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Old 07-16-2023, 08:34 AM   #1
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Starboard side idle stronger than Port

My starboard side reacts much stronger when it is put in gear than the port side. Forward and reverse. Props were just tuned in March so I don’t think it is related to anything under the water. At speed there doesn’t seem to be an issue. Assuming the tachs are calibrated I’m thinking the issue might be in the transmission? They are twin disc transmissions. Is there another variable I need to consider? Or by chance is this “normal” for a 42?
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Old 07-16-2023, 09:22 AM   #2
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I guess I am curious about what is meant by reacting stronger?

Does it shift more quickly? Or shift more quietly? Or shift with less vibration? Or produce seemingly more thrust at idle? Or seemingly produce thrust more quickly at idle?

I wouldn't at all assume tachs are calibrated. I'd check that with a phototach for sure. Being 200rpm off at 2000rpm, e.g.vone tachnreading +100rpm and the other reading -100rpm is no bug deal at 2000rpm, but if it is 600rpm +/- 100rpm at idle, there is a world of difference in how an engine may run at 500rpms vs 700rpm on a cold day.

Which brings me to the other question, how do the engines sound to you? Any difference out of gear? In gear? While shifting gears?

When transmissions have problems you can usually feel it and hear it as the slipping changes the load on the engjne and sounds bad.

Also, a transmission problem coukd be as easy as a fluid change.

Also, with those transmissions, fluid is always a good place to start. If the fluid has debris from some higher speed shifts or just particles from years of use, they won't work as.well.
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Old 07-16-2023, 09:32 AM   #3
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Originally Posted by PatrickF View Post
My starboard side reacts much stronger when it is put in gear than the port side. Forward and reverse. Props were just tuned in March so I don’t think it is related to anything under the water. At speed there doesn’t seem to be an issue. Assuming the tachs are calibrated I’m thinking the issue might be in the transmission? They are twin disc transmissions. Is there another variable I need to consider? Or by chance is this “normal” for a 42?
Hi PatrickF. Tachometer accuracy is often a big "maybe". Have you actually checked with a phototach that your tachometers (and you've got four of them) are accurate? I do not know the brand or model of your tachometers, and therefore I'm not sure they can actually be "calibrated". However, you might check them for accuracy, and verify actual engine speed at idle. It's likely your engines do not idle at the same speed; thus the difference in "reaction" when engaged at idle.

It's unlikely there's any issue with your transmission, nor is this "normal" behavior for a GB 42, or any other twin-screw powerboat for that matter.

Regards,

Pete
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Old 07-16-2023, 09:44 AM   #4
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Thank you for the reply. By reacting stronger, the starboard engine produces noticeably more thrust when put into gear. The motors sound “normal” and shifting seems normal as well. I haven’t verified the tacks, but that is a good point on the difference between low and high rpm. The transmissions are due for an oil change so I’ll do that and hope it helps. Thanks
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Old 07-16-2023, 03:23 PM   #5
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Do the trans oil change and then see if you can hear the engines. What I mean is throttle up on the tacks to the same rpm and then stop looking at tach, pick one engine add and/or reduce rpm and make the engines sound the same, in sync. Then look at the tachs.
You said the props were just done. Please tell me they are not the same pitch, but compensated for the diff trans ratios to cut thru water in sync.
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Old 07-16-2023, 03:48 PM   #6
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Do the trans oil change and then see if you can hear the engines. What I mean is throttle up on the tacks to the same rpm and then stop looking at tach, pick one engine add and/or reduce rpm and make the engines sound the same, in sync. Then look at the tachs.
You said the props were just done. Please tell me they are not the same pitch, but compensated for the diff trans ratios to cut thru water in sync.
Shouldn't the trans ratios be the same? Never heard of them not being unless you're running one in reverse to get the counter rotation. And that's just wrong.
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Old 07-16-2023, 04:15 PM   #7
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They sometimes have a minot difference in gear ratios.
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Old 07-16-2023, 04:38 PM   #8
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I agree that they are sometimes a little different. I don't know that I've ever seen it be enough that props are set up to compensate. Just really small difference.

But, I suppose that given the symptom, they could be off. Something seemingly is.

Any chance you can find the transmission model number on the housing? We might be able to look up the ratio.

I remember the person on the forum a while back that bought a twin-screw to discover that a transmission had been swapped and both props turned in the same direction.

I kept insisting that no one on Earth would commit such a terrible crime against humanity. But he got the numbers and it was what it was.
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Old 07-16-2023, 04:54 PM   #9
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Shouldn't the trans ratios be the same? Never heard of them not being unless you're running one in reverse to get the counter rotation. And that's just wrong.
My last boat had a slightly different ratio between the two transmissions. It was factory built that way. It had to do with having one counter rotating engine, the parameters space they wantes for the props, the lack of reversibility of that model of transmission, and the lack of an exact partner transmission available.

Ultimately, the 185hp contra-rotating engine fell in a weird spot in BW's line w.r.t. the prop parameter space Marshall wanted and the contrarotating engine didnt help. They could have gotten out of the space with appropriate prop parameters, but that would have left the props less tunable by owner, so they tolerated the small disparity, instead. Apparently over propping was common at the time to be able to cruise.more quietly at lower rpms.

More or less hp would have led to ratios with more options and not having thst contra rotating engine would have led to more options. But there wasn't a beefier Perkins 6.354 at that time and the Cat wasn't considered an option until later. RIght where it was and with the prop parameters they wanted...they just decided to accept a small difference.

I never noticed until it was pointed out to me. Seas and wind made waaaaay more difference.
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Old 07-16-2023, 06:50 PM   #10
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MY GB had 2.1 & 1.91 trans with 24x17 & 24x18 props.
The props can be the same if the trans ratio is the same. I have read many threads where people wonder why one engine burns more fuel than the other, just maybe?
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