A question for You Yamaha Pros out there...

[pilot4net]

I have a 2005 F115 on the back of a 18'3" aluminum bass boat which has a high performance hull and pad underneath it. The motor is turning a 13x19 SS prop. I was looking through the internet and found a slip calculator for props so I plugged in my engine rpms per the in dash tach (5850), gear ratio (2.15:1), pitch (19+1 for cupped prop), and speed as per gps and it calculated a 5% slip. I posted these findings in the prop forum and they are saying that the 5% slip is not reasonable and that if all other numbers such as gps speed, gear ratio, and prop info or correct, then the tach is inaccurate. I know for sure the numbers are correct because I have gotten these results consistently.

I should also add that I added Mobster Tabs to the boat to fix a porpoising problem I had as well as low rpms. With the tabs, I was able to trim the motor up a good bit more, pick up 6-7 mph on the top end and now my rpms can get up there near the top of the rpm range (5000-6000 rpms) which people on here say is the ideal place to have the WOT rpms. Before I was getting 5400 rpms at WOT and 42 mph; that's it! Now it is much better.

My question is that if these numbers are correct, do you Yamaha tech gentlemen think that the 5% slip is impossible? If so, the only possible fault would be the inaccuracy of the tach. If the tach is in fact inaccurate, am I possibly hurting the motor since I must be running higher rpms than what I think to achieve the 50 mph? Is there a rev limiter on these motors to keep them from damage in this case. The motor seems to be turning up just fine and doesn't sound like it is "screaming" or anything.

If someone would chime in on this and let me know what they think of this situation. I hate to be harming the motor since in order to get the amount of slip that the prop forum guys are saying I should be getting (at least 8%), the rpms would have to be at least 6025 rpms and that is over the recommended top range of 6000 per Yamaha.

I would appreciate any advice or information you guys can give me on this. Thanks.

 

[rodbolt]

Re: A question for You Yamaha Pros out there...

a quick run with an inductive tach or the laptop can confirm the tach, its rare that the yamaha digital tach is off but an aftermarket analog could be.best I recall, the crankshaft position sensor and pulser coils feed RPM information to the ECU and the ECU generates the tach signal. however its been a while since I looked at one closely.
the rev limiter will grab ya at 6200 or so. above about 6400 it drops spark and fuel on all 4.
if you use an inductive tach remember the coils are siamesed and its a waste spark system.
if your inductive tach has a two stroke setting then use it or divide the 4 stroke setting by two. seeing as its all theory anyway I dont have an isue with 5% slip.
nor do I have an issue with cracked dilithium crystals or warp drives.

 

[jerkin]

Re: A question for You Yamaha Pros out there...

Cracked dilithium crystals, now that there's funny, I don't care who ya are.

 

[Ray Neudecker]

Re: A question for You Yamaha Pros out there...

The biggest problem with the prop calculator is the pitch numbers. They can be off because of either adjustments such as cupping the prop or where the pitch number on the prop is not accurate due to design. Many prop manufactors use either a regressive or progressive pitch in their props where the pitch is not constant through the blade.
Sounds like you are dialed in with your setup. I have found most tachs and in particular the Yamah digital tach to be very accurate.
Still you want to be sure you don't have cracked dilithium crystals.