# gearbox ratio calculation

Discussion in 'Props' started by enmico, Apr 10, 2013.

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### enmicoJunior Member

how can i calculate ratio for my 2x550 hp volvopenta to reach 20 knot speed

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question

Where are these engines installed ?

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### enmicoJunior Member

ratio

Engines are instald on the 19 m.composit semi displacment boat.wt 40 tonnes.

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### gonzoSenior Member

It is a rather complicated calculation. Are you an engineer or Naval Architect?

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### enmicoJunior Member

ratio

i,m an engineer .

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Info

I think that will give you generally a form to work.

#### Attached Files:

• ###### Propellers Nomogram.pdf
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7. Joined: Jan 2010
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Info

You have only to graph a straight line which connect the point on the power
curve (HP or kW) at your engines power level to the Curve of propellers diameter
that is indicated for your ship hull and if you extend this line to
the third curve you will read the number of Propellers rpm that is recommended.

Then divide the Engines rpm at referred power by the number of Propeller's rpm
that you read in the Nomogram's last curve and you will find the asked Gearbox ratio.

This a preliminary calculation to have an idea about that you asked.

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### enmicoJunior Member

by this way propller D.will around 1750mm and it,s too big for 19M. boat propller could,nt be more than 800mm.
thanks

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Info

I made two lines in the graph.
You can see for example two cases.
a) with 800 mm propeller corresponds to 1500 propeller's shaft rpm
b) with 700 mm propeller corresponds to 1900 propeller's shaft rpm.

so in case that your engine full load rpm are for example 3000 you need ratio 3000/1500 = 2 for a propeller diameter 800mm

or in case b) 3000 /1900 = 1.578 for propeller diameter 700mm.

I think now you understood how will use the Nomogram.

Over that on the second attached file see the limit tolerances between propeller
and the hull. So you can choose a Propeller with a Diameter that is suitable for your case.

#### Attached Files:

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• ###### Propellers Nomogram Red Line.pdf
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71.9 KB
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### gonzoSenior Member

The value of those nomograms is for open water. The turbulence at the stern will influence the values greatly. Also, the maximum diameter and shaft angle will change the propeller choice.

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Point to discuss

That was obvious through the way provided the data. No drawings. Not even photo. Few Owners do that often.
The points you have mentioned should be calculated in a basis of these "missing" information regarding to define the optimum propeller.
Add to that the answer on your question to enmico , if he is NA or Engineer , was not supporting his next handling of the Nomogram data.
Let say that "enmico" first of all is an Owner and I suppose you agree on that.
Your valuable indication and concern regarding the method on propeller's study, is up to him , his future plans and definitely will be based to hull drawings that we haven't seen yet.
Maybe the ship is in order process and they are still discuss the speed issue...
Maybe that will be the subject of his next thread.
Anyway the use of the Nomogram, when will be defined the Propeller's dimensions according and in respect to the factors you mentioned, will exerts a value of gearbox's reduction ratio correctly.

This Nomogram is not used to calculate Propellers' data/dimensions but for selection of gearboxes reduction ratio when these are available - and never in reverse use.

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### gonzoSenior Member

I agree. Maximum propeller diameter is the first parameter and absolute limit of the calculations

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### Roberto CertainJunior Member

Hi everyone:
I bought a Silverton 330 boat totally wrecked the only thing working is the hull. I stripped the boat completely. It used to have two volvo penta engines.

I searched the web and was able to find two New diesel marine engines 120hp, im not sure of what gear ratio to use because the boat normally the boat has 550-600 hp and im only installing 240hp. I really dont care for speed, I would be happy at 10 knots. The transmission does 3000rpm, I would not like to force engine or transmission so I need a gear ratio and propellor that works.

My boat dry weight is about 15000 lbs.
Total length: 10,7 meters
Beam: 3,76 meters
Draft: 0,89 meters
Displacement: 7115 kg

Can some one out there help me pleeease I beg you.

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### comfishermanSenior Member

Google Victoria prop calculator, select the displacement calculator and enter your data. Planning hulls used at displacement speeds is fine, but they usually are relatively higher resistance.

If you keep original props it's going to be a pretty deep gear ratio.

I'll try and run some numbers in the morning.

In the 80s lots of commercial fishing boats targeted herring at high speed. The value for herring dropped and salmon and cod rose to prominence. Lots of hull speed repowers. Pulled a lot of 100hp man's and 715 hp Detroit's and repowered with 300 hp john Deeres. Usually required a fair bit of pitch pulled out combined with deeper gear ratio.

Play with the numbers and see.

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### Roberto CertainJunior Member

Thank you. I know that with the 120hp diesel twins I wont achieve the original boat speed. I Just want to move the boat at a low speed. Originally the volvo penta came with Stern drives but in Hot weather sterns get damaged very fast, so I want to change to a vdrive with a shaft and a propellor, what I dont know is what gear ratio and propellor size to use.

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