Thinking too much about keel forces

sailersteve · Apr 13, 2015

The boat ( was ) racing a fractional rig sloop. Its very fast with the keel up & yes i've learnt to sail in a sideways drift method with the keel only 2ft down. When i say very fast , i have a hand held knot o meter with a 10 knot max. Sailing i can easily be past 10 knots . The boat is a fiberglass shell w/rudder & keel. I really don't care to race , i'm turning it into a liveaboard w/ water & gas tanks, shorepower , 4- 31 AGM batteries, wind turbine, gas generator, Gen 4 battery charger, big TV-theater system, sink, ice maker ,safe, dinghy & motor, 6 man life raft, etc, etc. the list goes on & on. Alot of heavy stuff. If i don't do something about the keel when down , i'll sail at 1 knot . I'll be trying it out on lake Erie in June. If things are no better with my keel , i'll put it back on the trailer back to hanging from the ceiling of the pole barn. I'll try grinding the front of the keel rounded a bit. And if that don't help , so be it . I'm heading for the east coast ,then south anyway . I will be done living on land period.
I would like to thank everyone for their input on keels , a very learning thread !

justsomeguy · Apr 13, 2015

sailersteve said:
snip- I will be done living on land period.

Now yer talkin'.

popeye the sailor · Apr 13, 2015

Tim M said:
Theory is an interesting way to talk about reality. In theory I know and understand that the vacuum in front of my mainsail sucks the sail, the rig, the boat forward on a run. In reality I know that the air that I can feel is what fills my sail and pushes the sail, the rig, the boat forward on a run. Is theory truth, or is reality truth? Think about it some more.
Cheers,
Tim

ok. there is no actual conflict between theory and reality in your example. your premise is incorrect. when running before the wind, lift is not what moves your boat. drag is. lift, the low pressure on the lee side of the sail that you mention, only drives a boat up wind or on a reach (where lift is combined with drag to drive the boat).

we all know how drag slows a car. a tall, slab fronted car will be slowed by the air pressure that builds up as the car moves through the air. the same applies to sails down wind. the difference is that the air moves and piles up behind the sail, creating higher pressure than that on the lee side of the sail; thus moving the boat.

so, in theory drag pushes your boat on a run ad in reality the wind pushes your boat on a run...using the force of drag. no contradiction.

popeye the sailor · Apr 13, 2015

Daveinet said:
OK, I'm missing something here. If two forces are acting in almost opposite directions, would there not be a pinching affect? I realize the forces are in two different planes, but the nature of the affect would still be the same. What am I picturing that is not accurate?

he is absolutely correct. what you are missing is that, when pinched between your fingers, the seed is being affected by forces pushing towards each other; both trying to push the boat and the sideways element of both forces is converted to forward motion because of the oppositon of the forces.

on a sailboat, lift is acting on both the keel and the sail, but it doesn't push towards the boat. instead the lift generated by the sail and the keel pull away from the boat, in opposing directions. the sail lift pulls the boat to leeward and forwards and the keel lift pulls the boat to windward and forward. being simplistic, the sideways elements cancel out leaving only forward force. instead of being pinched between the forces, the boat is pulled in opposite directions by the forces.

in fact, this pull in opposite directions is responsible for heel. the boat is like the centerpoint of an old airplane propeller. one blade of the prop is like the sail and the other is like the keel. since the opposing lift is on opposite sides of this pivot point, the keel helps to heel the boat. much like spinning the propeller. this is where your comment about the forces being on two different planes comes in.

if the opposing lifts were somehow on the same side of the pivot point/boat (thus, the same plane), sailboats would not heel. deeper keels cause more heel that shallower keels because the lifting arm is farther from the pivot point. of course, this can be more easily countered if ballast is near the end of the keel because the righting arm is longer.

anyhow, i hope i helped clarify why he said your analogy, which is often used, is not really an accurate one.

Kermit · Apr 13, 2015

Daveinet said:
Actually, there is no such thing as lift, only the absence of pressure.

So flaps produce absence of pressure plus drag I suppose.

Jackdaw · Apr 13, 2015

Daveinet said:
Actually, there is no such thing as lift, only the absence of pressure. The air on the leeward side does not pull your sail. Air does not consist of glue. The foil only reduces the air pressure on the leeward side of the sail. There is only force against the windward side that moves the boat. (before anyone freaks out - that force is made up of wind pressure and atmospheric pressure)

Sure Dave, but what's your point? It's like saying that there is no cold, just the absence of heat. True, but I still like my beer cold. Or should I say not warm.

Jackdaw · Apr 13, 2015

Daveinet said:
OK, I'm missing something here. If two forces are acting in almost opposite directions, would there not be a pinching affect? I realize the forces are in two different planes, but the nature of the affect would still be the same. What am I picturing that is not accurate?

If the forces were pushing TOWARDS each other, then yes. But they are not.

And the reason a seed pushes out is not the same reason a boat moves FORWARD and not AFT when 'squeezed'. The analogy is so full of holes that I cannot see how it helps anyone really understand what is actually going on, which I hope would be the reason for trying to explain it in the first place.

Unless not, then carry on.

jibes138 · Apr 13, 2015

Here is an article from a research scientist on how sails create lift. You should all read this to understand what is really going on.

jibes138 · Apr 13, 2015

Another article on lift from Arvel Gentry.

https://www.facebook.com/notes/alej...y-arvel-gentry-january-2006/10152573021219520

Kermit · Apr 13, 2015

I have the final answer. It's all a conspiracy. The fact is sailboat manufacturers need wind to push the sails to make their boats go so they can sell them to make money. This is where the word "payola" comes from. There. That's it. Easy peasy lemon squeezy. All you really need to know. And that's also why sailboats are so daggum expensive.

