# Lifting Spinnakers:does it lift the bow?

Discussion in 'Sailboats' started by Doug Lord, Nov 24, 2006.

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### PI DesignSenior Member

Okay, here are some fairly crude calcs for a 12' Skiff. I don't have the time, inclination or information to do a more refined calc.

A quick flick through the rules shows a mast height of 6.8m. Assume a mast head kite and that the mast is deck stepped 0.5m above the water. Assume also that the vertical CE of the kite is 40% of the way between the foot and the head (ie something between a triangular and square sail). Thus the drive force has a lever of 0.5+(0.4*6.8) = 3.22m.
Also from the rules: Hull length is 3.7m. Bow extension = 0.7m. Pole length = 2.5m. The end of the pole is therefore 6.9m from the stern. Assume that the foot of the kite reaches from the end of the pole to the mast, which is 1.5m from the bow. The foot length (clew to tack) is therefore 1.5+0.7+2.5=4.7m. Assume that the longitudinal CE of the kite is one-third of the distance from the clew to the tack = 4.7/3 = 1.56m from the end of the pole, or 6.9-1.57 = 5.33m from the stern. Assume, whilst planing, that the LCF (the point about which the hull trims) is 1.0m from the stern. Thus the lever for any vertical (lift) component of the kite force is 5.33-1.0 = 4.33m.
Using the Cherub data supplied earlier, the drive (horizontal) force from the kite is 680N, whilst the lift (vertical) force is 600N. It is reasonable to use the Cherub data here because the two classes are very similar (a Cherub measures as a 12' Skiff).
So, the bow down moment is 3.22*680 = 2190Nm.
The bow up moment is 4.33*600= 2598Nm.

Therefore the net moment is bow up.

Note however, that the two values are quite close. It is quite likely that in a real world, dynamic, environment that the moment will switch between bow up and bow down as things like sheeting angle, boat trim, wind strength etc change. There is a great video clip (go to the UK class website) of a 12' Skiff sailing along which seems to show this happening.

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### PI DesignSenior Member

RHough, a mainsail on a vertical mast won't produce an upward force. I am talking about spinnaker in my example. Although the wind flow may be horizontal, the spinnaker will produce a force with some upward component, due to the angle of the luff. The Cherub data shows that the force from the kite is 68kg forward and 60kg up. These are not my figures, they have been derived by a well respected sail maker from a computer simulation. As they are the only figures anyone on this forum has presented, I have used them in my calcs. Even if you find the figures suspect, the general principal is that the kite will produce a resultant force with some upward component. Dave Culp (Kiteship) does not dispute this, he merely refutes the amount of upward component. Kiteship, WaterAddict and I are in agreement on the form of the free body diagram, but not the numbers to plug in. Kiteships original assertion was that the moment was ALWAYS bow down. I hope I have been able to show that this is not necessarily the case.

Kiteship wrote:

"Try every boat you can think of. Any combination of sails. Feel free to cheat. Unless you use a free-flying kite, you will get a bow-down pitching moment. Period. This isn't a close thing, not by a very long shot."

I have done this. I have posted my findings. Without cheating, I can find a sail that produces abow up moment.

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### Man OverboardTom Fugate

PI Design states.
“I thought I'd managed to show that, with the right sail plan geometry, it IS possible for the bow to lift? If anyone still disagrees with my demonstration, can they explain why in a clear manner…?”

Theoretically, you certainly can imagine setting a spinnaker at a 45 degree angle or more; or another way of stating it - is to set the sail on a longer bow sprit, and lower on the mast so the moment generated from the sprit is greater than the moment generated from the mast. You certainly could show a situation where the spinnaker would lift the bow. I am not sure that this would be beneficial, as the whole point of a sail is to generate forward thrust.

In real life there are some things to consider that have not been discussed. Spinnakers generate force by two methods dependent on their angle of attack.
First: a force produced by parasitic drag, similar to the way a parachute operates. I am uncertain if it is correct to state that there is a leading edge in this situation, but for the sake of illustration, we would say that there is full separated flow on both the leading and trailing edge. Because of the wind gradient, there tends to be greater flow, and hence more violent separation towards the top of the mast, therefore greater driving force higher up on the mast. (When sailing down wind of course) There is no lift component in this type of scenario; simply a drag force parallel to the wind. The second way a spinnaker generates force is due to lift. This generates a much more powerful force than in the first scenario, but not without a high penalty. (Pun intended) Remember that with the component of lift, also comes the component of induced drag. You of course have heard it said of spinnakers ‘no big deal, the drag aids in driving the boat’. But it is a big deal, because like with most airfoils, the greatest portion of drag occurs towards the tip of the airfoil, which is again high up on the mast. The total force component on a sail, especially a spinnaker of any type is most certainly not near the geometric center of the sail, but quite a bit higher up, contributing to the moment generated by the mast.

