HALF FULL BALLAST VS FULL BALLAST

Catigale · Post by **Catigale** » Mon Dec 11, 2006 3:59 am

I think we hashed through this once before ...the density of the ballast material contained in the tank doesnt change the fact the heeling involves lifting mass against gravity, and hence the ballast works to prevent heeling

RIch is right in that the

is very tender for the first 10 degrees of heel in my experience I dont see a lot of difference with ballast in or out. (Be careful with ballast out - light air only and no kids on board !!)

Frank C · Post by **Frank C** » Mon Dec 11, 2006 2:32 pm

... It is not until this "ballast" is lifted above the waterline that the weight of the water becomes true ballast. It is at this point that the boat will stiffen up.

Agreeing w/ Stephen, it's fallacy that water ballast only works as it rises above the waterline.
But I heard this slightly different explanation from a professor of physics.

Proof: Imagine a bucket of water as a 'pseudo' ballast container. Submerge it into the lake ... as you lift it from the lake, you feel a gradual increase in its 'weight' as it emerges from hydrosphere to atmosphere - this is the effect that many ascribe to water ballast (erroneously). But, now carry that filled bucket down into your cabin, and gradually lower that bucket to the cabin sole, which IS below the waterline.
Does that bucket weigh less as you lower it closer to the sole .... ???

NO! Truth is, the entire ballast tank IS ALWAYS above the effective water level, since it's contained within a vessel - the hull. All mass contained within the boat's hull are suspended fully within the atmosphere, not the hydrosphere. Hence all materials within that hull are in the same locus ... subject to the same laws of physics.

baldbaby2000 · Post by **baldbaby2000** » Mon Dec 11, 2006 3:59 pm

I think if one views the problem as the righting moment defined by the center of gravity and the center of buoyancy the water ballast issue always can be explained.

If the ballast is full, the center of gravity (COG) of the boat is low and doesn't change as the boat heels although the center of buoyancy (COB) does.

If the ballast is partially full, the center of gravity is higher, but the important thing is that both the COG and COB change as the boat heels. And the center of gravity changes in a bad way (it moves in the same direction as the COB) until the boat is heeled enough that the ballast doesn't move anymore.

BB

Divecoz · Post by **Divecoz** » Mon Dec 11, 2006 5:51 pm

eric3a wrote:Just as a quick reminder: Free surface effect and its stability reduction is only a matter of the surface, not the volume (ie weight) of the liquid.
It's also independent of where in the boat it happens to be and independent of the angle of heel.
So whether you have one inch in the bottom or your ballast or are 90% full you loose the same amount of stability due to free surface effect.

Of course the transfer of liquid from one end of the tank to another changes the weight distribution, so that's where the volume actually counts.
Transfer enough weight and you may capsize, or just no come back up.

Probably a little too theoratical for the question asked, but people tend to get confused by the "surface only" element of stability loss due to free surface effect.

Eric

Are you trying to tell us. That it makes no difference if I have a 100 lbs of water running willy nilly in the ballast or 500lbs, that it has the same effect on stability?? I don't buy that.

James V · Post by **James V** » Mon Dec 11, 2006 11:19 pm

Eric - Please do not have the ballest partially full.

With or without - well that is going to be up to you. I motored the ICW several 100 miles and it is much easier motion to have the ballest full. The motion left to right is not as sever and easier.

The best thing to do would be try it without and with ballest.

Please note that I would not be in my boat is seas over 2 feet without ballest and when at anchor.

50/50 · Post by **50/50** » Tue Dec 12, 2006 1:44 am

Duane Dunn, Allegro wrote:The only times I've ever experienced stability issues with my Mac have been when the tank is partially full (while draining under way). An empty tank is always preferable to a partially full one. We motor empty 95% of the time. Almost 1500 nautical miles under power this way with no stability problems even in rough seas. All with a crew of 5 on board.

I read about your adventures on your web site. Very interesting I must say. I'm interested though, as to your lack of sailing on your trips?

Regards
Gerry

Zoran · Post by **Zoran** » Wed Dec 13, 2006 4:03 pm

Ok, How about filling the ballast with one person on board (single handed, me for example) and when you have 6 people on board. When I am single handed with full balllast I can barely reach the water in the tank (1997 with vent under the step) while with more people on board the water level in ballast tank is higher for about 1" (for obvoius reasons, no need to discuss that). Does it mean that singlehanded I have partialy filled ballast? Other question how do you fill the ballast completely since water slashing in the tank on anchorage can drive me nuts, especialy if there are kids chasing around with dinghies. I tried rocking the boat to get the air bublles out but it does not work. There is always some air left in the tank.

Zoran

Catigale · Post by **Catigale** » Wed Dec 13, 2006 4:20 pm

The last air bubble in the 2002

tank at least is only about 5-10 gallons in volume typically...not enough to allow the dangerous sloshing and weight transfer you are trying to avoid. I cant speak to what the bubble size is on the other boats

Frank C · Post by **Frank C** » Wed Dec 13, 2006 4:54 pm

I agree with both ... Catigale's 2002 ballast bubble, like mine, is deminimus. But the bubble left in the '97 or '98 ballast tank is reportedly quite frustrating. That's why I suggested (in different thread) drilling the hole into the forward tank and plumbing a hose to the sink drain (or even directly into the top of the CB trunk -

). This would permit using your crew to move to the bow, minimizing the bubble just before closing the transom valve.

Craig LaForce · Post by **Craig LaForce** » Wed Dec 13, 2006 7:30 pm

I don't have any problems getting my ballast to fill completely on my 97.

If I hear a bit of slosh after filling, I just stand at the galley and rock the boat around a bit with my weight and put the plug in. I don't hear any sloshing after that. Maybe it fills better if the boat has some load in it with gear, fuel and water tanks. Not sure how much I have in mine, but maybe it is enough to make the tank fill better. I have no forward vent on mine and would not see a reason to add one.

Don T · Post by **Don T** » Wed Dec 13, 2006 7:36 pm

Hello,
Early boats like mine had the vent under the step with a center fill valve. I always had a huge bubble in the tank. Because the original hull (#3) I test sailed had a forward bump with vent, so did the mold. I drilled a vent up forward on my boat and vent both (one at a time) when filling. This is not an insignificant amount of water. It takes an additionl 2 minutes with both valves open (small transom & center) to completely fill the tank. It makes no noise when rocking at anchor (I'm a light sleeper too) and the boat is much more stable. I figure it's an additional 200 lbs. The tanks are full to the top when lightly loaded. The center vent will spurt up like an artesian well when we are loaded for a cruise.

Zoran · Post by **Zoran** » Thu Dec 14, 2006 10:56 pm

Interesting posts. Looks like I might be drilling a new vent.

Zoran

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HALF FULL BALLAST VS FULL BALLAST