The Controllable Pitch Propeller, a summary.

Richard,

I have a 46' motorsailer under construction. I've done a bit of research on CPP systems and consider the concept as the premium option for my particular application. I have contacted West Mekan and while they have been responsive, I do have some concern about ongoing service and support without any U.S. base distributors. If I remember correctly they were still recommending a reduction gear for a Beta, 2600 rpm, 75 hp engine. In addition there is a minor issue of clearance in order to fit the equipment in my alloted space.

I have also spoken with the Auto-Prop folks and they tell me that the Auto-Prop does exactly what a CPP systems does, just more effeciently. In other words, the Auto-Prop adjust itself just as an operator would using a CPP setup. I'd like to believe this, but it seems a bit to good to be true. I would be interested in your comments regarding this claim.

Regards

 

Hum...my joke does not make any sense anymore now that the question is deleted.
So...

Do you know why the chicken crossed the road?

 

You're partly correct here, over half of the WWII US subchasers were built with the GMC 'pancake' 16-184A diesels (two engines per ship) and CPPs; they were indeed more maneuverable than those not so equipped, but their top speed, with a clean bottom, was only a little over 20 knots. The rest were built with straight-eight GMC 8-286A diesels, and without CPPs; their top speed was only 15 knots.

Several of the more powerful, more maneuverable CCP SCs were converted to gunships for close in support; they were chosen for conversion because of their greater speed and maneuverability.

They looked very similar to the somewhat lighter WWI subchasers, which had triple 220hp Standard gasoline engines; a few of these remained in service with the Navy during WWII.

source: Splinter Fleet, The Wooden Subchasers of World War II, by Theodore R. Treadwell, Naval Institute Press, Annapolis 2000. I have a personal interest in the subject, as my father was CO of SC1039, one of the slower ones; the information in the book as to the slower ships' top speed is confirmed in his diary.

 

You beat me to it..

Richard, is some part of the advantage lost by using the gear provided by Autoprop?
I was enamored by it but could not make a fit in my application.

http://autoprop.com/

 

I've not personal experience of the Autoprop, but to me it seems like good alternative for installations less than 40hp and for easy installation in excisting drive trains. However if building a new boat/new engine/total drive train overhauling I'd choose (>=40hp range) CPP gearbox (reduction and pitch control, no reverse gear) and appropriate CPP shaft propeller combination..

 

As long as propeller power curves will remain hidden by manufacturers i do not understand how we will be able to compare.

About AUTOPROP, they claim a lot of things that seem to me a bit optimistic until i can read the power transfer function

 

On really fast boats , 40K= a high shaft speed might wotk OK.

Onn a 50 ft motorsailor under 1000shaft would be desired , even lower if you could.

The problem is prop diameter (bigger IS better) but shaft angle and prop prices are a hassle.

About 20 inches of diameter is all that will be "normal" , although you might get 24 with lots of work , or a prtop pocket.

DIAMETER is what uses the HP , the pitch does change it some , but is usually used to refine the cruise speeds.

2.5 to 1 reduction on a Kubota would do fine , use a Twin Disc , instead of a Hurth , or get the Hurth 2 sizes larger than their book recomends.
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Its my guess that CPP are not installed in USA boats as we have low fuel prices and 70% of the planets lawyers.

With Manual control it would be possible to overload a CPP engine by an untrained idiot.

Perhaps with cheap computers the prop mfg will imbecilize the units , tho it would be hard without a tranny for neutral starting.

The one unit I worked needed to be set for neutral thrust at about 800rpm before shutdown .
The warm engine would idle at 600 but on start up it needed 800 for the first few min of operation, which meant the boat would move . Imagine the fun of a full throttle start, in a marina!

"As long as propeller power curves will remain hidden by manufacturers "

Power curves are freely given , what is secret is the "Fuel Map" that shows the efficiency at different rpm and loading.

FF

 

i never had this luck when asking for "fuel map" or power curves.

Has anyone WWW links to download them for usual popular prop. like GORI, AUTOPROP or other MAXPROP ?

 

Fred,

You hit the numbers right on. I would like to use a 2.71:1 ZF transmission but AutoProp seems to have a problem with it. I'm not to the point where I have to make a decision about this, and want to keep an open mind. However, where the CPP (WestMekan) looses me is that I still need a reduction gear (and likely neutral and reverse). At this point the economics arguments for CPP loose a bit of steam. Of course the AutoProp ain't cheap either!!! Anyway here's what the AutoProp folks had to say...

"For the Beta 75, 2600rpm we would recommend th H6-566 (22.25" dia) and the 2.1:1 ratio, & no larger. The engineers view is the 2.71:1 for the Autoprop is over the top."

Richard,

I went back and did a little reading about your project and see that you and I are building two different animals. I suspect the AutoProp is probably not a viable option in your case. Nonetheless, I would be interested in your opinion if you have one on the subject. I don't want to hijack your thread, but I think we are talking about the same thing; changing prop pitch to match conditions and rpm. CCP does it via an operator, AutoProp does it through voodoo (best as I can tell ).

Regards,

SeaJay

 

The autoprop is a lot of hype and little advantage! Yes it is more efficient than a fixed prop, in some cases, but has nothing in common with a "Controllable" pitch system.
Like the Gori for example it adjusts itself to a sort of "optimal" setting. Rarely that is really the case.
But being manually adjustable "controllable" is, what makes these CPP systems so efficient.

