Does Stability Decrease with Length?

[jtrieck]

If I have a boat that is 16' and decrease it to 14' without decreasing the beam, have I increased or decreased stability?

[johngsandusky]

Decreased. Bouyancy too.

[Thorne]

I'd guess that much depends on the boat, but it sure seems that for many designs (think barge or garvey) that making it 5-8% shorter while leaving the width the same would make it more stable but slower (shorter waterline). I'll agree with the less bouyancy.

[Kaa]

If I have a boat that is 16' and decrease it to 14' without decreasing the beam, have I increased or decreased stability?

Well, you didn't increase it, that's for certain.

Further than that, and it depends. If your boat is a sailboat and by "stability" you mean the righting moment, then it probably didn't change all that much (keeping in mind, of course, that your displacement changed and so did the sail-carrying capability). On the other hand, if by "stability" you mean "is not likely to capsize in rough water", then it's hard to say anything useful without looking at the boat in question.

Kaa

[boylesboats]

Less bouyancy.
Safe load limit may be reduced as much as 25-30%

[kenjamin]

If you shorten a boat's length without changing its width, you've decreased wetted surface and mass of the boat so it will feel less stable to you assuming you haven't lost a whole bunch of weight since the boat was longer. If you shorten your boat and gain a bunch of weight, the boat will feel very tender and your cholesterol will probably be through the roof.

[ahp]

Stability varies as the first power of length, if the other dimentions remain the same. You will reduce the stability by the factor 14/16, or 7/8.

[P.I. Stazzer-Newt]

If I have a boat that is 16' and decrease it to 14' without decreasing the beam, have I increased or decreased stability?

Depends.

In Roll probably not much change.

In Yaw - a little more twitchy.

And Pitch can be real fun.

[Tom Hunter]

agree with the answers above. It is not going to get more stable if the length to beam ratio changes. Stability is not purely a function of legth to beam ratio.

[johnw]

Waterplane area is an important factor in stability. Reduce that, you've reduced stability unless you've added it back some other way, like increasing ballast.

[Woxbox]

There's no question you lose stability with length. You also gain stability as a hull gets longer, even if all other dimensions are the same.

Think of it this way -- to heel the longer boat 5 degrees, you've got to push more chine down into the water. It's going to take more force, or to turn the idea around, you have more righting moment with the longer hull.

Any chance a short version of this would stay upright?

http://www.battle-fleet.com/pw/his/air-carrier/aircraft-carrier-0.jpg

[dredbob]

Stability varies as the first power of length, if the other dimentions remain the same. You will reduce the stability by the factor 14/16, or 7/8.

AHP has it right. The stability will linearly increase and decrease in the same proportion as a change in a single dimension, length or beam.

Read chapter 13 in Dave Gerr's excellent book _The Nature of Boats_, or do a search for an article on Relativity and Similitude as it relates to changes in the size of a boat.

When you change more than one dimension at the same time, you get exponential changes in qualities.

Bob

[boylesboats]

There's no question you lose stability with length. You also gain stability as a hull gets longer, even if all other dimensions are the same.

Think of it this way -- to heel the longer boat 5 degrees, you've got to push more chine down into the water. It's going to take more force, or to turn the idea around, you have more righting moment with the longer hull.

Any chance a short version of this would stay upright?

http://www.battle-fleet.com/pw/his/air-carrier/aircraft-carrier-0.jpg

Whats up with all those big "mothballs"?

[Tom Lathrop]

This is a fine example of a question that is incomplete and therefore gets incomplete answers. If you decrease the length of a boat, you have changed other factors not included in your question. The main one is its load capacity to the same waterline as before. Johngsandusky caught the clue to this.

Look at it this way. Suppose you decrease the length to make the boat square, or waterline length = beam. When loaded to the same waterline as your 16' boat, it should be obvious that the transverse stability will be much greater in the shorter boat.

That makes the consensus wrong but also correct if the same weight is applied to the rail of both boats. Which case did you really mean to ask?

In a design sense, stability (resistance to heeling) is inversely proportional to length. Longer boats are less stable than shorter ones when all are loaded to the same waterline.

