Water Ballast

[Guest]

How are water ballast tanks on a trailerable sailboat set up? How are the seacocks set up so that the water is able to enter and leave the tank? There has to be some kind of air venting, where is that most often run to? Is it common to have access hatches into the tank space?

I have seen this included in plans but never seen the details of how the system is actually supposed to work.

Marvin

[Pointyendfwd]

Hi Marvin,

I think the designers are too lazy to draw all the details, and sometimes that they don't know. Same story with retractable bowsprits etc.
However, back to the question, I will describe a water ballast system on a boat which we built.

Water inlet is through a Sofomarin scoop, which is a pipe inside a pipe.
The inner pipe can be slid down and turned so that the hole, which is at the side and not the bottom, faces forward or backward.
When facing forward, the speed of the boat through the water (about 5 kts works well) will force the water into the pipe and from there to the windward tank (selectable with a valve system, pretty simple.)
To get the water out of the tank, the pipe is pushed down once again and turned to face aft, and the process is reversed.

Air vents are required, they protect the tanks and serve as in indication when they are full. Normally led to the trandsom, high up.

Access ports to the tank are required, get a nice transparent one which is watertight (obviously), then you have access and you can see how much water is in the tank.

A transfer tube is also required, largest possible diameter, for tacking and taking the water across to the new windward side.
Tacking is interseting, and has to be timed well.

A top-up pump is also required, because you never get all the water across using gravity alone, and the stuff does not seem to want to run uphill without some mechanical motivation.

Holy mackerel, I just realised you were actually referring to the normal dinky little centerline ballast tanks ;) , and here I am describing a transferable water ballast system. Sorry, but I think the principles still apply.
Remember also to put a few extra kilo's of resin on the inside laminate of the tank, with some glass tissue to make really sure about water ingress into the core, it's worth the trouble.

[gonzo]

Mac Gregor boats have a simple system. There is a vent pipe amidships and a hole in the transom. You launch the boat with both holes open and let the double bottom fill up. Close the holes and the ballast is in. When you haul out, open both holes and pull the boat out slowly while the water drains out.

[kudu]

Do you suppose that a similar idea can be designed for a retractable keel? I'm kickin' around the idea of having a small day sailer designed for my grandson. Something in the range of 17' to 18'. It will be an enclosed two seater,for winter time fun as well. I want it to have a water ballast system.

[gonzo]

Do you mean a keel that fills with water? The double bottom has the advantage of extra security. If you run aground and make a hole, the boat will float anyway.

[kudu]

Hi gonzo...Yes, a retractable keel that could be filled via a manual bilge pump or other device???

[gonzo]

Considering the density or water, the retractable keel would be quite large and create enormous drag. If that is what you need though, a dagger board is the easiest way. On the top, leave a hole. A bar goes through it to a valve in the bottom. Open the valve after launching to fill the daggerboard.

[tspeer]

I think you have to be clear as to just why you're adding ballast to the boat in the first place.

All you're doing by filling the keel with water is removing the buoyancy of the keel. There's not enough volume there to make that big a difference to the initial stability of the boat. It might help with nuisance factors, like the centerboard tending to float up when you want it down.

The main reason to use ballast is to move the center of gravity laterally relative to the center of buoyancy. Keel mounted ballast does this when the boat heels. Water ballast near the sheer strake does this when the tank is filled.

Water ballast under the floor does not have the lever arm to move very far laterally when the boat heels. So the main thing it is doing is to make the boat sit deeper in the water, making the waterplane wider. Effectively the boat's sections are now the trapezoidal shape formed by the top of the ballast tank and the sides of the boat, and the stability is that of a shallower flat-bottomed scow. But you still have the drag of the rounded-bottom ballast tank volume under the water. And if the bilge is already under water before the ballast is added, there won't be all that much increase in the waterplane area.

At extreme angles of heel, as in a knockdown, the ballast might be effective if the topsides, cockpit and cabin could be counted on to provide buoyancy. So the ballast would be effective in reducing the region of inverted stability, moving the point of neutral stability past 90 degrees of heel.

For a trailer sailer, I think it would be more effective to design the boat with adequate form stability in her lines to begin with, and dispense with the water ballast. The ballast tanks take away headroom, which a trailer sailer desperately needs. The boat will perform better and have more room inside.

But I recognize you may want to trade some performance for a more forgiving boat. So I think you ought to define what you want in the way of behavior at extreme angles of heel and analyze your ballast design with respect to those requirements. And other parts of the design have to be compatible with these requirements, too, like preventing flooding as the boat is heeled to the angles the ballast is supposed to overcome.

[kudu]

My thought for a design was somewhat of a two seater/day sailor where the occupants sat side by side in an enclosed mini pilothouse due to weather. Because sailing in the waters of the Strait of Jaun de Fuca can be bumpy at times the craft must be stout. Would like it to be 17 to 18' and still light enough to be lifted by two people and placed on a trailer. I'll ask the question again...Can a retractable keel be designed, similar to a daggerboard then be filled with water via a manual bilge pump for stability? At the end of the day reverse the flow of water via a valve to extract the water to dump the weight. Would a sketch of my idea help?

