Got BOAT Prop PROBLEMS? Cavitation Vs. Ventilation!!!

a couple of weeks ago we were talking about what causes a boat to porpoise whether that be the weight distribution of a boat a hook or a rocker in the hole or the height of the engine based on how this will affect and where this will position the anti-ventilation plate now this leads us into the discussion of these hooks and rockers in the anti-ventilation plate and the effects this has on the prop and the performance of the boat's engine there is also this phenomenon called cavitation which is what we want to cover today and how all these different things tie in together and at the same time how they are all different specifically how there is a difference between ventilation and cavitation because it isn't uncommon for the plate on the lower unit to be called an anti-cavitation plate or an anti-ventilation plate even a cavitation or a ventilation plate this even goes for looking into manufacturer service manuals they are using these terms interchangeably and some people can get so politically correct with this that they miss the importance of helping people to actually know and understand what these things mean how they happen and what it is that you need to know about when looking at something like this and trying to fix a performance issue with your boat so let's start by breaking down what the difference is between ventilation and cavitation ventilation comes from what you would think it comes from the venting of air from somewhere to the propeller because a propeller is designed to grip the water and think about it like this it is holding onto the water pulling itself through the water and at the same time pushing water on the backside of the propeller's blades pushing the propeller through the water we'll expand on this in a little bit but just keep this gripping of the water in mind because what this does when the prop spins around in the water is that it creates two different pressures of barometric pressure just to make it easier let's just call it water pressure there is a low pressure side and high pressure side to the propeller think about it like the water in front of the propeller doesn't have any pressure to it so it's the low pressure side of the prop then once the propeller goes through the water the water coming off of the back of the prop is going to be pushed away from the prop meaning it has a higher pressure to it than it did before the prop went through it so when we relate this idea of the propeller gripping a hold of the water we know that we want that prop to have the best grip that it can get taking this back to the ventilation issue if the propeller is allowed venting to get air to it then it will now have a space of air between it and the water allowing it to lose its grip on the water and if the prop loses its grip the common results that we will see as the driver of the boat is that the rpm of the engine will shoot way up and the boat will lose its momentum going forward this is the same feeling that we will get when the prop hub spins on the propeller which is what happens when the hub to the prop that attaches the prop to the prop shaft fails and the power of the engine is able to spin the prop shaft without the hub allowing the power of the engine to transfer over to the propeller to make the prop spin through the water and push the boat forward now ventilation can come from many different factors that we have talked about in the other videos of the purposing the engine height increasing your fuel economy and even the video on how to select which prop is right for you and your boat the two main reasons that a prop will ventilate though is due to the engine being mounted too high or the engine being trimmed too high out of the water not allowing the plate that is on the lower unit to do its job of stopping the prop from being able to suck in the surface air from above the water which is why we call this plate on the lower unit the anti-ventilation plate then the other reason is going to be anything that is connected to the hull of the boat whether that be transducers water pickups through-hole fittings imperfections in the hall like a hook or a rocker or any other thing that will disturb the water coming from the bottom of the boat to the propeller as soon as something is put in between the whole of the boat and the propeller whether that be any of these things that will disturb the water and create air bubbles these air bubbles if they get bad enough will allow the prop to lose its grip on the water and begin to ventilate just like if it was able to suck the air from the surface of the water there is also another main reason for ventilation but it's a little less common than the other two reasons and that is if the prop is able to suck in exhaust gases from the engine most commonly through ventilation holes in the propeller or also depending on the engine and style of prop it can get it from the space between the propeller and the gear case here there are some models that have rings like these that will help to prevent this issue from happening now since we are talking about all this air being able to get to the propellers let's move over and talk about the other thing being the cavitation issue now cavitation is basically where water gets to the point of boiling and once the water is given the ability to boil it will create air bubbles these air bubbles will run along the blades of the propeller in the same way that ventilation bubbles will but in this case these boiling air bubbles will pop while on the surface of the propeller and it's when they pop that they will begin to damage the surface of the propeller it's like thousands of little mines exploding on the surface of the prop slowly damaging the propeller and eventually creating pits all across the prop blades creating the damaging effect of cavitation they can have different kinds of signs to you as the driver in a lot of cases you will either just see the damage on the prop but in other worse cases you can find vibrations and shaking happening that you feel as the operator with all these little explosions happening on the propeller this can be caused by quite a few different things and there's actually like six or seven sub-categories or types of cavitation that can happen but we'll not get into all of them today instead we'll talk about a couple of the main causes that can cause this most commonly this can happen by either nicks or damages that can be found on the prop either from ground strikes or other damage that will change the angles and the ability of the prop to move through the water and another major factor is simply the angles and design of the prop if it's the wrong type and design of propeller for the application being the boat hole design engine weight and power of the engine misapplication of a prop can cause this cavitation issue bringing us back to the discussion of the different pressures on the different sides of the prop blades these different pressures can allow the water to boil at less temperatures than normal normally water will boil at 212 degrees but with the right pressure change water can boil at temperatures as low as room temperature but that can get into all kinds of different aspects that will leave up to the professionals and the scientists just remember if you see pitting and burning on your prop you might not be propped right if it is a newer prop and if it's an older prop with lots of hours and previous damage it's just time to put a new prop on the boat now we want to know have any of you ever had to deal with your boat ventilating or have you ever had a prop cavitate on you let us know in the comment section below and if you haven't already make sure you are subscribed to the channel and check out some of these other videos that you might have missed you can visit us at bourneiganboating.com and we want to thank you all for hanging out with us this week and we look forward to seeing you next week you

Channel: Born Again Boating

Views: 83,605

Rating: 4.9379845 out of 5

Keywords: Prop, Propeller, Cavitation, ventilation, anti-ventilation, anti-cavitation, boat, boating, problem, issue, diy, how-to, outboard, inboard, sterndrive, hull, mercury, yamaha, tohatsu, honda, suzuki

Id: KbFa1fmlAXg

Length: 8min 8sec (488 seconds)

Published: Tue May 04 2021