Calculation of HP relative to speed

[boatmike]

There must be some way to calculate or estimate the HP required for different speeds. I have looked at calculators on the internet but they seem to give wildly optimistic results. I think my Aquastar 33 semi-displacement boat has about 7.5 - 8 tonnes displacement fully loaded. It has 2 TAMD41A diesels and achieved just short of 22 knots flat out on a test run before purchase. Running at about 12 knots bringing her home to the Solent from Ipswich, I think I was burning about .75 gal per mile (perhaps a bit less) which I was happy with but I haven't had her long enough to have experience of running under slow conditions and what I am trying to calculate is at 6-8 knots or lower (below displacement hull speed) how much HP will be required. Dont tell me its not good for the turbos running at this speed, I know that, but am trying to calculate the fuel required to get her through the canals to Port Napoleon, or thereabouts. Theoretically running both engines at this speed they will be very lightly loaded but I shall probably run both rather than one as I will probably want both alternators running to balance the load of the AC running from an inverter. I tend to think running one engine won't save much fuel anyway as its the total HP required that is important, but am aware that they wont be running efficiently at this speed and it might be more economic to run on one if I don't need to run AC....
Anyone know how to calculate this or have experience of similar at canal speeds? Opinions?

 

[Grampus]

Can't help with the horsepower but at low displacement speeds ~ 1xhull length^0.5, I believe that our consumption might be similar.
We have a Rodman 1250 sitting in the water at about 14 tonnes with full fuel, water and cruising stuff.
At canal speeds through the Caledonian, we recorded the following.
4.5 knots - 9 nmpg
5 knots - 5 nmpg
6 knots - 3.7 nmpg
7 knots - 3.2 nmpg
8 knots - 2.3 nmpg.
These are total fuel used running both engines.
galps

 

[Latestarter1]

I have some experience of barges on French canals.

Your challenge is going to be getting your minimum in gear speed down which is where running on one engine comes into play, on some small canals even at the legal limit of 6Kph or just over 3 knots you need to exercise care, on larger canals 8kph or perhaps a little higher may be acceptable, but very dependent on how much wash you are creating. This is where alternating running on one engines comes into play.

Never use gallons per mile, complete nonsense, work off gallons per/hr.

 

[Nick_H]

.

 

[boatmike]

Well indeed I have some experience of French canals too having traversed the system several times in a number of different boats. Regarding your statement that gallons per mile is a complete nonsense, disregarding the arrogance that is inherent in that statement, may I respectfully point out that this is not an acedemic competition it was a genuine attempt to calculate the fuel consumed to travel a certain distance and gallons per hour is unhelpful unless one knows the distance travelled in that hour.
Your reply does nothing to answer the question I asked.
The post by galps on the other hand was extremely helpful and goes some way towards answering my question although I was really looking for some formula that links HP to speed on a displacement hull. There are plenty of ways of translating HP consumed over time to fuel consumed but what I was looking for was the HP required to achieve a certain speed which will be relative to hull type and waterline length.

 

[Latestarter1]

Well indeed I have some experience of French canals too having traversed the system several times in a number of different boats. Regarding your statement that gallons per mile is a complete nonsense, disregarding the arrogance that is inherent in that statement, may I respectfully point out that this is not an acedemic competition it was a genuine attempt to calculate the fuel consumed to travel a certain distance and gallons per hour is unhelpful unless one knows the distance travelled in that hour.
Your reply does nothing to answer the question I asked.
The post by galps on the other hand was extremely helpful and goes some way towards answering my question although I was really looking for some formula that links HP to speed on a displacement hull. There are plenty of ways of translating HP consumed over time to fuel consumed but what I was looking for was the HP required to achieve a certain speed which will be relative to hull type and waterline length.

Ooo contretemps, seem to have touched a nerve!

Back in the 1960's I was working on the brochure for Meakes Sealion, consumption gallons/hr, moved on to bigger stuff, consumption was measured in tonnes/day and years later moved back into realm of liters/hr as well as gallons Imperial and US/hr. No clue when inappropriate use of MPG in relation to boats ever crept in??

Going back to the original question, propeller demand at very low canal speeds is correspondingly very low, could be as little as 5kW/6.7 hp per engine therefore you have to consider that engines burning not much more than idle fuel i.e no more than .25 gallon/hr per engine.

Next time I compile a service data sheet for a marine diesel engine I will have to consider dropping service interval in hours in favor of miles/nautical miles for the non nautical.

Ho hum

 

[boatmike]

There you go again with sarcasm this time!! For your information I am far from non nautical and have spent much of my working life on the design and build of Superyachts, Warships and Passenger vessels. Many of these superyachts and commercial vessels were fitted with Floscan readouts that give the current consumption of the engines and compare this with a GPS readout of speed over the ground. This gives either litres, gallons or other fluid measure PER NAUTICAL MILE. The reason for this is that this is the ONLY measure that can indicate the most economic throttle opening in the prevailing conditions. Fuel burn per hour tells you nothing. In fact it is often the case that running engines at too slow a speed against foul tides and high winds will give a worse consumption over a distance run and it is better to run them at higher output regardless of the fact that fuel consumed per hour will be higher. N Miles per gallon is therefore a very legitimate and useful figure and there is no way you could give any accurate calculation of the fuel burn for a given trip without taking the distance into account.

 

[jdc]

You can calculate HP against speed, and from that work out consumption against speed, and thence to consumption against range. But while straight forward it is time consuming to do. What I did for my boat is described in http://www.awelina.co.uk/hull_drag/fuel_consumption.pdf. It's for a sailing boat under engine, but apart from adjustment for wetted area and LWL should still apply to any boat in displacement mode.

The gist is that, without any wind or stream, you always get better range (more 'mpg') when going slower. But if there is wind or adverse current there is now an optimal speed.

The steps were:
1. Knowing hull form and dimensions calculate drag as a function of speed
2. Calculate thrust from the prop as a function of rpm and speed
3. Thereby work out speed as a function of rpm and drag as a function of rpm by balancing thrust against drag
4. Knowing the prop's size and form work out shaft torque as a function of rpm
5. Thus get shaft HP as a function of rpm, and so consumption
6. Relate back consumption to speed and thus range
7. Re-do steps 2 - 6 with wind and / or current