Why is my boat faster on a stbd tack?

[thomm225]

Sorry to disappoint you engineer types, but it's the elements.

Wind, waves, current ...................and the fact that folks don't steer a perfect course on both tacks

[belizesailor]

Originally Posted by briendaniel

Hi,
My understanding is that if you are in the northern hemisphere in 'stable air' then ALL sailboats with mast height greater than 5metres will be faster on starboard tack and slower on port tack. The reverse is true for Southern Hemisphere, where here, port tack is faster. This phenomenon can be exploited by proper sail trim. IE if you twist the top of your main and headsail away from the boat (vang off and traveller up and sheet out so that the boom is just to leeward of the centreline, plus headsail car back). (This won't work on the opposite tack, where the only thing a trimmer can do is "untwist" and flatten the sail and point higher but slower)You should be able to exacerbate the difference by up to 25% to 30%. It will win or lose a sailboat race when one uses this characteristic of the wind. 'In stable air' wind exhibits "wind shear" ,where the direction of the wind at the base of the mast can be 30 degrees 'rotated' to the wind at the top of the mast. The reason is that wind slows down as it gets closer to the water due to friction at the surface. If the air is stable then the coriolis effect 'turns' the direction of the wind as it slows, closer to the surface.
On port tack in the northern hemisphere, the top of the is not effective so your boat is slower on this tack. All boats exhibit this characteristic if their mast is high enough. The opposite to 'stable air' is turbulent air. A good example of turbulent air is wind coming off a large land mass that has been heated by the sun.....wind shear in these conditions is not as predicable as in 'stable air' and will result in similar speeds on both tacks.
Hope this helps.

Very skeptical that Coriolis Effect would have any relevance at typical cruising boat mast heights...got any references for this?

[Oceanride007]

In my opinion, not Coriolis, but friction. Look up wind shear.

Thomm, Engineer types?. Yes, and your point being... TIC.

[StuM]

Originally Posted by SURV69

Do most sailboats track straight from bow to stern?

My Bristol 29 tracks about 10 degrees off in calm waters and virtually no wind, under power.

Anytime I try to point the boat via the bow, I have maintain a vigilant eye on the course ... a few seconds of inattention results in the bow going to port ... quickly and substantially.

When I aim for a course between the bow pulpit and the forward shrouds, the boat tracks rather nicely.

The full keel(with a forward cutout), and what appears to be a straight rudder, line up when sighted with the naked eye.

Maybe ... there's an established reason/cause for some sailboats to crab through the water, and maybe, it might be the answer to the OP's issue?

I presume you are sitting on the centreline when sighting with bow?

If it's doing it only under power, have you looked at the prop alignment?

[briendaniel]

Originally Posted by belizesailor

Very skeptical that Coriolis Effect would have any relevance at typical cruising boat mast heights...got any references for this?

There are many references to this phenomenon. The one I refer my students to is a book called "Wind Strategy" by David Houghton and Fiona Campbell. You will find a whole chapter devoted to it. It is easy to read and understand and definitely not reserved to "engineering types". World champion and Olympic coaches and sailors reference these two authors and their coaching results speak volumes. Fiona had a brilliant site called Skye weather (after the isle of Skye in the Hebrides). I am asked this question by boat owner clients many times....it puzzled me too before I found the answer. Cheers and take care.

[briendaniel]

There are many references to this phenomenon. The one I refer my students to is a book called "Wind Strategy" by David Houghton and Fiona Campbell. You will find a whole chapter devoted to it. It is easy to read and understand and definitely not reserved to "engineering types". World champion and Olympic coaches and sailors reference these two authors and their coaching results speak volumes. Fiona had a brilliant site called Skye weather (after the isle of Skye in the Hebrides). I am asked this question by boat owner clients many times....it puzzled me too before I found the answer. Cheers and take care.

[UNCIVILIZED]

Originally Posted by briendaniel

There are many references to this phenomenon. The one I refer my students to is a book called "Wind Strategy" by David Houghton and Fiona Campbell. You will find a whole chapter devoted to it. It is easy to read and understand and definitely not reserved to "engineering types". World champion and Olympic coaches and sailors reference these two authors and their coaching results speak volumes. Fiona had a brilliant site called Skye weather (after the isle of Skye in the Hebrides). I am asked this question by boat owner clients many times....it puzzled me too before I found the answer. Cheers and take care.

I'm not doubting you, though it does see odd that in the dozens, & dozens of books I've read on racing trim, none of them mention it. Even those by Olympians hailing from both hemispheres.


