Are you in the market for a new or used boat? Have you decided that marine diesel engines are more suitable for your power needs than gasoline engines?
If so, then this is the article for you. We will explore how to choose the best marine diesel engine for your particular application. Read on, captain!
Marine Diesel Engines Are Widely Available
In today’s marketplace, there are three major diesel competitors: Caterpillar, Cummins, and Detroit Diesel. There are many smaller competitors, such as Volvo, Yanmar, M.A.N., and MTU. Reliability of all manufacturers is good overall.
While you may be prone to choose one over the other due to brand familiarity, that certainly should not be your primary decision point.
Much depends on how you plan to run your boat and the level of performance and service life you expect. There are a large number of variables to consider.
If you were to ask your dad or your grandpa about the longevity and reliability of diesel versus gasoline engines, chances are they would tell you diesels win that matchup. And, that used to be true – when diesels were much simpler engines.
Today’s modern marine diesel is very complex, lighter weight and very powerful.
Two-Cycle vs Four-Cycle Marine Diesels
You should know there are two basic marine diesel engine types: two-cycle and four-cycle. Of the manufacturers listed above, only Detroit Diesel makes a two-cycle marine diesel. Four-cycle diesels are made by all the other companies listed.
The differences between the two types are significant, with each having their pros and cons depending on how the engine is used.
Two-cycle and four-cycle diesels are also known as two-stroke and four-stroke diesels, respectively. Because only Detroit Diesel (DD) builds a two-cycle marine diesel, we will refer to this type of engine as a DD from here on.
DD engines produce much higher torque and higher power at lower speeds (RPMs) than a four-cycle diesel.
A “stroke” in engine terms is defined as a single movement through the cylinder by the piston (either up or down). The piston is connected to the crankshaft by a push rod.
In a DD engine, the diesel cycle of intake, compression, combustion (power), and exhaust occurs in two strokes, resulting from one complete crankshaft rotation.
The piston moves up to the top of the cylinder (the first, or compression stroke) then back down (the second, or power stroke) as the crankshaft rotates. Both exhaust and intake occur simultaneously.
A four-cycle engine requires four strokes during two crankshaft rotations to perform these same four functions.
So, even though DD marine diesels are more complex with way more internal parts than their four-stroke competitors, costs are held down by economies of scale due to higher production numbers. Fuel consumption is similar either way.
How Much Power Do You Need?
This is the central question when choosing a marine diesel for your new or used boat. Power equates to speed.
The boat manufacturer makes it easier for you to select the right engine because they install different models, with different options, at the factory.
You just need to test them by taking them out on the water. You should test different option packages and then go with the most horsepower you can afford. Why? Because when you’re testing out a boat, it is not loaded out with your gear.
The power-to-weight ratio is key and even a 50 HP difference between engines could be very noticeable. You don’t want to power your boat on the margin. You should buy a bit more power than you think the boat needs.
The push rods of DD engines are about one-third longer than most four-cycle engines, permitting more rotational torque as the crankshaft turns. Because of this, a DD engine delivers more power from less displacement and lower RPMs.
Detroit Diesel has been making two-cycle engines for over 60 years. They are the largest manufacturer of diesel engines in the world. Many of their engine models use interchangeable parts. They have a great reputation for reliability.
When there is enough horsepower and RPMs can be held lower, engines don’t have to work as hard to give you the speed you want. You will have lower maintenance and fuel costs over the life of the engine.
You need to consider engine weight itself in figuring power-to-weight ratio. And, if you’re fitting a used boat with a new engine, the physical size must be looked at. The new engine will have to fit into the existing engine area (i.e., footprint).
What About Servicing a Marine Diesel Engine?
You need to be able to service whatever engine you select. So, being able to gain access to do-it-yourself service areas is important. It’s true, though, that modern marine diesels do not lend themselves to many DIY tasks.
Unless it’s just routine changes of engine oil and filter, cooling system maintenance, or replacing a fuel filter, the complexity of electronically-controlled boat diesels requires either special tools, special training, or both.
All diesel-powered boats, regardless of craft size or engine horsepower, are required to meet EPA Tier-3 emission requirements. The need to reduce emissions is the main reason engine computer controls are so complex.
Don’t Get Confused By All the Data!
We’re experts in marine diesel engines and we can help lead you through the dense fog of data that is out there. If you have any questions about buying the right marine diesel, contact us today!