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Old 12-05-2019, 19:23   #31
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Re: Water tank capacity/yacht stability.

Ah, Raymond! You were a Merchant Mariner? I'm always happy to have my errors picked up by people with superior qualifications, so thank you :-)

One way or the other I think we agree that Coopec is worrying needlessly. Calculating ballast for a keel yacht is obviously done for the purpose of putting the LPS beyond all practically attainable angles of heel. IOW a sailing keel yacht never runs "light" in the sense a cargo vessel can.

We had a nifty case of a "light" cargo carrier having to shift berths at Surrey Docks a few years ago. As she was being eased away from her berth by tugs, one of the tug skippers didn't like the way it felt and jammed her up against the dock at his maximum revs and held her there till they got a dozen handy forklifts deep down into 'er hold :-)!

I hope the Super bought that skipper a few beers :-)!

TP
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Old 12-05-2019, 21:53   #32
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Re: Water tank capacity/yacht stability.

Do remember that most cruising voyages are designed for as little windward work as possible. The loss of windward performance from tenderness is a small factor for most! However, adding several tons of liquids to the burden, no matter how organized vertically, will surely slow the boat when sailing off the wind. So, some consideration of how you tend to plan your cruises might influence the choice of full vs limited tankage.

We carry 800 L of water, and can easily feel (and measure) the loss of performance when the tanks are full.

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Old 12-05-2019, 23:47   #33
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Re: Water tank capacity/yacht stability.

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Originally Posted by TrentePieds View Post
Ah, Raymond! You were a Merchant Mariner? I'm always happy to have my errors picked up by people with superior qualifications, so thank you :-)

One way or the other I think we agree that Coopec is worrying needlessly. Calculating ballast for a keel yacht is obviously done for the purpose of putting the LPS beyond all practically attainable angles of heel. IOW a sailing keel yacht never runs "light" in the sense a cargo vessel can.

We had a nifty case of a "light" cargo carrier having to shift berths at Surrey Docks a few years ago. As she was being eased away from her berth by tugs, one of the tug skippers didn't like the way it felt and jammed her up against the dock at his maximum revs and held her there till they got a dozen handy forklifts deep down into 'er hold :-)!

I hope the Super bought that skipper a few beers :-)!

TP
Not merchant marine, oil driller.

The weirdest Cross Curves of Static Stability you have ever seen are those for a semi submersible drilling rig. These things acquire their sea state motion minimization by spreading their water plane areas over a wide area which, whilst it minimizes pitch and roll, makes them very sensitive to load shifts to a degree which requires them to have a ballast control officer permanently employed in keeping them level.

Doing a stability plan for a deep casing job on the smaller ones can be a bit of a nightmare.
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Old 13-05-2019, 02:38   #34
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Re: Water tank capacity/yacht stability.

Geez, I wish I was qualified to comment, but I canít find a certificate to put on my wall that proves I can sail a boat.

If I was qualified I would comment that I canít tell the difference in stability between when my 450 litre keel tank is empty or full on my 17 ton boat.

Maybe a qualification would help me tell the difference.
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Old 13-05-2019, 04:16   #35
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Re: Water tank capacity/yacht stability.

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Geez,
If I was qualified I would comment that I can’t tell the difference in stability between when my 450 litre keel tank is empty or full on my 17 ton boat.

Maybe a qualification would help me tell the difference.

If your yacht is a Swanson 42 it obviously is not 17 tonne? (FYI a Swanson 42 would weigh closer to 10 tonne)
I don't think a qualification would help!
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Old 13-05-2019, 05:25   #36
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Re: Water tank capacity/yacht stability.

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If your yacht is a Swanson 42 it obviously is not 17 tonne? (FYI a Swanson 42 would weigh closer to 10 tonne)
I don't think a qualification would help!


So you know more about my boat than me now?

So, please, elaborate. How do you know this? I am absolutely intrigued.
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Old 13-05-2019, 05:42   #37
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So you know more about my boat than me now?

So, please, elaborate. How do you know this? I am absolutely intrigued.
You can't be serious?

As I said before I don't think any qualification will help you understand how full/empty water tanks affect stability.

Let's leave it at that because I'm moving on.........

Cheers
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Old 13-05-2019, 05:45   #38
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Re: Water tank capacity/yacht stability.

Oh. Right. Sorry. I stand corrected.

Good luck and thank you for letting me know what my boat weighs.
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Old 17-05-2019, 07:22   #39
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Re: Water tank capacity/yacht stability.

One concern that needs to be addressed if you make a boat stiffer, is that that stiffness affects the loading on sails and standing rigging. The stiffer the boat, the stronger these need to be as a mono subject to a massive gust will heel to relieve that loading, but a catamaran is designed to accept that extra stress, by having thicker mast section, heavier sailcloth and thicker standing rigging.
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Old 17-05-2019, 09:05   #40
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Re: Water tank capacity/yacht stability.

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I very much appreciate these forums because there are a lot of knowledgeable people willing to share their expertise. (But then, sadly, there are the "others" who just think they know........)

Currently I have three 40 gallon (200L) water tanks buried down in the keel but now I've decided to install a water maker with a capacity of 21gph. I am wondering what a sensible tank capacity would be?
https://www.ebay.com.au/itm/21-GPH-W...53.m1438.l2649

If I left the tanks in and filled them with water it would add to the yacht stability but if I took two out and replaced them with 300kg (660lb) of lead, the yacht would be more stable and I could then store heavy items such as anchors low down in the keel.

I would appreciate any advice from skippers of Cruising Yachts who would be qualified to comment.



Change them over to fuel tanks. A gallon of diesel will make 16 gallons of water.
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