Yamaha F150 RPM spike

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DeepDiver

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I have a 1999 Grady White Tournament 192 with a new Yamaha F150 that has 100 hours on it. A month ago I took the boat out and noticed that while cruising the RPMs would spike from 4400 to 4900 for a few seconds and it would lose about 5 knots. We took the boat to the dealer (Bob's Marine) and they said they couldn't find anything wrong with it. After putting the boat back in the water it still had the same problem. One of their mechanics came out with me and hooked up their laptop and after an hour and a half the problem finally occurred again. No error codes showed up but it did register the RPM surge. We thought it was maybe a spun hub and had them put a new aluminum prop on, but this did not fix the issue. Bob's Marine says they still can't find anything wrong with it and claims it could be cavitation. I usually have the motor trimmed half way (3rd bar) and never had this happen last year. It happens most frequently when going over a wave or making a sharp turn. It doesn't seem to me like it is a fuel issue because it occurs only intermittently, but I guess I should hookup an alternate fuel source to eliminate that as a possibility. I have contacted Yamaha and they said to fax the laptop report, but Bob's says that it doesn't show anything other than an RPM spike. I'll have to wait and see where Yamaha goes with this, but so far this has been extremely frustrating.
Does anybody have any ideas?
 
I'm no expert but an RPM spike couple with loss of speed definitely points to a cavitation problem (you seem to have eliminated the prop). As for it not happening last year, I'd look at what's changed such as weight aboard or bottom paint or possibly marine growth; something increasing hull drag. Another possibility is if you've added something like a hard top which increased air drag. Lastly, the trim guage may be wrong and you're trimming higher than you think.

Good luck finding the cause.

Gary 89 Overnighter
 
Spun hub, seaweed picked up on prop, cavitation. End of story

Are you sure boat slows? What proof do you present?

Are you sure it is not prop spun? What proof do you present? Did you try another prop?

At the time it increases, did you pass a boat wake or turbulent (white) water?

When it occurs each time, what are common conditions?

Does motor trim stay put, or does it rise and have found risen by itself ?
 
I just thought of something, perhaps gearcase is involved, never heard of such a thing since the dog is usually pulled in and stay putat speed, but may be something new with F gearcase.
 
My boat will do that in really rough water and sharp turns with the motor trimmed at '3'. and I have a huge cavitation plate on there, a Permatrim. Without one of those they can suck air even easier.

I also suspect that you're trimmed higher than you think. See if you can get it to do it trimmed at '2'.

I can't recall if that gear case has a friction hub, so I would get an exploded diagram or call Andy at SIM and ask him. If it has one it could be slipping intermittently - easy warranty fix there. I don't recall there being one in these motors, I think they moved that feature to the prop.

Frankly, I don't think it's a slipping hub, I think it's cavitating. What kind of prop did they put on? If that didn't fix it, then it wasn't the flex hub in the prop. I run a 15x15 stock yamaha aluminum. Lots of dig to that one, but top end is about 37-38.

It is possible they mounted the motor a bit high, I would check that. The plate should be less than 2" above the bottom of the hull V. Could be you need to drop the motor a hole on the mounts.

I would also highly recommend a Permatrim - Andy has them. I really like mine, they are a nice add-ons for these applications. Together with the Bennet's, I can dial in the attitude of my boat precisely.


EDIT: Oh, I almost missed that you were at post #1 - Welcome to the forum!!
 
Sounds like a cavitation problem to me also. Chris' advice is good. Try trimming the motor down a little bit more to see if you can get the problem to go away.
 
Thanks for your replies.

I guess it probably is cavitation, but I'll admit I'm a little surprised. The only difference with the boat from last year is it has more marine growth on the bottom. (We didn't sand it during the offseason.) The trim gauge has a tendency to drop down a bar so I usually hit the switch to raise it up again. I'll try running with the motor down a little more and see if I can get it to cavitate. I took the boat out yesterday for over an hour and didn't have any problems.

BTW, this is a great forum. 8)
 
I have the same setup - Tournament 192 with the F150, and am very happy with the combination. If I have the motor trimmed up 3 bars and turn sharply I get cavitation and the RPM increase. I still trim it up at least that far if I am just cruising. If I am going to be turning sharply a lot (for waterskiing for example) I trim up much less or not at all. Good luck - I hope that is all it is for you.
 
Your getting an increase in rpm because your trimmed up too much spinning air. Try 1 or 2 bars and your increase will go away

2005 300 Marlin w/F250's
 
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