flatt bottom, where to put balast

[mizzenman]

I’m just speculating here, about a project I’d like to start in about 2 years time…

On most inside ballast designs I have seen, the ballast is close to the centerline of the boat. On a flat bottomed hull I would rather putt the ballast as close to the shines as possible. This would give the boat a greater turning momentum and make it less likely to be knocked down in a gust.

What do are your thoughts?

[Tristan]

Physics and math have always been a problem for me, but it seems to me that whether you have the ballast at the chines or at the centerline will make no difference, as the center of the total ballast weight will still be at the centerline.

[Thorne]

General knowledge states "Keep the weight out of the ends" -- and I imagine that this would apply to the sides as well.

You don't give us much info on the design, but if we can assume it is a sailboat, you'd lose some of the righting 'force' of the overall ballast when heeled to starboard, as half of your weight would be to starboard already -- sorta like water in the bilges.

If you are concerned about it taking up space in the bottom of the boat, why not consider outside ballast?

[Uncle Duke]

Ballast at the chines will end up a rocking boat... It'll make it too tender, it'll roll over easily
I'm confused by this - my understanding was actually the reverse - that the further the weights are spread, the greater the resistance to movement.
Simple test anyone can do: get a broomstick and a couple of 5 lb. weights.
1: Put both weights at the center of the broomstick. Grab it with your hands about shoulder width apart (outside the weights, obviously). Rotate it 20-30 degrees back and forth.
2: Put one weight at each end of the broomstick. Repeat the grab and rotate.
You'll find that it is much harder to rotate it back and forth in the second scenario, even though the total weight is exactly the same.
In the same way, if you find that your boat is too lively over waves, move your anchor chain farther forward and move something correspondingly heavy farther aft. That will slow down the action.

Now - having said that, I might keep the weights at the center point, depending on the beam of the boat. A simplistic view is that the righting moment is the product between the weight vector(s) and the center of bouyancy. If you're going to spread the ballast then the ballast on the 'down' side of the heel still wants to add to the heel, but less and less as the center of bouyance moves closer to it. The ballast at the 'high' side wants to right the boat, more and more as the heel increases and the center of bouyancy moves farther away. The questions would be whether that righting effort (the 'more' for half the ballast - the 'less' for the other half - the vectors, of course, adding up to centerline more or less) exerts more force than 'all' the ballast at the centerline distance from the center of bouyancy.

edited to add: that "more" is really hampered by being 'flat bottomed' - that's a big part of the reason that boats which really need righting moment keep it low, low, low - to the point of adding a ballast keel. Standard mental experiment is to think of a fin keel hanging down - righting moment actually increases as the angle of heel increases. Now thing of one pointing up.... righting moment decreases. I think.

[Christopher Locke]

I'm not a physic expert either, and so to the degree I'm wrong or muddy the waters here, I apologize.

My initial take is that this seems to be a case of initial stability vs ultimate stability. A simplistic explanation of the difference between the two (again per my limited understanding) is that initial stability is the amount of force it takes to heel the boat; ultimate stability is how far the boat can heel before it capsizes. Flat bottom boats generally have more initial stability than round or V bottomed boats, meaning that it takes more force (eg wind) to heel them to begin with, but they have less ultimate stability, meaning the angle of heel they can bear before the balance shifts and they capsize is less. Which you want depends on the application of the boat. Flat bottomed boats are perfect for things like fishing where you have people standing and want a lot of initial stability, but are unlikely to experience 45 degrees of heel and so don't care about the ability to recover from that. A seagoing boat that expects to routinely roll through 90 degrees of heel wants a lot of ultimate stability so it can recover, but doesn't need to stay rock solid most of the time.

It seems to me that putting the ballast on the chines causes a flat bottomed boat to have even more initial stability than it otherwise would, at the cost of less ultimate stability. If that is what you want, that is fine. I think another effect is that not only would the boat capsize at an lower angle of heeling, but would also be harder to right if it turned turtle (completely upside down), because again it takes more force to heel the boat back up.

Again, to the degree I'm wrong, let me know. Always interested to learn more from others who are better informed.

Chris

[Uncle Duke]

It occurs to me that I should have made a clearer distinction between 3 things: (1) 'tenderness' - how much/little force is needed to induce some angle of heel, (2) 'quickness' - how fast/slowly that heel is manifested and (3) 'righting moment' - which is the calculated vector-sum which defines tenderness.
Spreading the weight reduces quickness, but probably lowers righting moment - at least until the angle of heel is such that the center of bouyancy is directly above the ballast at one chine. At that point the righting moment is approx. the same as having all the ballast in the center. Beyond that point (which you probably never want to get to!) the righting moment would be slightly more than having all the ballast on centerline, but not much.
On the other hand, I could be mis-stating - I'm at work and don't have the opportunity to draw out the actual forces. So I'm open for education on this also.

