What the heck- I have this propeller problem

Hi Bntij.

That would be my question... for a yacht engine, that if anything, normally suffers from under loading.... is that outcome actually an improvement? If the engine had the torque/power to carry full speed some where near the peak of its torque curve with some revs in hand is that really that much of a problem? I can see why the approach might be different for a planning power boat and I'm assuming some of the earlier posts had that type of vessel in mind. I'm really not sure I get why we must be at max revs near hull speed?As long as the engine is not struggling, is loaded up to a reasonable level and revving to say the 75% to 80% range I think I (in my ignorance) would be happy with that.

If I remember I will ask and see what you think by next season.

Cheers
MBz

 

I believe that the ideal is to use the hp developed at rated rpms to carry the load instead of counting on the torque developed at the lower end..........?
Something about the load on the bearings?
Have to admit- I would like to know a bit more about this subject.
I will now use 2200 to 2400 as my cruising rpms while the top end is in reserve.
In the prior case the top was at 2500 and cruising would have been at 1800 to 2000.
From the torque curve for the engine I can see that there is a specific range- ~2200 rpm where the torque is the greatest. I don't really understand the objective of dialing in propeller load to revs in relation to this curve and how hp is related to torque in general. Another area I need to get a handle on is how these adjustments effect fuel use at a specific boat speed.
To your point Beanz- these Kubota's give really long service in industrial use. I hear of 10s of thousands of hours service life in agricultural pumps and gensets. Like any other marine engine a service life of 5k hours seems like the standard in a boat. Is this difference partially due to load regime.... don't know.

 

I need to get a handle on is how these adjustments effect fuel use at a specific boat speed.

Usually the closer you are to using most of the HP that is aviliable at the torque peak the better the engines efficiency.

So the better the mpg will be .

FF

 

Kubota's are great, I know that from direct experience. The shorter life in the marine environment is probably more due to lack of use, sitting around in a hostile environment, salt water, salt air and all that. I'd wager much better service life in hrs would be had from a working boat than a yacht. Anyway they are sweet little engines that's for sure.

Cheers
Beanz

 

Thats it Fred.... just what is the sweet spot, and why?

 

The "sweet spot" is that place where the engine is 80% loaded (for that specific rpm's hp )and the boat procedes at an acceptable speed.

Unfortunatly it only can be optimized for a single load condition, flat water , flat but 20K headwind , climbing combers.

So the best compromise is usually to accept some inefficiency and have RPM left to increase power if needed.

BIG Ships get over 50% efficiency from their propulsion engines by accepting a slowdown in heavy weather.The engine is run at almost full speed 105 rpm, and full load almost all the time .

FF

 

Marine Engine RPM

Dave Geer wrote a great book titled "Propeller Handbook". It goes into the details behind the calculations of propeller design, explanations behind the whys of engine - prop configurations. I have used his book as a basis for prop calculations and adjustments for 20 years. Dave even goes into the clearances necessary for proper water flow to the prop as well as end tip clearances. and other minutia. Very useful.

I used information from his book to redesign an rebuild some of the underwater shapes of my Allied Princess Ketch to improve water flow. The improvement in performance was dramatic.

I have also used the information to improve the prop configuration for high speed sports fishermen (50+ knots).

"His suggestion is for a Light duty or high-output diesels to be 80-85% of Max RPM." His book is published by International Marine in Camden, Maine and is available in paperback. You won't regret the $20.00 or so it costs.

 

Fast Fred,

Torque = Hp x 5252 / Rpm (5252 is a constant)

 

So the "cruise prop" should go, I know the revs at hull speed will be more now - but a 18x10 seems a little too much to me. From the graph, what should my best cruise rpm be then? 2200?

The cruse prop should STAY , unless your cruising style changes.

If a good cruising speed is reached at 1500, YES the engine would be overloaded at 2000 , but SO WHAT?

Simply use 1800 as a red line (which probably wont raise the temp ,) or 1750 and continue to criise at 1500.

By heavily loading the engine at 1500 it will live longer and produce as good as fuel burn as you could expect.

By raising the power band , (more rpm at full throttle and cruise ) you will be better able to perhaps cruise at a higher speed but at the cost of heavier fuel burn. Is that what you need?

FF

 

I think I have a good working solution for my boat but continue to be interested in susing out this whole issue.

FF- How is higher used power at a lower rev reconciled with the "net continuous" rating at the lower end? That is, it looks like this power curve stipulates that the engine can only deliver the power as shown on this curve. How is it that a "cruising prop" can load the engine higher than this curve at the lower end with no ill effect?

 

How is it that a "cruising prop" can load the engine higher than this curve at the lower end with no ill effect?

The cruising prop will ordinarily be using just a bit less HP than the net cont AT its cruising rpm.

The danger is a prop that does load the engine at low rpm will usually be too much to use at higher rpm.

Unfortunatly most engine converters do nor publish (although they probably have) a Fuel Map.

The map looks like a series of clouds , one drawn around the next, sorta like a target.One side is HP the other is RPM.

The inner circle is all the areas where the fuel consumption is lowest, next out higher consumption ect till the outer edge.

By picking an area inside the inside circle , and matching it to the prop used and power/rpm the least fuel use can be figured.

Unfortunatly the ability to overload the engine with more RPM is what keeps the mfg's requesting that props be selected for max RPM, which is insurance for them.

The cost in wasted fuel and shorter engine life is NOT their concern.
Just getting by the warantee period is.

FF