prop help - cavitation

[heavy-chevy]

well i went from my beat up 10x13 3 blade to a 10.5x14 4 blade and it planes faster, jumps out of the hole better, and completely eliminated the porpoising with one person... but i was very disappointed with how easy it blows out compared to the old prop. i also lost a little top end with one person on the boat but gained a little loaded.

anyways i want to know what causes it to blow out when 4 blades are known for not slipping or blowing out as easy. im thinking it could be one of two reasons, one - the 4 blade is not cupped, my old one was, or two - being that its a half inch bigger its too close to the cavitation plate and is pulling air from the surface. i mean its pretty bad, i can be cruising about 18-20 and it will just totally lose bite for no reason and wont hook up until i pull out of it. any slight cornering blows it out. i cant run trimmed up at all sometimes it was hard to even get it to plane, it would just keep slipping until i totally stop and start over.

the other thing is its slower, im thinking maybe the 10x13 has been taken to a shop and cupped out, making it actually deeper then a true 10x13. anyone that knows props please help me, is my cavitation theory correct?

btw - the hub is not slipping.

 

[Quackrstackr]

I think you probably answered your own question if your 3 blade had been cupped.

Also, some 3 blade designs will actually run semi-surfaced (like a cleaver) but I have never heard of a 4 blade that would.

 

[heavy-chevy]

well i was reading up on them before i bought it and it seemed like cupped or not 4 blades were supposed to be superior for running aerated, which is why i was still surprised at how bad it did in that department. of coarse some 3 blades are designed to run aerated and have more surface area and less angle to basically do what a 4 blade does. but what i had was just a regular old omc aluminum 10x13 possibly with an additional cup but not a double cup.

the more i think of it, my cavitation theory must be it, because most regular props can handle being trimmed up cant they?

btw, i have seen a few bass boats with 4 blades, so im sure those are designed to run above the surface.

 

[Quackrstackr]

Most props are not designed to be run above the surface. There are only a few designs that are and they are usually mucho $. Bass boats run 4 blades for the hole shot and load carrying capability, not because they are a surfacing prop.

All props should be able to run properly trimmed.

It's really not a cavitation plate issue if the larger diameter is the problem. Your motor is simply too high. How far below the transom is the centerline of your prop shaft?

If you are running your motor high enough that the prop is surfacing, you really need to install a water pressure gauge. I bet you are dangerously low on pressure if that is the reason the prop is blowing out.

 

[Zum]

Maybe you not only dented the bow but also the bottom,creating air??
Some tunnel hulls have trouble with cavitation.

 

[heavy-chevy]

Most props are not designed to be run above the surface. There are only a few designs that are and they are usually mucho $. Bass boats run 4 blades for the hole shot and load carrying capability, not because they are a surfacing prop.

All props should be able to run properly trimmed.

It's really not a cavitation plate issue if the larger diameter is the problem. Your motor is simply too high. How far below the transom is the centerline of your prop shaft?

If you are running your motor high enough that the prop is surfacing, you really need to install a water pressure gauge. I bet you are dangerously low on pressure if that is the reason the prop is blowing out.

of the designs for running at the surface would be a 4 blade, in fact that's what theyr best at because by design they have alot more surface area...

all bass boats run a surfacing prop, which is why a 4 blade does well on them. do you think they would throw the kind of roost they do with a subsurface prop?

my old prop WAS able to run above the surface, the cav plate sits 1 inch above the bottom of the boat. when trimed up all the way it looks like the prop should break the surface. im thinking the taller prop now sits high enough to break the surface too much. not to mention the idea of a cavitation plate is to prevent cavitation by guarding the prop from sucking surface air, this would seem to be less effective the closer the prop gets to it?

i dont understand the water pressure thing, my boats not a tunnel hull if your assuming it is. the pickups are well below the waterline at all times.

 

[Quackrstackr]

Whatever you say, brother. My boat throws a roostertail too but I can assure you that the prop is not running surfaced.

If you are surfacing a prop, your water intakes are not well below the surface.

Since you keep making reference to bass boats, go check out one of their dedicated forums and ask around if those guys run water pressure gauges with jack plates and why they do it. Matter of fact, ask them how many of them are capable of intentionally surfacing a prop. I think you might be surprised. :wink:

 

[heavy-chevy]

hold up now, what exactly do you mean by surfaced? all im saying is a prop should be able to operate with part of the blade running out of the water. where i run my motor is standard height for most fiberglass boats i see, in fact maybe a little lower then most.

