length and stability

this probably has been asked before but…in general, keeping a canoes width the same and only stretching the length will it increase it’s stability?

teeny, tiny bit
not enough to even notice but spreading the footprint a bit fore/aft will make the boat a little more stable but…designers do it with beam and shape.


A lot more than just a tiny bit
Actually, increasing the length increases stability quite a lot. It makes a lot of sense too, if you consider the geometry involved. Increasing the length by say, a foot-and-a-half or so increases the amount of displacement along the extreme edges of the widest part of the boat by a whole lot more than if you were to take the same boat and widen it by a few inches. Notice also that the shorter boats which are designed primarily for good primary stability are so wide and unwieldy as to be impractical for paddling any real distance (that’s not to say people can’t enjoy them). In any case, climb in and out of a few very similar boats of different length and you will immediately feel the difference.

but only immaterially
Simply ask yourself, does a floating 24-foot log have more stability than a 12-foot log?

The friction force of water that assists in stability on a round log would increase with greater length, but it is neglible, at best.

On a hull with some shape, perhaps you’d gain a little more stability, but not enough to write home about.

Try this
Best example I can think of: Do a side-by-side comparison of a Wenonaha Vagabond and a Sandpiper. These boats are practically identical, except the Vagabond is 12" longer. After you do that, come back here and try again.

Friction is pretty darn negligible as you say, but friction isn’t what provides stability. Displacement and the distribution of that displacement are what make the difference.


– Last Updated: Jan-23-06 12:11 AM EST –

Cross section shape an overall beam make most of the difference.

I kayak but the principles of stability are similar enough.

My primary boats are:

17'2" X 17"
21' X 17"

Waterline beam is similar. The 21' boat is slightly more stable. But it has a deep seat and a slightly flattened bottom under the paddler. The short boat has a rounded bottom and a high swiveling seat. If I take the seat out of the short boat I can't feel much difference in overall stability. Either one will dump a novice in a heartbeat. The boats are 4' different in length so I can only conclude that hull cross-section and seat height are much bigger factors in stability than length. I kind of like redneck's log analogy.

Then again, maybe my boat too closely approximate Redneck's logs.

Perhaps in a boat with a 26" BWL and a relatively flat cross section, if you carried that flat section further toward the ends, perhaps by retaining the general shape but going longer overall, you'd get greater stability in the longer boat.

I need to test a Q700 versus my Q600.

great word
I learned a great word back in my college accounting classes – immaterial.

In this thread – just like the comments on your wind speed thread – what you state is factually correct, but wholly insignificant as the difference you cite are, well, immaterial when looking at the entire picture.

If you want to debate technicalities for the sake of technicalities, count me out of this discussion.

700’s more stable than 600
Subtle maybe - but definitely more solid.

Just citing experience
I have done a side-by-side comparison of the Sandpiper and Vagabond, and the difference in stability between those two boats is remarkable and undeniable. You will notice it as soon as you step in, and the degree to which the Vagabond “firms up” when leaned is much more pronounced that the same action with the Sandpiper. You need to call this immaterial for your own needs. I call it a night-and-day difference because that’s how it felt to me at the time. Somebody else here posted the same comments about these two boats a while back, but I can’t recall who it was.

An increase of length (keeping everything else equal) will noticeably enhance stability. No doubt about it.

Cheers…Joe O’

i did you side-by-side comparison


Do you think maybe the fact that the waterline width is a full inch wider for the boat that is longer has anything to do about it?

And we haven’t even looked into hull shape yet.

I rest my case.

Sure it makes a difference,
but so does length. Also, since the Vagabond can accomodate a heavier load than the Sandpiper and I weighed the same when paddling both boats, I was sort of expecting that the waterline width of the Vagabond was on the low end of its range when carrying just me and no gear, while observers at the dock told me the Sandpiper looked a bit too heavily loaded to be a good choice for me if I ever indended to carry along some camping gear (in the Vagabond I can slip over logs that have less than two inches inches of water over them without touching). Hull shape of these boats is pretty much the same since one is just a lengthened version of the other. Anyway, I didn’t hesitate to conclude what I did about my experience with these boats because I’ve read too many articles already which say that length makes a noticeable difference to just chalk it off as having nothing at all to do with the length difference.

