Potentially Serious Problem

[Guest]

I have a problem that I hope someone can solve!
I have a Seamaster 925 with a mitsubishi herald diesel engine which is maintained well, but I have had a problem with the drive shaft coupling on two occasions.

On both occasions I have been entering a berth or rafting along side, giving blasts of power in forward and reverse to manoever myself, and suddenly I lose all thrust, due to the shaft coming away from the coupling.

Luckily I have been in a situation where I got help from other boaters to secure myself.

The shaft slots into the coupling with a key inserted into the groove between the coupling and the shaft.
An M8 SS bolt slips through a hole in both to secure them.

On examination of the bolt, it had sheared cleanly at the centre.
What I don't understand is how any load is being applied to the bolt, surely the key should take this, and how it is shearing at the centre?

Luckily I could change the bolt while the boat was still, but with the engine in the usual awkward position on most yachts It is very fiddly, I wouldn't like to try it at sea.

Can anyone suggest either another type of bolt, or method of securing the shaft into the coupling, or any other advice?

Regards

Sean Foster

 

[Trevor_swfyc]

Sean,
It sounds as though the bolt is suffering fatigue, is it getting bent in use possibly one way in reverse and then the other in forward gear.
On my Volvo 2002 the shaft is held by 6 alan screws 3 one side and 3 on the other side of the clamp this is then in balance.
It might be possible to change your clamp system without to much of a problem.
You do mention bursts of power from forward to reverse and back but I am sure you do not mean 2000 revs forward then immediatly 2000 revs in reverse as this might cause bits to fall off.
Other than that, do not invite Uri Geller onto your boat again!
Good luck.
Trevor.

 

[dickh]

Not sure of your exact arrangement, on mine the shaft (1" Dia) slips into the
(split)coupling about 3" and is clamped by 4 bolts, two either side. Relies purely on friction and has never let me down. I do not have a key, and it has never had one.
The only suggestion is that the bolt is not fully tightened, perhaps coming to the end of the threads before pulling up tightly? I'm also surprised you only have a single bolt - most of the couplings I have seen have at least 2 bolts.
Is the coupling correctly bored for your shaft?
The bolts should be Hi-Tensile, fine thread(UNF) for maximum strength - on mine they are 3/8"UNF. I notice you use an M8 SS bolt - does this go through both the shaft and coupling? - Try replacing the bolt with an M8 Socket Head Cap Screw(grade 12.9) not Stainless Steel.
Alternatively fit a better coupling!!
Good luck - keep us informed.

dickh
I'd rather be sailing... /forums/images/icons/smile.gif

 

[neil_s]

I'd also suggest making sure the bolt is a good fit in the hole, and that it has a substantial length of shank (without screw threads) where it passes through the shaft.

Neil

 

[Fergie]

Sean
It sounds to me like you (1) check your engine mountings havent perished or gone soft ,causing exessive movment, i.e. forward and reverse rocking about, (2) that your engine allignment is ok in respect to your shaft, causing fatige to the bolt,
(3) change bolt to HT steel, unusual to have 1 bolt ?????. scuse spelling.

 

[charles_reed]

It is unlikely the bolt has fatigued - in any case fatigue failure is fairly easy to observe - the faces are granular, easily picked up when looked at under a microscope.

If you look at it logically the broken bolt is a safety feature preventing worse damage.

The cause is the important factor, certainly the coupling you describe is not usual and the single clamping bolt is possibly inadequate.
I'd suggest checking engine alignment and your engine mountings as a first step - the other possibility is a bent shaft or too great a distance from stern tube to clamp.

Meanwhile avoid stirring things by sharp bursts of forward/reverse.

You could replace the current bolt with a high-tension one - there is a remote possibility that the coupling was assembled sometime in the past with the wrong bolt.

By the way - this may be a blessing in disguise (if you have a sailboat) as a chance to brush up on coming alongside and taking off under sail alone. You defuse the "potentially serious problem" by assuming your engine will never work.