edited to add: at 'level rest', initial stability is much more dependent on 'form stability' than on ballast stability (righting moment)

[Bruce Hooke]

It seems to me that if you spread the ballast out evenly on both sides then the moment arms cancel out, so the stability is not changed. What does change is the moment of inertia, or how hard it is to get the boat to change its angle of heal. Yes, this means the boat is slower to heal in a gust, but it also means it is slower to recover afterwards. It also means that as a wave comes up and tries to lift one side of the boat, the boat will be slower to lift to the wave, which is not a good thing because it makes it more likely that the wave will come over the side. Then again, flat bottom boats may be overly inclined to lift quickly to waves due to their high initial stability, so toning down on this effect might not be a bad idea, but I would not go too far. The best approach might be to make it easy to reposition the ballast so that you can try out different configurations.

[mcdenny]

As long as the ballast at the chines is just as low as if it were in the centerline (truly flat bottom) the boats center of gravity is the same either way, hence no difference in righting moment, no matter what angle of heel.

The weights on a brooomstick example above illustrates the difference in polar moment of inertia. Rotating things want to keep rotating; the further from centerline the cg is the more rotational inertia they have. Ballast at the chines would increase the inertia in the roll axis. In theory the boat would be more resistant to start rolling but also less resistant to stop rolling. In practice I bet you'd never notice the difference in a small boat.

[mizzenman]

The stability would not be affected sins the center of mass does not change location.
However, as Uncle Duke notes, the momentum of the hull will change, as the momentum is dependent of the distance of the mass to the axis of the movement.
An increased momentum would mean that although the ultimate stability is unchanged, the hull would resist quick rotational forces to a higher degree. By “quick rotational forces” I mean for example a sudden gust of wind.

I would be very happy to hear from someone that has practical experience in this.

[brian.cunningham]

How about a flat plate mounted to the bottom on the outside like a skid plate?

[mizzenman]

mcdenny and Bruce Hooke, sorry I cross posted you. The answer you gave is just what I was wondering about. Thanks!
Of course moment of inertia is the right word, not momentum.

The design:
Loa 20 feet
Beam 4-5 feet
Sail about 140 sqft
Dplm about 1000lbs where ballast is 50% of the weight (inside or outside, don't know yet).
Half decked. Whith Biiig flotation chambers.

[Thorne]

Brian - That's what I was suggesting with outside ballast -- you see it sometimes as a central plate with a slot for the centerboard, other times as two plates on either side of the CB slot or keel.

Since the boat seems to not have either CB or keel, a long external central plate / ballast mass should work well and keep the righting motion where it needs to be. Christopher L's perspective seems correct to me.. but I'm no NA.

;0 )

Another benefit of external ballast is that it provides a bit of a skid-plate or bottom protection -- which I suspect you will want as the boat seems designed to operate in thin water.

[mizzenman]

It will have a CB

[Thorne]

Ah -- I thought it was either a catboat or a sharpie.

Why not put at least some of the ballast in a swing keel (heavily-weighted centerboard) design? Some of the glass boats out here have 600lb + swing keels, and use small trailer winches to raise and lower the cable pennon. My San Juan 21 had this setup and it worked really well for an otherwise unballasted boat. Just avoid smacking your head with the winch handle when you release the swing keel in the water -- don't ask me how I know this...

;0 )

That puts most of the ballast where it will do the most good under sail.

[mizzenman]

Yes a swing keel would be most efficient. But I’m afraid of making the boat to complicated. A chunk of lead bolted to the hull and a CB I can raise with my hands will never brake or need much maintenance.

Have you seen the keel design on these boats?
http://www.bluelightning.co.uk/

[Jay Greer]

On L. Francis Herreshoff's "Meadow Lark and Golden Ball", boats that are modified lee board sharpies he laid lead ballast blocks on either side of the main keel stringer inside the boats.
JG

[Paul Fitzgerald]

I built a Vandestadt mini tonner twenty five years ago. The plans also included a trailable version.
The ballast was under the seats, about a foot either side of the midline. The boat bottom was flat, so they were at the same depth as the keel but further outboard.
Seemed like a good idea, and quite a few of them were made IIRC.
Also seemed like a good idea for water ballasted boats, put the tanks under the seats rather than under the floor.

[Paul Fitzgerald]

On L. Francis Herreshoff's "Meadow Lark and Golden Ball", boats that are modified lee board sharpies he laid lead ballast blocks on either side of the main keel stringer inside the boats.
JG

I thought Meadowlark also had an external iron shoe along the keel.

