Issues with oversized engine

[eddystone]

As far as I understand it (probably very little!) you need to match the engine to gearbox ratios and prop so that not only is maximum hull speed achieved at reasonable revs, but you don't want say 75% cruising mode to require too few revs (although there seems to be some divergence about what revs do lead to engine longevity).The other requirement is to be able to go dead slow at tickover, but I'm not sure how a) conflicts with b).
Question - would a variable pitch prop (Kiwi Prop) make it easier to optimise a large engine? Expensive but cheaper than replacing the engine.

 

[Tranona]

Question - would a variable pitch prop (Kiwi Prop) make it easier to optimise a large engine? Expensive but cheaper than replacing the engine.

It is not a variable pitch prop, although the pitch is adjustable. However, once adjusted to the required blade angle it is fixed. A true variable pitch prop as used for example on the old Sabb engines allows the engine to run at constant revs and varies speed by mechanically changing the pitch. That enables a low powered engine to move large heavy displacement boats by reducing the pitch and uses a larger diameter prop than normal single gear units. Same effect with old Seagulls where the same powerhead was used with different reduction ratios to give large slow turning props for heavy boats and smaller props and lower reduction for lighter boats.

 

[EuanMcKenzie]

I was reading a report about the HR Rasmus 35. They have a 75hp engine which is said to produce only 42hp on a continous basis. Many have replacement engines of around 50hp.

In the middle of that one at the moment. The MD21A is not recommended to run beyond 3000 in a displacement boat. Its rated at 50HP at 3000rpm. It can give you 75 in a planning power boat at higher revs. I barely cruise at more than 1600 RPM as all you do is try and push a bigger hole in the water and burn more fuel for little gain. It is however very useful into a sea or strong winds. I'm about to re-engine with a Beta 35 and all the calculations indicate its more than enough. It has also worked successfully in other Rasmus I know. I can see its easy life hasn't done it much harm it is now 41 years old and I'm changing it because of the state of the electrics and ancillaries rather than the engine itself.

You really need to look at the power / torque/ consumption curve and this has been said before

 

[Coaster]

Question - would a variable pitch prop... make it easier to optimise a large engine?

Have you looked at the Bruntons Autoprop?

The other requirement is to be able to go dead slow at tickover...

That's the only slight disadvantage of the Autoprop, as far as I know. We cope with it by engaging forward gear for short periods, with longer intervals of coasting in neutral. When parking our boat (34', 8 tons) I usually take it out of gear about 200m before our berth and have ample forward momentum, including several turns, finishing with a short burst astern to halt.

 

[chinita]

I have always been a slave to this 'always run an engine under load' mantra; it seemed to make sense to me.

However, this summer whilst walking along the Oxford Canal I wondered how narrowboats, which often have modern marine diesel engines like ours, cope with bore glazing etc.

They spend about 95% of the time at tickover, trickle charging batteries and when they do move it is at about 3kts and negligible load.

From what I can see, it does not seem to be an issue with them?

 

[Rivers & creeks]

Perhaps their engines are sized to use about 75% revs at walking speed?

 

[Searush]

Perhaps their engines are sized to use about 75% revs at walking speed?

Not normally, from my experience, most just seem to run at tickover for 95% of the time - and that can be for 10-12 hours a day, day after day.

 

[Coaster]

...I wondered how narrowboats, which often have modern marine diesel engines like ours, cope with bore glazing etc.

They spend about 95% of the time at tickover, trickle charging batteries and when they do move it is at about 3kts and negligible load.

From what I can see, it does not seem to be an issue with them?

As stated in post #13, Beta states categorically that its engines are happy operating all day at low revs.

Modern engines are evidently better than old ones in this respect.

 

[chinita]

As stated in post #13, Beta states categorically that its engines are happy operating all day at low revs.

Modern engines are evidently better than old ones in this respect.

But narrowboats have all sorts of engines - from ancient, and highly cherished Gardners and the like - to the same Japanese thing I have in my yacht engine bay.

So why do they not suffer from the issues we fear?

 

[[2068]]

No, you are and, by the way, I did serve an apprenticeship in the motor trade. Bore glazing is a symptom of running a diesel off load for long periods such as charging batteries at low revs. Have a read of -

http://coxengineering.sharepoint.com/Pages/Boreglazing.aspx

Not long after we bought this boat, the engine (Thornycroft 80D, Mitsubishi K4D base) blew all its oil out. When I stripped it, the rings were stuck in no.2 piston and the bores looked like mirrors. Phoned Diamond Diesels the UK Mitsubishi engine importer for rings, gaskets etc. and the first question asked was "been motor sailing a lot then?", followed by "DO NOT run at low revs for long periods or it may happen again". The K4D and many other similar engines are marinised plant engines, designed to run under load at around 3,000 revs constant, mainly driving generators or hydraulic pumps in small diggers.

I've since re-engined and the operating instructions clearly state Minimum Load Operating Revs 1,500 rpm.

Not getting into a pie throwing competition with people that can't read what I said, but ...

OFF LOAD

You said it yourself.

Not running a fractionally larger engine at a slightly lower load! It's not the same thing at all.

 

[GrahamM376]

Not running a fractionally larger engine at a slightly lower load! It's not the same thing at all.

The opening line of the OP's post - I have encountered an number of boats on the market with what I would consider seriously oversized engines - not fractionally larger.

 

[[2068]]

The opening line of the OP's post - I have encountered an number of boats on the market with what I would consider seriously oversized engines - not fractionally larger.

Keeping on track, even a seriously oversized engine ... if run under reasonable load (e.g. driving a propeller), is not going to suffer from bore glazing.
In the motorboating world, there are many planing powerboats on a rivers or lakes, and these boats have engines that are much larger than will be found on any sailing yacht. You might ask why anyone would do such a thing, but that's a different issue.
e.g. There are dozens of 600hp+ boats on Windermere or trundling around the Norfolk Broads at displacement speeds, probably never exceeding 1000rpm.

Running any engine at idle speeds off load, or very low load (e.g. charging batteries only) for hours on end is not a good idea, but it's not the same thing.

 

[Rivers & creeks]

So if I re-engined what should it be? Designer suggested 30hp to 110hp. Currently there's a 40 year old Thornycroft 75hp that never needs more than 75% revs and cruises best at 55%. Ideal replacement is a 58hp, too little or maybe more horses than I actually already have?

 

[Tranona]

So if I re-engined what should it be? Designer suggested 30hp to 110hp. Currently there's a 40 year old Thornycroft 75hp that never needs more than 75% revs and cruises best at 55%. Ideal replacement is a 58hp, too little or maybe more horses than I actually already have?

Both Nigel Calder and Dave Geer publish spreadsheets to help you calculate power required based on your boat's data. For a given waterline length the main determinants are size of prop you can swing and displacement of the boat - plus some recognition of bulk to reflect wind resistance.

 

[Rivers & creeks]

Thanks that sounds good - any chance of a link?

 

[pmagowan]

My engine is a bit oversized as a 20hp Beta on a 28ft sailing boat. I have never had any problems with it and get over the space problem by moving all the filters etc to the front of the engine bay. This makes it neat and easy to change without the risk of dropping bits into the bilge. I can get her up to 7knt through the water at full tilt but don't do this for very long. I prefer to cruise at 5 and push at 6 if in a hurry/bored/needing a drink. It has been very useful having the little extra poke on a number of occasions but it sucks fuel at this speed. She starts on the key without preheating even in winter and there are no signs of things getting glued up. I think, in general, for the amount of use a marine diesel gets all you really need to worry about is giving her clean fuel.