Heeling Question

When a boat heels, does it rotate around the waterline(approx) or around the center of gravity(about halfway down the fin on an IOM)???

Yes & Yes

There is a coupling of a number of forces that influence the “turning moment” that heels the boat. The actually centre point of rotation is C.O.G but it will not always be half way down the fin, think maybe you are confusing this with CLR ?

Cheers Alan

Actually, when the boat heels, it will rotate around the lateral center of buoyancy. However, keep in mind that since the shape of the hull is not usually perfectly round, the lateral center of buoyancy changes as the boat heels. That is where the form stability of wider boats comes into play. As they heel the center of buoyancy moves to leeward, increasing the moment of the center of gravity.

Thanks guys(I think)
To see where my thinking is going lets assume two possible rotation points. Either the COB or the COG(and since on my boat it is about halfway down the fin we’ll put it there). When the boat is heeled and on a beat the fin is creating lift along it’s length. If the boat rotates around the COB then all the lift is trying to rotate the boat in the direction of heel(read more heel). If the boat rotates around the COG the the bottom half is trying to rotate the boat in the direction of heel but the top half is trying to rotate it in the other direction (read less heel). My thinking is if the the latter is true then it helps make the case for a fin that has a longer chord at the top and possibly a shorter chord at the bottom.

Unfortunate, but true.

It would be nice, but no such luck.

Sort of true. The wider top gives you more lift in the area of less moment arm, so less of a heeling force component.

Question: If the boat is pivoting around the CoG, what exactly is it pivoting around? Gravity is pulling down and heeling force is pushing down. Naturally the fulcrum would have to be pushing up.

This (I think) is what I am having trouble getting my head around. If you threw the boat in the air it would spin around the COG, it doesn’t need a fulcrum. It’s that damn “working in two mediums” that screws with my head. The water surface sort of works like a fulcrum, but it’s a weak one and would give way. Sort of like a shock absorber on a car, it slows the movement but doesn’t stop it.

Hi Don, the finer dynamics & forces involved in sailing is something most of us have trouble getting our heads around, even the experts have trouble which why the perfect sailing craft has never been found, but we can fly to the moon :rolleyes:

Dumb question but what are you wanting to achieve, I’m guessing from your thread heading, it is to reduce heeling?

Staying with your thinking that the surface of the water as sort of fulcrum point, then if you take the vertical distance between the CE and CLR you can think of this as a arm using the hull as central pivot point, the distance between CLR & CE determines how much the boat will heel (longer arm = more heel at any given wind speed).

I’m assuming you have maximum class rules for keel fin length and bulb weight, then one other option to reduce your heeling angle at any given wind speed, is to change your sail plan to have a lower CE ?

I must be missing something

surface of the water as sort of fulcrum point - How is the surface of the water a fulcrum point, sort of? This should be the center of buoyancy.

CE - Where the heeling force is coming from, OK

CLR - How does the CLR resist the force of the CE? In actuality, as was sort of pointed out earlier in the lift discussion, the CLR is acting in the wrong direction to resist heeling. The Center of Gravity is what resists the heeling force. That is why we use the big heavy weights way down low.

The distance from the CE to the CoB causes heeling. The distance from the CoB to the CG resists heeling.

Hi Greg,

I was simply using Don’s interpretation of his word of “fulcrum” to keep it simple e.g “water surface sort of works like a fulcrum, but it’s a weak one and would give way”.

First question back to you is what does “R” mean in CLR ….is it not RESISTANCE ? which by loose definition is the underwater sideways at approximately the geometrical centre of the submerged hull, rudder, keel and bulb when viewed from the side.

The forces acting on CE (air) and CLR (water) create the “turning moment” which heels the boat. The weight of the boat and its buoyancy form a coupling that counteracts the turning moment. These CE/CLR forces will be equal magnitude but in opposite directions when the boat heels. (see pic in post #2 for visual)

I think we are saying the same thing only using different words ? but open to correction if I have misunderstood.

Cheers Alan

I understand that you did not originate this statement. The surface of the water exerts no forces on the boat, except if you want to get technical, surface tension is pulling on the boat, but it is small and isotropic so it cancels itself out. The thing holding the boat up is buoyancy.

