Boat Speed Variation between tacks

[Jesse]

Is your test area effected by any current?

[senormechanico]

Originally Posted by Connemara

And speaking of paddlewheels... ours works fine for the first few days after a bottom wash and then stops. We sort of think it's marine life clogging it, and if so would love some ideas about how to prevent that. But are there other explanations?

Connemara

Critters hiding from bigger critters.
After we installed our ultrasonic bottom system, no more fouling of the paddle wheel no matter how long the boat sits at the dock.

[Jim Cate]

Originally Posted by Connemara

And speaking of paddlewheels... ours works fine for the first few days after a bottom wash and then stops. We sort of think it's marine life clogging it, and if so would love some ideas about how to prevent that. But are there other explanations?

Connemara

Well, we remove the wheel from the body (just push out the axle pin), clean carefully (soaking in white vinegar if there is hard fouling), wipe with acetone and then using an artist's brush, put a thin coat of antifouling paint on the wheel and in the cavity. Let dry, check to see that there are no lumps and then reassemble. Lasts for several months, easy enough to do that it isn't a big deal to repeat.

Jim

[44'cruisingcat]

Originally Posted by StuM

Good evening Naughty Cat (well, it's evening here)

With a difference that extreme, the first thing I'd look at is your wind indicator.

I don't know what sort of boat you have (you may like to update your profile with that info so that it shows beside your post), but I find 10.1 knots in 10.4 TWS at 44° AWA hard to believe for a cruising cat.

If your wind indicator is 10° out, you are pinching at an actual 35° AWA on Starboard and sailing at 55° AWA on Port - which could make your figures much more believable.

This. In fact I no longer bother climbing the mast to calibrate the wind instrument, but simply adjust it while sailing so the boat performs the same on both tacks.

[cyan]

Originally Posted by 44'cruisingcat

This. In fact I no longer bother climbing the mast to calibrate the wind instrument, but simply adjust it while sailing so the boat performs the same on both tacks.

I like this simple answer- the instruments are not quite telling the truth. However, if the OP can easily "feel" the difference on the different tacks, then would have I assumed he already tried pointing higher on the bad side as a sanity check for the instruments. No?

[44'cruisingcat]

Originally Posted by cyan

I like this simple answer- the instruments are not quite telling the truth. However, if the OP can easily "feel" the difference on the different tacks, then would have I assumed he already tried pointing higher on the bad side as a sanity check for the instruments. No?

It would make sense, but you never know... but
you'd try pointing higher on the good tack.

[Dave_S]

I'm probably the least experienced one here but another vote for the wind direction being off by 5°.

10kn out of 10kn apparent at 44° seems optimistic. 8kn out of 50° is probably realistic. In theory with 5° change in you wind direction reading would give you 8kn on each tack at 50° apparent which sounds like a good result.

Other factors could be current and swell direction.

[Dave_S]

Set the main traveller car as close to centre, raise the main, no jib, tight on the main sheet. Head into wind on motors, find the point where the main starts to luff on both tacks. I expect if the wind direction is miss calibrated it will show that the mid point between your 2 tacks will be 5° to port of where your instruments say is directly into wind.

[hoolie]

Originally Posted by boatman61

My rationale is pretty simple.. the owners side is loaded much lighter so on one tack the heavy hull is being lifted by wind and hydro dynamic's, less resistance more speed.. on the other tack the wind is pushing the heavier hull under increasing resistance and slowing the boat down.
But.. I'm probably wrong..

You've got my vote. I think that nails it, in spite of any possible instrumentation/sensor calibration errors

Just to emphasise the OP says the boat's a heck of a lot slower on one tack compared with the other, not even looking at the instruments

[cyan]

Originally Posted by 44'cruisingcat

It would make sense, but you never know... but
you'd try pointing higher on the good tack.

Ah I see. I may have been a bit mixed up. I guess I would try sailing by feel in a steady breeze and vary the heading higher and lower on BOTH sides until the optimum sail-shape and speed are achieved, ignoring electronics. When racing, we rarely trust many instruments except the compass.

