Re: Hard on motor
A boat that is set up to achieve it's optimum RPM, in this case 5800, does NOT need to be run at that RPM to prevent coking or other lugging damage from occuring. HOWEVER, if something in the equation changes, in this case, increased drag due to the tow-behind, then it is doubtful that the motor could achieve the recommended wide open RPM. Again, the boat need NOT be running at or near wide open to be lugging.
Think of it as driving your car, towing a trailer on flat ground. You might keep it in 4th gear. Now you approach a hill. If you leave it in 4th gear you will lose speed and the engine may start pinging, even if you push down on the accelerator. So, you would downshift to bring the RPM up and maintain your speed. Same thing happens in a boat. If you increase the load (.ie the tow behind approximates going uphill) then you would need to downshift (.ie go to a lower pitch propeller), so that the motor COULD reach it's optimum RPM, even though you would certainly not want to do so with kids behind you. It's not about running the motor at top RPM, but rather propping it to run so it could potentially hit that mark. If it means swapping props around periodically, then so be it. As DHadley's tests show, it doesn't take long during a heavily loaded situation to cause damage.
That's why you'll often find propellers labeled as "ski props" or "power props". They're good at moving a heavier load. There are also shiftable propellers that achieve the same thing by altering the pitch to match the load. Some composite props claim similar benefit by flexing the blades under heavier loads to essentially lower the pitch.
