diesel woes

[wheels 543]

Got to get to work. I look forward to seeing what you find out when I am home again later.

[Guest 000]

I have spent the morning with the engine, checked everything and all is as it should be, except..

The engine in neutral runs as smooth as silk at all rpm. It runs in astern without any problems. In ahead it has these momentary misses or jerks, but the shuddery motion is only fore and aft, and only up to 1800rpm. The engine moves forward on its mounts with each shudder by 3-4mm. I am sure the gearbox is not slipping.

I now suspect the engine mounts in that there is possibly by guess minimal wear on the rubber parts in astern but plenty in ahead. I don't know how old they are but looking at them I would guess their vintage to be early 17th century except for the fact they have a machined stud.

I would like to identify the issue for myself because if I call in a mechanic, it's going to be a case of 'possibly it's this', and 'possibly it's that,' and each 'possibly' is going to cost magabucks.

[Guest 000]

Further to the above..engaging astern causes the engine to move back on the mounts by 2mm at 1200rpm. Engaging ahead causes the engine to move forward on the mounts by 6mm at 1200rpm. I don't know what the permissable movement is, but it sounds as lot. So at moderate revs the engine can lurch around fore and aft, but at high revs it is forward on the limits of the engine mount movement, hence smoth running.

Maybe that's the problem, getting excited now

[Fish 0]

We recently changed our engine mounts. Made an absolute world of difference to noise and vibration, but I started that job because of a noise and vibration issue.

if you are getting 6mm movement forwards (deflection in the mounts) then that is way too much. Soft mounts are good for a 'smooth' running engine, but they also need to resist the sheer force of the propeller pushing the boat forward, as all this force passes through the engine mounts.

Rubber perishes, so if they are of indeterminate age, they may be stuffed. 

 

All that said, they wouldn't cause the issue you are describing. They could be the symptom of the issue, i.e. the engine jumping forward being the manifestation of the problem, but not the cause. 

 

I've got no idea what could cause an issue like that though, the engine to jump forward in a certain rev range. I would note though that the last thing you changed was the gearbox, and the last gearbox failed cause it had a nice smooth slippy action.

 

If the engine is moving that much in a dynamic situation, I wonder if something in the drive chain is catching something in the dynamic state of the engine moving. We also have an R&D flexible coupling, the flange bolts on ours have very little clearance from a hull rib and from the back face of the gearbox. If the engine is moving that much, so is your propeller etc...

[Fish 0]

PS, really good mounts are cushy-floats. When spec'ed for the engine weight and thrust, deflection should be around 2 mm, max 4 mm, and that is at full wide open throttle, not 1,200 rpm. Other synthetic mounts such as isoflex etc, have deflection in the order of 1-2 mm, i.e. much stiffer.

 

If your installation is old, and you have an R&D coupling, you may well have R&D compression mounts. They are circular in plan view and cylindrical in side view, with a steel top cap and a bolt sticking out. They are not shear mounts, which resist the forward thrust.

The cushy float mounts are rectangular, longer in the fore and aft, to give more forward thrust resistance. They also have a steel cap over the whole mount (covering the sides), to prevent oil etc perishing the rubber, not that you get oil or fuel coming out of marine diesel engines ever....

[Guest 000]

I have a 2.5 to 1 gearbox reduction. I was thinking that at low revs the engine mounts aren't loaded up and because they are so soft and probably near death, they allow the engine to leap around fore and aft. Once the mounts load up with increased thrust they can't really move any more so everything settles down. The prop on the H28 is very close behind a big fat deadwood so at low revs the blades are alternatively behind the deadwood and then in the water flow. I know that these situations are not ideal for smooth running at low revs. Faulty mounts would then exacerbate an already less than perfect condition.

Just a theory I'm trying to talk myself into.

[Black Panther 1,607]

Don't run the engine at 1600 to 1800 rpm?  (Dives for cover)

[Guest 000]

Fish, what you described are the mounts I have.

[Fish 0]

How many blades on your prop?

[Fish 0]

Fish, what you described are the mounts I have.

You mean the round R&D ones? with no metal cover?

They will almost definitely be stuffed.

[Guest 000]

BP, you are probably right. Conventional wisdom says not to go easy on your engine, but to run it hard. But just to open another can of worms I read a research paper reasently where a lot of experimentation indicates that slow speed running causes no detrimental effects at all.

[Guest 000]

Three bladed fixed.

[erice 732]

I now suspect the engine mounts in that there is possibly by guess minimal wear on the rubber parts in astern but plenty in ahead. 

 

how easy would it be to loosen the top nuts, removed the bottom 2 bolts and rotate all mounts 180 and test again? 

[wheels 543]

6mm is way too much movement. You need new mounts.
Well....that was easier than reconning a fuel system

[wheels 543]

 

Conventional wisdom says not to go easy on your engine, but to run it hard. But just to open another can of worms I read a research paper reasently where a lot of experimentation indicates that slow speed running causes no detrimental effects at all.

