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H5000 Leeway coefficient?

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What is this number? If i enter this to B&G, should I put the same value to Expedition as well?

Now I have nothing bunched in, Compass is calibrated and aligned so that HDG vs. COG difference is more or less the same on both tacks and no leeway (HDG-COG) motoring downwind. So pretty close I think.

I also started using the "SOG as Boat Speed" as it is close to impossible to get the calibration spot on on both tacks (and slow speeds). I sail in an area with practically no tide/currents. Seems to be the best option for me, and gives most constant polar% and targets, etc.

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Leeway=(heel*coefficient)/bsp^2, right?

23° and 8 knots is 3,6° leeway

15° and 7 knots is 3,1 leeway.

Above with coefficient 10, seems ok values, maybe slightly on the high side? 

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We also normally use the coefficient for the upwind angles - much less leeway downwind means errors are negligible.

For an example, I took the VPP targets for a Bene 44.7

                     
tws 4 6 8 10 12 14 16 20 25 30
Up.Vs(kts) 3.68 5.17 6.19 6.79 7.07 7.26 7.35 7.44 7.51 7.53
Up.Bt 47.3 45.6 44 42.4 40.7 39.7 38.9 38.4 39.4 41.3
Up.Vmg(kts) 2.5 3.61 4.45 5.01 5.36 5.58 5.73 5.83 5.8 5.65
Up.Heel 2.8 6 10.3 15.3 18.1 20.2 21.9 24 24.4 24.8
Up.Leewy 2.42 2.58 2.93 3.35 3.54 3.7 3.87 4.21 4.67 5.18
                     
k 11.70 11.49 10.90 10.09 9.78 9.65 9.55 9.71 10.79 11.84
                   
                     
                     
                     
                     
                     

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For all practical purposes zero heel means zero leeway. If you aren’t heeling there is very little side force. As Expediton said, leeway is an upwind/reaching deal. Not so much downwind. 

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On 5/24/2018 at 11:43 PM, Joakim said:

What about zero heel or slightly negative heel at 4-8 knots + crew?

You can be going sideways to leeward, if there's a force on the rig in that direction, independent of the heel angle, especially if in heeling to weather, you pick up a lee helm and have to put the rudder over to keep the bow up... 

A leeway coefficient is only an estimate that's based on the loss of lateral resistance as you heel, assuming a near constant righting moment. 

Different hull shapes, rudder angles etc will change the effect. 

At 4-8 kts of wind depending on your boat, you may be sliding sideways if downspeed +/or pinching, and not so much if footing, with your keel and rudder loaded up with attached flow.

If you have the capability, log your COG/SOG vs BSP/HDG coming out of tacks, it's rather eyeopening how long it takes to get a heavy long keel boat moving with minimum leeway after a downspeed tack, with the keel stalled. 

 

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7 hours ago, LionessRacing said:

A leeway coefficient is only an estimate that's based on the loss of lateral resistance as you heel, assuming a near constant righting moment.

No, it's not. It's a simple model, that assumes lateral force to be a linear function of heel. Which is accurate as long as there is no crew and the vertical center of effort of the rig is at constant location. This model comes from basic wing theory and stability theory. Lift generated by the keel is increases linearly with leeway and in square of water speed. Heel increases linearly with lateral force.

With crew on the windward reel you get more lateral force, especially at low or zero heel,  and at higher winds you need to lower the vertical center of effort and get higher lateral force.

Expedition shows how well the model agrees with 44.7 polars. But all the accuracy is lost, if you choose to sail the boat with some other heel angle at low wind speeds when you can easily choose the heel angle by changing crew position. E.g. at 6 knots you can easily choose to sail at from over 10 degrees to zero or even windward heel. The model would then give anything from over 4 degrees to zero or even negative leeway (maybe negative is not allowed?) while in real life heel would have almost no effect on leeway at that situation. Or if it has any effect, it is more likely to be reversed (less leeway with increased heel due to rudder loading). E.g. a DIV II windsurfer was sailed with a lot of heel (30 deg?) in light winds in order to reduce leeway, but obviously the rig could still be upright.

A more accurate model would ignore heel at low wind speeds until the boat is loaded up. Once loaded up the model should have on offset for heel to count for the crew weight (how much a hiking crew can heel the boat without sails). When the boat becomes over powered, the model should be corrected for lowered center of effort.

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6 hours ago, Joakim said:

No, it's not. It's a simple model, that assumes lateral force to be a linear function of heel. Which is accurate as long as there is no crew and the vertical center of effort of the rig is at constant location. This model comes from basic wing theory and stability theory. Lift generated by the keel is increases linearly with leeway and in square of water speed. Heel increases linearly with lateral force.

With crew on the windward reel you get more lateral force, especially at low or zero heel,  and at higher winds you need to lower the vertical center of effort and get higher lateral force.

Expedition shows how well the model agrees with 44.7 polars. But all the accuracy is lost, if you choose to sail the boat with some other heel angle at low wind speeds when you can easily choose the heel angle by changing crew position. E.g. at 6 knots you can easily choose to sail at from over 10 degrees to zero or even windward heel. The model would then give anything from over 4 degrees to zero or even negative leeway (maybe negative is not allowed?) while in real life heel would have almost no effect on leeway at that situation. Or if it has any effect, it is more likely to be reversed (less leeway with increased heel due to rudder loading). E.g. a DIV II windsurfer was sailed with a lot of heel (30 deg?) in light winds in order to reduce leeway, but obviously the rig could still be upright.

A more accurate model would ignore heel at low wind speeds until the boat is loaded up. Once loaded up the model should have on offset for heel to count for the crew weight (how much a hiking crew can heel the boat without sails). When the boat becomes over powered, the model should be corrected for lowered center of effort.

And that is why more sophisticated instrument systems have a leeway K factor matrix - a single number does not work incredibly well however it does a reasonable job for most situations.

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2 hours ago, Campbell said:

And that is why more sophisticated instrument systems have a leeway K factor matrix - a single number does not work incredibly well however it does a reasonable job for most situations.

Having a K factor matrix doesn't really solve the fundamental problem of the model. You still need to pick K with a specific heel angle and you may have very different heel angle in light winds depending on crew position. It would be much better to have a matrix for K*heel for light winds and ignore the real heel.

Or even better have a VPP running in the instrument and calculate leeway with the all the models in the VPP.

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Tested the leeway coefficient few days ago. I set the coefficient to 10 (H5000) Seemed reasonable, when downwind, close to zero leeway and gradually more when heeled over.

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