Why is my boat faster on a stbd tack?

[fb4skipper]

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

[catsketcher]

Wind shear from coriolis - what a load of baloney.

Coriolis is an incredibly weak force. It only comes into play with huge distances that magnify tiny amounts of differences in radial velocity (which is parallel to the plane of latitude) and the gravitational force (which directs towards the centre of the earth). The discrepancy between the outward (centrifugal) force and the inward gravitational force pushes huge volumes of fluid slowly towards the equator. The Earth turns as the fluids are gently pushed and so we get the spiralling of the Coriolis.

But to then make the statement that this causes wind shear is so dumb that it flabbergasts me that a coach could say this. Then again I was coaching at a session where another coach told the youngsters that the seabreeze shifted 7 degrees every 7 minutes precisely. I had to hold my tongue. Had the guy ever read a book like Frank Bethwaite's Performance sailing?

Fluid dynamicists, chaos scientists and anyone who has seen a smokestack will know that wind can be laminar and turbulent. In turbulent conditions you get vigorous mixing and any discrepancy that could exist between layers gets swallowed up by the wind tumbling over itself. Turbulence exists in all breezes over 7 knots - Bethwaite again.

I have been asked in my racing career what is wrong with many boats. In almost all cases it is the mug on the helm and the sheets. This boat is not going 1 knot faster or if it is it is the only boat I have sailed on or raced against that does this. Strangely enough none of the boats we are in close contact with suddenly drop away depending on the tack they are on. If we are close on starboard the same happens on port. Once I was talking to a bloke after a race we won at a state champs. He lamented his performance and put it down to his old and slow model - number 924 he said. When he found out I had won the race he asked me eventually what my boat's number was - it was 923. As old and slow as his, it was the crew that made the 20 minute difference between the boats. 20 minutes in an 80 minute race - 25% faster with exactly the same boat. Why does anyone think everything is the boat when Lasers and Tasars and other one design classes show that huge differences are usually caused by the mugs on the strings?

It is always the crew, the differences in boat hull shape etc are way less than the skill of the crew. One of the hardest things about coaching racing sailors, especially adults, is getting them to stop looking for excuses in their boats. It is the instruments lying or the sailor doing something silly, everything else is worth 1/10 of a knot at most.

[carlo de leo]

Some boats are designed to be strait with the tanks fullfilled. So even if the boat is strait at the dock, you can have asymmetric ballast.

[jeremyitalia8]

We had the same on a Sun Odyssey 349 we chartered in Greece, we figured it might have been the fuel tank which sits under the stbd aft bed. So that weight helped the boat sit flatter and therefore faster.

[jtsailjt]

Originally Posted by thomm225

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

While no one can deny that current, wind, and waves are constantly having an effect on the boat and its speed, WHY on one tack more than on the other?

[thomm225]

Originally Posted by jtsailjt

While no one can deny that current, wind, and waves are constantly having an effect on the boat and its speed, WHY on one tack more than on the other?

Because on one tack these factors are hindering your boat's performance and on the other they are helping your boats performance.

I think the guy that used the racing example said it best.

When we are racing and two or three boats that have been racing close to each other on the same tack, tack over at the same time, their position stays the same IF all three boats performed the maneuver properly.

[StuM]

Originally Posted by thomm225

Because on one tack these factors are hindering your boats performance and on the other they are helping your boats performance.

I think the guy that used the racing example said it best.

When we are racing and two or three boats that have been racing close to each other on the same tack, tack over at the same time, their position stays the same IF all three boats performed the maneuver properly.

I think you are missing the point. While a boat will sail faster on one tack than the other, depending on conditions - if it always sails better on a stbd tack under different conditions on different days and different routes, it's not the conditions favouring the tack, it's be boats configuration. Otherwise on some days it would be stbd tack and on other days the port tack that is faster.

[thomm225]

Originally Posted by StuM

I think you are missing the point. While a boat will sail faster on one tack than the other, depending on conditions - if it always sails better on a stbd tack under different conditions on different days and different routes, it's not the conditions favouring the tack, it's be boats configuration. Otherwise on some days it would be stbd tack and on other days the port tack that is faster.

I think many of you are over thinking it.

The guy stated his boats are faster on one tack than the other.

He gave specifics on one weekend sail where he may have been headed into waves or current.

This assumes he doesn't have 200-300 lbs of crap stored on one side of the boat or something. In other words, hopefully he has eliminated all obvious things that could cause a difference...like jib cars being equal, tuned rig, good sails, crew in similar positions, tell tales installed equally, etc

Then it could be the driver. Maybe every boat he sails is faster on starboard. This could be debated forever

[mrohr]

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.

Well Sir ,Its still horsefeathers. ,If you found the "answer" you did not find it in the book you referenced here. I went out and purchased Wind and Strategy and am unable to find any statement to suggest that the Coriolis effect has any micro effect that would account for the discussion here. The Coriolis effect is a macro phenomenon ,not observable over the few meters that surround a boats mast.
The author who I find quite knowledgeable with excellent creds does have a chapter regarding the S. hemisphere ,but in the 21 chapters NONE are devoted to effects you claim.
Perhaps you are thinking of some other tome.......lets us know,I'll buy that one too. I'm here to learn as well.


.........................luv you all...........mike ..........................

