Water ballast

Question for the technically minded for a trailerable boat.
If using water ballast, does the ballast containment need to be closed off when full, therefore adding the weight to the boat, or is it sufficient to have an open ended container that will fill when the boat is launched and drain when being put back on trailer. My thought was that with an open ended container/s as the boat moved from side to side the weight in the container would add to the weight of the boat.
I am only talking about displacement boats obviously.
Only issue I can see is having more openings below the waterline.
I am looking at building a 16ft round bilge displacement hull, that may require a small amount of ballast and I dont wish to add that in the form of lead, steel or other solid stuff due to the extra trail weight.
Probably a silly question, it haven't found much on google yet to answer it. Still searching.

 

Welcome to the forum.

Short answer is to function as ballast it must be closed off from the supporting water.

A valve can be opened to allow drainage for trailering

 

I know I'm not answering your question, but for such a small displacement boat, the use of ballast should be avoidable. Why do you think it needs ballast?

 

OK, but since JC asked, and the answer applies when it is worthwhile, Blueknarr is also right, the water ballast has to move with the hull. If the ballast tank isn't almost completely filled and sealed, the water can slosh around and that shifting mass can make the boat less stable. Sealed in this case means an acceptable leak rate.

To fill the tank, the air has to be vented from the top. The lower hole can be left open once the tank is filled, but when it is above the waterline while tacking, the tank can drain, shifting the balance in the wrong direction.

Bolger used water ballast in the Martha Jane, and others(?), but I have sailed on that one. There was a 2 or 3 inch PVC threaded pipe plug/inspection hole in the top of the tank which was near the waterline level when it was filled. The inlet was around a 1" PVC plug on the side near the chine as I recall and could be reached by reaching over the side to screw in the plug after the tank was filled.

The key advantage of water ballast is for trailering. The strongest argument against water ballast is that the same mass of lead on the bottom of the boat would lower the cg more than the water tank. Having a flat 4 inch deep water tank is equivalent to mounting the lead plate 2 inches above the bottom. The strongest argument in favor is that without 900 lb of lead on the trailer the car stops in time, climbs steeper grades and gets better gas mileage.

 

Being round bilge it may need a small amount of weight down low. The plans I am considering have no details other than lines and profiles for the formers for strip planking. May not be necessary, but it will be trailed and therefore I need to keep weight down.

 

Thanks Dave.
Ballast would be in a displacement power boat. No internal engine but powered by high thrust 9.9 outboard, so may need some ballast to bring it down into the water. The hull will be strip planked and internal layout as light as possible.
Yes I could build a ply hard chine boat and not worry about it, but I have a thing for round bilge and the movement is a lot gentler on my old bones. I do not need to, or can, go fast anymore. Comfort all the way.

 

I agree, ballast may be needed, but before deciding anything and thinking about how and where that ballast goes, I would do some stability calculations to check the real needs. It is important, if ballast is really necessary, to know its exact quantity and its location so that the necessary weight is minimized. Would a removable solid ballast be viable? (I apologize if this, for whatever reason, seems silly). It has many advantages over liquid ballast.

 

Water in a keel / centreboard / container below the waterline, and open to freely fill and drain through small holes, can add stability via mass momentum, against sudden gusts giving rapid heeling, but is not exactly ballast. Many trailer sailers can have water filled centreboards, for that reason, and light trailering, Macgregors spring to mind. Not so effective for negating continuous heeling in a stiff breeze. Side tanks and pumping water to the high side on every tack would be more effective there, but has many components to fail on a bad day. Maybe calculate the amount of lead you can use, to stay within your local trailering road rules. EG. my father had a 22 footer, on a single axle trailer, with no breakaway required, carrying about 500 Kg of lead under the hull, either side of the centreboard slot, legally, towed by a family sedan.

 

Removable solid ballast would be better, obviously due to the extra weight being smaller, however I have back issues that would prevent me moving and fitting the pieces. Yes I could do 20 X 5 kilo pieces of lead, but they still have to be carried somewhere and then transferred to their respective positions..

 

Perhaps I should have mentioned it is a power boat, not a yacht.
My legal unbraked trailer aggregate trailer mass here is 750kgs, with 200kgs already taken up with the trailer, so I have 550kg left for the boat, hence the size and questions about ballast. I could go to the next level with a braked trailer, but the trailer cost blows out to over $3000 new. I got mine for $1300 never been in water, and that was a $1300 saving over the factory new price for unbraked.
I may be overestimating the need, but one of the designs I am looking at is a lifeboat style boat designed at 16ft to hold 12 people, a considerable weight at an average of 75kgs each for a small boat, so even after adding in some internal fittings and fixtures, some extra may be needed. I'm 105kgs and most of the cruising will be solo, with weekends adding one other lighter person (my wife, but she would be mortified if I posted her details)

 

For 16 feet, you shouldn't need to add much if anything. You'll have a battery for the 9.9 and an anchor and some chain. When you get the boat in the water, Play with the battery and anchor stowage location unit she rides right, then make permanent provisions. I would expect the bare hull to be around 650 pounds for a 9.9 powered craft. The motor, battery, and fuel is another 220 or so. That leaves more than 300 hundred pounds for trim ballast and odds and ends.

One suggestion, having trailered 16'ers about 100,000 miles in the last 50 years - for longer trips on the highway, remove the OB from the transom and clamp it to a permanent frame midship. Build or modify a frame to act as an engine caddy. I don't bother if I'm just running around the corner to the local ramp, but for highway travel, getting the trailer weight in the middle is important for stability on the road.

And I recommend at least a 2500 pound trailer, axle, and tire rating even for a 16' boat if you are traveling on the highway much even if the tow rating is a lesser number. That's where I ended up at when stuff quit falling apart on me. Having a trailer wheel hub come off on Alligator Alley at 2 am in the rain kinda sucks. So does breaking down when you are evacuating for a hurricane, or when it's four in the morning on Monday and you are supposed to be at work in two hours.

 

Just sail the boat.
If it's a proven design, it'll be fine.

Go to the gym, gain some muscle mass,
AKA "movable ballast", the best.

 

This is false as liferafts use these flexible fabric open water ballasts

While the mass of water is inside flexible or rigid walls in an open ballast it acts as individual mass so it brings a stability ballast

Even a righting lateral water ballast can be open too if the water discharge is little compared to the whole volume so when heeled the boat will be gradually losing that water ballast

Even with in the case of closed righting lateral water ballast you could fill them with a big water entry by heeling the boat the opposite way you want to sail and discharge the water and the next tack and repeating the operation

 

I like the answers from sailors. In small boats, crew weight is the dominant factor in stability.

 

All cargo ships in the world use water ballast stabilisation systems mostly for rolling and dynamic stability but also for pitching and heeling