(NOTE: This article has been edited from the version in B&WB: a few corrections, clarifications and changes due to HTML format.)
Do I have your attention?
Good! Let's set the egos aside and talk some basic mechanics, hydrodynamics, physics and math, then go to work on optimizing setup. Top-end speed is the yardstick by which we usually measure a boat's performance, but it should not be the objective. "I want to go faster" can be achieved by simply adding more horsepower and turning a higher pitch prop or more RPM, but the objective here is to make a particular hull/engine combination run as efficiently as possible. Each hull design has characteristics that limit its top end speed capabilities with a given amount of horsepower.
Done properly, setup is a time consuming process. It can be expensive, and often is, if pursued to the last MPH.
Perfecting boat setup requires the understanding of a few simple "truths" before you can begin. The theory of optimum boat performance is relatively straightforward, but affected by many variables:
- A prop of "X" pitch will move the hull forward "X" inches if there is zero slip
- A prop MUST slip to work effectively
- Drag increases prop slip
- Hull drag is a function of wetted surface (area)
- Lower-unit drag is a function of gearcase design and engine height
- Aerodynamic drag is a factor, but usually a small one until speeds approach 100 MPH
- Aerodynamic lift generally decreases hull drag
With these basic truths in mind, you must also understand another important factor. The baseline, TMS (Theoretical Max Speed), is based on WOT (Wide Open Throttle) RPM, gear ratio and (effective) prop pitch. Hull size, design and weight, horsepower and engine setback and height are irrelevant in these calculations. There are many formulas used to figure a boat's theoretical top speed, but the simplest mathematical statement is (Tach RPM * Pitch) / (1056 * Gear ratio) = TMS. This is straight math and not subject to debate. If the data plugged into the formula is accurate, the math doesn't lie. This number is the absolute maximum the boat could run with that particular pitch prop and engine gear ratio.
To figure out just where your boat's present setup is compared against the TMS, you need to know another factor: RWS (Real World Speed), or your boat's speed as measured by GPS, radar or timing over a measured course. That number, divided by TMS yields efficiency expressed as a decimal (.90). The difference between this number and one (1.00) is efficiency loss, usually expressed as a percentage (10%) and commonly called "slip". I use "efficiency loss" as it describes total system performance. A 10% efficiency loss is generally considered good performance for a typical performance bass boat, while 5-8 % is considered good numbers for a race boat.
For those of you who hate math, the "Theoretical Performance Chart" is designed to simplify the math to one division step.
IN REAL LIFE
Once you know where your boat is in the scheme of optimum performance according to the numbers, then it's time to work on set-up to try to improve the performance factor and reduce that "slip" percentage. The following techniques apply to any hull, engine and prop combination, whether a 12-foot aluminum jon boat with 9.5 HP or a 20-plus-foot ultra-high-performance bassboat hull with a 300 hanging off the transom. But there are no guarantees, nothing written in stone. Only real-world testing will establish optimum setup.
To do the job properly, the following items or accessories are required:
- An accurate speed measuring device such as GPS or radar gun
- An accurate tach
- A jackplate or some other method of readily adjusting engine height
- A water pressure gauge
- A notebook for recording test information
- An assortment of props (one or more additional props on hand can save trips to the prop shop or dealership)
You will also need the following information:
- Engine manufacturer's recommended operating range, peak HP RPM and rev limiter point
- Engine manufacturer's recommended minimum water pressure
- Lower unit gear ratio
- "Nominal" prop pitch (effective pitch is rarely known)
FINE TUNING SETUP
IMPORTANT: Remember to wear a PFD and hook up the kill switch.
Monitor the water pressure gauge closely.
Each test should consist of a minimum of two (2) runs at WOT, trimming to achieve maximum speed. If the boat begins to handle poorly (chine walk, loss of feel in the wheel, etc.), ease off the throttle and trim down. Repeat the test noting where performance degrades. Most pad v type hulls will require practice and careful driving when operating at their performance limits.
When RPM continue to climb with no increase in speed, the highest trim point for that particular setup has been exceeded. Once best performance is achieved, note RPM, speed and amount of trim used. Also note hole shot (B&WB uses 0-30 MPH stopwatch time as a standard), "roostertail" height and handling characteristics. The roostertail should be no higher than top of cowling for most setups.
This is a sample list of
problems that may be experienced and typical solutions:
1) RPM too high (and not overtrimmed) - a higher pitch prop is indicated
2) RPM too low (and not undertrimmed) - low engine height and/or prop pitch too high
3) Roostertail too high (and not overtrimmed) - engine too high or excessive prop slippage (change/rework prop)
There is insufficient room to list all problems and their solutions in this article. The true source of a problem can and will vary from one boat to another even if they are "identical" and must be addressed on an individual basis.
If the engine specifications have not been exceeded, raise the engine 1/4-1/2" and retest. Note performance characteristics. Repeat this process as necessary until water pressure reaches minimum allowable and/or further trim yields RPM rise without a corresponding speed increase.
You may want to try various pitch props. Typically, going up or down 1" in pitch can change engine RPM by 150-200. Going to a smaller pitch prop should make the holeshot quicker as well as increasing WOT RPM. Likewise, a higher pitch prop can degrade holeshot and decrease WOT RPM. Sometimes, however, a higher pitch prop will carry the hull's weight better, resulting in a gain in both WOT RPM and top speed. Only experimentation will reveal how your particular hull/engine responds to these changes.
Another factor to consider is prop blade style. In a nutshell, a three blade prop will typically run faster than a four blade of the same pitch. Conversely, a four blade will produce superior holeshot, handle better in rough water and have less steering torque than the three blade.
Once you have found the prop style you like, the engine setup where the holeshot is acceptable and top speed and WOT RPM are maxed out while maintaining minimum safe engine water pressure, you are at the optimum performance setup for this hull/engine combination.
Optimizing for maximum speed does not mean that total performance is optimized. A setup for top end often sacrifices holeshot, may cause porpoising at low and/or intermediate speeds and will degrade rough water ride. When set up for rapid holeshot and good overall top end handling, maximum top speed is usually sacrificed. Use of an hydraulic jackplate, allowing "on the fly" engine height adjustment throughout the operating range, can minimize or eliminate these issues as well as compensate for variations in loading.
Achieving optimum setup for maximum performance, although time consuming and often expensive, assures that you are getting the most out of your investment. Optimum or near optimum setup pays dividends in higher speed, improved handling and fuel economy.
Some boaters spend countless hours fine tuning their boat's setup, playing with different brands and styles of props, engine setbacks and so forth until they achieve what they feel is "Nirvana". Others are content to run their rig just as it came from the dealer. The majority of bassboaters will find a satisfactory compromise between these two extremes.
copyright Ken Cook 02/12/99, all rights reserved
Printed in Bass & Walleye Boats MAY/JUNE 1999
Reprinted by permission of Bass & Walleye Boats