Venting prop issue

[biglurr54]

I think i know the answer to this one but i might as well clear it with people who have way more knowledge then myself. My boat (1989 Crestliner Phantom 17.5) has a problem with the prop venting when moving along. When Im up on plane and do any turning the prop sucks in air and the motor revs until i back off the throttle, come off plane, and slow way down, then accelerate. Not the best when your skiing or wakeboarding. If i trim the motor all the way down it does it much less but the bow is doing some plowing while on plane that way. When i replaced the transom I noticed that the motor (1989 Evinrude 120) seemed to be mounted pretty high up.

The motor has the planing fins on it and it has a new prop on it as well because i'm messing around with pitches. Look at my other thread if interested.

Should I keep the motor mounted where it is or should I lower it? If i lower it will I have issues with drag? \

Right now if i trim it up just right and go in a straight line the motor revs up nicely and scoots right along. as long as I dont turn its great. Even with the motor trimmed level with the boat it sucks air on a turn.

 

[MikDee]

The angle of the motors cavitation plate should be level with the bottom of the boat to begin with. It can be flush with the boat bottom, or a bit higher, or lower, that can be found by trial, & error. As every boat is different.

 

[jestor68]

What "hole" is your motor mounted in?

Compared to all the way down on the transom, is in the 2d hole, 3rd hole?

Most older aluminum boats have that nice big keel sticking down right in front of the prop, so you usually cannot mount the motor as high.

My little Crestliner suffered from bad ventilation in turns using the BRP prop. Changing to a Solas prop pretty much got rid of the problem; i.e., in the same turn where the BRP would all but let go, the Solas hangs in there.

I guess all props are not created equal.

 

[steelespike]

Have you tried a Solas Amita or Rubex? 4 blade? You could try moving the motor off center port and/or starboard.
I figure to port would allow normal prop rotation to be in cleaner water before passing in back of the keel.
You might buy or fabricate a jack plate. This will allow some lowering and you can fine tune the height.I think third is a no brainer solution.
Don't worry about drag.I think your so on the edge now that your slip may be high lowering the motor gets it to cleaner water
perhaps improving slip and of course more dependable drive.
You could try adding a little weight at the stern.Just have a 100 pounder or less ride near the stern.

 

[Dukedog]

First. A water pressure gauge is really a must have when you start movin' tha motor up n down. Plus ya gotta keep an eye on it (gauge) at all times when ya test motor heights. When your testin', if pressure drops or fluctuates any at all in a straight line and/or turnin', your normally to high.

Normal "rule of thumb" motor height for starters.

With both boat (bottom/keel) and motor level (cavitation plate) settin' on level ground. Measure center line of prop shaft to ground. Measure bottom at tha very rear point to ground. Difference is actual motor height. 3 1/2 inches is normal starting point for most play/fishin' boats. From here you start with tha testin'. Move it, try it and so on. In your case it sounds like it needs ta go down a scosh. But tha prop also comes inta play. Type, pitch and manufacture all make a difference. Load carrying, bow lift, stern lift and etc. capabilities. Even "identical" box stock props will perform differently. Fine tuning motor heights/props is ONE of tha several benefits of tha "jack plate". All of this takes a little time but is worth tha effort.......

If you like I email or pm ya a site that has a drawing of "how to measure motor height"..........

 

[biglurr54]

I have to check the motor mounting hole its in when i get home. My prop is unknown make. I am tempted to try a 4 blade and hopefully that will help with venting. The boat is in the water for the season so maybe this fall when I pull it will lower it one hole. The lake I live on is very deep and im not racing so i guess the lower the better? There is a sizeable aluminum keel right in front of the lower unit. I know the bottom of the boat isnt smooth like glass boats and shooting a depth sounder through the aluminum is pointless if your moving at all because of the aeration under the hull. Found that out the hard way. Im sure that same aeration causes issues with venting.

Currently I am running a 17p prop down from a 19p. My tach says Im turning 4900 RPM. Maybe a 15p will do the trick but right now i want to make sure my tach is accurate. When I ensure accurate tach readings i will look into a solas prop in the correct pitch.

 

[Texasmark]

Ventilating, aka sucking in air, in a turn is associated with air bubbles generated by the hull that flow onto the prop and reduce the density of the otherwise solid water. Aluminum boats with things like a rib along the keel area where the two sides are welded together is a good example. In a turn, especially a tight one, the slide slip of the rib essentially cavitates, lowering the atmospheric pressure thus sucking the air molecules out of the water and making bubbles.

High trim angles and non cupped props are particularly susceptible to the problem in addition to high mounting positions on the transom. The GENERAL rule of the AV plate being right at or slightly above a straight line extension of the hull, for semiV hulls doesn't necessarily apply for that reason. I really doubt the 4th blade will be your problem solver in this instance. My boat is just such a critter and I know from experience that to which I am referring. I can go into a tight turn at max trim and WOT and get it to easily blow out. However, if I reduce the throttle about a thousand and tuck the trim in a couple of clicks going into the turn, it will hold till I get through about 100 dregrees of the turn. Then I can hammer down on it, punch it back out and it will hold. Great sport and my sweetie sitting next to me just loves it!!!!!

Mark