Why are diesel engines so much more expensive?

I understand why buying a NEW boat with diesels is considered an upgrade, and hence more expensive. Demand and marketing play a large role in that increased purchase price, but it's also usually a 10-20% price increase on a new boat.

But I've always been curious about why a repower with diesels is so disproportionately expensive compared to a repower with gas engines. I have a friend with my identical boat ('97 400DA) and he's on his 3rd set of 7.4s. He can pick up crate motors for a couple grand and swap them out over a weekend. Meanwhile, I hear anecdotal reports that replacing one of my CATs could run me $20k or more.

What is the economical and/or engineering explanation for why a diesel engine costs so much more compared to a similarly sized gasoline engine?

If you mean changing from gas to diesel, I've been thru the cost justification of a diesel repower on a 390EC.......loved the boat; hated the 7.4's......I discovered that there are lots of differences between a gas boat and the same hull with diesel engines. To change to diesel, consider the following:

-Add a fuel return system
-Change the gas generator to a diesel one
-Change most of the engine senders and gauges
-Change the tachs
-Change the shafts
-Change the props
-Change the transmissions....ratios for an engine that will turn 45-4800 are different than the ratio needed for an engine that has a WOT speed of 1/2 that.

If you mean just repowering a diesel boat with fresh diesel engines, then the difference in engine cost is due to the level of rebuild necessary. Diesel engines have a compression ratio of 21-23:1; gas engines have 8.5-9.5:1. A diesel rebuilder will install new liners, new pistons, new bearings, new valve seats and guides, new accessories (oil pump, circulating pump), shave the head and deck the block to ensure flat surfaces, pop test and replace or rebuild the injectors. All t he wear parts are either new or returned to factory spec. There is very little room for slop onan engine with 3X the compression ratio of a gas counterpart. On the other hand, gas engine rebuilders typically only replace the parts that are damaged or worn out of spec, grind the valves and install new stem seals, and replace the accessories. This is the reason that your friend is on his 3rd set of reman replacement engines and you are still on your original Caterpillars.

Gas engines used in the marine environment enjoy the economy of scale provided by the car and truck market. They are, for the most part, just marineized versions of truck engines.

They cost more because we will pay it.
When building an engine, you bore a hole in a block, fit a piston, and provide a fuel supply. It doesn't cost more to bore a bigger hole and make a bigger engine, but a car dealer charges another $1000 dollars. A diesel pickup truck doesn't cost any more to make, but it is a $9000 upcharge.

Bob, so how do those statement about boat engines benefitting from the car and truck market?

My Cats (3406C's) are in hundreds of thousands of over the road trucks. That's what they were originally designed for. It would seem the economies of scale should work to my advantage, but they don't.

I'm usually on the gas side of this argument. Millions of GM Big Block engines have been made and many of the parts are interchangeable with various models. There is also a huge performance aftermarket that surrounds them.

Cats and Cummins are in their own world and they largely control the service and parts for their products. As a result, their products are more expensive to buy and service.

That said....."more expensive" is relative to how many hours the engines have and how many gallons of fuel you burn a year. I believe there is a crossover point where diesels are far cheaper to run depending on fuel costs. The numbers close pretty quickly when you burn 50 gallons an hour (at cruise) in a big gas boat. Diesels in that same boat may burn 20. Arguably, diesels save you $100 per hour in fuel cost in that situation. Someone who uses their gas boat 100 hours a year pays a $10k premium to run gas. Depending how you boat.....it doesn't take long to prove diesels are actually cheaper.

How many 50 ft boats have you seen with gas engines?

How many road tractors (semi trucks) do you see with gas engines?

What about Agricultural tractors (100hp and over) or ever seen a gas powered bulldozer?

My observation would be the difference in torque curves and sustained operation.

I would suggest the cross over point is the amount of torque that you are applying to the turning of the propellers.

Diesels require much higher tolerances than gas engines - not interchangeable. Compression ratios are much higher in Diesels.

Just a couple of thoughts,

Mark

Z-Worthy said:

What is the economical and/or engineering explanation for why a diesel engine costs so much more compared to a similarly sized gasoline engine?

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In my view - heavy duty engineering accounts for the cost difference. Diesel fuel also negates the need for an ignition system and also allows the use of turbo's in combination with relatively high compression ratios, together with large displacement allows low rpm peak torque and thus accounts for the fuel efficiency of diesels as they don't need to turn the revs of an equivalent gas motor.

In economical terms, reliability of a diesel in commercial applications means less downtime and with the associated fuel savings, the cost/benefit argument favours diesel. In non-commercial applications the cost benefit analysis becomes more blurred as you have pointed out and largely comes down to the intended use of a boat of up to say 40 foot.

I think a lot of it has to do with materials and engineering. A car engine is probably designed to run 150000 mile at 50 mph. That's 3000 hours of running. Since a boat is like driving up hill all the time at high rpm, youreduce that longevity to 1500 hours. The diesel truck engine is designed to last 1000000 miles. At 50 mph that's 20000 miles. In a boat a well maintained diesel will last 10000 hours. So there has to be a significant difference in the materials, design and engineering of something that is designed to last 6 times longer and so it's not unthinkable that it would cost 6 times more.

Pete

Gofirstclass said:

Bob, so how do those statement about boat engines benefitting from the car and truck market?

My Cats (3406C's) are in hundreds of thousands of over the road trucks. That's what they were originally designed for. It would seem the economies of scale should work to my advantage, but they don't.

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The more widgets (engines, toothpicks, bath towels), a company makes, the less expensive they are to make. Granted your Cats number in the hundreds of thousands. However chevy small blocks number in the millions.

bobeast said:

The more widgets (engines, toothpicks, bath towels), a company makes, the less expensive they are to make. Granted your Cats number in the hundreds of thousands. However chevy small blocks number in the millions.

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Very true.
That’s why small block parts and small block crate motors are so much cheaper by comparison to other motors.

Why are diesels so expensive? Like divorce, because they're worth it?

ps - Married 32 years.