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Posts: 260
Location: Kentucky | Could someone please explain cavitation as it relates to outboard motors please. I just bought a new boat last fall and when I drove it at full throttle, there was water flying straight up off of the back of the boat. I had the dealer raise the motor a notch, which fixed the spray, but know the water intake on the lower unit comes out of the water on occasion when I hit waves. Plus now the cavitation plate is higher than the bottom of the boat!? That also is against everything I've been Told!
HELP!
Edited by mavmskyb8 2/12/2003 3:53 PM
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Posts: 44
| Cavitation is when your prop is losing it bite. You can tell, because your rpm's will go up but your speed won't. You will also lose your bite in the turns and if you have a fast boat this can be dangerous. Cavitation is normally caused by having your engine too high on the transom or a prop that is not right for your boat. Also a prop that has been reworked improperly can cause this.
The boat manufacture can usually tell you how high to run your engine on the transom and what prop to run on your boat.
You should never raise an engine on the transom without a water pressure gauge. You can destroy your engine very quickly, if it's not taking in enough water. Are you sure you didn't just have a rooster tail off the back of your boat. A rooster tail that doesn't go higher than your cowling is normal.
What boat, motor and prop do you have? What are your rpm's, water pressure and speed? Maybe I can help. | |
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Posts: 260
Location: Kentucky | Thanks Bassin- I have an xpress 1870VJB w/ Yamaha 90hp. Both year 2002 models. The rooster tail was about 5' straight up over the top of the motor. The boat is aluminum 18' long 96" beam. SS prop came w/ motor, goes between 30 & 40 mph I'm guessing? Side console if that matters. This was a package from the manufacturer. I guess i need to get them involved, i don't think it is set up properly. The engine is to heavy maybe, the rear of the boat is really low in the water. Tho its rated for a 115hp? I don't know!!!!!!!!!!!!!!!??? Thanks for your help!
Edited by mavmskyb8 2/12/2003 6:12 PM
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Posts: 3480
Location: Elk River, Minnesota | Actually, by definition, you are not cavitaing, you are ventilating. Cavitation is when the water behind the blade vaporizes (or in a sense boils) behind the blades. Cavitation is not good. It will cause deterioration of the blade.
Ventilating is when your motor increases RPM rapidly and is sucking surface air or exhaust fumes into the blades increasing slip and loss of handling.
By design, most motors are able to be lifted to it's highest bolt hole without damage to the lower unit. A pressure gauge is not a bad Idea, but as long as you have a good stream of water coming from your outlet, you should be fine.
In many cases, the cavitation plate can be above the lowest point of the boat. What needs to be remembered here is that the boat creates a hole in the surface of the water, which immediately starts to fill back in as the boat is moving. So...by raising the motor, you are optimizing the capabilities of the rig. (i.e. less lower unit in the water means more mph). In most instances, the plate can be above the lowest part of the boat by about 1 - 2 inches (possibly more) depending on the hull design. Deep V hulls can usually go higher, where as a flatter bottomed boat may not. Each boat is a little bit different so although there are some general settings that dealers can use, you usually can tweek it a bit more depending on a specific prop.
Now..my question to you is this...if your inlet is coming out of the water in waves, are you pounding the waves at full throttle, or is the prop just losing bite as you go along? Usually, that should not happen, but if you ar travelling at a slower speed with a vented prop, RPMS can possibly rise due to receiving some exhaust fumes into the blades.
Worst case is to drop the motor so it sits on the transom and start from there. try it out. If it handles well but you want a little more efficiency, move it up a hole and repeat. You are optimized when the boat will corner well without the prop blowing out or slipping. If you get to that point where the motor won't bite the water anymore, move back down one hole and you are all set. YOu can test out other props as well. Any good dealer will have props available to demo, so long as you don't damage them.
What I am thinking about (at this point in time...more of an assumption)
is that the water kick up behind the boat is due to where the cavitation plate was located. It very well could have been right at the water line where some water will naturally go straight up. By moving the plate up, the water is being split by the lower unit (by design will flow around better)rather than the front edge of the cavitation plate. I would bet that if you did move the motor down from it's original position, you would find the spray would dissipate as well.
