Outboard mounting height 1985 Johnson 115 v4

[scottcab]

Hello all, first time poster, so please bear with me.

I recently purchased a 1985 115hp johnson with the 25" inch shaft. I just mounted it on my 1992 concept cc that is 17'6" long. The original motor was a 70hp johnson with a 20" shaft. From what I have read on the forums, I believe that I want the cav plate even or 1inch above the bottom of the boat. Unfortunately because of the differance in shaft size, using the bottom holes the cav plate is 2 inches below the bottom of the boat. I understand that the motor being low will create more motor drag and thus decrease top speed.

My question is can anyone estimate how much my top speed is being affected? The top speed as it sits is 35mph. Also I have noticed that it doesnt "jump on plane", I know that this is not to strange, but would the motor height affect this and if so how much? My last question is,when it gets planned off and is running, it feels good, but is throwing up a serious amount of water. Rooster tail ish water. The water comes almost straight up and hits the motor cowling. I will attempt to attach a video. Is the motor height the cause of the water show and is it a problem. I would like to get a jack plate eventually and if the results justify it. But if my current setup wont damage the motor I would like to run it as long as possible.

Thanks in advance for yall's response and expertise,
Scott

 

[sutor623]

Re: Outboard mounting height 1985 Johnson 115 v4

Yeah it sounds like your shaft is 5" too long. Have you tried trimming the motor in different positions? Also are you using the proper pitched propeller?

 

[sutor623]

Re: Outboard mounting height 1985 Johnson 115 v4

One other thing, I would try to avoid getting a jack plate with a motor that large, and look for a 20" parts motor that you could swap the midsection and lower unit or driveshaft with. It will probably cost you the same amount of money and would keep your motor from sitting up another 5". You should be fine running the motor how it is in the meantime, just make sure that the water intake is not cavitating (sucking in air and water instead of just water) and you can use what ya got until you find a standard shaft....

 

[jere1972]

Re: Outboard mounting height 1985 Johnson 115 v4

Like Sutor states motor to is too long, boats/transoms designed for 20" engines can not be expected to perform with a 25" engine, I would start looking for another motor entirely, you may find someone with the opposite situation 20" motor on a 25" transom, the anti-ventilation plate should be even with your keel bottom to around 1" above keel bottom, I would not be so concerned about speed loss, but extreme poor handling, she might never plan off either!

 

[scottcab]

Still looking for answers :-D

Still looking for answers :-D

Thank y'all for the responses. Unfortunately, my questions are still in the air. I get that a 20" is what is preferred, but I don't have a 20" or the money to purchase a different motor. The 25" inch is the motor that I have, and it runs very well.
As previously stated. The boats planes off and run 35mph with 500lbs of passengers on board. The handling is good, it responds and is not difficult to operate.

I am still trying to find the answers to my previous questions.

I also, understand that a boat operates best as designed, but I plan to make this motor work with this boat. What I am trying to do is decide whether a braket or jack plate would make this motor work much better on this boat.

Thanks again,
Scott

 

[sutor623]

Re: Still looking for answers :-D

Re: Still looking for answers :-D

Okay well there are two types of people that ask questions here. 1) the kind that wants to hear what they want to hear, and 2) the one that wants realistic answers that stem from experience. The guys on here are not known for their buttering up answers!! Not being a jerk just being honest. You shouldn't have many issues with your setup, it is just less than ideal. The water is spraying up from the extra drag of the lower unit. It will take longer to get on plane with the extra drag. Also, your top speed sounds a little low for that motor and that setup. It is surprising how much top speed is affected by the smallest factors, I.E. slight changes in prop pitch, trim of motor, weight being off kilter in the hull....

 

[sutor623]

Re: Still looking for answers :-D

Re: Still looking for answers :-D

P.S. not sure how much or how well a jack plate will work with a motor of this size.

 

[bob johnson]

Re: Still looking for answers :-D

Re: Still looking for answers :-D

why do some of you think a jack plate is a bad idea? and why it wont work...a 115 is NOT a big motor.....bass boats use jack paltes with 300 hp outboards that weight 800 lbs!!

