Issues with oversized engine

[eddystone]

I have encountered an number of boats on the market with what I would consider seriously oversized engines. On a practical level, one issue is obviously access - they can be such a tight squeeze in the engine box that difficult to access filters, lift pump etc. Operationally, if the excess weight isn't a problem but assuming revs have to be restricted, how far is that an issue? I know extended running at idle causes varnish to build up and ultimately, if that stops the piston in its tracks equals bent connecting rod. But I would have thought that running at low to moderate revs was good for a diesel engine. The one in my Land Rover doesn't usually go much above 2000rpm except for the occasional burst to clear the turbo's passages. However, having to run a marginally underpowered engine at high revs isn't going to do much for it either. Also fuel consumption at 75% throttle is about half of that at full throttle, so a big engine run at half throttle may actually be more economical.

 

[eddystone]

I have encountered an number of boats on the market with what I would consider seriously oversized engines. On a practical level, one issue is obviously access - they can be such a tight squeeze in the engine box that difficult to access filters, lift pump etc. Operationally, if the excess weight isn't a problem but assuming revs have to be restricted, how far is that an issue? I know extended running at idle causes varnish to build up and ultimately, if that stops the piston in its tracks equals bent connecting rod. But I would have thought that running at low to moderate revs was good for a diesel engine. The one in my Land Rover doesn't usually go much above 2000rpm except for the occasional burst to clear the turbo's passages. However, having to run a marginally underpowered engine at high revs isn't going to do much for it either. Also fuel consumption at 75% throttle is about half of that at full throttle, so a big engine run at half throttle may actually be more economical.

Further thought - is the gearbox ratio critical to allowing the prop to turn fast enough to be effective in manoeuvring - I'm not sure of the theory here?

 

[GrahamM376]

Can't compare a road diesel load with a marine one in some ways. Your Landrover engine is under variable load and revs when you're driving, whereas a boat isn't as most people just set the revs for a particular speed and don't vary it much.

Motor sailing and running at low revs to charge batteries is bad news for a diesel. They need to be run under load and many modern engines are marinised industrial plant engines, designed to run at quite high revs. Incorrect running will, over time, glaze bores and gum the rings up. To vastly over-engine, thinking it can be run at very low revs, isn't a good idea.

 

[eddystone]

Can't compare a road diesel load with a marine one in some ways. Your Landrover engine is under variable load and revs when you're driving, whereas a boat isn't as most people just set the revs for a particular speed and don't vary it much.

Motor sailing and running at low revs to charge batteries is bad news for a diesel. They need to be run under load and many modern engines are marinised industrial plant engines, designed to run at quite high revs. Incorrect running will, over time, glaze bores and gum the rings up. To vastly over-engine, thinking it can be run at very low revs, isn't a good idea.

Thank you - appreciate the advice

 

[knuterikt]

I have encountered an number of boats on the market with what I would consider seriously oversized engines. On a practical level, one issue is obviously access - they can be such a tight squeeze in the engine box that difficult to access filters, lift pump etc. Operationally, if the excess weight isn't a problem but assuming revs have to be restricted, how far is that an issue? I know extended running at idle causes varnish to build up and ultimately, if that stops the piston in its tracks equals bent connecting rod. But I would have thought that running at low to moderate revs was good for a diesel engine. The one in my Land Rover doesn't usually go much above 2000rpm except for the occasional burst to clear the turbo's passages. However, having to run a marginally underpowered engine at high revs isn't going to do much for it either. Also fuel consumption at 75% throttle is about half of that at full throttle, so a big engine run at half throttle may actually be more economical.

What kind of engine/boat combinations with oversized engines have you encountered?
In many of the new boats I think the standard engines is on the small side.

 

[oldharry]

Optimum running speed fro a modern diesel is arond 3/4 of max RPM. Much depends on how you get rid of the excess power. Propos should not be spun mch over around 1800rpm in a displacement boat, and it is better to keep prop speed to around 1600, otherwise cavitation and slip start eating up the available power. So a diesel with a max sustained speed of 3000rpm, needs a prop that will allow it to 'cruise' at around 2200rpm, and a box reducing shaft speed accordingly.

