Need to do some adjustments on my tinny, 115 optimax

[vroom ZOOM]

Hello

Got a couple of questions. My boat is a 2010 lund 1725 explorer ss with a 2008 optimax 115.

First, I think I am overpropped. WOT rpm spec is 5000 - 5750 rpm on that motor. My current prop is a michigan wheel aluminum 13x21 and at WOT and light load I get 75 km/h at 5200 rpm. Little too low IMO. So I can go down to a 13x19, will that do the trick? Also will staying with a 13x21 and going stainless do the same thing? IMO its a pretty fast boat right now lol.

Second thing is that I think I have an issue with the engine height. I can't go past 1/4 trim on the gauge without over revving and losing lots of speed. If I trim past 1/4 from full down the prop loses all grip and the boat drastically slows down. The way I have been driving it right now is I trim full down and then get out of the hole. It has a little bit of bow steer but not really noticeable. Then I start trimming up, and the boat starts going a lot faster and the engine starts spinning faster. When I get into the readable range on the trim gauge, I an pretty sure it starts ventilating, but still if I trim up more the boat goes faster. If I take a look back the prop wash is shooting out of the water kind of in a very low rooster tail (maybe 1ft tall at its highest point) but still no loss in speed or crazy rpms. If I trim farther up it will rev to the moon but I am only 1/4 way up! Now here is another weird thing. If I start to turn in this highest position, it will ventilate and loose all grip, so before I turn I have to trim down (this is where the trim down bow steer starts getting noticeable). So yeah I think this rig needs some tweaking. I bought my current prop brand new, and after a few times out one of the blades has a cavitation burn mark already. So maybe I should leave the engine where it is and get some sorta surfacing prop? Will putting the motor down one hole help with these problems, or should I just get a smaller prop and run it the way it is? What is your ideas on this?

 

[WesNewell]

Going to SS might bring your rpm up a bit. Lowering your engine will bring the rpm's down. Sounds to me like you are at a sweet spot now unless you want to run an obstacle course, then lowering your engine one hole or more and lowering pitch will help in turns, but will slow you down on top end.

 

[vroom ZOOM]

Ok, so a little bit of ventilation is ok? I'm just concerned about the burn mark on the prop. If I go stainless, will it hold up to that and the burn marks won't be an issue? I just really need like 200 rpm to be right on so yeah maybe a stainless would make it run good.

 

[Texasmark]

First off, over steer as you come out of the hole is normal for a bow heavy condition.....caused by your tucked in trim for the shot. That is to be expected. To eliminate it, you could start trimming out as you come out of the shot....will make for a faster shot also.

Trimming out lifts the bow, lets the boat "float", steers easier and picks up speed of which you are aware. Any porpoising that is encountered is cured by getting the right combination of speed and trim position....just trim in or out, increase or decrease speed to suit what you expect the boat to do at the time.

Trimming out too far causes the prop to loose grip, pick up air and ventilate. Fastest trim position is by feel, not instrumentation and you do your hole shot as you mentioned and as the boat picks up speed and you continue to bump the UP button (a little at a time, giving the boat time to respond), at some point the speed will start to fall off and simultaneously the RPMs will start to increase. For your given "Rig" that's all you are going to get.

If you want your RPMs up at the top of the suggested operating range (which is where I set up my boats) you will need to drop some pitch (150-200 RPM per inch of change is the going rate....excluding any boat performance change that will impact that number).

If you want better grip to reduce ventilation at high trim positions, get a SS (which has goodies to improve performance where you are having problems). Since you are in a different world with SS from GP aluminum, lots of things change and you can't expect a direct conversion number. Gotta run something and get some data to see where you go from there. For that, you need a supplier that will work with you with loaner props to find which works best for you. Expect there to be a fee attached for the convenience but overall should be cheaper than a crap shoot.