Water Ballast vs. ???????

I've been having discussions in a different thread about my design for a "25' trailer sailer" that has led to disscussions concerning water ballast. Most input concerning water ballast has been less than positive. I'm just curious if there are any proponents water ballast. My intent is to minimized the total use of fixed ballast in order to make trailering less of an ordeal. I'm starting this thread with a title more appropriate to the discussion.

 

There is NOTHING inherently wrong with waterballasted boats. As long as you don't expect them to be oceangoing! If you go for a solution where almost all the ballast is water (just a centerboard) you will not have good AWS, and a high Center of gravity. This increases the chance of the boa going "turtle". But if you use water in junction with a weighted lifting keel that is heavy enough to make the boat selfrighting in case of a knockdown, it would work fine. But that is fine for a daycruiser/weekender that is only intended to use in relatively sheltered waters.

OTOH if you are talking about using waterballast as a way yo increase righting moment and shifting it's position from tack to tack, that's nother story.

 

The best boat in world uses it. See the "secrets in yacht design thread". tic

But seriously, I reckon shifting water ballast is a good idea when used in conjunction with a weighted keel/centerboard etc. Personally I would be a bit scared to go into rough weather without a decent lump of lead down there somewhere. But a purely water ballasted boat can easily be made light enough to float if it capsizes or gets holed. I guess every way has its own benefits.

 

my opinion is that it's a decent source of ballast IF the boat is big enough to contain it well down in it's bilge and or where there is enough area to contain enough volume to make it effective. from my readings as a rookie here, i have surmised a water system, in order to be truely effective, should be shiftable athwartships, i think, primarily because it's wieght to volume ratio is inadequite in a smaller displacement vessal, especially one of shoal draft as in your plans. the requisite shiftability is what makes the system soewhat costly and maintenance intensive in a boat that should be basically low maintenance and easy sailing, simply put, it seems to me that the system needed to make it effective is just too elaborate to make worth doing in a small boat, let alone the fact that it requires so much more space and expense than other materials. i have been looking into water ballast systems in depth, because the boat i will be building is a small cruiser, who's plan offers the option of water ballast. in that i will be trailering my boat everytime it's sails, it is a desirable option, for sure. still, i can't see it being as serviceable as more conventional ballast materials. at worst, in some cases, depending on the amounnt of ballast needed and design of the hull, with lead ballast, it might need to be unshipped for trailering which leaves the possibility of it knocking around, but if done carefully, it can be made to stay in place quite well. i am firm believer in K.I.S.S., especially when it comes to an activity that is supposed to be relaxing and enjoyable.

 

Dear Lear:

I have some design experience with water ballast. I had a preliminary design that was to be self rightinting using almost exclusively water ballast. It did work. On paper. Doing a 180deg. righting curve, the design was found to be stable (with the weight of the solid wood mast counted, but not its boyancy) to 140deg.

There was one rub, however. In order to do this, the boat had to be very narrow and high sided. The ultimate stability was good (better than a lot of 'lead boats') but its initial stability sucked. Its preliminary rig had to be cut down from 170sft (even that being modest for a 3,000lb boat) to 140sft for the poor thing to stand up to enough wind to even move it.
Now it was under rigged as well as having much of its carrying capacity and its internal volume taken up. It required 1400lbs of water!

In order to save the design, I had to compromise. I first put 300lbs of concrete in its bilge, then decided to just chuck the water for the volume it would yield. It was to be replaced with sand bags (which are almost twice as dense) which, presumeably, can be transfered, all 600lbs, to the tow vehicle. All this was to be augmented with 320lbs of drinking water that was to be stored in 2 liter bottles that were to be refilled with sea water as they were consumed. That, along with the cut down rig, gave it respectable initial stability.

The things to remember with water ballast (at least from my experience), iare:
1.) that the best hull form is either a flatish bottom or a hollow fin, or fins, to hold the water. The water gives you ultimate stability, but next to zero initial (sail carrying) stability. So keeping the ballast tank as wide and shallow as possible, or having it augment denser ballast deep in a fin, should be a primary design objective.
2.) that using it for primary self righting stability will ALWAYS mean a shorter , less powerful rig for your boats displacement.
3.) that even with these statements being true, it may be still worth doing. Like for a motor sailor, for instance.
4.) that it (theoreticly) puts less strain on the hull. The weight is less dense, but it is also very well distributed for that reason. This can mean savings in bottom scantlings. Don't believe me? Drop a water balloon from mast head height into the water and see if it bursts. Most keel boats need elaborate re enforcement to their bottoms to distribute the concentrated loads of not just the keel but the surounding (that has no ballast weight against it) water pressure as well.

For your particular purpose, let me suggest the following:
1.) that you go with a long narrow centerboard that is about 1/3rd the boats WL length. (has a shallow case and, when extended, gives geat righting moment for both initial and ultimate stability)
2.) that you arrange a locking system that is strong enough to hold it down if the boat flips, but weak enough so it will shear if you ever hit bottom.
3.) That this 'swing keel' make up about 1/3rd the total ballast.
4.) that you put two huge water ballast tanks, one on either side, to 'finish the job'.
5.) that the widest section of your hull (deck, or cabin, if the cabin is water tight and strong enough, to keelson) be at least 3/5ths as deep as it is wide.
High, narrow cabin trunks, if they are strong enough, really help. Even though they do look dorky.
6.) that the rig be as low as possible for its intended area.

I hope I have answered some of your questions.

Bob

 

A fun aspect of water ballast missed by these serious designers, is watching the faces of powerboaters at the ramp when you tell them you're emptying your holding tank. A couple of Baby Ruth bars stuck in the tank through the vent, ala Caddy Shack, would really get 'em going! Nasty stuff can grow in the tank, so I wouldn't pick one up and eat it for maximum effect.