View topic - Primary Stability, Secondary Stability

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PostPosted: August 11th, 2018, 8:12 pm 
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OK I think I know what the first one is - some boats feel tippy until they are loaded with gear. Is that primary?

My Alchemist is like this. My buddy's Alchemist as well (different model).

I'm not quite clear on what secondary stability means.


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PostPosted: August 12th, 2018, 12:52 am 
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A flat bottom boat will resist leaning - this is primary stability.
When the conditions are fine and flat, this boat shines.
When the conditions are rough and rolling, this boat will pitch and roll to find the flat water below it (there is none).
As the boat rides up the wave it will pitch one way and then pitch the opposite way on the way down the other side.
This becomes very hard on the paddlers and is very inefficient.

A rounded bottom will lean with less resistance - this is secondary stability.
When conditions are fine and flat, this boat feels a little tippy all of the time.
When the conditions are rough and rolling, this boat will ride level because the rounded bottom will feel the same no matter what the water is doing below you.
As the boat rides up the wave the canoe rides level up the wave and level on the way down the other side.
This is very comfortable for the paddlers and is very efficient.

A boat that seems to do a good job of acquiring both of these characteristics is the shallow V-hull.
A little roley poley while level but stiffens right up when leaned over.

All boats will get more stable as more weight pushes the hull further into the water... to a point.
Once past that point it gets incredibly unstable.
Some hull shapes will provide different stabilities. Flared hulls are super stable and can take lots of weight whereas heavily tumblehomed boats are a little more stable and can have a maximum weight increase.


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PostPosted: August 12th, 2018, 3:36 am 
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Prospector16 wrote:
OK I think I know what the first one is - some boats feel tippy until they are loaded with gear. Is that primary?

My Alchemist is like this. My buddy's Alchemist as well (different model).

I'm not quite clear on what secondary stability means.

Not clear to me too,
but I have been told by canoe designer John Winters
that this is because "The term is really just one of convenience"...
as it is just a "laymans way to describe something that is quite complicated".

Personally I now judge boats on having good or bad stability:
good is if it is easy for me to balance a canoe even in difficult situations.
Which means sufficient initial but also good secondary stability that will be there in time when I need it ;)

For a better 101 explanation I would recommend reading this:
https://sites.google.com/site/barendsnoot/cchoosing.pdf

Dirk Barends

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PostPosted: August 13th, 2018, 8:46 am 
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I always thought that primary stability was a measure of how "tippy" the boat feels when you step into it, whereas secondary stability is a measure of how hard it is to actually tip the boat over after you have tilted it over to one side.

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PostPosted: August 13th, 2018, 8:58 am 
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My two pesos... primary stability is felt most when the hull is level with the water surface... when the hull is submerged by an equal amount port and starboard, more or less. The hull will resist tipping to the left or right as long as it's in that level position.

Secondary stability shows up when the hull is tipped significantly left or right so that the water surface begins to approach the gunwales. In a canoe with good secondary stability, the hull will resist leaning further to the left or right, so that there's more stability there than in the level position.

Flat-bottomed hulls should have good initial stability paddled level but once the canoe begins to tip over to one side, stability disappears and the canoe may tip over and swamp easily once it's past some critical angle. Arched or round-bottomed hulls with flared sides should resist tipping more and more the further they are tipped over from that level position.

All in theory, there's probably some combination of initial and secondary stability in most hulls... try 'em all, some hulls should show better stability when leaned over close to the water and others will feel very secure and stable when level.

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PostPosted: August 13th, 2018, 9:09 am 
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Initial stability is what you feel when you get in the canoe. Secondary stability is what happens as the canoe is subjected to change in conditions. That could be shifting loads like a fidgety paddler or a dog that won't sit still or it could be waves. Neither of those terms are a technical definition of stability, they relate to a paddler's perception.

The secondary stability is affected more by the sides of the canoe than the bottom. The most dramatic example is to consider fishing dories that have very pronounced flare sides. That kind of vessel is used in rough waves often while heavy loads are being hauled out of the water. The dory may bob a weave on the water but as the sides are tipped closer to the water the boat becomes more buoyant and rights itself.