[merlinron]

let me see if i can explain this clearly...this is how i see it.
for the best initial and overall stability, you would want the most wieght acting against the force by moving it the farthest. when it is at the centerline, all of the weight resists heel the instant the center of gravity starts moving away from the centerline of the hull. with the ballast at the chines the ballast closest to the c/g doesn't start to resist heel untill the c/g moves far enough to have both ballasts on the same side of the c/g. in graphic explination.... two people of equal weight on opposite gunwhales, the boat heals and the leeward person moves to the center of the boat, what does the boat do?....... that is the same most ballast being on the same side of the c/g.

[donald branscom]

I’m just speculating here, about a project I’d like to start in about 2 years time…

On most inside ballast designs I have seen, the ballast is close to the centerline of the boat. On a flat bottomed hull I would rather putt the ballast as close to the shines as possible. This would give the boat a greater turning momentum and make it less likely to be knocked down in a gust.

What do are your thoughts?

Balast - Ballast
Shines - Chines
A flat bottomed boat still has a part of the hull that is lower, that is usually in the middle. Ballast can be put in the skegs . Also a centerboard can have more weight added.

[Jay Greer]

I thought Meadowlark also had an external iron shoe along the keel.

Actually "Golden Ball" has three four foot interlocking blocks of lead bolted along the bottom of the keel. lnternal ballast is also specified but not as to the material.
JG

[Mark Parratt]

Hi everyone on the WB forum, having visited the forum quite often I thought it about time to put in my two bobs worth. The Bolger AS29 had ballast boxes on either side of the boat about midways,which my friend who built it found quite satisfactory.
I in turn put ballast boxes in my slightly modified Bolger 25' Jesse Cooper lee boarder and found them OK but I put more ballast in the centre of the yacht after a little mishap with the Jesse (called'Dugong') when she failed to right after a knockdown.
But everything's hunkydory with the Dugong now-at present on anchor in the Great Sandy Straits (otherwise known as the Great Sandfly Straits) in Queensland

[Woxbox]

Bolger's Martha Jane, roughly the overall weight and size of what you have in mind, also places the ballast (water) in two boxes under the seats, port and starboard.
But the center of gravity is the same, no question about it, if you take ballast in the middle of a boat, cut it down the center, and spread the two halves out on a flat bottom. And thus the righting arm of that ballast is exactly the same as when it was one piece.
As explained above, the rotational moment of intertia of the boat does, however, change when you spread out the weight. Flat bottom boats have a reputation for being too quick in a seaway and making people seasick, so an increased moment (laterally) of intertia on a flat bottomed boat is probably a good thing. You still want to keep the weight out of the ends of the boat, for sure. And it's not just the ballast in question, here, it's every last thing you stow aboard. Ballast, after all, does not know that it's ballast. It's just more mass in the mix.

[ahp]

Keep the ballast concentrated as much as structural integrety permits. By spreading out the mass, you increase the moment of inertia AND you reduce the damping ratio. Less damping ratio menas that rolling and pitching damps lomger. Your boat goes through more cycles of these before settling down.

[Thorne]

Mark raises another reason to keep the ballast along the centerline -- righting after a knockdown. Ballast along the chines would make the boat harder to right than ballast along the centerline/CB slot.

[boylesboats]

Mark raises another reason to keep the ballast along the centerline -- righting after a knockdown. Ballast along the chines would make the boat harder to right than ballast along the centerline/CB slot.

Thorne,

Go tell 'em that...... I tried to explain way.. I just gave up....

[PVanderwaart]

Ballast along the chines will not affect the righting moment. It will only increase the rotational moment of inertia, i.e it will slow the reaction to a gust. Good idea.

[ahp]

If I ever build that sharpie, besides putting as much lead into the CB as I can I will buy some sheet lead at Home Depot and secure it to the bottom, on the inside of course. The old working sharpies used cobble stones, but I don't like that idea.

[boylesboats]

If I ever build that sharpie, besides putting as much lead into the CB as I can I will buy some sheet lead at Home Depot and secure it to the bottom, on the inside of course. The old working sharpies used cobble stones, but I don't like that idea.

Cobble stone, eh? If boat start to sink, you surely start bailing rocks, before water... Just kidding... Some of those large ships have a large cannonballs piled in the hold as ballast

[Bruce Hooke]

I'd be cautious about adding a lot of weight to the centerboard. You end up with a board that is a lot harder to raise and lower, and also a board that puts a lot more stress on the centeboard trunk. "Drop keels" (i.e., heavily weighted centerboards) are best left to the high-performance crowd with highly engineered boats. The beauty of a sharpie is its simplicity. Adding centerboard winches, highly loaded centerboards, and so on is the wrong way to go in my opinion.

[Bruce Hooke]

Thorne,

Go tell 'em that...... I tried to explain way.. I just gave up....

Actually, assuming you are talking about righting a boat that has swamped rather than just recovering from a gust, the greater moment of inertia will hardly matter. When righting a boat that is swamped, everything is moving pretty slowly. So, the moment of inertia has much less of an impact. Try this experiment:

1. Hold your arms out horizontal to the ground and spin around slowly. As you are turning bring your arms into your side.