[George Roberts]

"This is a fine example of a question that is incomplete and therefore gets incomplete answers."

We get a lot of those here.

[Cuyahoga Chuck]

If I have a boat that is 16' and decrease it to 14' without decreasing the beam, have I increased or decreased stability?

In marine design every rule seems kind of fuzzy to the inexperienced. Naval architects can do a dance adding something here and taking something off there to keep a design within certain limits.
For the unwashed , like us, the game has to be dumbed down.
Planing hull or displacement hull? Motor, sail or human power?
In paddleboats increases in length are said to aid stability.
In displacement hulls in general longer means better hydrodynamics up to a point. In paddleboats the longer hull has a greater speed potential and will, generally, have more glide per paddle stroke. Short boats require more urge to keep up with their longer bretheren.
Short canoes are all the rage today. There so cute and so easy to store. But they are also so slow, don't go straight and can't accomodate more than a 40 lb. passenger. And you have to sit on the bottom to give the boat good initial stability.
Sorry for the ramble but you didn't supply a lot of hints about what you were building.

[Osborne Russell]

Depends what you mean by stability. Generally, length means mass and mass is inertia, or, resistance to change of direction.

OTOH if a boat is too long relative to its beam, the extra inertia may mean that once it begins to roll, the roll is more difficult to stop, which is instability of a kind.

[Daniel Noyes]

If I have a boat that is 16' and decrease it to 14' without decreasing the beam, have I increased or decreased stability?

decreased stability.

I redrew the Ipswich bay 18 as a 21' just streached the hull, no changes to width, and when I redrew thw rig I gave her 4' more mast 3' more boom and toatal of about 75sq ft. more sail... think of a boat 1 ft long would it be as stable side to side as the 16?...
Dan
http://dansdories.googlepages.com

[Peter Eikenberry]

So far most everyone has been right. Your stability would decrease, but why? Well it is true that length has soemthing to do with it, but more important is the relation ship between the center of gravity, center of buoyancy and metacentric height. To try to simplify this, if you lop a foot off of each end, basically your Longitudinal CG will be the same, your transverse CG will be the same and your vertical CG will be about (give or take a little bit) the same, But your center of buoyancy (CB) will move significantly up ward because the boat weighs less and displaces less water. This decreases the distance between your CG and CB and reduces the metacentric height. Thus lower stability. You initial stability may feel greater but once you heel more than a few degrees your righting moment will be less due to the reduced displaced volume. If you lop the two feet off only one end the CG parameters are going to shift and so is the CB but the result will be the same, less displaced volume, less righting moment, less stability.

But as was said, more info is needed, length, beam, draft hull shape, waterplane area wetted volume and so on.

[Don Z.]

Whats up with all those big "mothballs"?

RAdio Detection And Ranging.

[Woxbox]

If I have a boat that is 16' and decrease it to 14' without decreasing the beam, have I increased or decreased stability?


Jtrieck's original question is pretty straightforward, so there's no need to cloud the issue with theoreticals that may or may not apply in this situation. What's really being asked is "what will the shorter boat feel like when I step into it?" We're talking about a person who may well weigh close to the dry weight of the boat, maybe more, getting into that boat and wondering if it will stay upright.

So where's the center of gravity? Could be way up high. Could be moving about quite a bit. But the question is, how does that shorter boat behave and feel? And it will feel and will indeed be less stable than if it were two feet longer, other things being the same or close to the same. It's possible, for example, that the 16' version boat will let Jtrieck put all his weight on the gunnel and not take on water, whereas the 14' boat will start to flood. The reason is simple, you have to displace more water to get the 16-footer's rail down that far.

[jtrieck]

Thanks for both the theoretical and practical answers. I appreciate those that explained the forces are work - it helps me visualize how the changes in dimensions works on the characteristics of the hull. I also appreciate the practical answers - what I essentially wanted to know (but didn't ask!?) was how can I make a boat less massive and yet more stable.

If I understand y'all - in general, for small craft, if you decrease the physical size of the boat (length, weight, width), then you have decreased stability (being the ability to stand and move around the boat without losing your balance)?

[kenjamin]

I also appreciate the practical answers - what I essentially wanted to know (but didn't ask!?) was how can I make a boat less massive and yet more stable.

