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RPM, speed and fuel consumption question for BF225

W

wlin98004

Member
My boat is back into the water and running smoothly. I am a happy camper. Hopefully new issue won't come up soon. The fun of owning a boat.

Here is something that puzzle me. Before my problem, I normally cruise about 4200RPM at about 20 knots. WOT RPM is 5500 RPM. In the last month to fix my engine problem I have cleaned IAC valves, took off and cleaned port side manifold, adjusted the throttle cable a bit, checked and greased up the lower unit and propeller shaft. Now the engine runs more smooth, by my RPM has gone up. Boat load is the same and no change in propeller. Now I have to run 4600RPM to cruise at 20 knots. My WOT RPM is about 6100RPM now. What would cause the increase in RPM to achieve the same speed as before.

I don't have a fuel consumption display or gauge. Does fuel consumption totally depending on RPM? Higher RPM means higher fuel consumption?
 
If nothing changed in weight or propeller pitch, did anything change on the outside surface of your hull to create more drag? I assume similar weather conditions and currents?

Your outboard will plane based on hull speed and to reach plane, the rpm/torque reaching an engine speed spinning the propeller fast enough. If the changes you made improved engine operation, in most cases you should have gained torque and seen an improvement in plane speed.

No change in speed measurement- are you using a GPS to measure speed?
 
Strange sounding set of circumstances.

Like NSDON, I too was thinking dirty hull. But it sounds like you're trailering.

Could it be that it was under propped and your tinkeeing has increased engine power enough to have it show up? I'm no guru aa to sizing props for boats so a questiin, not an answer.

How old is your prop? A worn hub might slip and cause something like this but I would think it would also effect take off and have the condituon get increasingly worse.

You could match-mark the inner and outer prop hub sections with a Sharpie and monitor for slippage that way.

Just guessing but I would expect fuel consumption to go up with engine speed but maybe not significantly in this case. I really don't know.

Good luck and please reporr back if you find an answer.
 
For consumption, it’s a combination of load and rpm- on a planing hull, your outboard is pushing a lot of water at slow speed, and when it gets on plane, it’s pushing a lot less water, and burning less fuel until you push it faster. I’d say the most fuel efficient speed is just above planing speed- other than a low speed off plane. Measuring it in mpg or gallons per hour? Or enjoyment per gallon?

I usually hammer down to get on plane, then drop rpm back to stay on plane.
 
Well, the war is not over. I am still having some issues. See my other thread. This time it won't keep me from fishing.

I am using GPS to measure speed. No change in hull condition. I did not pay attention to see if there is any change in WOT speed. On my recent trip, my RPM and speed are back to the way it was before.

Can I assume the fuel consumption is totally depending on the RPM. If an engine is running at 4000RPM, it will have the same fuel consumption rate regardless boat load or speed. Or the fuel consumption is also depended on how hard an engine is working? Can an engine inject different fuel amount depending the power needs?
 
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