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Choosing the Right Diesel Engine for your Boat

“A Different Perspective”

A very important question and I typically go about answering these types of questions very differently than everyone else. Why? Because I’ve “been there and done that” for 25+ years on 100’s of different types of boats in this size range with every conceivable type of power train installed, and with every conceivable type of vessel operator who wants the “moon” as to performance but doesn’t understand what moving a boat forward really means (i.e. Horsepower & GPH, and ultimately his wallet…

The real and only correct answer you are going to get out of me is the following:

Speed First

Your boat’s cruise speed will come down to 2 things…  The TOTAL operating weight of the vessel and how much fuel you are willing to burn to go from Point A to Point B. Fuel burn must be balanced between your wallet and the business you are in trying to make it all work financially… In so many words, it’s dollars and “sense”.

It’s really that simple because we will all assume that the hull you are working with is well proportioned & designed, the boat is in good trim as it relates to weight distribution,  the prop and running gear is in first-class condition, etc, etc.  This also means that the boat is set up and equipped per your needs, and all the rest that comes with owning, or building, a new boat…

I have not really answered the question on engine selection quite yet, I’m going to throw this at you… Your understanding of how any boat moves forward and how much fuel it uses moving forward.

You must understand this concept, and it does not matter what color the engine is anymore:

Concept #1

Every engine made today in this general class (high-speed turbo charged emission certified EPA 2 or 3 diesel, 4-25 liters), develops about 19-21 HP for 1 hour of every gallon of #2 it burns. Take an 800 HP CAT… Without even looking at the spec published by CAT for that exact engine, I’d bet that at WOT on the dyno making 800HP, the engine is consuming 40 GPH (or more).  Drop it down to 400HP at 1800 RPM or something like that while cruising, and it’s burning 20 GPH. That is IPSO FACTO & you can take that to the bank!

Concept #2

It takes a certain amount of HORSEPOWER to move any given boat under any given set of circumstances… Think about that a tad. Under any given set of circumstances.

This means: what exact boat (all the info), what is total working weight, at what speed are we going to travel, and in what type of weather conditions, or put this way:

The particular set of circumstances is controlled by all of the following variables such as boat size, weight and shape, vessel trim, bottom condition, running gear design and underwater drag, factor along with other external forces such as weather (wind, waves, & current).

So now that we have that fixed in our noggins, now you have to answer the real questions:

1) How fast do you really want to go? Fast cruise, a slow leisurely cruise, and at WOT… What matters most?

2) Then we have to look at your Duty Cycle—In other words, are you a weekend warrior that puts 100-400 hours a year on the boat?  Or, are you a commercial guy that runs 1500+ hours a year, or maybe something in between?

3) And last, what is the speed worth to you? You not only have to look at the cost per mile of travel, you have to look at the cost of the initial engine purchase (MORE HP always costs more), the cost of carrying around extra HP that you may never use other than on seatrial day, and the fact that many times, more HP can also add more weight to the boat kinda making the cost per mile a vicious circle (more weight means a slower boat at any HP output). Things that come into play here can be a bigger transmission, larger prop, bigger shaft, bigger exhaust, etc. etc. See where this can lead?

4)  So now, let me know again …what is your final answer about how fast you want to go?

Once I get that answer, the rest will come with questions about the boat itself. The exact model of the boat, your application or use, engine room layout and room to work with (we both want to be sure that the engine(s) are easy to maintain), and some budget questions…

See, I told you I go about this differently!