Lest you scoff, if there's wind, I'll sail off a mooring or the anchor 80% of the time under sail and marina berths about 50%.

PS Singlehanded.

 

[Trevor_swfyc]

Potentially Very Serious Problem

Charles,
I was just about to give you an ear bashing for suggesting sailing into a marina. Luckily I check your details and note you sail in the med, now i'm really envious.
If you sailed in the UK I think sailing into a marina would not be appreciated either by the marina owner or other yacht owners, problems such as no brakes being the main objection.
There I go stirring things up again!
All the best.
Trevor

 

[jleaworthy]

I remember a coupling like yours - solid without any clamp bolts - on an old Sabb, I think. To take the thrust, not the torque, it had a tapered pin driven through the whole coupling and, of course, the shaft. This might be what you have but someone has replaced the tapered pin with a bolt which could only be of the smaller diameter of the taper. Suggest you measure the hole on each side of your coupling. If different sizes get a pin turned up in your local engineering workshop and give that a try. Incidentally, when driven well home I never knew the pin to move. Good luck.

 

[jleaworthy]

Thinking again, I don't think it was a Sabb but a Wortham Blake gearbox fitted to a Ford petrol engine (called a Fisherboy I think but I'm probably wrong about that too).

 

[charles_reed]

Berthing under sail

You really can't expect any marina authorities to know anything about the sea or sailing - I always used to sail up to the Berthon office @ Lymington and then into my allocated berth, which used to cause a lot of expectant twitching and surreptitious peering from the go-faster brigade moored on that starboard pontoon.
They were pretty stuffy in the RMYC marina in Poole as well, but the one from who could expect the best performance was the bridge-keeper on the Poole lifting bridge.
When I got up to N Wales, I was generally deemed OK, if eccentric, because Bill Williams (as opposed to Will) was the ex-cox of the Pwllheli boat, knew I was on the Dovey relief and put in a word for me.
In France it's the done thing, because they never learn boat-handling in any case, though I did rather embarrass myself by leaving Loctudy under sail (there'd just been 2 blue ensign mooring travesties and I was trying to correct the locals' judgement of Brits).
Getting away from the pontoon and out of the marina went perfectly and it was whilst short-tacking down the channel, that I left it just too late by Perdrix and, whilst going about, came to an ignominious halt amongst ironic cheering from ashore.

I'd opine that it's a matter of practice, you should aim to be equally competent bringing a boat alongside under sail as power (though it requires split second timing with the spring, coming in downwind).
Most don't practice sufficiently and, you may be right, expect others to only have their level of expertise.

With regard to the Med - the Western Med is awful - give me the Atlantic coasts of Spain, Portugal and France (S of St Mattheiu) for far better sailing.
If you think the standard of seamanship is lacking in the Atlantic you need to see it in the Med!!! talk about waterborne caravans!!!

 

[frilaens]

Forward/reverse

Would someone with more engine savvy comment on what happens when shifting from forward to reverse? I'd not thought too much about it when I first started sailing with an inboard and was quite happily (OK, nervously) manouvering into the berth and quickly changing from forward to reverse. After a while, when my girlfriend began to take her in as well, I had more time to think about it and have wondered about it since; tending to be a bit more cautious! Should you wait for the prop to stop before changing direction? How does the gearbox do it?

Andy

 

[Trevor_swfyc]

Re: Berthing under sail

While anybody who can handle his boat with the confidence to sail into a marina gets my respect. The rules of the marina are for the safety of all, eventually somebody backs out in your path or you get an unexpected wind shift. I wonder what your insurance company would say in the event of an accident, as I am sure a risk assessment would show its far safer with the engine regardless of your ability which is not doubted.
Some boat owners I know have done well if they have remembered to get out the fenders and lines.
All the best to you.
Trevor

 

[mtb]

Reatience is required

All this hard rev this way and that way !!!!!!
Think about it most of the time you should be able to come along side gently and manage to control your boat by just stunning it in gear so to speak.
Even in fast currents you should be able to balance your boats speed so your able to gently make headway . I was taught on a 40ft passenger carrying boat to take my time, taking into account the current and wind .
Try practising some where safe, in mid stream will do learn what happens with slight throttle and at idle putting it in gear ( stunning ) to see the effect.