Just trying to put my view in perspective.

"Volume distribution"
Distribute volume further out with length and it will be more stabil. Make it longer with pinched ends and it won’t help. My simple brain tells me that more wetted surface will add stability because it does add drag - drag forward, sideways or whatever doesn’t matter, it’s just drag.

The log anaolgy IMHO will work too because a longer log will roll slower it’s just simple friction. But how much different I don’t know.

According to what you’ve said, the QCC 700 at 18’ x 21" should be slightly more stable than the Q600 at 16’9" x 21". The ends of both boats – say the first 3 feet from bow and stern are roughly similar – leaving the 700 with a 12’ mid-section (18’ - 6’) which contains most of the above waterline volume of that boat. In contrast the “midsection” of the Q600 is 10’9" in length – the same max. beam but with a lesser above-waterline volume. The Q700 has the same maximum beam as the 600 but has more volume in its midsection because it is “wide longer.”

My experience in these boats seems to support this. Not a huge difference but definitely enough to be noticeable.

whether you are perceptive enough to notice or not, a stretched version will be more stable, even if its a log.

See “waterplane coefficient” etc.

There is science to apply here. We don’t have to guess on something as basic as a length change (all else equal).

This page was very helpful to me designing my first. I ran nearly all the numbers on a rough approximation of the hull I had in mind - plugged them into the Matt Broze spreadsheet for a speed estimate too. I’d have to take new measures off the finished boat to know what the actual hull numbers are, but it was nice to know what ball park I was in and what the game was before I started playing. I can conform 100% it made a difference over just winging it (though some claim that’s “traditional” with SOF).

I think …
I know you love to argue for the sake of argument, but as the topic of this thread is an IDENTICAL hull stretched out and made longer (i.e. not wider & not shaped differently as are your examples) I think that the best thing you could do right now is to stop talking and cut your losses.

Like I said, “simple mind” over here
I like guessing. I put the “error” in trial and error.

A log is a bad example. A cylinder (log) is an inherently UNSTABLE shape. It has zero righting moment at any heel angle.

Righting moment is partly a function of the sum of each of the cubic inches of volume and eachs’ distance from the center line.

So, all else “being equal” (which is an impossiblilty), a longer boat of the same general shape will have more volume further away from the center line, and will thus have more stability.

However, the shape of the cross-section is much more influential, as that is what dictates the relationship between center of bouyency and center of gravity.

Just don’t claim to know what my …
…motivation is. Assigning an unlikeable character trait to someone you can’t even pretend to know is one of the more insulting things you habitually do. I don’t mind a disagreement that deals with facts, but why on this and every other thread I’ve seen you post to more than once you insist on tossing insults is beyond me. (In this case the insult is making a derogetory comment about my personality and motivation).

As to these boats, why fall back on the same spec-page argument after I’ve given a decent reason why a lightly loaded shallow-arch hull has a narrower waterline width than it would have at its full rated load? Believe me, I heard you the first time, which is why I clarified that part for you. Go ahead and respond by telling me my geometry is wrong, or tell me that the local gravity gets stronger whenever certain people sit down in bigger boats, but don’t simply ignore the explanation and re-state an old argument.

I wouldn’t have posted again except for your claim that I’m only here to fight. By my understanding, that is outside the bounds of this kind of discussion. I never would have thought this was fighting or “arguing for arguing’s sake” so I figured I’d clarify that. As to cutting my losses, I’m not concerned about that because the comments I’ve gotten through off-the-board e-mail have been anything but insulting (in fact I’m pretty happy about them).