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S&S Swan Maintenance - gearbox reduction
18 September 2017 - 18:21
#1
Join Date: 01 April 2007
Posts: 88

gearbox reduction

Dear Friends,

This summer my gearbox broke down. My engine is a Nanni 4.150 and the gearbox has a reduction of 2:1. And I have a maxprop.

I am now offered a new gearbox with a reduction of 1.8:1. My mechanic said it will not make a big difference. And if necessary he can adjust the pitch of the propellor.

Does anyone has experience on this matter ? Or perhaps the professor can advise ?

I would say there is a difference of 10% and that would mean the prop will rotate 10% less....... 

I would be very glad to hear your opinion.

 

fair winds,

Jan

YULUNGA 38/110

19 September 2017 - 07:31
#2
Join Date: 02 January 2008
Posts: 1403

Dear Jan

For a comment it is necessary to know the propeller diameter, and the number of blades. The present pitch, or maximum speed and rpm at full throttle would also be helpful.
Kind regards
Lars

 

19 September 2017 - 08:25
#3
Join Date: 01 April 2007
Posts: 88

Dear Lars,

 

Thanks for your reply !!

Maxprop is 3 blad,

maximum RPM=3000 at full throttle

speed ad 2200 RPM=6knots (crusing speed)

I have to check the diameter, that information is in the boat. I will visit the boat next weekend.

 

Kind regards,

Jan

YULUNGA 38/110

19 September 2017 - 20:16
#4
Join Date: 01 April 2007
Posts: 88

Dear Lars,

Maxprop diameter is 15 inch, and max speed at max 3000 rpm is 7 knots.

kind regards,

jan

 

24 September 2017 - 14:50
#5
Join Date: 01 April 2007
Posts: 88

Dear Lars,

The max-prop has code B34531 according to the instruction manual and 18 degrees RH code : EH.

Kind regards,

Jan

25 September 2017 - 10:13
#6
Join Date: 02 January 2008
Posts: 1403

Dear Jan

Thank you for the information.
Change of reduction ratio to 1.8:1 is OK, and will provide slightly better speed with the proper pitch setting, both at full throttle and cruising.
 
It deserves to be mentioned that with your 2:1 reduction and 18 deg pitch setting the propeller absorbed 19 hp at 3000 rpm. You had the same power available as with the original 20 hp engine.😁
 
With the 1.8:1 reduction 20 degree pitch setting is recommended. You will then have the full power available. 
The propeller rotates faster with the new reduction ratio.
Kind regards
Lars

25 September 2017 - 11:39
#7
Join Date: 01 April 2007
Posts: 88

Dear Lars,

Thank you very much for your comment.

I just spoke to the mechanic to give the oké, but then found out it is the other way around: the original reduction is 1.86:1 and the new gearbox is 2.05:1.......

My mistake off course. Sorry.

Kind regards,

Jan

 

 

25 September 2017 - 13:14
#8
Join Date: 02 January 2008
Posts: 1403

Dear Jan

Good that you discovered this in time.
10 min of number crunching discloses that also your 1.86:1/18 deg setting used less power, 24 hp max, and 2.05:1/24 deg will improve the performance.
 
Would appreciate to hear from you after the sea trials.
Kind regards
Lars

 

25 September 2017 - 15:58
#9
Join Date: 01 April 2007
Posts: 88

Dear Lars,

Thanks very much. It is quite a complex matter !

So your advise is to increase the pitch from 18 to 24 degrees.

In the manual of the maxprop it is mentioned that an increase of 2 degrees will decrease the engine pm by 15% at the same speed.

Will it also effect the max-rpm of the engine ?

Kind regards,

jan

25 September 2017 - 16:44
#10
Join Date: 02 January 2008
Posts: 1403

Dear Jan

24 degrees will allow 3000 rpm, but if you increase to 26 degrees the max rpm and achievable power will drop - the engine is overloaded. 
It was already pointed out that using a smaller pitch will limit the available power - the engine is underloaded.
The proper pitch is a balance between the two 
Kind regards
Lars

25 September 2017 - 18:18
#11
Join Date: 01 April 2007
Posts: 88

Dear Lars,

Thank you so much.

I will set the pitch to 24 degrees and will let you know the results.

kind regards,

jan

04 October 2017 - 16:01
#12
Join Date: 01 April 2007
Posts: 88

Dear Lars,

 

Today performed seatrials.

Pitch is set to 24 degrees.

Speed is much better, but it was difficult to say how much better because there was about 30-35 knots of wind. But it was obvious there was more power. 

Max. rpm however only 2750.

What is your advice ?

 

kind regards,

Jan

YULUNGA 38/110

 

04 October 2017 - 18:47
#13
Join Date: 02 January 2008
Posts: 1403

Dear Jan

Thank you for the information
Here a checklist for sea trials under power, there are quite a number of possible reasons for a reduced rpm reading. 
 
In case you would like to get a Speed-Consumption-Range diagram for YULUNGA you should take the six readings suggested, and forward to me.
 
