What the heck- I have this propeller problem

So they reckon the engine should be working harder then? Tell us where you end up when its done.... plz

 

Correct procedure, take some pitch off the prop, keeping the diameter as it is.
The engine must be allowed to reach its maximum desugned rpm. The engine will in fact then be able to deliver to the prop the best of its capabilities. In real life, the engine should be running at 70% of its max recommended speed for 70% of the time. This is an old rule of thumb that has been reached after years of experience running diesel motors. Lower speeds cause glazing of the engine walls, higher speeds are detrimental to the engine as it is being worked at its maximum rating, and this is obviously not what we want to do, you do not get into your car and run it flat out at every gear, unless you expect to pay for the performance.
One inch of pitch is a lot to remove from a small prop, but the difference is very noticable. It is most likely the amount that will be needed to increase rpm up that extra 700 that is being loaded out of existance at present.

 

Good to know... but I am not sure I understand why?

 

Good call from the prop shop, you were at max torque but a long way from max power. To quote Dave Gerr "if RPMs are held too low the engine will smoke and foul its valves"

*Dave Gerr, Propeller handbook, 1989

 

So why is it bad to set up so that at 70/80% of RPM you are at max speed? Why do you really need to be able to hit max RPM, its something I rarely have attempted to do. Just trying to understand this one.

Cheers
MBz

 

There is a big difference between reducing the loading on the engine by throttling back to 70% and having an engine that can't get past 70%. One is having a holiday while the other is working its guts out.

 

For engines where it is controllable (a series of gen sets) the long life rule is 80/90

You load the engine to 80% of its RATED load and run it at 90% of its rated RPM.

Grand in theory , but hard to do on a cruising yacht.

The problem on a sailboat is that when sea conditions kick up , and you wish to punch at speed into big combers the HP required will increase by about 300% of smooth water requirements,to maintain speed.

So If you have enough power to climb 15 ft combers , the engine is severely underloaded 99% of the time.

Simplest setup today is actual measurements underway.

Run the engine at 100 RPM increases logging on graph paper the (GPS) speeds,start at 1000 and go till you run out of rpm.

You will soon notice a speed where large RPM increases are required for small increases in speed.

The bottom of that range , is usually matched RPM wise to the torque peak, or lowest fuel flow on a Mfg graph.

This gives the best cruise that 99% of the time BUT the prop will be "oversized" (a cruising prop) and you will NEVER pull max rpm.

The technique if running 10% or 300rpm under max observed rpm will be used in heavy weather and will not overload the engine.

You're choice set up for 1% of heavy weather punching or 99% of normal cruising.

To set it up to punch at heavy weather the prop diameter should be about the same , but the pitch much lower so the boat moving at only 2 or 3K will be efficient.
Getting max rpm tied to the dock might be a good check for the ability to climb walls of water almost stopped.


FF

 

That is sounding good to me FF...

So what he needs is a variable pitch prop really... no?

Cheers
MBz

 

So what he needs is a variable pitch prop really... no?

That works great on a newbuild , where a thrust bearing and the slightly larger more complex shafting can be installed , and for some the cost of a clutch system can be avoided.

In our more complex world , where slips are common its probably better to pay for the clutch system as the engines always need more rpm on starting cold , so the boat will want to walk in the slip.

A CPP is a great idea , but hard to justify its about $10,000 price tag , if fuel keeps climbing tho.......

FF

 

I have only used one boat with a variable pitch prop and it was a dream. Yes... expensive although the boat I used was only a $15K yacht at the time. Yachts and props, it all seems to be a compromise.

Thanks Fred.

 

"Yachts and props, it all seems to be a compromise."

Are there truer words?

 

"Yachts and props, it all seems to be a compromise."

Are there truer words?

For folks working on a prop set up,

"Third Time Lucky" seems to work.

FF

 

Is that sort of like Goldilocks and the Three Bears?

 

very good thread - My current setup must be over propped - hull speed at 1500 rpm, very quiet, however engine will not reach its max power of 3800rpm. Continuous power 3000 is reached but engine will begin to run hot.

current prop is 18x15, Westerbeke people said a 18x10 was spec'd originally.

Albin 27, Westerbeke 70hp, ZF 1.5

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?

engine specs

 

Update-
First off- I am using a Campbell Sailer prop with all its idiosyncrasies.
I was 500 down in Rpms.
The prop shop said in a standard Michigan wheel I would need to loose 3" to get these revs, they also could work with the cupping which is on the CS.
The manufacturer told me I would get about 300 rpm per inch of pitch removed and advised against touching the cupping.
I had the shop pull 1.5" off the pitch. On the water I am seeing just slightly under 3000 rpms at full throttle, which is about right for this engine. I am happy but will have to take a bit cruise to see how it all works out under varying conditions.
As noted above a small change on this prop made a big difference. The character of the boat was remarkably different with the prop change. The boat has a lot less push at the lower end an yet has the same speed reached over the full range.
I will drop another post as I get a few miles under the keel and know whats up.

Thanks all