slow and fast and long

i am having problems trying to effectively design a boat which will intermittently require to run at 20kts but for around 70% of the time of more run at 12 kts. the vessel is 30m long, and is of a semi displacement hull form. does any body know where on the internet i can find accurate NPL graphs ? the problem with the vessel is is need a rather long range (3000nm) and the amount of fuel required is a serious stumbling block so far. using the De groot series i havs calculated that i need an insanly powerful engine (4500kW) to achieve the 20 kts. what are the most fuel economic engines out there?

 

You need atleast a CPP to get the best performance and economy out of the engine, whatever it may be..

 

Look at multiple engines (3-4) and a mix of props (low speed) and boost waterjet or jets. Look at MTU series engines.

 

New here?

Search NPL series in this forum.

 

Is there a certain manufacturer who makes slow speed and medium speed engines with low fuel consumption?

 

The problem you will be facing with long range vessel is the diminishing displacement. At the start of the voyage, the vessel will be fully laden with fuel, FW, stores, and provisions. Propeller will be at optimum efficiency as it is deep in the water. At nearly the end of journey, the vessel will be very light and probably part of the prop is out of the water with reduced efficiency.

To compound the problem, as the vessel consumes the load, you need progressively less power to move it. There is a formula given in the Cyberiad site of Leo Lazauskas (I have forgotten the title at present) which should be usefull for determining range of vessel with progressively diminishing load.

 

If you vessel is not weighting in the 500 - 600 tons ball park, there is a problem in your figures (20 kts, 4500 kw needed, 30m/100 ft length).

If your vessel is the 550 tons, what kind of surfing device are you tring to create . trying to imagine the wake ...

 

not sure how i got such a high number before but thank god that i was wrong. now have used a powering program to calculate required power and at 12 knots :i will require a little under 500kW and at 20 knots i require a little over 2600 so might slow the top speed a bit but around 2400kW for max speed.

 

the porblem is that is the vessel will require about 20tonnes of fuel at 18-19 knots travelling 500nm or for 27 hours. which is a stupid number. i think the only thing i have heard of which could do this i saw in star trek!

 

Not that stupid.

See http://www.vicemyacht.com/index.php/vintage-line/cruiser/92-2

It is only the hard life of engineering.

If you want to improve your figures, you can lighten the boat, increase length, or of course reduce speed. As some said, TINSTAAFL

 

will it be possible to have one main engine and say two booster engines as the installation. will the main engine still give the required thrust when the boosters are working?

 

does any one know if a main slower prop will still give thrust in combination with maybe two smaller props.Or will it just cause added drag?

 

If you multiple post it is really confusing.

 

sorry, gonzo. thanks for the assistance though. Now i think i will have two engines of the same power. Does any one know of any thing similar to what rxcomposite has written about above(lowering draft and load decreases)?

 

Cargo vessels in light condition have the propellers sticking out of the water. I don't think the fuel in your project has the same percentage of the displacement as the cargo on a ship.