Jump to content

Diesel engines


Guest 000

Recommended Posts

I have two questions for the engine experts.

 

(1) Why is it that the rule of thumb for marine diesels appears to be that they will deliver 3000 hours of service and then require a rebuild? My H28 had a 30hp 3 cylinder Perkins. Years ago I had a Ford Dexta tractor with a 30hp 3 cylinder diesel and I sold it with 9000hrs on the clock - no problems, still going strong. This is normal for land based diesel applications. Why the disparity in expected hours run time between land and sea diesel usage?

 

(2) In European motorboats up to the 12-13 metre LOA mark it is quite popular to tuck the engine away in some unused corner down aft, put a hydraulic pump on the end of it and run a couple of hoses to a little hydraulic motor on the end of the prop shaft. The installations I have seen are very neat, save a lot of premium boat space and I would think easier with regard to alignment - better to connect the prop shaft to a smooth running hydraulic motor than to a diesel engine that's leaping around on its mounts. Presumably I'm missing something because if it's such a good system, wouldn't everyone be doing it.?

Like to know because I'm going to spend some money in Europe on another boat.

Link to post
Share on other sites

Losses in transmission. It's not as efficient.

There is no reason at all why a marine diesel can't last A lot longer than 3000 hours, if installed and maintained correctly. Most are not, and corrode away rather than wear out.

I know several commercial marine diesels in excess of 20000 hours. They run all day every day...

Link to post
Share on other sites

(1) Several factors at play here. Firstly, the little modern engines tend to be high reving. 4000RPM is the norm. This means that they are revolving around nearly twice the distance to the older slower reving. Then you need to consider the power produced from these things today. Compare it to the Old Perkins figures.
Basically summed up, High revving, high power, light weight Vs the old faithful reliable heavy workhorses.

(2) Yep it sounds wonderful, but there are two big problems with Hydraulic. The first and major one is the poor efficiency. Ruffly 50% of the Hp the engine produces goes into pushing the Oil around the system. There are huge losses with hydraulics. That loss ends up as heat and there is a lot of it.
The second part is that because of this loss, the engine needs to be running at full noise to produce it's best power to match the power required by the Hydraulic motor. So you end up with a revving engine most of the time to produce enough power to overcome the loss and give you some drive.
And then there is the oil. Great when everything is where it is supposed to be. But bust a hose and it quickly becomes a mess. Have a seal leak and you are forever trying to clean up oil.
The better system is actually Diesel over Electric. An electric generator on the engine directly powers the electric motor on the prop. As you rev the engine, the prop spins faster. This is the basic principle behind all Diesel Electric Trains and big Earth moving equipment. The Electric motors being within the Wheel itself. And of course, some Ships use this kind of drive system also.

Link to post
Share on other sites

Thank you men.

I knew there had to be some downside to hydraulics.

On the engine longevity issue, the boats I'm looking at operate in fresh water, if you discount all the poo in the French canals. Most have closed circuit keel cooling. Most will run at around 1600rpm so as not to exceed the canal speed restriction of 3 knots. The average, and what we are looking for is around 12m, 8 tonnes and engine power 60 -80hp. So with a well maintained engine one could expect a long service life?

Link to post
Share on other sites

I've never understood how you match the duty point of an engine (target revs) with the gearbox ratio and propeller size when you want to spend 95% of your time at 3 knts. I've got this crazy idea that you get a badly undersized engine, run it at full noise, put a really big reduction ratio in the gearbox and match the propeller to the shaft speed for a 3 knt cruise.

 

But if it helps, we've got a 37 Hp Izuzu 3 cyclinder on a 37ft, 6 T yacht. Max Revs is 2,800 rpm but we normally cruise around at 1,800 rpm. The engine is about 35 years old and hasn't missed a beat. Its a classic big, heavy slow reving thumper. Weighs twice as much as a modern equivalent of the same horsepower, and has the same displacement as a turbo charged model that produces 180 Hp.

 

I'd be interested in what type and size of engines they actually use in these canal boats.

Link to post
Share on other sites

 

The average, and what we are looking for is around 12m, 8 tonnes and engine power 60 -80hp. So with a well maintained engine one could expect a long service life?

Pretty much, but there is also one other thing that Kills Diesels. Love. They hate Love. Never be kind to a Diesel. They are wild animals that need to be harnessed up and run hard every now and then. They hate idling around the place all the time. It causes them to glaze up and burn oil.

 

 

I've never understood how you match the duty point of an engine (target revs) with the gearbox ratio and propeller size when you want to spend 95% of your time at 3 knts. I've got this crazy idea that you get a badly undersized engine, run it at full noise, put a really big reduction ratio in the gearbox and match the propeller to the shaft speed for a 3 knt cruise.

if I understand you correctly here, a Diesels power is related to it's RPM. Not exact, but you could say for explanation, A 100Hp engine develops 100Hp at Full RPM. Lets say that is 4000RPM. 50Hp would be at 2000RPM. 25Hp at 1000RPM.