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### gggGuest...

You're grossly over simplifying. Go to the table at the end of the paper and look at the actual pitch moments. That's the critical point.

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### raykSenior Member

A moment is the change from one stable state to another.
Using a moment to describe lift of the bow or depression of the bow means it must start from an equilibrium and end in an equilibrium, of forces.

Only four forces act on the 2D illustration. Lift>Gravity, and Thrust>Drag.(Up>Down Forward>Back...whatever)
All vectors are made up of these forces.

A moment starts when one of these pairs is out of balance.

Thrust>Drag
If the wee boat is tied to the wharf with a rope, when it reaches the end of its tether, drag becomes infinite.
Depending on where the rope is tied (fulcrum) a moment will occur rather quickly.
If the wee boat is tied to the wharf with a bungy cord, drag will increase to infinity over time with an increasing moment.

Lift>Gravity
The small amount of spin lift is some meters ahead of the CG.
Spin lift is the sum of all up/down forces around that point roughly.
When spin lift is lost bouyancy moves forward as the bow comes down.

Greater amount of bouyancy lift only a little aft of the CG.
Crew move forward and CG moves forward. Spin lift stays in CE but CB moves forward.
CB is the center of bouyancy lift. LCF goes out the window with two (CB CE) lift forces.

Thrust and drag for the illustration are balanced. In a dynamic sort of way.
Vertical forces are balanced in a static kind of way.

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### MalSmithIgnorant boat designer

You're all idiot's! (except Rayk)

At the end of the day, if the boat is sailing at a roughly constatant angle of trim, the pitching moment of the boat/sail system is zero. For simplicity, the pitching forces can be broken into horizontal and vertical components. The horizontal thrust of the sails reacts against the hull resistance force to create a bow down pitching moment. The vertical thrust of the sails reacts against the weight of the boat, through the centre of gravity to produce(usually) a bow up pitching moment. The sum of these two moments is the net pitching moment of the sails. This moment must be balanced by something, and that something is an equal and opposite moment which is usually created by offsetting the centre of gravity with the centre of lift of the hull. The centre of hull lift is the sum of of the buoyancy force and the net vertical component of any hydrodynamic forces.

To determine qualitatively the sense of the sail pitching moment (Ms), estimate the location of both the centre of gravity (CG) and the centre of hull lift (CL). If CG is aft of CL then Ms is bow down. If CG is forward of CL then Ms is bow up. Put more simply, if you pile the crew aft (usually the case), the pitching force is bow down. If you pile the crew forward (not often seen) the pitching force is bow up.

To answer then question, when sailing without the spinnaker, the vertical thrust component of the rig is fairly negligable and hence the bown up pitching moment will be insignificant compared to the bown down pitching moment. When the spinnacker is raised, a significant vertical thrust component may added well forward of the CG in addition to the added horizontal thrust of the spinnaker. The sum of these moments may result in a net reduction of the bow down pitching, giving rise to the impression that the spinnacker is lifting the bow.

Mal.

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### raykSenior Member

Extremely close MalSmith

Bold is mine
Forces not moments, that is my only change.

By the way I quoted MalSmiths post because it was the most reasonable one yet!

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### RHoughRetro Dude

From the article:
"The third column, SinkF, is the sail force component perpendicular to the sea surface. Negative sink is LIFT, so the kite is lifting the boat at a force of 586 N, or about 59 kilos. The asymmetrical is driving the boat at 67 kilos, so the sail is lifting the boat nearly as much as it is thrusting it forward."

"lifting the boat" does not mean lifting the bow.

"Even all sections of the main are lifting (negative SinkF). I have assumed a realistic bow-up trim of the boat (from photos) - the mast leans so heavily aft that the main is actually working like your book cover opened. Nevertheless, both sails are pitching the bow down (positive pitching moment), the asymmetrical even more so than the main.

Together, the lift of the main and the asymmetrical corresponds to approximately 30% of the boat's displacement."