Good luck on getting set up, and if we can help more, feel free to ask.
Steve | |
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Posts: 44
| I totally disagree with the statement about a motor beung designed to run in any bolt hole position. Boat transoms are not all the same, that's why they have more than one set of holes. You must maintain a minimum of 12 pounds of water pressure or you will damage your engine. I suggest you do two things.
1. Call Express and ask them the proper engine heigth and prop for your boat.
2. Call Mercury and ask them what the minimum water pressure should be without blowing your engine.
A new Mercury 90HP powerhead is $3500.00 so don't take chances. A water pressure gauge is about $45. | |
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Posts: 260
Location: Kentucky | Thanks to all for your input. I talked to the folks @ express boats and the told me to go get the dealer and take him out on the water. They would do what ever it takes to fix the problem. He mentioned that it sounded like the motor was to high on the transom. Again , thanx for all your help!! Bret | |
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Location: Minneapolis, MN | Your boat may benefit from a jack plate. What's nice about plates is that you can adjust them 1/4" at a time watching the water pressure each time until you find the right height for your rig.
Also, it is very likely someone has been through this before with your identical boat/motor combo. I would bet that person(s) visits www.wmi.org/bassfish and would share their setup ideas with you if you post on the boat/motor page at that site. | |
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Posts: 3480
Location: Elk River, Minnesota | Bassin,
The bolt holes are there to optimize the rig rather than have the motor sit in it's most inefficient postion on the transom. I agree not all transoms are built the same, but when motors are designed, engineers are not going to put bolt holes where the motor will guarantee itself to fail due to bolt hole position and no water pressure (UNDER NORMAL OPERATING CONDITIONS). Nor will boat designers make a transom that guarantees the same thing (UNDER NORMAL OPERATING CONDITIONS). Even though a motor starts to ventilate, it does not mean that it is losing water pressure. Engineers design these motors and test these items extensively before full production takes place, in many different conditions to make sure that losing pressure would not be due to their miscalculations. That would have law-suit written all over it. Same goes for boat designers as well.
Do you run the risk of losing pressure? Yes..to a degree, depending on how hard the operator pushes their rig in adverse conditions that are not part of normal usage by disregarding everything (hard turning without trimming down a touch to keep the prop hooked up and the front end to dig a little as an example...letting it blow out, but not down throttle)
In normal situations, without a jack plate, the motor will function just fine..it's a matter of how the operator chooses to have it run, beit high or low on the transom. Higher is faster, but less favorable handling is the result. Lower allows for nicer handling, but increases drag (and thus speed).
The idea of the bolt holes is to find a good balance between handling and speed. Once that position is found, everything works at it's best.
One also has to remember the hole in the water the boat makes. that hole fills in faster than one may think...It has to by nature...
Steve
Edited by VMS 2/19/2003 12:34 PM
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Posts: 44
| Steve,
Most times if your running a high performance set up (65MPH and up) you run a jack plate not only to ruduce drag by moving your height 1/4" at a time, but also to get the prop back into "clean water".
The hight of the cavitation plate in relationship to the planning surface of the boat is not usually as important( however it should be above the water when running) as the center of the prop to running surface of the boat.
I would level my boat in the garage and measure to the pad. Then I would measure to the center of the prop. Then I would divide the diameter of the prop by 2. I would then add that to the number to the center of the prop number. I wanted that number to be 1" greater than the distance to the pad. This allowed the prop to tick 1" out of the water on the highest up stroke. This is not alway's a final setting, but a good place to start.
If your running a repaired prop, hooked hull or a low performance boat none of this matters anyway.
But, getting back to the original topic, I can't believe Express can't give this person a starting point. I'm wondering if he has a speedometer pickup or transducer that's not causing the water to shoot straight up? | |
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Posts: 260
Location: Kentucky | Xpress did give me a starting point. They told me to take it back to the dealer and get him in the boat with me in the water.
Thanx guys for all your help!!!
Bret | |
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cavitation