boats and fuel consumption

[[2068]]

>Bit like some pillock above suggested

Oy! There was a serious point there, which is that you can't just consider engine efficiency: the hull and drivetrain come into it too.

e.g. big heavy displacement hull and engine 200rpm below WOT, the engine might be working efficiently in terms of getting the most power out of each gallon of diesel, but if that power gets spent on pushing a huge bow wave, then you won't be getting the best mpg.

 

[Divemaster1]

Thank god someone showed linear figures .... I would actually have thought that the longer the hull lenght, you'd get a more linear curve.

These are my figures, based upon load ... and as I like to be on the cautious side, I have reduced my speed by 1 knot against average actual SOG with a clean hull.

The figures are for our two-stroke, straight six Detroits in our 45 ft Deep V hull. As someone else mentioned, I see little reason (particular in the Detroit's case) to calculate anything below 1250 RPM as that is when the Turbo chargers kick in and I get a good operating temperature on the engines. Engines will idle smooth at 450 RPM under load and we'll tick along at about 4.5 knots, but I get a carbon buildup after about 8-10 hours like that (Canal journeys), which clears with a great black puff once pushed up above 1250 RPM.

 

[peterb26]

Very interesting thread. My own boat is (presently) most economical at around 22kts. (This is a single D4-260 installation).

Dont mock me too much (I'm old....) but I actually prefer to travel at 15-16kts, which is not only quieter but softer on the body as well.

However at this point the boat is happily planing, but the natural angle of the hull is very "bow high".

I dont have trim tabs, but am considering fitting them in the hope that I can get a better hull angle at 15-16kts, and also therefore better fuel economy.

But then I thought that when the trim tabs are pushed down, they must increase the hull resistance through the water. So maybe they wont help the economy at all??

Hence I am confused.. /forums/images/graemlins/frown.gif

 

[DAKA]

[ QUOTE ]
IF the question is "where is MPG and range the highest, assuming you dont want to plod everywhere at displ speed (ie you want to plane)?" the answer is always just past the hump

[/ QUOTE ]


My post above claimed I could get 4 knots free.

This wasnt a off the cuff remark and was based from this graph

You will see the sweet spot is 3200 3400 rpm, all modern engines have sweet spots and the manufacturer can choose where it is via the engine management 'chip'.

Torque drops but HP increases 50 hp for virtually no extra fuel
Once on the plane the extra 200 rpm (50hp) gives an extra 4 + knots.

Everyone should study their manufactures details as most on board fuel consumption calculations are not as accurate as the test bed !

 

[duncan]

[ QUOTE ]
Very interesting thread. My own boat is (presently) most economical at around 22kts. (This is a single D4-260 installation).

Dont mock me too much (I'm old....) but I actually prefer to travel at 15-16kts, which is not only quieter but softer on the body as well.

However at this point the boat is happily planing, but the natural angle of the hull is very "bow high".

I dont have trim tabs, but am considering fitting them in the hope that I can get a better hull angle at 15-16kts, and also therefore better fuel economy.

But then I thought that when the trim tabs are pushed down, they must increase the hull resistance through the water. So maybe they wont help the economy at all??

Hence I am confused.. /forums/images/graemlins/frown.gif

[/ QUOTE ]

you are right that the trim tabs will increase drag but they will wnable you to vector your thrust more efficently which more than makes up for the drag - no tabs and your drive is trimed in and using some thrust to push the stern up rather than along.

but

generally if a boat is not fitted with tabs as standard, or has them offered as an option, the manufacturer has run tests and determined that whilst they are usefull from an attitude perspective they don't add to the efficiency off the drive train.

I suspect my boat is similar to yours and whilst the attitude around the 10-13 knots always seems (well is!) steeply bow up the economy doesn't seem unduly affected.

also like you I tend to cruise at 19 knots simply because the water is softer and the engine quieter - when it is glass calm and the sun's setting over the water why hurry!

 

[gjgm]

I havent had the boat that long,so havent endured all weathers, but so far the boat does seem to cover distances very quickly. Whether I can stand 3 hours in some crappy weather is another matter.
Anyway, I only looked this data up after another thread. I d always assumed that the faster you go, the more fuel you would use for the journey. It seems that isnt necessarily the case.
We ll be going from Hythe to Falmouth in July, so then I ll know a bit more.