posted 11-27-2005 12:08 PM ET (US)        

In my case, my current boat averages about 1.8-MPG overall, and even at optimum cruise only reaches 2.1-MPG. I began to look at performance reports of other types of boat to see what sort of fuel economy their hulls and motors were producing. What I found surprised me.

GO-FAST BOATS

Many people think that the epitome of bad fuel economy must be the high-power offshore-hull go-fast boat. It turns out these boats are not as thirsty for gasoline as you might think. In a recent issue of POWERBOAT MAGAZINE (May 2005) the editors reported on the performance of a 24-foot V-hull runabout, the ULTRA 24 STEALTH. This boat is roughly similar in dimensions to my Boston Whaler. It is 24-feet long, has a beam of 8-foot 6-inches, and has a V-hull with 22.5-degree deadrise at the transom. It is powered by a 375-HP stern drive, and it weighs about 3,800-lbs. The engine has a displacement of 496-cubic-inches, or 8.1-liters. This combination of boat and motor has enough power to reach a top speed of 63-MPH. That is about 50-percent faster than my boat. But here is the surprise: This boat can cruise at a comfortable 25-MPH and get 3.5-MPG fuel economy! That is about double the fuel efficiency of my two-stroke outboard and Boston Whaler combination at that speed.

You can cast a cold eye on the ULTRA 24 STEALTH's gaudy paint job, on her plush upholstered race-style seating, on her impracticality for fishing or cruising, but it is quite hard to ignore this boat's very significantly better fuel economy when compared to my classic Boston Whaler!

HOW THEY DO IT

What is the secret to the improved fuel economy of this go-fast boat? Of course, the first advantage it has is the four-stroke engine. A four-stroke engine guarantees some improvement over my old two-stroke outboard due to its more efficient fuel use and better valve timing. Less unburned fuel goes out the exhaust port of a four-stroke. But what really helps the go-fast boat is the choice of engine horsepower and propeller pitch. With a 375-HP engine, this ULTRA 24 STEALTH can swing a 24-inch pitch propeller, even though the gear ratio on the stern drive is a tall 1.5 to 1. As a result, that 25-MPH cruise speed come with the engine only turning over 2,000-RPM. In contrast, my Whaler is turning a 15-inch propeller through 1.86 to 1 gears. To reach 25-MPH the outboard has to turn over at 3,800-RPM, almost double. The go-fast boat's engine is loafing along at 25-MPH cruise, while my outboard is running about 66-percent of throttle.

WHAT WE CAN DO

Using the performance of this go-fast boat as a model, what can we learn from it to help with the fuel economy of our Boston Whaler boats? The most obvious factor is to switch to a more efficient engine, either a four-stroke or a direct-injection two-stroke. Next, choose the engine horsepower to be at the very high end of the hull's rating. This will allow you to choose a propeller with a higher pitch. And this will permit the boat to run on plane at cruising speeds with a lower engine speed.

MORE PROPELLER CHOICES

Naval Architect Dave Gerr notes in his excellent technical booklet, Propeller Handbook, that "unless your boat speed will consistently be above 30 or 35 knots" propeller efficiency will increase with slower shaft speed and larger diameter propellers. (Note: 35-knots is 40-MPH). Most Boston Whaler boats are operated below 40-MPH most of the time. Thus Gerr's advice seem applicable. Go for lower shaft speed (which implies larger pitch) and larger diameter propellers.

WHERE THIS LEADS

If all this advice is followed, where does it lead? Using my boat as an example, to maximize fuel economy it seems I should do the following:

--re-power with a four-stroke or direct-injection two-stroke
--re-power as close to hull maximum (300-HP) as possible
--look for larger diameter propellers and slower shaft speeds

The engine that comes to mind that best fits these criteria is the SUZUKI 250-HP four-stroke outboard. What distinguishes it from others is the ability to turn larger diameter propellers with its 2.29-to-1 gear ratio. Before bolting one on the transom, let's look for some performance reports on other boats with similar dimensions.

Suzuki's website provides some good information. They have tested the 250-HP on a 22-foot 5-inch center console with 8-foot 6-inch beam that weighs 2,700-lbs. This is somewhat comparable to my Whaler. On this boat the engine is turning a 16-inch diameter 23-inch pitch propeller. Now for the fuel economy numbers:

At 3,500-RPM and 28.6-MPH = 3.9-MPG !<--see below for comment; this number is in error!

The fuel economy is impressive, to say the least. That is just about double what my current boat can obtain.

In another test on a heavier MAKO 23-foot center console with a lower pitch propeller (21.5-inch), the big four-stroke still achieved an impressive 3.9-MPG fuel economy at a slower cruise speed (23-MPH).

