What is porpoising and cavitation???

[mtduckhunter]

what does porpoising and cavitation mean? im thinking about getting a hydrofoil and it says they eliminate this.

 

[Boatist]

Re: What is porpoising and cavitation???

First forget the hydrafoil.

They lift the stern from the center and to me make the boat kind of like a bike where it is balanced from the center. I would much rather have it be balanced from the Chines and be more stable.

Porpoising is when the bow starts bounching up and down like a dolphin. Many boats will do this if you trim the bow up to high. After you get up on plane in nice calm water it is normal to trim the motor out some and raise the bow. This give you more speed and better fuel mileage, but if you go too far the bow will start to bounce up and down (porpoising).
Cure is very simple just trim the motor in or down a little at a time until it stops.

Cavitation is when the prop starts turning in air. Of course the prop looses bite on the water and it can cause the motor to over rev while the boat slows down. Most common cause is the motor in mounted to high or motor trimed too far out. Also common in turns.
If the motor is too high then there is also a chance that the water pump is also sucking air and the motor can overheat while water pump burns it's self up. I belive a Hydrafoil would make this worse as it lifts the stern of the boat more. Cavitation is also where you have air bubbles on the face of the prop. These bubbles create mini explosions and the face of the prop and can cause it to wear. The paint or coating go first then the metal of the prop.

Different props can do a lot to cure Cavitation. Stainless steel props are oftened douplecupped which cause it to get a better bite.

I think you will find more people who have taken off hydrofoils than swear they are great.

If you want to lift the stern of the boat better to go with Trim Tabs on the boat.
Smart tabs if you just want to get on plane faster at a slower speed and with less bow rise.
Helm adjustable tabs if you want complete control of your boat in all conditions and speeds.
Trim Tabs lift from the corners of the stern and make the boat more stable and are much better than any hydrafoil.

 

[5150abf]

Re: What is porpoising and cavitation???

I have never tried smart tabs so I can't say if they work or don't and they may be as good as everyone says but I have run a foil.
I haven't found that it makes my boat unstable at all, always runs straight and true.
I have a 16' Sea nymph V bottom and my foil works really well on my boat with my set up,the boat runs faster with it than without it and planes way earlier.
Either is a viable option, tabs are $120 a foil is $40, tabs require drilling holes in the hull,foils require drilling holes in your motors cavitation plate.
Look into both and see what fits your boating style and budget.

 

[tashasdaddy]

Re: What is porpoising and cavitation???

i have taken every hydrofoil i ever had off. smart tabs are the way to go.

 

[ziggy]

Re: What is porpoising and cavitation???

Cavitation is when the prop starts turning in air.

that, i think is ventilation. cavitation is when water boils and turns into steam, then goes back to water form. when this occurs, the water drops actually hammer on the prop and eat chuncks out of it....
i'm another one voteing for no foil. i've had one. removed it after it broke my ventilation plate on my drive. i've never had smart tabs, but from the read, they work way better than a whaletail does + don't stress yer drive.....

 

[nlain]

Re: What is porpoising and cavitation???

Smart Tabs all the way, no foil.

 

[Silvertip]

Re: What is porpoising and cavitation???

Ziggy has the cavitation/ventilation discussion correct. I am a user of Smart Tabs and can vouch for the amazing difference in performance between no tabs, foil, and tabs. Smart Tabs are worth every penny. I too have removed foils every time I tried one.

 

[fdmsiv]

Re: What is porpoising and cavitation???

cavitation is when water boils and turns into steam, then goes back to water form.

The function of a prop is to create an area of low pressure that the blade is then sucked into. Near the blade tips the pressure is very low compared to the surrounding water. This ultra low pressure is essentially pulling on the water molecules. The tensile strength (force holding the molecules together) of the water is the vapor pressure. This means that if you prop is turning too fast you are creating a pressure that is lower than the vapor pressure of the water. The water molecules are being pulled apart and steam is filling the gaps between the molecules. When this steam bubble passes to the back side of the blade, the water pressure is much higher and the bubble is smashed and the steam condense. This is a very very violent action that releases quite a bit of energy. That energy is what pits the prop blade.

