When a boat heels does it rotate around its center of gravity or a point on the waterline?
Don
Don Case
Vancouver IslandNeither,
First of all, it is not that useful to think of the boat as rotating “about” something as it is heeling. If you are not moving forward and you try to heel the boat, the boat will actually move sideways as well as heeling. If you are moving forward, a similar effect occurs. Your leeway angle generally increases as you heel such that if you mathmatically ignored the forward motion of the boat, the boat would appear to be moving sideways when heeled.
Looking at it another way, consider a boat as it heels: for any given angle of heel, there is a vertical position of the waterline that displaces the weight of the boat. For most boats, you will find that the center of that submerged volume moves off of the centerline by a fair distance. This is partially what leads to the prismatic of the boat. If you look at the CG of the boat, it usually rises slightly as the boat heels. If you look at the any line on the cneterplane of the boat, these also tend to rise slightly. So it is very difficult to find any geometric point on the boat that remains at the same height as the boat heels through the full range of heeling.
There are some special situations (such as a torpedo shaped submarine hull) where the heeling angle would not change the shape of the submerged volume. In that case, you can find a point that the boat rotates about (in the case of the cylindrical hull, it would be the center axis of the cylinder). But in general, a “U” shaped hull does not exhibit this behavior.
- Will
Will Gorgen
Don, I tried to figure this out a few years ago in order to build a jig to tune a complete sailboat. Found that real water conditions compared to the jig and a fan were difficult to relate to each other in a way that would create an advantage. Maybe the wave action and the nose down attitude when on the water create too many variables. Let me know if this is the path that you are trying, perhaps my ideas were not sound, and we can come up with a better system together. Clyde
I wasn’t really headed in any direction with my thoughts, I was just thinking. It occured to me that if the fin provides lift, and if the boat rotated around it’s COG(Which moves up and down the fin depending on the weight of the boat vs the weight of the bulb) that the lift would be pushing one way above the COG and the other way below it. Now to straighten this out in my thick head, does the lift generated by the fin on a beat pull the boat to windward or leeward? I’ve read that it pulls to windward(helping pointing) but I can’t figure that. If the boat has leeway it would mean that the bow is falling off slightly (if that is putting it right) which to me would mean that the lift would be to the leeward side. Or-it just occured to me- when I hold the boat into the wind am I forcing the the boat to sit the other way(bow to wind).
Don
Oh, I tried the fan thing and I don’t think you can duplicate the real wind properly. If you were like me you had the boat up about 3 or 4 ft. which would mean the the wind gradient would probably be wrong. I tried getting around this by stacking two fans with the lower one on low speed and the upper one on high speed. I didn’t accomplish anything but breaking a fan when the pile fell over.
Don Case
Vancouver IslandDon,
The sails are trying to push the boat sideways (to leeward). The keel fin is reacting that sideways push of the sails by generating lift to windward. In order for the fin to generate lift, it must go through the water at an angle of attack. Thus, the boat is headed a few degrees to leeward of where it is pointed. That angle is called the leeway angle.
Hope that explains it.
Sorry about your broken fan.
- Will
Will Gorgen
O.K. Now we have the boat on a beat and the fin is generating life to windward. Is that lift creating more or less heel? I’m still thinking that the forces involved have to sort of rotate around something. We have the sail making lift sort of downwind (at least downwind from the direction of travel) and the fin making lift upwind. If this was an airplane wing it would roll. The part that confuses me is the two medium (not pizza) thing. Water and air. If it wasn’t for that boundary the whole thing would rotate on the COG. Sometimes I’m kind of thick.
Don
Don Case
Vancouver IslandThe lift is creating more heel. The heel is actually caused by the moment created by the sail force AND the keel fin force. In fact in order for it to be a moment, you need both (the hull would not create enough lateral resistance on its own to creat the equal and opposite force to keep the boat from side slipping).
So yes, you have roll.
Now that heeling moment is counteracted by a righting moment generated by the keel weight (force in the gravity direction) and the boyancy force (force in the up direction). When that moment equals the heeling moment, the roll stops and the boat sails along happily at that angle of heel.
Are you trying to solve something here. There is probably an easier way to get to your final destination than worrying about what “center” the boat heels about…
- Will
Will Gorgen
The faster you go while heeled over, the more you will heel over. It may be imperceptable, but the lift that the keel creates that prevents leeway also forces the boat to heel over more. If one was to say a boat “rotated” about something, that rotation point is not a constant point- it would move as the boat heeled more.
No, I’m not trying to solve anything; I just have this insatiable curiosity about how stuff works. In this case I’m trying to visualize all the forces involved in sailing to windward. Sometimes (if you do most of the thinking yourself) you arrive at a new way of doing things. My two current interests are sailing and bonsai. Both were pretty much perfected before modern science that means that a lot of the methods of doing things are “because that’s the way it’s done”. Sometimes the historical explanation of why something works is wrong but because it arrived at the correct solution it has persisted. It’s fun to just puzzle these things out and see if my conclusions match the real ones. Sometimes I get bogged down and I have to ask something. I like to keep the questions simple so as to develop my ideas myself. Sorry if I seem vague at times.
Don
Don Case
Vancouver IslandThe heeling centre is that not the “DOCTORS”
Sorry my spelling again.
Don
I throught that bonsi sailing was model yachting.
Ahoy Don! The point you are looking for is called the Center of Bouyancy,
The point all Bouyant forces have no rotational effect. Similar to how the CG is the point where gravitional forces will produce no rotation.
From this point one derives the Hulls metacenter which in place defines righting moments and other measures of stability.
Hope this helps
Rob