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CruisinGA
01-28-2013, 07:30 PM
So I have a little camo 12' jon boat that I use for frog gigging and duck hunting. It's a little too small, but the price was right.

Neighbor gave me a 9.9 Johnson, but boat is rated for 3 hp... really showing my redneck side now :D

With a little motor tuneup and trim adjustment, this sucker does 22 mph now, but the transom is showing signs of having 3x the rated hp on it.

It appears to me that the motor sits too low in the water, lots of spray around engine at WOT. Anyone have any experience with jack plates on small boats? What is the ideal height of the lower unit with respect to the bottom of the transom?

While I am beefing up the transom to handle the increase in power, I might as well get it set up for the ideal height too.


Note: picture shows my '65 Evinrude 6 hp that I used until I came into the 9.9hp.

KahunaCraft
01-28-2013, 08:10 PM
That's about the top end for what you have... You might be able to adjust the manual trim on it get a little more.

Here are some similar posts on some other forums...

http://www.crappie.com/crappie/arkansas/35007-how-beef-up-transom.html

http://www.catfish1.com/forums/showthread.php?82576-Strengthening-the-transom

Good luck. I'd tie a 25ft rope with a floaty to one end and the motor to the other...just in case.

psychobilly
01-28-2013, 08:14 PM
The horizontal fin on the lower unit should be level with the bottom of the skiff. Oh, and you may want to add a stiffener piece of wood on the transom now before the aluminum starts cracking under the stress of the 9.9. Don't ask me how I know this. 😄

AJones
01-28-2013, 08:44 PM
Not sure if anyone makes a jackplate for small motors and jon boats. From my bass boating days, you need to run a straight edge along the bottom of the boat extending beyond the transom. The cavitation plate on the engine above the prop should be setup approximately inch above the bottom of the boat. This would be a good starting point. If the motor has adjustment holes in the mounting bracket, raise the engine a notch at a time until cavitation starts and then back down a notch. Warning, running too high will cause cavitation and overheating because of lack of water to intake holes. The shaft length on the 9.9 hp may be too much for the transom of your 12 jon boat. You may need to modify/rework the transom to mount the motor high enough.

Good luck.

boater8987
01-28-2013, 11:58 PM
I have discussed proper outboard height with Mercury Marine for my 300 Verado's before. Water just touching cav plate under way at level trim close to wide open. Can be used with mostly any outboard they said. There are many varibles besides cav to keel placement. Transom setback, bottom design, speed, etc. This one is still a little deep.

http://www.youtube.com/watch?v=4TavDjfe0Ng


Here you go this is about correct.

http://www.youtube.com/watch?v=3TOAmLi43nI

TRBenj
01-29-2013, 12:03 PM
Lots of opinions on the exact placement of the cavitation plate relative to the hull... some say slightly above, some say slightly below. I found that going perfectly level yielded pretty darn good performance (much better than several inches below, as youre describing). I found one of these T.H. Mini Jackers on ebay and its a nice piece. I use it for the 15hp Johnson on my 12' aluminum boat.

http://www.boatersplus.com/t-h-marine-mini-jacker.html?utm_source=googlepla&utm_medium=cse&utm_content=EB-6201164&utm_campaign=googlepla&gclid=CNDFgYb4jbUCFQqe4AodXQ0AgA

Sodar
01-29-2013, 12:09 PM
Bob's has mini adjustable jack plates, too...

http://www.bobsmachine.com/Products/jackplate_manual.cfm

cbryan70
01-29-2013, 12:13 PM
depends on shaft length. is it a 15 or 20 inch shaft on the motor? How tall is the transom now?

boater8987
01-30-2013, 12:58 AM
Quote: much better than several inches below, as youre describing.

If your refering to my post. I did not describe several inches below. I gave the expert opinion of the manufacturer that designs, engineers, builds and has to warranty their motors. Not my opinion their expert advise. I am not an expert by any means and not trying to P.O. or step on any toes. No harm no foul. They stated many varibles. Here is a example of a set back transom step hull / note cav plate several inches above keel. Water travels up off bottom to set back motors allowing less drag more speed. As a motor gets closer mounted to a flat transom boat it needs to be lower than this aplication, thus their statement about water touching on cav plate under way is a safe position. Could very well end up level with keel depending on varibles. That is their way of getting it close to correct. I realise jon boat is a considerbly different application with only one motor, but their procedure is a proven method. And as AJones stated, another varible, motor too high water starvation occurs, then motor meltdown and or prop blow out (loss of speed and steering) Also stated setback jack plate is a good way to be able to safely move motor up and can create less drag and more speed gives more distance and height from transom so water has more time to roll upwards to feed water intake and prop. Most setback jack plates will list the setback distance and height. Then the fun begins dialing it in. I have a 12' G3 with a 10HP 1956 Johnson to play with. Good fun.

