# For some easy,for me difficult.



## Robert D (Mar 21, 2009)

Hi to you in the engineroom,
this is not a thread as such, just a question, easy for you folks difficult for me. I was reading the users manual for my boat engine, there it says the engine has reduction gear of 1,91:1 which will enable me to get full effect with the propeller turning at 1000rpm. Question is if the propeller is turning at 1000rpm with 1,91:1 reduction , how many revs should I expect to see on revcounter? I know, my brain has rung full back and stop on this. Many thanks beforehand.
Regards
Robert.


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## R798780 (Oct 27, 2004)

Robert D said:


> Hi to you in the engineroom,
> this is not a thread as such, just a question, easy for you folks difficult for me. I was reading the users manual for my boat engine, there it says the engine has reduction gear of 1,91:1 which will enable me to get full effect with the propeller turning at 1000rpm. Question is if the propeller is turning at 1000rpm with 1,91:1 reduction , how many revs should I expect to see on revcounter? I know, my brain has rung full back and stop on this. Many thanks beforehand.
> Regards
> Robert.


Far too many revs for me. Mawana did 15½ knots on 97 rpm with the turbines doing a bit more, all on 42 tons of heavy a day, and Luminetta with variable pitch did everything on 115 revs - with the main engine/s doing 450 rpm thats a reduction of 3.91:1


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## makko (Jul 20, 2006)

Answer: 1.91 x 1000 = 1910 rpm

The gearbox is a reduction unit, therefore, the engine will be turning faster than the propellor, in this case almost twice as fast.

Rgds.
Dave


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## Satanic Mechanic (Feb 23, 2009)

Are you sure you want to ask this - did you see where the last 'question' thread ended up 

By the way the answer is 1910 revs


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## surfaceblow (Jan 16, 2008)

With a reduction ratio of 1.91 to 1 at 1000 rpm on the engine the prop should turn 523.56 rpm. (1000/1.91) 

Small propellers turn a lot faster than the big propellers on ships.


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## makko (Jul 20, 2006)

surfaceblow said:


> With a reduction ratio of 1.91 to 1 at 1000 rpm on the engine the prop should turn 523.56 rpm. (1000/1.91)


Oh! Surface!
Didn't they tell you - Read the question! The prop needs to turn at 1000 rpm! It reminds me of my Thermodynamics professor. He deserved the title - As a young whippersnapper, he had worked at Frank Whittles jet engine lab in Lutterworth. He showed us how to do numbers on one side of the page and units on the other. Years later, I was checking a calc where a graduate had mistaken Giga for Kilo - His answer was a little out! He didn't even have the sense to look at it from a Common Sense point of view such as that we had learned from the old professor!

Rgds.
Dave
(Only an HND!)


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## Robert D (Mar 21, 2009)

Thank you folks for the prompt replies, I was really caught out on this. I am very grateful thank you. You have most likely saved me from, maybe, an expensive repair job. There is a varibel pitch propeller involved, is seems it was too much opened up because the engine would not go over 1000rpm and when I opened the cover much too my "fright" I saw the engine swaying about like it had come loose from its mountings. At the time I was somewhere between Norway and Sweden. I lowered the revs and it went better but not as I would like. Thank you all very much for your help.
regards
Robert.


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## Satanic Mechanic (Feb 23, 2009)

Now you've done it.

Robert - you will need to check the optimum design pitch setting for the prop if it is variable pitch. 

The propeller should be at this pitch and doing 1000 rpm in other words the engine should be capable of turning the propeller at this pitch and at rated revs.

If the engine is not capable of doing 50% revs at maximum pitch - you may have a potential problem i.e undersized engine. However it is more likely that it may be as simple as setting limits on the pitch - what are the specs for the propeller and the engine?


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## Robert D (Mar 21, 2009)

Satanic Mechanic said:


> Now you've done it.
> 
> Robert - you will need to check the optimum design pitch setting for the prop if it is variable pitch.
> 
> ...


 Thank you for your caring aswer, yes yesterday afternoon I was out trying different pitches and found out that by reducing the pitch the engine had no trouble in doing 2000rpm. With this engine there are three alternatives. 1. an ordinary 3 blade propeller. 2. varible pitch without reduction. 3. varible pitch with reduction. The 1st alternative is supposed to go best at 1000rpm, which seems to have got stuck in my mind However on reading the manual more carefully I started to experiment but the thing I had difficulty with was with getting to grips with this reduction issue, I was so baffled by it and thinking about it I took the wrong buss and ended up miles away in the opposite direction Hehe.
Anyway I am glad I turned to you folks for advice, thank you. By the way, At times I have seen folks from the engineroom maintaining lifeboat engines. This brings on another question just out of intrest. These freefall lifeboats that hang high up pointing downwards, they look like bullets. How do you service or maintain one of their engines? Surely it cannot be a good thing keeping an engine permantly on a slant, would think there to be a good deal of condensation.
Regards
Robert.


