When you're trying to dial in your boat's setup to optimize performance there are few key pieces of information that you need to know before you start making any adjustments. One of which is how deep your lower unit and propeller are submerged when you're ripping down the lake.
This measurement is commonly referred to as your prop-to-pad measurement (PTP), or just "pad height" in short. It's basically a measurement of how far the propeller shaft centerline is below the the bottom running surface (the pad) of the boat.
The pad height is a critical piece of information because it tells us how much of your propeller is submerged vs surfacing during normal on-plane operation, as well as how high the water is contacting the lower unit. Both of these are critical to get correct if you're looking for optimum performance, but the magnitude of impact is much larger for higher-speed applications like bass boats and higher performance v-hull power boats. That being said, we're primarily going to focus on those here.
Let's first look at the proper way to measure the PTP, and then we'll discuss some of the ins and outs of adjusting your PTP by raising or lowering you motor, and typical rules of thumb for different applications.
Basic Propeller to Pad Measurement Procedure:
- Park your boat and trailer on as flat and level a surface as you can find.
- Adjust the angle of the trailing edge of the hull to be as level as possible by raising or lowering the tongue of the trailer using the trailer jack. Placing a bubble level along the last few feet of the bottom of the hull in front the motor works great.
- Now place the level on the anti-ventilation plate of the lower unit, and adjust the motor trim to level the lower unit.
- Next you simply measure and record the vertical distance from the ground to the trail edge of the bottom of the hull, and from the ground to the center line of the bullet of the lower unit where the propeller shaft is.
- Subtract the hull measurement from the lower unit measurement, and that's your current prop to pad measurement. Note that a negative number indicates that the propeller centerline is below the bottom of the boat, zero would be even, and a positive number means it's above the bottom of the boat.
There will be a ton of variation in what the ideal PTP will be based on hull design, lower unit design, gear load and weight distribution, amount of fuel onboard, etc. etc. However, below is are some general ballpark rules of thumb based on wide open throttle boat speed capabilities:
0 to Mid 70's: -3.5" to -2.5" below pad
Mid 70's to Mid 80's: -2.5" to -0.5" below pad
High 80's and Up: -0.5" below to +2.5" above pad
Now let's dive into some the details regarding what to expect when raising or lowering your motor.
Raising Your Motor Will:
- Reduce the wetted surface area of both the lower unit and the propeller that is in contact with the water. Less surface area = less drag = more RPM.
- Reduce the amount of bow lift generate by the propeller
- Reduce the amount cornering "traction" the propeller can support before blowing out or ventilating
- Improve the hole-shot (up to the point where blowing out starts becoming an issue.)
- At some point start raising your water pickups above the water line reducing cooling water pressure. On lite weight boats this is typically the limiting factor for how high you can go.
- Decrease mid-range cruising fuel economy
Lowering Your Motor Will:
- Increase the drag associated with having to drag more lower unit and propeller around in the water
- Increase the amount of bow lift generated by the propeller
- Increase the propeller "traction" in corners and reduce ventilation in tight cornering maneuvers
- Improve cornering response and tracking with the larger amount of lower unit in the water acting like a larger rudder to steer with.
- Increase the time to plane
- Increase water pressure.
- Increase mid-range cruising fuel economy
I hope this helps clarify the basic prop to pad measurement and some of the general implications of engine height and why you might want to adjust it.
In closing I'd also like to mention hydraulic jack plates. If your motor is direct-mounted to your transom or if you have a fixed or manual jack plate, then you're essentially stuck with selecting a single PTP engine height that can either do one thing awesome and everything else poorly, or a height where it can do everything Ok but nothing to the boat's full potential. This is where hydraulic jack plates shine. With a hydraulic jack plate you can raise or lower your engine height on the fly and achieve the best performance possible regardless of the situation or conditions. Obviously, being able to adjust the motor height at the press of a button in 2 seconds also saves an ENORMOUS amounts of time when testing vs having to stop and physically move your motor with tools.