CruisinGA
01-30-2013, 08:56 AM
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.... :D

Dylan
01-30-2013, 11:24 AM
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
01-30-2013, 11:59 AM
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
01-30-2013, 01:04 PM
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. :D
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. :cool:

CruisinGA
01-30-2013, 01:10 PM
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 :D
Also good to know that 15/9.9 carb switch is a real thing and not internet myth. :cool:

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.
http://i183.photobucket.com/albums/x29/lonewolf9029/DSC00068.jpg

You have PM on the carb.

Willski
01-30-2013, 01:54 PM
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.... :D

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

CruisinGA
01-30-2013, 02:07 PM
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! :D

boater8987
01-30-2013, 10:38 PM
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
01-30-2013, 11:02 PM
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
01-31-2013, 12:11 AM
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
01-31-2013, 11:45 AM
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.

CruisinGA
01-31-2013, 03:21 PM
I generally agree... but my buddies single cylinder 6 hp 4 stroke turned lower rpms and was quieter than my old 6 hp... really quiet under 1/2 throttle. I think thru-hub exhaust plays a role here.

TRBenj
01-31-2013, 04:55 PM
I generally agree... but my buddies single cylinder 6 hp 4 stroke turned lower rpms and was quieter than my old 6 hp... really quiet under 1/2 throttle. I think thru-hub exhaust plays a role here.
Thats consistent with my experience. The later 2-strokes from the 70's and 80's rev quite a bit higher than the 2-strokes from the 50's and 60's (5000 vs. 6500 rpm WOT). Neither is particularly quiet or efficient.

The new 4-strokes are way quieter than any of them, and way more efficient. Theyre also significantly heavier, which is particularly noticeable on these light weight, low hp applications- giving them a significant performance disadvantage.

boater8987
02-01-2013, 01:34 AM
You can tell its winter time. Talking about jon boats. LOL I have been looking around for one of those 18 Hp 1950's Evinrudes. My neighbor with his 90's 15 Evinrude says his is fast. We have BS ed about having a jon boat radar run shootout. Like they have a LOTO for performace boats.

TRBenj : Do you have a pic you can share of the jack plate mounted motor & boat?

TRBenj
02-05-2013, 11:48 AM
No need for a radar gun... a handheld gps that captures max speed (like a garmin etrex) works great. We use it on the aluminum boats and ski boats alike. The 15hp's we have of various vintage all run 24mph consistently, and have touched 25-26mph briefly under ideal conditions. With a light load, they really haul- enough for my 150lb buddy to ski behind!

I checked and dont have any good pictures of my mini jacker installed... here is the best that I have. I mounted mine as high as I could on the transom, but could not go quite as high as I needed due to the lip of the aluminum at the top (hence the additional board under my bracket)... but its high enough to get a nice solid surface to clamp against.

EJ OJPROP
02-05-2013, 01:14 PM
We used to call those boards "rev sticks" when we were racing stock outboard hydros. Used the sticks to run the engine as high as we could and still pump water at speed. Too high = no water as well as not being able to steer.

CruisinGA
02-07-2013, 11:50 AM
Mocked up the motor last night while fabricating some transom reinforcement brackets.

1st pic - cav plate relative to transom with motor sitting all the way down on the transom
2nd pic - cav plate relative to transom with motor sitting raised on transom
3rd pic - shows transom bracket position when above pic was taken. Can the O/B be mounted like this? I don't think so?

CruisinGA
02-07-2013, 11:56 AM
Pic 1 & 2 shows where the transom started to come apart from the increased stress.

Pic 3 shows the brackets I made to prevent the transom from pushing 'into' the boat and continuing to pull the transom apart. I am also going to run a piece of angle across the bottom of the transom board to stiffen it and help transfer the force to the brackets and sides of the boat.

Unfortunately, I don't have an tig welder with AC (yet) so I tried some of this (http://www.durafix.com/)stuff but my Mapp torch wouldn't get the aluminum hot enough. Just too much surface area to dissipate the heat.

We'll see how well this all works! https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcR93dFakWVr6LTj5FFKR9grfWAsRy9Ry FLdcHl8O8D3mClQvigz

TRBenj
02-07-2013, 01:01 PM
No, I wouldnt run the motor with a gap under the bracket like that. Build up the transom higher or use a jack plate. Looks like you need to gain another 1/2" anyways.

I would reinforce the transom better if its flexing that bad... a board that covers the majority of the inside (as far down to the floor as possible) is a good start. Those brackets are a good idea too.

captain planet
02-07-2013, 05:30 PM
The horizontal fin on the lower unit should be level with the bottom of the skiff. Oh, and you may want to add a stiffener piece of wood on the transom now before the aluminum starts cracking under the stress of the 9.9. Don't ask me how I know this. 😄

Thanks for that, I got a chuckle out of it.