KEY WEST 225: [Hyperlink: www.suzukimarine.com]
MAKO 232CC: [Hyperlink: www.suzukimarine.com]

OTHER OPTIONS

HONDA—Another four-stroker with a reputation for fuel efficiency is the HONDA BP-225. This engine has a more conventional gear case, with a 1.86-to-1 ratio. It does not swing large diameter propellers. Curiously, the Honda website provides a performance test on the same MAKO center console mentioned above. With the smaller engine, smaller pitch, and smaller diameter propeller, the fuel efficiency was not as good. Optimum cruise:

At 3,500-RPM and 22-MPH = 2.9-MPG

MAKO 232: [Hyperlink: www.honda-marine.com]

E-TEC—To discover how two-stroke engines compare we can look for results using the E-TEC engine. For performance reports we have to turn to the excellent resource of e-tecinfonet.org . They have more test reports on the E-TEC than anyone else, including Bombardier! Unfortunately the MAKO hull is not among them, but there is a test of a 23-foot sport boat with 8-foot 6-inch beam that weighs 2,700-lbs. This appears to be the closest comparable boat tested. The E-TEC shows strong fuel economy. Despite turning only a 14.75-inch diameter, 19-inch pitch propeller, the modern two-stroke achieved:

3,5000-RPM and 27.4-MPH = 3.5-MPG

NAUTICSTAR 230: [Hyperlink: http://www.e-tecinfonet.org]

posted 11-27-2005 01:48 PM ET (US)            

VERADO—Using a Boston Whaler 210 OUTRAGE with a length of 20-feet 6-inches, a beam of 8-foot 4-inches, and weight of (a rather husky) 3,400-lbs as a comparable boat, the 200-HP Verado turned in some amazing fuel consumption numbers:

3,000-RPM and 19.5-MPH = 4.06-MPG

posted 11-27-2005 02:10 PM ET (US)            

[Hyperlink: whaler.com]

Even more amazing is the Verado 175 on that boat which is reported to provide 4.69-MPG at 3250 RPM and 21.1 MPH.

This tends NOT to support the notion that one will achieve better mileage with a larger, more powerful motor on a given boat.

posted 11-27-2005 03:49 PM ET (US)            

As well there is a link as follows comapring the Verado and E-TEC. I have to tell you I though the Verado's were the be all to end all, but this does not appear to be the case as can be seen in this report: [Hyperlink: www.e-tecinfonet.org]

All always new I liked Evinrude and now a whole lot better, thanks for the link for the great information. What really impressed me is how the E-TEC has much better fuel economy at the top end and how it burns way less fuel at the top end. Why would anybody buy a Verado based on the reports seen here. And I am not trying to stir it up but the figures do not lie. Especially the way I boat with the throttle down most of the time.

posted 11-27-2005 05:16 PM ET (US)            

Another area where fuel economy could be improved is total weight. You will recall that according to Crouch's formula, boat speed is in proportion to the ratio of horsepower to weight (to the 0.5 exponent).

SPEED = C X (HP/LBS)^0.5

where

SPEED = boat speed
C = a constant which depends on hull type
HP = shaft horsepower at the propeller
LBS = total displacement (weight) of the boat

Solving this for HP produces

HP = LBS x (SPEED/C)^2

In other words, the horsepower needed is directly proportional to weight. In the case of my boat, which I estimate to weigh about 4,800 pounds when underway with normal fuel, gear, and crew, if I could reduce the weight I should also reduce the horsepower needed to maintain the same speed. Let's try an example:

At 4,800-lbs, I need 225-HP to reach 39-MPH (using C=180). If I could find 400-lbs of gear to leave ashore, I could reduce the boat weight to 4,400-lbs. This is a reduction of (400/4800)=0.083 or 8.3 percent.

Using this lighter weight and solving for horsepower, I would only need

HP = 4400 x (39/180)^2
HP = 4400 x 0.0469
HP = 206.5

Fuel economy should improve if the horsepower is reduced to 206.5 from 225. This is a reduction of (18.5/225)=0.083 or 8.3 percent.

Therefore, every pound I can take off the boat will produce a direct and proportionate improvement in fuel economy.

This is one area where not much attention has been paid. Both engines and hulls have been increasing in weight. A boat manufacturer with a strong but light hull that was powered by a modern light-weight fuel efficient engine should be able to produce some very significant improvement in fuel economy compared to existing hulls and older two-stroke engines.

Will higher fuel prices provide the incentive for boat builders to turn to lighter weight construction?

posted 11-27-2005 09:29 PM ET (US)            

I have to reject the simplistic results of Crouch's formula predicts. Reality does not seem to be quite so simple. I say this based on my own observations in recording performance data from my Revenge 25 Walk Through.

