Pontoon Boat performance
I participate in several pontoon boat forums/groups. A topic that comes up every day is the performance (or lack thereof) of pontoon boats with outboard engines. I've grown tired of repeatedly presenting much the same information to people who didn't take time to search first to find the answers to their questions, so I am going to address this matter here and simply link to this in the future.
First, a word or two about me. I am not a marine engineer, but I am an aerospace engineer and retired pilot who has an strong understanding of how fluids (e.g., air and water) flow, how objects move through and around them, and how engines and propellers work. I owned my first boat more than sixty years ago and I have owned and operated many since then. As of the day of this writing, I own two pontoon boats - an old one that I refurbished several years ago, and a newer Bennington two-log with a Yamaha 115 outboard.
Pontoon (and tritoon) boat performance varies greatly from boat to boat. It is affected by the number of logs, the number of lifting strakes, the length and weight of the boat and its load, and the environment in which it is being operated (e.g,, temperature, altitude, winds and currents). I will also note that for the rest of this article I assume the boat is properly configured (e.g., correct engine leg length, correct mounting height, etc.). For the above reasons it is rarely useful for one boat owner to tell another how to maximize his boat based on the other's experience. As you will see below, a small difference can produce a very noticeable difference in performance.
Here are some terms I will be using:
Here are three basic principles that determine the maximum speed of a pontoon boat with a displacement hull (i.e., does NOT lift up on strakes and plane like a bass boat).
The most common questions involve the relationship between wide open throttle (WOT) RPM, maximum boat speed and propeller pitch. Most of those questions can be answered by understanding the relationship between power required and power available as illustrated in this diagram.
First, a word or two about me. I am not a marine engineer, but I am an aerospace engineer and retired pilot who has an strong understanding of how fluids (e.g., air and water) flow, how objects move through and around them, and how engines and propellers work. I owned my first boat more than sixty years ago and I have owned and operated many since then. As of the day of this writing, I own two pontoon boats - an old one that I refurbished several years ago, and a newer Bennington two-log with a Yamaha 115 outboard.
Pontoon (and tritoon) boat performance varies greatly from boat to boat. It is affected by the number of logs, the number of lifting strakes, the length and weight of the boat and its load, and the environment in which it is being operated (e.g,, temperature, altitude, winds and currents). I will also note that for the rest of this article I assume the boat is properly configured (e.g., correct engine leg length, correct mounting height, etc.). For the above reasons it is rarely useful for one boat owner to tell another how to maximize his boat based on the other's experience. As you will see below, a small difference can produce a very noticeable difference in performance.
Here are some terms I will be using:
- Speed - as measured by a reliable GPS source (and may include effects of winds or currents if present)
- Power - the amount of energy transferred from a device per unit of time. In the case of an outboard engine it indicates how rapidly the chemical energy in fuel can be converted to mechanical energy at the propeller and is commonly expressed in units of horsepower.
- Propeller pitch - typically measured in inches, it is the hypothetical distance the propeller would move forward through the water in one revolution if their connection was 100% efficient. For example, a wood screw with ten threads per inch would advance 1/10 inch with each turn as the connection between the wood and screw is 100% efficient.
- Propeller slip - an indication of how inefficient a propeller is at moving forward through water. Explanation
- Propeller slip calculator - an online tool to quickly calculate propeller slip Sample
- Gear ratio - an indication of how much the propeller speed is reduced below the engine speed. For example an engine turning 4,000 RPM with a gear ratio of 2.0 is turning the propeller at (4,000 / 2.00 =) 2,000 RPM.
Here are three basic principles that determine the maximum speed of a pontoon boat with a displacement hull (i.e., does NOT lift up on strakes and plane like a bass boat).
- The amount of power required to move a boat at a specific speed is determined by the total drag (wind and water) acting on the boat at that speed. The amount of drag is a function of the square of the speed. So when a boat doubles its speed from 10 MPH to 20 MPH, it's drag increases by a factor of 4.
- The amount of power provided by an outboard engine typically increases linearly from about 1,000 RPM to about 4,500 RPM and then gradually levels off or declines as it approaches its maximum rated RPM (e.g, 5,500 - 6,000). See the green line on the chart below.
