I wouldn't touch it the difference you are seeing is down currents and the probe is telling you that. it's not so much what the numbers are but repeatability to what's working .Gps speed is totally different than speed over water some guys use it but again it's all about repeatability . You will learn to trust the numbers .
I had that problem on two Merc drives had to pull the engines and transom brackets . A lot of work for a two bit seal got lucky the pins were not corroded they cost big bucks.
It is a bracket with a scale and line and ball you hang it over the side when trolling . A lot of us started with the set up and I think they still make it to day. Basic cheap and they work fairly well.
Good points Sk8man great unit to have. Early spring in the skinny water no so much early summer offshore looking for temp breaks , cross currents down deep the unit is invaluable . Late summer early fall with the pre stage and later staged fish not so much. Would I go without one not really .
I guess I am not the only old fart around here I grew up with those things they were called June Bug spinners . We trolled and drift fished them with night crawlers in Erie out of a old cedar strip with 7.5 Evinrude
The 4 blade I used was made by Turning point it's the Hustler model is compressed aluminum with a stainless profile . I talked to several owners and everyone said they got a speed increase and better control. There is no magic prop out there lots of variables involved
I went from a 3 blade on a Thompson 4.3 IO the 4 blade picked up 7 mph and I could hold the boat on plane at 3000 rpm .Don't have to use my trim tabs anymore. Great improvement for me on my rig anyways but they are all different.
Any opinions on what a 9.9 Yamaha electric start high thrust long shaft tiller 2004 with low hours is worth ? Looking at one looks clean and in good shape.
I dropped one off the boat in the driveway bought another boat and made a double keel out of it . Kind of took the hurt out of it a keel sells for $70 .
My Garmin Vision card , Blue Chart and Canada Lakeview HD cards work well on Lake Ontario ,Lake Erie , And Lake Simcoe for ice fishing . Use them on a 4210 ,4212 , 720s , 421s , and a Echo 70s and Montana 600. Great detail more than happy with the product and service. Sorry it didn't work out well for you.
That's why I like my Brutes the flex in the booms is very forgiving on cables and probes a weights when the water is rough and choppy . Any solid boom rigger is hard on the boat and the gear.
Anything over 18" is overkill on a boat your size . We run two boats a 20 and 22 ft . Standard 18' Amish bags work fine on both boats a 4.3 & 5.7 inboard outboards. My two cents.
I live on a farm and it is tickville central a drop of gasoline will make it let go. I keep the ticks in a jar for further testing should any health concerns arise .