Basic but perplexing stability question

I was saying that it is intuitive for a layman person to think that adding some volume below, or under, the submerged chines would give a sensible increase in stability. The way that board manufacturer did it, to be more precise. A small increase is possible, due to effects explained by Ad Hoc and TANSL.

Adding the flotation around the chines (increasing the beam) is clearly going to increase the stability, due to the increase of the second moment of waterplane area. No news there, anyways. You can't escape the laws of physics.

By the way, the picture of items you've used as weights for your inclining experiment is great!

Cheers

 

The difference, like on a cat, would only start at the angle when the sponson or bump comes out of the water.

 

I wouldn't say "like on a cat". For the same waterplane area, a cat has a bigger initial stability than a monohull (even when both hulls are in water) because the distribution of areas on a cat gives a much bigger moment of waterplane area.

This way of distributing the volume works on a cat (and it doesn't work here) because a cat has a void in the waterplane area around the centreline which this board does not have, being a monohull.

Cheers

 

I was considering an inclination at which point the bump is completely out of the water. The COF would be farther out than on a flat bottom hull.

 

The way I usually think about it is to draw the sections, assign a Vertical Center of Gravity (VCG) to the ceterline, Draw the inclined WL, then draw a dead vertical line from the VCG to the very bottom of the largest section. Once it has been determined that the sections displace as much heeled as the they did when level (this may take several WL height tries), I see how much sectional area is on the high side as opposed to how much is on the low side.

If it is clear that there is more on the low side, I know I still have some righting moment left. If there is clearly more on the high side, I know the boat will be capsizing at this heel angle. (it is amazing how soon this happens with a double ended boat, with sharp ends and a deep, flatish bottom)

This, of course, is a bit of a simplification, but, in the case of the 'bumps', it is clear that if both are submerged, one cancels the other out. Only if the boat can be rolled to the point where one leaves the water, without the other one crossing the line, does the one, still in the water, add stability.

Looking at it this way, it should also be clear that it is better to have the bumps, out to the sides, than it would be to have them next to each other, under the center line of the hull.

This is one reason a boat, with water ballast keels, will often be better off with widely spaced bilge keels than it would be with one keel under the center line.