prop ventilation

[kevinfishman36]

Hi, i recently bought a new to me boat and am having issues with it, its a 19' deep v aluminum lowe. It is powered with a 115 evenrude. The motor is lowered all the way down on the mounting holes has power trim ( for further explination) Motor came with a 19 pitch 14 diameter prop. At full throttle, the boat jumps out of the water and at top end is running at 6100 rpm with 4 men in it, max at around 34 mph. So after researching a little on the engine, max wot is supposed to be 5500 rpm. OK so i better not run the boat at full throttle!!! First problem that i am having is that when in a little choppy water, i am getting some prop ventilation when doing any turning- and yes, i have the trim all the way in or down, any adjustment up, and it will ventilate more. So after researching why this is happening i figured i better go up a few pitches in prop, to drop the r's down to recommended specs. So i buy a 3 blade 23 pitch 12 3/4 diameter prop off ebay and tried it out yesterday. Had 3 people in it, and some gear, when i floored it, the boat got on plane good, and seemed right away to be the ticket, but as it hit 23 mph, the prop ventilated and engine went sky high, and had to shut the throttle down. Made sure the motor was trimmed all the way in and tried again and again, only to realise that after 23 to 25 mph the prop ventilates. So my question, after doing alot of internet searching, is = what is my best option... have looked at extending the lower unit- have looked at cutting and drilling another lower set of holes in the transom (which i don't think there is really enough room to lower it. Or do i try a 4 blade prop? OOps forgot to tell you that the cavitation plate is about even with the bottom of the boat, maby 1/2 inch above bottom of boat. Thanks for any help and good advise!!

 

[kevinfishman36]

Well, I just ordered a 4 blade 21 pitch, we'll see what happen's

 

[steelespike]

I suspect your tach is not accurate. 6100 at 34 mph calculates to 38% slip. Highly unlikely.Typical slip is in the
low double digits. If we use 13% slip we get a rpm around 4300 at 34 mph.I think I would confirm your rpm before ordering a prop.
With the venting issue your slip may be a little higher which could result in too much rpm but 6100 seems out of the ordinary. A 4 blade should reduce the venting and lower slip. Is the mounting bracket all the way down on the transom.

 

[alldodge]

How does the ventilation plate line up with the bottom edge of the keel?

 

[Texasmark]

How does the ventilation plate line up with the bottom edge of the keel?

How about a picture or two. The main one I'd like to see is from the side with the camera even with the bottom of the boat, straight out to the side from the transom about 4' with the engine vertical, not tucked all the way in. This is for the same reason as above.

Second take a straight edge and lay it up against the bottom of the boat and take a picture from the side as you did for pic #1. Would like for straightedge to be at least 4' long.....looking for a hook in the hull.

Your comment about "jumping out of the hole", if with 4 people would support your overrevving number but on that much boat, even with it being alum. with 4 folks and that much prop and only 115 hp something isn't right......I didn't run the prop slip numbers nor any of what steel commented.

A 14" prop on a 115 is a bit much if you ask me. Most props that run engines of that size are in the 13 ish inch category. The 14's run on the larger diameter gearboxes like found on 150's and up.

Mark

 

[steelespike]

How does the ventilation plate line up with the bottom edge of the keel?

He covered that in the last 2 lines of his post.

 

[Texasmark]

He covered that in the last 2 lines of his post.

Oops, I missed that too.

 

[Chris1956]

Does that hull have a constant Vee all the way to the transom, or does she flatten at the transom, somewhat?

Sometimes prop ventilation can be lessened with the correct prop. One that holds the water better can ventilate less. What brand/style props are you buying? Also, some motors have the ability to be adjusted up and down on the transom, usually by changing the mounting bolt holes on the motor transom bracket. Could you lower your motor in this fashion?

 

[kevinfishman36]

The motor is lowered all the way down on the mounting holes has power trim ( for further explination)

The boat has a tapered v all the way down to the back of the boat. I suspect someone has had some problems propping this boat, because the 14 inch prop is special order only.

The 21 4 blade that is supposed to be here today, is a solas. The 3 blade 23 that i bought off ebay is a stainless, and i don't know what brand it is.

The ventilation plate lines up with the bottom of the keel at about 1/2 inch above the bottom of the boat.

 

[alldodge]

If your prop is ventilating then it's getting air. Most the time this is caused by the plate being set to high. You state the plate is 1/2 inch above the keel, which isn't much. Can you take a pic of what your seeing?