Model Boat Mayhem

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Author Topic: Another question......  (Read 2003 times)

Bryan Young

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Another question......
« on: February 03, 2012, 04:37:13 pm »

Just another question.
Why do some vessels...particularly tugs and trawlers have a much deeper draught aft than for'd? That is, why is it thought necessary to have a keel that is nowhere close to being parallel to the waterline. Depth in the water of the prop? Possibly. But that's only a proposition. BY.
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Xtian29

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Re: Another question......
« Reply #1 on: February 03, 2012, 04:57:06 pm »

Hello

Depth in the water of the prop is the main reason, there is another the position of the turning point aft as possible (for tug)  but for this one, it still possible to do it with parallel keel/waterline.

Xtian (When I was student as Naval Architect I made my end of year work project on a tug/supply  :D)   
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Bryan Young

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Re: Another question......
« Reply #2 on: February 03, 2012, 06:06:46 pm »

Thanks Xtian...I thought that the depth of the prop was the minor consideration....should have thought about the turning point myself, ....but I didn't. Thanks. BY.
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Xtian29

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Re: Another question......
« Reply #3 on: February 03, 2012, 11:01:50 pm »

Not, it's major consideration as air melting with water is very very bad for bollard pull : the main purpose of tug.

 From zero to 10 meters depth the water pressure double !  Then, this zone is the most important for propeller, the deeper side of propeller is working better than the higher one and it's why it's a turning effect on ships when going astern in one time.

Xtian   
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irishcarguy

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Re: Another question......
« Reply #4 on: February 04, 2012, 08:02:25 am »

If we pay attention, we learn something new every day on this forum, thank you Xtian & Brian, Mick B.
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Mick B.

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Re: Another question......
« Reply #5 on: February 04, 2012, 03:05:06 pm »

Hi Guys

That was very interesting. How can that be applied to modelling a powerful tug?

I've long wondered how you you get the best out of a tug anyway.

Dave
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Xtian29

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Re: Another question......
« Reply #6 on: February 04, 2012, 03:35:19 pm »

Hello

This thing is well applied to model boat, specialy the ones with propeller larger than 6-8 cm. 

If there is air suction with the propeller you will see soon that your bollard pull will decrease.

Then the turning effect is a thing that every modeller, specialy the ones with large one screw tug must know as there is a better side to dock the boat .  You come to the pier at slow speed with a small angle and when you go astern for a short time the aft part of your boat is going transversaly.  If you well know your boat and this turning effect you will do a perfect manoeuvrer   8)  Then if not and if you want to dock on the wrong side of the boat (depending the turning or your propeller right or left) then you will ruin your manoeuvrer  <:(

Xtian
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Netleyned

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Re: Another question......
« Reply #7 on: February 04, 2012, 03:50:28 pm »

Air suction is called cavitation in english and turning is called paddle wheel effect Xtian

Ned
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Xtian29

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Re: Another question......
« Reply #8 on: February 04, 2012, 04:13:06 pm »

Thank for this info Netleyned, specialy the paddle effect.

About cavitation I'm not sure. In France many people confuse with this term as the real cavitation is a bubble created because of the void. Because of depression on the front side of the propeller blade, void appear and then the water is instantaneous boiling. When these bubble implose as the pressure is back it could be some damages, it creat also a singular noise.

Maybe there is not distintive word for that in English but here many people confuse with this words, air suction and cavitation.

Xtian

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Netleyned

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Re: Another question......
« Reply #9 on: February 04, 2012, 04:20:26 pm »

Yes Xtian
I think that any aeration around the propeller is called cavitation as a general term
by most people .Obviously it is not technically correct as there can be other causes.

Ned
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Xtian29

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Re: Another question......
« Reply #10 on: February 04, 2012, 04:38:18 pm »

Quote
I think that any aeration around the propeller is called cavitation as a general term
exept that the real cavitation is not an aeration, there is not air, just void and boiling water (not because of temperature but because of depression)

off course it's very technical and as I said many people by confusing and to simplify (wrongly) are talking about cavitation for aeration.

I checked on the English version of wikipedia and they are talking about cavitation only on the real and properly meaning   http://en.wikipedia.org/wiki/Cavitation

Xtian
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Netleyned

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Re: Another question......
« Reply #11 on: February 04, 2012, 04:47:13 pm »

Paddle wheel effect also known as prop walk

Ned
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Bryan Young

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Re: Another question......
« Reply #12 on: February 04, 2012, 07:21:07 pm »

Also causes the "transverse thrust" effect. I'm glad I asked, nice to see the interest!
But just as I thought of that........what is known about "squat"? The effect on a ship when (for instance) going through the Suez Canal, when a ship gets sort of sucked deeper as the bottom water gets blown away. There must be hull forms that lessen this problem. BY.
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