Help! How to trim/tilt OMC Stringer, Part II

[Costello]

boaterntexas, Snake Bit, and McKenzie: Thanks for the valuable info.<br /><br />What are trim tabs?<br /><br />Also, I just bought a stainless steel prop to replace my chewed up aluminum one. Should I be concerned about damaging the shaft if the prop hits the bottom of the lake? (I can't afford disposable props!) Will I notice much difference going from a 19 pitch to a 21 pitch prop?<br /><br />I guess a fish/depth finder wouldn't hurt, too. Any recommendations on make and model?<br /><br />As always, you guys are the best!

 

[McKenzie]

Re: Help! How to trim/tilt OMC Stringer, Part II

There are formulas to calculate a props performance, i don't remember them, but you can do a search for it in the forums. I believe that it is 200rpms per inch you add or subtract from the pitch (correct me if i'm wrong prop experts). I run a 14.25 x 21 inch prop on my boat (about the same HP) and i get a really nice ride out of it. I turn 4300 rpms at 48/49 mph (GPS). I would expect you would get something close to that if your engine is running at what its rated.

 

[Doug Durako]

Re: Help! How to trim/tilt OMC Stringer, Part II

If you ar hitting rocks, doesn't matter what prop you have---you'll do damage.<br /><br />19 pitch prop moves the boat 19 inches in one revolution (in theory). 21 pitch moves it 21 inches. Does no seem like much, but big difference if you are trying to get a 200-pound skiier out of the water, or you are pulling a tube---then you want the lower pitch number--boat reaches plane faster because the motor can turn higher RPM.<br /><br />For top speed, the boat may go 4 or 5 mph faster at wide open throttle with the 21 pitch. You have to try different props to really know. Ideal prop should get you out of the water and reach the upper end of the wide open throttle rpm range for your motor.<br /><br />Find a shop manual for that stringer, you will need it. Don't run with the drive unit up, you'll be replacing ball gears next season---if you can find the parts.

 

[tmclendon]

Re: Help! How to trim/tilt OMC Stringer, Part II

Yep, in general a 21" will give you a bit more top speed and less acceleration than a 19", and ~400 less RPM at WOT. There are way too many variables to say for sure, you need to talk to your local prop guy. A SS prop will usually give you up to 5-10% better performance over aluminum of the same dia/pitch, both top speed and acceleration.<br /><br />I've never really bought into the idea of using aluminum to minimize damage; as a previous poster said, if you hit a rock or whatever hard enough, you're gonna break something in the lower unit, no matter if you have aluminum, stainless, or titanium! In theory, the hub will spin (Thats the rubber bushing around the hub of the prop) and prevent damage. Sometimes does, sometimes doesn't. The advantage of a SS prop is that it will survive hits that would really tear up an aluminum prop.<br /><br />The trim tab that I am familliar with is that little 'fin' that hangs down from the anti-cavitation plate, in front of the prop. You can loosen it (2 bolts if I remember correctly) and adjust it's left-right angle. This is used to compensate for p-torque, the force applied by the prop spinning in one direction. If it's out of adjustment, the boat will be really easy to turn in one direction, and really hard to turn in the other. The catch is, this force varies with how fast you are going, so the trick with the trim tab is to find a happy medium.<br /><br />The guy talking about trim tabs may have been talking about something else...<br /><br />Tom