The CE and CLR do cancel each other out, as you say. But these are in the lateral direction (leeway) and not rotational (heel). But one is above the rotation point and one is below. So they are both trying to rotate the boat in the same direction. What cancels the rotational force generated by the sails (centered at the CE) is the center of gravity. When the boat is straight up, as in no wind/no heel, the CG is directly below the Center of Buoyancy. As the sideways force on the sails increases it starts to rotate the boat (around some pivot point, the original question). as the boat rotates, the center of gravity moves to windward, farther away from the pivot point (fulcrum) and increases the resistance to heeling. The CG is pulling down, naturally. The CLR, assuming the boat has some leeway, is pushing the fin to weather, which is in the same rotational direction as the sails are trying to push the boat, adding to the heel.

In the pictures above, The forces are acting in the following directions around the hull:
CE - Anticlockwise
CLR - Anticlockwise
CG - Clockwise
CoB - Up

These are the main forces acting on the boat in the transverse direction. So to recap, the CE and the CLR are trying to turn the boat clockwise. The CG is trying to turn it anti-clockwise and the CoB is holding it up.

Hi Guys
Sorry to confuse things by saying"surface of the water". I knew that there was a point near the surface but I wasn’t sure whether it was the LCB or the LCF. Hullform defines them as:

“Centre of buoyancy (LCB)
the fore-and-aft location of the point about which the buoyant forces acting on the hull have no rotational effect (“moment”). It
is expressed as a percent of the distance from the foremost to the sternmost extent of the waterline.
Centre of flotation (LCF)
the geometric centre of the area enclosed by the hull’s waterline. For small pitching motions, it defines the line about which the
pitching occurs.”

I kind of assumed that if pitching occurs around the LCF that heeling would also and since the LCF is the center of the waterline area I used the word “surface”.

If the COB and the LCB are at the same point(I may be wrong) and there is a force “up” at it doesn’t that kind of make it a fulcrum? Also in your last post shouldn’t the lift of the fin enter into tho rotational forces?

Anyway, two people answered this post and one thinks the boat heels around the COG and the other around the COB. Anyone else want to take a shot.
Thanks for the input guys

Don, I’m sorry I failed in trying to understand the point behind your question :confused:

Cheers Alan

According to my understanding ,

the “fulcrum” is the place where all the masses are in equilibrium.
Ease to see when the full boat is suspended like in the picture below :

Since in the water the masses receive a push up equivalent to the volume immersed, the “fulcrum” will tend to move forward the hull as in the sketch below :


Hi Alan
Don’t be sorry, I’m not very good at composing questions. I confuse people a lot. Basically what I’m after is understanding. The more you know about a subject the easier it is to think about it. A question doesn’t need a point. In this case I’m just curious about the “point” about which a heeling boat rotates. Maybe once I know I will be able to apply that knowledge to something.


Not sure what the LCB is, but yes, I absolutely agree with your statement. If it is the Lateral center of Buoyancy, than definitely yes.

I was lumping the lift of the fin in with the CLR. Either way, it is pushing to weather below the rotational point (anti-clockwise) and therefore contributing to the healing force.

The LCF is kind of a measure of where the boat will pick up the most volume the quickest as its attitude changes. It is more of a rate thing. It is relevant to healing in the sense that it can be an indication of how quickly the hull will pick up submerged displacement as it heels and how quickly/far the CoB moves to leeward as it heels. Remember that the farther the distance between the CoB and the CoG (horizontally) the more force to counteract the heel.

I’ve been thinking about Claudio’s post. Since in all normal things(except boats) the COG would be the center of rotation it stands to reason that we should start there. In boats the COB provides an alternate possibility. If they both have some effect maybe somewhere in between is the answer.
That would be the wishy washy Canadian solution

Curious. How did you find the fulcrum in the water? Every time that I tried, the hull floated on the surface and the keel sank below leaving the boat upright.

That’s how your boat will balance on the fulcrum when it sinks ! let us know when you try …if it is correct ? :lol:

Gotcha :slight_smile: then here’s an interesting document about some of the basic principles of naval architecture for small craft design ratio’s that Eric Sponberg published following similar discussion on another forum recently …hope you find it as informative and entertaining as I do.

Cheers Alan