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Old 11-23-2010, 03:27 PM   #21
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RE: Filling the keel with cement

Quote:
skipperdude wrote:*Just gross 16 ton *and net 10 ton. *She is 31 &9/10*ft* waterline* 11 and 9/10 beam
OK, judging by the beam and extrapolating your roll period, the boat is a bit stiff. It*may have*too much weight down low already, probably because someone worried about the top hamper.

And since you mentioned it, gross tonnage is enclosed volume, net tonnage is the volume available for cargo. Neither has any connection with the displacement of the boat., they are regulatory and taxation figures.

*
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Old 11-23-2010, 03:29 PM   #22
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RE: Filling the keel with cement

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Delfin wrote:

Yes, RickB.* I have heard of 'Free Surface Effect,' which is why I suggested that Skipper fill the tank to eliminate the effect.
*It wasn't a tank you suggested he fill, it*was the*fish hold. Big big difference!
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Old 11-23-2010, 03:35 PM   #23
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Filling the keel with cement

Quote:
RickB wrote:

OK, judging by the beam and extrapolating your roll period, the boat is a bit stiff. It*may have*too much weight down low already, probably because someone worried about the top hamper.
I am going to redo the roll test and get it right this time.* I assume a roll*period should be about 4.0*seconds or there about. for my boat.

SD

*


-- Edited by skipperdude on Tuesday 23rd of November 2010 04:35:34 PM
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Old 11-23-2010, 07:06 PM   #24
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RE: Filling the keel with cement

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RickB wrote:

*
Delfin wrote:

Yes, RickB.* I have heard of 'Free Surface Effect,' which is why I suggested that Skipper fill the tank to eliminate the effect.


*It wasn't a tank you suggested he fill, it*was the*fish hold. Big big difference!
Isn't the free surface effect present in a tank, too?* As I understand it, this is one of the things truck drivers who haul fuel*trailers have to understand and be careful of.* Isn't the free surface effect the force(s) generated by the sloshing of a fluid in a defined space like a tank or fish hold?* The fluid gets moving back and forth by the movement of the container and once it's moving it*begins generating unbalancing forces independent of what the container is doing?

Or do I have the definition wrong?

*
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Old 11-23-2010, 08:00 PM   #25
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RE: Filling the keel with cement

Hiya,
* I think the free surface effect is much more pronounced in a boat as opposed to a tanker truck.
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Old 11-23-2010, 08:21 PM   #26
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RE: Filling the keel with cement

My*last vessel, a pocket cutter*(Bluewater Blackwatch), had sand as ballast, well sealed.* Never a problem.
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Old 11-23-2010, 08:51 PM   #27
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RE: Filling the keel with cement

If the tank is full, the effect is nil.* If it is not, you have to deal with a moving center of gravity.* Mypoint was to determine whether adding weight easily with water to what one would presume was designed to add significant weight below the waterline effected the apparent stability.* If it did, then it might make sense to go to the trouble of adding ballast.* If not, why bother?* However, it sounds like that might be a problem for Skipper, so please assume I never said a thing....

Next, we'll get a lecture from RickB on the subject, to be sure.
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Old 11-24-2010, 03:39 AM   #28
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RE: Filling the keel with cement

Hiya,
** I think Mr Delfin you've hit the nail on the head.* If the tank is totaly FULL, there's no free surface and thus no effect.** I'm ready to be corrected.* I recall boating in a leaky canoe.* Even 1" of water in the bottom completely destroyed stablility.* That was when I was a lot younger and HAD a sense of balance.* Now, I'd be swimming.
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Old 11-24-2010, 05:11 AM   #29
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Filling the keel with cement

Quote:
Delfin wrote:
Next, we'll get a lecture from RickB on the subject, to be sure.
If you tell someone to do something very dangerous and stupid and I read it I will post the reasons why it is dangerous and stupid. Too bad if it offends you.

It's a fish hold, not a tank. Most of the volume of that hold is above the vertical CG. If it were pressed in order to avoid free surface effect the boat would probably be dangerously overloaded with a GM that made it liable to capsize without warning. If it was only filled to some arbitrary level then the very large free surface would flip it in a heartbeat. That is why your "advice" is dangerous and stupid.

To top it off, what has the waterline got to do with CG?

And for Marin, the sloshing you mentioned is also very dangerous, especially to a road vehicle where the tank is a separate structure. On tankships it can cause structural damage when the ship is subject to surging in a seaway. Both tank trucks and tankships use multiple smaller tanks with baffles and divide them on the centerline to minimize free surface and sloshing problems.

The difference in it being a hold rather than a tank is that, except for fishing vessels that use a refrigerated seawater fish hold like you see on the Bering crab boats, the boat is not designed to operate with the hold completely full of water and "batter boards" are used to prevent fish and ice from causing a free surface effect.