[Dangen]

I did my due diligence before starting this thread:

-Port and Stdb water tank are equal volume and are interconnected from the bottom, so they level out. Diesel and waste tank are on the centerline.(Maybe too much beer in the fridge on port side though!LOL)

-Rigging is perfectly tuned, Nice rake and good mast bend, equal tension on the shrouds. The mast is perfectly in line with the halyard at the dock when there is no wind.

-Instruments are brand new, I have SOG and SOW. Wind vane is perfectly aligned.

-Consistently getting more speed on STBD tack no matter how I trim my sails. Basic stuff like Genoa traveler are always in the same position on each side.

I suspect that it may have something to do with the current. Last weekend I had a long friendly race with a boat the same category as mine and he couldn't catch up on me on either tack!

Current flows from West to East, dominant winds are SW. I will pay attention next time we have wind from North are East.

[barnakiel]

Now and then you can see a boat with keel blade or rudder NOT exactly aligned.

Such a boat will effectively sail different angles even when you wind instrument shows same wind angle on each tack. Thus speeds will differ too.

Next time in the boatyard have a good long look at your keel and rudder.

Cheers,
b.

[Stumble]

Originally Posted by Dangen

I did my due diligence before starting this thread:

-Port and Stdb water tank are equal volume and are interconnected from the bottom, so they level out. Diesel and waste tank are on the centerline.(Maybe too much beer in the fridge on port side though!LOL)

-Rigging is perfectly tuned, Nice rake and good mast bend, equal tension on the shrouds. The mast is perfectly in line with the halyard at the dock when there is no wind.Like I mentioned before, if the rig is perfectly symmetrical in tension at the dock IT IS NOT perfectly tuned. No boat, short of massive post-construction optimization, is symmetrical side to side. You have to tune it on the water to really get the rig tuned correctly.

-Instruments are brand new, I have SOG and SOW. Wind vane is perfectly aligned.

-Consistently getting more speed on STBD tack no matter how I trim my sails. Basic stuff like Genoa traveler are always in the same position on each side.

I suspect that it may have something to do with the current. Last weekend I had a long friendly race with a boat the same category as mine and he couldn't catch up on me on either tack!

Current flows from West to East, dominant winds are SW. I will pay attention next time we have wind from North are East.

You may also have issues with an asymmetric underbody. Again unless the boat has been optimized it is almost guaranteed, how much it is effecting you is something of a crap shoot, but I can promise it is happening.

There is a reason why racers spend tens of thousands getting their boats worked on new.

[Dangen]

Originally Posted by Stumble

You may also have issues with an asymmetric underbody. Again unless the boat has been optimized it is almost guaranteed, how much it is effecting you is something of a crap shoot, but I can promise it is happening.



There is a reason why racers spend tens of thousands getting their boats worked on new.

I get your point and maybe the word "perfect" was a bit too strong, however, I don't understand how a boat with a 6500 pound keel cannot be levelled at the dock when there is zero wind? I know the last step in tuning your rig is on the water, but I have 1040 pounds of preload on each shroud and even in strong winds I get no slack on either tack. Pleas keep in mind that I am not a racer trying to gain a tenth of a knot... I'm on my boat as we speak and everywhere I check, everything is square.
The more I think about it, I believe it's because of the current. Otherwise whenever I race or go in the same direction of other boats, I would notice doing better on stbd tack than port tack which is not the case.

[Stumble]

Originally Posted by Dangen

I get your point and maybe the word "perfect" was a bit too strong, however, I don't understand how a boat with a 6500 pound keel cannot be levelled at the dock when there is zero wind? I know the last step in tuning your rig is on the water, but I have 1040 pounds of preload on each shroud and even in strong winds I get no slack on either tack. Pleas keep in mind that I am not a racer trying to gain a tenth of a knot... I'm on my boat as we speak and everywhere I check, everything is square.
The more I think about it, I believe it's because of the current. Otherwise whenever I race or go in the same direction of other boats, I would notice doing better on stbd tack than port tack which is not the case.

At the dock tuning is meaningless. And frankly there is no reason to measure the tension on the wires until you are pulling the rig out of the boat for some reason.

As for your keel... It is almost guaranteed to be cockeyed. Every single keel I have ever seen new from the factory is, so while its possible yours isn't I really doubt it. How much this matters... well thats up to you. But a few degrees off center isn't uncommon. Having an asymmetric shape is also pretty common (say 90/100 boats) because lead casting isn't that precise. if one side of the mold is warmer than the other as the lead goes in, say from being in the sun, that side will cool differently than the other, which leads to different amounts of lead in one side than the other.


As for current. If this happened one time, then sure current is the easy explanation, but if you are always faster of starboard no matter which way you are pointing then its likely the boat. But its an easy test, just go out at slack tide and see how the boat handles.

[lateral]

When I bought my boat it sailed like a pig, anything other than downwind.
It wouldn't point anything like it should, and the rudder stalled out quickly.
The sails were baggy & everything unbody was wrong.

I bought a new set of class sails & new rigging.
I refoiled the keel, Naca profiled the rudder.

It is a different boat upwind. Now it has a groove, around mid 30°'s apparent.
Profound! Rudder stalling all gone but for unreasonable AOT's.
Just wish I had done it progressively to see what was the biggest contributor.
Difference on tacks can be noticeable, working on that, this thread is a great help. Thanks.