Planing Hull Efficiency
What speeds are efficient for a boat with a planing hull?
Many boaters say they go slow or they go fast, either way it is the same efficiency. It sounds correct but how do we know it is correct? To get the numbers there is a little math after collecting some base data. We will need the length of the water line in feet, the total weight in pounds, top speed in knots, total horsepower and at what speed the boat planes. Working from the details of Viatori for reference, the length of the water line is 44 feet, total weight is 46500lbs, top speed on calm water with little current is 18.9 knots, horsepower is 740 and Viatori gets on plane at 10.5 knots. To determine the pre-plane efficiency of the hull we multiply 1.34 X square root of the water line for a speed of 8.9 knots.
Since Viatori planes at 10.5 knots, speeds between 8.9 and 10.5 are the least efficient, burning more fuel. It is only a 1.6 knot difference but the loss in efficiency comes from the force of the water acting on the bow of the boat before it reaches planing speed. As water builds up at the bow, the force created hinders the ability to reach planing speed, the speed at which the boat start to rise on top of the build up of water it is creating. The equation is showing how fast the hull can travel through the water before building up enough water to cause significant resistance to forward motion. At 8.9 knots, our engines are at 1250rpm. To get Viatori on plane, we have to achieve 10.5 knots at 2400rpm. Once the boat gets on plane, the speed significantly increases to 16 knots with no further increase of engine rpm.
This happens because the bow starts to dip over the top of the wave, the angle of the boat changes, bringing the props more parallel to the waters surface. The gain in thrust from the angle of the prop change increases the boats speed with no additional rpms, this is the cruising speed. Increasing the engines to maximum rpms from this point only increases fuel burn minimally. We can confirm what our top speed should be with more math using the Crouch Coefficient, a calculation to determine the maximum speed of a planing hull. Crouch generalized the value of a planing hull in knots to be between 150 and 156 and we use 150 first in the calculations for Viatori to make sure we are close to what our known numbers are.
We get the top speed by getting the square root of the horsepower divided by the weight multiplied by the Crouch Coefficient. This shows that our top speed is calculated to be 18.9 knots.
To check that we are using the correct Crouch Coefficient we divide the top speed by the square root of horsepower divided by the weight for a Crouch Coefficient of 150. Had this been a different value than 150, we would use the new Crouch Coefficient to determine what our top speed should be.
We use this information to check the calculated performance to the factory specifications. Luckily we have access to the original delivery paperwork and manufacturer specification sheet for the particular vessel and engine combination. A search of the internet for performance information may result in a generalization and not the specifics for any one vessel and engine combination as a Google search shows that the maximum speed of a 2003 Carver 444 is up to 25 knots. A lofty goal for any vessel of the 444 size with 740 horsepower. The engines specs from Volvo for a 2003 Volvo TAMD63P-A are that the engines produce 370 horsepower at maximum rpms of 2800 each. Volvo shows that the engines themselves have a maximum rpm of 3500, but in the marine environment are reduced to 2800 or 80% of wide open throttle to reduce the risk of engine failures.
Full throttle rpms at our gauges are 2687 or a 4% (113) loss of RPM from factory specs. This would not be a concern under normal operation for an engine that is 20+ years old. There can be a lot of factors that account for this drop but in our case it is because we are almost 14,000lbs over the factory specified weight. To confirm our weight theory, we used the worksheet above and change the weight back to factory specification and our top speed increased to 22.5 knots, which is a slightly higher than the delivery specifications of 22 knots.
This math and data helps get us on the right track to find the best fuel efficiency for our planing hull so we can stay at the most efficient speeds, track performance issues and assist in diagnosing potential problems related to performance.