some tweaking help please

[Birdseye]

Well you are a group of sail tweakers arent you?

Coming back across the channel yesterday ( Bristol that is not the soft southern version) I was a bit surprised to find myself with far more rudder on than my failing memory suggests I should have had. But I am a bit rusty having spent the last summer ( wasted should be the word) on home improvements not sailing and the previous winter out of the water.

The boat is a Starlight 35 which is masthead rig and medium displacement. Barn door rudder so it has never rounded up even once. Anyway, the windex indicated 20kn on the beam. I had one reef in the main and one in the nice new laminate genoa. The log isnt installed but the gps was showing 7.5 to 8kn. I was mostly cross tide but possibly getting a bit of tidal help.

The pilot was doing a good job but the wheel angle was 45 to 90 degrees so had I not been single handed I would have put in another reef. Yet I remember racing the boat 2 years ago in such conditions with just one reef in the main and none in the genoa.

Lots of backstay tension on. Main well down the track and sheet out to the point where it bubbles behind the mast.

Any ideas? I suppose the windex could be under reading but no reason to suspect that. What else could it be?

 

[flaming]

My first thought is too much kicker, and possibly too much backstay.

Had you moved the genoa car to get the right lead with your reefed and eased genoa?

 

[Birdseye]

The genoa car was too far back . Set for the full genoa. They might in theory be towed but taking them to bits and getting rid of some friction is a winter chore I missed. But then why would an overpowered genoa lead to too much rudder?

The backstay was about as hard on as the shrouds. But I always tighten the backstay as wind gets up rather than loosen it. Not that it has a lot of effect other than on forestay sag.

 

[flaming]

Backstay on a masthead boat will rake the mast aft, which increases weather helm. Ease it a bit on a reach, as you don't need a bar tight forestay unless going upwind.

Overpowered anything leads to too much rudder! However, if your car was too far aft, rather than too far forward then I think it's unlikely to be the cause.

To solve your problem I'd have been checking I wasn't oversheeted, easing a bit of kicker, standing the mast up straight and making sure the halyards were nice and tight.

 

[Ingwe]

I would also have some serious doubts as to the wind speed that your instruments were telling you as you will almost certainly find that they are assuming that the "speed" that they are receiving from the GPS is at a vector pointing straight ahead where in reality that vector speed could be pointing anything up to 45 degrees off the line of the bow if you have a couple of knots of tide pushing you sideways combined with a bit of leeway.

It is possible to calculate an accurate TWS in reasonable amounts of tide but you need log speed, the vector of the COG and the SOG from the GPS as well as the apparent wind speed and I am not aware that any of the basic systems can manage this, so I wouldn't be shocked if your system wasn't under reading TWS by a couple of knots in the scenario you described.

 

[flaming]

I would also have some serious doubts as to the wind speed that your instruments were telling you as you will almost certainly find that they are assuming that the "speed" that they are receiving from the GPS is at a vector pointing straight ahead where in reality that vector speed could be pointing anything up to 45 degrees off the line of the bow if you have a couple of knots of tide pushing you sideways combined with a bit of leeway.

It is possible to calculate an accurate TWS in reasonable amounts of tide but you need log speed, the vector of the COG and the SOG from the GPS as well as the apparent wind speed and I am not aware that any of the basic systems can manage this, so I wouldn't be shocked if your system wasn't under reading TWS by a couple of knots in the scenario you described.

Oh dear, not this stuff again...

The wind that is calculated using the wind speed and the GPS speed (known as Ground WInd by most instruments and most sailors) is only of interest to the navigator and is of, and is absolutely no use to the trimmers, as in this case which is a "how much sail should I have up for a given wind strength?" question.
The best reading for this is known as True Wind by most instruments and most sailors, which is calculated using the wind speed and the log speed, and is the measure of the difference in velocity between the water and the air, which is all that powers your boat.

For example, my instruments tell me I have a true wind of 10 knots. There is a 3 knot tide. The ground wind range is between 7 and 13 knots depending on which way the tide is going. If you used ground wind as your trimming guide, you're going to get very different results! Use True wind and you'll always get a repeatable number, so you'll know that in 10 knots true I should be doing 6.5 knots at 40 degrees. Using ground wind is obviously not going to work for that use, as it's the same power available to your sails but a number that could be between 7 and 13. You can't get good trimming guides from that!

Given what Birdseye has said, my suspicion until he says otherwise, is that he was quoting apparent wind, and therefore probably actually overstating the wind by about 3 knots, maybe more.

 

[markhomer]

Well you are a group of sail tweakers arent you?

Coming back across the channel yesterday ( Bristol that is not the soft southern version) I was a bit surprised to find myself with far more rudder on than my failing memory suggests I should have had. But I am a bit rusty having spent the last summer ( wasted should be the word) on home improvements not sailing and the previous winter out of the water.

The boat is a Starlight 35 which is masthead rig and medium displacement. Barn door rudder so it has never rounded up even once. Anyway, the windex indicated 20kn on the beam. I had one reef in the main and one in the nice new laminate genoa. The log isnt installed but the gps was showing 7.5 to 8kn. I was mostly cross tide but possibly getting a bit of tidal help.

The pilot was doing a good job but the wheel angle was 45 to 90 degrees so had I not been single handed I would have put in another reef. Yet I remember racing the boat 2 years ago in such conditions with just one reef in the main and none in the genoa.

Lots of backstay tension on. Main well down the track and sheet out to the point where it bubbles behind the mast.

