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Old 09-19-2014, 03:28 PM   #1
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Question Different ratio gears on twin-screw boat

Hi all,
First of all - thanks for the warm welcome on this forum.
Now here goes my first question, and it doesn't seem to be an easy one:
My new to me trawler (Aquila, you can see her in the "welcome mat" forum) is fitted with twin Peugeot Vetus engines from 1982 combined with (C71 or something) Velvet-drives.
Strangely (to me anyway) the reduction ratios of boxes are different, or at least so their labels state: one is 1,91:1 while another is 2,10:1. Sure enough the screws themselves are also different in their pitch.
Previous owner knows nothing why this is so, only that he bought her this way from the first original owner.
Does anyone know if there is a specific reason for such a setup?
I'm about to replace the props, and have been thinking of maybe re-gearing one of the boxes and having similar pitch props.
Thanks in advance for ideas.
Boris
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Old 09-19-2014, 03:36 PM   #2
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Not uncommon. Probably your engines both rotate the same direction. On the BW gears on the back is a planetary reduction. One reverses direction, one does not. So you get counter rotating shafts. Apparently the design of the planetaries is such that the ratios are slightly different. The pitch of the props should be higher on the 2.1 side. Divide pitch by ratio on each side, should be very close.

More modern transmissions have the same ratio in either direction, avoiding this minor annoyance.
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Old 09-19-2014, 03:44 PM   #3
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Ski, thanks a lot!
What you say makes perfect sense.
Both engines turn in the same direction indeed, but…
From what I saw in the Velvet's manual - they did have 1,91:1 available in both directions. Strange they didn't fit similar ratio drives both sides. Or is it also normal?
Boris
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Old 09-19-2014, 04:14 PM   #4
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The 1.91 was available in both rotations. Provided the engine could spin either way!!!

Many old engines could be ordered in either rotation, so this was not an issue. Modern engines all spin the same way, unless you get up to really big machines like tugboats.
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Old 09-19-2014, 04:21 PM   #5
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Which means that I'm mistaken, and my boxes can not be re-geared to have the same ratio - right?
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Old 09-19-2014, 04:27 PM   #6
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I do not think the planetaries are available that both change rotation AND have identical ratios. Most solve the problem by adjusting pitch 10% from side to side.

I have seen BW gears with equal ratio reductions, but those were drop centers and that would require changing engine mounts, etc. Easier to fix with prop pitch.

I am not a BW expert, so maybe some searching may reveal other options.

As long as your engines both get the same full power rpm, it is not an issue.
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Old 09-19-2014, 04:44 PM   #7
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Indeed you're right.
The only ratio available in "counter-engine" rotation for BW71C is 1,91:1.
The closest to it in "same-as-engine" rotation is 2,10:1.
I was mistaken 1,91 exists in both rotations.
Thanks a lot for clarifying! You saved me a lot of a headache!
Boris
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