Question on why different tacks produce different speeds

[Gitane]

I sailed to Brightlingsea this weekend and noticed that when beating on port tack I can achieve much greater speeds than when beating on starboard tack.

(I was sailing out of Bradwell, so with the SE winds this weekend and tide still against me, that means a lot of beating to get down the Blackwater!!!)

Gitane is a bilge keeled Vivacity 650 and the sail area and angle to the wind are theoretically the same irrespective of which tack she is sailing on. Weight distribution is probably more or less the same, with the weight distributed equally between port and starboard.

I was at the top of the tide, so the water was slack, so tide effect should not be an issue.

Does anyone know what causes a difference in speed between tacks? Is it standing rigging tension, or is it something else?

Thanks
Ron (Gitane)

 

[johnalison]

There are several reasons for this, some to do with the boat. Of these, there are: mast not straight, log impeller not aligned fore & aft wind indicator not aligned ditto.

In my experience, the commonest reason for the discrepency is that the wave pattern is not directly downwind, so that on one tack you may be meeting waves almost head-on and on the other tack at a greater angle. With a relatively light craft like yours (say, compared to my 5 tons) this effect will be noticeable even if the angular difference is quite small.

 

[Gitane]

Thanks,


I haven't thought about wave direction. That would make sense.

I will keep an eye on speeds on different tacks over the next few sails to see if the faster tack is consistently the port tack. If the faster tack changes with each sail, then it was indeed wave patterns that caused the problem.

I'll also look at the other suggestions.

Ron (Gitane)

 

[Saguday]

Wind shear might also be a factor, if there is a lot of shear the wind direction at the top of the mast will be noticeably different to that at the bottom and this means that sail twist will need to be adjusted from one tack to another - tighten the kicker on one tack to pull the top in and flatten the sail, let it off on the other to let it out. Not sure how significant this is on a heavy cruising yacht but it's noticeable in certain conditions racing a dinghy, took me ages to figure out what was going on. Wave patterns are likely to be more significant though.

 

[Athene V30]

Had a similar problem on my last boat - turned out the mast wasn't stepped centrally on top of compression post and the keel was moving however these are not common faults (I hope).

I assume you are talking about spped through the water and not spped over the ground which would be affected by currents. Have you lchecked your rig tension? Have a look up the mast track on both tacks when close haulled. If there is any sideways bend this needs correcting. The top of the mast should not be fwd of the bottom. Check mast is central when alongside by reaching the hallyard to opposite sides of the boat etc

 

[Otter]

Also tide height sometimes does not predict current - perfectly possible to be high tide but still some tidal current.

 

[Pye_End]

Are you sure that your log is giving you the correct informtion on both tacks? If the impellor is not on the centreline there is a good chance that it will give different reading for different tacks. A trailed log might give an accuate double check.

GPS is may give a useful check, but only if you are sure that the conditions are right for using it - eg slack water etc.

 

[Saguday]

I think the conclusion is inevitable - it's impossible to tell what your speed is on either tack, so you may or may not have a problem but you'll never know