Range its all about efficiency

I'm sure almost everyone knows If you go a bit slower you get much better range. if you accept a diesel uses about 0.55Usg per horsepower per hour You can calculate you will get a fantastic range if your engine is running at just above tick over speed and you have the patience and time to get to your destination. But the calculation is only accurate at the speed at which the prop has been pitched at. The prop eff changes with speed and makes things wrong. Most power prediction formula give results for average conditions and a prop efficiency of about 55%. In the attached document I have calculated speeds and predicted ranges in red, then adjusted then to account for varying efficiency in blue. mik

 

Sorry the doc was invalid so I made a second attempt. ranges are based on 100lt of fuel mik

 

2000 rpm 2.75 hp as an example
Where did you get these numbers?

 

The power curve ends at 16hp. The propellor curve should cross the horsepower curve at 16hp. (3600revs/3600)^3 * 16hp =16hp. (2000/3600)^3 = 2.74hp.
Note most engines use the indeses 3 or 2.7 for there propellor curve.
mik

 

hope this makes things clear

 

I would like to comment on the horsepower to gallons per hour assumption. Any engine will have different hp/gph depending on load and rpm. The best a small/medium size diesel can do is between 15 and about 21 horsepower per gph near peak torque and rpm. When any engine is run outside the optimum rpm and load the fuel consumption per horsepower is different. My point is even a Gardner will be hard pressed to match a Detroit Diesel if mismatched load to rpm. Overall I wish to bring attention to the fact that people keep referring to the x horse power to gph as if it were chipped in stone. The fact of the matter is that an engine capable of say 20 horsepower per gallon per hour is just as capable as 10 horsepower per gallon per hour, depending on load and rpm. It's interesting to me how large, fast boats can idle along at 7-8 kts with incredible range, and rev up to the teen's and twenty's with good range. Is there any big brains on this forum that can explain to me how to design a boat that can roar around locally at 1.34-1.5 the square root of the LWL, AND be able to slow down to 1-1.2 the square root of the LWL for long distance passage making? A 2 speed boat. Fast locally, slow to cross oceans. I'm not really interested in sail...as I would like to spend a lot of time in the doldrums. Thanks all

 

First let me say in my chart 3900 rpm for 16hp I meant to write 3600rpm.
The boat in question is designed to be 8256lbs on a LWL of 35ft 16 hp gives about SL1.34. Now what I am about to say can be proven mathematically but I want to keep this simple so please accept what I say without formula. The displacement length ratio is 86 (very light). It is a waste of power to try and force a boat with D/L ratio of 450 past SL 1.34. 8.26/DL^.311 gives the speed length ratio the boat can travel at. My boat works out at just over 2 for 11.8kts.
The boat is a catamaran so you can sail toward the doldrums and when you get there you can motor through at up to 7 - 8kts.
Now if you fit a 45hp engine the boat will make SL 1.87(11kts) 9.26hp will still produce about 6.4Kts and will use a little more fuel than the 16hp engine. It is not slow running that wears diesels it is allowing them to cool to much which promotes wear. As long as the diesel temperature does not drop below minimum operating temperature things will be fine. It would be safer to run the 45hp engine at about 15hp.
mik

 

What better way to spend a lot of time in the doldrums than a sailing boat?

 

A Prop is close to100% efficient at 1 RPM so large diameter is important along with low pitch. There is much more to be learnt from an engines diesel usage as all diesels consume about 1 litre per 5HP or 1USG per 20 HP per hour even with the most modern rail injection engines

 

Where does that info come from?

 

If 1 rpm gave 1kt of speed 1.4/ 1^.57 gives 1.4% slip. 1 - slip = 98.6% efficiency If you could have that efficiency at 1rpm I reccon you could need a prop like a helicopter blade. 1rpm is so slow you might get near 0 slip but 1rpm is about no thrust.mik

 

I haven't studied prop design but it seems to me that for a start the prop would have to be designed for 1 rpm, is that right?.
A fast outboard motor prop doing 1 rpm will largely be pushing the water around in a circle instead of helping it on its way out the rear, so I don't see how that could be efficient at all.

 

At one knot for one hour you would move 72,931 inches.

The prop would turn 60 times in an hour at 1rpm

The theoretical pitch with zero slip would be 1,215 inches. a little over 100 feet
The blade angle with respect to the horizontal would be 11 degrees, ie almost flat. ouch

 

These calcs are a bit silly. First you calc the rpm at the prop and what diameter prop that engine can swing.