'04 Yamaha 115 2 Stroke w/21K Prop (Lower RPM than expected)

Th3Krah

Recruit
Joined
Aug 28, 2014
Messages
3
I bought an '04 Skeeter SX170 last year which came with an '04 Yamaha 115 2 Stroke outfitted with a 13.5x21K stainless prop. The boat runs great in my opinion with the exception of holeshot. I don't expect this boat to jump up on plane like an aluminum Xpress or anything but seemed a little sluggish but manageable. The top end with just me and a light load is right 50mph. I'm getting really good gas mileage as well and I guess that is attributed to my RPMs being around 4700-4900 RPM. I did some research and I believe this outboard is supposed to be running upper 5000's which is obviously concerning.

I borrowed a 19K prop a couple of weeks back and the holeshot didn't really seem to improve much if at all. The RPMs went up just about 5000-5100 and I lost about 4mph on the top end. What gives? I would think that this outboard should be able to turn that 21K and 19K prop faster (recommended RPM Range). I checked my outboard mount height and the cavitation plate is level with bottom of the hull when trimmed half-way to 3/4 up.

Could my tachometer be off or set incorrectly? Do you think I really need to drop down to a 17K or 15K prop? Again I'm happy with the boat's performance as is, but worried about undue stress on the outboard if I'm not running at the recommended RPM range. Please help.
 
Last edited:

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
The SX180 runs a 19" prop at 5500 and 46.
Your numbers don't add up. Can't go 50 at 4900. If we assume 50 is accurate and use 10% slip we get 5587 rpm.
We need a gps speed to be sure it is accurate.The gps speed will pin down an accurate speed.and give us a better idea about actual rpm.
I would expect a sluggish hole shot with a 21" Being a ss it probably has bow lift, that will also retard hole shot a little.
 

Th3Krah

Recruit
Joined
Aug 28, 2014
Messages
3
I am using a Lowrance Elite GPS 51 is my absolute top speed. My speedo doesn't work but in South LA nobody uses speedos due to constantly being gunked up. The 19K prop I tested with did push me 46 on the dot WOT but that was with a 2nd person and loaded down.

I just spoke with Yamaha Outboard Support in GA and they looked up the my exact outboard and said that 4500 - 5500 RPM is the recommended WOT range. This is consistent with the document that I found as well but inconsistent with what guys I've spoken with have said. They told me that I need to be running 5800. My understanding is that if I'm within the recommended range, then I'm good to go. If that's the case, I will just stick with my 21K prop as it felt smooth, faster and better fuel efficiency.
 

YoungMind

Seaman
Joined
Feb 25, 2014
Messages
69
You need to be withing the upper half of the recommended rpm range for optimal performance. You have verified speed by GPS, you also need to make sure the tach is reading right. Run the 19P by yourself, lightly loaded, with an external tach to verify your tach reading. Your original readings all indicate negative slip which is impossible. So...

Verify the tach readings

Verify the GPS reading with another GPS just to be sure.
 
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