HotTommy
Lieutenant Junior Grade
- Joined
- Mar 15, 2013
- Messages
- 1,025
Folks often come here asking advice or seeking to know if their boat is set up right. I offer the following as one person's experience with three different setups on one boat. Here is are the basics. The boat is a 25 year old pontoon boat with two U-shaped logs and three sofas. My usual load is 4-5 adults plus ice chests and gear. I use it primarily for cruising, swimming and towing a two-person tube.
The first configuration used a 90 HP two cycle engine with three bladed aluminum prop (unknown pitch) and a 20" shaft. At that time I had a large canvas top that likely produced considerable drag. Top speed (GPS) with a light load was 18 MPH. Towing a tube in a hard turn it would slow to 12 MPH. RPM was unknown due to inop tach, but the engine sounded like I expected it to sound.
The second configuration was with a 150 HP two cycle engine with the same three bladed prop as before and a 25" shaft. The bottom of the boat was skinned, the engine pod was replaced with a stronger one and I replaced the canvas top with a more streamlined flat aluminum top. Top speed with a light load was about 27 MPH. Towing a tube in a hard turn would slow to 21 MPH. RPM was again unknown. I suspect some speed was lost due to the engine sitting too deep in the water.
The current configuration is a 115 HP four cycle engine with a 20" shaft and a four bladed aluminum prop (14.8" dia x 15" pitch). Top speed with a light load is 28 MPH at 5900 RPM. I have not towed a tube yet, but I expect it to be as fast or faster than the 150.
Given the unknown prop and RPM numbers for the first two configurations, it is hard to compare results precisely. But it seems obvious that horsepower alone is not a good predictor of boat performance. If the shaft length is wrong or the propeller is wrong, performance can suffer while fuel useage does not decline. It seems well worth the effort to find out exactly you have to optimize performance and fuel efficiency.
One last observation for folks who, like me, ran an older engine. My new 2015 engine includes an engine monitor that provides a variety of readouts including fuel useage in gallons per hour (GPH). I charted the GPH versus GPS speed at the full range of engine RPMs. I observed that for my boat and engine combination, I get about 5 MPG from 1500 to 3000 RPM. Above that the MPG drops quickly to about 2.5 MPH at 5500 RPM. So if you don't know the data for your boat, you might want to use 2500-3000 RPM as the speed that provides the best combination of range and speed.
The first configuration used a 90 HP two cycle engine with three bladed aluminum prop (unknown pitch) and a 20" shaft. At that time I had a large canvas top that likely produced considerable drag. Top speed (GPS) with a light load was 18 MPH. Towing a tube in a hard turn it would slow to 12 MPH. RPM was unknown due to inop tach, but the engine sounded like I expected it to sound.
The second configuration was with a 150 HP two cycle engine with the same three bladed prop as before and a 25" shaft. The bottom of the boat was skinned, the engine pod was replaced with a stronger one and I replaced the canvas top with a more streamlined flat aluminum top. Top speed with a light load was about 27 MPH. Towing a tube in a hard turn would slow to 21 MPH. RPM was again unknown. I suspect some speed was lost due to the engine sitting too deep in the water.
The current configuration is a 115 HP four cycle engine with a 20" shaft and a four bladed aluminum prop (14.8" dia x 15" pitch). Top speed with a light load is 28 MPH at 5900 RPM. I have not towed a tube yet, but I expect it to be as fast or faster than the 150.
Given the unknown prop and RPM numbers for the first two configurations, it is hard to compare results precisely. But it seems obvious that horsepower alone is not a good predictor of boat performance. If the shaft length is wrong or the propeller is wrong, performance can suffer while fuel useage does not decline. It seems well worth the effort to find out exactly you have to optimize performance and fuel efficiency.
One last observation for folks who, like me, ran an older engine. My new 2015 engine includes an engine monitor that provides a variety of readouts including fuel useage in gallons per hour (GPH). I charted the GPH versus GPS speed at the full range of engine RPMs. I observed that for my boat and engine combination, I get about 5 MPG from 1500 to 3000 RPM. Above that the MPG drops quickly to about 2.5 MPH at 5500 RPM. So if you don't know the data for your boat, you might want to use 2500-3000 RPM as the speed that provides the best combination of range and speed.
