Re: Can a bracket on a boat be a performance problem?
HokiePog:<br /><br />Porpoising is a very common problem with I/O or Outboard powered boats because the weight balance is far aft of center. In most cases trimming the motor to a negative trim angle helps minimize the problem because, the prop is now pushing upward on the transom which rotates the boat down in the front. In other words you are using the prop to create lift on the stern and pressure on the bow. <br />Balance is the key to boat performance, and some are more out of balance than others. <br /><br />I am no expert on brackets, but they do change the balance or axis - toward the rear. The bow comes up easily because the distance from the axis point to the bow creates a nice long leverage arm, and any amount of upward pressure ((i.e.: wave) in the front will push the bow up. What goes up will come down, and reentry into the water will be deeper than the mean running angle so the water pushes the bow back up. That is why it seems difficult to stop the porpoising once it starts. A quick test of this theory is that you can lift the tong of the trailer but not the back.<br /><br />when you look at your boat and most others from the side it is evident that the transom is designed so that the motor can be tilted under the boat. The manufacturer knows that the boat needs the help of the prop to get on plane, and to control porpoising. Using the prop to help the boat plane is not so bad but using it to control porpoising is really inefficient because the prop is force to run through the water at an angle which slips the outer edges of prop. Loss of prop bite = loss of fuel economy and top speed.<br /><br />The engine bracket usually allows for more cockpit room, and some say it extends the hull length which may make the boat faster. Not sure about that. In looking at your pictures, the bottom of the bracket angles up at the rear, which changes the hull design to look like a "rocking horse". This could facilitate the porpoising.<br /><br />After you have re-powered, and making sure that the motor is at the correct height, test it again. Keep in mind that the balance may change again with the new motor, along with prop characteristics. My guess is that the boat will still porpoise. Adding trim tabs will allow you to lift the stern (putting pressure on the bow) without using the prop angle. If you use hydraulic or electric (helm controlled) tabs you should keep in mind that the amount of deployment needs to be carefully adjusted, other wise you may be adjusting the motor trim to compensate for too much stern lift (not efficient either. They will also need to be adjusted in relationship to the boat speed. This is not hard - just takes some practice.<br /><br />Someone mentioned Smart Tabs, and the first thing you should know is that I have a proprietary interest. That said, they have been designed for boats up to 22 ft, and 240 HP. The principle is somewhat different. Instead of using hydraulics or electric actuators they use a nitrogen gas fill actuator. The tabs are held in the deployed position at rest, and as the boat increases speed and goes on plane, the tabs are automatically pushed up to the horizontal position. In this position they continue to be under pressure, which is the pressure needed to control the porpoising. Since the system is active, the position of the tabs will change in coordination with the water pressure ( speed, current, weight balance etc.) Since the system is adjustable upon installation (or after installation and testing) the correct amount of running pressure is easily achieved.<br /><br />Keep in mind that the more efficient you make the hull the better the economy and the less wear on the engine. I hope this has helped.
