Jon boat O/B height

[Dylan]

Just wanted to say I am totally in for a video of a Johnboat with a jackplate, buzzing across the lake wide open. Preferably trimmed out too, but I won't complain.

Make sure you reinforce the transom. I would also make sure your boat will float, and not go down coming off plane.

[TRBenj]

Originally Posted by CruisinGA

boater8987- I think TRbenj was referring to what I was describing when he said "several inches below transom"

Bingo.

Comparing cav plate vs. hull height gets much trickier on a v-hull (or other complex shape)... Very simple to do on a basic flat bottomed jon boat.

Ive got a small collection of those 74-92 Johnson/Evinrudes... I grew up with a '86 Johnson 9.9 just like the one you have pictured. I threw a 15hp carb on it and it flew... pushed my 12' v-hull 26mph on the GPS. The later 15's (starting around '81) got tuned exhausts in addition to the carb change which made them a little more powerful... but you can add that carb alone and make a big difference. That 9.9 with the 15hp carb ran just as fast as my '80 15hp. I may have a spare 15hp carb from a parts motor (in need of a rebuild, which is super easy)... the prices they fetch on ebay are crazy. Shoot me a PM if youre interested.

Thats a short shaft (15") btw, which is what you need. The way they measure is from the transom mount to the cav plate (not the centerline of the prop, bottom of the keel, or anything else).

My 12' boat is very low freeboard which can be a pain at times, but the very slight rearward mount that the Mini Jacker provides didnt make any difference in that respect (vs. mounted on the transom itself). Ive seen some fancy wood transom reinforcement done that also raised the transom where the engine mounted, but if you need to raise it more than an inch or 2 (and even that may be pushing it) I would opt for a metal jack plate.

[CruisinGA]

Originally Posted by Dylan

Just wanted to say I am totally in for a video of a Johnboat with a jackplate, buzzing across the lake wide open. Preferably trimmed out too, but I won't complain.

Make sure you reinforce the transom. I would also make sure your boat will float, and not go down coming off plane.

Dylan- It is coming.
When I got the boat up to 22 mph I was too nervous to hold my iphone with one hand...

The boat is pretty stable at speed, but when coming off plane it does some weird stuff.

[CruisinGA]

Originally Posted by TRBenj

Ive got a small collection of those 74-92 Johnson/Evinrudes... I grew up with a '86 Johnson 9.9 just like the one you have pictured. I threw a 15hp carb on it and it flew... pushed my 12' v-hull 26mph on the GPS. The later 15's (starting around '81) got tuned exhausts in addition to the carb change which made them a little more powerful... but you can add that carb alone and make a big difference. That 9.9 with the 15hp carb ran just as fast as my '80 15hp. I may have a spare 15hp carb from a parts motor (in need of a rebuild, which is super easy)... the prices they fetch on ebay are crazy. Shoot me a PM if youre interested.

Thats a short shaft (15") btw, which is what you need. The way they measure is from the transom mount to the cav plate (not the centerline of the prop, bottom of the keel, or anything else).

My 12' boat is very low freeboard which can be a pain at times, but the very slight rearward mount that the Mini Jacker provides didnt make any difference in that respect (vs. mounted on the transom itself). Ive seen some fancy wood transom reinforcement done that also raised the transom where the engine mounted, but if you need to raise it more than an inch or 2 (and even that may be pushing it) I would opt for a metal jack plate.

Perfect, always wondered how you measure o/b shaft length
Also good to know that 15/9.9 carb switch is a real thing and not internet myth.

The low freeboard, especially in the back is a pain. I just found out about these floatation pods for jon boats, I don't think I'll be spending $300+ on some for my $100 boat, but I may try to make some. Seems like it would be the ticket to help my boat float level with the oversize engine, not to mention I could move my gas tank back to the back out of the way.


You have PM on the carb.

[Willski]

Originally Posted by CruisinGA

boater8987- I think TRbenj was referring to what I was describing when he said "several inches below transom"

Regardless, some good info in this thread.
I'm not sure about the jack plates- don't really want to move the o/b back because the boat already sits low in the back if there is only one person in it. I think I will just look at O/B height with regards to the bottom of the transom, and if I have to just raise it vertically.

See pic, I guess this is a 20" shaft motor. Measured my transom and it is 15"

See other pic with red circle- I have always set the motor all the way down on the transom, just noticed this shoulder- is it meant to be set on the transom if you want to raise the motor?

I couldn't set the motor on the boat yesterday, but knowing the transom is 15" and looking at the motor with the tape measure, it looks like I may already be close.


Next step after setting transom height is to find a carb off a Johnson 15hp.... apparently the 9.9 and 15 of this vintage are exactly the same outboard except for the carb....

I have been through water with my 4 wheeler, but never deep enough to need outboard assistance!!

[CruisinGA]

Originally Posted by Willski

I have been through water with my 4 wheeler, but never deep enough to need outboard assistance!!

I don't have an outboard stand other than my fourwheelers... they work perfectly!

[boater8987]

I think TRbenj was referring to what I was describing when he said "several inches below transom

No big deal guys, just gave me more time to ramble. Has anyone had any good results with the small jack plates? Will these old 1950 motors even get close to the speed of new models 4 strokes and 2 strokes?

[CruisinGA]

Originally Posted by boater8987

I think TRbenj was referring to what I was describing when he said "several inches below transom

No big deal guys, just gave me more time to ramble. Has anyone had any good results with the small jack plates? Will these old 1950 motors even get close to the speed of new models 4 strokes and 2 strokes?

Lol, see post 12. I am 100% confident that TRbenj was not referring to your post.


I know that my 1965 Evinrude 6hp is not close to as fast on my 12ft boat as my buddies new 6hp 4stroke mercury. My '65 rude runs great. Not sure where the difference is.

[thatsmrmastercraft]

Originally Posted by CruisinGA

Lol, see post 12. I am 100% confident that TRbenj was not referring to your post.


I know that my 1965 Evinrude 6hp is not close to as fast on my 12ft boat as my buddies new 6hp 4stroke mercury. My '65 rude runs great. Not sure where the difference is.

Mercs were always wound a little tighter than Evinrudes. That and almost 50 years of service may have some effect.

[TRBenj]

Originally Posted by CruisinGA

Lol, see post 12. I am 100% confident that TRbenj was not referring to your post.


I know that my 1965 Evinrude 6hp is not close to as fast on my 12ft boat as my buddies new 6hp 4stroke mercury. My '65 rude runs great. Not sure where the difference is.

Correct... was referring to the position of the 9.9 on the flatbottom jon boat, not the 300 Mercs on the v-hulled offshore. Apples and oranges and I dont know jack about oranges!

Motors from the 50's and 60's do run very well, even compared to more modern motors. My buddy and my father both have a few 1955-1962 15hp and 18hp Evinrudes, and they are great performers. Very similar performance to the 15hp '74-92 motors Ive played with. Im actually considering trading my newer motors for an older one- they just look and sound so much cooler. They run much lower revs to boot, so running them is a lot more relaxing.

Compared to modern motors, I'll still take the 70's and 80's stuff over them any day. General construction quality seems lower nowadays, and the 4-strokes are not good performers on small boats due to their extra weight and complexity, IMHO. They are quieter and more fuel efficient though.