When you want the power, you need those revs and the potential boat speed or at least response. You might be wanting to get through a current or stop in a berth or whatever. 3kts is pretty slow for most boats. It would mean you are idling around. It takes very little power, so no need to have the engine revving to produce unneeded power and high fuel use.

 

Link to post
Share on other sites

I know where you're coming from, Fish. It's confusing as heĺl.

It's hard to generalize but the typical steel Dutch motorboat, 10 - 12meters, 8- 10 tonnes. Most are 4 cylinder, 60 - 80hp, most popular brands in order Daf, Mercedes, Mitsubishi, John Deere. They have big gearbox reductions, often 2.9 to 3.3 to 1.

On the other hand the old barges converted into cruising boats get along quite nicely on the Rhone against the current. I've seen these barges often, 15-18metres, up to 25 tonnes,40 - 80hp. How do they go it on such little horsepower?

Link to post
Share on other sites

if I understand you correctly here, a Diesels power is related to it's RPM. Not exact, but you could say for explanation, A 100Hp engine develops 100Hp at Full RPM. Lets say that is 4000RPM. 50Hp would be at 2000RPM. 25Hp at 1000RPM.

When you want the power, you need those revs and the potential boat speed or at least response. You might be wanting to get through a current or stop in a berth or whatever. 3kts is pretty slow for most boats. It would mean you are idling around. It takes very little power, so no need to have the engine revving to produce unneeded power and high fuel use.

 

Yes that is a good summary, but lets say you have a new engine, and it needs to reach max revs to validate the warranty. You need to size the prop and shaft speed so the engine can reach max revs. Then if you cruise around at an idle, aren't you just going to glaze over the bore and basically wreck the engine? As per your comment above, never love an engine.

 

Same applies to those big launches with 600 plus Hp engines that live up the various canal developments like Whitianga Waterways. Have to cruise for miles at 3 knts. I can just imagine some massive Cummins engine worth 10's of 1,000's idling along the waterways. Either that or putting a trolling motor onto the swim platform of a $2mil launch...

Link to post
Share on other sites

Some power vessels use wing engines for that, as it really isn't good to run the main engines too lightly loaded all the time.

That does not mean they can't run at 50%, just that from time to time they should be run wide open, while watching the temp and oil pressure...

Link to post
Share on other sites

As IT said. As long as you open it up and give it a good workout every now and then.
As for Prop and Gear ratio, it depends on what you are wanting to achieve with a design. Sailboats are about just that. Sailing. They are not designed to be used as a power boat and certainly not at 3kts all day. The main criteria is to get you in and out of the berth and then have the least drag as possible under sail. A Bare is about moving weight, getting it moving quickly and stopping that weight quickly. It does not have to get in the plane. Then you have the Fast machines that need to do just that. So just like any Vehicle, you have gear ratios and tire sizes. A Tractor may only have 100Hp and yet will out pull a Car with a 600

Hp performance engine. But the Tractor will never do 250Kph down the back straight. A little Toymotor Wishitcouldgofaster would be economical with little narrow tires fitted. But fit wide 255 wide tyres to it and you will find it slow and heavy on fuel.
It is all about the propellor and how they work in a fluid.

Link to post
Share on other sites

Yep not good for diesels running under light load. Our 60hp Yanmar (Turbo) had about 1000hrs on it when we bought the boat 15years ago and used to have a bit of blue smoke from being babied .Had to clean the turbo as choked with carbon. Luckily have managed to save it by cruising at around 2500rpm and regularly revving it out to around 3200rpm for about 5 or 10 minutes (making sure not to overload ie no black smoke). - max RPM is 3400.

It has now done around 4000 hours is clean burning, uses no oil and touch wood will remain trouble free. Turbo also clean as a whistle.

Link to post
Share on other sites

If a turbo diesel isn’t pushed hard enough to get the turbo working properly, the turbo vanes collect carbon, as they don’t get sufficiently hot to keep themselves clean. This will lead to turbo failure. Sometimes this can be heard just before the failure occurs, as a squeak or chirp from the turbo, as the built up carbon begins to touch the housing. Turbos turn very fast, and the vanes need very little deflection to have a catastrophic failure....

If your turbo still has paint on it, it’s likely not working hard enough. Running the engine at wot for 30 mins now and then is no bad thing

Link to post
Share on other sites

For me this post is becoming the sum of all fears.

In a peculiar turn of events we don't own a boat anymore, and are looking to replace the dearly departed H28 with a canal boat In France.

