Thank you for all the replies!!! Greatly appreciated!
OK, so I just came back home after a pretty interesting day both in and out of the water!
First off, I went for another dive and looked for funny
things - which is quite a challenge since visibility is about one foot and the water was about 10degC (50degF).. nothing strange. I felt like a navy
clearance diver... Checked prop angle again, nothing amiss.
Nothing snagged on the keel
(rudder is also about 2m deep, so quite possible..) ... all good apart from being really cold (I wish I had a hood
on my wetsuit!)
Fired up the engine and went for a lap, no joy, still same numbers.
I did check my email
on my phone
and read the earlier replies on CF and tried a few things including measured distances and observing the location of the bow wave at full speed. Using these other methods gave me different numbers! Also - the bow wave was roughly 3/4 of the way to the stern... something smells fishy.
OK, so back to the mooring
, tie up, dry off, dinghy
back to car, drive back, pack blah blah blah and drive to chandlery
"Please give me that handheld GPS
$424 later and armed with a Garmin
GPSMAP 78stc, drive back home, pick up dinghy
, drive to boat ramp
blah blah blah...
Lo and behold! It turns out that the mounted Maretron GPS unit twice the price
of the handheld unit displays almost EXACTLY HALF the speed of the handheld!!!! Bizarre!!!
So hitting the maximum throttle-stop and at 3000 rpm, I managed 7.4
knots under power today according to the handheld.. WOOHOO!!! Big sigh of relief!!!
Now considering that the Maretron GPS has no real configurable settings or calibration etc and the display is using the right units etc - what on earth is with this half speed business!?!?
Thank you all again, I can finally have a good night of sleep!!!