Gearbox Ratio

[Paul McCabe]

Hi, I am hoping that someone can help me with a gearbox issue.

I have a Princess 368, 1992, running with 2 x 6BTA5.9M2 300hp and I have recently (June 2022) replaced both my ZF220A gearboxes. The original gearboxes were IRM  220A 1:1.533 (Red info plates) and the new ones are ZF220A 1:1.533 (Blue info plates). Old and new gearboxes identical apart from the info plates.

Since installing the new gearboxes I have had a reduction in power. With the old gearboxes I would get the full 2800 rpm at WoT and now I get a max of 2100 rpm! We lifted the boat a couple of weeks ago to get the prop pitch checked and our surveyor commented on the cavitation damage on both props. The boat was previously lifted in June and the props were cleaned, burnished and balanced etc before being put back on. No evidence or comment from the engineer about cavitation. Size and pitch of props have been checked against Princess Data sheet and the props are correct.

I am being told that the cavitation damage is a result of the transmission being over prop’d. My dilemma is that the size and pitch of the prop has not changed. Same props gave me the Princess spec 2800 rpm/28Kts. Engines have been checked and sea trialed and are performing within Spec. Both engines rebuilt and the turbos rebuilt in the last two years and no issued with power before fitting new gearboxes.

The boats hull is clean and apart from fitting deep sea dripless stern glands nothing else on the boat has been changed.

The only thing I can think of is that the Gearbox ratios are not as labelled. I am hoping that someone with experience of a similar problem can shed some light on this issue.

Many thanks and Happy Christmas

Topic

[Paul McCabe]

Yes, posted earlier: Engine revs at idle (under load) 670rpm,  shaft rpm 438. Both sets fo readings for both engines were very similar.

[Rob Schepis]

Did you check the shaft speed vs engine speed yet with the laser?

[Paul McCabe]

Managed to find a short video clip dated aug 2017 that shows the boat doing 22kts at 2400rpm.

[Paul McCabe]

Sorry Tony. I can’t give you information that I don’t have. I’ve given all the recent mechanical history of my boat. The new props were fitted at the end of 2019 and the condition remained sound until at least 1st June 2022 as per photo uploaded. Since the boat went back in the water on the 1st June the props have deteriorated significantly as per second photo and I have lost 700rpm and 7kts of boat speed. 

[Tony Athens]

I can’t seem to get the answer I am looking for..

All is good.

 

Tony

[Paul McCabe]

Hi Tony.  I can’t give you accurate historic rpm/speed data because I never took that much notice or recorded it. I know we could get 2800 rpm and 28kts in no tidal flow as we would very occasionally open her up to WoT to stretch her legs so to speak.

As previously said we would normally operate her at 2200 rpm and get 20kts. Thats an average as we operate in tidal waters. We will have accurate stats for what we are getting now when we take the boat for another sea trial next week. We will record rpm and speed throughout the range.

The company who handled the props are well respected and have shown me the records (size, pitch) for the old props, the new props and the recent measurements of both the props. All checked against serial numbers stamped on the props and confirmed by the surveyor who takes photos of the props as part of the coding survey.

[Paul McCabe]

We had a valve blow on one engine that trashed the piston so we rebuilt that engine and we had a CaC fail resulting in a rebuild of the other engine.

[Tony Athens]

Way too hard for me to really follow, so let’s try this (below) ..  But is does sound like  after the props  had something done to them,   you never saw 2800 RPM again, only 2100 RPM..

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You said you made or now can only make 2100 RPM    –    What was / is the actual vessel speed at 2100 RPM (NOW) vs. 2100 RPM months/years back when you said you always made 2800RPM?

[Rob Schepis]

Thanks for the further detailed info.  Please elaborate as to the cause of the “catastrophic engine failures” resulting in rebuilds needed.

[Paul McCabe]

To clarify, the boat is used as a school boat and is used for 46 weeks of the year conducting RYA training courses on the RYA Yachtmaster (Motor) scheme.

The engines have in the region of 3500 hours. Both have been rebuilt in the last 2 to 3 years due to catastrophic failures. One engine is an original and the other block/head was replaced last year by local Cummins engineers.

The boat was lifted to change gearboxes as we had had a persistent problem with the Port gearbox and eventually the FNR valve failed and we lost ability to change gear. At the same time we had been experiencing some vibration at lower rpm. I decided to order two new gearboxes as the originals were 30 years old and do go through more gear changes in a day than most boats do in several months worth of use. Whilst the gearboxes were out awaiting arrival of new gearboxes (4 months) we had the shafts checked and the pitch and balance of the props checked. We also fitted new cutlas bearings and changed the log bearing and replaced the dripping stern gland for a dripless, deep sea gland.

I had new props made and fitted a couple years ago. They were made to the spec of the old props so we checked the data sheet to make sure that the old props were in fact the correct ones and had not been replaced with a different size and pitch at any point before I purchased the boat.

