for the boaters ... and interested people

[mikechell 0]

This is a very interesting video clip showing cavitation in a glass tube.

Cavitation is what happens when pressure drops low enough for water to "boil" without heat. But as soon as the pressure rises, the resultant bubble collapses. This is what etches, or eats away at your prop if it's not the right one for your boat.

This video as little long, but the explanation of what he's doing is good, and the high speed camera images are really neat!

[Poopdeck 0]

Perfect timing as I just uncovered, prepped and readied my new old boat for sea trials, or river trials, tomorrow. Cavitation will be one of my on the water tests as well trying to catch a fish. I liked the high speed footage of tiny talking bubbles.

[Dave G. 0]

What I know is I think my boat needs a new battery this year, despite several charges over the winter. It doesn't hold well overnight.

 

Cavitation is an issue too with too much power going into turns under full plane. More cup in the prop can help that, more pitch can help that. But if the prop otherwise seems perfect for the boat, it may just be that you have to trim power back a bit and trim the engine angle down a bit while making the turn.. Cavitation can also be magnified with too high an install height of the engine for the particular hull design. So there are a bunch of variable that enter into this. But it was cool to see what happens gas wise in the water !

 

RPM wise the prop on my boat is perfect, it maxes out just about on the red line without sounding the alarm ( 5500 rpm and 5550 it will sound). It gives good acceleration and I can troll on the motor generally speaking without throwing buckets over the side for drag . It can be a bit slippery in turns though, I do as mentioned above, especially in left hand turns. The cup has been slightly modified on the prop so it is less slippery than it once was. It's a 17" pitch prop, I've been temped here and there to put on a 19 for the bigger lakes and bring the cruise RPM down a little bit. I know the hole shot will suffer but it has plenty of guts to get on plane anyway, not a huge issue to me. But for the record, more pitch doesn't always equate to what one thinks it might. A good prop guy can message a prop to suit your boat, by doing compound pitch and cupping.

[Dave G. 0]

Perfect timing as I just uncovered, prepped and readied my new old boat for sea trials, or river trials, tomorrow. Cavitation will be one of my on the water tests as well trying to catch a fish. I liked the high speed footage of tiny talking bubbles.

File Apr 11, 12 34 32 PM.jpeg

I envy your top on that boat ! Nice rig.

[mikechell 0]

Good call on most of your post, Dave. But don't confuse cavitation with aeration.

In turns and with the engine too high on the boat, the tendency is for the prop to pull air from the surface ... that's aeration.

Hole shot (torque) and max RPM (horse power) are your main goals when sizing props, and it sounds like you've got yours dialed in perfectly.

[Dave G. 0]

Hmm, I hadn't considered aeration. LIKE

[vicente 0]

Nice boat poopdeck.

[Saltybum 0]

What I've seen eat up props faster than anything is running in too shallow water and sanding the blades down and or hitting hard rocky bottom.

[mikechell 0]

Prop damages are often misdiagnosed. I know you run a lot of flats, Salty, so you probably do have more strike and sanding damage ... but nation (world?) wide, that's not the most common reason.

Cavitation is THE most common reason for Prop failures, even though it's often not properly "blamed".

This is cavitation on a stainless steel prop ... and easily seen as the fault.

 

 

This is a gear case and prop that have been run in too shallow water, and has sanding damage. Note the sand paper effect is seen on the skeg, too. Usually, if you don't have that skeg damage, you don't have sanding on the prop, either.

 

 

This prop has failed due to extended run time with cavitation. There are no sand paper effect marks to indicate that it had ever been run in shallow water.

 

 

This one, too, although it did hit something, has no sanding marks.

 

 

All the snapped off pieces in the last two examples are due to cavitation making the aluminum weak and brittle ... not prop strikes.

[Saltybum 0]

Very interesting Mike.