Rooster tale

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Warpa

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What causes this? I have seen them on 8000 BHP turbine boats and 100 BHP outboard boats, so highering the trim i would say is not the issue, nor speed:confused: is it the type of prop used, although i cant see that making a difference either:rolleyes:
 
A fast power-cat will leave an impressive rooster tail which sits well aft of the boat, I guess something to do with the converging of the hull and drive wash.

If you talk to raggies that sail on super-fast sailing cats, the conversation invariably gets to the moment when the eyes light up describing the huge rooster tail the boat leaves screaming along at an "untold" amount of knots, with the helmsman bearly holding on :rolleyes: :).

I think most folks like the look of a rooster tail.
 
It's caused by a type of propeller and transmission known as a surface drive I believe. The effect is easily demonstrated at home in a domestic surrounding. You will need a powerful handheld liquidiser, a large bowl and a pint or so of raspberry coulis or similar brightly coloured liquid.

Fill the bowl with the raspberry coulis, put the blades of the liquidiser well below the surface and switch it on to full speed. You will see some surface movement but nothing dramatic, similar to the wash produced by a stern drive that is fully down at displacement speed. Gradually move the liquidiser upwards - as the blades approach the surface you will see more disturbance and clear signs of ventilation around the blades, exactly as with a powerful high-speed prop that is not deeply immersed. Note this is ventilation caused by air being mixed with the fluid, not cavitation (which is caused by a very low pressure occurring behind the prop blade and does not involve air) though many mistake the two.

Continue moving the blades upwards until they eventually break the surface - you should now see a great deal of disturbance and have effectively created a surface drive system, albeit a very inefficient one with minimal pitch and little thrust.

If you now run around the room as quickly as possible to simulate the forward motion of a fast boat you will clearly see the rooster tail appear, exactly in the way a surface drive creates one.
 
perhaps there is another factor in the tail as well as the angle of trim ?

The water exiting from the propeller (any propellor) is going faster than the ambient water past the boat (otherwise it wouldn't work !).

If the prop is pushing a fast jet of water into the sea, then in effect the jet hits a block of water, and the weakest point in that 'block' is between the prop and the surface. So the jet pushes upwards, and if there is enough power will create a rooster tail.

You can see this effect in all power craft, rowing boats, where there is a differential between a fast-moving jet of water and the surrounding slower-moving sea. The jet goes for the weakest link and creates turbulence above the median level of the sea.

It would be interesting to see the dynamics of such a jet from a side view.
 
grumpy_o_g said:
It's caused by a type of propeller and transmission known as a surface drive I believe. The effect is easily demonstrated at home in a domestic surrounding. You will need a powerful handheld liquidiser, a large bowl and a pint or so of raspberry coulis or similar brightly coloured liquid.

Fill the bowl with the raspberry coulis, put the blades of the liquidiser well below the surface and switch it on to full speed. You will see some surface movement but nothing dramatic, similar to the wash produced by a stern drive that is fully down at displacement speed. Gradually move the liquidiser upwards - as the blades approach the surface you will see more disturbance and clear signs of ventilation around the blades, exactly as with a powerful high-speed prop that is not deeply immersed. Note this is ventilation caused by air being mixed with the fluid, not cavitation (which is caused by a very low pressure occurring behind the prop blade and does not involve air) though many mistake the two.

Continue moving the blades upwards until they eventually break the surface - you should now see a great deal of disturbance and have effectively created a surface drive system, albeit a very inefficient one with minimal pitch and little thrust.

If you now run around the room as quickly as possible to simulate the forward motion of a fast boat you will clearly see the rooster tail appear, exactly in the way a surface drive creates one.
Click to expand...

Just brilliant!
 
AndieMac said:
A fast power-cat will leave an impressive rooster tail which sits well aft of the boat, I guess something to do with the converging of the hull and drive wash.

If you talk to raggies that sail on super-fast sailing cats, the conversation invariably gets to the moment when the eyes light up describing the huge rooster tail the boat leaves screaming along at an "untold" amount of knots, with the helmsman bearly holding on :rolleyes: :).

I think most folks like the look of a rooster tail.
Click to expand...



I've heard of Dragonfly Trimarans reaching 24 knots+. Fancy a go on one of those but dunno if I could handle the G forces. Aahaha.
 
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Particularily good at 8:25



The OB here seems to struggle more



For jetboats its a natural occurance.



A regualr 19' with OB




So you dont visit your local chandlery and say 'escuse me mr, can i have a rooster tale making prop' part with a large emount of beer money and of youy go with a big smile on your face:rolleyes:
 
grumpy_o_g said:
The effect is easily demonstrated at home in a domestic surrounding. You will need a powerful handheld liquidiser, a large bowl and a pint or so of raspberry coulis or similar brightly coloured liquid.

.
Click to expand...

Just tried it but didn't have any raspberry coulis, or "thin runny jam" as we call it, so instead used chocolate Angel Delight. We had three packets with a use by date of 1987 kicking around so poured 'em into the washing up bowl and kicked up the soup liquidiser. Cor pretty good effect. Lounge now looks as if the dog's gone beswick on E's and got a nasty dose of diarroeah into the bargain, but the experiment was worth trying and I now fully understand how rooster tails are made. Good job we're leaving this rented apartment in the morning though.
 
coliholic said:
Just tried it but didn't have any raspberry coulis, or "thin runny jam" as we call it, so instead used chocolate Angel Delight. We had three packets with a use by date of 1987 kicking around so poured 'em into the washing up bowl and kicked up the soup liquidiser. Cor pretty good effect. Lounge now looks as if the dog's gone beswick on E's and got a nasty dose of diarroeah into the bargain, but the experiment was worth trying and I now fully understand how rooster tails are made. Good job we're leaving this rented apartment in the morning though.
Click to expand...

:D:D:D
Just spilt tea all over the keyboard
 
MapisM said:
I wouldn't mind spending some time on that boat, too - but hopefully with another helmsman.
Either in the clip he/she was showing off for the camera, or he/she doesn't have a clue about how to use surface drives.
Click to expand...

How are they doing it wrong MapisM , is it trim or speed ,i think that video has been slowed down a bit .
 
coliholic said:
Just tried it but didn't have any raspberry coulis, or "thin runny jam" as we call it, so instead used chocolate Angel Delight. We had three packets with a use by date of 1987 kicking around so poured 'em into the washing up bowl and kicked up the soup liquidiser. Cor pretty good effect. Lounge now looks as if the dog's gone beswick on E's and got a nasty dose of diarroeah into the bargain, but the experiment was worth trying and I now fully understand how rooster tails are made. Good job we're leaving this rented apartment in the morning though.
Click to expand...
You just got my vote for the post of the week.
Good thing that I wasn't drinking tea as Bassplayer did!
 
bamboleo said:
How are they doing it wrong MapisM , is it trim or speed ,i think that video has been slowed down a bit .
Click to expand...
Trim. Way too high: the water is supposed to be pushed astern, not to the sky.

PS: on second thought, I suppose they did it purposedly, with tabs fully lowered to keep the bow down and trim fully raised to push water up for the video.
 
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