Keel dynamics question

[Lakesailor]

I think Jim has the best answer yet. As the circumstances usually involve a gust, normally with a wind shift, and there is an acceleration (or at least the attempt, if you are already at hull speed).
The interesting thing is that it's all seat-of-the-pants. There isn't really that much change in course. My little boat doesn't stall the rudder very easily so I can avoid rounding up in the biggest gusts, and even if I let it follow it's head it just comes up to the wind a few degees. But it makes a load of difference to the heeling angle.
I reckon he's right about the forces from the rudder.
Better check my pintles, they're not much bigger than a dinghy's.

 

[mikejames]

Re: Aquadynamics

If you round into the wind, the apparent wind over the sail increases as you spin the rig towards the wind, and the apparent wind comes faster from ahead with the addition of the boat speed. That pushes you over. If you bear away the apparent wind goes aft and slows as you sweep the rig the other way, the boat speed takes you away from the wind.

The keel drag and keel lift work to heel you more not less as you go to windward. The difference you are imagining is that when you hang a weight from the mast it keeps pulling more and more (sine of heel angle) as the boat heels, while the wind force drops off with the heel angle until its just the wind on the hull holding you over (cosine of heel angle) . Generally sails + keel self-stabilise...


If you are in a fast boat (catamaran , open 60 ..) and bear away then you will heel more as the boat actually accelerates as the wind goes aft. The apparent wind increases with the bearing away and the force on the sail goes up.... You are then in the s**t whichever way you go unless you free the sheets as you go.

 

[peteandthira]

Andy

"Did the keel bit lower down"

Not sure what that sentence means! But thank you for your reply.

Pops

 

[Omatako]

IMHO this is all too complicated.

When you turn a car to the left, the passenger wants to come onto your lap. When you turn it to the right, you want to move into the passenger seat. Assuming a starboard tack, when a boat rounds up (turns to the right), the mast wants to be in the passenger seat. When you turn off the wind (round down), the mast wants to be in the drivers seat. If you hold a course, the heel will stay the same (assuming pressure on the sails remains the same).

Hydrodynamics, aerodynamics? No, simple G forces.

The same condition exists whether you have a long keel, fin keel, swing keel, bulb keel, whatever keel. How can this be driving by hydrodynamics?

 

[Lakesailor]

I think you are all confusing the situation by assuming the boat is turning and generating forces with that motion.
I'm sure pops is experiencing the same as me in that the boat is wanting to turn and either being allowed to, gently, or not. There isn't any sudden movement to generate acceleration or centrifugal forces.

 

[kengill]

There is an interesting part to this. Boats are generally balanced fore and aft so they float level when sitting still in calm conditions. As heel is induced there may be different levels of fore and aft buoyancy due to the differing underwater shapes forrard and aft of the Cof G.

Hence the vast amount of sail shifting at various points of sail and angles of heel by mad keen racers who effectively balance out the forces by re-distributing the mass inside the vessel to minimise the rudder input needed to maintain a straight course and thus reducing the braking effect generated by the rudder blade turning in the waterstream.

I think!! /forums/images/graemlins/confused.gif /forums/images/graemlins/smirk.gif /forums/images/graemlins/grin.gif

Thus a boat as it heels may have its keel angle of attack to the water vary as it heels.

Also as a yacht is powered by its sails the lateral forces generated by the sails are balanced by the combination of the keel forces and the force input from the rudder until the yacht heels too much and the rudder force input reduces as it surfaces and the yacht broaches.

 

[Lakesailor]

It used to happen to a greater degree on my bilge keeler and I put that down to different underwater geometry as one keel came up and the other took more responsibilty.
With the fin keel I'm not sure.

 

[peteandthira]

LS

So it looks like just you and me then. I am about to start some research on the net (sad really, I know!) and will publish any definitive findings here once SWMBO has got comfortable with me poring over the internet for a few days!

Pops

 

[Lakesailor]

Pops,
I think you're right. Mind you the Bennies and Jennies of the lake seem to nearly fall over when they round up in gusts, so either no-one with a boat like that has answered, or our boats are stiffer than most and we notice the effect more when it happens. I don't know how stiff Leg-Ends are.

 

[peteandthira]

Our Leg End seems only slightly stiffer than a Bav36. with which I used to sail in company quite regularly. I think we are a ton heavier according to the specs. However, having seen the same as you actually on Windermere ourselves, I was of the opinion that they were all overcanvassed for the conditions at the time.

I still have a feeling "in my water" that there is a hitherto unmentioned hydrodynamic thingy going on under there somewhere. Being a tenacious old git, I plan to get to the bottom of it!

I will report back in due course.

Pops