Polar chart

 

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This topic contains 20 replies, has 7 voices, and was last updated by Bruce Hubble Bruce Hubble 6 years, 3 months ago.

Viewing 21 posts - 1 through 21 (of 21 total)
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  • #1831

    John Schnellback
    Participant

    Does anyone have a polar Diagram or chart for VMG for the Olson 30? Have not been able to find it online no inboard model. Thx John

    #2308
    Bruce Hubble
    Bruce Hubble
    Participant

    I have one…sistership. I’ll send it to Scott to post.

    #2309

    John Schnellback
    Participant

    That’s great, I had better brush up now how to decipher it now.

    #2310
    Bruce Hubble
    Bruce Hubble
    Participant

    I’m missing the graphs…I have them, but have to do a dreaded windows search. At least you have the speed stuff. BTW…GIGO…speeds are predictions…the O30 is capable of going faster than these, especially downwind.

    #2311

    Bill Vosteen
    Participant

    Here’s a link to the polars for the Express 27 with some commentary by Carl Schumacher. These are very similar to the Olson 30, just a bit slower target speeds. http://express27.org/articles/polar

    #2313

    Jason Adamson
    Participant

    Bruce do you know if there is a difference between the polars for an IB vs OB? I too am interested in them. Hopefully Scott get’s them up.

    #2314

    Bill Vosteen
    Participant

    The polars for the inboard and the outboard should be the same within reason. The calibration variations of your instruments and sea conditions will cause more error than the weight and drag difference between the inboard and outboard versions of the O30.

    My opinion is the polars are a good reference and should mainly be thought of as educational tools. Trying to sail an Olson 30 to your instruments will send you to the back of the pack as quickly as anything.

    #2315

    Old School
    Participant

    As long as the copyright holders don’t give me a hard time, the polars can be found HERE

    #2317

    Old School
    Participant

    Bill has kindly forwarded me his copy of the apparent wind diagram. It can now be found on the above link.

    #2318

    Derek Lay
    Participant

    It may be the dumb engineer in me, but I’m having a hard time making sense of the diagram. How can 85% ever be an optimum run angle? Your VMG would be negative . . .

    I can attest to the outside line though. We had our IB at 11.5 knots with the main alone during an Irene-assisted delivery on Sunday.

    #2319

    Old School
    Participant

    The diagram is for apparent wind, not true.

    #2320

    Derek Lay
    Participant

    Hmmm. If I’d have read the top of the chart I guess I would have noticed that . . .

    I can attest to the angle then. We needed to get to nearly 90 degrees apparent to move well during the runs last Saturday. TWS was probably 4-5 knots.

    #2321
    Bruce Hubble
    Bruce Hubble
    Participant

    Rather than use the graph…make your own bulkhead chart in Excel using the upwind/downwind best angles at the various wind speeds; also showing target speeds and vmg. I find the speeds credible until 18 knts…then waves, etc make them so, so for the O30. Remember..these are predictions!

    #2322
    Bruce Hubble
    Bruce Hubble
    Participant

    Once we have a bulkhead spreadsheet..Scott can ‘bury’ the polars somewhere for later publication.

    #2332

    Jonny
    Participant

    My partner in the boat and math genius pal Malcolm has been trying to wrap his head around the polar charts and optimum wind angles and has hit a wall. Since I can’t articulate to him why the angle is what it is (due to Apparent wind) I’m posting his email to me up here in the hopes that one of you geniuses know the answer and can help.

    This is probably driving him crazy so the sooner we get an answer the sooner he can get back to sleep at night.

    Read:

    I keep looking at the Polars for our boat and thinking I’m missing something. The high end and low end of the wind ranges seem incorrect, at 6kts (True Wind Speed) they say we should be sailing 85 degrees, that’s not down wind. In 20kts they say sail 114 degrees.

    I was wondering do you know anyone who we can ask how to read these things. The chart is in Apparent Wind, so unless I’m missing something, this tells me to sail in 6kts of breeze 180-85 degrees or 95 degrees off the wind, in 20kts using the same logic 180-114 we would be sailing 66 degrees off the wind.

