Jack Plate Help

[awe1979]

I have a 1996 17 ft fish and ski cajun with a 1995 90 hp V4 crossflow johnson. When I bought it; it had a 17 pitch alum. prop on it. I could get 4900 rpms at wot. So after reading on this site that that wasent good I bought a 15 pitch alum prop and installed it. I get 5200 rpms at wot now. I have been reading up on maxumizing your set up. So I have decided to buy a jack plate and see if I can get more rpm's out of this crossflow. I just caint see having to go with a 13 pitch prop on this boat. Are there any ideas on how much set back I need 4 6 or 12? Is a manual jack ok?

 

[walleyehed]

Re: Jack Plate Help

On a 17ft'r with a 90hp, a 6" plate will help....If you go with the set-back and ability to raise the engine, you may have to follow with a Good SS prop...stiletto, turbo, raker, etc.....

And...don't get rid of that alum 17 yet...if you install a plate, we may need to run it for reference.

 

[awe1979]

Re: Jack Plate Help

I need to shoot for 5800 rpms? with the cross flow v4 right? The owners manual says 5000-5500 wot range. Do you think that I can get my rpms up with the jack plate? After getting the jack plate installed I want to go with a SS prop. I will post my numbers and hopefully get an idea of what kind of prop to buy. Also how do you lift this motor off the boat? I just dont know what to hook the chain to.

 

[walleyehed]

Re: Jack Plate Help

5800....yes.
Under the cowl, there should be a fixed loop ring to attach a lifting device to on a crossflow. Mounted behind (to the rear of) the flywheel.
The plate will allow us to raise the engine, reducing the amount of gearcase in the water which will reduce drag, in-turn increasing speed and rpm.

 

[Dhadley]

Re: Jack Plate Help

And if you look at something like the Bob's 4 in 1 you'll have a couple of set back options. Figure in a set of 2" spacers and you have all kinds of options.......

 

[awe1979]

Re: Jack Plate Help

What about this jack plate for my boat
http://cgi.ebay.com/ebaymotors/VANC...198608274QQcmdZViewItem?hash=item300198608274

 

[Dhadley]

Re: Jack Plate Help

Vance is a good plate. It's very easy to adjust the height with the center bolt. You do lose the ability to change the setback and it's a little heavier than the Bob's. It just depends on what you prefer. I don't think you can go wrong with either one.

 

[awe1979]

Re: Jack Plate Help

Do you think the Vance plate will bolt right up to my boat or will I have to drill new holes to mount it. I would like to use the holes that I allready have.

 

[SpinnerBait_Nut]

Re: Jack Plate Help

What ever you do, don't forget water intake to cool the engine.
You get too high and you will loose your water pressure and this is not good.

 

[steelespike]

Re: Jack Plate Help

While a jack plate would work really well;have you checked to see if you can simply raise the motor a hole or 2.You can do this without removing it and it could get you closer to the rpm and speed your looking for.Again be sure you are pumping water.

 

[Silvertip]

Re: Jack Plate Help

A 17 foot boat with a 90 should have no trouble twisting a 17P prop. Before buying a jack plate and working with props, I would suggest doing a thorough engine check (compression check, link & sync, thottles opening fully, and a general tune up) to ensure the engine is actually making full power. It would appear this engine is down on power. You can't make a poor performing engine perform well just by changing props and adding a jack plate.

 

[awe1979]

Re: Jack Plate Help

Had the dealer check it out. They said all holes withen 5 psi and they did a decarbon and tune up on it. They told me it was in A+ shape

 

[awe1979]

Re: Jack Plate Help

I have owned this boat for a long time. It has always done 4900 rpm with the 17 pitch prop. Its not like it has lost power or something. When I bought the boat it had 20 hours on it and I watched them do the compression test. It ran the same then as it does now. I dident watch the guys at the dealership do the test last time but they said it was withen 5 psi. The motor has always been decarbened every 20 hours since I have owned it. It has not lost rpm. It has just never had it. I dont think the motor sets right on this boat. I have to trim it up too high to get any rpm out of it. I just dident know I needed to turn it so many rpms untill I started reading up on this site.

 

[SpinnerBait_Nut]

Re: Jack Plate Help

With the engine tilted down, where does the cavatation plate set in conjunction with the bottom of the boat?

 

[steelespike]

Re: Jack Plate Help

If you can trim way out with no ventilation problem then you probably need a little more height.A stainless prop will help to get the max height with minimum
venting.

 

[walleyehed]

Re: Jack Plate Help

I have owned this boat for a long time. It has always done 4900 rpm with the 17 pitch prop. Its not like it has lost power or something. When I bought the boat it had 20 hours on it and I watched them do the compression test. It ran the same then as it does now. I dident watch the guys at the dealership do the test last time but they said it was withen 5 psi. The motor has always been decarbened every 20 hours since I have owned it. It has not lost rpm. It has just never had it. I dont think the motor sets right on this boat. I have to trim it up too high to get any rpm out of it. I just dident know I needed to turn it so many rpms untill I started reading up on this site.

At 4900RPM, I am pleased to hear that it still runs at all.
5200 is basically lugging that engine...5500 would be the minumum you would ever want with a load and 5800 would be the best target...
being 800-1000 low, in my mind, you're not going to get what you need with just a jackplate....I think a jackplate and the correct prop for that set-up is the way you'd have to go.

 

[Dhadley]

Re: Jack Plate Help

Glad to hear you're decarbing every 20 hours, very smart. That's what's kept the motor alive most likely.

A couple things come to mind -- first, we should run the numbers to verify that the tach is reasonably accurate. All we need to know is what the actual (real) top speed is with each prop. Perhaps the tach is off but the increase between the 17 and the 15 is right on. Therefore we can assume the tach is reasonably close. But it's best to check.

Second, and this is basically why I'm thinking a jackplate will help, every Cajun we've worked with has benifited from a jackplate. Low top rpm, running full trim etc seems to be common theme. Everything points to raising the motor. Obviously at some point we'll need to move to a ss prop.

 

[awe1979]

Re: Jack Plate Help

I bought a compression tester last night and did a test myself. The numbers are 122 127 125 125. And yes the carbs open up all the way. Checked that as well. I will post the numbers that I get from the jack plate and see what ss prop that I need. So how high do I go when tunning the jack plate? I have a water pressure guage. And with the 15 pitch prop that I have now I get 38 mph at 5200 rpms. Thanks everyone for your help.

 

[Dhadley]

Re: Jack Plate Help

OK, 38 mph with the 15 at 5200 doesn't add up. Theoritical speed is about 37.5. If the speed is accurate the tach may indeed be off. At 10% slip you'd need to be at about 5750-5800 to get an actual 38 mph.

What brand is the tach?

 

[awe1979]

Re: Jack Plate Help

Its a telefelx classic 1996 model. Do the tacs go bad?