Alternatiuvely one can overprop a heavier boat, putting a prop on that will hold the engine speed down, but this can result in pushing the hull too fast at tickover. Trying to manoever in the marina with a minimum tickover speed of 4knots can lead to some interesting moments, white hairs, and tears.

Running a bigger engine than is needed to maintain hull speed at around 3/4 throttle is a waste of time, money and usually (but not always) fuel. The exytra weight can be a consideration too.

 

[Beamishken]

I guess ot depends on how overpowered you are talking . we bought a searay 290 sundancer (29') & before we even used it we replaced the 165hp factory fit diesels with a pair of 300hp diesels which meant the boat was 80hp above its recommended hp

we had the boat surveyed & the surveyor agreed that the installation was fine & safe as you wouldn't use the excess hp in other than ideal conditions

the boat ran 40knots flat out (which was slightly slower than the 2X 260hp petrol version) so not over the boats max speed rateing so no safety concerns from the surveyor

in use we cruised at 27kts which was 2700rpm on the engines which made for very economical cruising as it was below 3000rpm where the fuel consumption graph took a dramatic upturn

in 5 years the engines never gave any trouble as they were very relaxed & at 2700rpm glazing wouldn't be a problem

I guess the only slight niggle was the boat was 200kg heavier in the stern so when full of fuel she sat 2" lower at the stern but it was never an issue

space was tight in the bay but was still manageable

slow speed at tickover was 4.5kts but we got used to that & always managed & we were coastal based so most time we were traveling longish distances at higher speeds although we did do all of the Scottish canals in the boat _forth & clyde crinnan & caledonian

so I guess from our experience it was all positive & I think I'd always look for a boat with the bigger engine options

 

[Tranona]

I have encountered an number of boats on the market with what I would consider seriously oversized engines. On a practical level, one issue is obviously access - they can be such a tight squeeze in the engine box that difficult to access filters, lift pump etc. Operationally, if the excess weight isn't a problem but assuming revs have to be restricted, how far is that an issue? I know extended running at idle causes varnish to build up and ultimately, if that stops the piston in its tracks equals bent connecting rod. But I would have thought that running at low to moderate revs was good for a diesel engine. The one in my Land Rover doesn't usually go much above 2000rpm except for the occasional burst to clear the turbo's passages. However, having to run a marginally underpowered engine at high revs isn't going to do much for it either. Also fuel consumption at 75% throttle is about half of that at full throttle, so a big engine run at half throttle may actually be more economical.

It is not the engine that moves the boat. It is the prop, so you decide the prop required and work backwards to determine the shaft speed and power required to turn that prop to achieve the speed the boat is capable of. The big constraints on most boats are the waterline length that determines the maximum speed, the weight and space to fit a propeller. So there is always some compromise to achieve the best balance, and as the prop usually has a fixed pitch the compromise is to achieve maximum hull speed at maximum revs.

Unlike a road application a boat engine operates largely at fixed revs and with only one gear. The control lever sets the required revs and the governor provides fuel to achieve those revs. Most manufacturers recommend running at revs that are around 70% maximum for continuous use, and if the prop is sized to allow the engine to run to maximum revs and achieve hull speed the 70% gives a comfortable cruising speed.

If, as you suggest a boat has too much power it might never run at high enough revs nor be loaded enough. However, in practice many yacht auxiliaries do not run enough hours to have much of an impact on ultimate life of the mechanical bits, but do suffer from other issues such as carbon build up from running lightly loaded for long periods. Different in your Landy as the gearing means that even though it is running at low revs when cruising it is fully loaded. It is possible to introduce gearing into boat propulsion systems to achieve the same effect, for example by varying the pitch of the prop, using a self pitching prop such as an Autoprop or have a selectable reduction gear as in some of the folding and feathering props but it is expensive and only really valuable in particular operational modes.

 

[doug748]

I always think:

Unsuitable cheap, knackered, secondhand engine fitted, by someone who did not understand the issues or did not care.

Of couse this could be wrong but it is one of the dire early warnings.

 

[eddystone]

I always think:

Unsuitable cheap, knackered, secondhand engine fitted, by someone who did not understand the issues or did not care.

