Uneven port/starboard boat speed

[TUBBY]

I'd go for the too close to the waterline/different depth on either tack option.

On the old 12s where the paddle wheels could be located comparatively deeply we would use separate paddle wheels on each side with a switch to select the leeward one all of the time.

 

[pete_nj]

Tubby - this seems like a reasonable possibility. I should ask my electronics guy if the dual setup is possible.  Perhaps we just need more crew forward of the shrouds!

 

[El Borracho]

I think the instrument should be turned off until is gives accurate speed. Sail another way until the botched boat design can be fixed. Either bore a hole on the centerline or install two transducers. 

 

[IMR]

Move the change from your left pocket to your right pocket. Turn your hat inside out. Oh last thing turn off the knot meter 

 

[Overbored]

He is using GPS , he knows the speed sensor is off read the first post again. he said the hull was laser scanned that would show bent keel. the battens could be the reason if there are 4 vertical battens tripping the air flow over the jib. if you had the same situation on one airplane wing it would not make the same lift on both wings. so why not on a sail airfoil. The sail with 4 vertical battens would have airflow changed on different tacks by over 2/3 of the sail. another factor might be the furler foil if it has two luff grooves on the foil , so not the same air flow over the sail on different tacks. both drag caused by the different shape and the loose of lift from the different set up will cause a speed change. if a different sail shape did not make a difference then why bother to trim the sail properly. In one design racing. just the slightest little thing will make a big difference in boat speed.  when passing another boat in light air you don't move anything until you have to, i find myself holding my breath so not to change the boat speed.

 

[AnotherSailor]

It is probably due to Russian interference with GPS signals.

 

[Frogman56]

Calibration under motor is not much use, best baseline is from full heel on 90 degree reach. Motor usually gives result 0.2 to 0.4 different, depending on transducer location.

 

[pete_nj]

Thanks Froggy - didn't know that. I suspect that given that the transducer is slightly off center line, I have an issue either way. In any case, I think I need to do what I can to minimize the issue, and possibly look to a different transducer setup. I suspect that with the triton setup, there is really no great solution here.

 

[Hitchhiker]

Calibration under motor is not much use, best baseline is from full heel on 90 degree reach. Motor usually gives result 0.2 to 0.4 different, depending on transducer location.

Read this a couple of times and not sure what calibration mode you are talking about, or if you are yanking some-ones chain.

@pete_nj  The generally accepted means of calibrating speed is to run a measured distance run (several times in each direction to counter effects of current) with instruments in the appropriate calibration mode.  If you run it at say, six knots over a mile that makes the maths easy and you can compare the timed run on a watch with what the instruments are displaying.  If you have an older system you make an adjustment as necessary to correct the speed number.  On newer systems you can make the Cpu do all the cyphering.

Btw, it really does sound like the speed sender is in the wrong location.  It ought to be closer to the front of the keel on centerline.  I have seen several J-111's in our area re-locate the speed sender.

For example: From NKE. 2.1 Principle of calibration : It consists in executing a course with your boat, with a true distance, D miles, that is known, and taking down the number of miles indicated by the log, L miles. Then, you calculate the calibration coefficient according to the formula : D / L. Example: The course measured on the chart between two sea-marks is : D = 1.43 MILES The number of miles indicated by your log for this course is : L = 1.10 MILES The calibration coefficient calculated is 1.43 / 1.10 = 1.30. To ensure the calibration is effective, you will execute a return journey, to cancel the effects of the current, and in excess of 1 mile. 2.2 Setting procedure of the calibration coefficient : To achieve a successful calibration, follow the indications below : ? Sail with the engine, on calm sea, with no wind and at slack water. ? Execute a return journey over a perfectly known distance.

B&G H5 system which is fundamentally the same as your Triton system.

 

[Varan]

screw the instruments. Line up with your sistership. Sail on one tack. Compare numbers and performance. Repeat on the other tack. Duplicate settings, and repeat again, and again, and again.

 

[Not for nothing]

take notes!

get another boat and go tack for tack, that would eliminate wind or tide conditions

Then check out the rigging , spreaders position , angle, top center and mast in a line , rig tension , This was a cause I had , turn out to be rigging

knot meter seem to be cover by others

rudder position and angle

 

[pete_nj]

Thanks all.

The issue is simply that the transducer is inconsistent from tack to tack. The boat performance is not in question. In slack current conditions, we sail at the same sog, heel angle, and AWA on both tacks. Our numbers match the polars. The issue is that even after calibrating the boat speed in a process similar to what @Hitchhiker describes, we get a large difference in the boat speed from tack to tack.

I do suspect it is related to the transducer position. Perhaps the offset position also leads to uneven wear of the paddlewheel over time, exaggerating the issue.