Here is a good example. last month I had the opportunity to test my boat on two consecutive days with similar weather and the boat set up in the same way but with very different total weights.

On October 21 I went out by myself with 35 gallons of fuel on board. I estimate total weight at 6200 pounds. Cruise speed data was:

4000 RPM -- 33.0 MPH -- 2.09 MPG
4500 RPM -- 36.2 MPH -- 2.16 MPG

On October 22 I went out with 45 gallons of fuel on board, two coolers and four female passengers in addition to myself. I estimate the total weight to have been 6900 pounds or 11 percent more. Cruise speed data was:

4000 RPM -- 33.2 MPH -- 2.14 MPG
4500 RPM -- 37.2 MPH -- 2.14 MPG

In spite of the 11 percent increase in total weight, there was no decrease in fuel efficiency, in fact my figures indicate an increase. While this may have been due to any number of reasons, error in my measuring no the least of the possibilities, it certainly does not suggest a decrease proportionate to weight.

Another observation (made from data collected just this afternoon while testing propellers) regarding fuel efficiency and total horsepower:

My boat gets considerably better fuel mileage while operating with only one of its 150 outboard motors. This in spite of the fact tat while operating with only one motor, the propeller is obviously not pitched correctly.

Let's look at the cruise data from today. Both motors running (300 hp):

3500 RPM -- 30.0 MPH -- 2.11 MPG
4000 RPM -- 33.8 MPH -- 2.32 MPG
4500 RPM -- 37.7 MPH -- 2.14 MPG

Just one motor running (150 hp):

3500 RPM -- 19.2 MPH -- 2.56 MPG
4000 RPM -- 25.0 MPH -- 2.78 MPG
4900 RPM*-- 35.1 MPH -- 2.34 MPG

* WOT, I did not get a measurement at 4500 RPM

I will say that in the case of the good mileage while operating only one motor it does seem that motor was burdened by a prop that has too much pitch, and maybe a high pitch prop is the key to good fuel economy even if it is at the expense of motor longevity.

posted 11-28-2005 05:11 PM ET (US)         

I am sold on Tom Clark's data...

Boating solo is the same performance as adding 2 full coolers, more fuel, and 4 Females! What other stat do you need to look at?

Oddly enough, if it was four men, performance would have been reported as "lousy" (reguardless of the data). They would have needed to pay for the beer and chip in for the gas.

In all seriousness, Tom thank you for the data. While I am not an engineer, I could only think that wetted surface area did not change that much, nor did the wind resistance of the additional passengers. Engine trim may have been able to negate the additional crew/weight while still providing efficient power. In addition, while you may not had intended to do so, weight was optimally distributed.

Jack

posted 12-04-2005 11:56 AM ET (US)            

I have suggested that outboards move to a higher pitch propeller, but you first have to move to a higher horsepower outboard in order to be able to turn a higher pitch propeller. My recommendation is to always select a propeller that will allow the engine to reach proper wide-open throttle speed with the typical loading on the boat.

Getting better fuel economy than a Boston Whaler with outboard motors is not limited to light-weight go-fast boats. Here is another example, just a random boat review from a random magazine issue:

A 26-foot cuddy cabin cruiser that weighs 5,350-lbs is powered by a single 320-HP stern drive. This gives the boat a top speed of 44.4-MPH, again faster than my Boston Whaler. But--hold on to your Boston Whaler baseball caps--at optimum cruise around 28-MPH this beast still gets 2.7-MPG! This is embarrassingly better fuel economy than I can coax from my two-stroke outboard motor, which only gets 2.1-MPG at about that same speed.

A rather larger and heavier boat is getting 28-percent better fuel mileage than I can. What is wrong with this picture?

And it gets worse! In the same magazine they test a 28-foot express cruiser. This monster weighs close to 7,000-lbs, and it is powered by a 8.1-liter, 375-HP stern drive with a duo-prop set up. You better be sitting down for this number: it gets 3.0-MPG at optimum cruise!

I am out there banging around in my 22-footer with an ice-chest getting 2.1-MPG and these guys are cruising around with microwave ovens and refrigerators in the galley, getting 3.0-MPG.

posted 12-04-2005 11:26 PM ET (US)         

I know that your comment was in jest, but I have both a Whaler and a 36ft "Blow-Boat." The main difference is that on the power side, most of the opperating money goes into the fuel tank.

I recently took my "power boat" friends out on the sailboat and one made the comment that "this (sailing) doesn't cost any money" and I just about fell over.

I know that this isn't the thread for this duscussion and will stop here, but if anyone is getting into sailing because they think that it would cost significantly less money - they are making a mistake.