As yachties, we bought or built boats with all our attention focused on rig, sails, running gear etc. The engine was secondary to all this. In considering a motor boat the engine takes on a whole new level of importance.

A detailed search online would indicate that a naturally aspirated diesel should deliver 5000 hours if not abused and one that is well looked after in terms of regular maintenance, proper operation in terms of revs,loading warm up, cool down etc should deliver up to 8000 hours.

So we decided in our search for a canal boat to limit our choices to vessels with a maximum engine usage of 2000 hours since new or rebuild, and preferably less.

However, even with a few hours engine longevity must be dependent on how the owner has treated it?

I was keen on one boat with 1100 hours on the clock but the owner told me with a clear conscience but in ignorance that his engine handbook said to change the oil annually. The book also said the leisure rating on his engine was for 1000 hours per annum. He concluded therefore that since he only did 200 hours on average yearly then he didn't need to change his oil for 5 years. How many owners are there with shonky ideas as to what proper maintenance is?

So it would seem that with engines, 'you pays yer money and you takes yer chance', and we are somewhat discomforted with that.

According to those who claim to know what they're about, no diesel engine has ever died from over use. They die from corrosion and neglect.

Going forward then,and still limiting ourselves to a max 2000 hours, if an engine survey shows compression to be within range, an endoscopic inspection does not reveal any internal corrosion and the oil does not contain any particulates, then it's a happy one?

Link to post
Share on other sites

Hi Chris, I know that in the UK Beta (Kubota)  or elsewhere Nanni (Kubota) are pretty near bullet proof, (given that normal maintenance is kept up) Vetus used and I think still do Mitsubishi conversions, up there with the Kubota's. Naturally aspirated is the best way to go, any of the common rail engines are more likely to have issues with stale / dirty fuel etc, top of the tree tech wise, just not casual user friendly. With your Dutch connections finding a goody should be a "breeze" (if it has a steadying sail.) Good Luck

Link to post
Share on other sites

Chris, if it was me, I'd look at the oil, make sure it looks like oil, not tar, not grey or milky. Then look at the filters, water trap, and belts. Do the filters have dates written on them, or in the log book? Are the belts tight, in good order, and does the engine room look clean and maintained? Does it start without much cranking, and run as expected. Can you run it under full load for 20 mins with no overheating.

If all that is good, then a compression test. If it passes all that, it's almost certainly fine.

By the way, my Volvo Penta 43 hp 2003 turbo is approaching 30 years old, has almost 10,000 hours on it, and still runs fine.

Don't over think it, if in doubt, make your offer subject to mechanical survey, and get a reputable mechanic to check it. If good, go boating!

  • Upvote 1
Link to post
Share on other sites

Thanks Steve,IT. You are right about over thinking. The trouble is the more I learn about engines the more I realise I don't know.

In the final analysis we will be guided by the good advice above and a surveyors report and hope for a good outcome.

I hope something happens soon - first time without a boat in 46 years and we've both got separation anxiety.

Link to post
Share on other sites

Chris, the one big plus of a diesel, most especially the older ones without all the computers and electronics, is that they have a great simplicity to their operation.
1: if it starts easily and idles smoothly, then the compression is good.
2: if there is a bit of blue smoke at start up, then it probably just wants a good hard run.
NEVER EVER, allow a diesel in a boat to idle to warm up. That is what causes issues. Start the engine, allow the oil pressure to come up, get it into gear as soon as practicable and let it warm up under some load. Don't thrash it, just make sure there is a bit of load and slowly lift the revs. For most of us, that all happens anyway. We start engine, lift anchor, get into gear, slowly motor out of a Bay or Marina and then ope the throttle.

One big plus with the Canal boats is that they are not running in Salt water. Damp Salt air is what causes corrosion and the big problem that can cause compression problems is the Salt air in the exhaust attacking the Valves when the engine is left sitting for long periods. in the very cold climates, because they don't use the boats in the winter period, they actually Winterise the Engine.
I also would not worry about the oil not being drained regularly. It is a rule of thumb maintenance thing only and not hard and fast essential. The idea is that the replacing at least at a 12month interval will help to remove any possible condensation that maybe inside the engine. That does not mean there definitely is water inside the engine. It is just good practice is all. Once again, not in Salt water, not as much of a problem.
Dirty oil as thick as treacle is the one that causes damage and that comes from very high hrs with no oil change and the owners that treat their engines like that, tend to have other more obvious issues with the engine that would make you think, hmmm, this guy doesn't look after his engine.
 

Link to post
Share on other sites

Thanks Wheels and others.

I am beginning go feel quite confident about venturing into the unknown world of motorboats. All we need to do now is to reassure ourselves that we are not too old to take on such a radically different venture in a foreign country.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...