Any ideas? I suppose the windex could be under reading but no reason to suspect that. What else could it be?

im guessing even on a starlight 35 , the difference in crew weight between sailing singlehanded and crewed up for racing would have significant effect on rudder loading , just a thought

 

[MikeBz]

The genoa car was too far back . Set for the full genoa. ... But then why would an overpowered genoa lead to too much rudder?

If the genoa lead is too far back the the genoa will have way too much twist and thus will be producing a lot less drive. Thus the centre of effort will move aft (main doing most of the work) and more weather helm will result.

 

[lw395]

Just overpowered with no fat on the rail.
A lot of masthead rigs will have weather helm under genoa only in 20knots in these conditions.
7.5 Knots is pushing this kind of 35ft boat quite hard for 'white sails'.

Sounds like a fine way to spend your day!

 

[Birdseye]

I would also have some serious doubts as to the wind speed that your instruments were telling you as you will almost certainly find that they are assuming that the "speed" that they are receiving from the GPS is at a vector pointing straight ahead where in reality that vector speed could be pointing anything up to 45 degrees off the line of the bow if you have a couple of knots of tide pushing you sideways combined with a bit of leeway.

It is possible to calculate an accurate TWS in reasonable amounts of tide but you need log speed, the vector of the COG and the SOG from the GPS as well as the apparent wind speed and I am not aware that any of the basic systems can manage this, so I wouldn't be shocked if your system wasn't under reading TWS by a couple of knots in the scenario you described.

Ah but I'm a bit more old fashioned. The wind instrument is a non digital ST60 so the data goes direct to the display with an NMEA copy to the B&G plotter. And without the log in its housing ( it just weeds up so I leave the blanking plug in) the wind speed is apparent not true anyway. I have had a look today at the weather station data from the area and if anything it suggests my wind speed reading was a bit high

 

[anoccasionalyachtsman]

But then why would an overpowered genoa lead to too much rudder?

I think that you're imagining that because the genoa's up at the front it will be 'balancing' the boat around the keel and therefore easing weather helm. You actually have to consider both sails at once, and if the centre of lift is creeping back along the genoa it's also taking the combined centre of effort with it.

 

[Ingwe]

Ah but I'm a bit more old fashioned. The wind instrument is a non digital ST60 so the data goes direct to the display with an NMEA copy to the B&G plotter. And without the log in its housing ( it just weeds up so I leave the blanking plug in) the wind speed is apparent not true anyway. I have had a look today at the weather station data from the area and if anything it suggests my wind speed reading was a bit high

That's fine then, I was just pointing this out because I seem to remember that on the system I had on my previous boat (Nexus NX2) if it wasn't receiving a speed from a log it would default to calculating TWA and TWS using the GPS's SOG data and treating it as a straightforward log speed in the calculations, if you then plugged a computer into the system it gave you the additional option of displaying ground wind speed and angle which was calculated from the resultant vector of AWS/AWA and SOG/COG - which is what Flaming was talking about.

 

[lw395]

I think that you're imagining that because the genoa's up at the front it will be 'balancing' the boat around the keel and therefore easing weather helm. You actually have to consider both sails at once, and if the centre of lift is creeping back along the genoa it's also taking the combined centre of effort with it.

It's not just about the centre of effort and CLR, it's about the boat presenting an asymmetrically curved shape in the water once it heels.
This can effectively move the CLR a long way forwards at speed.
If you have a fair sized genoa on a fast reach, the boat may well have strong weather helm with the main flapping or even furled.
I'm not sure of the exact mechanics of it, but I have experienced it a few times.
On one occasion when I was fairly new to bigger boats, I was racing in the RTIR. The boat was set up well, storming upwind on port with a No2 genoa, very well balanced with just a touch of weather helm, main well powered.
When I had to duck a starboard tack boat. we eased the main a good handful and bore off, but then needed to bear off more and the boat would not, even with the main dumped.
I could feel the rudder starting to stall.
The jib trimmer was sure the genoa would help us turn but he was very, wery wrong. When I shouted at him to dump the @@@king jibsheet, the boat came more upright and we turned quick enough to avoid the other boat.

Some non-racers making a short trip will travel under just a genoa, so called 'hangover rig', don't bother to take the cover off the main. A lot of yachts will go up to say 60degrees from true wind like this without a lot of lee helm apparent. If you try to go much higher, lee helm becomes obvious.
On a broader reach, many boats are much more neutral on the helm with the kite up, once you're moving a bit.

 

[MikeBz]

Whilst the asymmetric underwater shape of a heeled hull does have an effect I'm sure it is far outweighed by the fact that the centre of effort moves laterally as the boat heels., so the forward drive is actually somewhere outside the leeward rail with a good dose of heel on. Imagine strapping a beam across the boat athwartships with an outboard motor attached a few feet outside the gunwale, then open the throttle sufficiently to drive the boat at 7 knots - it's going to try to turn pretty hard.

 

[lw395]

Whilst the asymmetric underwater shape of a heeled hull does have an effect I'm sure it is far outweighed by the fact that the centre of effort moves laterally as the boat heels., so the forward drive is actually somewhere outside the leeward rail with a good dose of heel on. Imagine strapping a beam across the boat athwartships with an outboard motor attached a few feet outside the gunwale, then open the throttle sufficiently to drive the boat at 7 knots - it's going to try to turn pretty hard.

And yet the boat is well balanced when similarly heeled but close hauled...