi John.
    Just ad the spec on the component shop p94. You can use tank steering as the device has two speed controllers in built, but you need to have two channels for this i.e. One channel for each motor.

    Regards

    Kevin
    2
    kevinsharpe
    Master Seaman
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฌ๐Ÿ‡ง United Kingdom
    Online: 17 days ago
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    Hi John

    The purpose of having left hand and right hand props installed on a twin screw vessel is in order to stop one propellor sucking water away from the other, therefore making one of the props inefficient by starving it of water that it needs to develop thrust. The set up should ensure that each rotates throwing water inwards towards the keel. Even though the props are handed differently and rotate in opposite directions they should both be pushing the vessel ahead or astern together. The motors can be controlled by one speed controller, ahead or astern but with one esc both motors will move ahead or astern together and at the same speed as they both receive the same command from the esc and you only need one channel to achieve this. Alternatively you can use two esc's one for each motor and control them independently and create ' tank steering' two channelsare required for this set up, one for each esc. The component shop device uses one channel to control both motors, along with a feed from the rudder channel. When rudder is applied then the mixing part of the unit slows one of the motors to allow the vessel to turn. The disadvantage is that you reduce the speed of one of the motors on the inside of the turn and therefore the vessel will lose speed. Using this controller does create a type of tank steering but I don't think you can turn on the spot i.e. One motor virtually full ahead and the other full astern.

    Hope this helps.

    Regards

    Kevin
    2
    drspock
    Leading Seaman
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฌ๐Ÿ‡ง United Kingdom
    Online: 3 days ago
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    Hi Pete,
    Will check that one,cheers!

    John
    1
    drspock
    Leading Seaman
    ๐Ÿ“ twin propeller setup
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    Country: ๐Ÿ‡ฌ๐Ÿ‡ง United Kingdom
    Online: 3 days ago
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    Hi Peter,Thanks for your reply,will look into that.
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