It's not quite that way. The issue with being "over propped" is a lot more complicated with a Diesel, It is not about how fast you run the diesel. Although idling is not good for them.

For a Diesel, it is a two fold related problem. Firstly, Hp is also a relation to RPM. Maximum RPM is where max Hp is developed. I will leave torque out of this, because it adds complexity. So lets just say that you need 50Hp to push your boat to max displacement speed. Lets also say that you need 4000RPM to reach 50Hp. So at 2000RPM, lets say you get just 25Hp. If you chose prop that you can only ever get 2000RPM from the engine with, then you will never obtain 50Hp. Some may argue that they never want to push their boat at max hull speed. That is OK, but at the same time, the engine can never develop 50Hp. There are the odd time you may want that 50Hp. Like in and out of a berth or against wind and tide or getting out of the way of a Ship.

The second part is that because the engine is only developing 25Hp, you may have a wide open throttle and the fuel dose is for trying to achieve 50Hp. Excess fuel to air ratio causes fuel to wash the bore and seep down into the Sump and create black smoke and you are using fuel excessively for the Hp actually being developed. A properly loaded engine actually becomes more efficient. You may only want to run the engine at 50% RPM, which means you will have a 50% or greater reduction in fuel use.

   One rule I have always pushed is NEVER LET YOUR ENGINE IDLE TO WARM UP!!!!! Start the engine, get it in gear and get it working. You don't need to thrash it, but you do need to put a load on it. If the engine is producing blue smoke and yet has not done a great deal of hrs, then chances are you are killing it with kindness. Whenever I have seen this done by people sitting at the dock, i tell them to get it in gear and pull in the lines. The Dock can handle it and the lines will handle it. Get the load on it and let it warm up working. if on a mooring, put it in reverse and pull on the mooring line.

[Guest 000]

So, I'm going to buy new engine mounts. I guess I'll get over spending $800 eventually. Even if it doesn't cure the problem at least I know they do need replacing anyway.

[Fish 0]

how easy would it be to loosen the top nuts, removed the bottom 2 bolts and rotate all mounts 180 and test again? 

Its note wear on rubber, its the rubber perishing.

The deflection difference between forward and reverse will be due to the force produced from the propeller. Being a three blad fixed prop, it is not as efficient going backwards.

[Fish 0]

I can recommend cushy float mounts. They are supplied by Hancock Industrial on College Hill, Freemans Bay. The gentleman there, Alastair I think his name was, used to own a Stewart 34 and is very familiar with marine diesel installations. He also gave a discount from the list price.

 

There is also very good mount selection information on their website, especially a Trelleborg manual (the makers of cushyfloat) that gives all the technical theory on vibration control from engine mounts, if you are into that sort of thing.

 

http://www.hancock.net.nz/products/Trelleborg/mobile-installations/cushyfloat/

 

The original R&D mounts are also available (and cheaper) from Lighthouse Marine (and others I think). Do bear in mind these are most likely just compression mounts and not shear mounts though, i.e. aren't actually designed to adsorb the thrust of the propeller into the boat.

[wheels 543]

One of the big problems of rubber is that it softens with Oil and Diesel.
Marine engine mounts are stupid expensive, but they have two points of difference. They have a captivated design so that in the chance of a rollover, the engine doesn't decide to part company and fall through the floo....I mean roof. And as Fish says, they take push pull loads and not just sit there and look pretty

[Guest 000]

I spent a very entertaining day today replacing my engine mounts. The first thing of course was to remove the flexible coupling joining the engine to the prop shaft and yes, all the bolts were tight. An earlier poster to this thread suggested the possibility of something not being done up correctly and I thought 'come on, Wheels, what sort of moron neglects to check that?' I noticed however that the prop shaft flange was slightly loose. I had drawn the shaft to replace the cutless bearing at my last drydocking and apparently had not tightened the nut quite enough when I reinstalled it. However, I don't see the need to dwell further on that particular issue.

My old mounts had a 3mm steel bottom plate with mounting holes, a cylindrical rubber bit and then a 3mm plate on top from which protruded the engine mounting stud. According to R & D, these are extremely old industrial style mounts. On removal, all showed signs of deformation and on one the top plate and stud had separated from the rubber part. I had imagined that on a flexible mount the stud would go through the top plate and be embedded into the rubber. Not so. On my mounts the stud went through the top plate only and was welded to it on the underside and this was then attached somehow to the rubber portion. Super glue??

So,I I am reasonably confident that with these issues sorted, my engine will be behaving itself better than ever. Just the alignment to do now.

On this issue, I notice that the new mounts have slots to allow for athwartships adjustment. The mounting brackets on the engine also have athwartships slots for adjustment, which is a bit of a double up. I am thinking to nicely position the flexible mounts and fill up the slots with thickened epoxy before tightening the bolts. The slots on the engine brackets will offer more than enough adjustment. Would this be OK?