Maximum backplate height

[Thompsoncustom]

Built a jack plate the other day for my 1965 Johnson 6hp. The jack plate raised the motor 7 inches and at that height you have to be moving to prime the water pump which was my biggest concern. Turns out the water intake wasn't the biggest problem its my loss of speed.

I'm guessing the prop must be grabbing some air as it revs higher then normal, what are your guys thoughts on this. Looking to get it as high as possible.

 

[hardwater fisherman]

How much offset do you have? Which hole is the tilt pin in? How high is the anti cav. plate above the bottom of the hull?

 

[Thompsoncustom]

Not sure what u mean by offset. Tilt pin is in the bottom. Anti cav plate is probably 4 in above the bottom.

 

[hardwater fisherman]

Offset would be how far the motor is behind the transom. Or did you just raise the height of the transom? If the motor is 4 inchs above the bottom of the hull with no offset I think that is about 3 inchs too high. Also if you are running in the bottom hole the motor is probably too high.

 

[Thompsoncustom]

Thanks. No off set than I just went straight up.

 

[hardwater fisherman]

You need to try to see what gives you the best performance. Most people will say a good starting point is the the anti cav. plate should be level with the bottom of the hull. I found that when running full throttle and looking over the back the anti cav. plate should be skimming the surface or just below.

 

[Thompsoncustom]

Maybe I'll have to modify or just build a new plate if setback will get me more height. I'm not looking for speed with this motor, normally it would only do 7mph max anyways what I need from this motor is the ability to run in shallow water so the higher the better.

I'd be fine if I drop down to 5mph but right now it probably does 2.

 

[hardwater fisherman]

Yes the further you go back the higher you can raise the motor.

 

[Thompsoncustom]

Is there any ruff formula for setback and height. Say for every inch back you go you can raise the motor x amount. I would consider redoing the whole jack plate if I knew I could run the motor 7-10in higher than the transom.

 

[hardwater fisherman]

I dont know but I think that you will never get the motor that high. If your motor is a short shaft the best you could hope for is maybe a 17inch transom. So if you set back say 3inchs maybe you could go up an extra inch or so.

 

[Faztbullet]

What type of boat????

 

[Thompsoncustom]

1998 14ft alumicraft modified V.

 

[gm280]

At 4" above the hull bottom, you are playing with an over heated possibly seized engine...seriously. Offset allows a little height, but that is because the water vectors up after leaving the hull bottom allowing the water inlet to get some water. But unless you have a huge setback, your present setup isn't going to work. If you are doing this merely to tread shallow water, you will need a hydraulic jack plate configuration. Then you can run pretty much normal until you get into very shallow water (where I hope you are merely trolling across anyway) and you can press the hydraulic jack lift button and raise the engine to go slowly through. You can't run a typical OB that high and expect proper water cooling in the engine. Sooner, not later, you will fry the engine. JMHO!

 

[Thompsoncustom]

Ya that was my first concern so when I built the jack plate I ran the legs just above the bottom of the hull so I could run a water pickup tube off of them if need be but it appears I'm way to high now and it still sucks water. I'm gonna drop it to 3.5ins and see how it runs like that. The upside to 3.5in is i can run my bigger motor at that height and the cav plate would be close to flush with the bottom so I might gain some mph.

Yes I do have a trolling motor but was hoping to use that less and the 6hp more in the shallows.