Stabilizer: The Art of Going Slow & Fuel Cost

Some Tips for Avoiding Trouble

Many boat owners are talking about this. If you reduce your gallons per hour from 36 to 10 you reduce cost from $151 an hour to around $42, almost 400% savings. From what I read on the forums a lot of people are going to try cruising at displacement speed since a lot of diesel boats will do around 8-9 knots at 10 gph. Gas will probably be a bit more.

Table of Contents - Show/Hide

Introduction

The modern power boat is designed for going fast, not slow. Some will do okay at 9 knots while for others the ride will be uncomfortable at best. Much of the reason why has to do with aspect ratios, beam-to-length, height and centers of gravity. These were all optimized for planing speeds over 20 knots in the typical “family cruiser.” A thirty-five foot boat with a 13 foot beam has the characteristics of a beach ball on water; that is, it lacks directional stability and will be like trying to steer a balloon in anything but flat water. It relies primarily on speed for stability and control.

I discuss the problem with undersized rudder further on.

Hull Shapes & Speed vs. Stability

A lot of boat owners who formerly dissed my long standing criticisms of modern boat design are now going to experience my reasoning the hard way. Particularly the ones that look like three story condominiums, but also many of the Sea Ray express types as well.

Virtually all hull forms have strengths and weaknesses. If you want to go fast, you have to give up the qualities that make for stability at slow speed since stability at various speeds differs. Conversely, if you want to go slow, you can’t have the characteristics of high speed and still have stability. The obvious point here is that with fuel prices what they are, the only way the vast majority of power boat owners can afford to use their boat is by going slow, very slow, which reality has prompted a number of emails asking about stabilizers.

Hydraulic Stabilizers

I’m no expert on stabilizers but I do have some experience with them. When Joe Six Pack asks whether he should put stabilizers on his 40’ Carver motor yacht, my answer is probably not. The smallest boat I’ve ever seen stabilizers on was a 42’ Grand Banks, a deep draft, moderate speed trawler-style hull with a fairly deep draft and a keel. Bear in mind that hydraulic fin stabilizers are essentially rudders and they do steer the boat. The trick is to try to control roll without steering the boat too much. Either way, you would not be happy running your stabilizers while manually steering the boat because they will work you breathless. Autopilots and stabilizers go together like salt and pepper.

While I know of no engineering rules about stabilizers, I would suggest one based on hull-to-length ratio for this reason: Putting stabilizers on a short, wide-beamed hull is like putting fins on a ball. That’s an exaggeration, but it makes the point that when the ratio is low, stabilizers will tend to decrease stability, both vertical and directional. They will tend to steer the boat. This effect was apparent on the Grand Banks, albeit not to an excessive degree, but it did make the auto pilot work very hard.

I would suggest that there is a minimum length of 48 feet and a beam-to-length ratio of 1:4. Many boats like a Sea Ray are closer to 1:3 Unfortunately, this leaves out most later model boats in existence. Note that the smaller the boat, the lower the ratio is likely to be.

Boat Size Suitable to Use Stabilizer

The main problem here is that the three stabilizer manufacturers all make them sized for larger yachts and the fin sizes are huge and so are the mechanical parts. They simply won't fit on a smaller boat. Once they perceive a need for them on smaller boats, this may change. Wesmar offers the smallest stabilizer system which has a fin length of 25” which is probably too large for anything under 40’.

If your boat doesn’t meet these dimensions but you want to seriously consider stabilizers, I’d recommend that you pay a few bucks to a naval architect and get some sound advice before investing a huge sum. I don’t think I’d put my trust in a manufacturer for advice of this magnitude.

Boats like old Hatterases are prime candidates. Because of their weight, depth and keels, they have hulls much more suitable to trawler-like speeds.

Bilge Keels

Of course hydraulic stabilizers aren’t the only kind; there are fixed bilge keels as well that can increase stability without causing steerage problems. Any type of appendage added to a flat bottomed speedster will retard rapid rolling, slewing, yawing and even pitching somewhat. Bilge keels are longitudinal, shallow fins added to the chine area of a hull, usually a round bilge boat, but I can conceive of designing something for a hard chine as well. These would tend to be more horizontal than vertical, say 30-45 degrees from horizontal. The idea of adding a centerline keel is probably cost prohibitive.

