motor height questions

[spot_remover]

i just recently repowered my 14X42 aluminum with a 30hp johnson. my boat has a 15" transom and the motor has a 20" shaft. i mounted it with a T-H Marine fixed jackplate. according to my measurements, i should be getting about 5" of lift but in actuality am only getting about 3" due to the angles involved. i dont want to mount it any higher on the transom because of concerns about strength, but can get about 1 1/2" more lift using a spacer between the motor's mounting bracket and the jack plate. as it sits right now, my cavitation plate is about 2" lower than the bottom of the boat.<br /><br />i have yet to water test this new setup. my questions...<br /><br />in your experience, what will be the result of having the motor 2" lower than it should be?<br /><br />do you think i should add the spacer now or wait till after a real world test?<br /><br />should i just buy a new hull with a 20" transom???<br /><br />thanks for the replies. the boat is going out tuesday regardless, i'm just trying to get a feel for what i should expect.<br /><br />mitchell

 

[Silvertip]

Re: motor height questions

2 inches is way to low but you could probably get by on that boat. There is a cheaper way to solve the height problem than buying a new boat. Replace the lower unit with a short shaft version and eliminate the jack plate.

 

[walleyehed]

Re: motor height questions

After about 3" of set-back, the AV plate relation to the bottom of the boat becomes of less importance due to angle of attack of the hull. 3 or 4 " back may require a 1" raise to keep the water-line hitting the Lower unit in the same place as it would be by sitting directly on the transom...<br />That being said, Upinsmoke has a better Idea, to me...

 

[spot_remover]

Re: motor height questions

the new hull comment was a little tongue in cheek...which is kinda' hard to convey on the internet...<br /><br />the jack plate i'm using gives about 4" of set back. walleye, i'm a little confused by your comments. do you mean that because of the 4" of setback i can get by with my motor a little lower or do you mean that i need to raise it even more?<br /><br />what is involved with a long to short conversion? i've seen short to long conversions with an extension housing and longer drive shaft. can i keep my lower unit and just swap out the upper case and drive shaft or would i need a new lower unit? <br /><br />again, thanks!

 

[mnw001]

Re: motor height questions

you may have a long shaft spacer section in you L/U. Used to be able to take it out and change drive shafts..

 

[G DANE]

Re: motor height questions

I havnt seen your setup, but why cant you move holes and get the jack-plate higher up ? Running that motor too low will cost you lots of speed and fuel.

 

[Virago85]

Re: motor height questions

Originally posted by walleyehed:<br /> After about 3" of set-back, the AV plate relation to the bottom of the boat becomes of less importance due to angle of attack of the hull. 3 or 4 " back may require a 1" raise to keep the water-line hitting the Lower unit in the same place as it would be by sitting directly on the transom...<br />That being said, Upinsmoke has a better Idea, to me...

Let me start off by saying i Im not working with a johnson but thought it was relovent to the conversation I have a 89 renken inboard that I converted to a 175 mercury outboard after doing all my measurments I was ready for glass well after I did the glass I could only get my motor down low enough to have the cavation plate about 1.6 inches above hull well the problem Im having is at about 3500 rpm the prop looses bite I sent them off to have them double cupped and was gona get a jack plate with a 6" set back and 6" lift to lower motor are you saying that once I set the motor back I may not need to lower it man That would be great

 

[walleyehed]

Re: motor height questions

Yes, when you set the engine back, it usually is slightly deeper under the surface.<br />The angle of attack of the hull bottom is positive to the water's surface, so in most cases, the farther back the engine sets, the higher you can mount it.

 

[spot_remover]

Re: motor height questions

i ran the boat yesterday. after sorting out a couple of problems and attempting to make a fishing trip out of the day, i've realized that the motor needs to come up a couple inches. i'm kicking up a bunch of water and i can't trim it out. i also burned way too much gas for the little bitof running i did. i called my outboard mech and he said, even using some used parts, it would not be economical to convert from long to short shaft. in light of that, i've decided to convert my transom instead, retaining the jack plate for strength. that way, i will have some bolts in the original transom. if i install the motor directly to the transom then all the weight will be on the extension. here is a pic of my current, unsatisfactory set up. the chunk of wood under the mount is 1". by my best estimate, i need to raise the top of the jackplate mounting flange 3" higher. the mounting holes in the plate are 1" apart, center to center.<br /><br />thanks for the replies and feel free to offer transom advice...

 

[dajohnson53]

Re: motor height questions

Looking at your photo, I'd do what I think you're saying you'd do - modify the transom to make it taller. I've seen that a lot around here with boats that are running outboard jets. A good welding /fabrication shop, one that has experience with boat mods and repairs, should be able to tell you exactly what needs to be done. Good luck and have fun.

 

[Strauser24]

Re: motor height questions

I am totally confused by this post. I like to come home from work at night and read and learn some. i just put a new motor on my boat and took it out for the first time today. other that a few problems i need to try to fix it ran good. the motor is a 1984 115 johnson, and the boat is a 1991 stratos 266 f/s. does the height really make a difference in the speed? i opened it up and was doing about 35 or so at 5600-5800 rpm's.

 

[Strauser24]

Re: motor height questions

if i took a picture could you tell if the motor was too high or too low?

 

[walleyehed]

Re: motor height questions

Picture won't really help...Only testing with several props and several mounting conditions will tell you which way to go.<br />I went from 36-37 to 47-48 by playing with set-back and engine height. At the same time, I was trying many different props at each particular mounting location. It's all in what you want out of your rig...I could pull back to about 35MPH and burn about 40% less fuel than my original set-up at the same speed.<br />Dhadley walked me through every step of it, and I found out just how much difference it makes to get some lower unit out of the water(less drag-more speed) and quality props are the only way to go when shooting for optimum performance, and fuel economy/speed is a HUGE part of that.<br />I must also add that I was able to turn about 250RPM MORE with 2" more pitch by playing the game.