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.įor those of you who hate math, the "Theoretical Performance Chart" is designed to simplify the math to one division step. I use "efficiency loss" as it describes total system performance. The difference between this number and one (1.00) is efficiency loss, usually expressed as a percentage (10%) and commonly called "slip". That number, divided by TMS yields efficiency expressed as a decimal (.90). 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. ![]() This number is the absolute maximum the boat could run with that particular pitch prop and engine gear ratio. ![]() If the data plugged into the formula is accurate, the math doesn't lie. This is straight math and not subject to debate. 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. Hull size, design and weight, horsepower and engine setback and height are irrelevant in these calculations. The baseline, TMS (Theoretical Max Speed), is based on WOT (Wide Open Throttle) RPM, gear ratio and (effective) prop pitch. With these basic truths in mind, you must also understand another important factor. For example, you could raise the engine to the point where the prop can't adequately lift the bow of the boat, so speed slows. These variables interact and a change in one may adversely affect another. Nosecones with low-water pickups allow raising the engine higher while maintaining sufficient water pressure to keep the powerhead cool. Lower unit drag may be reduced by raising the engine, while adding a more hydrodynamic "nosecone" to the gearcase housing will reduce drag at higher speeds (but may increase drag at slower speeds) and a condition called "blowout", which occurs at speeds from the high 70s up. Hull drag can be reduced by lifting more of the hull out of the water (thereby reducing wetted surface), or by blueprinting the last couple of feet of the running surface. ![]() Careful reworking of the blades can reduce prop slip that is directly attributable to the prop. Some of these variables can be controlled and some cannot. * Aerodynamic lift generally decreases hull drag * Aerodynamic drag is a factor, but usually a small one until speeds approach 100 MPH * Lower-unit drag is a function of gearcase design and engine height * Hull drag is a function of wetted surface (area) * A prop of "X" pitch will move the hull forward "X" inches if there is zero slip The theory of optimum boat performance is relatively straightforward, but affected by many variables: Perfecting boat setup requires the understanding of a few simple "truths" before you can begin. ![]() It can be expensive, and often is, if pursued to the last MPH. Each hull design has characteristics that limit its top end speed capabilities with a given amount of horsepower.ĭone properly, setup is a time consuming process. "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. Top-end speed is the yardstick by which we usually measure a boat's performance, but it should not be the objective. "You can make neither a silk purse out of a sow's ear nor a race boat out of a fishing boat, but you can come close"ĭo 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.
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