Sailing faster one side than another

[tillergirl]

At the risk of providing Robin with ammunition about wooden boats, TG goes faster on port tack than starboad (about half to three quarters a knot) and points higher (the use of the term 'pointing high' is entirely relative). Accepting that it might be down to the way she was built and that I can't do anything about that, and, that I think I have got sail trim identical on each tack, is there anything 'adjustable' that I could play with?

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[Mirelle]

At risk of teaching grandmother....

(1) have you got an offset prop?

(2) where is the paddle wheel for the log? Is it in an even flow of water on both tacks?

Mirelle behaves differently on port and starboard tacks, but after allowing for the prop being on the port side, the remaining differences, I think, are due to differences in sea state - the tack that she does better on will vary from one day to the next depending on where the seas are coming from relative to the wind. the difference is most pronounced when sailing with a light air through a left-over slop.

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[tillergirl]

Central prop on this one, last one had a quarter installation. Paddle wheel is starboard side so will be deeper on the 'faster' tack. It was really noticeable yesterday on a very smooth Blackwater at slack water. What makes me convinced something is not set up right is that I noticed this tendancy before and it's always port tack which is better, both for speed and (it feels) for pointing. I accept that whether she does really point better is arguable as there is little to accurately judge this by.

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[penfold]

Things to consider:

Is the mast straight? Are the shrouds evenly tensioned?

Measure sheeting positions; are they the same on both sides?

HTH, cheers,
David

<hr width=100% size=1>What in the wide, wide world of sports is goin' on here?

 

[AlanPound]

Well, I read we are talking about sailing trim, rather than motoring...

Windspeed nearer the water 'backs' (comes from a lower compass heading) compared with windspeed higher up. This is as a result of the probably completely imaginary Coriolis effect that almost certainly doesn't cause your water to go anticlockwise down the plughole (in the northern hemisphere).

Anyway, this means that, given identical sail setting on each tack, one tack will (usually) provide more drive, and allow to point higher, than the other tack.

Another way to look at this is that you would need to organise different degrees of twist in the sail on each tack to compensate for this effect.


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[tillergirl]

Well I think so. I think I probably need to investigate a bit more closely to be certain. Prior to my ownership the mast was moved forward to counteract weather helm (so I have been told) so it might not be central. Everything looks and feels even, shroud tension, sheeting position etc. but I probably need to start to be a bit more exact. But it makes me wonder how fine some of the differences need to be checked.

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[tillergirl]

Well I think I understand! But wouldn't this mean that if it was port tack fastest heading East it would be starboard fastest going west or have i missed the point. I have noticed the difference recently heading both ways.

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[Avocet]

Avocet's log paddle wheel is on the port side and she always sails "faster" on starboard tack. She's a traditionally shaped long-keeler and I'm pretty certain the difference is due to the depth of immersion of the paddle wheel. If heeled more than 35 degrees on port tack, the speed drops to almost zero!

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[Mirelle]

Snap!

Mirelle, also a long keeler, has her paddle wheel on the starboard side and she is "faster" on port tack! It is "strategically located" in the seaboot locker, so I don't propose to change this! To be honest, it tells monstrous porky pies at the best of times, but is good fun for telling us how hard the tide is running, at anchor!

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[tillergirl]

Re: Snap!

So have we discovered the solution! As you say the accuracy leaves a lot to be desired. Which I wonder is closer, the fast speed or the slow speed.

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[LORDNELSON]

Just a thought: My boat sails better on starboard than on port, I am not only referring to speed but also pointing higher. Whilst not too worried about this I have always assumed that it was because the boat, at rest, has a distinct lean to starboard. Thus when sailing on starboard she is slightly "stiffer" than when on port tack and thus, under the same wind conditions, will sail better on starboard. Does your boat "lean" at rest?

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[Mirelle]

Re: Snap!

Next time I do a proper passage, with a couple of good long tacks in it, I am going to zero the trip meter and then compare it with my ancient towed Walker, which I do trust, for port and starboard tack, and build up a correction table for it.

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[qsiv]

I'd beg to differ - I dont believe that the Coriolis effect will be significant for boats with rigs of less than about 20m. You can get significant changes in wind direction and speed due to frictional losses in the air/water interface but these are unlikely to be significant in the conditions you describe.

