Rough seas stability: wide or narrow beam?

Hello,

What effect beam size has on a boat stability in medium waves 2-4 ft?
Like for a power runabout (<20') is it better to have wider or narrower beam?

Thank you.

 

If you look at several runabout designs, for a 20' one you may go from 5.5 to 8+ feet beam. Generally speaking oldies had less beam and moderns have more.
With 2-4' seas from the beam (and having enough freeboard, of course), I think there is a low risk of capsizing with whatever figure between those (Higher with the lower), but the higher the beam the less comfortable the motion.
You'll have to choose...

Also take into account that behaviour in waves is not only a matter of beam.

 

As the learned gentleman says it just ain't about beam but having said that generally the wider the beam the safer the ride (not necessarily the more comfortable) the narrower the beam the faster and more unstable the boat! This does not mean that you'll be nice and stable in a square 'biscuit tin', you might live throught the storm but your guts will soon wish they'd died (and you'll be wearing your breakfast)!

Other things do have a say in the equation, shape of bottom etc, and of course the safest most stable way is to stay ashore (but that too has it's risks)!

 

Nowadays I think it's safer to be at sea! But as for the topic at hand, I believe somewhere in the middle is the best place to be. Too much beam will pound your fillings out no matter how deep the V, too narrow gives a whole new meaning to rock and roll. For the 18 to 20' range, in my humble opinion, I believe 6 to 7 1/2 feet is a good beam. Lots of boats in this range have 8 1/2' beam, which is nice for interior space, but boy will they beat you up in a chop. I've run an 18' skiff with a 6 1/2' beam and a flat bottom that rode nicer than wider v hulls (with less power to boot, nothing to crow at with $2.50 a gallon gas!)

 

It's not really relevant but I once was in a 50m long submarine with a gale blowing from the side. We wern't allowed to dive. Hull is long and skinny and completely round except for the tower which turns into a nice breakwater. The sub was rolling about 45 degrees to each side with everything that wasn't duct taped to some pipe flying around inside. With the hatches closed and navigation through the periscope there wasn't any real danger. You just found out very quickly if you had problems with seasickness or not.

A further point is the centre of gravity. If it's far above the water line the boat becomes more unstable.

 

Just look at a hunk of board and a broom handle in a bay.
The broom handle simply goes up and down , a nice EZ ride.

The wide board will follow the surface undulations , heaving pitching & rolling to keep up with the waves. Beam sucks when the waves get BIG.

On multihulls the ride can get severe as the boat is on top of an unstable wave system , and the boat follows the wave surface.

The long skinney boat (a log ) just rides with a bit of roll.
How much roll is determined from how far from a log the boat is.

The current crop of 3 story plastic motorboats with an oxygen tent on top would be a sad companion in any sea state on either hullform.

FAST FRED

 

Can see which way your going on vessel shape Fred, nice Victorian style easily driven steamboat! Tis OK till you gets a beam sea then the log tends to roll over, as our submariner friend sez (never could get the hang of this submarine stuff, it's the inabillity to wash thing I think, the smell of the deisel don't help either - thought provoking stuff is diesel in a storm)!!

 

For your wee power runabout (<20') it's mostly a matter of wave height. True 4 footers will overwhelm a boat this size. Thus you will be going very slow, and the more beam the better. But in the 2 footers you can scoot along, then less beam will be more comfortable. As Craig noted, lots of these runabouts have 8' or more beam. But that's mostly to accommodate a load of skiers and their beer. Really has nothing to do with seakeeping.

Kelly Cook

 

I beg to differ with that one. Case in point, my Dad and I used to go 20 miles offshore in a Wellcraft V20, 8 1/2' beam (of course) in 4 to 6 foot Gulf of Mexico slop (read steep and close together) with no trouble at all, other than getting wet and banged up. The boat handled the conditions well, I never once felt unsafe, even when we got caught in a squall. Self bailing cockpits and decent gun'nel height are my only prerequisites, which those boats had. The only complaint I had about that boat was her excessive beam, which was the cause of the getting banged up part. I've gone offshore in an 18' Mako in the same conditions with no problems as well, though I don't think I'd want to go any smaller. As long as the boat is of an offshore design such as these two examples, you'll be ok. Just keep your eye on the weather!

 

You hit the nail on the head there Corpus "As long as the boat is of an offshore design such as......"

As another famous American said 'a lightweight steam roller don't do the job!'

 

I like that one Walrus You'd be surprised how many knuckleheads around here head offshore in "bay" boats, even jon boats Keeps the Coasties in business I guess.

 

And the original inquiry by glastron156 was concerning runabouts, not saltwater fishing boats.

Kelly

 

Details details I just wanted to point out that 18-20' boats could handle rough water.