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Water ballast and a bilge pump?


Tom the rower

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Generally it is preferred to put the water ballast in the center of the boat. A quarter turn access port mounted in the top of the ballast tank just above the waterline, allowing reach through access to a simple drain hole with a stopper will do the trick. It is important that the top of the ballast tank be as close to the waterline as possible to minimize sloshing of the water ballast.

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A deck plate as Brent describes will work well and let you reach down in there, for cleaning or unplugging if necessary. The height of the tank will have no effect on sloshing water (free surface effect), which is a fair concern with water ballast. Baffles are the only easy and good way to control free surface effect. There are other techniques, but baffles require no moving parts or other devices. If a water ballast tank is designed (and used) properly, this problem can be mitigated to a large degree.

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I wonder how you would fill and empty a port or stbd water ballast tank?  Use a bilge pump to fill and empty the tank?Thanks

There are several ways similar to the attached sketch... but you get the idea. Sketch is a home-made version of a commercial product made in France that costs hundreds of dollars if I remember. Home-made version typically made from various pieces of standard PVC plumbing materials or f'glass tubes and O-rings. The inner tube slides up and down, exposing the tube with it's intake hole below the hull bottom while filling the tank. The hole in the slider tube faces forward and the dynamic pressure from the movement of the boat is used to provide the impetus to fill the tank (probably needs an air vent hose/valve at the top of the tank). This is the approach used in most if not all the Classe Mini 650 Prototype division boats.The side tanks are usually interconnected with one or more large diam tubes/hoses that have ball or slide valves to open close flow from one tank to the other. In the right conditions, you can dump from the windward tank to the leeward tank using gravity (just before you tack/jibe) thereby reducing/avoiding most pumping altogether.The choice of location for one or more ballast tanks depends on what you objective is for the extra weight (increased righting moment vs increased momentum vs boat trim vs self-righting stability) or a combination thereof.Dual saddle/side tanks (maybe in/under the seats of a CS17/20) would be more efficient than a single/central tank if your primary use is to gain maximum righting moment. But there are obviously other considerations. Perhaps Graham will see this thread and add his thinking and elaborate on his choice and the tradeoffs for a single tank on his water-ballasted designs.As PAR indicated earlier, the location of the top of the tank relative to the waterline has nothing to do with the free surface effect. Controlling free surface effect is VERY important but easily accomplished - depending on the geometry of your tanks and whether or not you intend to operate with tanks partially filled. (I would use more vertical baffles in a very shallow tank and more horizontal baffles in a tall skinny tank.)As a thought exercise... consider that it's not uncommon for small offshore boats to use standard 7 gal water jugs (total 56 pounds in each, and that's also their drinking water stores) and manually shift them to various locations to trim the boat side to side (and fore/aft). Perhaps you could do something like that on a CS17/20 if the seats and seat hatches were configured for it. Four jugs (prox 224 pounds) would be like having a hefty crew member moving from side to side for increasing your righting moment. Or perhaps even better on small boats to use more but smaller jugs of shapes that fit specifically depending on the places you have/make available to store them. Depending on what you're trying to accomplish, maybe you don't need the complexity of permanent tanks and plumbing to get a majority of the benefits.PS - keep in mind that one pound of weight that is shifted from side to side (outboard in the seat area) has the approximate beneficial effect as two pounds of weight stowed near the center-line (under "normal" conditions). Even when cruising (I mostly single hand, so have no crew for ballast) I also keep all my heavy stores like canned goods & drinks in those single tier plastic soda crates and shift them from side to side along with my 7-gal drinking water jugs whenever sailing on long tacks. Works much better than expected.

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Like Howard, I was a little confused knowing that Tom has a CS20 mk2 kit. The mk2 has a single ballast tank on centerline between the cokpit sides. There is a centerline baffle, not two tanks.

 

The object of the water ballast was not to increase horse power at normal sailing angles, the CS20 has plenty of that but to give more righting moment at large angles of heel to prevent a capsize or self right if a capsize occurs.

 

Wing ballast while generating more hosepower could be counter poductive if you got it wrong and would be too busy for short tacking. The CG would be higher in a V bottom boat compared to CL ballast.

 

The couple of sails that I had in the mk2 showed that it worked very well and the capsize test proved that the math was correct. In reality it increased the steadiness of the boat a lot especially just moving around on board at rest and did have a lot more righting moment under sail.

 

What I think that what Brent meant about having the ballast tank top too high was that if you do not fill the tank to the top, the water would move to leward reducing it's effect.

 

It remains to be seen if it would be faster in a race like the EC but the increasd horse power will allow the boat to carry more sail offsetting the extra weight. The main objective was for safety.

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