What's your cruising speed.

[Baker]

I am not familiar with that Navigator or those specific engines but here is a boat test on a 2004 - 56' Navigator that may help out some....
https://www.powerandmotoryacht.com/boats/navigator-56

I have found at times you can also get additional information that is not online by heading to your local library and asking for help with the microfilm and past periodicals - helped me secure some old boating tests that I then copied.

So If I did the math right..... .53...almost 2 gallons per mile. Not bad for a boat that size. It is still almost 20,000 pounds lighter than the boat in question.

 

[smitty477]

So If I did the math right..... .53...almost 2 gallons per mile. Not bad for a boat that size. It is still almost 20,000 pounds lighter than the boat in question.

I do not know this sized Navigator but my math seems to agree at about 0.55 nmpg at WOT.
My best guess (and only a guess) is that with a 0.55 at WOT the nmpg at say 66% of WOT would be much higher. The weights are really unknown between the two since each of them would need to be confirmed.
Navigator had one hull design for a large % of their boats and just 'extended' it for various lengths of their boats.

 

[smitty477]

I do not know this sized Navigator but my math seems to agree at about 0.55 nmpg at WOT.
My best guess (and only a guess) is that with a 0.55 at WOT the nmpg at say 66% of WOT would be much higher. The weights are really unknown between the two since each of them would need to be confirmed.
Navigator had one hull design for a large % of their boats and just 'extended' it for various lengths of their boats.

Based on some online data it looks like the 58' Navigator is a big step up in beam from the 56' Navigator - therefore it is likely not possible to draw conclusions from the 56' Navigator specs or reviews.

 

[although]

We run about 1700 RPM for about 7 kts. Boat is a 40ft Marine Trader with a single 120 Lehman. I don't have a fuel flow meter, but the engine seems happy at that speed and it doesn't go through much fuel... I recently used a hand strobe to verify my tachs, and it runs about 2400RPM at WOT. So, I'm pretty happy with the way it is propped.