"That's non-sense.
Jack
"That's non-sense.
Jackplates have (non-standard) amounts of set back dependant on the mfg.
You will see 4", 5.5", 6", 10" and 12" as "common" with generally 5-6" of vertical travel.
Assuming the motor was correctly fitted in the first place, such that the shaft length matches the transom, making slight trim changes can give you better "gas mileage" and possibly an extra 1 or 2 mph at top end.
There is no real formula to figure how much back or how far up. It often comes down to simply "fiddling" with the vertical adjustment and then checking the result using a fuel flow meter.
A sorta starting point is, for every 4 inches of setback you can raise the motor about an inch and a half - but that really is a pretty loose rule because literally every hull (even if the same model) will be a little different.
Now perhaps other Javelin owners have found that a 10 inch setback is optimal on that particular hull but I would like to see their numbers because I believe you would be talking about infantismal differences in performance if you compared a 6" and 10" plate....
However, if it's just pure speed you are after, then I do agree that the further back and the further up the better.
Mounting a motor, particularly on a bass boat hull, as far back as possible with so much vertical lift that the prop breaks the surface occassionally will give you MORE SPEED, but it's really hard on the motor and it guzzles gas.
But hey, if you have to beat everyone else on the lake to that weed patch on the other side (read pros with deep pocket sponsors who will pay for a new motor when they toast it as long as they win the tournament), it can be accomplished with a jack plate.
Purely for fine tuning you need nothing more than 4" setback and 5" vertical adjustment.
One final note. Since this is going on a hull that is 15 years old, I really would have the transom inspected. If there is even a hint of delamination it could spell disaster (and a motor at the bottom of the lake to boot).
A plate puts a significant amount of stress on a transom. Think of a teeter totter. If someone sits close to the middle it's easier to raise them than if they sit back at the seat - so conversely, the further out they are the more leverage they have - and that force increases exponentially.
So while a 4" setback may exert "x" amount of stress, an 8" setback doesn't exert 2x it's more like 8x the amount. If the transom is weak due to existing un-noticed damage, it will quickly part company from the rest of the boat (or alternately, the plate, with the motor connected, will pull the bolts through the transom and head for the bottom of the lake)..."