There are some formulas to check for cavitation but I can not recall them off the top of my head.

 

[QC]

Re: What is porpoising and cavitation???

Porpoising is when the bow starts bounching up and down like a dolphin

Or like a porpoise

Helm adjustable tabs if you want complete control of your boat in all conditions and speeds.

There is a reason that Boatist and I always mention this . . . And please remember I have used both Smart Tabs and Helm adjustable, if you haven't well then . . . Oh, never mind.

The fact is that the Hydrofoil manufacturers do not even understand the difference between Cavitation and Ventilation or they would not claim they cure Cavitation . . . Friends don't let Friends use Hydrofoils

 

[mtduckhunter]

Re: What is porpoising and cavitation???

the reason i asked is because my boats front end is riding in the water when on plane. i was told it could have hook but i looked and it didnt seem to have. i can tell that the boat is being slowed down by this. how can i fix this?

 

[nlain]

Re: What is porpoising and cavitation???

Not sure what you have here, more boat, motor and drive info would help. If you have outboard motor or IO, are you trimming up? Trim up brings the bow up, trim down keeps the bow down. What is the weight distribution, 500 lbs in the bow and 100 in the stern, heavy bow would of course keep the bow down. Just some thoughts, more info will help get better answers.

 

[JCF350]

Re: What is porpoising and cavitation???

the reason i asked is because my boats front end is riding in the water when on plane. i was told it could have hook but i looked and it didnt seem to have. i can tell that the boat is being slowed down by this. how can i fix this?

Did you raise the boat off the trailer and run a string line from the stern to the bow (before it starts to rise) along the keel? Most times you can't tell when the hull is on the trailer.

 

[mtduckhunter]

Re: What is porpoising and cavitation???

i have a monark 15 ft with a johnson 70 hp w/o power trim.

i didnt lift the boat up and check it with a string line.

 

[Captain Caveman]

Re: What is porpoising and cavitation???

If you don't have power trim (like me), then you need to get used to the loads in your boat and adjust the trim prior to boating. Each time before I leave the shore, I assess if I'll have other people in my boat and then trim up the motor accordingly. You might need to do so as well, assuming that you can manually trim your 70hp motor.

 

[QC]

Re: What is porpoising and cavitation???

Check trim first. May be non-issue as Captain Caveman has described. Good to know about hook/rocker (the other way) too, but a little hook might be OK if you can change attitude with trim and weight adjustments.

 

[mtduckhunter]

Re: What is porpoising and cavitation???

this motor has five trim settings. i tried all of them and it seemed the same. the top two i couldnt even get it to go.

 

[jonesg]

Re: What is porpoising and cavitation???

The function of a prop is to create an area of low pressure that the blade is then sucked into. Near the blade tips the pressure is very low compared to the surrounding water. This ultra low pressure is essentially pulling on the water molecules. The tensile strength (force holding the molecules together) of the water is the vapor pressure. This means that if you prop is turning too fast you are creating a pressure that is lower than the vapor pressure of the water. The water molecules are being pulled apart and steam is filling the gaps between the molecules. When this steam bubble passes to the back side of the blade, the water pressure is much higher and the bubble is smashed and the steam condense. This is a very very violent action that releases quite a bit of energy. That energy is what pits the prop blade.

There are some formulas to check for cavitation but I can not recall them off the top of my head.

Nice, in aerodynamics its called aeroelastic seperation and produces destructive flutter or stall.

 

[QC]

Re: What is porpoising and cavitation???

What about weight in the bow duck guy? If it is already empty then hook sounds like a likely culprit . . .

 

[mtduckhunter]

Re: What is porpoising and cavitation???

there is rotted plywood under the floor. that all thats in the front. no trolling motor or anything. so i guess its hook and i dont know how to fix that.

 

[captbjack]

Re: What is porpoising and cavitation???

all this info is great. guess i should have read it first, just got inside from installing "cavitation plate" on my boat:redface: