Uneven port/starboard boat speed

[Vespucci]

I’m looking for thoughts and suggestions that might explain a pronounced port/starboard performance difference in my boat.  In medium/heavy air the performance seems fairly symmetric, no obvious bias on one side or the other.  In very light winds, though, the boat seems to go faster on starboard tack than port. 
 

a few notes:

1. The boat hull and appendages were laser scanned a year or so ago in order to get an ORC rating (that’s another story). From analyzing the 3-d data it is hard to find any discrepancy in the hull / keel shape from either side. Certainly if there is any it’s less than 1cm.

2. The speed sensor is on the starboard side and clearly ends differently depending on the tack we are on, BUT, GPS speed doesn’t lie, we are consistently almost a knot slower, and pointing lower on port than starboard.  

3. to the best I can tell, the rig is symmetric and the rig tensions about the same on either side.

4.  having local ‘good sailors’ on board always results in their comment about the feel of the boat being different, with no mention or ‘leading’ from me.

so any thoughts, or suggestions as to what I might do to figure this out?
 

this is a very long shot, but I begin to suspect that the battens on my jib are the culprit. The boat has a 100% non overlapping jib on a furler, and so has 4 vertical battens on the leach, each almost 5’ long. These battens protrude significantly on one side of the sail, and not at all on the other.  See the photos below which show either side of the batten to get an idea.  I’m starting to think these battens are really disrupting air flow when they are in the “underside” of the sail - which would correspond to the port tack, but hardly disrupt flow when they are on the upper side of the foil  Seems a long shot as an explanation.

thoughts or suggestions?

 

[Varan]

Let's go old school... hoist the boat, get an old step ladder, put it on a flat surface behind the boat. Tie a string on the top rung with a weight in the other end, dangling above the ground. Step back and sight along the string and keel (or mast). Shim the ladder as necessary so they line up. Now sight the mast (or keel). Do they both align with the string?

Surveyors can probably do this better, but you get the idea. The position of the boat is not part of this equation. 

Edit.. whoops, forgot to mention the rudder. Mast, keel AND rudder should all align with the string. The rest is just buoyancy or weight.

 

[Steam Flyer]

A full knot in speed difference is a HUGE difference. Jib battens aren't causing that. It's something much more basic.

When did this speed difference start? What did you change right before that?

Normally I would suspect a mast that is not centered, and / or is not in column on the slow tack (or is curve into the slot on one tack, and bows out away from the slot on the other). This latter is not the kind of thing that shows up only in light air, though.

I know of one boat that had a loose patch of fiberglass skin just aft of the keel, on one tack it flopped down and open and was a water brake.

FB- Doug

 

[Varan]

J24 I sailed did this. Bottom pintle was replaced (badly). Keel was 15 degrees offset from the mast, keel column. Easy fix once you knew what the problem was. Back to string theory...

Ok, these two strings walked into a bar. One string said to the other, "Hey, do ...:.

 

[Kenny Dumas]

Surface roughness/ slime on the south / sunny side?

 

[nota]

sail the boat without the jib or borrow a different jib

does it still have a speed difference ?

then

try less strong battens or even no battens

 

[nota]

why does a 100% jib need battens ?

 

[Overbored]

because the designer 100% designed it that way

 

[ExOmo]

hitting on Varan's comment...is the keel centered/aligned fore/aft?

 

[AnotherSailor]

I agree that the battens will not make that big of a difference. Alignment of mast and keel + rudder is indeed important. What you describe really sounds like an issue with the keel, rudder or rig out of wack. You say you think the rig is straight / symmetric. How did you measure?

 

[sledracr]

I'd give another look at the rig. 

Take a 100' tape and run it up the main halyard.  take a measurement at each rail, abeam the mast.  The measurements should match.

If this is not a new problem, just for grins, measure from the side of the mast to the rail on each side.  See if those measurements match (and, yes, I have seen boats that were not symmetrical, and/or where the mast partners were not equidistant from the rails.  ObNote, I've also seen boats where the jib tracks were not the same distance from the centerline... that might be worth checking as well)

I'd also ask, just because... what kind of speed sensor do you have?  If it is a paddlewheel, it could be in very different flow on one tack from the other.  Or might not be facing directly fore-and-aft.

Failing all that.... as others have noted above, take a good look at keel and rudder when the boat is out of the water.  I've actually seen early-in-the-production-run boats with mis-aligned keels (heck, I've seen one where the keel was mounted a foot farther forward than it was supposed to be.  You'd think that would have gotten someone's attention....)