As to the crabbing while under power comment, I'm thinking similarly to StuM. And also consider that prop walk isn't just something which is present when docking in close quarters. It's a phenemenon that affects anything that's spinning, that travels any distance through anything but complete vacuum.

[briendaniel]

Originally Posted by UNCIVILIZED

I'm not doubting you, though it does see odd that in the dozens, & dozens of books I've read on racing trim, none of them mention it. Even those by Olympians hailing from both hemispheres.


As to the crabbing while under power comment, I'm thinking similarly to StuM. And also consider that prop walk isn't just something which is present when docking in close quarters. It's a phenemenon that affects anything that's spinning, that travels any distance through anything but complete vacuum.

On prop-walk, you may find reference to this in some past editions of Cruising Helmsman and Yachting World. Prop-walk has more to do with the prop shaft exiting the hull at an angle on a sailboat. It is therefore not horizontal. Boats with sail drive units (where the small prop shaft is horizontal) do do exhibit anywhere near as much, or any, prop-walk. Angled prop shafts mean that the pitch at the top of the prop is different than the pitch at the bottom of the prop....the unequal pitch causes prop walk.

[briendaniel]

Originally Posted by UNCIVILIZED

I'm not doubting you, though it does see odd that in the dozens, & dozens of books I've read on racing trim, none of them mention it. Even those by Olympians hailing from both hemispheres.


As to the crabbing while under power comment, I'm thinking similarly to StuM. And also consider that prop walk isn't just something which is present when docking in close quarters. It's a phenemenon that affects anything that's spinning, that travels any distance through anything but complete vacuum.

While David Houghton has had great results from his coaching of World Champions and Olympians, his knowledge and explanations for wind shear are more in support of sailboats with mast heights greater than 5 metres....Olympic classes have tended to be skiffs for a few decades now. In other words recent Olympic classes don't have masts tall enough to take into account wind shear (although they will induce sail twist for other reasons). This may explain why the books you have read don't cover this issue....it may not be relevant. But it is relevant to masts higher than 5 metres (which is not terribly high). Happy sailing.

[UNCIVILIZED]

Star boats don't count? They've been one of, if not the, most competitive classes for many decades running. And they're primarily mainsail driven, with spars over 5m in height.

[Juho]

One topic that was maybe not covered well enough yet is the angle of wind. Both true and apparent wind angle displays could be inaccurate (maybe not well trimmed). The telltales in the sails can be used to double check that the wind angles are (about) the same on both tacks.

[Pascals Wager]

Houghton's assertions, (which make theoretical sense), I understood to be largely based on the observations of sailors. Without a proper study and reviewed results it's an idea. There are so many variables in the atmosphere close to the surface covered by the height of a mast. Inversion layers, gusts, tidal and wave effects, even the boundary layer can extend 100s of meters high and as mentioned the variables in yacht production. All these make me doubt the effect of coriolis could be observed or even adjusted for through trim as it would be changing constantly and by only 1 or 2 degrees at most. Allowing for it on a cruising yacht would be like measuring with a micrometer and cutting with an axe....Makes interesting discussion though, thanks for raising it.

[briendaniel]

Originally Posted by UNCIVILIZED

Star boats don't count? They've been one of, if not the, most competitive classes for many decades running. And they're primarily mainsail driven, with spars over 5m in height.

Good point! Thank you.... I missed the Star boats....they would be a great example to clearly demonstrate this effect. In any case, I'm pleased to offer a snippet of information to the discussion and hope to have improved some understanding of that wonderful element....the wind. Ciao

[thomm225]

Originally Posted by Oceanride007

In my opinion, not Coriolis, but friction. Look up wind shear.

Thomm, Engineer types?. Yes, and your point being... TIC.

As racers, we engineer types often get so stuck on boat setup, sail choice, etc we forget about everything else.

We reset our mast prebend sometimes before every race depending on wind strength, readjust shroud tension, figure out whether we want thin flexible battens or thick stiff ones, then it's to the sails. Older for light wind sailing or new for heavy wind.....then mast rake and so on

Then if we lose a race we are rethinking boat setup when later we realize it wasn't any of these things that caused us to win or lose many times by the course. (and course management)

We won because the leaders decided to sail the last leg against the current while we sailed in the shallows boards up against much less current even though we couldn't point quite as well.

Or we started at the pin end instead of the committee boat end because it was the most efficient tack to windward

Or we tacked away from the tall building on one side of the course while other boats stalled in their lee.......


Point is, the current, wind, waves, etc are probably affecting the boat speed more on one tack than on the other.

[fb4skipper]

One reason can be Wind shear and twist take a Look at

http://www.oceansail.co.uk/Articles/...istArticle.php

And of course the rigg