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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #1  
Old 11-26-2007
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Port vs. Starboard tacks

I've noticed a significant difference in my boat's performance, my starboard tack is about 1.5 knots slower than my port tack. I have a large water tank on my starboard side and thought that this might be a factor. Yesterday we had some very stong conditions and my destination was about 20 miles directly upwind, with the waves and current head-on as well. I had 2 reefs on both sails (25+ knots) and actually measured performance for the first time.

I would sheet in the genoa identically on both tacks, pulling the sheets back to the aft spreaders. I had adjusted the main until I had only about 2-6 degrees of weather helm on both tacks. This gave me 8.2 knots on starboard tack but only 6.5 or so on port tack. I don't have an inclinometer, but I was well heeled over and it seemed to be about the same angle, and I kept the angle to the wind identical on port and starboard tack as well.
I couldn't more than 7kn on port tack without letting out a reef. I'd emptied the starboard watertank completely, so I think that my boat is evenly statically balanced. The rigging looks straight and tensions are similar on both sides as well as fore-and-aft.

Is this rather large difference normal or acceptable? Is a rigger likely to find some way to balance this out? Could there have been some prior damage (i.e. grounding) which might have caused a slight deformation of the keel that can be causing this?
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Old 11-26-2007
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How were you measuring your speed? If with a log wheel, is the wheel symmetrically placed on th hull? If with GPS, what currents were predicted? Does your compass have the ability to measure/display heel angle? You mention the weather helm, was it the same on both tacks? Were the waves coming down wind or were they slightly offset?

I think there was another thread on this question and it seemed to end up with asymmetric rigging tension.
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Old 11-26-2007
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Zanshin—

First, Idiens points about how you're measuring things is very key... it is hard to accurately measure the difference between port and starboard tacks, since it is very difficult to have the exact same conditions on both tacks.. with respect to currents, and such.

Also, if your wind instruments are off a bit... say 5˚, you could actually be sailing ten degrees higher on one tack than the other, and that could easily cause such a difference in boat speed.
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Old 11-26-2007
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If everything's roughly symmetrical in your hull, straightness and plumbness of spar, sails, and weight distribution, then the conditions you're sailing in may be the factor. Double-reefed in heavy wind, seas, and current may not the best time to compare performance this way. The angle of the current (if not truly head-on), shape and angle of the seas, and difference in wind gusts can make a difference you're not aware of (assuming you're using GPS to get speed readings).

I've often found in sloppy conditions that sometimes one tack is just "better" than the other, in a boat which otherwise sails the same close-hauled on either tack. Try your boat in lighter air, no-reef, no current conditions if you can find them, and see what happens then.
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Old 11-26-2007
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One other thing, the main sail shape may be somewhat different on the two tacks with the sail reefed, since on most boats, the reefing tack cringle will be brought down to a cringle hook on one side of the gooseneck, leading to an asymetrical sail setup. The sail's reefing clew cringle is often also biased a bit to one side or another, depending on how the reefing lines are lead.

These differences in the reefed mainsail's shape could also account for the differences.

Nolatom's point about trying the test in lighter wind, with less swell/chop, an unreefed mainsail, and preferably with no current... and see what happens.
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Old 11-26-2007
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No one has mentioned rudder angle; the drag from which would affect boat speed, if not consistent for each tack.
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Yesterday was the first time that I took the effort to measure things, particularly as I'd emptied the asymmetrical weight of the water tank. I was sailing directly against a .25 knot current and the waves were head on to the wind which made measurement simpler. I was using a log wheel, which is forward of the keel and only about 4 inches off the centreline (it is to port). I didn't use a GPS, I'll do that next time out. In my case both Genoa and Main are furling (genoa closing to clockwise, main closing counterclockwise) and with 2 reefs there is a definate obstruction in the luff when on a starboard tack. Both directions had the same heel (a couple of inches shy of the toerial) and weather helm. I'll check today to see if the wind instrument reads true - as I was basing my heading on the wind angle and didn't cross-check with compass headings. The forecast is for choppy 4-6 seas and 20+ wind spees until later this week so I won't be testing further for a while. But for those interested, I'm using the free wifi at Saba Rock but can't take any more pictures. In the process of assisting a stranded kite surfer with a disabled kite about a half mile offshore yesterday (I saw the glint of something and decided to investigate on the dinghy) I managed to get my camera inundated...
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Depending on how your boat's keel and rudder are configured, prop drag could be creating a slight sideways component when the prop is either freewheeling, or atilt slightly one way or the other.
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Old 11-26-2007
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Get yourself a Loos tension gauge.
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If the headsail and main were furled, the airflow over them wouldn't be symmetrical, since they're both roller furling... so that alone might explain the differences..
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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