I do not think the battens are the problem

 

[sledracr]

Tie a string on the top rung with a weight in the other end, dangling above the ground. Step back and sight along the string and keel (or mast). Shim the ladder as necessary so they line up. Now sight the mast (or keel). Do they both align with the string?

Just a random note.... if you hang a plumb-line from the top of a ladder, no amount of shimming will change the angle of the plumb-line.  It'll be "plumb" no matter what angle the ladder is at....

 

[JMOD]

You checked for a straight mast right? Take your halyard And measure different points port and starboard. Are the jib tracks in the same positions?

indeed, check keel, mast and rudder alignment. 
 

is keel and rudder profile the same on both sides?

when sailing view up the mast, take a picture of the mast bend and sideway sag. Tack and repeat. 

 

[Mid]

(and, yes, I have seen boats that were not symmetrical, and/or where the mast partners were not equidistant from the rails.

more often than not .

 

[Chris in Santa Cruz CA]

I am liking the main only test as it might be possible the jib has a different shape tack vs tack in light air due to the batten pockets being asymmetrical in the way they are attached to the sail. My first thought was foil symmetry where rudder and keel were working together better tack vs tack. But I can easily see the jib issue going away with heavier loads. Kinda reminds me of light air drifters with wire in the luff to help with shape in light air. Attached(or not) air flow with big curves in jib and main in light air is where some voodoo shit can be happening. Rig alignment with the foils is also influential as well as CG in regard to all crap below decks of which an entire chapter could be written. 

 

[alphafb552]

Lots of good suggestions above, but the main rule is never assume that ANYTHING on board actually is symmetrical port to starboard!

That includes thing like spreaders, mast sheaves, etc, etc, etc

So measure everything you can think of and compare

 

[slug zitski]

I’m looking for thoughts and suggestions that might explain a pronounced port/starboard performance difference in my boat.  In medium/heavy air the performance seems fairly symmetric, no obvious bias on one side or the other.  In very light winds, though, the boat seems to go faster on starboard tack than port. 
 

a few notes:

1. The boat hull and appendages were laser scanned a year or so ago in order to get an ORC rating (that’s another story). From analyzing the 3-d data it is hard to find any discrepancy in the hull / keel shape from either side. Certainly if there is any it’s less than 1cm.

2. The speed sensor is on the starboard side and clearly ends differently depending on the tack we are on, BUT, GPS speed doesn’t lie, we are consistently almost a knot slower, and pointing lower on port than starboard.  

3. to the best I can tell, the rig is symmetric and the rig tensions about the same on either side.

4.  having local ‘good sailors’ on board always results in their comment about the feel of the boat being different, with no mention or ‘leading’ from me.

so any thoughts, or suggestions as to what I might do to figure this out?
 

this is a very long shot, but I begin to suspect that the battens on my jib are the culprit. The boat has a 100% non overlapping jib on a furler, and so has 4 vertical battens on the leach, each almost 5’ long. These battens protrude significantly on one side of the sail, and not at all on the other.  See the photos below which show either side of the batten to get an idea.  I’m starting to think these battens are really disrupting air flow when they are in the “underside” of the sail - which would correspond to the port tack, but hardly disrupt flow when they are on the upper side of the foil  Seems a long shot as an explanation.

thoughts or suggestions?

View attachment 388478

View attachment 388479

You interpret boat speed against wind angle 

check your wind instruments 

one knot of speed difference between tacks  is extreme 

your rudder tells the boat balance  story 

carefully measure  degrees of weather   helm ...port tack , starboard tack 

 

[IStream]

Also, just because nobody's mentioned it, does the boat have a list? Extra weight on one side or the other can lead to a performance differential between the two tacks. It's like you've got rail meat on starboard but none on port. 

 

[El Borracho]

check your wind instruments 

one knot of speed difference between tacks  is extreme 

Good points. Maybe you have not yet made accurate measurements. Perhaps do a test with wind instruments and fly ignored. The offset sensor can be a huge error. GPS is not a reliable indicator. There might be a consistent current in your sailing area that affects different tacks....there certainly is in ours. A half knot current could give a one knot difference. If you always sail the same course, like along a built-up shore, there could be wind aloft issues that affect the twist or different tacks.

I would doubt non-obvious symmetry or balance issues would cause such a difference. If the battens are damaging flow then leech tell-tails should be misbehaving.

Or maybe your helming is better on one side. My decrepit body is more comfortable on port tack...for example.