Small Rudders

Yet another problem with slow cruising are the tiny rudders found on most high speed boats. In most cases small rudders won’t provide adequate control at slow speed. While it seemed okay in calm water, in seaway the boat either becomes uncontrollable or it works you to death keeping it on course. Or, using the autopilot, the pilot becomes overworked and suffers breakdowns. The typical modern (and very cheap) autopilot is a lightweight and unlikely to hold up to low speed cruising. While I haven’t investigated this, the cost of changing to larger rudders where control problems are acute may not be excessive.

* * * * *

Diesel Engine Carbonization at Slow Speed

Another question I’m hearing concerns running diesels at slow speeds. It’s true that high speed diesels are not designed to run at slow speeds and can suffer from carbonization. I’ve long recommended “blowing the cobwebs out” after a long day of trolling, but slow speed cruising is not the same as trolling since it lasts longer. I’d recommend that you contact the engine manufacturer or a top-notch bolt-turner. Really Good mechanics (check with the guys at Boat Diesel.com) often know more than the guys that made the engine. For many engines there are possibly some modifications along the lines of “detuning” and cooling system adjustments that can be made that will reduce the carbonization problem. For all Detroits, among other adjustments, the injector sizes can be reduced to eliminate low speed over-fueling.

Note that low heat level is the primary culprit of carbonization along with over-fueling, so changing thermostats or other cooling or aspiration system adjustments to raise low speed temperature, or air flow may be in order.

Gas engines can experience the same carbonization problems as diesels, though true fuel injection systems (not throttle body type) are better at metering fuel at low speed. Check your temperature at your selected cruise speed. If it's not close to 180 you should probably change to a thermostat that will bring the temperature up to that range. My experience is that at around 160 heavy carbon build up begins to occur. But be aware that this may result in overheating at high speed. I check for carbonization in a gas engine by checking spark plugs occasionally; it’s a good tell. Another is sooty particles in the exhaust water when you first start up.

Running on a Single Engine

In case you don’t know, running on a single engine will damage some, but not all transmissions since the propeller on the down engine spins, turning the gears, but not fast enough to provide lubrication. Thus the bearings burn up and you don’t know why. Save yourself some expensive repairs and check with a dealer about your gearbox before running on one engine for long periods. A monkey wrench can make a decent shaft brake as long as you don’t forget it's there and try to run the engine with it in place.

Keep Both Tanks at Equal Levels

If you do run on one engine, don’t forget about fuel management. You don’t want to run one tank down and forget that. Rare is the modern sea rocket that has an equalizer system. No, not rare – unheard of. Fortunately, it is not difficult or particularly expensive to install one. An equalizer simply maintains both tanks at equal levels, which is a much better situation when using fuel at low rates. For one thing, you do not want to keep fuel in the tanks for too long because of bacteria build up, among other problems. You must have water separating filters with sight bowl so that you can monitor water and debris accumulation before fuel system failure occurs due to clogged filters.

It's best to draw down both tanks equally even if you only fill up half-way. Water will accumulate more rapidly when tanks are maintained full or near full. Yes, you read that correctly -- this is due to the lack of agitation that is much more pronounced when fuel levels are lower. This, in turn, results in scavenging water and debris from the bottom which is then removed by the filters. If you’re not using much fuel, it's better to maintain the tanks at half level so that you’ll refuel more often and keep it fresh. Most water comes either from the fuel suppliers tanks or leaking deck fills. Note here that many deck fills are going to leak a little. We’re talking a matter of droplet size amounts here, but they do add up over time. If you are getting water, check the O-ring on the cap, make sure it's there and not damaged. Many caps simply don’t have an O-ring seal and rely entirely on the threads to keep water out.

Posted May 14, 2008

 

David Pascoe - Biography