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  #1  
Old 10-16-2012
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Still confused about water ballast

So water ballasted boats like the macgregor 26D (not the X and the other "power" sailors, let's exclude them from this conversation) obviously work, and its phrf rating isn't too bad either. It is a huge advantage that the entire hull weight is under 2000#, meaning a regular jeep can tow it when necessary. But I'm still confused.

The widely held idea that the water ballast in the bilge has to be lifted out of the water before it starts creating righting moment has been debunked on the internet at least, and frankly I don't see how the concept of water ballast could possibly work if it had to be above the waterline to work. Hey, our bodies are made of 90% water and a boat is more stable (theoretically) with a person lying in the bilge than without a person lying in the bilge, right? That's how you balance a very small dinghy no? I have heard the explanation that a pound of feathers is the same as a pound of lead if the cg of that pound is in the same place. This makes sense to me.

But if this were true, then wouldn't a bulb keel, full of water, also act as ballast? Weight is weight, right? Yet this is obviously not true. So how can water ballast work when inside a hull and below the waterline, but not when inside a bulb suspended below the boat, also below the waterline?

Could someone explain this concept to me, dumbed down enough that I understand ?
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Old 10-16-2012
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Re: Still confused about water ballast


It's not that the water ballast doesn't work, it's that it's not nearly as efficient as lead ballast in a fin keel or a modern fin/bulb combo.

Water ballast doesn't work well. That means you need a smaller shorter rig, reducing performance.
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Old 10-16-2012
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Re: Still confused about water ballast

Here's Hunters explanation for the 260..

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But realistically I think the major benefit is the ability to shed the ballast weight for trailering. Water ballasted Ocean racers carry the water ballast outboard under the gunwales, on very beamy boats. This can equate to several bodies 'on the rail', as is typical of crewed yachts.
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Old 10-16-2012
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Re: Still confused about water ballast

Quote:
Originally Posted by peterchech View Post
But if this were true, then wouldn't a bulb keel, full of water, also act as ballast? Weight is weight, right? Yet this is obviously not true. So how can water ballast work when inside a hull and below the waterline, but not when inside a bulb suspended below the boat, also below the waterline?

Could someone explain this concept to me, dumbed down enough that I understand ?
Lets do a little mental experiment.
Picture a standard hull with standard fin keel.

You put some weight in the bilge of the boat.
Can be be feathers, lead, people anything.
What was their before was air you are replacing it with weight that weight will change the righting moment of the boat.

Same boat but this time you take a garbage bag fill it with water dive under your boat and tape it to the keel.
Yes if you try move the boat forward the bag will rip off but what about the righting moment.
No change.
If you attempted to tip the boat the effort needed to tip the boat with the bag of water attached to the keel would not change from the effort needed to tip the boat with no bag.

If you think about it anything that you attach to the bottom of the fin keel that is lighter than water will have the effect to float the boat higher and make it tip easier. Anything you attach that is heavier will have the effect to sink the boat a little and make tipping it harder.

If you had a boat design that had a very deep full bilge that was normally full of air and you flooded it that would change the handling of the boat. You are changing the displacement of the boat.

If you had a torpedo shaped tube at the bottom of your keel that could be pumped out or filled with water that would make a big change to the handling of the boat but not the way you would want.
When empty the lever action would make the boat very tender. You could imagine making it big enough that the boat would tip over automatically.

Hey you just invented something new. Design a reverse canting keel.
Your bottom mounted air bladder would be canted to leeword to stabilize the boat. I think you would need really hi-speed pumps plus computer controlled stabilization. It would be a bear to balance.
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Old 10-16-2012
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Re: Still confused about water ballast

Quote:
Originally Posted by Faster View Post
Here's Hunters explanation for the 260..

Mistress of Grand Traverse

But realistically I think the major benefit is the ability to shed the ballast weight for trailing. Water ballasted Ocean racers carry the water ballast outboard under the gunwales, on very beamy boats. This can equate to several bodies 'on the rail', as is typical of crewed yachts.
I agree and I don't think it is nearly as effective as a denser material. It is used purely to be able to lower the tow-able weight of the boat. They use some really dense and heavy materials like depleted uranium in the bulbs of racing boats correct? So water is not going to have nearly the affect of a more dense material. Even concrete would be more effective but of course it does not flow out well after it sets!

One thing on water ballasts on racing boats is that they can move it from one side to another so as to effect trim. I once saw a guy at a launching ramp explaining to the guy next to him how superior the water ballasts he said just like the racing boats use.
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Old 10-16-2012
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Re: Still confused about water ballast

I am just waiting for this one to devolve....
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Old 10-17-2012
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Re: Still confused about water ballast

Quote:
Originally Posted by davidpm View Post
Lets do a little mental experiment.
Picture a standard hull with standard fin keel.

You put some weight in the bilge of the boat.
Can be be feathers, lead, people anything.
What was their before was air you are replacing it with weight that weight will change the righting moment of the boat.

Same boat but this time you take a garbage bag fill it with water dive under your boat and tape it to the keel.
Yes if you try move the boat forward the bag will rip off but what about the righting moment.
No change.
If you attempted to tip the boat the effort needed to tip the boat with the bag of water attached to the keel would not change from the effort needed to tip the boat with no bag.

If you think about it anything that you attach to the bottom of the fin keel that is lighter than water will have the effect to float the boat higher and make it tip easier. Anything you attach that is heavier will have the effect to sink the boat a little and make tipping it harder.

If you had a boat design that had a very deep full bilge that was normally full of air and you flooded it that would change the handling of the boat. You are changing the displacement of the boat.

If you had a torpedo shaped tube at the bottom of your keel that could be pumped out or filled with water that would make a big change to the handling of the boat but not the way you would want.
When empty the lever action would make the boat very tender. You could imagine making it big enough that the boat would tip over automatically.

Hey you just invented something new. Design a reverse canting keel.
Your bottom mounted air bladder would be canted to leeword to stabilize the boat. I think you would need really hi-speed pumps plus computer controlled stabilization. It would be a bear to balance.
Ahhh, this is a great thought experiment. So when you replace what was air filled space with water (or feathers), you are adding ballast. This is starting to make sense.

I know it's not ideal, but everything is a compromise I'm just curious about the concept in general. Besides, if I'm correct the mac 26D has a phrf rating of 210, which is well below my lead ballasted hunter 25's phrf rating of 234. So it's no racer, but it's not bad for a cruiser either, water ballast or not...
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Old 10-17-2012
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Re: Still confused about water ballast

You can't overlook the fact that water-ballast, in the case of these trailerables is all about convenience not performance. In essence, MacGregor isn't saying that water ballast is preferable, but the design allows the boat to do something it wasn't intended to do, at least initially. If you have the choice, you're better served with a conventional keel, however not having one shouldn't exclude you from being able to sail.
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Old 10-18-2012
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Re: Still confused about water ballast

Take a look at some modern day sailors/racers and you'll see they're all going to lighter materials and changing to lifting keels w/ bulbs. Lifting, so they're easy to trailer. Your still stuck towing a big chunk of lead around, but at least its not 5' above your trailer.
The GP26 looks like its going to a lifting keel in their class rules, the Left coast dart, the J70, the i550, melges, Farr B/one, etc. It just makes sense.
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