Propulsion Problem

[thompsontwins]

Hi All,
Went to leave my berth last Tuesday, my LM24 struggled to reach 2 knots. DV24 engine was running fine, and the prop shaft was running fine, no hesitation. The prop was turning but lacking thrust. I thought it may have been a loose prop. I've had the boat lifted out of the water. The prop is not loose. The marine gear (gearbox) is a Bukh ZF BW7 running on ATF, which is full and done 50 hours. There have been no signs of clutch slip, but then, would I have noticed? Has anyone experienced a worn clutch? What are the symptoms? This happened very suddenly, I would have thought a slipping clutch would give warning? Has anyone had similar problems?
If the gearbox requires removal, can it be done in the water? Research shows I need to remove the rear engine mounts, loosen the front mounts and lift the engine enough to get to the lower gearbox mountings.
I have adjusted the gear arm linkage cable, but I won't know if this is a cure for the problem until re launch, hence the question of removing the gearbox whilst on the water, which will require a lifting rig. I don't want to pay for 2 lift outs!
If the consensus of opinion is a clutch fault, does anyone know who could repair it, preferably on east coast?
Paul

[Sula]

Are you sure it is not fouling of the hull and/or propellor that is causing the loss of speed? That is by far the most likely cause of such problems, though max 2 knots is drastic I admit, but it is surprising how much even a modest amount of fouling can sometimes slow a boat. (You would not be the first to go searching for some expensive and mysterious mechanical failure, only to find it was fouling after all.)

Note that ATF (Automatic Transmission Fluid) is the wrong lubricant for the BW-7! This might be causing the clutch to slip..

The Bukh Manual specifies use the same oil as the engine in the BW-7 gearbox - i.e standard mineral motor oil to minimum API CD, about SAE 15w-40, typically sold 'for older diesel engines'. (Don't use synthetic.) The Bukh manual originally specified monograde SAE 20 or 30, but that's hard/expensive to find these days and multigrade oil of about 15/40 is fine, was recommended by Bukh UK's now retired experts, and has proved itself satisfactory in years of use by many Bukh owners. (I think the manual for the current DV36 may actually specify API CD, about SAE 15w-40.)

You should be able to relatively easily check whether it is the gear cable adjustment at fault, by disconnecting it at the gearbox and operating the lever on the gearbox by hand into and out of forward and reverse gear. It is important that when the cable is connected and neutral is selected at the boat's control lever the lever on the gearbox is in the neutral position - i.e. exactly perpendicular to the drive train - across the boat. If it is angled forward or backward at all the clutch (actually there are two clutches, one for forward the other for reverse) will be partially engaged, and that can cause excessive wear and eventually slippage.

Also look for any misplacement or other problem of the gearbox lever itself (unlikely though that is) - it should have a detent (sprung notch) in the central position (exactly perpendicular to the the drive train) and move a similar distance forward and backwards (the gearbox section in the Bukh Workshop Manual specifies the correct full angular movement in each direction, which if I recall correctly is about 37 degrees).

If it does come to removing the gearbox, then according to my research (I haven't done it myself, but I'm faced with the doing it before too long) lifting the engine is only required if you cannot access the lowest of the nuts securing the gearbox to the engine to remove them (which is the case in some boats, not in others). If you can get that/those nuts off, then the alternative is to disconnect the propellor drive shaft (which you'll have to do anyway) and then drop the shaft back , which in turn may or may not require removing the clamp on the shaft and/or the rudder to give adequate clearance to remove the gearbox from the engine.

Good luck, and let us know how you get on.

[thompsontwins]

Hi Sula,

Thanks for the prompt reply and advice. The marina staff also suggested prop fouling. Apparently other vessels in the marina have had similar problems due to a weather related surge in marine growth this year. I haven't used the boat since mid April, so your theory may be correct. I noted small mussels, barnacles etc. around the shaft and rope cutter. I am awaiting a marina staff update on the condition of the prop when the boat was lifted prior to pressure washing. Incidentally, the prop was not loose.

I have spoken to the Bukh people today, they think it isn't the clutches. It's happened too quickly. A burning smell would be an indicator. The ATF fluid is clear, and doesn't smell burnt, which are symptoms.

I agree the manual says SAE30 is the recommended lube. I queried this with the mechanic who previously reconditioned the engine, and he insisted on ATF in the gearbox. I told the Bukh man today it was filled with ATF, and he was ok with that.

My marina has been bought out by ABP who have no facilities to assist an partial engine lift ashore, or any other boat services. So if gearbox removal is required, I will have the same logistical issue of (possible) engine raising to access nuts and bolts, whether ashore or afloat. so I think its a relaunch and hope a clean prop and accurate morse controls adjustment do the job. Fingers crossed!

Paul

[Sula]

Aug 29, 2023 15:30:12 GMT thompsontwins said:

. . .  The marina staff also suggested prop fouling. Apparently other vessels in the marina have had similar problems due to a weather related surge in marine growth this year. I haven't used the boat since mid April, so your theory may be correct. I noted small mussels, barnacles etc. around the shaft and rope cutter. I am awaiting a marina staff update on the condition of the prop when the boat was lifted prior to pressure washing. Incidentally, the prop was not loose.

Yup, prop and hull fouling can make a big difference.  The recent YBW Forum thread linked below may be of interest.  Original poster sceptical that his loss of speed problem could possibly be this - the second suggestion in the thread, repeated various times later - perhaps unsurprisingly as he had even dived to inspect his prop and considered it OK.  The thread ran for 3 weeks and 80 posts, exploring, dismissing and returning repeatedly to all sorts of theories about potential complicated/expensive engine, gearbox, drive train, folding prop, etc. issues.  You can guess what the eventual outcome was!
forums.ybw.com/threads/lost-of-speed.599072/
    

[thompsontwins]

Thanks Sula,

Well, the boat is back in the water, and a test down the river showed things are back to normal. I'm so relieved (and so is my wallet!) The problem was all caused by marine growth. I am amazed that this could have such a detrimental effect on performance. You never stop learning ....