Since the boat went back in the water on the 1st of June the boat had been in almost constant use. We noticed the loss of power almost immediately and started investigating with our engineer what the cause might be. At first I suspected a rope wrapped around the prop, had the boat lifted and the hull pressure washed etc. We checked fuel filters, air filters, checked blow by, fuel flow, boost pressure etc. We also checked with the company that we ordered the props through to make sure that the props were delivered at the correct spec. The  props were checked again in the last couple of weeks and they were still at the spec we ordered.

Pre- new gearboxes we would easily achieve 28 kts at 2800rpm (WoT). I have always been impressed at the fact that a 30 year old boat still performs to the advertised spec. Although I own the boat and am the school principle I am not the only instructor/skipper of the boat but my recollection is that we would get something like 20 knots at at 2200rpm.  Now I cannot get more than 2100rpm and nowhere near a full 2800rpm, under load!  So were have I lost 700rpm!

As a school boat she spends most of her time in idle doing manoeuvring but we do achieve 100nm per week and we tend to keep the speed to 20Kts for doing the required passages whilst training. We keep the fuel tanks full and the water tanks half full.

We have recently fitted CCV system but that has been since the low power issue started.

As the props were being removed a couple of weeks ago our surveyor commented on the cavitation so I have also had the earth bonding checked to rule out galvanic corrosion being the catalyst for the cause of the cavitation.

I don’t mess with the engines myself I always use qualified and experienced engineers for repairs and servicing and we always use genuine Cummins parts.

Tony, I don’t think I have left anything out. You will have spoken to our engineer in the past, Simon Green. He mentioned having spoken to you previously though not on this issue. He may be contributing to this thread in due course after todays tacho readings.

I hope the above helps.

 

[Tony Athens]

This paragraph is confusing– Since installing the new gearboxes I have had a reduction in power. With the old gearboxes I would get the full 2800 rpm at WoT and now I get a max of 2100 rpm! We lifted the boat a couple of weeks ago to get the prop pitch checked and our surveyor commented on the cavitation damage on both props. The boat was previously lifted in June and the props were cleaned, burnished and balanced etc before being put back on. No evidence or comment from the engineer about cavitation. Size and pitch of props have been checked against Princess Data sheet and the props are correct.  

 

If nothing was changed as to anything to do with the props, then why did you check the pitch?  

What does  the Princess data sheet on prop size have to do with anything years  /  decades later?   

Is this the 1st time you ran the boat since your “JUNE Prop tune-up”    

You said you made 2100 RPM– What was the actual vessel speed at 2100 RPM vs. 2100 RPM months/years back when you said you always made 2800RPM?

 

I think you are leaving something out  

[Paul McCabe]

Update

So today our engineer, Simon, came to the boat to re-tacho both engines to confirm the out rpm of the two gearboxes. Both came out as near as damn it to be 1:1.533 as labelled! Engine revs at idle (underlay) 670rpm, ZF 220A gearbox output shaft rpm 438. Both sets fo readings for both engines were very similar.

Taking the boat out in a few days for another sea trial to record engine revs against boat speed. Also check fuel flow, boost pressure and whatever else Simon can do.

Any suggestions gratefully accepted.

[donald roth]

You should be proactive on this, a photo tach is a very small expense, and you can verify your gear ratio in a few minutes without removing anything.          Also nobody can BS you then!

 

[Paul McCabe]

Thanks Tony.

My engineer has offered to come out between Christmas and the New Year to have a look and hopefully get to the bottom of this.

[Tony Athens]

IMO — Zero reason to remove to the gear to check the ratio — in the boat  is way simpler and dead accurate if you understand what to do. 

 

[Paul McCabe]

Hi Tony

Both gearboxes are new. Ordered from ZF (Italy) in Jan 2022 and received Jun 2022 from Lancing Marine here in the UK. 

One of the big differences that I have noticed is that the prop walk in idle is noticeably more pronounced. 

I will ask our engineer to re-check the ratio in situ by doing another tacho test. I intend to take one of the gearboxes to Lancing Marine, who can put it on a test rig to check the ratio officially.

Many thanks for the response. Happy Christmas.

 

Paul

[Tony Athens]

Paul,

Not sure where the gears came from, but I personally have shipped out new gears labeled wrong ( 3 times that I remember  over about 1000 gears) .. Last summer cost me big time as the gear went to Hawaii as the R&R  was really tough.. Still dealing with the admin  of it all..

You can check all at a idle in gear ( as Rob said) — Engine at 700-ish should yield 460-ish shaft RPM with a 1.53R. That fact that you are not making RPM would mean may have 1.25R’s?– but I’d think you know it as the boat would be noticeably faster in the harbor up to 1500-2000-Ish ??  Both gears or ??  That would really be odd.

If you, for sure,  determine you have mis-labeled gear(s) , you need to get a ZF authorized dealer/rep involved on the boat to confirm & let him to the paper work. 

Curious, where did the gears come from? They were factory “new” right?

 

 

 

… 

[Rob Schepis]

You can measure shaft speed vs engine speed using a handheld laser tachometer as a means of determining gear ratio. 

[Author]

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