Speed on different tacks

[tommyt]

Boat is a 2004 Tall Rig

This continues to bother me. Rig is set every spring, halyard and eye are used to make certain that mast is straight. Loos Gauge is used to make certain that tensions are correct, and is done again once or twice in the season to check for any change.

We have done everything we can think of to answer our question but have never corrected our issue. The boat, based on ST60 boat speed, loves port tack much more than starboard. Given, using GPS, it is closer than ST60, but still different. With consistent wind it can be .5 to 1 knot slower by reading on a starboard tack.

Initially I thought the issue was rig set up. After a couple of years, and having others check the rig, we have not been able to answer the WHY. As GPS puts us closer in speed, I thought that it must be that as the thru hull for the speedo is on port side of centerline near bow, that the water pressure would be different on each tack so info would be different.

The question: Does anyone else have this quandary or have opinions on the issue?

[Ron Hill]

Tom : I believe that you have answered your own question.  Your Knot Log is not going thru the same undisturbed water when you are on a starboard tack. 
However, your GPS speed should almost be identical. The only "Drag" difference to the GPS might be the thru-hulls between the different tacks. 
Don't know what point of sail you measured your speed, but I suspect you are beating rather than sailing with a following wind!!

My Knot Log is in the hull (starboard side) next to the mast step which is more aft and almost centerline.

A thought

[Noah]

Lots of potential variables to rule out if you have different speed measurements on each tack (verified by GPS) and your rig is tuned identically and varified under sail load; — you still need to consider/rule out current, wave action/direction, unequal weight distribution below, angle of heel, keel or rudder shapes/foils not being symmetrical or placed off-center, jib leads and or tracks not being equal/positioned/centered...lots of things/issues to sort through...

[britinusa]

Jib foil tape? Ours seem pretty substantial, definitely not a symetrical shape on both sides of the foil.

I think it's referred to as a Luff Tape.

Paul

[Geoffreykwright]

Ron's suggesting of the log impeller wheel being off-center seems very plausible to me.  But digging deeper...is it more prevalent on one point of sail or all points of sail (upwind versus downwind)?   If its only upwind then the luff tape theory offered by Paul would be more likely than if it also happens down-wind (I'd think anyway).   Could there be some asymmetry on the keel creating more drag on one side that the other?  What sort of prop do you have?  if its fixed (not folding/feathering) could you be getting more drag on one tack than the other?   

[Ron Hill]

Guys : I believe that Toms' problem is not the off center as all knot logs are somewhat off center.  I believe that his is that the impeller is too far forward.  Like I mentioned , mine is opposite of the mast, not in the Vberth!!
Mine is were the hull "flattens" out not in where the "V" of the hull is prevalent!

A thought

[Geoffreykwright]

Ron - thanks.  This makes sense - NOW I understand.