A canoe that's described as being "stable" will have a flat bottom that gives the impression of stability when you get in it but as noted in this discussion, the down side is that when the canoe is leaned past a point the stability goes out the window and the canoe quickly tips.


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PostPosted: August 13th, 2018, 8:43 pm 
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A dory,though, has a fairly small flat bottom with a strong flare. I guess the bottom is too small to provide much primary stability. Re secondary stability: They were traditionally used by fishermen who hauled quite heavy nets in over the side. Their gunnels can reach a few inches of the water surface and be very difficult to tip any further.

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PostPosted: November 7th, 2018, 7:53 am 
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Primary stability is what you feel when you get in a boat and operate it at it's designed displacement, level with the normal disturbances to perfect balance. It is going to develop X righting moment any time you lean it, or it experiences a tipped water plane to one side or the other.

So long as when you tip it, it immerses more net volume on that side of the boat, or moves the center of buoyancy outboard, and the other side of the boat does not move a countervailing buoyant force, then you will have an increased righting moment.

For canoes it may go back to an important time in the selling of the modern canoe, when guys like Galt where very popular, and at the other end of the spectrum The MR Explorer was very popular for tripping. Both these styles were tippier than the typical canoe sold at the time that was flat bottomed, had a keel, and otherwise tried to look like it was an upswept ended boat that Joe Two Rivers would have paddled.

Galt's thing was that he wanted to paddle a solo that ran fast, even to the extent of pro boat dimensions, so was difficult to turn. He would then use rail down techniques to wheel it around the Everglades' twisting rivers. Galt also designed from half molds, and he liked the shape of whaleboats or similar vernacular forms that were not canoes. When freestyle actually became a thing, people realized traditional canoes worked better, and then after that they realized that less straight keels were also faster, and more stable in the wind. But in earlier days, tippy with some stability as you leaned down to the rail was cool.

The explorer was more likely just one of the first designs to come out of a Royalex mold, and was made to be easy to build. A boat with flair can also have a wider sweat spot for loading because it is always gaining on inches of immersion. The moment the boat starts to loose that battle , it gets tippy. And no mater what you do, a high aspect ratio of the immersed section will eventually make a boat very tippy.

Some unexpected things can make a boat unstable. A few years back when SUPs were just catching on, I saw two women out on Frenchman's and one was small and standing, and the other was rather heavy, and kneeling. They seemed to have got a couple of the early cheap SUPs from maybe Costco, that were styrofoam covered in shrink wrap. The larger lady had her boat bent like a banana, and the mid-deck was awash. She was kneeling in that water. It was the first sunny day in spring and I was worried for them as the water was really cold. Anyway, that SUP was very unstable, as the deck was awash and there was no way that when it was leaned the hull would displace any more water, so she might as well have tried to paddle a log roller's log. Even though the thing was maybe 36 inches wide.


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PostPosted: November 7th, 2018, 8:42 pm 
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As has been stated, primary stability is felt when first entering a canoe. In calm flat water, if you heel it over it tends to want to stay level and becomes increasingly difficult to roll onto its side. But then suddenly, it reaches a point where it no longer resists and over you go, quite suddenly. Think boy scout grumman aluminum canoe with a flat bottom and flat straight sides. In a canoe with good secondary stability, it is relatively easy to heel or roll toward one side, then it resists going beyond to the point of actually going past a critical unstable degree, rather such that you may tip enough to actually sink the gunwale and fill the canoe with water without suddenly flipping any further. Think canoes such as a PlacidBoat Rapidfire or others with flared tumblehome sides.

Note, either way, you may still get wet, not from the canoe suddenly capsizing over (good primary to a bad primary) or filling with water (going beyond good secondary), but rather instead from loosing your balance when your center of balance going outside the gunwale and thus your body falling overboard out over the gunwale. Just be careful not to do it with an audience (as I have done).


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