[44'cruisingcat]

Originally Posted by hoolie

You've got my vote. I think that nails it, in spite of any possible instrumentation/sensor calibration errors

Just to emphasise the OP says the boat's a heck of a lot slower on one tack compared with the other, not even looking at the instruments

That's not exactly what the OP said. He said he could feel the difference in speed.

From the exactness of the numbers given, I got the impression that the boat was being sailed on autopilot set to steer to a wind angle.

[yachtingdirect]

Hi Naughty Cat,
This is a very common observation made by most boat owners at some stage in their sailing life. However most sailors never uncover the reason for it. "It" has a very simple origin.

It is caused by "wind shear" and is covered most comprehensively and clearly in a publication called "WIND STRATEGY" by Fiona Campbell and David Houghton. These guys are meteorologists and UK Olympic coaches. They know their stuff.
It has also been covered in other threads on this website. Sometimes without solution (frustrating!).

The bottom line is that in the northern hemisphere, if the wind is "stable", then starboard tack is faster (sometimes significantly so). Conversely, in the southern hemisphere, if the wind is "stable", then port tack is faster. There is more to it but read the book and you will end up being much more educated than the vast majority of sailors. (and no, I have no connection whatsoever to this publication or the authors....just a commitment to education!!
Fair winds to you and may you uncover more and more of the extraordinary pleasure of sailing. B

[cyan]

Originally Posted by yachtingdirect

Hi Naughty Cat,
This is a very common observation made by most boat owners at some stage in their sailing life. However most sailors never uncover the reason for it. "It" has a very simple origin.

It is caused by "wind shear" and is covered most comprehensively and clearly in a publication called "WIND STRATEGY" by Fiona Campbell and David Houghton. These guys are meteorologists and UK Olympic coaches. They know their stuff.
It has also been covered in other threads on this website. Sometimes without solution (frustrating!).

The bottom line is that in the northern hemisphere, if the wind is "stable", then starboard tack is faster (sometimes significantly so). Conversely, in the southern hemisphere, if the wind is "stable", then port tack is faster. There is more to it but read the book and you will end up being much more educated than the vast majority of sailors. (and no, I have no connection whatsoever to this publication or the authors....just a commitment to education!!
Fair winds to you and may you uncover more and more of the extraordinary pleasure of sailing. B

Wow, thank you! I learned something new. I'm amused to think back on a racing skipper who was convinced the crew was oddly screwing up on every port tack. DOH.
Here's a simple breakdown...
Ocean Sail Articles: Sail Twist and Wind Shear

[Paul L]

Originally Posted by yachtingdirect

Hi Naughty Cat,
This is a very common observation made by most boat owners at some stage in their sailing life. However most sailors never uncover the reason for it. "It" has a very simple origin.

It is caused by "wind shear" and is covered most comprehensively and clearly in a publication called "WIND STRATEGY" by Fiona Campbell and David Houghton. These guys are meteorologists and UK Olympic coaches. They know their stuff.
It has also been covered in other threads on this website. Sometimes without solution (frustrating!).

The bottom line is that in the northern hemisphere, if the wind is "stable", then starboard tack is faster (sometimes significantly so). Conversely, in the southern hemisphere, if the wind is "stable", then port tack is faster. There is more to it but read the book and you will end up being much more educated than the vast majority of sailors. (and no, I have no connection whatsoever to this publication or the authors....just a commitment to education!!
Fair winds to you and may you uncover more and more of the extraordinary pleasure of sailing. B

Are you saying wind shear caused by mast height/corriolis affect is the cause of a 40% drop in boat speed? Seems far too large a drop in speed.

[Jim Cate]

Originally Posted by Paul L

Are you saying wind shear caused by mast height/corriolis affect is the cause of a 40% drop in boat speed? Seems far too large a drop in speed.

I quite agree! Until instrument bias is ruled out, I'll still give that the greatest chance of being the cause of the observation

Jim