I'm a mechanical engineer, so I find this kind of thing interesting. It occured to me to figure out how hard an outboard is pushing on the back of the boat. To use round numbers I assumed 90 HP and 30 MPH.
One HP is 550 foot pounds per second. 30 MPH is 44 feet per second. So the calculation is 90 times 550 divided by 44 and the result is 1125 pounds. That's what 90 horsepower represents at 30 miles per hour. 1125 pounds of thrust.
At the other end of the spectrum, a 9.9 HP outboard pushing the boat at 6 MPH is putting out 619 pounds of thrust.
The slow speed case is displacement mode, plowing more of the hull through the water, and the high speed case is planing mode, plowing much less of the hull through the water. Hence the thrust only changing by a factor of two, while the speed changes by a factor of five.
I'm supposed to get my CD 22 this weekend, so it won't be too long before I see if these speeds are reasonable guesses. I take it easy on equipment, so I'll be breaking it in a while before doing any speed tests. My understanding is that outboards are rated at propeller shaft output, and I think I've read elsewhere here that these boats run at speeds close to these, so I'm sure I'm in the ballpark anyway.
I'm also aware that 30 MPH would require SMOOTH water, careful trimming and a LIGHT load in the boat. I'm sure 20ish is a lot more reasonable cruising speed. Still, it's fun to get a rough idea of the forces involved on our little plastic boats.
Jeff Lastofka
One HP is 550 foot pounds per second. 30 MPH is 44 feet per second. So the calculation is 90 times 550 divided by 44 and the result is 1125 pounds. That's what 90 horsepower represents at 30 miles per hour. 1125 pounds of thrust.
At the other end of the spectrum, a 9.9 HP outboard pushing the boat at 6 MPH is putting out 619 pounds of thrust.
The slow speed case is displacement mode, plowing more of the hull through the water, and the high speed case is planing mode, plowing much less of the hull through the water. Hence the thrust only changing by a factor of two, while the speed changes by a factor of five.
I'm supposed to get my CD 22 this weekend, so it won't be too long before I see if these speeds are reasonable guesses. I take it easy on equipment, so I'll be breaking it in a while before doing any speed tests. My understanding is that outboards are rated at propeller shaft output, and I think I've read elsewhere here that these boats run at speeds close to these, so I'm sure I'm in the ballpark anyway.
I'm also aware that 30 MPH would require SMOOTH water, careful trimming and a LIGHT load in the boat. I'm sure 20ish is a lot more reasonable cruising speed. Still, it's fun to get a rough idea of the forces involved on our little plastic boats.
Jeff Lastofka