Cavitating Tohatsu 3.5hp 4-stroke on Zodiac Zoom Air

[patrickkidd]

Hi there,

I have a ten-year-old Tohatsu 3.5hp 4-stroke mid-length shaft that cavitates like crazy as soon as I get on step on my 8' zodiac Zoom air inflatable. Adjusting the shaft tile to fully vertical helped a bit, I've tried mounting it higher and lower, and the propeller is apparently original with no adjustments available. Any thoughts? The prop is at least 3" under water.

Thanks!

 

[justsomeguy]

as soon as I get on step on my 8' zodiac Zoom air inflatable.

"On step"? Sure looks deep enough. Maybe something to do with the hydrodynamics of your Zodiac?

 

[patrickkidd]

"On step"? Sure looks deep enough. Maybe something to do with the hydrodynamics of your Zodiac?

On step = planing. I guess Alaska's super progressive lingo hasn't quite made it down here yet, haha.

 

[Bill Roosa]

Is it sucking air from the surface or actually caveatting? Sucking air is normally prevented by the horizontal plate above the prop. If there is a bur on the lower end that disturbs the water it can provide a path for air to get to the prop. Actual caveattion is caused by the pressure behind the prop (forward side looking at the boat) blades getting lower than the vapor pressure of the water and the water turns to steam. Note that caveattion does not normally cause loss of thrust but does erode the prop as the steam bubbles collapse and water hammer the prop while sucking air will let the motor rev up as the prop is running in air. Look at the low pressure side of the prop for pieces of paint missing, usually along the outer edges.
good luck

 

[mlpayne]

Sounds like the motor shaft is too short and does not accommodate depth of the hull and associated dead rise angle.

 

[patrickkidd]

Sounds like the motor shaft is too short and does not accommodate depth of the hull and associated dead rise angle.

That could be. The shaft is the mid-length version and the dinghy calls for a short shaft, so if this is so this is surprising. The prop is several inches under the water, as shown in one of the photos.

 

[Ken Cross]

Yes. When you speed up the hull rides higher in the water. It's a shaft length problem. If you could lower the mounting point (such as notching the transom where the motor sits) it should improve.

Ken

 

[Chief RA]

That motor looks like a standard length motor. Ken Cross says to cut your transom down and that is probably what you will have to do. Cavitation as almost always a prop too close to the surface. Porpoising of an 8' inflatable is usually caused by improper angle. Put your motor into a water barrel and submerge it exactly the depth it has while on the transom with it in gear to verify our contentions. As Ken points out though: this can't simulate the rise of the transom when moving. I think the barrel test will cavitate anyway. Chief

 

[Benny17441]

I guess it is a matter of how it is being used. I have the same boat with a wood slats floor and an inflatable V rib in the bottom. When the boat is on plane it will rise up in the water a couple of inches and that would be enough to get your prop too close to the surface of the water. I purchased mine to get to shore and back and carry provisions and never get it on plane nor have had any cavitation problems. Used to run a 5HP Tohatsu but replaced the heavy engine with one of the last 3.3HP Mercury two strokes engines to be legally sold (19 lbs.). Have you tried shifting your weight farther back? At 8' these boats were hardly intended to be fast anchorage cruisers.

 

[Brian S]

Is it sucking air from the surface or actually caveatting? Sucking air is normally prevented by the horizontal plate above the prop. If there is a bur on the lower end that disturbs the water it can provide a path for air to get to the prop. Actual caveattion is caused by the pressure behind the prop (forward side looking at the boat) blades getting lower than the vapor pressure of the water and the water turns to steam. Note that caveattion does not normally cause loss of thrust but does erode the prop as the steam bubbles collapse and water hammer the prop while sucking air will let the motor rev up as the prop is running in air. Look at the low pressure side of the prop for pieces of paint missing, usually along the outer edges.
good luck

THANK YOU, Bill Roosa! I HATE when people mis-use the terms "ventilate" and "cavitate" when discussing propellers. I've met plenty of boats that will ventilate in chop or aggressive turns, but few that cavitate. And why the industry calls the plate above the prop an "anti-cavitation plate" is beyond me, considering it's really an anti-ventilation plate.

Sorry, ChiefRA, I like your posts, but it's still ventilation, not cavitation.

God, I am such a pedant. It's my own fault.

 

[Chief RA]

No problem Brian, I just try to help and sometimes I can use the wrong term. This sure isn't one of my fields of expertise! Thats the main reason when in one of my fields of expertise I indicate I am an Engineer sometimes so people will know "from which it comes"!
Chief

 

[justsomeguy]

And why the industry calls the plate above the prop an "anti-cavitation plate" is beyond me, considering it's really an anti-ventilation plate.

I did not know that.