You're welcome.

Scott T-Bird · Apr 13, 2015

Jibes, I gave it a try ...

but the article lost me at "early mathematicians tried to solve this problem and they derived a set of equations". A cursory glance at the rest of the article convinced me that I had better go to bed.

Jackdaw · Apr 13, 2015

Daveinet said:
Because that is what I understand about the pinching aspect. The sail is not pulling, it is being pushed. Yes, the foil is an integral part of why the force occurs, but the forces really do oppose each other. Ok, yes the forces are not in direct opposition, they are both angled slightly forward depending on the point of sail.

My apologies for being so anal, but I'm just that way. I can not flip a switch and expect the light to turn on. I have to picture the flow of electrons through the switch contacts. Yes, I'm really that bad.

I was actually talking about your 'Actually, there is no such thing as lift, only the absence of pressure' comment. I was not sure how that factored here.

As for your personal tricks to keep it all straight, that's all good. Its the end results that matters. But I do have a hard time with those thoughts being publicly perpetuated as correct,. Because they are not.

This is being treated like some sort of option debate, but its not. The facts behind this (lift/windward progress) are scientifically well understood, and accepted by the sailing community at large. I'm not creating original thought here; I'm communicating what is understood as state-of-the-art thinking.

popeye the sailor · Apr 13, 2015

Daveinet said:
Actually, there is no such thing as lift, only the absence of pressure. The air on the leeward side does not pull your sail. Air does not consist of glue. The foil only reduces the air pressure on the leeward side of the sail. There is only force against the windward side that moves the boat. (before anyone freaks out - that force is made up of wind pressure and atmospheric pressure)

you are saying that drag is what drives a sailboat upwind or downwind and that lift is an illusion. however, nasa seems to think there is such a thing as lift since their site explains lift and calls it by that name and differentiates between lift and drag.

power driving the boat upwind is caused by a large reduction in pressure on the lee side; not a great increase of pressure on the windward side (there is only a small increase of pressure on the windward side due to the angle the wind hits it). since nature just loves uniformity and abhors a vacuum, it seeks to equalize the pressure difference on either side of the sail. this results in lift, moving the boat forwards. that is lift. compare that to drag and there is a difference. drag drives the boat downwind by greatly increasing the pressure on the windward side of the sail; not reducing it on the lee side of the sail.

it seems to me that you are just mincing words and for no real reason. lift and drag are the appropriate words to describe these two forces.

Kermit · Apr 14, 2015

Here's a quote from your original question. My question now is does lift exist or not exist?

Daveinet said:
...When considering the sail shape and the lift, it seems like most of the forces are sideways, and almost no force that pushes forward...

Scott T-Bird · Apr 14, 2015

Anybody else ever been to Moose Junction?

I'm sure more than a few have ...

Jackson Hole, Wyoming may be my favorite place on earth but that's a different topic.

Anyway, there is a ferry at this location which crosses the Snake River and the principles seem to apply in a different but similar way. It's simple and ingenius ... they chose a location on the river where the channel is straight and the current is relatively constant (& strong). They stretched a cable across the river and the boat is attached to the cable so that it can pivot.

When they load up the ferry to travel across the river, they simply pivot the boat so the bow is pointed toward the far shore. The current creates high pressure on one side of the keel and the low pressure on the other side creates the lift which causes the boat to scoot across the river at surprising speed ... no auxiliary power necessary! When they come back, they simply pivot the boat the other way. Obviously, the cable is what prevents the boat from drifting leeward. The boat essentially sails against the current to get to the other side.

Sue and I went there for a chuckwagon dinner (what a great time) but I really wanted to ride that ferry!

I think the river current can be correlated to the wind ... the pressure on the keel is similar to the high pressure on the windward side of the sail. The force that causes the boat to "sail" across the river in the direction that the bow is pointed is the lift generated on the lee side of the current. The cable prevents slippage in the direction of the current, just as a sailboat's keel prevents slippage to lee. What prevents the boat from overturning?? I suppose ballast, hull form and weight distribution just as a sailboat. The more surface area the keel has, the faster it would probably sail across the river. But like a sail, if it has too much area, the current causes an overturning moment which is too great to resist.

The pressure differential between the windward side of the sail and the leeward side provides the lift which causes the boat to move forward. Without that pressure differential, for instance when we take the sails down, the pressure of the wind is simply drag on the hull which causes the boat to drift downwind. The keel and rudder does provide some lift on its own ... but not enough to allow you to "sail" upwind without a sail. If you are drifting downwind, you can use your rudder to orient the bow to create some pressure differential underwater to give the boat some directionality across the wind, even if you are just drifting.

If you look for the entrance to Grand Teton N. P. and find the bridge at Moose, Wyoming, look upstream and you can see the ferry landings. It's a great place for bicycle riders to take off from as well!

jibes138 · Apr 14, 2015

Kermit said:
Here's a quote from your original question. My question now is does lift exist or not exist?

Well, the force pushing the boat sideways creates a moment about the center of buoyancy that can lift say a 5000 pound hunk of lead on a 35 foot boat until it is at a 30 degree angle without much difficulty, but the force pushing the boat forward only gets it going maybe 7 knots. Now the heeling moment is a lever arm so it only takes maybe 1000 pounds on the sail to create the same moment as the righting moment of the keel (just a guestimate). Many small jet engines produce 1000 pounds of thrust, see attached
http://www.jet-engine.net/civtfspec.html.
I am guessing a jet engine would get the boat going over 7 knots despite the hull speed limitations. So yes the component of force from the center of effort of the sail in the forward direction is a lot smaller than the component that heels the boat over. The PW610F would be a good choice for an experiment to see how fast the boat would go. Just over 1100 pounds of thrust.

Thinking too much about keel forces

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