Cavitation or popping out of gear?

[exodave]

Finally got my 1957 Evinrude 35 out in the water today, running fantastically. The issue now that we are out on the water, is that we will be cruising along 3/4 throttle or so, hydroplaning, then suddenly the RPMs go up and we slow down until we just bump the throttle down.

There aren't any shakes or knocks or kicks from the engine. it just suddenly whines high until we bump the throttle down.

The reason I expect cavitation, or prop not low enough, is that this transom was hacked up by a previous owner to lower the engine, which clearly wouldn't reach low enough otherwise. It is model 25930, which I think has a twin model which has a 5" longer shaft.

The top of the prop is maybe a couple inches below the lowest part of the hull.

btw, I have, indeed, sealed up all the exposed transom wood, which for some crazy reason wasn't done whenever this modification was made. Clearly must not have been in the water more than a few times this way before me.

 

[Bifflefan]

Re: Cavitation or popping out of gear?

Well first a smaller pic would be nice so we dont need to scroll back and forth to read the post.
Then a pics of the prop and one of the anti-ventalation plate in relation to the keel of the boat.

If yor prop has a rubber hub, that would be my first guess, but need a pics of the set up.

 

[wilde1j]

Re: Cavitation or popping out of gear?

I would expect a spun prop hub from what you describe. If the AV plate is even or a little higher than the hull bottom, cavitation is not too likely. What does the engine do in tight turns at planing speeds? Another check is to look at the motor at full speed and look to see if the AV plate is under water or not. If it's under water or just dry, cavitation is not very probable.

"Popping" out of gear (worn clutch dog/fwd gear) is not a smooth affair ... it's more like hitting a log, when it happens at power.

 

[Vic.S]

Re: Cavitation or popping out of gear?

Put a mark on the prop and another in line on the prop shaft end nut. ( or just mark the prop in line with the head of the cotter pin)

Run it until misbehaves. If the marks no longer line up then the prop hub is spun.

 

[exodave]

Re: Cavitation or popping out of gear?

Great info thanks! I'll pop it off tomorrow morning before we head to the beach, mark the hub, then take a look when we get back and post pics and update! Hope everyone has a nice 4th on the water!

 

[exodave]

Re: Cavitation or popping out of gear?

I took it out today with one more person and about 100 pounds more of gear than the aforementioned trip, to the beach. First, marked the prop at the cotter pin. Kept the throttle low with no planing on the way out. There seemed to be none of the previous issues, expectedly.

Upon arrival, I looked at the mark and it had not moved. On the way back, I punched it up to WOT near our port, and planed for quite a while with no issues, except turning back downriver into the wind, it acted like it was losing power and RPM spiking a couple times, although not to the previous level.

Upon arrival back home, the prop mark was still lined up on the pin, as if it had not moved.

I notice the plate above the prop (AV plate?) is exactly level with the bottom of the boat. there is a small notch missing at the stern of it, as shown below. Would this let enough air in to cause my problem?

 

[Bifflefan]

Re: Cavitation or popping out of gear?

except turning back downriver into the wind, it acted like it was losing power and RPM spiking a couple times, although not to the previous level.

The engine is mounted to high. When you turn your pop is blowing out.
Basiclly sucking air from the surface causing it to loose the bit on the water. It may have a little to do with the missing piece of the AV plate.
If you can lower the motor it will help. If not then one of those hydrofols may help, I cant say for sure on that. Get the flatest one can find and dont spend a ton on it, it may not help.

By the way, is the last pics of the AV with the motor tilted all the way down? The AV plate should be under water.

 

[exodave]

Re: Cavitation or popping out of gear?

By the way, is the last pics of the AV with the motor tilted all the way down? The AV plate should be under water.

Actually its tilted up as far as it goes, part of the botched mod by the previous owner, lowering the engine made it hit the rail before tilting out of the water

 

[wilde1j]

Re: Cavitation or popping out of gear?

I took it out today with one more person and about 100 pounds more of gear than the aforementioned trip, to the beach. First, marked the prop at the cotter pin. Kept the throttle low with no planing on the way out. There seemed to be none of the previous issues, expectedly.

Upon arrival, I looked at the mark and it had not moved. On the way back, I punched it up to WOT near our port, and planed for quite a while with no issues, except turning back downriver into the wind, it acted like it was losing power and RPM spiking a couple times, although not to the previous level.

Upon arrival back home, the prop mark was still lined up on the pin, as if it had not moved.

I notice the plate above the prop (AV plate?) is exactly level with the bottom of the boat. there is a small notch missing at the stern of it, as shown below. Would this let enough air in to cause my problem?

NO.

 

[jtexas]

Re: Cavitation or popping out of gear?

Definately not popping out of gear, you'd feel that. And only props with thru-hub exhaust get spun. So everything except ventilation/cavitation is ruled out.

Hard to tell exactly from the photo, but it looks to me like it could be raised...about...as much as it was lowered before.

I had a 6hp with about that much missing from the a/v plate, never noticed it at all, so Jim might very well be right, but here's my theory: guy buys a perfectly matched boat/motor combination, somehow manages to break the motor, and in a vain attempt to resolve the resulting ventilation, busts up the transom instead of just fixing what's broke. Finally, he gives up & sells the boat to you.

You'll find a competent welder of aluminum, who will fix it for a reasonable cost. Then, when you get around to it, you'll find a way to reverse engineer dumbo's slick transom work and raise the motor back to where it belongs, which will improve your performance.

hey, it's just a theory......

 

[wilde1j]

Re: Cavitation or popping out of gear?

Where is the AV plate @ WOT???

 

[exodave]

Re: Cavitation or popping out of gear?

I'll have someone in the boat lean over and check out the level of the plate, hopefully snap a photo and/or video, and report back tomorrow.

jtexas - thanks for that info. my exhaust comes out of a port that hangs down diagonally out down behind the prop. Next time its out of the water, I'll snap a photo of where exactly the plate is, but from my best measurement underwater, since its impossible to raise the prop out of the water at dock, the plate is level with the bottom of the hull. It looks lower in the pic, but I think it's just a bad photo.

 

[Outsider]

Re: Cavitation or popping out of gear?

Where the plate is when on the trailer doesn't much matter except to establish a starting point. The key is where the plate is when you're on plane and at speed. Should be at water level or very slightly above (but getting splashed). Too high and ventilation becomes a problem, to low and drag/spary/inefficiency results. With no correlation to position of the plate, your motor 'tilt pin' would nominally be in the second from most forward position with nominal loading. If it's too far out (high, doesn't let the motor tilt down enough), bow rise would be a problem as well as less than optimum efficiency. Too far in and bow plow could result ...

 

[exodave]

Re: Cavitation or popping out of gear?

Update, out today, with basically no real issues, we were really flying, and I was pretty much satisfied. Looking back at the av plate at WOT, it seems to be maybe 1/2 inch under water.

 

[bktheking]

Re: Cavitation or popping out of gear?

Definately not popping out of gear, you'd feel that. And only props with thru-hub exhaust get spun.

Normal non-exhaust prop's can have spun hubs as well.

 

[wilde1j]

Re: Cavitation or popping out of gear?

Update, out today, with basically no real issues, we were really flying, and I was pretty much satisfied. Looking back at the av plate at WOT, it seems to be maybe 1/2 inch under water.

I guess your issue may have been related to boat loading. If the AV plate is under water, cavitation is very unlikely.

 

[jtexas]

Re: Cavitation or popping out of gear?

Normal non-exhaust prop's can have spun hubs as well.

you mean in addition to shearing the shear pin? Guess I was mistaken. Thanks for setting me straight.