Proper WOT RPM for my outboard?

MAArcher

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WOT for my 2004 Yamaha 90hp 2 stroke on my 19ft flat bottom skiff is around 4,300 rpm's. I just read that it should be between 4,500 and 5,500. What does it mean that mine is lower? Is it the wrong prop? It does seem to use more gas than I anticipated, are these issues connected? How do I figure out what prop I have on there and what one I should try? Or is it another issue all together?
 

Jjammer

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You need to drop the prop pitch, which should be labeled on the prop somewhere with a number like 13x15 -- 15 is the pitch and 13 is the diameter.

Every 1" of pitch drop should get you about 200rpms at WOT. I'd prop that thing close to 5500...at least 5000.
 

Old Mud

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Jjammer

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But not making WOT isnt necessarily a prop issue. It could be an ignition, carb or compression issue.

I'd start with a compression test then start playing with props.

Mud makes another good point: most people mount outboards too low. Ventilation plate can typically be higher than your hull bottom by about 1"depending on hull design...the lowest you want it is even though, definitely not lower.
 

Old Mud

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BOSBoatMan

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You should be able to clearly see the cavitation plate when you running, i.e. no almost no water goes above it.

I'd really examine the mounting of the boat first, then start playing with the props. And have her turn all the way up her rated range.
 

Jjammer

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You should be able to clearly see the cavitation plate when you running, i.e. no almost no water goes above it.

I'd really examine the mounting of the boat first, then start playing with the props. And have her turn all the way up her rated range.
100%. Cav plate out with no spray is the clean way to rig. If that's not the case now, raise that engine in its mounting holes.
 

steveinak

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i'd bet your over propped, look on the prop for the size numbers. You may want to go down in pitch size.
 

MAArcher

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I mounted the motor with the cab plate dead even. I’m thinking of a jack plate so I can play with it. Right now I get cavitation some times when the swells are larger and the water at the mouth of the river is doing squirrelly things.
 

leaky

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A hydraulic jack plate is the way to go, adjust on the fly to match conditions.

On a single engine utility sort of boat I want to be able to go over the max RPM with a light load (ie in your case 6000 RPM maybe).. or conversely with the heaviest load I want to see the bottom of the operating range...

If done this way the boat will still perform normally and not be all doggy with a heavy load, which is more important than being able to run it safely for long periods at wide open IMO. Running over max RPM occasionally is also not much of a thing with 2 strokes, ask the guys who race them.

With a twin engine boat that makes longer trips this is a little different, they do not tend to get doggy if over propped a bit and sometimes you can be more efficient over propped without hurting acceleration.
 

leaky

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I mounted the motor with the cab plate dead even. I’m thinking of a jack plate so I can play with it. Right now I get cavitation some times when the swells are larger and the water at the mouth of the river is doing squirrelly things.
This problem may self resolve when you go down in pitch, lower pitch props are usually larger.

What prop model is on there today? Should have a number stamped on it someplace.
 

BOSBoatMan

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A jackplate on a boat that ain't a Florida flats boat that's ramped up with horsepower is stupid unless you get it for free or have one laying around. OP should start with the basics and go from there.
 

leaky

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A jackplate on a boat that ain't a Florida flats boat that's ramped up with horsepower is stupid unless you get it for free or have one laying around. OP should start with the basics and go from there.
I don't disagree that the #1 problem he is facing is probably setup/prop or maybe even maintenance. That far out of the RPM range, provided the engine is running right, surely requires a prop change.

Jack plates though make even a modestly performing boat run better. It's just that you can lift the engine squarely vertical instead of using the trim (which changes the angle of thrust, tries to lift the bow up or push it down, causes porpoising, etc..) - it's very handy to be able to just drop the engine when you are experiencing blow outs due to sea conditions without having the boat drag ass all the time when it's calm. The jack plate provides height adjustment and the trim can be used for angle, without the two tied together.

Lately up north have been seeing more boats with them, particularly when people go and re-power older hulls with lots of HP as you mention.

I've got one on my 17 ft aluminum center console; it's not a matter of performance but in shallow water I can lift the engine straight up and get through 18 inches without churning bottom - if I tried to do the same using only the trim the engine would be squatting the hull down making things worse, can actually trim the engine full down so the thrust levels the hull & lifts the stern - a nice feature when you pushed your luck a little too far with the tide running out at 2:00 AM :).
 

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