 

[Quackrstackr]

That's what I'm saying.

Most all props are not designed to run with their blades breaking the surface.

It takes a whole lot of hp with a special setup to run a surfacing prop intentionally, not your standard outboard/prop. If your standard outboard and prop is running surfaced, it is not set up properly and will have very poor performance, which you are experiencing.

Also, you have to have water pickups mounted extremely low on the foot for surfacing or your pump is sucking more air than your prop is.

 

[heavy-chevy]

okay, the top of the prop is higher then the bottom of the boat, does this nescisarily mean its surfacing? the point of when a prop is actually surfacing is becoming blurry here.

 

[Quackrstackr]

It may not be surfacing just because it is above the bottom of the boat. There is a low pressure swell that comes up immediately behind your transom that the prop usually operates in. That's part of the reason for a jack plate with a lot of setback. It gets the motor further back into that area.

Surfacing means that the blades are physically breaking the surface of the water. It takes a blade with a lot of rake and a lot of surface area to pull that off.

The only props that I know of that are designed for a 25hp to surface are cleaver style props meant to run on the F1 style racing boats.

 

[heavy-chevy]

okay so then my prop is not breaking the surface... which leaves me where i started, there must be a reason why the .5" bigger prop is not listed as compatible with my lower unit.

 

[Quackrstackr]

Not listed as compatible?

Most of the time it is because they have become such a large diameter that it will interfere with the cavitation plate or torque tab. Yours may be so close to the plate that it can not get clean water flow around the tip as it passes.

Is this prop stainless or aluminum? Who is the manufacturer?

If it is aluminum and cavitation is the problem, you will soon cook the paint off of the leading edge of the blades. The water actually will boil (literally) at that point and cook the paint right off.

 

[heavy-chevy]

Not listed as compatible?

Most of the time it is because they have become such a large diameter that it will interfere with the cavitation plate or torque tab. Yours may be so close to the plate that it can not get clean water flow around the tip as it passes.

Is this prop stainless or aluminum? Who is the manufacturer?

If it is aluminum and cavitation is the problem, you will soon cook the paint off of the leading edge of the blades. The water actually will boil (literally) at that point and cook the paint right off.

it doesn't interfere with the torque tab as there isnt one on a 25hp, and its still about 1" from the plate. but what you said about clean water around the tip is what im talking about, im thinking that it being 1/4" closer makes it harder to draw water and easier to suck air from the surface around the cav plate.

its and aluminum omc prop, and it actually hasn't lost any paint yet. it was a NOS i got on ebay, they don't make this prop anymore for some reason, 4 blades probably aren't popular on smaller motors.

 

[Quackrstackr]

it doesn't interfere with the torque tab as there isnt one on a 25hp.

I guess some of them do and some of them don't. I have a buddy with a 25hp Johnson in the same year range and it has one. It may be a difference in the short vs long shaft, I'm not sure.

 

[heavy-chevy]

realy? i wouldn't see it necessary on a motor that small.

 

[aborgman]

Lets all remember there is a difference between "cavitation" and "ventilation" also...

Cavitation is when pressure decreases causing water to boil at much lower temperatures. When the bubbles from this boiling burst they can cause pitting on the prop.

Ventilation occurs when air from the water's surface or exhaust gases from the exhaust outlet are drawn into the prop blades. The normal water load is reduced and the prop over-revs, losing thrust. Most commonly occurs in turns, or with an excessively trimmed-out engine or drive unit.

--
aborgman

 

[heavy-chevy]

Lets all remember there is a difference between "cavitation" and "ventilation" also...

Cavitation is when pressure decreases causing water to boil at much lower temperatures. When the bubbles from this boiling burst they can cause pitting on the prop.

Ventilation occurs when air from the water's surface or exhaust gases from the exhaust outlet are drawn into the prop blades. The normal water load is reduced and the prop over-revs, losing thrust. Most commonly occurs in turns, or with an excessively trimmed-out engine or drive unit.

--
aborgman

well then ventilation would definitely be my issue as there is no pitting on the prop after 3 outings. i just don't understand how my pos hammer tuned 3 blade worked so much better then this fresh 4 blade under the same conditions.