Motor height question????

[coopermark7]

I have a 1448 semi-v jon boat with a new 25hp mercury fourstroke on it. The motor is mounted on a mini-jack plate from bass-pro. It has four inches of set back on it. The motor now is an inch and 3/8 above bottom of boat with ventilation plate. When the motor is trimmed slightly up the motor spins out. Trim it slightly down runs great. I guess I'm asking is it a hole to high on the plate?

 

[jacoboregon]

Re: Motor height question????

Is that with one person or more? What kind of load? How is the bow running when on plane (high, low, just right)? If you have the right leg length for your transom and not trying to put a long shaft on a short (15") transom then you would be better off without the mini jack. If you are trying to run in shallow water you'd be better off with a jet or get a boat with a tunnel hull. I don't know your abilities, but there are tunnels available to convert your boat. I'd recommend you take the mini jack off.

 

[coopermark7]

Re: Motor height question????

Its with two people onboard the boat. It really runs flat it seems just right to me, but I am know expert at this at all. I kinda want to keep the jack plate on as I like the tiller handle a little more behind me the way it is now. I just don't want to worry about getting the Merc. to hot.

 

[jacoboregon]

Re: Motor height question????

It shouldn't be "really flat" while on plane. That has the potential to have some steering problems in rougher water. You need to have the option to trim the motor out to bring the bow up some while on plane. If the mini jack is set too high to allow that and you want to keep it, then you should set it down a couple of hole and try it there. You don't want to get into a situation that can cause bow steering while going fast.

 

[coopermark7]

Re: Motor height question????

I was afraid of that the way it seem to want to unload the prop when slightly trimming it up. I have not really opened it up yet. Probably only half throttle at most.

 

[5150abf]

Re: Motor height question????

Just drop it a hole and try again, with a setback you can run the engine a bit higher.

 

[coopermark7]

Re: Motor height question????

The big problem I have is the screw clamps on the motor will hit the back of boat and not let me drop the extra hole down.

 

[steelespike]

Re: Motor height question????

How much space do you need to clear.Perhaps you could add a 1/4" or 1/2" spacer to get the clamp handles to clear.
Perhaps you could raise the whole jack plate just enough so when you drop the motor
it just clears the transom.
You could try a prop with some cup; Could eliminate venting.might squeeze out a little speed.You might be able to raise the motor on the jackplate just enough so when you drop
the plate it the clamps just clear.Obviously you won't gain the whole drop but maybe enough to ease the venting.

 

[coopermark7]

Re: Motor height question????

Well I was able to get it down from an 1 3/8 down to an inch right on the money... next warm not windy day here in the ozarks I will test it out. The spacer idea is a great idea... May have to try it if this does not work....

 

[coopermark7]

Re: Motor height question????

Well finally got to get her out on the water for some run time. It carries the nose a little not running flat now. And no venting the prop when you get all over it on take off.. It is wanting to vent it on tight turns ,but other than that it seems fine.

 

[steelespike]

Re: Motor height question????

Some folks tollerate a little venting in turns to keep as little as possible in the water.

 

[coopermark7]

Re: Motor height question????

Yea I think I will as well. I like it to run shallow as well so we can get up the creeks in the spring for the white bass.. I may try one of the new Mercury Spitfire props. They say they have more hold in the turns. Other than the turns love the way it runs 29mph at 5700rpm so I think not bad with two adults and alot of gear onboard.

 

[steelespike]

Re: Motor height question????

Pretty respectable. Especially considering a jon usually doesn't lend itself to speed very well
and sometimes their bottoms can be banged up some.

 

[coopermark7]

Re: Motor height question????

Yea I am really pleased with the numbers that I get out of it. I am wondering about changing to a fourblade prop to help the cornering more. It now has a 9.5x11 blackmax on it. I have looked at the Solas props but am unsure how the are. Mercury's spitfire prop says a 9.3x11 will do the trick just cannot find one and the price is very high for those.

 

[Texasmark]

Re: Motor height question????

The problem in cornering is the hull ribs that Johns have. When turning, they skid across the water generating bubbles (turbulence) which, when applied to the prop blades, creates an air/water environment which causes your engine to overrev.

You could lower your engine to get below the bubbles but that would defeat your other goals. 4 blades, cupped might help, or you might take your prop to a prop shop and have them increase the cup which will also change the load on the engine at WOT especially (more cup more load...maybe more mph, certainly a decrease in rpm's).

Why not just avoid tight turns at WOT?

Mark

 

[coopermark7]

Re: Motor height question????

Yea I don't normally make tight turns at all at high speed. I heard on here somewhere that is the way to tell if you are on the edge of being to high.

 

[Texasmark]

Re: Motor height question????

Yea I don't normally make tight turns at all at high speed. I heard on here somewhere that is the way to tell if you are on the edge of being to high.

Not a valid test on your current hull for reasons given.

Mark