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Old 08-08-2010, 11:36   #1
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Jib Presses Bow Down

I keep reading that people say to ease the jib first to ease the pressure on the bow. This didn't make sense to me, especially since I started with the premise that the boat was upright. With the boat upright, given the same height of centers of efforts for the main and jib, then the lever arm is the same. If anything as the bow gets depressed, the lever arm on the main could increase and give it more power to depress the bow.

Finally it occurred to me that some people could have been only talking about when the boat is heeled over. Now since a component of the drive is pointing down, and the center of effort of the jib is generally forward of the center of buoyancy, then the jib is pulling the bow down (to the side).

So maybe when people say this they automatically have been assuming that everyone knows that they are only referring to situations where the boat is significantly heeling. (Admittedly many cruising boat points of sail while overpowered involve heeling.)

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Old 08-08-2010, 13:06   #2
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The centers of effort for the main and the jib are rarely of the same height.
Almost every point of sail of a cruising boat involves heeling, even if not overpowered.
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Old 08-08-2010, 20:46   #3
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Originally Posted by FSMike View Post
The centers of effort for the main and the jib are rarely of the same height.
Almost every point of sail of a cruising boat involves heeling, even if not overpowered.
OK, the center of effort of the main is normally higher than that of the jib, so it has the bigger lever arm depressing the bow more. They're probably close enough together in height to not make a difference.

Upwind case, 15 degrees of heel gives approx. 25% of the sail's drive is down. Yes this is pushing the bow down, but how far forward is the center of effort of the jib in front of the center of buoyancy? I would argue not very far, making this a fairly small lever arm.

Broad reach, most of the sail force is forward and I don't believe that vector is going much at a down angle with heel, I'll have to play with it. The small crossboat vector still goes down, but that's not much force.

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Old 08-08-2010, 21:01   #4
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I think we know what you are concerned about but just to make sure, you are asking if the headsail is pushing the bow "down" in terms of downwind as opposed to vertically down, right?
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Old 08-08-2010, 21:39   #5
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Illusion -
I have a feeling that when I find out exactly what John is talking about I'm going to feel uneducated.
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Old 08-08-2010, 23:45   #6
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what's the question? What to do if over powered? If that's the question, the answer is to twist off the top of both sails.
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Old 09-08-2010, 00:41   #7
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Maybe it's the attachment point of the jib sheet that needs to be further back, allowing more twist in the sail and lowering it's centre of 'push'. The other factor is the hull, perhaps the bow shape is a little tender so you'll have to learn to accept the boats limitations.
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Old 09-08-2010, 00:44   #8
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Ease the jib first? Not on a beat. Keep the headsail wound on hard. Maybe move the sheet block aft to flatten the foot and add twist.

Depower the main first, unless you want to reach.

But I don't follow the pushing the bow down idea. Who cares? The evolution of boat design has already handled this. Trim the main to get a few degrees of weather rudder. I like 7 degrees of bow down rudder.
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Old 09-08-2010, 00:48   #9
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I think we know what you are concerned about but just to make sure, you are asking if the headsail is pushing the bow "down" in terms of downwind as opposed to vertically down, right?

I've been looking for an example and now I can't find one. I'm not talking about lee and weather helm being changed with sail trim. Perfectly familiar with steering with the sails in pre-start manuevers.

What I believe I have read is that some people are claiming that the jib depresses the bow down into the water. I can see that this is true, particularly upwind heeled over, but it seems that it would be a minor effect. You've got me doubting myself now since I can't find an example. It's possible I misread what they wrote, or perhaps it was written poorly and they were talking about turning the bow down as in downwind.
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Old 09-08-2010, 01:02   #10
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I suppose anything that pulls the hull forward that's above the waterline tends to push the bow down. But, overall, the force needed to pull the hull thru the water at beating speeds is not much (the hull drag). So the bow down force is pretty small.

Easing the headsail implies sailing a little 'fatter' which increases the speed and therefore the hull drag, and therefore (presumably) the bow down force. So...?? Hmmm.

I understand on some hulls, downwind, it's advantageous to use the spinnaker to lift the bow a bit by easing the tack and clew up.
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Old 09-08-2010, 08:08   #11
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Look at any set of pictures where monohulls are racing to the upwind mark and you will clearly see that the bow is down. Link takes you to an example:

12+HOWL+upwind.JPG (image)

Quote:
With the boat upright, given the same height of centers of efforts for the main and jib, then the lever arm is the same. If anything as the bow gets depressed, the lever arm on the main could increase and give it more power to depress the bow.
The CE of the headsail is generally going to be forward of the mast, while the CE of the main will be behind the mast. Therefore, the "lever arm" effect of the jib will pull the bow down.

Fair Winds,
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Old 09-08-2010, 08:46   #12
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Modern hull forms exhibit a "bow down" attitude when heeled becasue the wider transoms tend to lift and it force the bow down.
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Old 09-08-2010, 09:48   #13
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John,

I think you hit the nail on the head. The jib only gives downward pressure when the boat is heeled. It might help the discussion to talk about levers relative to the longitudinal metacenter. The longitudinal metacenter is the axis about which the boat will pitch. It is not constant when heeled, but for this exercise, this shouldn't matter. Both the main and the jib impart torque on the hull about the metacenter, in proportion to both the size of the sails and the geometry of their lever arms. The distance of jib's center of effort relative to the metacenter may be greater then the corresponding measurement on the main, so when it gets a vertical component, it shoves the bow down harder even though the main is the bigger sail. Combine that with lots of lift from a wide transom, the bow goes down.

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Old 09-08-2010, 10:10   #14
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Brett,
Think you hit on a major factor when you mention the lift from the wide transom that's almost universal in modern sailboat design. On an older boat (like mine) with a much narrower transom the downward thrust in the bow is much less apparent. If you run hypothetical numbers, the downward lever is not all that great but when you combine that with the lever provided by the lift at the stern you get what seems to me a significant downward force at the bow.
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Old 09-08-2010, 10:18   #15
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Quote:
Originally Posted by daddle View Post
I understand on some hulls, downwind, it's advantageous to use the spinnaker to lift the bow a bit by easing the tack and clew up.
Negative. Consider where the forces are attached and their vectors. Does not seem like easing the tack and the clew will lift the bow.

In real life, I have never seen any bows lifted (like that). I have seen spinnakers trimmed for max drive so that the hydrodynamics lift the bow though.

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