 

[Charlie Jones s/v Tehani]

THANK YOU, Bill Roosa! I HATE when people mis-use the terms "ventilate" and "cavitate" when discussing propellers. I've met plenty of boats that will ventilate in chop or aggressive turns, but few that cavitate. And why the industry calls the plate above the prop an "anti-cavitation plate" is beyond me, considering it's really an anti-ventilation plate.

Sorry, ChiefRA, I like your posts, but it's still ventilation, not cavitation.

God, I am such a pedant. It's my own fault.

Thank you Brian. Been biting my tongue to not say that. Takes a VERY very hi speed prop to "cavitate", and when it does, it make s a lot of noise. But since we can't SEE each other, precise terminology can be important

 

[Brian S]

Thank you Brian. Been biting my tongue to not say that. Takes a VERY very hi speed prop to "cavitate", and when it does, it make s a lot of noise. But since we can't SEE each other, precise terminology can be important

And submarines. They turn those props slowly so there isn't enough of a pressure drop to cavitate across the blades, 'cos that would be VERY noisy. And subs don't like to be noisy.

 

[Brian S]

No problem Brian, I just try to help and sometimes I can use the wrong term. This sure isn't one of my fields of expertise! Thats the main reason when in one of my fields of expertise I indicate I am an Engineer sometimes so people will know "from which it comes"!
Chief

Oh, you're an engineer? NOW we're in trouble! Just kidding, I'm just a pedant! I really do appreciate your posts. Heck, you don't have to know the right words, or the physics of stuff, to know what works well from experience. I only know most of my stuff because of others I've learned from!

 

[Bill Roosa]

your welcome!
words have meaning and being a former commander of some special forces soldiers I can tall you those rascals pushed that concept to the limit (as they should BTW). things like "Don't be coming in after curfew" So they didn't! Curfew came and they did not come in after curfew (little snots!) Should have said "Be sure you are in before curfew"

 

[justsomeguy]

Oh, you're an engineer?

No, he's an Engineer. The capitalization makes it muy special.

 

[patrickkidd]

I guess it is a matter of how it is being used. I have the same boat with a wood slats floor and an inflatable V rib in the bottom. When the boat is on plane it will rise up in the water a couple of inches and that would be enough to get your prop too close to the surface of the water. I purchased mine to get to shore and back and carry provisions and never get it on plane nor have had any cavitation problems. Used to run a 5HP Tohatsu but replaced the heavy engine with one of the last 3.3HP Mercury two strokes engines to be legally sold (19 lbs.). Have you tried shifting your weight farther back? At 8' these boats were hardly intended to be fast anchorage cruisers.

That is very helpful to hear that the boat may rise a couple of inches, and it makes sense.

And submarines. They turn those props slowly so there isn't enough of a pressure drop to cavitate across the blades, 'cos that would be VERY noisy. And subs don't like to be noisy.

Wow that's really interesting, and so pertinent to the discussion!

That motor looks like a standard length motor. Ken Cross says to cut your transom down and that is probably what you will have to do. Cavitation as almost always a prop too close to the surface. Porpoising of an 8' inflatable is usually caused by improper angle. Put your motor into a water barrel and submerge it exactly the depth it has while on the transom with it in gear to verify our contentions. As Ken points out though: this can't simulate the rise of the transom when moving. I think the barrel test will cavitate anyway. Chief

Thanks for that. The PO said that it was a "mid-length," but maybe it is just a "short" 15" shaft. Do you know exactly where to measure it? The dinghy manual calls for a "short shaft."

I just bought this boat brand-new and I am very apprehensive about cutting the transom! Maybe a motor with a longer shaft? Shame because I got this one for $400 and it's an immaculate fresh-water only motor.

Is it sucking air from the surface or actually caveatting? Sucking air is normally prevented by the horizontal plate above the prop. If there is a bur on the lower end that disturbs the water it can provide a path for air to get to the prop. Actual caveattion is caused by the pressure behind the prop (forward side looking at the boat) blades getting lower than the vapor pressure of the water and the water turns to steam. Note that caveattion does not normally cause loss of thrust but does erode the prop as the steam bubbles collapse and water hammer the prop while sucking air will let the motor rev up as the prop is running in air. Look at the low pressure side of the prop for pieces of paint missing, usually along the outer edges.
good luck

It is a black plastic prop with no signs of wear.

 

[Bill Roosa]

Caveatation would erode a plastic prop in short order so I'm thinking that looking at the lower end while on plane might show where the air is getting in to ventilate the prop. Should see a stream of air "clinging" to the lower end and leading to the prop

 

[Fred]

You can get a zodiac to plane with a 3.5 hp? I have a 4hp (2 stroke) on my air deck Seabright 230 (7.5 ft) inflatable and can't get it to plane