Possible gearbox trouble - oh woe

[Robert Wilson]

I have power problems on my Kubota powered craft.
Since Khamsin was last in commission in 2015 I have fitted a new three blade propeller (no thread drifts, please!) and can only obtain half revs/power in gear. The loss of power was immediately evident on launching in May, so unlikely to be a rope around the prop.
I am about to replace it with the old two blade prop to check, but local marine engineers think it may be "slipping plates" in the ZF gearbox.

This is a new and very alarming concept to me. Quite apart from the near-impossibility of removing the gearbox in the cramped and awkward space I don't know anything about how to go about it or what I might find.
I really don't want the hassle and expense of bringing the boat ashore into it's cradle, especially so soon after re-launching.

I am rather fed-up, to say the least.

Anyone any thoughts, comments, advice, warnings and/or experiences of this problem. What am I faced with, apart from yet another expensive scenario? A figure of £600 - £1000 has been muted for a new box alone - and money is "short".

TIA.
Mr "pi**ed-off"

 

[earlybird]

First thing to check is the cable adjustment and travel, as per engine handbook.
A gear-box oil change is said, by some, to help with this type of problem.
If still no joy, then it could well be either a worn selector or worn clutch plates. If the former, an in-situ replacement might be possible if you're lucky.
If the latter, it's gear-box removal and strip-down. If your Kubota unit is like many small Beta engines, to remove of the gear-box can mean first lifting the engine from its mountings.
Add: re-read post, as said below, half revs of the engine won't be slipping plates.

 

[Talulah]

Won’t be slipping plates. With slipping plates you get full revs on the engine but lack of drive. Sounds more like Prop too big/wrong pitch.

 

[sailorman]

what are the dims of the 2 props

 

[macd]

As said, symptoms are the very opposite of slipping plates, and your "marine engineers" should certainly know that. Even if they don't, it's easy enough to check with a £10 optical tacho on the prop shaft.

And rule one of fault finding: suspect the last change you made = prop.

If the prop is a wildly over as your symptoms suggest, it's highly likely you'd be making lots of black smoke with a wide-open throttle.

 

[Robert Wilson]

Thank you one and all for the rapid responses and thoughts/comments.

First thing to check is the cable adjustment and travel, as per engine handbook.
A gear-box oil change is said, by some, to help with this type of problem.
If still no joy, then it could well be either a worn selector or worn clutch plates. If the former, an in-situ replacement might be possible if you're lucky.
If the latter, it's gear-box removal and strip-down. If your Kubota unit is like many small Beta engines, to remove of the gear-box can mean first lifting the engine from its mountings.
Add: re-read post, as said below, half revs of the engine won't be slipping plates.

Cable adjustments are OK. What where is the "selector"? Inside the gearbox? Or to do with the Teleflex system?
I have checked that the "gear lever" on the side of the box works properly.

what are the dims of the 2 props

I'm sorry, but I don't have the dims to hand. I bought the new prop from FAL at Buckie after giving them as much detail as possible (re engine, gearbox ratio etc).
FAL are a reputable company, up here at least.

As said, symptoms are the very opposite of slipping plates, and your "marine engineers" should certainly know that. Even if they don't, it's easy enough to check with a £10 optical tacho on the prop shaft.

That could be reassuring news!
I have just fitted a Tiny Tach. I get about 3500 rpm out of gear, but can only get about 1900 in gear - forward or astern

And rule one of fault finding: suspect the last change you made = prop.

If the prop is a wildly over as your symptoms suggest, it's highly likely you'd be making lots of black smoke with a wide-open throttle.

No black smoke. Exhaust cooling water spurting as before. Cooling water temperature gauge normal.

Keep the help coming, please :encouragement:

 

[macd]

I have just fitted a Tiny Tach. I get about 3500 rpm out of gear, but can only get about 1900 in gear - forward or astern

First of all, are those figures under way or when moored/berthed? It's perfectly normal not to reach max revs when not moving through the water.

Secondly, my tacho suggestion on the prop shaft was so that you can compare crankshaft revs versus final drive revs. Factor in the gearbox reduction ratio, and you will know for sure whether there's any slippage.

Thirdly, sack your "marine engineers": they should be telling you all this.

 

[Robert Wilson]

As said, symptoms are the very opposite of slipping plates, and your "marine engineers" should certainly know that. Even if they don't, it's easy enough to check with a £10 optical tacho on the prop shaft.

And rule one of fault finding: suspect the last change you made = prop.

If the prop is a wildly over as your symptoms suggest, it's highly likely you'd be making lots of black smoke with a wide-open throttle.

