Jack plate info needed

brappley

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Apr 17, 2011
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I have a 1998 sylvan deck boat that when I bought it had a jack plate with a 90 hp mariner , my question is does it really need to be there ? Have no idea why the previous owner put it on. The reason I ask is when I use the trim if I move it up just a little like 3 to 5 degrees it seams to cavitation. I have to keep it tucked down . Did check the distance on the cavitation plate to bottom of the boat and its bout an inch below. Also it is a tri hull , which I've never owned before so I'm just not us to it. Any ideas ? Or I just might take it off , also it sticks out about 10 to 12 inchs off the transom.
 

Teamster

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Have you tried raising the motor up with the jackplate?

The cavitation plate should be even with or slightly higher than the bottom of the hull,.......

Might be running to big of a prop as well,..
 

jbcurt00

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Yep ^^^ when running a jack plate the anti-ventilation plate (the horizontal 'fin' just above the prop) can be raised a couple to several inches above the keel/bottom of the boat.

Which reduces the amount of the lower unit IN the water, so less drag.

When the prop comes up out of the water, in a turn or by being raised by motor trim or a jack plate, it is said to be ventilating. The motor will often overrev as the prop is experiencing significantly less drag.

Cavitation is something different, and is what causes props to pit....

The terms are often used interchangeably, but are 2 distinctly different things. Neither is good....

Does your 90hp Mariner have tilt/trim from the factory? If not, thats propably why the PO added the jack plate.

Is the jack plate electric or manual?

I've seen cases where just a 1/4in higher motor mounting changes handling, overspray and WOT speed.

How about some pix of the boat in profile and then closer details of the motor/jack plate and mounting....
 

gm280

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Jack Plates can be really great add-ons when needed. But if not needed, it merely is a show piece. If your adjustments doesn't improve the handling and operation of the engine and boat, then remove it. But give it a good try before opting to remove it. You may be surprised. And yes, like jbcurt00 stated, pictures would help answer your questions a lot better. JMHO
 

Dukedog

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I move it up just a little like 3 to 5 degrees it seams to cavitation. I have to keep it tucked down .

a jack plate doesn't move in degrees.. it moves straight up or down in inches, manual or hyd... tilt and trim moves in and out in degrees.. sounds more like its an aftermarket tilt n trim?

jan.. might do some investigating on it.. might have been added so all tha controls, fuel lines, steering etc. don't get pinched or cause splash well damage when fully tilted up...
 
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fhhuber

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Local boat guy calls jack plates "transom breakers"

They give the engine more leverage on the transom by moving them away from it.

If the outboard can be mounted properly (including correct height) without it, you are better off without it.
The engine should be able to get down into proper running position and be tilted up so the skeg is not below the bottom of the boat at the transom.

Pictures of what you have would help in advising how to adjust and if you really need the jack plate (to make that outboard fit that boat) or not.

A good side view with the anti-cavitation plate just having the edge visible and the engine down in normal operating position
And pictures that show the mounting and your jack plate
 

brappley

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Apr 17, 2011
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I tryed to upload pics but couldn't , jack plate is a manual, I measured it , it's a 10 inch offset, after reading a lot of posts bout this a lot of forums say that for every inch of offset it should be 1/2 inch above the bottom of the boat. So that works out to 5 inchs ? Also I leveled the boat and the cavitation plate and ran a straight edge to the bottom of the hull and raised it up but it would only go 1/2 above the bottom of the boat no more adjustment, also I could be wrong but I think the reason why it was put on is because the boat has a very short transom so it should have had a short shaft motor , that's why they added the jack plate to raise up the motor, I'll try to upload pics again today.
 

Teamster

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Which mounting holes are used to mount the motor to the jackplate?

Might be a was to raise it there,........
 

Dukedog

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am i thinkin' right in this after readin' your post again?.. your not rasin' or lowerin' tha jp but when you "trim" tha motor out and/or up very much at all is when ya get tha cavitation?
which deck boat is it (model, size)?.. prop?.. max hp rating?
 
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brappley

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1998 sylvan deck boat w/ 90 hp mariner that is a 1996 elpto
It's 19 foot long and is rated at 1500 lbs per the tag. Max hp 125, it runs out at 5000 rpm at 32 mph by my gps.
 

Dukedog

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am i on tha same page with you in my last post... it blows out and/or cavitates when ya hit tha trim button?

does it brerak over on plan like ya feel it should?
carry tha bow at all or run very wet?
not enough motor for that heavy weight (not counting tha 1500 lb. load cap.) unless ya happy with it.
might jus be a prop problem, to much prop... size and type thats on it now?... ie: 5 grand ain't quite there, closer ta 5500
 

brappley

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No Title

Here are a few pics I was able to load, hope it helps,
Dukedog to answer your question the prop is correct according to my mariner / merc dealer, the noise I'm hearing is exhaust noise , I found that out today, I know when I bought the boat the dealer had lowered the motor bout an inch below the bottom of the boat , why I don't know.
It's one of those things that I've never had a jack plate on any of my boats and I've had several over the last 25 years and kept upgrading according to my needs. Hope the pictures help. I think I said I raised the motor up 1.750 inchs and it did help on plain. Take a look at the pics and give me your opinion, so you know the cavitation plate is .500 above the bottom of the boat.
 