    To further the point

    6kts wind at 180 degrees – preferred port course 275, starboard course 85 degrees

    20 kts wind at 180 degrees – preferred port course 246, starboard course 114 degrees

    If you were to plot this as they have on the polar you wonder how the added VMG does you any good because you’re sailing the opposite direction of the mark in 6kts, not much better in 20.

    I’m doing something wrong, but, I don’t know who to ask. I’ve done a bunch of reading but, I’m not getting how to use this information when the polars say such weird things.

    #2333

    Old School
    Participant

    No, the apparent wind is always measured relative to the boat. 85 degrees apparent means that the windex is pointing five degrees ahead of the beam, either on port or starboard. It true life, the boat course will be more like 130-140 degrees from the true wind direction, due to the skewing effects of apparent wind sailing

    http://www.sailingmagazine.net/how-to/technique/936-understanding-apparent-wind

    Now the real question remains…can you ever sail DDW faster than windspeed?
    http://forums.sailinganarchy.com/index.php?showtopic=82175

    #2334

    Old School
    Participant

    BTW, I have another set of polars I have just uploaded. This set is for another boat with an inboard. It has the true diagram as well as the apparent diagram.

    I would love to see a set for an outboard.

    #2335
    Bruce Hubble
    Bruce Hubble
    Participant

    The outboard would not be different. The only difference might be a tenth of a knot of boat speed at lower wind speeds. The sailing characteristics are the same. Everyyone should remember this is a velocity PREDICTION program. The speeds are not real, but are relative targets to angles. This is why you use this chart to make your own charts. I would guess the optimum speed for VMG is 5.8 knots..for example. We know we can easily go 6.5 by cracking off 3-5 degeees, but VMG suffers. This is all about VMG. That is why 85 apparent is best VMG downwind with light air..but look at the true angle for the same VMG and all will be clearer!

    #2336

    Old School
    Participant

    I would suggest that the shape of the curves should be different for outboard boats. Different weights, waterlines, drag, etc. If you look at the two curves we currently have, you will notice a difference between the two inboard boats (one “folding prop” and another “folding tractor prop” whatever that is):

    http://www.olson30.org/wp-content/uploads/2011/08/o30polar4.pdf
    http://www.olson30.org/wp-content/uploads/2011/08/POLAR_DIAGRAM.pdf

    Mind you, there are not huge differences, probably not enough of a difference for us mortal helmsmen, but there are differences none-the-less. It would be interesting to see if an outboard curve would be enough different for a helming change.

    #2337
    Bruce Hubble
    Bruce Hubble
    Participant

    Points of sail should not change. Speed…you fill in the blank. BUT..you can make your own polar at that point. Angles will be the same. I think you are putting too much emphasis on the inboard having drag & weight. There are times when that weight can help…like keeping the rudder in the water! :)

    Point of sail, point of sail, point of sail= VMG

    I dare say no one can match (or should try) the speeds. New vs old sails for instance! It is a reference only…leave the speeds OUT!

    A personal column chart: (pasted to bulkhead)

    TWS AWS AWA TWA (upwind)
    TWS AWS AWA TWA (downwind)

    Fill in @ 6 knts/8knts/10knts & so on below each. You can add VMG and BS only if using GPS, not a knotmeter. Those speeds will be a reference guide. If you don’t have a wind instrument…you need to fix your windex to a true 30 degrees AWA (60 degrees spread) and get used to figuring out where the AWA’s are downwind. Upwind…sailing tails anyway. (don’t forget wind shear messes with the angle up there)

    #2338
    Bruce Hubble
    Bruce Hubble
    Participant

    Reviewing the two different boat charts, I’m guessing the differences are related to a program change on the 1993 VPP program. I note the heel angles and best vmg differences as evidence. Tractor refers to the aperture…possibly a sail drive? These two charts would only be good for you makeing your own predictions based on your experience and the charts for guidance. VMG for a wind speed..using the best angle YOU like. On runs: @10 knts there is very little VMG difference between 113 & 148 AWA. Waves would make a difference. Boat speed difference is VERY apparent… 6.2 down to 5.4, but near the same result on VMG.

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