What WILL be significant is the difference in apparent windspeed on differeing tacks when in a tidal zone - a boat sailing at 5kt in a tide of 2 knot can easily have 3 to 4 knots more on one tack than the other - this will be most noticeable in calmer conditions.

Having said all that - I'd say it is just as liley to be either rig or hull (you dont get more foulding on one side than the other?)

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[penfold]

Re: Snap!

Time to do some trials with a GPS. That will give a fairly definitive answer, as will using your walker towed log.

cheers,

David

<hr width=100% size=1>What in the wide, wide world of sports is goin' on here?

 

[tillergirl]

Re: Snap!

The trouble with a trial will be the inconsistency of the wind won't it over the period of the trial. On Friday I was trying to compare apparent wind speed against boat speed on each tack so that I could be exact. However, boat speed builds doesn't it; get her in a groove, less helm alterations etc and the faster she goes. What did surprise me was how the speed of the vessel built for a relatively minor increase of windspeed. Now I know this is obvious really as the wind has got to get the mass moving with all the resistance/friction etc but once over 8kts of wind, the increase in the speed of the vessel was quite significant for a small increase in wind speed.

Lord Nelson has a point that's worth considering - diesel tank is port side, water is starboard - diesel three quarters full at present say about 30 gallons, water is full - 25 gallons (yes I know that's the wrong way round, I should have more water capacity than diesel b ut that's how they were when I got her). Is diesel heavier? There's at least 5 gallons more on the port side anyway which would help.

the other point is that whilst it was pretty well slack water any tide on the starboard tack would have been meeting the vessel on the beam while on port tack it was more fore and aft. Whilst I was doing this the ebb finished and flood started.

As for the bottom, its still clean and newly antifouled from a week last Friday's scrub but it is interesting that starboard side seems to get more waterline fouling so I guess it gets more sun on it than port side. I need to think through whether this is possible and since we off to Madiera tomorrow, I shall ponder this problem for a week.



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[Avocet]

Re: Snap!

Diesel isn't quite as dense as water - about 0.8kg per litre compared to 1.0 kg / litre.

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[oldharry]

Either there is a fundamental problem with the structure of the boat - like the keel isnt on straight, or there is a warp in the hull - neither of which is very likely - or the mast is not set up vertically to the hull. This assumes that the mast itself is straight when under sail. Sight up the mainsail track with the sail down, then in a good F4, to check the mast has no uneven bends on either tack under load.

A quick and simple check; use any halyard that goes to the mast head, and take it down to one of the main shroud chainplates. mark the shroud exactly where it passes an identifiable fixed point - e.g the toerail cap. Now repeat the process on the opposite side to the same position. Not 100% accurate, but accurate enough to allow you to get the mast vertical enough for most cruising boats. Racing, where the last 1/8th knot counts needs more precise trimming.

Jeremy Howard Williams' excellent book 'Sails' describes this problem precisely, and as a direct result of the mast not being set up exactly vertically to the beam line of the hull.


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[castaway]

Re: Snap!

I have the same problem. Not so much "speed" as pointing ability. My boat sails far better on Stbd and points 5 to 10 degs closer, according to the instruments.

I've even checked the wind intrument to ensure that its reading is not biased either way.

If I force her to sail the same heading relative to app wind on port tack, the speed is well down, whether checked against the log or GPS.

"Fairweather" is quite a heavy boat, and I would not expect her to be effected by small inconsistancies. The only thing I can think of is that the 'screw' on the prop effects the flow leaving the hull differently on one tack to the other.

<A target="_blank" HREF=http://www.yachtsite.co.uk/fairweather>http://www.yachtsite.co.uk/fairweather</A>


Regds Nick

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[andy_wilson]

Paddlewheel transducer

Is it claen and not binding in one side of the housing or other?

Is it correctly aligned fore and aft?

If so, does changing the alignment on each tack lead to different readings?

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[vyv_cox]

It's worthwhile checking that the chainplates are in the same position on each side, by measuring from the mast foot or from the stem.

I agree with most other posters, having the mast vertical and straight is usually the key. Lower shroud tension is another cause, check on each tack that when you sight up the mainsail groove the mast is not sagging away. On a masthead rig shroud tension should keep the mast in column under all conditions, apart from a small amount of pre-bend from the babystay or forward lowers.

My boat performs quite differently with the water tank empty or full - 60 gallons at maximum beam under the starboard side saloon seats

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