 

[Vespucci]

He is using GPS , he knows the speed sensor is off read the first post again. he said the hull was laser scanned that would show bent keel. the battens could be the reason if there are 4 vertical battens tripping the air flow over the jib. if you had the same situation on one airplane wing it would not make the same lift on both wings. so why not on a sail airfoil. The sail with 4 vertical battens would have airflow changed on different tacks by over 2/3 of the sail. another factor might be the furler foil if it has two luff grooves on the foil , so not the same air flow over the sail on different tacks. both drag caused by the different shape and the loose of lift from the different set up will cause a speed change. if a different sail shape did not make a difference then why bother to trim the sail properly. In one design racing. just the slightest little thing will make a big difference in boat speed.  when passing another boat in light air you don't move anything until you have to, i find myself holding my breath so not to change the boat speed.

Thanks overboard, and thanks for noting the details in the original post, ie laser scanned hull etc.  Many of the comments are missing the challenge, some are helpful. As also mentioned in the post, this asymmetry ha been noticed by numerous (at least 5) hotshot skippers who have come on board, which rules out my own asymmetry as the culprit! Also, the asymmetry is noticed in a variety of locales with very different current conditions.  It’s not obvious what is going on.

i think remeasuring the mast and rig position is worth doing, as is sailing both without any jib and with a jib from another boat, preferably one that doesn’t have any battens. Hard to find such a sail though, not many boats of this size sailing around here. I’ll report back once I have done a bit more sleuthing,

thanks for all the input

 

[Frogman56]

Hitchhiker 

Calibration under motor almost always gets you something out by a material amount, as the flow around the hull and keel when heeled and with leeway is completely different. Quickest way to get close to ok is to find 12 knots of wind, smooth water, no current and reach at full heel and 90 degree twa, comparing to gps on both tacks. Job done in 20 minutes.

On, for example, a Farr type like f40 or Cookson 12, the difference motor vs sail is about 0.3 at 7.4 approx.

 

[IMR]

If you want accurate boat speed, you need two or better yet four sending units. With two you would have them port and starboard about 2 feet in front of the keep. With four you would have two more aft in the boat between the keel and the rudder again port and starboard. 
 

we found the at high speeds the bow sending units would pop out of the water. 

 

[Steam Flyer]

If you want accurate boat speed, you need two or better yet four sending units. With two you would have them port and starboard about 2 feet in front of the keep. With four you would have two more aft in the boat between the keel and the rudder again port and starboard. 
 

we found the at high speeds the bow sending units would pop out of the water. 

So, you need a correlating factor of air speed to water speed.

FB- Doug

 

[IMR]

It’s was only an issue above 20 knots and not all the time just when the bow was over a wave.  
 

but yes air speed correction factor would have been the fix if we could get the calibration right. Time to top off the blinker fluid. 

 

[Hitchhiker]

Hitchhiker 

Calibration under motor almost always gets you something out by a material amount, as the flow around the hull and keel when heeled and with leeway is completely different. Quickest way to get close to ok is to find 12 knots of wind, smooth water, no current and reach at full heel and 90 degree twa, comparing to gps on both tacks. Job done in 20 minutes.

On, for example, a Farr type like f40 or Cookson 12, the difference motor vs sail is about 0.3 at 7.4 approx.

Fascinating.  I have to say in 35 plus years of racing, working on race yachts, including installing race instruments and navigating race yachts, this is the first time I have ever heard of this technique.

Something new every day, I suppose.

 

[Left Shift]

Hitchhiker 

Calibration under motor almost always gets you something out by a material amount, as the flow around the hull and keel when heeled and with leeway is completely different. Quickest way to get close to ok is to find 12 knots of wind, smooth water, no current and reach at full heel and 90 degree twa, comparing to gps on both tacks. Job done in 20 minutes.

On, for example, a Farr type like f40 or Cookson 12, the difference motor vs sail is about 0.3 at 7.4 approx.

Hmmm...Having owned, raced and tended to one of the mentioned boat types for quite some number of years, that difference never showed up in any of our calibrations or test runs.  0.3 knots is not something you fail to observe.  

 

[ctutmark]

Thanks Froggy - didn't know that. I suspect that given that the transducer is slightly off center line, I have an issue either way. In any case, I think I need to do what I can to minimize the issue, and possibly look to a different transducer setup. I suspect that with the triton setup, there is really no great solution here.

On j105s with a similar setup they see the same tack to tack differences. The tack where the knotmeter is on the low side is always the faster tack. Learn to accept there will be fast tack and a slow tack unless you have a system that will allow boat speed calibration with heel as well as speed