I've noticed too that the internal computer data does not match manual fill-up gals vs miles on cars. OCD plugins have been standard on cars since the 1990a so they all have the ability but when I've compared to actual mileage, the onboard computer data is always a bit optimistic - at least 5%. Most cars these days have the mpg read-out on the dash cluster too. Notionally accurate - and always optimistic.
Fuel burn about 1800 litres and tanks is about 2* 610 litres, speed about 15 knots
No kidding, my current car bought used at 4 years old came with the best consumption still not matched or beaten. Makes me wonder if was run at idle to create that entry. Great sales tool though.
That's a shame.
Not to get too far off topic, but this is always more about the driver than the vehicle.
On your first post you posted the formula but for S/L ratio in the formula you show squaring instead of square root. Really makes a difference in numbers.
That is correct. The great mileages from low CP quoted in this thread were all low S/L ratios. If you want 1.3 or above you will give up fuel mileage at the lower S/L ratios by quite a margin. The reason for all the higher prismatic coefficients is that new boats cost a lot and new boat owners don't care much about real mileage because that is a minor cost. They want the ability to go a bit faster. By the time the boat gets well used and cheap the poor guy buying it thinks the mileage isn't cost effective and runs the boat less and sit on anchor.
I think context is everything, and those are relevant data.
Oh and I do get better mileage at 7 knots and under by quite a bit. I would be better off with an SD boat because I run 7.5 knots and would prefer 8.5 knots. In fact, I would prefer 10 knots. the problem come in because my boat is such a small percentage of my net worth. I am cruising full time starting next fall and will burn through $25,000+ dollars a year in dockage. I fit the real profile of a Hatteras owner. I really don't give a darn about a few more gallons and hour in the scheme of things and the oversized Detroit's and the stabilizers would be happier at 8.0-8.5 knots.
Good catch. I can't go back and edit now but it should be:
Also good points about prismatic coefficient. The stern shape of many SD boats is also very draggy at low S/L ratios. That shows up in the prismatic, but also has its own issues.
That's a very good point. The amount of submerged transom you drag along on a flat transom hull has a significant impact on drag at lower speeds.
If it's practical to do it may help a bit. I don't have a good way to adjust trim easily on our boat, but for efficiency's sake I tend not to fill the fuel tanks all the way unless we need the range.
Yes, but what speeds are we talking? On a 36' WLL low S/L is under 6 knots.
I think the crossover speed might be closer to hull speed. At 1.15 S/L we're dragging an awful lot of turbulent water behind us.
Fuel flow and fuel economy are blank. I get figures for both with my Cummins was 6.7 with Garmin mfd.
I took that pic when the engine wasn't running. When it is, the figures show real values.