Apologies.
One is a young second engineer on a sea-going boat and the other, his father, is a local fisherman.
That said, I have seen them undertaking all sorts of minor and major work on their fishing boat, and have heard very good reports of the progress through the ranks of the son.
A third man, another local crofter, fisherman and general boat worker had the same "opinion" about the internal plates.
To be fair to them, they have not been aboard or made any investigative work.

I shall be taking the boat alongside (18nm trip to harbour), hopefully tomorrow, there to await low tide for a quick change of prop.
I'm desperately hoping that it is the new prop, and that somehow either I have given FAL the wrong info, or they have made a mistake.

P.S. Another point just come to mind:-
In calm conditions Khamsin can make about 4.7kts/5kts ahead, compared to before anything up to 7.5kts.
Into a slight to moderate headwind the attained speed drops to circa 3kts/3.5kts or even less.
Does that help with the deliberations?

 

[macd]

[/I]In calm conditions Khamsin can make about 4.7kts/5kts ahead, compared to before anything up to 7.5kts.
Into a slight to moderate headwind the attained speed drops to circa 3kts/3.5kts or even less.
Does that help with the deliberations?

I'd say those figures are broadly consistent with the lack of revs you describe (and clearly they were taken when the boat was under way, so answer my earlier question). They are not suggestive of: drive slippage, poor gear engagement, or anything similar.

They are suggestive of excessive load on the engine. The obvious causes of this are overpropping or excessive drag on the drivetrain. The latter can easily be tested by turning the propshaft by hand with the gearbox in neutral. (And, for safety's sake, the engine not running). If it's a struggle, something's probably wrong aft of the gearbox.

(I'm assuming the engine is running sweetly, otherwise you'd have mentioned it. Ditto that the hull's clean.)

 

[sarabande]

Can I just check that the last time you had decent revs and speed from Khamsin was when she was in the water 3 yrs ago, pls ? And also that you have full revs in neutral ?

If so, and assuming that you have made no changes to the gearbox and controls, it looks as if the new prop is to blame.

BTW, have you made an oil change in the gearbox ?

Also, is there decent clearance between the prop blades and the prop aperture / deadwood ?

 

[vyv_cox]

I agree with Mac's detailed analysis. The earlier point about gear cables is that sometimes a tight cable can prevent complete engagement. The trick is to disconnect the cable at the gearbox end and engage gear with the lever on the box. It does not sound as if this is the problem.

You may have discovered the difference between a mechanic and an engineer. A man who can fix his fishing boat does not necessarily know a lot about the mechanics of gearboxes.

 

[macd]

If so, and assuming that you have made no changes to the gearbox and controls, it looks as if the new prop is to blame.

Aka Occam's razor. Do you think it significant that the archetypal old salt is bearded?

 

[Robert Wilson]

First of all, are those figures under way or when moored/berthed? It's perfectly normal not to reach max revs when not moving through the water.

Secondly, my tacho suggestion on the prop shaft was so that you can compare crankshaft revs versus final drive revs. Factor in the gearbox reduction ratio, and you will know for sure whether there's any slippage.

Thirdly, sack your "marine engineers": they should be telling you all this.

Ah! Gotcha.
I can confirm that I slipped the mooring and motored around in calm conditions for about half an hour, testing various engine speeds in both ahead and astern.
Broadly speaking, my comments stand, full revs in neutral (c.3500rpm) and much reduced going ahead/astern (1700/1900rpm).

In fairness to the "engineers", they were not employed to rectify the problem, but we discussed the problem in general.
Their first response was, "Bet there's a rope around the prop." Their second was "slipping plates." Third was "there's a fault with the pitch of the new prop", but they too highly rate FAL .
See my following post #8

Thanks

 

[Tranona]

Another vote for the prop being too "big". Did you check the exact gearbox reduction ratio? if so what was it?

 

[Robert Wilson]

I'd say those figures are broadly consistent with the lack of revs you describe (and clearly they were taken when the boat was under way, so answer my earlier question). They are not suggestive of: drive slippage, poor gear engagement, or anything similar.

They are suggestive of excessive load on the engine. The obvious causes of this are overpropping or excessive drag on the drivetrain. The latter can easily be tested by turning the propshaft by hand with the gearbox in neutral. (And, for safety's sake, the engine not running). If it's a struggle, something's probably wrong aft of the gearbox.

(I'm assuming the engine is running sweetly, otherwise you'd have mentioned it. Ditto that the hull's clean.)

