Grand Banks Classic Fuel Burn
By Geoff Holmes
We recently returned home to the Chesapeake Bay after taking our boat, a 1987 Grand Bank 42' Classic, to the east coast of Florida for the winter. The trip was great, the weather caused a few delays during our transit both southward and northward and the boat performed as she should, dependable and with comfort for her crew. What was a shock however, was the increased cost of diesel fuel. On departure in October, our first fueling in Portsmouth, VA was an eye-opener with a price of $2.90 per gallon. By March, when we left Vero Beach, FL, the price was $3.75 per gallon and we heard it had approached $4.20 per gallon further south, OUCH!
A salvation, however, was a Grand Banks trawler can be very economical, if you treat the boat as a trawler and run at essentially hull speed. The following description of hull speed was extracted from a Google search on the internet:
Heavy Boats, Light Boats, and Hull Speed
Here's an attempt to explain these things in layman terms.....
by Jim Antrim
A boat displaces its own weight in water. When the boat is moving, it must push that much water out of the way as it goes forward. Since a heavy boat has to push more water out of the way, it makes bigger waves. (As a boat moves faster it has to push aside more water in less time, so that makes the waves bigger too.)
Each boat creates a bow wave and a stern wave. When a boat reaches "hull speed" the bow and stern waves coincide to make one huge wave system. A heavy boat gets trapped in its own wave system. (For a 20 foot boat, hull speed is about 6 knots. For a 30 foot boat, hull speed is about 7.3 knots.)
The best example of this is a tugboat. Tugboats are very heavy, since they have huge engines for shoving ships around; and when they are not shoving a ship, they are racing as fast as they can to the next job. That's why you see them with a huge bow wave, a huge stern wave, and a deep wave trough in between. In spite of their enormous horsepower, they can't break loose from the trap of their own wave system. They dig a big hole in the water, and can't climb out of it.
A light displacement boat such as a dinghy or a multi-hull doesn't have so much water to move out of the way - so they make smaller waves. When they reach the speed that would be hull speed for a heavy boat the wave system is not big enough to trap them. They are able to exceed the "speed limit" where bow and stern waves coincide. A planing hull actually climbs up its own bow wave and is lifted partially out of the water.
Now, a Grand Banks trawler has a semi-planing hull and is a relatively heavy boat thus making for some of its creature comforts. Our boat when fully loaded displaces ~40,000 pounds. It has a waterline length (WLL) of 40'9.5", is powered by two Caterpillar 3208 naturally aspirated engines each turning a 28x23 three bladed propeller through 2.5 to 1 reduction via a Twin Disc transmission. The calculated hull speed (V=1.4X?WLL) for our boat is 8.94 knots or 10.35 statute miles per hour. An important point to note is this is the speed through the water not necessarily the speed over land which is what is displayed on a GPS chart-plotter.
When the Grand Banks trawler is operated at hull speed it is essentially functioning as a displacement hull vessel and becomes remarkably efficient. To achieve the best economy, we started to operate the engines at ~1,400 RPM and realized a speed through the water of ~10 MPH as noted on the electronic read-out from the Datamarine speed wheel. What became remarkable was the calculated fuel burn for the boat including both engines which was ~4.5 gallons per hour (GPH). If we increase the engine speed to 1,600 RPM, the fuel burn was ~6.0 GPH and at 1,800 RPM we recorded ~8.0 GPH. The speed through the water increased but not proportional to the increased engine speed. The boat was not able to reach planning speed and was "trapped" in its own wave system.
During the course of the trip, we encountered a variety of weather conditions and were able at times to maintain the more efficient speeds for along enough time periods to re-fill the fuel tanks, determine distance traveled and calculate statute miles per gallon. Given the fact that at times you run against a current or wind and at times you run with a current and wind, we were able to approach a fuel burn that indicated ~2 miles per gallon. Not bad at all for a boat that provides the comfort and accommodations of our winter home.
Dickerson Harbor
3831 Trappe Landing Road
Trappe, MD 21673
410-822-8556
dickerson@oya.com