Merc 300R Fuel Consumption

[Hebert519]

Get a vessel view or smart craft gauge, either will show you real time GPH. Load changes it a lot. I have a 20' Lake and Bay backwater flats boat 300R. 10" bob's jack plate. I get on plane with the motor down, at 4000 rpms trimmed down I burn 13.8 gph and only like 42 mph, once I start trimming up speed will increase to 46-48 mph, BUT GPH will also go down to 11.8 when trimmed properly. So load has a lot to do with GPH even at the same rpm. 4500 rpms, I'm around 52 mph usually and 15.8, but again if I trim down and run with the bow buried (maybe cause it's rough), GPH will go up to as much as 17.2 GPH. If you're spending this kind of money on the motor, why not spring for a vessel view, or at least a smartcraft gauge that will show fuel burn, as well as other functions like water pressure.

I have a smartcraft gauge, didnt even know it would show GPH. Ill look into accessing that through the gauge

[David]

If I ever get the antenna to work, my Merc Monitor (smartcraft) will show mpg

[Toffen]

Hi!

My boat is approx 1350 kg with me in it. Around 2970 lbs. When I had a 400R my milage at 48 mph was 4,36 MPG. My milage with the 450R at the same speed is approx 4,1 MPG. However, I prefer to run 3500, so then I get 4,12 MPG. Max torque is at 4,250, so the engines likes to get up there .. I used a single Promix 300X back in time..... That one swallowed fuel ... !

Cheeers Toffen

That is really impressive Toffen. My 300R pulls about 6 gph @ 3000 rpm in my 24 Checkmate, but it is only doing 29 - 30 mph. So you should really compare the fuel consumption at similar speeds not similar rpm. Your 450R pulls 5.26 gph at 29 mph!! That is actually slightly less than the 6.1 gph mine pulls at that same speed!! Will it stay on the plane at 2000 rpm?? How heavy is your Phantom with the engine, fuel, you in it and ready to run? I would guess mine is around 3100 - 3200 lbs (about 1430 kg) with my wife and me in it.

Best Regards,
Glenn

[Pulsare2400]

Get a vessel view or smart craft gauge, either will show you real time GPH. Load changes it a lot. I have a 20' Lake and Bay backwater flats boat 300R. 10" bob's jack plate. I get on plane with the motor down, at 4000 rpms trimmed down I burn 13.8 gph and only like 42 mph, once I start trimming up speed will increase to 46-48 mph, BUT GPH will also go down to 11.8 when trimmed properly. So load has a lot to do with GPH even at the same rpm. 4500 rpms, I'm around 52 mph usually and 15.8, but again if I trim down and run with the bow buried (maybe cause it's rough), GPH will go up to as much as 17.2 GPH. If you're spending this kind of money on the motor, why not spring for a vessel view, or at least a smartcraft gauge that will show fuel burn, as well as other functions like water pressure.

I read your post, and I know that the ASC function on my 300R will not allow the rpm to increase if I trim up, but always keep the rpm constant at any rpm below WOT. At WOT the ASC does not operate, so trimming out at WOT will result in more rpm and speed. So I ran mine up to 4000 rpm today, trimmed fully in, and it was pulling around 11.8 gph and running at 40 mph. I then trimmed it out to about half way, and the rpm stayed at 4000 rpm and of course the speed stayed at 40 mph because the rpm did not change. The fuel flow dropped to around 9.1 gph, which makes sense and means the additional trim freed up the hull and it would have run more mph and more rpm but the PCM closed the throttle blade (even though I never touched the throttle) to maintain 4000 rpm, but since the throttle blade was further closed the pulse width of the injectors was likely less which reduced the fuel flow. So mine appears to operate a bit different than yours, in that my rpm at part throttle (and speed in mph) will not change with additional trim, but the fuel flow will be reduced like yours.

[Capt j]

Pulsare- My RPM stays the same when I trim up, but my speed increases a lot at the same rpm's due to less drag.......and fuel burn goes down.

[Pulsare2400]

Pulsare- My RPM stays the same when I trim up, but my speed increases a lot at the same rpm's due to less drag.......and fuel burn goes down.

Interesting. Neither my speed or rpm changes when I trim out at part throttle, but the fuel flow decreases. At WOT, both the rpm and speed increase when you trim out. If I raise my jack plate at part throttle, the same thing happens, the rpm and speed stay the same and the fuel flow decreases. At WOT, raising the plate will change the speed and rpm because the ASC is not in effect.

[Capt j]

Different boat hull designs I guess. Your prop must be efficient at any trim, so when you trim/jack up it just lessens the load on the motor (drag). I have a 20' Lake+Bay backwater flats boat.

[Pulsare2400]

Different boat hull designs I guess. Your prop must be efficient at any trim, so when you trim/jack up it just lessens the load on the motor (drag). I have a 20' Lake+Bay backwater flats boat.

That makes sense to me. Under 35 mph, I have to run full negative trim to keep the boat from porpoising, and the bow is still running very high. After about 40 mph, I can use a little bit of trim. After about 55 - 60 mph it will start to push the bow down and you have to give it trim to raise the bow. If I am cruising say at 3000 rpm / 30 mph and I turn the boat, the rpm will stay constant but then the mph does drop due to the drag and the fact that it lost lift from strakes and hull. I have tried at least 6 different props, and all reacted the same regarding no speed increase with trim when the ASC was active, so I think your theory of the hull design is likely correct.