I'm not a raggie but read in a few books that if you know what your about you can use your sails to back a boat and even balance her against the tide using the wind.
Cheers
Mick

smile you'l be on your boat soon ;-)
<A target="_blank" HREF=http://homepage.ntlworld.com/boats/>http://homepage.ntlworld.com/boats/</A>

 

[Trevor_swfyc]

Re: Forward/reverse

With gritted teeth!

I think if you slam immediatly from forward/reverse the gearbox and shaft coupling are put under considerable load if the revs are high. The prop will rapidly slow in water so even counting one two should be long enough to engage reverse.
I sold a small vivacity 650 many years ago with an outboard and the new owner wrecked the drive shaft by changing gear at high revs.
Gentle is best!
Trevor

 

[colvic]

Re: Forward/reverse

There are two basic types of gearbox, mechanical and hydraulic. With the mechanical you MUST pause between changing from forward to astern otherwise you will soon destroy the gearbox. Though desireable on the hydraulic type you'll get away without pausing but always try to do so.


Phil

 

[colvic]

I think you have a drive/mounting problem here. To keep shearing even a small 8mm bolt takes some force and I didn't think you be'd creating that much.

Check
1) Engine mountings for excessive movement while engine is running.

2) Is the shaft alignment with gearbox OK? Can you slide the shaft in and out of the coupling when the bolt is out?

3) A Vetus clamp round coupling such as a Bulflex, size 4 by the sound of things are very good and though not offering as much misalignment allowance as a Aquadrive are about half the price.

4) TAKE IT more slowly when changing direction, as MTB said.


Phil

 

[Guest]

Thankyou

Thankyou everyone for your replies, I think the first thing to do is check my mountings.

Regards

Sean

 

[trthebees]

from your description, i get the impression that you have a parallel shaft end (ie not tapered) with a keyway entering into the coupling. Then you have a bolt through the coupling at 90deg to the key., held on with a nut.
Hope thats it.
first thing is that keyways in parallel shafts are miserable things when torque reverses, they fret and burr and knacker shafts etc. they are only any good when in one direction, or for reversing when a taper or clamp system is used. Prop is fine, the taper does most of the driving, the key stays a fine fit, snug as the 'roach under the fridge, and against one drive face.
But that isnt your problem. now, guessing again, is thrust taken at the gearbox? not a thrust block?
If this is the case, then this poor little M8 bolt is taking all the fwd/rev thrust. By now, the hole in the shaft is shaped, in the thrust direction, like a shapely lady's waist--although it might not appear so to the naked eye. So the bolt bends back and forth, pivotting on the high point in the centre of the shaft, the point of max stress in bending. BING!!
80 shp or so through this arrangement? hhhmmmmmm.....
So what to do.
Best answer, same as other respondents, is to fit a keyed and clamped coupling. Not too expensive if you measure shaft and flange and go to an industrial supplier.
For now, temporarily, I might suggest that you don't tighten the nut, use a nyloc and let the bolt float. Two reasons. One is that the bolt isnt holding anything together, its just a "thrust" pin, so can float, and this will reduce the compound tensile/bending/compr. stress in the bolt as it bends. Second reason is that it may revolve a bit to reduce fatigue of the same area all the time.
But get the clamp system.
Good luck

 

[trthebees]

in my previous note, 80 should be 18, my guess at the power.
This would give, with a, say 1.5in shaft, around 1400 lbs on the key, and at 5 knots and .5 prop eff, a thrust of around 5/600 lbs. torque.

 

[trthebees]

in my previous note, 80 should be 18, my guess at the power.
This would give, with a, say 1.5in shaft, around 1400 lbs torque on the key, and at 5 knots and .5 prop eff, a thrust of around 5/600 lbs.