If the checks below do not bring an improvement, you can either consider reducing the pitch one step, or leave the pitch as is, and take care not to use more than 2450 rpm continuously (300 rpm below maximum in any specific condition).
I guess you rarely use over 2000 rpm, and with this pitch you can enjoy slightly more cruising speed at the same rpm.
Kind regards
Lars
 

Before sea trials
The bottom including propeller to be clean, no fouling allowed
Log calibrated (this is not required if GPS is used for speed measurements)
Engine rpm counter calibrated, or alternatively external digital counter used during trials
Engine fuel filter clean
Engine intake air filter clean 
Fuel water separator clean
Fuel to be proper quality
Cooling sea water strainer clean
Engine fresh water coolant level checked
Engine lubricating oil level checked
Throttle cable free to turn injection pump lever to maximum position
Stop solenoid not set too tight so it restricts maximum fuel flow
Engine space air intake channels open (recommended that engine space hatch kept open)

Sea trials
Calm and deep water, at least 15 m, if using GPS eventual tide effects to be eliminated
When engine has warmed up, record idling rpm as well as high idling rpm, i.e. full throttle with gear in neutral 
Record rpm/speed for six runs at 1500/1800/2100/2400/2700/2850/3000 rpm.
Record air and water temperature, percentage relative humidity
Each run to allow speed/rpm to stabilize for a moment before taking reading
If engine does not achieve 3000 rpm, or exceeds this value record also the maximum rpm/speed.
Keep an eye on cooling water temperature when full throttle is used, if temperature is rising excessively interrupt the run and find the reason for this.

04 October 2017 - 20:11
#14
Join Date: 01 April 2007
Posts: 88

Dear Lars,

Thank you for your rapid reponse. I think al conditions for sea trials are okay. RPM was okay before the reduction gear broke down.

I am glad that I can leave it for now without any problems. I will make sure not to go above 2450 continuous. And I will reduce the pitch by 2 degrees to 22 during winter when she will be ashore. Then rpm and everything should be normal.

Next year I will record speed and rpm for a fuel consumption diagram. Thanks for that as well.

Thanks again !!!!!!

Kind regards,

Jan

YULUNGA 38/110

 

 

21 June 2020 - 19:21
#15
Join Date: 01 April 2007
Posts: 88

Dear Lars,

Last year I fitted a new alternator of 100 amps. The previous one was the standard of 70 amps. Is it possible that it decreased the rpm of the engine by about 150-200 revs? 

kind regards

Jan 

yulunga/ 38-110

21 June 2020 - 19:22
#16
Join Date: 01 April 2007
Posts: 88

Dear Lars,

Last year I fitted a new alternator of 100 amps. The previous one was the standard of 70 amps. Is it possible that it decreased the rpm of the engine by about 150-200 revs? 

kind regards

Jan 

yulunga/ 38-110

21 June 2020 - 20:22
#17
Join Date: 29 January 2007
Posts: 799

Dear Ian,

the answer is definitely "yes", a more powerful alternator absorbs energy, so revs decrease. Did you notice more smoke than before changing the alternator?

Fair winds!

matteo (47/069 Vanessa)

22 June 2020 - 06:02
#18
Join Date: 02 January 2008
Posts: 1403

Dear Jan

Matteo already confirmed this, but there are additional aspects.
As long as the batteries are in a low state of charge, the alternator is hard at work charging them, and this causes an additional load on the engine.  
But as soon as the batteries become approx 2/3 charged (assuming lead/acid), the charging regulator cuts the charging power ever more, and the engine sees less alternator load.
Kind regards
Lars

22 June 2020 - 17:28
#19
Join Date: 01 April 2007
Posts: 88

Dear Professor,

Does that mean that if the batteries are fully loaded (shore power) then a larger alternator does not reduce the rpm ?

kind regards

Jan

23 June 2020 - 12:12
#20
Join Date: 02 January 2008
Posts: 1403

Jan

Yes, a fully charged battery does not load the alternator if there are no other instant electrical loads. You are right that after having shore power connected for a sufficient time - usually overnight - the batteries are fully loaded. The same state of charge is not achieved by running the engine for shorter times.
Whenever there are some electrical loads the alternator assists the battery in meeting them provided the engine is running. If you run a big consumer like an anchor windlass it is worthwhile to run the engine at the same time, the alternator will then help to reduce the heavy load on the batteries.
Kind regards
Lars

23 June 2020 - 19:44
#21
Join Date: 01 April 2007
Posts: 88

Thanks Professor.

Remember (see earlier posts in this thread) that after installing a different gearbox  the revs were about 2750 rpm? In that following winter I changed the pitch in he maxprop, to obtain the normal 3000 rpm, and I installed a different alternator. 
But the max revs were 2550 rpm when she was back in the water.

If the alternator is not the cause, then I must have made a mistake in changing the pitch!

I am stupid......

jan

yulunga 38/110

 

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