Posts: 3508
Location: Elk River, Minnesota | Hiya.
I would say if you are getting porpoising when trimmed out, it could be two different things or a combination of both.
First off, the torque tab adjusts for steering torque (that hard pull on the steering wheel when you want to turn left but easy when you want to turn right.) you can get some of that torque to be reduced when you are trimmed in by adjusting the tab so the back end is pushed more toward the left as you look at it from the rear (port side). It should not be too far from center, and with the tab on the OMC/Bombardier lower units, the back-right edge of the tab should be close to the centerline of the lower unit....maybe a touch to the left of that centerline.
Unfortunately, though, the porpoise would not be caused by the torque tab... The porpoise is due to either of the following, or potentially both combined...
Case 1: Just trimming out too far. If the motor is way down on the transom, the prop has a bunch of grip through all trim angles, but at som e point, the weight of the bow is overcoming the motor's power to keep the bow up. Thus...the bow falls, and the process repeats.
Case 2: The prop on the motor has too much pitch, which is not allowing the motor to reach the mid to upper end of the recommended RPM's. The motor cannot overcome the weight in the bow again, so the same process as in case one starts to happen.
In some situations, it may be a combination of both, but here are some things to try before doing any major changes to your set-up. First, how much weight do you have in the bow area? Many boats have the livewell placed in the bow area as part of a front deck. Combine that with a battery compartment that could potentially hold 2 deep cycle batteries, a bow mount trolling motor, a potential gas tank (if built in) located toward the front, some stowed gear in compartments, musky boxes (which can add up in weight).... All of these things can affect the overall balance of the boat.
So..first thing to try if it is applicable... move weight toward the rear of the boat. Sometimes it may be just a move of 100 pounds, but when you are trimming, that 100 pounds up front actually works as more than 100 pounds since the trim point of the boat is at the rear of the boat. Moving weight back allows the bow to trim easier.
If this does not solve the issue, I would be considering where are your RPM's on the motor at the best trim setting you can get without a porpoise. Being a you have a 2 stroke 70hp, you should be reaching 5500 rpms on a light load (you and your gear only along with full gas). If you are only reaching 5000 on a light load, I would say you are over-pitched by about 2 inches. Going down in pitch will allow the motor to reach it's optimum RPMs through all weight ranges which should help the overall performance. You will lose some speed here, but the low end will be better and planing time will be much improved.
Something to start with, at least...
Steve
|