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Dukedog

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is tha prop beat up on one blade or is it jus tha picture?.. looks bad! i like what teamster says, ya need ta drop at least one pitch.. looks like ya have one more hole in tha clamp bracket ya can go to if ya think it needs ta go higher.. but i'd address a prop issue first...

one thing you will learn if ya keep foolin' with these things is about props... no two will be tha same.. dealers jus sell things that are "supposed" ta work.. two new props, same numbers, same pitch, one might run a hole in tha wind and one will be complete turd.. jus part of it...

don't worry 'bout where tha cav plate ends up.. should be more concerned with where tha center of tha prop shaft ends up and how tha boat acts.. one thing you really need when foolin' with motor height is a water pressure gauge....
 

brappley

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Apr 17, 2011
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33
Yep ^^^ when running a jack plate the anti-ventilation plate (the horizontal 'fin' just above the prop) can be raised a couple to several inches above the keel/bottom of the boat.

Which reduces the amount of the lower unit IN the water, so less drag.

When the prop comes up out of the water, in a turn or by being raised by motor trim or a jack plate, it is said to be ventilating. The motor will often overrev as the prop is experiencing significantly less drag.

Cavitation is something different, and is what causes props to pit....

The terms are often used interchangeably, but are 2 distinctly different things. Neither is good....

Does your 90hp Mariner have tilt/trim from the factory? If not, thats propably why the PO added the jack plate.

Is the jack plate electric or manual?

I've seen cases where just a 1/4in higher motor mounting changes handling, overspray and WOT speed.

How about some pix of the boat in profile and then closer details of the motor/jack plate and mounting....

It has trim tilt on both the motor cowl and the throttle handle / forward reverse
 

brappley

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Apr 17, 2011
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33
is tha prop beat up on one blade or is it jus tha picture?.. looks bad! i like what teamster says, ya need ta drop at least one pitch.. looks like ya have one more hole in tha clamp bracket ya can go to if ya think it needs ta go higher.. but i'd address a prop issue first...

one thing you will learn if ya keep foolin' with these things is about props... no two will be tha same.. dealers jus sell things that are "supposed" ta work.. two new props, same numbers, same pitch, one might run a hole in tha wind and one will be complete turd.. jus part of it...

don't worry 'bout where tha cav plate ends up.. should be more concerned with where tha center of tha prop shaft ends up and how tha boat acts.. one thing you really need when foolin' with motor height is a water pressure gauge....


It does not have a pressure gauge .
 

brappley

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Apr 17, 2011
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is tha prop beat up on one blade or is it jus tha picture?.. looks bad! i like what teamster says, ya need ta drop at least one pitch.. looks like ya have one more hole in tha clamp bracket ya can go to if ya think it needs ta go higher.. but i'd address a prop issue first...

one thing you will learn if ya keep foolin' with these things is about props... no two will be tha same.. dealers jus sell things that are "supposed" ta work.. two new props, same numbers, same pitch, one might run a hole in tha wind and one will be complete turd.. jus part of it...

don't worry 'bout where tha cav plate ends up.. should be more concerned with where tha center of tha prop shaft ends up and how tha boat acts.. one thing you really need when foolin' with motor height is a water pressure gauge....

The pic does look misleading, it has a small ding in the blade but seams to be ok.
There is one more hole in the transom mount I can raise it up .750 if I need to and would be an easy do
 

Dukedog

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i'll be tha first ta tell ya i'm not that comfortable with "little" motors and that type hull.. very little experience with either but its still a boat and motor so basics should still apply..... my first impression tells me that dealer sold a way under powered rig for whatever reason... i wouldn't worry 'bout tha small amount of trim you add before "blow out/cavitation" till ya get tha prop thing done.... this is where i'd go....

do this first since ya don't have a pressure gauge.. get tha prop "fixed" or try a different pitched prop (one inch smaller in pitch).. a slight ding, especially with aluminum, will be very detrimental to performance.....5 grand is workin' tha motor unnecessarily..


then this can be safely done without a gauge... this is what we use for initial set up on boats we have no clue as where to start with motor height....best done on level ground or pavement....

level "bottom" of boat along tha keel or pad, which ever yours is, last 4 feet or so....
level tha motor on tha cav plate...this also levels tha prop shaft...

measure bottom/pad to ground
measure "horizontal center" of prop shaft to ground..
you want tha horizontal center of prop shaft 3 1/2 to 3 3/4 inches "below" tha bottom/pad....
this will get ya close enough to run hard and tell if there's any other problems.. like prop!

with a pressure gauge and "good" prop you can "tweak" on tha height from there ta "get all its got"......gl
 

brappley

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Apr 17, 2011
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Got it , that's a little different than what other guys have told me to do . The way I did it was like you said level the boat then level the cavitation plate which I did , then adjusted the motor up , I was told that for every inch of set back I needed 1/2 inch above the very bottom of the boat ,since it jack plate is a 10 inch off set that works out to a 5 inch lift above the bottom of the boat which sounds very high. But the jack plate would only let be go 1/2 above . I'm also getting the prop fixed and should get it back by Friday and I'll see what happens
 

Dukedog

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gottcha... has this been a problem with your boat or is it jus more of a concern not knowin' if its supposed to act this way?
 
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