cavitation plate height question

[Meul]

hello<br /><br />i have read some postings about the height of the cavitation plate pertaining to the bottom of the boat (is that called transom? i have some difficulties with all the new terms in english).<br /><br />i have a few questions about this subject.<br /><br />this is my setup<br /><br />a yam 360 s inflatable with a 20 hp evinrude 1972 shortshaft<br /><br />what is the benefit of placing the plate in line with the bottom and is it making a significant difference?<br />on this moment the plate is not comming out of the water when i am in plane.<br />by other boats from the same type i can see the plate drifting on the water when they are going in plane.<br />what i do not understand because i thought that all shortshafts where the same length.<br />i have the feeling that my prop is a little to deep in the water.<br />are there symptoms whereat i can see that the prop is to deep without having to take the boat out of the water?<br /><br />and then another question about the performance of my boat/motor<br />does it make a big difference in speed when i buy a propeller that is made for more speed?<br /><br />i guess that you guys maybe need to know more details about the motor, etcetera.<br /><br />but maybe there are some tips or answers that can be given in general for this subject.<br /><br />excuse me for my ignorance but we all have to start somewhere, right !?! <br /><br />thanks in advance,<br />jan

 

[NathanY]

Re: cavitation plate height question

The anti-cavitation plate is the plate just above the prop. That should be level with the bottom of the boat or hull.<br /><br />If it is set to high, this will cause the motor to cavitate, which means that air is getting in the prop and not allowing it to bite into the water to create forward thrust.<br /><br />If it is too low, it will create extra drag, which will not allow the motor to reach full RPM's and plane out properly, which in turn, will not let the boat reach optimum performance.<br /><br />Unfortunately, that old of a motor, that doesnt have the thru-hub exhaust, you are very limited to props that you can use.

 

[Spidybot]

Re: cavitation plate height question

Hey Jan<br /><br />The plate above the prop (anticavitation or antiventilation plate) is supposed to keep water around the prop when it is spinning. This plate should ideally be just in the surface line when planing - or at bottom line level. The trim angle of the motor should keep this plate parallel to the surface. At least as a starting point for adjustments. If this plate is way lower than the bottom (keel) your prop is running too deep.<br /><br />You also want to have your motor high to minimize the drag that pulling and object through water will create.<br /><br />Transom is the vertical rear panel of yout boat - the thing your motor hangs on.<br /><br />A propeller is driving your boat forward by turning - like a screw. Depending on the propeller pitch, it will cover a theoretic distance per revolution. This distance, measured in inches, designates the pitch. A 19" pitch will move 19" forward per turn.<br /><br />High pitches will make the boat go faster but also put more stress on your motor, or take more hp to get up to the right rpm.<br /><br />Your motor manufacturer recommends a max rpm range at wot (Wide Open Throttle or full speed). You need to stay within this range, for example 5000-5500. By choosing the right prop you can make sure you achieve this. You must know what rpm you reach with the current prop. If it outside the range, you calculate the change by 200 rpm less pr. inch higher pitch. And vice versa.<br /><br />As inflatables are normally quite light, my guess is that you will need a higher pitch than the motor came with standard from new.

 

[iRich]

Re: cavitation plate height question

Just for the record it is the anti-ventilation plate. It is supposed to keep the prop from drawing air down to it. It is often mistakenly called an anti-cavitation plate.<br /><br />Rich

 

[Meul]

Re: cavitation plate height question

thanks for explaining,and the fast reply on my posting<br /><br />ok, when reading your comments i have the feeling my prop and plate are to deep into the water.<br />am i right about this?<br /><br />my cavitation plate is about 1 inch under the surface when i am going in plane.<br /> <br />also water is squirting up from the left and right side of my motor into my boat.<br />i checked the depth of my cavitation plate pertaining to the bottom with my hand under water, the plate is also about 1 inch under the bottom of my boat<br />is this acceptable or do i get significant beter performance when i put up the motor?<br />if so, how can i raise the motor, i see a big screw for the trim positions, but see nothing whereby i can raise the whole motor?<br /><br />i can't say how much rpm my motor is running now because i have no equipement to measure it.<br />i do not know much about the motor, i bought it second hand and have no manual. also i can not find much about the motor on the internet.<br />so, for now i have to do it with the prop thats on the motor, thats ok with me.<br /><br />thanks again,<br />jan

 

[Hawaiian]

Re: cavitation plate height question

Did you read the post above yours? It is called an ANTI-VENTILATION plate. The pate should be even with or slightly above (1 inch) the bottom of the hull. With the anti-ventilation plate at the proper height, the boat will perform better.

 

[Meul]

Re: cavitation plate height question

Hi Perry<br /><br />yes i read the replies on my questions!<br /><br />i have an old motor and i think i must improvise to put it up.<br />now i want to know if it is worth to change the motor height in my specific situation. <br />i mean if it will make a big difference in the performance when i raise my motor only about 1 inch.<br /><br />jan

 

[Meul]

Re: cavitation plate height question

Hey perry again<br /><br />now i understand your comment.<br />sorry for using the wrong word.<br />english is not my first language, but i think you noticed that already )<br /><br />jan

 

[Boatman]

Re: cavitation plate height question

Meul<br /><br />One thing I have seen done to slightly raise the motor hieght on the transom. Take a small 1" board, lift your motor up and slide the board underneath the motor and set the motor clamp down on it. Tighten your clamp down and try it. If it cavitates too much, you can alsway remove it.

 

[wayne h]

Re: cavitation plate height question

i am going to try that <br />Boatman<br />i am having problems getting on plain i was not sure if that would put more stress on my boat <br />12 foot aluminum with a 9.5

 

[Meul]

Re: cavitation plate height question

Hey Boatman,<br /><br />i thought about that but i am affraid to damage my stern (hope this is the good word).<br />when i raise my motor 1 inch the screws of the bracket are almost on the transom i don't know if this is risky? <br /><br />greets,<br />jan