Prop blow out, needing help

[jbeener1]

I have a 92 20 hp Johnson short shaft, last year I kept having trouble with prop blow out at about 3/4 speed, the prop was wore pretty bad so over the winter I put a new 3 blade 10x13 on it. I ran the boat yesterday with blow out just one time out of 5-6 runs, so I moved the motor trim pin up one notch and ran great everytime. Went fishing today, the lake had about 6" chop to it. Every time I ran it I had prop blow out even tryed moving motor back down towards transom, still blow out. The cavataion plate is 1" below bottom of boat. Any help? I appreciate it.

 

[jestor68]

Re: Prop blow out, needing help

What sort of boat(hull) do you have?

What brand/material prop did you get?

What sort of rpm are you turning at WOT?

 

[MikDee]

Re: Prop blow out, needing help

The leg should be straight down, perpendicular with the bottom, or cavitation plate parallel with the bottom.

By the way, how is your weight distributed? If the boat is heavier in the bow (lower), rather then the stern (preferred), this can happen.

 

[dazk14]

Re: Prop blow out, needing help

What sort of boat(hull) do you have?

What brand/material prop did you get?

What sort of rpm are you turning at WOT?

+1.

Sounds like an obstruction/construction disturbing the flow.

Is it aluminum, they're famous for that...

Cupped prop usually is the fix (sometimes only fix), then to a 4 blade cupped. Some rake helps as well, but obviously there's likely very limited selection for that motor.

Depending on budget and available prop selection, you may need to have the current prop cupped (or double cupped) and on small props it can be done in house.

 

[steelespike]

Re: Prop blow out, needing help

Does the boat get up on the water or does it seem to be laying down flat and possibly plowing.
What is the wot rpm and gps speed in normal operation?
Is the bottom clean and smooth No dents or irregularities?
Any additions to the bottom or the bottom of the transom?
A 4 blade or a turning point Rascal prop may help.

 

[jbeener1]

Re: Prop blow out, needing help

Its a 14 ft flat bottom. The prop is a solas aluminum. I dont know on rpm's

 

[jbeener1]

Re: Prop blow out, needing help

+1.

Sounds like an obstruction/construction disturbing the flow.

Is it aluminum, they're famous for that...

Cupped prop usually is the fix (sometimes only fix), then to a 4 blade cupped. Some rake helps as well, but obviously there's likely very limited selection for that motor.

Depending on budget and available prop selection, you may need to have the current prop cupped (or double cupped) and on small props it can be done in house.

It is aluminum and the aluminum prop came from Iboats. It is a 3 blade Solas

 

[jbeener1]

Re: Prop blow out, needing help

Does the boat get up on the water or does it seem to be laying down flat and possibly plowing.
What is the wot rpm and gps speed in normal operation?
Is the bottom clean and smooth No dents or irregularities?
Any additions to the bottom or the bottom of the transom?
A 4 blade or a turning point Rascal prop may help.

I dont know on rpm's. the bottom of the boat is smooth but there is a kiln right down the center of the boat. No additions to boat or transom. It gets up almost on the water then it blows out.

 

[jbeener1]

Re: Prop blow out, needing help

The leg should be straight down, perpendicular with the bottom, or cavitation plate parallel with the bottom.

By the way, how is your weight distributed? If the boat is heavier in the bow (lower), rather then the stern (preferred), this can happen.

Weight is more from middle of boat back to transom

 

[jbeener1]

Re: Prop blow out, needing help

I went fishing today and the lake had about a 6-8 inch waves, about three qtr throttle it went to blowing out. I fished for a couple hours and had to run back across to the ramp (down poor of rain) with the same size waves and it ran great with no blow out, no weight change in the boat,ice chest and all was the same as when i first ran across it.

 

[jbeener1]

Re: Prop blow out, needing help

Its a 14ft aluminum flat bottom boat. Prop is Solas 3 blade aluminum from Iboats. I dont know the rpm's. What do i need to find the rpm's??

 

[jestor68]

Re: Prop blow out, needing help

I'd be willing to bet that when you experienced ventilation, you were going against the chop and on the way back you were traveling with the chop/waves.

Those flat bottom boats are famous for producing lots of air bubbles under the boat when traveling over an kind of chop, whereas a V hull tends to push the water aside. The flat bottom passes the air bubbles right under the hull and out the back into the prop.

As suggested, a 4 blade should help to reduce the ventilation issue due to more blade area in the water. In fact; that's one thing that the 4/5 blade props are good at; maintaining grip under less than ideal conditions.

