Lowered motor, still blowing out.

zeek7pc

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I have a 1992 Invader 18.5 ski boat with a 1996 Johnson 90hp with a 13.5X17 cupped prop. Whenever trying to make a corner while on plane the prop will grab air and the boat will over rev. I've lowered the motor to the last holes and it won't go any lower, still the same result. Also while running near WOT bumping up on the trim has no effect whatsoever, until the motor basically raises too high and blows out. Any thoughts or ideas? Thanks in advance..
 

BonairII

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Re: Lowered motor, still blowing out.

At it's lowest setting, where is the cav plate in relation to the keel?
 

zeek7pc

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Re: Lowered motor, still blowing out.

The cav plate is about an inch above the lowest point on the keel.
 

BonairII

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Re: Lowered motor, still blowing out.

If you're bumping the trim up(raising the motor even higher out of the water), you're only making it worse. Unless there is a different prop that could give you more bite, you might end up having to install a jack plate and lower the motor further.

How hard are the turns you're making?
 

steelespike

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Apr 26, 2002
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Re: Lowered motor, still blowing out.

Does the boat get up on the water with the bow up slightly?
Or does the boat lay down flat like its glued to the water?
 

dazk14

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Jul 22, 2008
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Re: Lowered motor, still blowing out.

A few pics would help.

What is the specific brand/model prop are you using?

Has this configuration ever functioned to your satisfaction?
 

zeek7pc

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Re: Lowered motor, still blowing out.

Thank you for all the replies! I did some more trolling on here and found some interesting things. I measured my transom, it's 23 inches. My motor has a 20" shaft. During the early 90's several crappy boat makers (one of which I own) used 23-25" transoms. I think i've found my problem. Would a jack plate allow me to lower the motor?
 

Philster

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Sep 15, 2009
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Re: Lowered motor, still blowing out.

A jackplate will not allow you to lower your motor. It's current lowest setting will be the jackplate's lowest setting. With the jackplate, you will be able to raise the motor vertically from the current lowest setting you have now. With a jackplate, you won't be able to go below the current lowest setting you are fighting with now.

Is your tilt/trim holding on properly as you operate your boat? Do you think the engine in tucked in/down but it's actually rising up a bit under speed?
 

zeek7pc

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Re: Lowered motor, still blowing out.

Everything looks just fine with the tilt/trim, nothing moves unless I tell it to. Guess I'm in the market for a longer shaft motor.
 

Dhadley

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Feb 4, 2001
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Re: Lowered motor, still blowing out.

Before you look at changing motors lets look at the prop. What brand is it? Can we assume it's stanless? Is it worn of has it been repaired?

With a jackplate you will gain some "down". The transom is on a slight angle so the further back the motor goes the (slightly) lower it goes. And as water comes off the hull it tends to rise up too.

The bottom line is that a good ss prop should run just fine at a 1" X dimension on that boat.
 

sschefer

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Re: Lowered motor, still blowing out.

Before you look at changing motors lets look at the prop. What brand is it? Can we assume it's stanless? Is it worn of has it been repaired?

With a jackplate you will gain some "down". The transom is on a slight angle so the further back the motor goes the (slightly) lower it goes. And as water comes off the hull it tends to rise up too.
The bottom line is that a good ss prop should run just fine at a 1" X dimension on that boat.

Sounds like the hull has a hook designed into it. Lay a straight edge fore and aft along the bottom at the transom. If you see air in the last 6" or more then the hull was designed with a hook so that it would come up on plane easier. It's thought that most people don't know what the trim switch is for so to get around that they designed a hook into the hull that forces the bow down early and lifts the stern just as if you were trimming the motor. The bad part is that a hook creates a lot of turbulent water in turns and it blows out the prop. You could try a four blade prop or add a blowout ring to the lower unit.
 

Sea Rider

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Sep 20, 2008
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Re: Lowered motor, still blowing out.

Sounds like you need to chop your transom down a bit untill water flow at wot passes slightly under small upper plate, at that position will have best engine performance and best tight turns with no prop cavitation al all. A trail and error isue.

Happy Boating
 

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dazk14

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Jul 22, 2008
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Re: Lowered motor, still blowing out.

This seems to be getting a little crazy...

I think the OP needs to respond with the prop he's using.

If the OP has a 23" transom and a 20" motor, he's in the hunt.

As Dhadley requested, prop info would help and a 4 blade prop may just be the ticket.

I also think it would be helpful to re-measure the x-dimension. 1" up should NOT be creating any problems.
 

zeek7pc

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Re: Lowered motor, still blowing out.

Sorry for taking so long to respond. The prop is an old steel prop without any markings whatsoever, I've even painted it with machinists paint and sanded it off, no marks. The paperwork that came with the boat said it was a 13X17. It also has a ring around the end of the exhaust side. Given I have no clue what the prop actually is, any recommendations? I"ll get some pics uploaded in the morning..
 

Dhadley

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Feb 4, 2001
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16,978
Re: Lowered motor, still blowing out.

Pics will definately help. My guess is that the prop has little tip cup and probably not much rake. The ring on the aft end is a defusser ring.
 

dazk14

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Jul 22, 2008
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966
Re: Lowered motor, still blowing out.

sounds like an old OMC SST1...1970's technology.

That would be an easy fix.
 

zeek7pc

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Jul 2, 2009
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Re: Lowered motor, still blowing out.

Thanks for the help, will have several pics uploaded by about 8:30 AM central time. Also this damn thing has a whale tail bolted to the cavitation plate. Forgot to mention that, I'll get the pics on here and let you experts figure out what I need to do.
 

zeek7pc

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Jul 2, 2009
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Re: Lowered motor, still blowing out.

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Here's the pictures of the setup. The motor will not trim down any more. After taking the pictures I removed that pin and was able to get the motor to lower more. What do you all think?
 

dazk14

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Jul 22, 2008
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Re: Lowered motor, still blowing out.

I think you don't need a new motor - that's for sure. And sure as "EL shouldn't touch that transom. Just need a prop.

The stick was helpful, but the cav plate is below it. Where your doel-fin is mounted. Post a pic...and another couple further back and much further/angled, so we can see MUCH more of the boat and the hull angle - all with your stick.

Is the front of the trailer sitting on the ground?

What is your WOT RPM and GPS speed? If you pull the prop nut and spacer you may find the prop numbers.

"Perfect" prop selection is always a challenge and not an exact science. What is your budget?

A $98 15" pitch Solas 4 blade, would stick well and probably be about right...but more info is needed. There are other props and you may not need a 4 blade. A turning point hustler prop 3 blade is possibly an option.

Lots of good prop guys on this board.
 
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