Tanks are calculated for their stability effects in the design stage and the vessel stability is calculated with tanks in empty and pressed condition. The stability documentation may include prohibitions against operating the boat with certain tanks part full, empty, or pressed.

*


-- Edited by RickB on Wednesday 24th of November 2010 06:21:00 AM

-- Edited by RickB on Wednesday 24th of November 2010 06:28:07 AM
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Old 11-24-2010, 08:25 AM   #30
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RE: Filling the keel with cement

Thanks for the correction RickB, and for being so predictably true to character.
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Old 11-24-2010, 08:36 AM   #31
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RE: Filling the keel with cement

"If you tell someone to do something very dangerous and stupid and I read it I will post the reasons why it is dangerous and stupid. Too bad if it offends you."
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Old 11-24-2010, 08:38 AM   #32
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RE: Filling the keel with cement

As I said, always true to character; and always amusing.
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Old 11-24-2010, 09:01 AM   #33
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RE: Filling the keel with cement

OK, so you know that your*ignorant advice is dangerous and stupid*and having it pointed out somehow amuses you?

At least you*seem to be*capable of understanding that one person here will point out the danger of truly irresponsible "advice." And you don't have to "predict" that, it is a fact.*

If you don't enjoy being*shown as a fool then spend a bit*more time considering what you suggest other people do with their boats.*Trying to make it look like your total f-up is merely an amusing to you troll makes*your advice even more*outrageous. I would have said ridiculous but it was so dangerous as to be bordering on criminal negligence.
*
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Old 11-24-2010, 09:06 AM   #34
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RE: Filling the keel with cement

LOl RickB.
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Old 11-24-2010, 10:11 AM   #35
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RE: Filling the keel with cement

now children be nice, namrod
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Old 11-24-2010, 04:30 PM   #36
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RE: Filling the keel with cement

" Can't we all just get along?"
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Old 11-24-2010, 05:19 PM   #37
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RE: Filling the keel with cement

"Can't we all just get a lawn?"

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Old 11-24-2010, 05:33 PM   #38
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RE: Filling the keel with cement

Quote:
RickB wrote:

*
Delfin wrote:
Next, we'll get a lecture from RickB on the subject, to be sure.
If you tell someone to do something very dangerous and stupid and I read it I will post the reasons why it is dangerous and stupid. Too bad if it offends you.

It's a fish hold, not a tank. Most of the volume of that hold is above the vertical CG. If it were pressed in order to avoid free surface effect the boat would probably be dangerously overloaded with a GM that made it liable to capsize without warning. If it was only filled to some arbitrary level then the very large free surface would flip it in a heartbeat. That is why your "advice" is dangerous and stupid.

To top it off, what has the waterline got to do with CG?

And for Marin, the sloshing you mentioned is also very dangerous, especially to a road vehicle where the tank is a separate structure. On tankships it can cause structural damage when the ship is subject to surging in a seaway. Both tank trucks and tankships use multiple smaller tanks with baffles and divide them on the centerline to minimize free surface and sloshing problems.

The difference in it being a hold rather than a tank is that, except for fishing vessels that use a refrigerated seawater fish hold like you see on the Bering crab boats, the boat is not designed to operate with the hold completely full of water and "batter boards" are used to prevent fish and ice from causing a free surface effect.

Tanks are calculated for their stability effects in the design stage and the vessel stability is calculated with tanks in empty and pressed condition. The stability documentation may include prohibitions against operating the boat with certain tanks part full, empty, or pressed.

*


-- Edited by RickB on Wednesday 24th of November 2010 06:21:00 AM

-- Edited by RickB on Wednesday 24th of November 2010 06:28:07 AM
*

Alrighty then, if no one else will do it, I will. Rick is right on the money with what he said here. And the seriousness of the matter truly required it be a little harsh. Marine architects should be employed to make decisions that might effect center of gravity. It's simply not a subject *for us backyard engineers. Thanks for helping us to be safe Rick.

*
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Old 11-24-2010, 06:40 PM   #39
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RE: Filling the keel with cement

Its the way and manner that RickB says things.* Most of us moved from PMM because of RickB crass inappropriate replies.* A person can make their point with out flaming and being so negative.* I was hoping that this site would be monitored and not allow such replies.*


*
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Old 11-24-2010, 07:11 PM   #40
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RE: Filling the keel with cement

Thanks Carey, for the insight and caution.* The idea that someone would put water into a hold designed to have water and fish put in it to see if there was any stabilizing effect before pouring concrete in the bilges was, as RickB said and you have confirmed, criminally stupid.* Who could suggest such a thing?

I move we forward this thread to the Bristol Bay fishing fleet immediately.* Who knows how many lives could be saved?
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