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## Satanic Mechanic (Feb 23, 2009)

Robert D said:


> Thank you for your caring aswer, yes yesterday afternoon I was out trying different pitches and found out that by reducing the pitch the engine had no trouble in doing 2000rpm. With this engine there are three alternatives. 1. an ordinary 3 blade propeller. 2. varible pitch without reduction. 3. varible pitch with reduction. The 1st alternative is supposed to go best at 1000rpm, which seems to have got stuck in my mind However on reading the manual more carefully I started to experiment but the thing I had difficulty with was with getting to grips with this reduction issue, I was so baffled by it and thinking about it I took the wrong buss and ended up miles away in the opposite direction Hehe.
> Anyway I am glad I turned to you folks for advice, thank you. By the way, At times I have seen folks from the engineroom maintaining lifeboat engines. This brings on another question just out of intrest. These freefall lifeboats that hang high up pointing downwards, they look like bullets. How do you service or maintain one of their engines? Surely it cannot be a good thing keeping an engine permantly on a slant, would think there to be a good deal of condensation.
> Regards
> Robert.


Stick to the manual settings and you should be fine.

That modern wonder that are free fall life boats (I'm a huge fan of them) - the easy answer is - you don't run them unless they are in the water - most manufacturers let you start and stop them very briefly as a weekly routine. I have never had a problem with condensation


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## Steve Oatey (Feb 6, 2008)

Controllable pitch, not variable.


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## Derek Roger (Feb 19, 2005)

Steve Oatey said:


> Controllable pitch, not variable.


Thanks for correcting this ! 

All propeller of modern design are variable ; only some vessels have controllable pitch propellers .

Derek


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## Derek Roger (Feb 19, 2005)

Also all controllable pitch props are also variable pitch . Enough to make the brains of non engineers hurt .

Derek


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## Derek Roger (Feb 19, 2005)

Steve Oatey said:


> Controllable pitch, not variable.


Thanks for correcting this ! 

All propeller of modern design are variable ; only some vessels have controllable pitch propellers .

Derek


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## MARINEJOCKY (Nov 25, 2007)

Robert D. 

I would ask what kind of boat and engine you have. 

Let me give you some examples. 

Lets say it is a sailboat with a small inboard diesel like a Volvo Penta. I have seen these with a feathering propeller to cut down drag while sailing. 

The volvo engine could be rated at anything from 1500 up to the latest ones running at 4300 rpms, engine rpms'. That means the maximum horse power of the engine as noted in the owners manual could be something like 27 HP at 3300 rpms'. 

Most small engines have a maximum RPM rating and then the "rule of thumb" cruising RPM would be about 10 to 15% below that. 

For that volvo with a maximum rating of 3300 the cruising rpms would be about 2800 to 3000 rpms. Going alot below that reduces the HP available. 

In your case if the owners manual states that the best performance from your propeller is when the propeller is turning at 1000 RPM's. That means the engine is turning at 1910 which if that is the cruising RPM would mean the maximum RPM would be about 2300. Those kind of RPM's are mostly seen on the larger Volvo's, Scania's, MAN, MTU and Caterpiller diesels in pleasure craft. 

What is important is that the engine and transmission are working correctly, the shaft is the correct size for the horsepower and then the propeller can be addressed. Sometimes I have seen a bronze propeller that is not strong enough for a new engine installation and this changed to a stainless steel shaft instead of having to change the shaft log (tube). 

Is the boat new to you, have you installed a engine etc. 

If you either post the boat and engine details on here or send me a PM I will try and help you further.


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## Satanic Mechanic (Feb 23, 2009)

Derek Roger said:


> Thanks for correcting this !
> 
> All propeller of modern design are variable ; only some vessels have controllable pitch propellers .
> 
> Derek


Nope - you better explain that one to me. I think I know where this is going but I will withhold any comment until I know for sure(Thumb)

Actually I'll just edit this - I know exactly where this is going, I am curious as to your take on it - it is still quite a good term to throw at Naval architects - it gets them a bit hot under the collar. Also a great one for the lovers of semantics. (note to BNP supporters - this does NOT mean Jewish)


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