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Maintenance

Troubleshooting Boat Performance

Compression test

Dear Capt. Thiel,

I’m hoping you can help me with a problem I’m having with my 2006 Cruisers, which is powered by Crusader 350 gasoline inboards. I’ve owned it since it was new, and it’s always hit 30 mph at WOT and cruised at 25 mph. Starting last fall, both top and cruising speeds dropped by three or four mph. My mechanic says both engines should be rebuilt since they have 700 hours on them. But my boatyard guy says my problem is propellers, that mine are old (they’re original) and probably have too much pitch, and I should go to a smaller propeller. Which sounds more plausible to you?

      — M. C. Handley
      Hilton Head, South Carolina

Dear Mr. Handley,

Boat performance is one of those things that a boater can usually troubleshoot himself. There are four main factors that affect it: engine output, propeller efficiency, hydrodynamic drag, and load. Let’s start with the easiest one—load.

Unlike a displacement hull, a design like yours must be lifted partially out of the water to achieve planing speed. Simply put, the heavier the load, the greater the engine has to work. Boaters are notorious for bringing things aboard, then never removing them—stuff like fishing and watersports gear, clothing, cooking utensils, etc. While each may seem inconsequential, they add up. So a good first step in troubleshooting your problem is to inventory these stowaways. You may decide to deep-six some or, for convenience sake, keep everything onboard and just accept the performance penalty.

Hydrodynamic drag is a simple concept: It’s resistance resulting from the friction between your boat and the water. The most obvious cause is a fouled bottom. If your boat hasn’t been hauled and pressure-washed in a year, it’s probably acquired a slimy film and maybe even a “beard.” A little marine growth will slow down a boat considerably, so a haul-out and pressure wash is an integral part of your investigation. And don’t forget the running gear—props, rudders, shafts, and struts. Bottom paint doesn’t adhere well to them, and is quickly replaced by barnacles that produce a lot of drag.

Now on to the expensive stuff. Assuming load and drag are not excessive and your engines are producing their rated horsepower, your original props should work fine. (The obvious exception is if you’ve damaged something, but in that case you should feel vibration.) Even so, if you haul your boat, remove the props and send them to a prop shop to ensure they’re within spec. (I doubt that normal wear and tear would account for the kind of performance loss you’re describing.) Note that if you have decided to keep all that extra gear aboard, however, you may need to change props to ensure your engines turn up to their rated rpm.

And finally: engines. I assume you haven’t experienced hard starting, black smoke, missing, or hesitation. If you have, some adjustments and a new set of spark plugs may restore your boat’s performance. That said, know that engine hours alone are not a meaningful yardstick for measuring wear. Equally important is how you ran your engines (i.e., at what rpm) and maintained them. At 700 hours your engines could be ready for rebuild or replacement—or they could be good for another 700 hours, or even more. To decide which, do three things. First, note how much oil they use between changes. Older engines usually burn some oil, but if you’re adding a quart every time you go out, or every other time, (or leaving a trail of blue smoke), your Crusaders have probably already seen their best days. 

Second, run the boat up to full speed. Your engines should come to within 100 rpm of their rated rpm. If they don’t, they may be not be putting out all the horsepower they should. (Or you may have excessive loading or hydrodynamic drag.) 

Third, perform a compression test, something any good mechanic can take care of, but which you can also do. All you’ll need is an inexpensive compression gauge (shown) and a socket wrench that fits your spark plugs. To begin, remove the coil wire so the engine can’t start. Remove the plugs, and insert the gauge into each spark plug hole. Have someone turn over the engine a couple of times, then note the reading. Normal should be around 150 to 160 psi; Crusader can supply you with the exact reading for your engines. At 700 hours, your engines probably won’t hit 150, but if they’re below 125 or there’s inconsistency among cylinders, things don’t look promising. Oil analysis is a good alternative to a compression test, and a lot easier. It’s cheap and will give you a precise picture of the condition of each engine. Look for “spectrographic analysis” in your yellow pages or search engine.

This article originally appeared in the May 2013 issue of Power & Motoryacht magazine.

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