How to optimize performance on 1775 w/ 115

[T Rev]

I am currently running a Lund Pro-V 1775 SE w/ Mercury 115 2-stroke. Running a stainless ballistic prop (13" 18 pitch) motor is mounted in top hole (lowest). It runs 32 - 34 MPH @ 5200 RPMs. (speed measured same on speedo, GPS & sonar) This is with full tank of gas, 2 guys, and moderate amount of gear.
Now, I have seen many posts on this site with similar sized boats and motors claiming 38 - 43 MPH. I am wondering what changes I need to make to my current set-up, to achieve this kind of performance.
I also need to mention that while running WOT, I occasionally am hearing a vibrating kind of sound coming from the motor. I have had it back to the dealer 4 times since picking up last summer, and they assure me that there is no problem. However, when this sound disappears (which it does, a few seconds at a time) the speed jumps by 1-3 MPH. Could this be the prop slipping? Please advise.

[PJM]

T Rev

The operating rpm on a is 4750 to 5250 so 5300 rpm is about right. The prop could be the problem and I would be inclined to taking a look at a Trophy four blade or maybe a High five. You could take five props with the same pitch and get five completely different results. The question is what is the boat doing, is the boat a dog out of the hole or is the bow not coming out of the water and it plowing water. The vibration could be that prop and you might want to demo a different one.

[Bitman]

I've got the same boat, with a 125 Merc. runs about 48 mph.
Speed over ground on GPS.

[jigtugger]

I agree, that combo should be faster. Tell me, how far can you trim up the motor before it cavitates at WOT? And as you trim it up, do you feel the bow rise out of the water(lift)? Have you ever checked where your splash line is along the side of your boat at WOT? I am guessing that your prop is not lifting the bow enough, and you are pushing too much water. I had a similar issue with a prior boat until I tried a high rake prop (stiletto or rapture). I didn't believe it would make a difference but it really did. The feel of the boat completely changed once it lifted. If you are in Minnesota, I have a prop that you can try on it to see if it helps. (13 1/4 x 17 stiletto with added tip cupping).

[Hawgeye]

I run everything identical to you and my motor is a 125 which is the exact same motor as yours with a few more HP. The RPM range is exactly the same on both the 125 and the 115. I run a 19 pitch aluminum prop on mine at 5250 and get 45MPH range depending on load and with lotsa weight, never get below 40MPH. First of all you need to get that motor in the middle hole. My boat also came from the dealer in the 1st hole which "plowed" way to much. It had a 17 pitch and the best I could do was about 39. I gained 6 MPH with the proper set up. The RPMs were wrong too. It is amazing that I had to argue with my dealer that the 1st hole was the wrong spot. I suppose he wanted to protect his butt from the warranty if it blew up...I am not sure. Move your motor, the prop should be good for that motor.

Something else on the vibration issue, maybe your prop is out of balance and a bad one. I too question the quality of the prop, the pitch and diameter should be pretty close to get you close to 40 for sure.

[T Rev]

Jig Tugger...
I can trim the motor up just past red line on trim gauge, haven't really looked to see how much that is. It does raise the bow when trimed up. I will have to check where the splash line is, not sure. The hole shot seems at least average. Although, I admit I am 2 years removed from a 17' tiller w/ a 60, And maybe I am not sure what the hole shot should be. But as long as I am trimming up while accellerating it gets on plane pretty quick. Sounds like I need to head back to the lake for a few more observations. How much work is it to raise the motor, or should I leave that up to the dealer?
I am in SD, but grew up in MN and get back there often. Where are you at? Shoot me an e-mail. Tjrevier @sio.midco.net

Thanks for the replies.

[Moose]

I was wondering if anyone had a 115 hp Johnson. Because have the problem that Hawgeye had only 39 maybe 40 mph? Just would like 45 MPH.
Thanks for the info.