Porpoising

[KevinTee]

I know there are a lot of postings on this topic but I have a couple of things that I can't find discussed.<br /><br />I have a 1998 Lowe 160SPL with a 1988 Mariner 45hp w/trim/tilt. Bought the boat used. It had two home made trim tabs which were nothing more than some 1 1/2 x 8 inch aluminum angle. My depth finder will not work once the boat gets on plane. Plenty of trim movement before the boat porpoises. I removed the trim tabs and the the depth worked fine at WOT. <br /><br />This created a Porpoising problem. After reading other threads I belive I have a weight problem. Motor trimed full down, my son and I in the boat it porpoises. No seats for crusing in the front, no room for a battery up front. In the back is 9 gal of fuel & two batteries. <br /><br />What would you think of adding a 100lb sand bag in the front?

 

[Chris1956]

Re: Porpoising

Kevin, It unusual for a boat like that to porpoise with the motor trimmed down. Is the bottom true? It sounds like the previous owner created a fix with those tabs. Does the wind get under the boat? (is it aluminum?) Is is rated for a 45HP motor? Sometimes if the boat is severly overpowered handling can be adversely affected.

 

[shep70057]

Re: Porpoising

Kevin, drop me a line. I know a guy that may be able to put you on track. Believe he has a Lowe and has it pretty dialed in. asheppard@oceaneering.com

 

[Jdeagro]

Re: Porpoising

Kevin2037;<br /><br />It is not unusual for stern loaded boats to porpois, but it is unusual for it to continue with the motor trimmed down. I do not believe that the motor should be trimmed down to get rid of the porpoising as this means the motor is running at an angle and that is not efficient. However, the fact that you can not get rid of it means that the boat has a severe problem.<br /><br />I would leave the fixed trim tabs off simply because they cure one problem and cause others. What they are doing is creating a permanent hull configuration change. Once you are on plane they will eventually drive the bow into the water. Not good for handling.<br /><br />I sell trim tabs (Smart Tabs) but the first thing you should do is make sure the boat is set up correctly, without the aid of other planing devices (even hydrofoils). I am guessing you have a haydrofoil on the motor. Check the motor height. With the motor down in the running position, the antiventilation (commonly refered to as cavitation) plate should be even or slightly higher than the bottom of the boat.<br /><br />Also check the hull (bottom) for convex or concave areas especially near the transom, as has been suggested. Give us more information about the boat balance and layout. The problem is simply boat balance.<br /><br />Do not install fixed trim devices.

 

[KevinTee]

Re: Porpoising

Thanks for the quick replies.<br /><br />The boat is aluminum and I do not have a haydrofoil on the motor. It is rated for a 50HP so it should not be overpowered. In the current configuration it will run 30mph (gps)at 5600 rpm. I will check the engine height and inspect the hull again.<br /><br />Is there a stearn to bow weight ratio that should be followed?