- A propeller optimized for max speed has a pitch that allows the engine to operate at its maximum power output RPM while the boat is traveling at the speed where the total drag on the boat requires that amount of power. So if the engine produces a maximum of 115 horsepower at 5,500 RPM, and drag on the boat requires 115 horsepower to travel at 25 MPH, the optimum propeller is the one with pitch that advances it through the water at approximately 25 MPH when the engine is turning at 5,500 RPM.
The most common questions involve the relationship between wide open throttle (WOT) RPM, maximum boat speed and propeller pitch. Most of those questions can be answered by understanding the relationship between power required and power available as illustrated in this diagram.
In this chart the green line represents a typical power output for a 225 horsepower engine. The three other lines represent the power required to propel three different boats through the water by that engine using an efficient propeller. As the speed of the boat is tied to the RPM of an efficient propeller, you can consider the engine speed to also represent the boat's speed through the water. So the boat represented by the purple line will stop accelerating at about 4700 RPM because the engine cannot provide enough power to go faster. The next boat (blue line) will achieve a higher speed because it has less drag and does not demand all the engine can provide until a higher RPM and speed. The third boat (red line) has even less drag and can go even faster with that same engine. All of the above assumes an efficient propeller with little slip.
So, here are the usual questions and answers:
So, here are the usual questions and answers:
- I'm in the purple boat and my boat only reaches 4700 RPM at WOT at XX MPH. Can I go faster with a different propeller?
- Answer: The chart shows your engine is already producing it's peak power, so RPM is not a problem. But it is possible your propeller is not as efficient as it could be and may be wasting some of the engine's output. The way to find that out is to use the Slip Calculator to find out how much your propeller is slipping. If it is in the normal range (5-20%), the propeller is ok and your boat is not likely to go faster unless you find some way to reduce its drag (e.g., clean the logs, reduce weight, etc.). But if the propeller is slipping much more (e.g., 50%), you likely would benefit from a propeller with less pitch. Reducing the pitch will allow the speed of the boat to better match the speed of the propeller. It will also reduce the likelihood of cavitation that can damage the propeller if it slips too much.
- I'm in the blue boat and my WOT RPM is 5500. The engine manufacturer says the WOT range is 5500-6000. Should I change props to get my RPM closer to 6000?
- No. As you can see from the chart, engine power actually decreases above 5500 RPM. Going to a different prop to achieve a higher RPM will allow the prop to spin faster, but that will not cause the boat to go any faster. And it may be harder on engine components.
- I'm in the red boat and I'm getting only 35 MPH at WOT and 6000 RPM. My buddy says he is getting 42 with a similar boat. Can I get more speed by changing from a 14 pitch prop to a 17?
- Maybe. As your engine produces more power at lower RPM there is the potential to gain some speed. But the increase is not likely to be 7 MPH. As drag (and power required) increases with the square of the speed, a 20% increase (from 35 to 42) would require at 44% increase in power, and there is not that much power to be gained by reducing RPM. . . . If gaining even a little speed is important enough to buy a new prop, your best path would be to calculate the slip on your current propeller. If it is very low (less than 10%), then go up one step in pitch. If it is higher than that, a higher pitch propeller is more likely to increase slippage than boat speed.
- I'm in the red boat and last year I was seeing 32 MPH at 4700 RPM (WOT) and this year its down to 28 at 4500 RPM. What's wrong?
- The red line on the chart represents both the total drag and power required to propel the boat through the water and wind. Total drag can increase over time for a variety of reasons - increased weight (e.g., more equipment in storage, water in a log, more people on boat, etc.), buildup on the logs (e.g., algae, mussels, minerals, etc.), and wind drag (bigger bimini, stronger headwind, etc.). If any combination of these things increase drag, the boat will go a little slower and that will cause the propeller to turn a little slower. That can reduce the engine RPM, and as the chart shows, this engine produces less power as the RPM goes below 4700. So in this case it is likely that a small but significant increase in drag caused a slow down that subsequently caused a decrease in RPM and engine power. This caused a further slow down until the total drag again matched the power available.