spinnaker, CE, and attitude

Technical discussion of ARC products
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havliii
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spinnaker, CE, and attitude

Post by havliii » December 4th, 2018, 8:21 pm

Bill, your thoughts and related theory please.

When you deploy a fore sail such as an asymmetric spinnaker or a screecher, does the movement of the CE of the entire sail plan add a force pressing the bows deeper or a lift component that pulls the bows up? or is it neutral? I don't own either of these type sails but would like to understand more about the associated effects and how the CLR moves to keep the total forces in symmetry.

Thanks!
Andy

Bill Roberts
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Re: spinnaker, CE, and attitude

Post by Bill Roberts » December 6th, 2018, 8:33 am

havliii,
Remember that the force from a sail is perpendicular to the sail cloth. Therefore if a spin or gen is leaning aft from the side view and inward at the head from the front view of the boat, then there will be some vertical lift from these flattish cut sails. These sails are made/shaped to produce sail force due to wind flowing across these sails, luff to leech, just like a jib.

On the other hand a symmetrical spinnaker with a head that flows forward from the mast to big shoulders in the top third of the sail and then the sail from a side view profile bends downward with the majority of the sail area being vertical and bending back and down toward the bow of the boat. This sail is acting like a catcher's mitt with the wind and it does tend to push the bow of the boat down deeper into the water. This is an older classical keel boat spinnaker sail shape; like an old Americas Cup boat spinnaker. Maybe this was the best spin shape for them because the boat frequently was sailing at boat speed limit and so reaching up to go faster did not/could not make the boat go faster, so the best course for them was to sail dead downwind toward the leeward mark, minimum distance travelled point to point with a maximum area spinnaker.

havliii
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Re: spinnaker, CE, and attitude

Post by havliii » December 6th, 2018, 8:41 pm

Where the sail curves around at the luff, combined with the rake angle from bow to mast, there will be some perpendicular vector lines coming off the sail that have a component of vertical. Am I understanding that correctly? Thanks Bill, for helping me visualize this.

Bill Roberts
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Re: spinnaker, CE, and attitude

Post by Bill Roberts » December 7th, 2018, 10:16 am

haviii,
For this discussion, think of this headsail as a flat plate, tack to head to clew. That flat plate has a resultant force perpendicular to its surface at the centroid of the flat plate. This resultant force is made up of three component forces. One component force is sideways. Another is forward and the third one is vertical lift upward.

havliii
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Re: spinnaker, CE, and attitude

Post by havliii » December 9th, 2018, 5:41 am

Thanks again. Are the three component forces described as pitch, yaw and roll. I think I'll make a little model to better visualize the situation as the boat heels. I framed houses as a young man, the boss was always screaming, plumb, level, square and straight, referencing a similar coordinate system maybe?

Bill Roberts
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Re: spinnaker, CE, and attitude

Post by Bill Roberts » December 9th, 2018, 6:23 am

havliii
Pitch, yaw and roll describe the three types of motion of a yacht, the dynamics of a yacht's movement about its three major axes.

Something to think about: let's consider a mainsail and keep in mind that the force from a sail is always perpendicular to the sail cloth. When normal sailing a mainsail is vertical or very close to vertical. That means that the resultant force from the vertical main sail is in the horizontal plane only. Therefore this resultant force can be divided into a forward force and a side force. There is no vertical force. OK, this sounds alright. Well, let's let the boat heel over at 30 degrees, one hull flying. Now the sail is not vertical and there is a vertical component of sail force. This vertical force, with boat heeled over, adds to the total down force, the total weight, pushing down on the hull. So when you heel over, the hull thinks the boat got heavier due to the added down force from the sails. Now the hull sinks deeper into the water and makes more drag and the boat slows down.

havliii
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Re: spinnaker, CE, and attitude

Post by havliii » December 9th, 2018, 4:03 pm

Okay so I made my model today, using a flat plate for SA and the centroid of the flat plate for the CE. I constructed the model so that I can change the angles of the three major variables, heel angle, sheeting angle and the luff angle. Here are my conclusions (possibly still wrong) but what I see is this. As you sheet the sail in, the positive (in the up direction) vertical force grows smaller and smaller. Likewise as the heel angle increases the positive vertical force grows smaller and smaller. If the heel angle increases great enough and the sail is sheeted in tight enough the vertical force can go through the neutral point and into negative territory. (a downward force) The angle of the luff is also a contributing factor. If the angle of the luff (between the deck and the stay) is decreased (a more acute angle, a longer pole) and all other things remain the same (heel and sheet) the positive vertical force increases.

The interesting thing about the model is that you can see the representative total force vector (that pierces through the flat plate at right angles) change direction in real time as you move the sail/plate about.

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Re: spinnaker, CE, and attitude

Post by Bill Roberts » December 10th, 2018, 10:00 am

Hi havliii,
You are educating yourself and that is good. I point out that on high performance boats the spinnaker is usually trimmed in close. That is because the boat goes faster on a beam reach initially and then the boat accelerates which bends the relative wind forward and now you are sailing within 20 degrees of straight downwind and doing it at faster than true wind speed. The increase in boat speed is much greater than the increase in distance travelled. On high performance boats, a lot of wild and crazy things happen sailing downwind. When the hull gets too high you bare off to suppress the relative wind speed and the hull comes down. If you head up in this situation, this will increase the relative wind speed and the boat turns over quickly. So, you have to turn your hat around and think backwards when sailing downwind on a fast boat.

And guess what? People who sail monohulls have no idea that this is going on. They sail to windward tacking on the windshifts until they get to the weather mark. Then they all get in line and head downwind to the leeward mark. Then they round the leeward mark and turn the race back on again sailing to windward. In high speed catamaran racing, sailing downwind is just a complex if not more so than sailing upwind.
Last edited by Bill Roberts on December 10th, 2018, 4:24 pm, edited 2 times in total.

havliii
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Posts: 264
Joined: March 27th, 2011, 5:12 pm
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Re: spinnaker, CE, and attitude

Post by havliii » December 10th, 2018, 4:17 pm

Bill, I hope you are enjoying this conversation, I certainly am! I'd like to understand mast compression loads a lot better. I understand a bit of the Euler slender column stuff, but not how the sails create more compression in the column as they load up. Maybe the hint is in your post about the main sail load as the boat heels.

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