What RPM should I be running?

[boatingdudebmk]

Hey iBoats

I am new here, and I need some advice on what RPM my boat should be running at. I know there is a specific range of RPM that any engine should be running under load. I have a Evinrude 25 hp ELCOC (pretty sure thats right) Its a 1985 and it will be mounted on an Addictor Mini Boat, 8-9' Long, 4'4" Beam. Boat weighs 300 dry, so with 6 gallons fuel, small lawn tractor battery for electric start, and the engine (100-125 lbs roughly) and two passengers, it should run about 775 lbs. I haven't run the engine on the boat yet (engine currently on boat went under) and have to get a tach reading. What prop should I be looking for? (pitch and diameter, and a recommendation of material [stainless, aluminum]) I will be running in muddy bottomed water, and when I get a trailer, in lakes around my area. Thanks so much!

 

[jsarnoski]

Re: What RPM should I be running?

Get the manual for the engine you've got, it will have the manufacturer's rating for most efficient RPM, and also should have a chart for best all around prop pitch based on boat size/weight.

Think of it as a car; you always want it at the most efficient RPM to get best torque vs fuel efficiency; but imagine instead of the transmission you only get to pick one gear (the prop). Some props will improve torque and hole shot as if you are in 1st gear, while others are for top spead and fuel efficiency like cruising in 5th. In general though the manufacturer's chart for the engine will define the best all around prop to use (kind of like 3rd gear).

Once you have the baseline experiment and try out new things, props are not that expensive (comparatively) and it can't hurt to have a spare anyway. Also for these little boats, I've found so many other variables more important to performance that changing the prop. Get the basic and tune it by adding a whale tail or just adjusting the wieght/balance in the boat to get the handling you want.

One other thing though if you are running in shallow/muddy water. Keep an eye on the **** stream from that outboard. And keep some tools and a spare water pump in boat (along with new sparkplugs) in case you gum it up in the silt. Not much can go wrong on outboards, but fouling the plugs and gumming up the water pump are 100% likely atleast once during the time you own it... and you don't want to be stuck just becuase you forgot $20 worth of spare parts.

Here's a pick of me stuck on the San Francisco tidal bay... good thing it stalled out becuase the cooling tubes were full of silt and any more running would have cooked it! I had to tow it back in, but if I had a socket wrench I could have opened it up and cleaned it out before the tide came back in.