Expedition Navigation Software 
1 of 1- DuncanR
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Offline - From: Up and down East Coast Aus
- Registered: 9/16/2015
- Posts: 471
Leeway in TP52s
I've gained the job of trying to get instruments on a TP52 to work well before the annual pilgrimage S-H in 3 weeks or so.
First question, what leeway might one reasonably expect in 12 knots of breeze, with an inclinometer showing 20 degrees of heel - and I'm not sure about that, but my leeway figures are - I think - too small.
- Ernie M
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- Registered: 11/05/2015
- Posts: 419
Re: Leeway in TP52s
Duncan,
Great question!
The formula in the Exp help file shows Leeway angle = k times (Heel Angle / BSP squared)
It further states that the constant (k) "value of 10 or 12 will suffice"
Looking at Will Oxley's book he says "11 on a TP52"
So I built a small MS Excel table and for BSP 10kts and Heel = 20, it shows 2.2 degrees
BSP being a large factor in the formula is reflected by what you kinda know; if you're slow and heeled you make a lot of leeway. The formula for BSP=5 and Heel=20 nets a leeway angle of 8.8 degree
On the other hand at 14 kts BSP heel=20 the angle would be only 1.1 deg.
Don't know if the theoretical helps, maybe someone with a lot of TP52 hours can chime in.
If you want the interactive spreadsheet I built, PM me an email
21 days isn't a lot of time to fine tune this stuff, guess that's why Nav gets the big bucks!
Cheers,
Ernie
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- Registered: 12/20/2015
- Posts: 91
Re: Leeway in TP52s
In a TP in 12kts of wind - so at about 8.5kts of boat speed - and a heel angle of 20deg you might expect around 3° of leeway.
A k factor of 11 is a good starting value for a TP. You might also have to try 10 and 12.
What I find helpful in fine-tuning the k factor is enabling current arrows in the chart window while tacking upwind.
If the current is changing direction from tack to tack you might have to adjust the k factor to either increase or decrease the calculated leeway.
With the k-factor you just have a linear function. Upwind this works alright. Downwind this is not as accurate. The good thing there is that leeway angles are quite small, so the influence on the current calculation is not too bad.
I posted below the k-factors, that would be required for a TP52 based on the results of a VPP, where the ouptut is boatspeed, leeway and heel. As you can see on average a K-factor of 11 works okay.
I guess if you'd ask the design office of the TP you sail on you will get the VPP results and then you can create similar diagrams for yourself.
- DuncanR
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- From: Up and down East Coast Aus
- Registered: 9/16/2015
- Posts: 471
Re: Leeway in TP52s
Thanks Nav
This is an interesting exercise ; wen one comes in from outside an organisation - and a TP52 is an organisation - then the problem that got you in there becomes a consulting assignment, especially when there is no OQE about what the key problem to be solved really is. And everybody has views about that, all in marginal harmony, each with another.
Long and short, a k factor of 9 isn't a million miles off the mark. And when you see TWAs upwind of 60 degrees, changing the k factor from 9 to 11 (or whatever) won't be The Answer.
S-H in 2 weeks. Sheisen!
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