Yes, I've checked the prop shaft turns well "by hand". And yes the engine was turned-off

Can I just check that the last time you had decent revs and speed from Khamsin was when she was in the water 3 yrs ago, pls ? And also that you have full revs in neutral ?

If so, and assuming that you have made no changes to the gearbox and controls, it looks as if the new prop is to blame.

BTW, have you made an oil change in the gearbox ?

Also, is there decent clearance between the prop blades and the prop aperture / deadwood ?

Yes, all was well with propulsion 3years ago. I changed the prop because firstly there was a "rumble" caused most likely wear on the key-way and secondly the prop blades/hull clearance was only about a centimetre.
The latter was possibly caused by my fitting a new stern gland (PSS) and not relocating the shaft far enough back in the coupling.
Said "engineers" doubted if this mis-locating would be possible, but as far as I can remember before the new stern gland was fitted there was adequate prop/hull clearance.
Odd!
No other changes have been made and the gearbox fluid is still clear and clean, although not changed.

I agree with Mac's detailed analysis. The earlier point about gear cables is that sometimes a tight cable can prevent complete engagement. The trick is to disconnect the cable at the gearbox end and engage gear with the lever on the box. It does not sound as if this is the problem.

You may have discovered the difference between a mechanic and an engineer. A man who can fix his fishing boat does not necessarily know a lot about the mechanics of gearboxes.

I'll try the disconnecting of cable, just for clarification, but (without measuring) the travel of the lever on the side of the gearbox appears correct; both astern and ahead.
Your second observation may well be to the point; however, in the past they have given good, helpful advice.

Inverness is 70 miles away across the "hills" ​

so there being a lack of handy, good, local, "proper" marine engineers, there has been a build-up of competent "do-it-yourselfery" in general.

Myself included to a minor level!!

Thank you all for your continued comments.

 

[Robert Wilson]

I agree with Mac's detailed analysis. The earlier point about gear cables is that sometimes a tight cable can prevent complete engagement. The trick is to disconnect the cable at the gearbox end and engage gear with the lever on the box. It does not sound as if this is the problem.

You may have discovered the difference between a mechanic and an engineer. A man who can fix his fishing boat does not necessarily know a lot about the mechanics of gearboxes.

Just a thought - being ignorant of the internal workings of a marine-type gearbox.

When selecting forward or reverse the Teleflex cable moves the lever on the side of the gearbox. IF the cable was damaged/tight and not travelling far enough, what effect would that have on the transfer of power from engine to prop shaft?
I ask this because after two+ years ashore the Teleflex was stiff/stuck and I had to apply more force than usual to move the gear handle in the cockpit.


Could it be that the lever and internal mechanism have not fully engaged the cogs/plates/do-hickies?

I think I'm grasping at straws, but the replacement of the Teleflex kit would avoid a 36nm round trip to Aultbea harbour and all the drying-out/refloating hassle, is a lot less expensive and much easier to fix than a duff gearbox; and would avoid a confrontation with the prop supplier.

IF the Teleflex cable is stiff/damaged enough not to fully engage the gears, then how little a travel would be enough to cause this?

 

[macd]

Could it be that the lever and internal mechanism have not fully engaged the cogs/plates/do-hickies?

You're getting things the wrong way round, Robert. Think of a manual gearbox car: if the clutch slips, the revs soar but the vehicle speed doesn't. Your boat's revs do the opposite of soar. That's why there's a deal of unanimity that it's the prop.

 

[sarabande]

Robert

pending a resolution of the prop issue by physical checks, can you plug in the figures to one of the online prop calculators and see if they come up with something wildly different from the one you have been given (FAL will surely be able to give you the figs for the prop) Is it on the invoice from them ?

e.g.

https://www.vicprop.com/displacement_size.php

and

https://veempropellers.com/propeller-calculator/

 

[Pye_End]

Prop was certainly my first thought as well, but I just wonder if you can be assumed that the engine itself is pumping out full power - eg. would fuel or air starvation give similar symptoms?

 

[Robert Wilson]

You're getting things the wrong way round, Robert. Think of a manual gearbox car: if the clutch slips, the revs soar but the vehicle speed doesn't. Your boat's revs do the opposite of soar. That's why there's a deal of unanimity that it's the prop.

Thanks.
As I said, I'm ignorant of the internal workings of a marine ZF box (road vehicles with cogs n shafts :encouragement - and I was grasping at straws!
I just wondered if the power was coming from the crankshaft but "plates" were not being fully engaged.
I have no idea how the lever operates the "plates" or whatever.