[Chris Krumm]

Saltwater has a density of 64 lbs/ft^3. Lead has a density of 708 lbs/ft^3. A 18'-19' sport sailboat with a retractable lead bulb keel might have an empty weight of 1200 lbs, with 500 of that in a keel bulb 4' below the waterline. A very beamy, mini full keel cruiser might have an empty weight of 2500, with 800 lbs of that as lead near the bottom of the keel 4' below the surface .

These are both very approximate examples, but the lead sitting 4' below the waterline would be .71 ft^3 in the first instance and 1.14 ft^3 in the second. That translates to 7.74 ft^3 saltwater in the first instance and 12.61 ft^3 saltwater in the second. A 55 gallon drum of water weighs somewehere around 440 lbs, not including the drum. Instead of a compact lead bulb, you'd be slogging around 1 or 2 oil drums under your boat at the end of your retractable foil or strut.

So back to Tom Speer's point of looking for form stability first, and if you're going to use water ballast, have tanks out toward the gunwales, inside the hull, that make efficient use of the hull structure.

Or how about a multihull? 2 or 3 easily lifted hulls & a simple x-beam system. Might take some time to assemble on the beach or trailer, but maybe no more hassle than pumping water around a water ballast system.

[tspeer]

Originally posted by kudu
...Can a retractable keel be designed, similar to a daggerboard then be filled with water via a manual bilge pump for stability?...

Technically, yes. Practically, no. The change in stability by flooding a retractable keel is not a significant contributor to the boat's righting moment.

I second the recommendation for a multihull. Having sailed across the Strait of Juan de Fuca in 20 kt of wind last May in my folding, trailerable F-24 trimaran, I found it eminently capable of handling the conditions. Of course you're looking for something smaller - FYI, Ian Farrier is working hard on the design of an F-22 as an entry level cruising multihull.

Phil Bolger has some small cruising boat designs that depend on form stability for the normal range of heel and buoyancy above the waterline for capsize resistance. I believe he had an article on one in Wooden Boat magazine two or three years ago.

[kudu]

Let me throw this out to you...Can a hull similar to a kayak be designed allowing two persons sitting side by side? Also, incorporate a watertight pilothouse enclosure, sail and ballast for offshore exploration.

[gonzo]

Sure, but you can't use a double blade paddle.

[kudu]

Yeah gonzo that paddle is an issue! Two oars will be in order. :) But could it be designed and built as a light weight "rocket" on the water under sail?

[Guest]

Here is a sketch :
http://www.bleunbrug.com/article.php3?id_article=4

[tspeer]

I think that sketch is very revealing as to what underfloor water ballast is all about. What you really have is a catamaran with a bottom skin connecting the two hulls. The water could be captive or it could be flowing through - either would be the same with respect to the initial stability. If the ballast tanks are not filled, the boat will sit higher in the water, and the waterline will be narrower, perhaps at about the location of the stringers, reducing the form stability.

The same stability could be obtained with less wetted area and wave drag by omitting the center bottom skin and making it a true catamaran. Or by moving the bilge out to provide a wider waterline. So evidently the water ballast in this case is not for the purpose of adding stability. There must be some other motivation.

Probably the biggest contribution the ballast would make is to enlarge the range of positive stability, and make the boat self-righting after a knockdown. The ballast would also add to the inertia of the boat, making it less lively in a seaway.

[Guest]

TSpear, your catamaran analogy is really good. It shows the inefficiency of this type of design. I think there is some benefit, however. If we imagine the boat heeled at say 30 degrees, the CB of the hull does shift to leeward of the CG of all that contained water, which starts to lift above the waterline. So it does provide some stability besides increasing the BWL.

I suspect that some of the impetus for having water ballast is the marketing angle of having a water-ballasted boat, just like Volvo 60's, Open 60's, etc.

Another note on the water ballasted keel idea. If you had a keel which was already hollow, then the air-filled keel would tend to decrease the righting moment of the boat (imagine heeling the boat a bit and having the bouyant force of the keel pushing upward). Filling it with water would neutralize that effect. But to purposely make a high volume keel to fill it with water, will just add wetted surface. After all, you are only replacing the weight of the water that you just displaced. You will gain some small benefits. As someone pointed out earlier, you will add inertia, which would help stabilize motion of the craft (it won't feel so tippy). The lateral surface of the keel will also resist rolling. And finally, if your craft does find itself upside down, that water filled keel sticking up in the air will tend to right it. But these small benefits will come at the expense of speed, manouverability and acceleration. It would be better to take your keel bulb and fill it with air and stick it out to the side, which is really just an outrigger.

[shu]

Sorry, did it again....that last post was me.