Of couse this could be wrong but it is one of the dire early warnings.

In the cases I was thinking of they may well have been fitted (more likely purchased) by people who didn't understand the issues, but they were anything but cheapskate engines. In one case apparently the owner ignored advice and fitted an engine so big the companionway steps had to be built out! However it was subsequently owned by a quite well known reviewer of yachts who seemed quite happy with it.

Tranona, could changing the prop be a partial/interim solution then putting off a replacement engine for a few years at least?

 

[lw395]

Hull speed is not a brick wall, more power will make the boat go faster, just not much faster in relation to the amount of power.
Some boats with narrow or curved stern sections will squat and go very little faster with more power of course.

Also more power will drive through waves and into the wind.

The issue with running a boat engine at constant low power is usually said to be bore glazing, which is also strongly related to running at a constant RPM for a long period.
Particularly a constant RPM which is not ideal for the oil system.
Generator engines run happily at low load at their specified RPM.

Even a small engine in a yacht is not doing much work when motoring quietly on a smooth sea. Unless you always barge around making a bow wave, the power may only be a few HP once the boat is up to speed.
Possibly more power goes into ancillaries like the water pumps, alternator etc?

 

[Searush]

Strong head winds & short steep waves have made me very glad that I upped my engine power by 30% when I re-engined.

 

[Coaster]

Strong head winds & short steep waves have made me very glad that I upped my engine power by 30% when I re-engined.

I did the same and have been very pleased with the extra power. Having an Autoprop makes even better use of the larger engine, especially for motorsailing.

Several comments about detrimental effects of continuous low revs use have been made on this thread. I spoke with Beta before buying our new engine and was told, categorically, that it's not a problem with the modern Kubota block. Apparently there are many Beta engines in canal boats that run at low revs all day with no adverse consequences.

 

[[2068]]

Anyone who talks about bore glazing happening because the engine is a bit too big and running at a lower load, is just spouting rubbish.
The main disadvantages of a larger engine that wasn't needed would be the extra weight, noise, and generally higher service costs.
Fuel consumption wouldn't be a lot different pushing the same boat at the same speed.

 

[Rivers & creeks]

Do marine diesels loose horses over time? We have a 1977 Thornycroft 75hp and we're considering replacing it, we've never used more than 70% revs in even the nastiest weather and run at 40-60% revs in most conditions. We want to replace it with a modern 58hp unit and wondered how many horses we actually have now as a comparison. We had a compression test and it came out first rate, oil pressure is also nice and high. Is it possible to guess how many horses we might have lost?

 

[Sybarite]

Do marine diesels loose horses over time? We have a 1977 Thornycroft 75hp and we're considering replacing it, we've never used more than 70% revs in even the nastiest weather and run at 40-60% revs in most conditions. We want to replace it with a modern 58hp unit and wondered how many horses we actually have now as a comparison. We had a compression test and it came out first rate, oil pressure is also nice and high. Is it possible to guess how many horses we might have lost?

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.

 

[richardm47]

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.

We have an HR Rasmus 35. The old Volvo MD 21a (21b?) had cracked its mounting brackets and we couldn't replace the leaking water pump at a sensible price. So we put in a new Beta 60 4 years ago. That was already many kilos lighter than the Volvo, we didn't want to have a smaller Beta in case the weight distribution got altered too much. Most of the time we run at 1500 revs getting an unstressed 80% of hull speed. Occasionally do 20 mins at max hull speed to work the engine harder, but that is still only 2000 revs. I do worry that such light loading will polish the bores but no trouble yet. We chose a large alternator so maybe that loads the engine a bit. Glad to read above that the Beta's Kubota block doesn't get polished. Is there anything else I could do?

 

[penfold]

Is there anything else I could do?

Not really, an 'Italian Tune-up' now and again is all you need.

 

[GrahamM376]

Anyone who talks about bore glazing happening because the engine is a bit too big and running at a lower load, is just spouting rubbish.

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.

 

[DownWest]

Back to the OPs comment about glazing stopping a piston dead and bending a rod. No. It will result in loss of compression and power, starting problems. Or the blow past detailed above.
DW