Fuel consumption on twin mercruiser

[Meps1983]

Hello fellow boaters! I'm currently looking at upgrading my current boat (viking 22) to a Maxum 2800scr. The boat I've viewed is fantastic to look at with bags of extra space. She is a 2000 model with twin 4.3 mercruiser mpi in the back. I'm going from a small outboard to these big monsters, and have no idea on fuel usage etc!? I understand there are a lot of factors to be aware of in terms of weight, weather, speed etc. Can anybody give idea of what the mpg on these twins would be. The boat would have 4 people on board, half a tank of fuel, and a calm day let's say!! Doing river speeds which never exceed 8 knots. Tricky to answer I know but I have no idea what im getting into😬. Thanks for your time reading

[mmwjr]

Welcome Aboard!

IMO the 4.3 will burn ~ 6-8 gallons per hour each at 3400 rpm. This is based on having owned a boat with a single 4.3 and a single 5.7. Funny you call these monsters as I am in the process of buying a boat with twin 7.4's. It's a matter of prospective.

[Meps1983]

This is a good point mmwjr. My current boat has a 15hp outboard, 25 litres of fuel does me for the weekend lol. To most in the UK they are monsters (but I love them). What kind of speed would you be going at 3400rpm?
As I said were on the river going 5-8 knots max. Would the engines be running at those revs to cruise at walking pace down a river? I'm hoping to buy this maxum with the inlaws and they are concerned about fuel economy of those 2 "muscle car engines" 😁. Thanks for the reply

[mmwjr]

At 3400 rpm I'm going 20 - 25 mph. At 5-8 knots you would be around 1500-2000 rpm and using much less fuel. As you get close to 2000 rpm you are starting to come up on plane which is not a good place for long term operation as the bow will be high and the stern gets loose. Also efficiency is bad at this point. BTY the 4.3 is basically the same as a 5.7 with 2 less cylinders. Since I never operated my boat at this speed for long time I'm guessing that you'll burn ~3-4 gallons per hour per engine.

[Meps1983]

At £1.60 per litre in the UK, that sounds quite pricey for a day trip down the river! The figures sound reasonable but I won't be getting on the plane at all while river cruising. So hopefully it will be better still 🙏.
The river/sea trial might give me a better idea, but I'm not sure it will tell me my mpg. Id like to ask the current owner, but you can guarantee that will be on the lean side lol! Gonna have to work the inlaws a little more I think.

[mmwjr]

I leaned towards the high side not to over set expectations that I’m not sure about.