I have a jack plate on my boat that has 10" of setback and i am running a 115 hp outboard!

right now it is impossible to nail down your problem or prpblems totally....for sure you are dragging more motor in the water than boats are designed to drag...but that is just slowing you down some...but not a ton...

since you didnt mention your rpms. and what you did onece you were going 35 mph to try and trim your motor.. we cant fully answer your questions


take the boat out....get up to speed and play wth the trim until you get the highest rpm and speed combo... tell us what they are

tell us what prop you have....

you can buy a manual jack plate that you set up and leave....for maybe $250 or less for that motor...dont get much set back.. get the least you can...you are only looking to set up the motor you have the best you can.

just buying the jack plate will give you another 3" of adjustment...with out even moving the jack apart for lift! the holes available on the transom side and the holes available on the motor side, plus the holes in the motor bracket should offer a lot of variables alone

bob

 

[sutor623]

Re: Still looking for answers :-D

Re: Still looking for answers :-D

All questions that I asked in the beginning of the thread, that were neglected to be answered.

 

[emdsapmgr]

Re: Still looking for answers :-D

Re: Still looking for answers :-D

Concur with Bob. Get a manually adjusted jack plate as he suggests. That will get the engine up higher. Some of them have little setback. You've got two bad things going on with the engine currently being so deep in the water. 1. You could be 5-10 mph slower with the engine that deep. 2. The drag from the lower unit in the water will create huge water spray off the back of the boat. This drag will reduce your fuel economy by maybe a third? You'll be running engine harder through it's whole life-when the boat is on plane. No need to put that kind of stress on an engine. I'd be careful mouting it above the transom using visual methods. A jackplate with setback will change that mounting measurement. Specifically, the bottom of the antiventilation plate should be just above the water flowing from under the transom when trimmed and on plane. As you noted, up to an inch above the water flow is probably fine.

 

[scottcab]

Re: Outboard mounting height 1985 Johnson 115 v4

Thanks again for the responses. Also Sudor, no need to sound offended or irritated; Information is received differently based on how it is presented and thus interpreted.

I will try to post new info as soon as possible, I am in the hospital with my wife for the next few as she just delivered my daughter. Unfortunately, I don't have a tach on the motor currently so I don't know my rpms. I plan to try to get one next week. As soon as I can get it mounted and get to the
Lake, I will post more numbers. Also, how do I know what pitch my prop is and how do I figure the best prop set up.

In the meantime, if anyone has found themselves in a similar situation, I would be interested to hear what ur results were.

So far I have gathered that elevating the motor so that the cav plat is 1 inch above keel will improve planning speed, top speed, and handling.

On a related note, would adding stabilizer fins/hydro fin like the stingray change my situation. Just from the related physics, I am guessing that a fin would increase planning speed, possible hurt handling, and drastically reduce top end speed by further increasing a already elevated drag. Does that sound about right or am I way off?

 

[SparkieBoat]

Re: Outboard mounting height 1985 Johnson 115 v4

they make jack plates that will work for as big of a outboard as is made...a v-4 is on the small side. so you can get a manual jack plate for under $200 that will have a 400-500LB capacity. your 115 is about 320LB so no problem there. mount it in the bottom hole so it gets the motor as high as possible, then jack it all the way up. (5") you probably would still be 1 or 2 inches low, but it should work..get a set of trim tabs to stop bow rise and porposing. total investment will be about $400. but you could sell the 25" motor and buy a 20" motor. which is what I would do, and I did, I mounted my 20" on the jack plate and it worked great, I was able to get my motor really high up, it reduced drag, improved speed and MPG. put your motor up for sale, when it sales, go buy a 20". I got a 1999 60 degree ois 115 johnson for sale now with a 20" shaft. it would work great on your boat, you would love it way more than what you have. you would probably have to add about $1200 to get a newer model like mine but boy is it worth it. these motrs will save you gas and run out better specially at idle and mid range RPMs. here is one for $150
http://www.jamestowndistributors.co...adwords!6456&keyword=product_ad_9136&type=pla

heres an adjustable for $190
http://www.marineengine.com/products/accessory.php?in=3428889

congrats on the new baby girl.

 

[wilde1j]

Re: Outboard mounting height 1985 Johnson 115 v4

A really simple question: Why did you ever buy a 25" shaft motor???