Here are the forces:

Driving force......0.650........0.357........1.007 kN
Heeling force......0.520........0.274.........0.793 kN

Heeling moment...1.265........0.781........2.046 kNm
Yaw moment.......0.076.......-0.450.......-0.374 kNm
Pitching moment..1.682........1.146........2.827 kNm

I'm not sure how it is possible to take some of the data, cite it as a reputable source, then go on to ignore the conclusion. If you want to think the the A-Sail lifts the bow on a Cherub ... fine ... where is the crew weight compared to the CG or LCF? fore or aft? Do they run forward in gusts?

The information was posted in post #3 ... over 90 posts later, people are still arguing about it, yet no one has refuted the data.

Making statements like: "A moment is the change from one stable state to another.
Using a moment to describe lift of the bow or depression of the bow means it must start from an equilibrium and end in an equilibrium, of forces.

Only four forces act on the 2D illustration. Lift>Gravity, and Thrust>Drag.(Up>Down Forward>Back...whatever)
All vectors are made up of these forces.

A moment starts when one of these pairs is out of balance."

Indicates no understanding of what a moment is: "a turning force produced by an object acting at a distance (or a measure of that force)" Yes to be in equilibrium the forces and moments must balance, to state that moments only exist until equilibrium is reached is wrong.

Are we here to debate the premise, dispute the facts or argue with Dave Culp?

Now if you set out to create a sail the lifts boat and provides very little drive, I'm sure you could do it. What would be the point? (As Tim B pointed out in post #8)

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### raykSenior Member

• A moment occurs when two opposing forces become unequal.
• A moment is the difference between two forces.
• There are no moments when forces are equal.
• A moment is not opposed by another moment.
• A force is opposed by another force.
CB not LCF.
In a gust the CB moves aft as spin lift increases.
Moving forward during a gust is not on the cards, boat wont go faster.
When the gust passes lift decreases, residual speed plus CG forward increases the chance of submarining, on a wee planing skiff thingy.

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### RHoughRetro Dude

Search definition of moment on the net. You are completely wrong.

The force of the sails act on a lever arm to heel the boat, the boat rotates until the righting moment is equal to the heeling moment. The boat stops rotating. The moments do not disappear, they balance each other. Only the motion resulting from the moments stops.

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### raykSenior Member

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### RHoughRetro Dude

Zero sum does not mean zero moments, It means just what it says, the sums equal zero.

To open the door wider the opening moment (force times distance) must be greater than the closing moment.

Not all forces create moments. The force of gravity on your monitor creates a force that is opposed by your desk. Move your monitor on a plank hanging off your desk and the same force creates a moment. The magnitude of the moment is equal to the force times the distance.

Just because something is not moving is no indication that there are no moments.

A moment is a force acting through a lever. A moment is the result of force and the length of the arm. 10 pound on a 1 foot arm creates the same moment as 1 pound on a 10 foot arm.

A moment occurs when two opposing forces become unequal.

No, a Moment exists when a force acts on an arm

A moment is the difference between two forces.

No, a Moment is a force acting on an arm.

There are no moments when forces are equal.

No, two equal forces acting on different length arms both have moments.

A moment is not opposed by another moment.

No, heeling moment is opposed by righting moment.

A force is opposed by another force.

??? Scotty ... beam me up!

I'd say that you are not clear on the concept.

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### raykSenior Member

Fair enough.
I cant argue with Star Trek. You have trumped me.
I was hoping you wouldnt raise to see me.

How about I draw wee planing skiff things which tear down wind dragging their wide arse through the water.
The competition is pretty tough for me. I havent really had any thing new to offer in this thread.

You can draw for the crowd that need a wide spoonlike planing bow to compliment the moment of the sail plan.
I think the racers are ready for a revoloution and your time has come.
Start now while no one else is doing it.

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### PI DesignSenior Member

The 12' Skiffs are pretty fast, and as far as I'm concerned their kite can lift the bow. The point would be that nosediving is reduced, a worthwhile goal many would argue.

I agree with you about the moment definition. A moment is a force applied on a lever. Nothing more, nothing less. You do not need a pair of forces and you will still have a moment when forces are equal and opposite if they have different lever lengths.

PS Sorry about the bold text, I'm not trying to shout, just trying to differentiate my text from quoted text.

PPS Anyone know what a typical moment to change trim (MCT) is for a skiff type boat?

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### gggGuest...

Depressingly it appears to be very easy to do so. We appear to have several budding politicians in this thread

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