stability: length vs. width

[river runner]

What just happened?
Increasing the LWL and the beam both increase stability, but what affects stability more? Does increasing LWL 10% incrase stability the same amount as increaing beam 10%?

[Easy Rider]

Too many variables but something to keep in mind is that fullness in the ends have a profound effect on stability what ever B/L ratio. Full ends are probably more beneficial to long narrow boats generally speaking.

[DCockey]

Originally Posted by river runner

What just happened?
Increasing the LWL and the beam both increase stability, but what affects stability more? Does increasing LWL 10% incrase stability the same amount as increaing beam 10%?

Assuming the hull form stays the same and the changes in length and/or beam are made by stretching, beam has the greater effect.

[tom28571]

Among other things, it depends on the L/B ratio you start with. Such a nebulous question cannot be answered. Think about your questions a bit before sending them out.

[river runner]

Hey Tom, I'm a baker in a grocery store, not an NA or ME. You might cut me some slack.
I've been working on a river dory design that is, currently, about 7 feet wide and a waterline length of 17 feet. I was trying to find out if I wanted to change these dimensions and the info I asked for would help make this decision.

[tom28571]

Originally Posted by river runner

Hey Tom, I'm a baker in a grocery store, not an NA or ME. You might cut me some slack.
I've been working on a river dory design that is, currently, about 7 feet wide and a waterline length of 17 feet. I was trying to find out if I wanted to change these dimensions and the info I asked for would help make this decision.

You might get some more slack if you asked more complete questions. Now you fill in the beam and length you are starting with. Those are absolutely minimum starting points. If you think I am being harsh, you should have asked that original question of my mentor. Now, is that the waterline beam or the max beam?

[river runner]

That would be maximum beam, Tom. Not sure what the waterline beam would be at this point. I'm attaching a rendering of the design, if that helps. I don't need an exact answer, just enought information to point me in one direction or another or tell me I'm on the right track.

[Ad Hoc]

Originally Posted by river runner

What just happened?
Increasing the LWL and the beam both increase stability, but what affects stability more? Does increasing LWL 10% incrase stability the same amount as increaing beam 10%?

One assumes you’re talking about Transverse stability?

The GZ curve, which defines the amount of stability (statical) that you have, is governed by the GM of the boat. So what affects the GM? GM is found by obtaining the BM of the vessel which si found from

BM = I/V

I is the inertia of the waterplane area and V = volume of the vessel.

I = LB^3/12, thus increasing the beam, increases the I, which increases BM, which, assuming the KG remains the same, increases the GM and thus, increases the GZ, the righting lever.

Simply:- dGM = 2BM.(dB/B)

dGM = change in GM
dB = change in beam

The effects are shown here:



Curve A is orginal hull. Curve B, is an increase in beam. Curve C, is original hull, but increase in freeboard.

If you’re talking about longitudinal stability (or trim), the formulas are the same, except the axis of rotation are different. BUT, the stability longitudinally is always greater than transverse. The BM(L) of a vessel is very close to the GM(L), and can be used as a rough approximation of stability. Thus we always refer to stability in the transverse sense, very rarely longitudinally.

Thus, since transverse stability is more sensitive to minor changes than longitudinal, increasing the beam has the greater effect.

[river runner]

Thanks, guys. I had always assumed that width had the greater affect, but I'd recently read something that put that into question. I wanted to be sure, in case I build another boat of my own design.
There seemed to be some thought that this conclusion was dependent on starting dimensions, but realistically, would a sane boat ever have starting dimensions where increasing length would have a greater effect on stability than increasing width?

[river runner]

I'm looking at AdHoc's graph and it looks like it is saying that to prevent capsize, you would be better off increasing freeboard than increasing width. True?

[Ad Hoc]

Originally Posted by river runner

I'm looking at AdHoc's graph and it looks like it is saying that to prevent capsize, you would be better off increasing freeboard than increasing width. True?

It really depends upon the SOR of your boat and where you intend to use it, that shall dictate your design.

[tom28571]

I'm a bit late in getting back river runner but no need to confuse with more math or suppositions. Ad hoc's post gives the groundwork. All of the principle dimensions have an effect on stability in one plane or another although we generally think of lateral stability as most important.

[DCockey]

Originally Posted by river runner

I'm looking at AdHoc's graph and it looks like it is saying that to prevent capsize, you would be better off increasing freeboard than increasing width. True?

It depends on the amount freeboard is increased versus the amount beam is increased, which in turn depends on the requirements and constraints.

[gonzo]

More freeboard will give you higher angle of positive stability (when the boat can recover from heeling over) and a higher maximum angle of flooding ( the boat can heel over more without flooding. More beam will give you more initial stability.

[Ad Hoc]

riverrunner

Once you have looked at your SOR, as i noted above, you need to be careful. Since, increasing the freeboard, does as Gonzo and the graph shows. This assumes all things equal. But rarely is that the case in design.

The downside to increasing the freeboard is the obvious one, always over looked too. If the majority of your heavy equipment is higher up, at or above main deck, then all you're doing is moving the KG up, which would lower the GZ, righting arm. So whilst curve C looks like the ticket, you can shoot yourself in the foot simply because the rise in overall KG negates the benefits shown by curve C.

Thus best investigate, via your SOR what you really want, rather than assumptions. The graphs i posted are indicative, as information. But you really need to establish the GZ curve for your design. One assumes you are 'designing' using electronic means, if so, it would be simple to dump the lines into a hydrostatic stability suit of programs for a quick check. Then see what GZ you have and what measure of dynamic stability and does it pass the recommended stability criteria for your design and area of operation.

The "playing about" bit, is the design!