If I understand y'all - in general, for small craft, if you decrease the physical size of the boat (length, weight, width), then you have decreased stability (being the ability to stand and move around the boat without losing your balance)?

That's pretty much it as a practical answer. The other thing you have to look at with sailboats is the shape of the bottom while it is at rest. Some sailboats, like a "V" bottom for instance, may feel a little unstable at rest but may get much more stable as it heels over while sailing. In that instance the hull is said to "harden up" when it heels over.

If you're really interested in decreasing the mass of the boat while maintaining stability, take a look at catamarans. I built a 450 lbs., 20 ft. x 6' 6" mono-hull and I think I can build a 350 lbs., 16 ft. x 8 ft. catamaran that is faster sailing and more stable most of the time.

[JimD]

...- what I essentially wanted to know (but didn't ask!?) was how can I make a boat less massive and yet more stable...

Presumably you don't really mean 'massive'. Mass and volume are quite different.

[Peter Eikenberry]

We are having a problem understanding what you are attempting to do. If you have a boat and do not want to make any severe changes to the boat such as adding weights, increasing the length, making it wider, some ways to increase the stability are

1. move as much weight in the boat as low as you can. That lowers the center of gravity while not shifting the center of buoyancy. (That's what sailboat keels do) Thsi is why they tell you to sit down in a small boat, or why you sit in the bottom of a canoe or kayak. It lowers the CG making the boat more stable.

2. Move weight that is out board (that is closer to the sides of the boat), toward the centerline. Reduces the moment of inertia.

3. Add an outrigger, or a flotation collar. This will not change the initial stability, but as the boat heels the outrigger or flotation collar will add buoyancy, and righting moment, farther out from the centerline.

[jtrieck]

Thanks for the clarifications. I am not really trying to do anything to a particular boat, instead, I am trying to get an conceptual idea of what generally happens when the various design levers are pulled.

[Paul Pless]

Thanks for the clarifications. I am not really trying to do anything to a particular boat, instead, I am trying to get an conceptual idea of what generally happens when the various design levers are pulled.
You're all over the board with you're questions. Nothing wrong with that, but there's an easier faster way to the understanding you want.

http://books.google.com/images/cleardot.gifhttp://books.google.com/images/cleardot.gif
Dave Gerr's book, The Nature of Boats.

http://boatdesign.net/assets/images/007024233X.jpg

Its an excellent reference explaining a wide range of boat design and construction characteristics in layman's terms.


http://www.amazon.com/gp/reader/007024233X/ref=sib_dp_pt/104-5871684-4011921#reader-link

[Paul Pless]

Here's another decent book, not as detailed as Gerr's but still good with basics.

Ted Brewer's Understanding Boat Design

http://images.amazon.com/images/P/0070076944.01.__PIt.arrow,TopLeft,-1,-17_SCTHUMBZZZ_.jpg

[Paul Pless]

Read anything you can by Bolger. You may not like his aesthetics, but there's no arguing with his engineering acumen. When he writes about boats that he's designed he's very clear about how design decisions affect the performance of his boats. Very enlightening stuff.

[jtrieck]

Thanks for the suggestions

[pila]

Dave Gerr's book is a fun read for me, even thought it's way above my head for the most part.:)

[PatCassidy]

The answer is..... maybe. Your question asks what woul happen if you change one dimension of you boat. Stability is a function of the opposing forces of gravity and buoyancy - the centers of which are determined in three dimensions. Your question involves changing just one dimension.

You lop off two feet of "boat": what has happened to your center of gravity? Same question regarding center of buoyancy. You change one or the other and you have changed your righting arm.

Here is an example of the center of gravity being raised and offset from the centerline of the vessel:

http://i7.photobucket.com/albums/y267/Pat558/Stellamare1.jpg

The weight and height of the generator being lifted would be offset by shifting ballast outboard ( the lower the better). As the weight of the generator being lifted approaches the centerline, the offsetting ballast also has to be reduced.

Otherwise.....


http://i7.photobucket.com/albums/y267/Pat558/Stellamare2.jpg


Three sailors died here.