2. Now try the same thing spinning fast.

In case 1 I don't think you'll notice any difference in how much work it is to spin. In case 2, if your balance is good enough to spin fairly fast (do this where you've got some clear space), you'll see a big difference in how much effort it takes to spin (you'll probably speed up a good bit as you bring your arms in).

While recovering from a gust is in theory an issue, the same issue would apply to a keel boat with a deep keel. Also, there seem to be plenty of successful flat-bottomed boats with the balast out near the chines. In the end I'm not sure it really matters that much. Where you put your weight while sailing a small boat probably matters a lot more...

[Woxbox]

Supposedly some fishermen in days gone by preferred round stones for ballast -- if the boat got knocked over, the stones would roll right out, rendering the rest of the wooden craft unsinkable. This would not be in a flat-bottomed hull, of course.
So what's the modern analogy? A good way to recycle old bowling balls? I just read or heard someplace that the 10 and 12 pounders float, and the 14-16 pounders sink. Nah, might as well stick with water ballast.

[Lars Silen]

In a V-bottomed boat internal ballast should be located as close to the center line as possible simply because the lower in the boat it is located the better righting moment. I think everybody can accept this.
In a complete flat bottomed boat (example my Mystic sharpie) the location should be close to the center in the long ship direction. This ensures again that we get the ballast as low as possible. The location sideways doesn't matter for static stability. On a flat bottomed boat moving ballast out to the sides will improve dynamic stability through increased moment of inertia. The increased moment of inertia lowers the roll frequency which is a good thing on a flat bottomed boat where roll otherwise may be very "snappy". In my own boat the main ballast (led pigs) is located under both center bunks well out to the sides. The important thing to remember regarding internal ballast is that it should be very well fixed. Assume at least that the boat any time may lie on its side at 90 ... 100 deg. No ballast must move or get loose in this case. A 30 kg (60 pound) led pig falling 3 metes (8feet) will probably go through the side planking causing the boat to sink very fast. Loose ballast could of course kill a person lying in the bunk on the lee side.
The speculations that the boat with ballast spread out sideways in a _flat_bottomed_ boat would be more difficult to right is simply wrong. The center of mass is still at the center. From a static point of view it thus doesn't matter.
Ballast should from a dynamic point of view be close to the center length wise to allow the boat to turn fast during tacking. We want to minimize the moment of inertia in this direction.
Using plates for ballast is ok. I think steel boats often have significantly thicker plates at the bottom than at the top and sides simply to serve as ballast. Spread out ballast of this type could give the boat a fairly strong roll. Of course puncture resistance will also improve.
(Physicist)

[Thorne]

I still don't understand how it is OK to have weight at the sides of a flat bottomed boat, but not OK to have it at the ends of the same hull.

Isn't a boat's hull really just a two-axis form, and when not in forward motion, can move in both directions and combinations of same.

In other words, imagine a fairly short, beamy flat-hulled boat sitting 'still' in the water. Waves, winds and combinations of same make the hull move in various directions - sometimes pitching, sometimes yawing, sometimes rolling -- and occasionally a combination of the three.

So why is it good to have weight out at the 'ends' of the sides, but not OK to have weight out at the ends of the bow and transom? The same forces apply to each, modified by the hull shape and the fact that one dimension is twice the other dimension?

http://www.pcmag.com/images/pcm_spacer.gif From Techweb.com -
pitch-yaw-roll

Movements of an object that are measured as angles. Pitch is up and down like a box lid. Yaw is left and right like a door on hinges, and roll is rotation.

[neilm]

I think Mizzenman's theory is correct but in reality I doubt you tell the difference because the distance is too short. This reminds me of a tight rope walker. The longer his balancing pole the easier it is to balance. If he had a pole with all the weight in the center it would not balance as well. Let us know how it goes.

Thorne, the only reason I can think of for not having weight on the bow and stern is to keep the weight as low as possible. The keel is the lowest point. That, and maybe it's too hard to turn quickly.

[Woxbox]

Well, as stated when this thread began, in general principal you want the mass of a boat -- ballast and all -- as close to the center and as low as possible.

However -- in lighter flat-bottomed boats, this sometimes results in a really snappy roll that makes things uncomfortable to the point that some may vomit. The easy fix is, if there is ballast, to spread it out athwartships, thus increasing the lateral rotational inertia and calming the crew's stomachs.

You don't want weight in the ends, especially up in the bow, because it would tend to bury rather than lift.

The effects are very real. Next time you're out in a chop in a light boat with two or more aboard, have people move around -- keep the overall balance the same, but with, say you and your buddy first centered, and then in opposite ends of the boat. The effect is obvious, you may well start taking water over the bow with crew in the ends of the boat, but be just fine with the weight centered.