Power Fluctuations

[theonecalledtom]

Summary: Engine power drops at higher boat speeds accompanied with an increase in rpm.

Engine history: Hours from factory ~20, sunk in salt water for about 1 hour, cleaned thoughrougly since then, new gearbox oil, new engine oil, new carb.

The story: So, having finally given up on my old clogged up carb and splashed out on a new one our Tohatsu 5hp fired up and ran smoothly again.

This coincided with our trial run of a new compact RIB, replacing our old inflatable which was mostly like an innertube with a floor that was falling off all the time.

Turns out that we can get up on a plane with both of us in, it takes 30+ seconds and a bit of physical encouragement to break free but it gets there....

To celebrate we decided to go for lunch a few miles up the bay. On the way we explored the Coronado Cays and nosed around some mooring bouys previously beyond our safety threshold. Lovely sunny day, happy times.

A few times on the way to lunch I noticed a little surge in revs, a little drop in power, but put it down to the slight chop.

On the way back we decided to time the run.

As we travelled back the rev surges and power drops became more frequent. Eventually I realised that they coinicded with getting up to higher (but steadily reducing) speed. For the last 10 minutes we couldn't maintain a plane.

Obviously we were discussing potential issues.

Firstly our fuel level was quite low and the tank not mounted ideally for the pickup point, so perhaps air bubble were making their way through....

... however why would the revs *increase* at the same time as the power dropped?

Perhaps this is a gear box problem?

[Captain Bill]

There are two possibilities that generate this symptom that I know of. The first and most likely is that the engine is mounted a bit too high or the angle is set wrong and you are getting air into the prop. As the prop increases in RPM it then begins cavitating and resulting in more loss of power (thrust) and further increasing RPM. The second is a bad hub in the prop. This usually manifests itself more when trying to get up on a plane and the prop is under high load. This and the fact that your motor only has about 20 hrs on it makes this less likely.

[Hud3]

I'd agree with Captain Bill that its cavitation. What kind of dinghy do you have? If it's an inflatable, high pressure floor model, and the floor's not pumped up enough, you'll see those symptoms--loss of speed and reving engine.

[neilpride]

Bill is right , it hapen to me in my 4 stroke 9.9 yamaha, the propeller hub, is like a piece of ruber glued inside to the propeller, i dont think the problem come from a bad carb , check the engine lever in the transom and the prop . Good luck.

[Vasco]

Sounds more like a spun prop than cavitation but I think Captain Bill has it all covered.

[GordMay]

Ventilation (or Aeration) is a phenomenon that occurs when surface air or exhaust gas (in the case of motors equipped with through-hub exhaust) is drawn into the spinning propeller blades. With the propeller pushing mostly air instead of water, the load on the engine is greatly reduced, causing the engine to race and the prop to spin fast enough to result in cavitation.

Cavitation is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure, at which point no thrust is generated at all. The condition continues until the prop slows enough for the air bubbles to rise to the surface. The primary causes of ventilation are: motor mounted too high, motor trimmed out excessively, damage to the antiventilation plate, damage to propeller, foreign object lodged in the diffuser ring, vegetation caught on the lower unit, or anything that interrupts the flow of water into the propeller blades.

Motor Mounting Height is an important factor in achieving optimal performance. The motor should be as high as possible without ventilating or loss of water pressure. This minimizes the effect of hydrodynamic drag while underway, allowing for greater speed. Generally, the antiventilation plate should be about the same height as, or up to two inches higher than, the keel.

The ideal trim angle is the one in which the boat rides level, with most of the hull on the surface instead of plowing through the water.

If the motor is trimmed out too far, the bow will ride too high in the water. With too little trim, the bow rides too low.

The optimal trim setting will vary depending on many factors including speed, hull design, weight and balance, and conditions on the water (wind and waves).

See also:

Tilt and Trim for Small Engines
➥ Small Outboard Engine Tilt and Trim: Theory and Practice

[theonecalledtom]

Thanks all for your responses.

Cavitation might make sense given that the worst behavior was exhibited with the largest following swell encountered.

It also makes some sense as while this was happening I was peering over the back of the boat and thinking that the little built in wing above the prop was rather higher against the transom than looked correct.

Not sure if this makes me happy or sad. Now I have a motor with a short shaft that may be too long and will need to replace it to get the most out of the RIB. Unless I could fix this by getting one of those big trim tabs added.

-Tom

[Ex-Calif]

"As we travelled back the rev surges and power drops became more frequent. Eventually I realised that they coinicded with getting up to higher (but steadily reducing) speed. For the last 10 minutes we couldn't maintain a plane."

This sounds like a spun prop to me. Cavitation would occur under pretty much the same condittions not steadily reducing speed.

I am assuming you were backing down the throttle each time the RPMs surged and then the boat would go again, abeit a slower pace each time.

We had this happen on a J24 earlier this year. $5 shear pin and tighten the prop nut.

[theonecalledtom]

Thanks Dan, I'll check the prop nut while we're there.

That part of the journey coincides with the largest following swell so I also figured we could have been lifting the tail higher in the water on a regular basis.

Going to add a stabaliser anyway I think, would probably have difficulty hitting a plane into a headwind or any kind of swell with the current setup and this sounds like a cheap solution without repowering. Anyone have experience with them on a small (5hp) motor?

-Tom

[osirissail]

I would go with Neilprides' answer. I had a Merc 9.9 that did the same thing and the problem was the Propeller hub would slip causing speed loss and increased rpm's. The propeller and hub are separate parts with poured rubber connecting them together. Over time the hub or prop blades break loose from each other and when load is applied to the prop it starts to slip. This happens quite often enough that there is a "common knowledge" fix. Because a new propeller sells for around $140, we take the old slipping prop and drill three holes through the prop/rubber/hub and thread the holes. Then we put three bolts into the threaded holes to keep the mounting hub and blades together. These engines do not use "shear pins" so the rubber in between the hub and blades is the "shear" protection.

[Cheechako]

Your tank vent IS open right?! A short shaft ought to be good on a RIB..... The bad hub would usually show up when trying to get up on a plane rather than just cruising along.... Cant remember for sure but I think the Cav plate should be 1" below the hull in front of it...?

[GordMay]

Originally Posted by Cheechako

Your tank vent IS open right?!...
... Cant remember for sure but I think the Cav plate should be 1" below the hull in front of it...?

Excerpted from post #6:
Generally, the antiventilation plate should be about the same height as, or up to two inches higher than, the keel.

[theonecalledtom]

Vent confirmed open - that one's bit me before

[Ex-Calif]

Originally Posted by osirissail

I would go with Neilprides' answer. I had a Merc 9.9 that did the same thing and the problem was the Propeller hub would slip causing speed loss and increased rpm's. The propeller and hub are separate parts with poured rubber connecting them together. Over time the hub or prop blades break loose from each other and when load is applied to the prop it starts to slip. This happens quite often enough that there is a "common knowledge" fix. Because a new propeller sells for around $140, we take the old slipping prop and drill three holes through the prop/rubber/hub and thread the holes. Then we put three bolts into the threaded holes to keep the mounting hub and blades together. These engines do not use "shear pins" so the rubber in between the hub and blades is the "shear" protection.

I would defintiely do this as a get home action but would be worried to do it long term.

The shear device whether rubber or a shear pin is there to protect the engine in case you hit something with the prop. The idea of course to decouple the prop and avoid damaging the gears.

If my prop had the rubbe couple and it had gone I would consider drilling adding a shear pin then putting it back together.

3 bolts seems like too "hard" a coupling.