Quote:
Originally Posted by Yankee29
Your motor length will depend on the setback of the bracket. It's an open ended question with too many variables.
For a baseline I'll give you specs of someone I know's rebuild. He had a 25" shaft motor, installed a hermco bracket and needed a 30" shaft motor to keep from blowing out when turning. There is a rule of thumb that for every foot the motor moves back you need to drop the motor.
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You're right that there are a lot of variables and one of the biggest is the prop! In general a prop without much cup in it will tend to "blow out" easier than one with a lot of cup.
The rule of thumb that's often quoted is that you have to
RAISE the motor an inch for every foot of setback, because the water flows upward after it clears the transom. The size of the boat and the speed you run may affect that. Both the BRP and Verado web sites say that optimum motor height is when the AV plate is above green water. Most dealers seem to mount the motors too low, I suspect because inexperienced operators will complain if the motor "blows out", even though that may be due to operator error (trimming motor too high), but if they mount the motor too low, there is less chance of blowout complaints, and most folks aren't savvy enough to realize they're losing performance due to excessive drag from a too-low motor!
In my experience with a 30" Hermco bracket on a 20, the 1"/foot-of-setback rule is not nearly enough! Attached pic shows the bottom of my AV plate at 3.25" above keel, AFTER I raised motor 2 holes (as high as possible) from where Don Herman originally mounted it. With motor at this height, the AV plate was completely underwater, so I had Don redrill the motor mount holes to raise it another inch, but AV plate is STILL under water when I'm up on plane at about 25 kts and fully trimmed out! Don't have any problems with blowout if running at or below 50% trim, where prop shaft is parallel to keel, but I'm running a good PowerTech 15x15 4B prop with a fair amount of cup. Bottom line is that the AV plate on my 25" shaft motor is now about 4.25" above keel, and it still needs to be raised at least another 1/2", so I would say there is a reasonable chance that a 20" motor would work
if you mounted it as low possible and run a good cupped 4B prop! Worst case might be that you'd have to buy a Bay Mfg. extension kit if it didn't work out, so might be worth taking a chance! Only downside is that your powerhead will be closer to the water than mine. The E-TEC cowl is very watertight and air intake is at extreme top of cowl at the back, so I wouldn't worry too much about an E-TEC but I don't know how the Merc cowls and air intake compare.