 

[bob johnson]

Re: Outboard mounting height 1985 Johnson 115 v4

..............?Also, how do I know what pitch my prop is and how do I figure the best prop set up........

almsot all props have their specifics stamped or cast into the prop somewhere...on the side in between the blades on the main dia is one place. Another is on the very end under the washer and nut just around the id on the face of the prop where the exhaust exits.

it will say 13-1/2 X 17, or 13-3/4 X 15, ect...the first number is the dia of the prop, the second number is the PITCH of the prop.

to set up the boat as best as possible you need to have the motor running well to start with, then you need to know the prop pitch, and you need to monitor the RPMs.....it MAY be benficial to monitor the speed as well...but isnt really needed...but nice to know.

you will get up to speed and start to trim up the motor a little at a time and watch the rpms... watch them on flat water at speed for 15 -20 seconds...then make another trim up adjustment...do that for a while making notes of your rpms... and even the speed...

you should do thjis with the cavitation plate at least close to the bottom of the boat.....if you have another person in the boat let the steer while you walk back and see where the cav plate is in relation to the water going under the boat and past the motor...if you can see the cav plate right at the waterline...thats close enough for most boaters...

you will want your rpm max to be about 5500 rpm....when you have your max rpm trim setting .....if you load up the boast with 3-400 more lbs those rpms may drop...which is ok.... you dont want to set the boat up to eve go too much over max rpm...with its lightest load.

if you find yourself going out with a heavy load often you may need to purchase a second prop to install on those days.....one that has a lower Pitch number

bob

 

[wilde1j]

Re: Outboard mounting height 1985 Johnson 115 v4

Motor height is very simple. The top of the AV plate (bracket or jack plate present) should just run dry. This is reached by gradually raising the motor until it happens. Of course motor needs to be propped to run 5800 RPM @ WOT.

 

[scottcab]

Re: Outboard mounting height 1985 Johnson 115 v4

Jim,
I purchased the 25" because it ran great and the price was right. On limited funds, sometimes you have to make things work. My 70hp that was on it was having issues and needed to be traded out. Obviously, if I could have got as good of a price on a 20 inch I would have. Another reason I went with the 25 though was because I was having some cavitation problems (sucking air) with the 20 and figured this would help.

Bob,
Thank you very much for the prop info. I will try to do some trial runs next week. Should the stainless prop off my 1992 70hp evinrude fit on my 1985 johnson 115? If so maybe this will give me another option for trial runs to get multiple data sets.

Thanks again

 

[wilde1j]

Re: Outboard mounting height 1985 Johnson 115 v4

25" leg won't cavitate, but won't perform well either. I would consider modifying the 25" and make it a 20" leg, which is pretty easy. Ditch the 5" spacer, get a replacement drive shaft and shift rod, correct the water tube length, maybe need an exhaust adapter too.. One of the salvage yards listed here would have the parts ... then you'd have a correct motor for your boat's transom.

Years ago I had a neighbor with a deal like your ... his boat wouldn't even plane, even though the motor was plenty big enough. 5" too long is NG.

 

[bob johnson]

Re: Outboard mounting height 1985 Johnson 115 v4

scott, i am pretty sure the prop will fit.. i have a 1988 70 hp and some mid 90s 115 v4s...and my props interchange between the motors.

I bet with that light of a boat you have....you would end up with a 15-17 pitch prop...more likely 17....the SS prop will turn faster rpms than the aluminum of the same pitch most likely....

on your motor i wouldnt go over 5500.....on the 60 degree looper V4s you could run the rpms up to 5800-6000

but I dont think that applies to the crossflow V4s....

get yourself a temperature SENDOR ( not sensor) and a guage and keep track of your motors temps....it can be a motor life saver.

I PERSONALLY think.....that if you are on a budget you could run the motor as is....once you figure the rpma nd prop selection..

but that it would be better to move the motor up....via a manual jackplate....get the adjustable kind to tweak the height....

for a guy who isnt a mechanic type it should be WAY WAY EASIER and about the same money or less than replacing the lower unit parts w to make your motor a standard shaft length

short= 15"
standard or long shaft=20"
extra long shaft= 25"
extra extra long shaft=30"

bob

 

[scottcab]

Re: Outboard mounting height 1985 Johnson 115 v4

The water show. Also, you will note there is some water running up the motor and funneling into the motor well area on the boat. This gets drastically worst/more water as I tilt the motor. I am guessing this is also related to the motor height.

 

[sutor623]

Re: Outboard mounting height 1985 Johnson 115 v4

Motor runs strong, and looks great for an '85. Congrats on the baby, I can see why you dont want to funnel money into the motor