RPMs increase while power decreases, not the prop

[2strokesmoke]

I was cruising at near full speed, I would loose power and the RPMs would rev up, just as if the prop was spun. My prop was nearly new and hadn't taken any hits or damage but changed it out anyway with a brand new one I had, same issue. A couple times I could get nearly up to speed and ride for a min or two then it would happen again.
This happened last fall, a lot of leaves in the water and a guy at work mentioned I was probably hitting patches of leaves slightly submerged out of view... but this went on many times that entire day in many wide open areas with no visible leaves.
It wasn't slipping out of gear because as I slowed I would regain power. I recently drained my lower unit and refilled. Dark-ish color but no metal flakes or water present. The oil seemed low though, but couldnt get the top screw out until the next day so its hard to say how low, if at all. I changed it myself about two years ago and know that I filled it then. If a seal is bad wouldn't that also let water in?
I'm not sure what else the issue could be, hoping some can give me some insight or other things to check.
I have a 2000 9.9 2 stroke converted to a 15. Been riding this for nearly five years with the same pitch prop, same load, same tilt etc.
Thanks

[rodbolt17]

my guess is the prop is ventilating.
either something got added near the prop,the engine height or trim angle is wrong or the diffuser ring is missing or damaged.

[TownsendsFJR1300]

my guess is the prop is ventilating.
either something got added near the prop,the engine height or trim angle is wrong or the diffuser ring is missing or damaged.

I'll bite, what/where is a diffuser ring? Function?

[rodbolt17]

it is that flared ring that on some props is a press fit.
lose it and the prop vents on ex gas.
some props its a part of the casting.

[TownsendsFJR1300]

Oh, ok. thanks..

[2strokesmoke]

my guess is the prop is ventilating.
either something got added near the prop,the engine height or trim angle is wrong or the diffuser ring is missing or damaged.

Nothing was added near the prop, and the engine height and trim has always remained the same with the same Solas prop/props. I'm not sure what the diffuser ring is... I don't see it on the diagram. Theres a deflector ring but only for dual thrust props. I read Solas props aren't very good, get what you pay for?

Also, stupid question. When installing a prop should it be tight or have some play? When I tighten down the cotter pin is too high and I have to back off the castle nut so it holds.

Here's a link to my lower unit parts diagram.
2000 9.9MSHY Yamaha Outboard LOWER CASING DRIVE 2 Diagram and Parts

Thanks for all the replies

[99yam40]

what boat, and what prop are you running on this motor?
what is the WOT RPMs with your normal load?

Could be some disturbance in the water from the boat that causes air bubbles to hit the prop making it loose bite

I have had a prop spin out in open water before. had to run back with low power to keep it from slipping. you can mark the hub and prop to see if they change alignment

[99yam40]

When I tighten down the cotter pin is too high and I have to back off the castle nut so it holds.

might want to rewrite this statement, so it makes sense.
prop nut should be torqued to spec and moved a little to get pin in, but prop should not be loose

[scofflaw]

His post makes perfect sense to me, mine is the same way.
When torqued down the cotterpin is aft of the prop nut.

[scofflaw]

His post makes perfect sense to me, mine is the same way.
When torqued down the cotterpin is aft of the prop nut.
I use the lock washer with the bend down ears, and install the cotterpin even though it doesn't contact the prop nut, but would prevent the prop nut from coming completely off.

[2strokesmoke]

I have a 1436 Jon. I use the same Solas prop Ive used for years. As for the exact model/pitch I'll have to look when I get home. No clue on the RPMs ar WOT, been looking at the tiny tachs in order to find this out - I know its a key part in diagnosing issues. Good idea marking the hub and prop, I'll have to try that to rule it out.

Yep, this is what I meant... although I didnt torque to spec. Not sure what spec is or where to find it.
"His post makes perfect sense to me, mine is the same way.
When torqued down the cotterpin is aft of the prop nut.
I use the lock washer with the bend down ears, and install the cotterpin even though it doesn't contact the prop nut, but would prevent the prop nut from coming completely off. "

Regardless, and I know it doesn't help to diagnose the issue, I've been running this same set up for years with no change to anything which to me is what makes it so strange.
I'm going to run the motor again here soon. If the problem persists or not I'll get a tach, research prop nut torque spec, mark the prop and hub... then go from there. Open to more conversation but not trying to beat a dead horse considering i dont have all the needed info.

Thanks for the replies

[99yam40]

OK so when you tighten the nut the pin/hole is too far from the nut to be in contact with it.
Maybe wrong thrust washer or spacer is being used.

you did change the prop from when it was a 9.9 to a different one when you converted it to 15 hp right?
with more hp you should have a different prop to make use of the extra HP unless the 9.9 had the wrong size on it when it was only 9.9 hp.

if the hub is not spun then something has to have changed from when it ran without problems.
Just need to find out what it is

[TownsendsFJR1300]

When installing a prop should it be tight or have some play? When I tighten down the cotter pin is too high and I have to back off the castle nut so it holds.

Here's a link to my lower unit parts diagram.
2000 9.9MSHY Yamaha Outboard LOWER CASING DRIVE 2 Diagram and Parts

Torque the prop nut on the shaft to spec's. If the grooves in the nut don't align, tighten slightly more till they do- This is per the Yamaha SHOP manual

[99yam40]

Torque the prop nut on the shaft to spec's. If the grooves in the nut don't align, tighten slightly more till they do- This is per the Yamaha SHOP manual

Sounds like his problem was the cotter pin when installed is too far from the nut for it to go between the slots in the nut( not touching it at all) so nut can back off some before it touches the cotter pin.

As I said before sound like the wrong washers or prop

[TownsendsFJR1300]

Sounds like his problem was the cotter pin when installed is too far from the nut for it to go between the slots in the nut( not touching it at all) so nut can back off some before it touches the cotter pin.

As I said before sound like the wrong washers or prop

If that's the case, he's probably missing that large "spacer" (round like a washer, but much thicker and machined to fit into the hub and prop shaft).

I'm batting 0 for100 trying to interpet posts so......


Part #19 or 20 at this parts fisch(I don't know his model # but should be close enough)

http://www.boats.net/parts/search/Ya...202/parts.html

For the OP, can you post your model # and or check under your prop and see if you have the parts as shown in the parts fisch ^^^?