Height of Anti-ventilation plate

B

brian ziegler

Member
Hey all,
I took out the newest build today for the first time and im starting to question my motor height since it ran so poorly. from what i read, the anti-ventilation (anti-cavitation to some) plate should be just skimming the water surface at WOT (wide open throttle). Another piece said that the plate should be mounted even with the bottom of the boat. The bottom of my boat is flat at the transom except for a keel i made from a piece of 2x2 lumber. Should the the plate be even with the bottom of my boat or the bottom of the keel because thats about an 1-1/2" difference. Id appreciate any info on the matter. Thanks in advance for your response.
 
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,,,,,,,,,,,,,,,,,,,,,,,,,,,since it ran so poorly.
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Brian,

Not to be a smarty pants but your info does little to help trouble shoot a solution. Poorly in what respect? Slow hole shot? [/font][font=Verdana,Arial,Helvetica]Bow porpoising? [/font][font=Verdana,Arial,Helvetica]Prop blowout in turns? Prop blowout at speed? Engine bogging down and not achieving proper RPM?

Usually the plate will be at or within 1 1/2 inch above the hull (NOT) the keel. Some props run better much higher. Surface drive props for instance run best with only 50% of the prop engaged in the water. Typically the higher the prop in the water column, the more prone it is to blowout. The lower it is in the water column, the more "bite" it has at the expense of increased drag.
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Yeah i supose i kind of skipped on the details but im having blowouts while im trying to bring it up to speed. i started at 3 clicks up from the transom and moved down one at a time until i had the pin totally out and when i did that the plate went under water which caused enough drag to keep the bow from coming down. I know i have to drop the motor down but that is going to require me cutting the transom down. just trying to decide how much i gotta hack off. thanks for the response though dave.
 
Dave the merc manual says the plate be 1" below the bottom of the boat .Could it be that 2x2 keel disrupting the water flow
 
I have a couple of thoughts, make sure the cooling water pick up ports are under water when running full speed. Are you running empty? It might make a big difference when ya have a full/normal load spread out as when you are normally using it . If ya determin its the keel causing problems, try cutting it off at a 45 angle so its not blocking flow to the motor. Good luck getting it set up as you want!
Dennis
 
Brian,

There is quite a bit of info available on choosing and adjusting for the correct motor mounting height. You will find that some of it can contradict other recommendations. I copied a portion from one article out there.


Following are examples of outboard to boat mounting combinations. These, along with the genereal rules for mounting outboards on planing type boat hulls, can be used as a guideline to assure adequate water supply to the engine and gearcase/powerhead durability.
Example #1 - Boat with 12-13 degree transom angle (typical of most boats) provides good bow lift when outboard is trimmed out and a top speed in the low 50 MPH (80 km/h) range. With this combination, the anti-ventilation plate runs parallel to the water line and can be mounted 1 in. (25 mm) above the boat bottom.
outboard_mounting_Ex1.gif

Each particular hull shape will react differently. The point was brought up concerning the keel disrupting the water flow, yes this could be causing a problem. Trimming the keel as suggested or stopping it short of the transom by several inches should make a positive improvement in the bite of the prop. If you are getting excessive prop blowout, this might be a very good area to consider changing.


One other thing, most of the info is for planing type hulls. If yours is a displacement style hull, these recommendations will not be correct.
 
Simple question before you start hacking or stacking to move the motor. Any chance it's your prop? Has the hub spun? Not knowingthe age of the motor or the style of boat, it just seems strange that it would"blow out" on the way up.

Best,
Scott
 
Dave saw Brians boat today it's flat bottom' the v plate is 3/8 below the bottom. Your pic's show the v plate above' now i thought the only reason for the plate was to stop the motor from sucking air in the prop . I have always set mine 1" below like merc says .
 
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