Since you're running a 13 pitch 3 blade, I'd suggest a 12 pitch in the 4 blade. It's nice that Solas makes 4 blade props for your motor.

It would be nice to verify the rpm's, but regardless, it's usually recommended to drop an inch of pitch when switching to a 4 blade prop to maintain rpm.

 

[jbeener1]

Re: Prop blow out, needing help

I'd be willing to bet that when you experienced ventilation, you were going against the chop and on the way back you were traveling with the chop/waves.

Those flat bottom boats are famous for producing lots of air bubbles under the boat when traveling over an kind of chop, whereas a V hull tends to push the water aside. The flat bottom passes the air bubbles right under the hull and out the back into the prop.

As suggested, a 4 blade should help to reduce the ventilation issue due to more blade area in the water. In fact; that's one thing that the 4/5 blade props are good at; maintaining grip under less than ideal conditions.

Since you're running a 13 pitch 3 blade, I'd suggest a 12 pitch in the 4 blade. It's nice that Solas makes 4 blade props for your motor.

It would be nice to verify the rpm's, but regardless, it's usually recommended to drop an inch of pitch when switching to a 4 blade prop to maintain rpm.

When i prop had blow out today the waves were hitting the side of the boat and when the prop didnt blow out the waves were hitting the the right front corner of the boat. I can see where the right front corner would break the wave. Im hoping i dont have to buy another new prop. I will find out the rpm's and speed,its just going to take a few days. Thanks for your help.

 

[MikDee]

Re: Prop blow out, needing help

I dont know on rpm's. the bottom of the boat is smooth but there is a kiln right down the center of the boat. No additions to boat or transom. It gets up almost on the water then it blows out.

Whats a kiln? A keel?

 

[steelespike]

Re: Prop blow out, needing help

I'm sure he wrote keel and spell check didn't like it.

 

[steelespike]

Re: Prop blow out, needing help

Does the keel run right up to the transom?Usually should end 8 or 10" from the transom.
In the early days when boats were wood and built before boats were easily motorized it was suggested to remove
some keel ahead of the transom.
I'll bet your boat probably has 3 or 4 reinforcing keels pressed into the bottom.These could be disturbing the water as well.
The Solas props are very good and usually improve performance.
You could try mounting the motor an inch or so off center port and/or starboard.
A home made jack plate would allow you to lower the motor a little and get the motor farther back in (cleaner water".
While it seems counter intuitive you might try raising the motor slightly.
You can do this by simply resting the motor on a 1/4" or 1/2 " stick while you tighten the clamps.

 

[dazk14]

Re: Prop blow out, needing help

Can you check the Prop and see if it has any noticeable cupping on the trailing edge...

I have seen smaller Solas props in the past without cupping.

Edit- fyi, all the small stainless Solas props have been cupped (Saturn). Fast too.

 

[jbeener1]

Re: Prop blow out, needing help

Whats a kiln? A keel?

Im sorry a mistake in typing a keel.

 

[jbeener1]

Re: Prop blow out, needing help

Does the keel run right up to the transom?Usually should end 8 or 10" from the transom.
In the early days when boats were wood and built before boats were easily motorized it was suggested to remove
some keel ahead of the transom.
I'll bet your boat probably has 3 or 4 reinforcing keels pressed into the bottom.These could be disturbing the water as well.
The Solas props are very good and usually improve performance.
You could try mounting the motor an inch or so off center port and/or starboard.
A home made jack plate would allow you to lower the motor a little and get the motor farther back in (cleaner water".
While it seems counter intuitive you might try raising the motor slightly.
You can do this by simply resting the motor on a 1/4" or 1/2 " stick while you tighten the clamps.

The keels stop about an inch from the transom and there are 4-5 of them. i had a home made jack plate of 4 inches but when the motor kicked up the arm on the front of the motor where you would attach steering rode to would hit the top of the transom. The motor ran great with the jack plate on there but i didnt like the motor hitting the transom. Dont know if i can remove a couple inches off the keels?? Ill have to look but i think the keels are make into the boat,you can see them on the inside of the boat.

 

[jbeener1]

Re: Prop blow out, needing help

Can you check the Prop and see if it has any noticeable cupping on the trailing edge...

I have seen smaller Solas props in the past without cupping.

Edit- fyi, all the small stainless Solas props have been cupped (Saturn). Fast too.

This prop was to come with cupping but i will check.