In our common installations with high revving engines one needs a reduction gear, that is true. A reverse gear as mentioned here, is a nonsense! The prop provides full reverse thrust, without changing the direction. that makes a much faster and smoother change of thrust direction! And you do´nt need shaft brakes on bigger installations.
A vessel with a CPP maneuvres in half the time than one with a gearbox! And much much less stress on the drivetrain.

One of the main advantages is the fact, that you can turn the largest wheel possible without any worry about pitch! The larger the prop, the better in general. (not always valid, though, see high speed craft)

Yes, a untrained idiot could overload the engine (and underload as well). But that is possible with every setup, he does´nt need a CPP for that!

SeaJay,
I did not refer to my own project when opening this thread! That is a completely different world, having much more in common with a commercial ship, than with a yacht. Just the engine / CPP and gear is 14 metric tonnes!

But in your application sure a CPP is the first choice. (as in almost ALL inboard boats)

And again, it is neither more complex, nor more costly than a conventional drive!
Of course being of premium quality and having a fully encapsulated shaft, one cannot compare it with some el cheapo scrap.
When the whole shebang is only 5000 dolores Americanes, you are cheaper away with that. (if thats true over the whole service life of the boat is another question)
But you do´nt compare a premium quality car with a standard Chevy either! So, apples and apples please.

The absurd question about a power curve could only be asked by........yes you guessed it.
For a real comparison one would not need the power curve (and consumption) of a fixed prop either!
The simple fact, that a FPP is eficcient at ONE single rpm only, and a CPP at EVERY rpm makes a comparison senseless.

And do´nt start mentioning commercial applications again please! None of you is going to build a Cruise Liner, and that was´nt the auditorium this summary was adressed to!
Yes I mentioned the fishing fleet, right, but that is a range of boats pretty similar to many of our yachts. Cruise ships and rig supply vessels are´nt.

Regards
Richard

 

Is this right ?

In the case of a diesel motor you would have 2 settings, one for idle and one for where the motor makes the optimum torque, say in this case 2000 rpm.

You'd start the motor at idle and probably allow it time to heat up a bit.

Then set the revs to 2000 and use the CPP from there on to adjust speed.

 

Richard,

I kinda, sorta, agree with your assessment of the AutoProp although I haven't seen any sort of hard evidence to either support or refute their claims. Their website shows some "test data" but does not really address the claim made by their statement;

"An additional feature of the Autoprop is that when motoring in adverse conditions, the blades can automatically reduce and optimize pitch, minimizing engine overload and resulting in power being available when it is most needed."

AutoProp has told me explictedly that their product does exactly the same thing as CPP only faster and that it adjusts automatically to changing conditions. I'm not questioning the integrity of the organization, but it seems to me that a controlled test with a pyrometer would really tell the story.

My statement regarding reverse stemmed from this comment by West Mekan:

"The hand pump is turned to change the stroke of the cylinder which again is
attached to the tension rod. The tension rod is connected to the propeller,
and when it is moved inside the shaft in one of the directions, the pitch is
changed.

The system is very simple and reliable, and if you accept that it can be
heavy to change the pitch at full load this system is OK. For sailing boats
you should also be aware of the amount of turns needed to reach sailing
position. Because of this we recommend the EHW/EHWS for bigger boats/yachts and boats for professional use."

It appeared to me that the manually operated hydraulic system was likely the most robust (and economical system), however, if I was being cautioned about the number of turns required to reach "sailing postion", I supposed that reverse was also going to be problematic. Also, am I not also still in the position of purchasing a reduction gear anyway? Can you clarify this point for me. Maybe I am not fully understanding the operation of this machinery.

For anyone who is interested, here is a link to the WestMekan site where there is a short video of the CPP in action.
http://www.westmekan.com/index.php?option=com_content&task=view&id=31&Itemid=31

 

Thanks Richard.

I came across a few CPP installations in older US yachts.
Perhaps just out of vogue for a time..?

 

No Fanie thats wrong! In that case a CPP would´nt have much advantage.

I said it before, I´ll post some sort of "howto", maybe tonight.


SeaJay

the "automatic" adjustment is the disadvantage of Gori and Autoprop!
And their claims are completely wrong! That is not a CONTROLLABLE prop. It is a auto adjusting one! Never in the optimal setting (or nearly never)

Especially in adverse weather you need to fine tune your settings. In nasty weather you need to respond fast on the throttle, or better on the pitch!

The simple Westmekan system is´nt the solution. Too slow the handpump. You have found the worst out of all the links I posted!

bntii

dunno, but it is impressive to see that a system is well proven since more than 80 years, widespread in Northern Europe (where the conditions are really tough), and almost unknown in the rest of the world.
Of course compare a Mediterranean fishboat and a German or Scandinavian one, the difference in cost and quality becomes very obvious. But why do´nt they know it in the US?

Regards
Richard

 

Right on the spot there; a check on the data given by Autoprop for SeaJay above (75 hp/2600 rpm/z 2.1:1, dia 22.5") tells us that the propeller selected is about 8% bigger dia than a "standard choice" fixed pitch propeller. This is where the hype comes in; in their sales documentation, they show a comparison between propellers, and surprise surprise, the AP comes out on top.

Now, if we really compared apple to apple, a low blade area std prop with the SAME diameter should be used, but then the hyping possibility would disappear of course........ I feel that the blade shape necessary to gain authority of the control mechanism is not quite optimum from a hydrodynamic point of view.

Since the FP torque/rpm characteristics (Torque ~rpm^k, where 1.5<k<2) is far steeper than wanted with regard to engine (~diesel) characteristics, a simple "constant torque" control of the CPP would be fairly close to optimum over a significant operating range!