On plane, motor will rev higher, and slow down. Not cavitation

[Cady8special]

Hey guys,

I was on my friends boat not long ago and experienced something strange. 14 foot aluminum, 5 HP 4 stroke. While crossing the lake at full throttle, a few times the engine seemed to rev higher, and the boat would slow down. Just for an instant. It felt similar to cavitation, but the engine was all the way down, and there would have been no reason for it. All the parts in a motor are gear driven right? I couldn't imagine anything "slipping" without a horrible grinding noise. If it was related to fuel starvation, then I don't understand why it would rev up and slow at the same time. Then my father told me he had the same issue last time out with his 90HP Johnson V4. He's looking to sell the boat, but we don't want to screw anyone. Any ideas on what this might be?

Thank you

 

[alldodge]

Look at the prop hub, if its rubber it might have spun and needs replaced

 

[ziggy]

a few times the engine seemed to rev higher, and the boat would slow down

prop ventilation would cause that scenario
google the definition of cavitation vs ventilation.

 

[82rude]

Sounds like a spun hub.If not addressed you will never be satisfied that your brother is beating you with a mere 9,8 hp and you will get so mad you go buy a 25 hp.OPPS sorry that was me ,lol.

 

[Texasmark]

Cavitation is little bubbles following the blade tips caused by irregularities in the blade tip or a poorly designed prop (1971 Chrysler 85 hp). Ventilation is sucking surface water into the prop which reduces the density of the medium in which the prop is turning, reducing the load on the engine causing increases rpms and reducing the thrust causing boat speed to drop.

A slipping hub in your prop, usually with rubber hub props as plastic isn't setup like rubber. Remove your prop and mark a line from the splined inner metal usually bronze, straight out to the outer casting of the prop. Replace and torque to spec and run it. Remove the prop and look at your line. If no longer straight, your hub is slipping. Prop shops can fix them or if a 99 buck alum bought on iboats, get another.

 

[jimmbo]

You weren't running the boat through weeds?

 

[Sea Rider]

Could it be : OB revs up while speed descends ? if so, the prop hub is shot. Will need a new prop....

Happy Boating

 

[QBhoy]

Likely the shear pin in bits or if it’s a rubber hub, same.

 

[QBhoy]

Or...just thinking about it...that’s a fair size of boat for that wee engine. If the engine height isn’t right, it would occasionally draw air from the surface. Often with big heavy boats and wee engines, it’s easier for the engine to draw air than it is to push the boat through the water.
Or the hub or shear pin is buggered

 

[Sea Rider]

As per post:Engine being fully down does not necessarily means it sits at the sweet transom height. 2 issues to consider, engine transom height and or spun hub, or even a combo of both ?

An extremely underpowered engine correctly height seated on transom no matter if boat is large, ocean liners are excluded, will be propelled correctly. Add a correct prop for engine to rev middle to max wot rpm range and volia!!

Happy Boating

 

[valvebounce]

Could be as simple as a partially blocked air vent in the fuel tank cap.
The motor could be struggling for fuel because of the vacuum caused by the partial blockage.When there is plenty of air the motor runs as per,once it uses the air in the tank it will faulter until it the tank re-vents itself.
Try cleaning the cap vent with carb cleaner and blowing it out with an air line.
Inspect the vent carefully by eye,some times just a mechanical procedure doesn't always clear a blockage.

 

[NYBo]

Could be as simple as a partially blocked air vent in the fuel tank cap.
The motor could be struggling for fuel because of the vacuum caused by the partial blockage.When there is plenty of air the motor runs as per,once it uses the air in the tank it will faulter until it the tank re-vents itself.
Try cleaning the cap vent with carb cleaner and blowing it out with an air line.
Inspect the vent carefully by eye,some times just a mechanical procedure doesn't always clear a blockage.

A partially blocked air vent would cause fuel starvation, which would cause the motor to slow down, not rev up.

 

[valvebounce]

Not questioning your opinion Bob,but I have had a similar situation with a small outboard.The vent in the cap was causing fuel starvation at high revs,then once the tank re-vented itself,it would temporarily accelerate.At lower revs there was no problems.
Of course it could be another problem in this case,but trying possibilities sometimes helps."V"