Exactly how high should I mount my outboard

Jimbeau

Active member
Joined
Aug 12, 2022
Messages
43
Reaction score
1
Location
Memphis, TN
I’m in the process of mounting my new engine and someone posted on another thread that I might have this engine too low. I never thought about the height, figured it’s a short shaft and one size fits all. Can someone give me advice as to the proper height and how I should go about adjusting that if needed. Thanks!

P.S. The trailer bunks actually extend out beyond the transom. I just had the boat scooted aft during the picture. Thanks again
 

Attachments

  • 4F45D303-7312-4AC5-82B8-DFFDF812417A.png
    4F45D303-7312-4AC5-82B8-DFFDF812417A.png
    5.5 MB · Views: 105

Jimbeau

Active member
Joined
Aug 12, 2022
Messages
43
Reaction score
1
Location
Memphis, TN
I see now there’s a ton of information available online about engine height. Had no idea it’s so important to obtain the correct height. The problem with this 20hp motor is it’s not exactly adjustable. I hate to drill my new transom unless it’s where I need it set. What’s the best way to go about dialing this in without drilling unnecessarily holes? Thanks.
 

Pappy

Well-known member
TinBoats Supporter
Joined
May 15, 2010
Messages
3,362
Reaction score
26
Location
Central Florida
You can use wood on the top of the transom if your engine has clamp screw type mounting.
A bit premature to tell you how far up you need to go though. Would be nice to see how far under the bottom the ventilation plate is for starters. In order to get that right you must make the vent plate parallel with the bottom of the boat and then run a straight edge off the bottom back to the engine or from the bottom of the vent plate forward to the transom and measure. Longer the straight edge the more true your measurement will be.
 

airshot

Well-known member
Joined
Feb 18, 2009
Messages
237
Reaction score
67
One simple method is to take a straight edge up against the bottom of your boat, slid it toward the rear until you get to the cav plate on your motor. That cav plate should be same height as the bottom of your boat. A half inch one way or the other wont make a big difference. Generally the cav plate should also be parallel to the bottom, so adjust your trim before setting your height. This quick and easy setup will get you very close. The only other issue I can see is the weight of that big 4 stroke...let us know what you figure out....
 

airshot

Well-known member
Joined
Feb 18, 2009
Messages
237
Reaction score
67
Sorry Pappy, we must have typed and been thinking at the same time !!!
 

DaleH

Well-known member
TinBoats Supporter
Joined
Dec 12, 2014
Messages
3,352
Reaction score
73
Location
Eastern Mass
I find having the vent plate up to 1” higher than the bottom of the hull to be ideal!
 

Jimbeau

Active member
Joined
Aug 12, 2022
Messages
43
Reaction score
1
Location
Memphis, TN
Thanks for all the info and tips. In regards to the weight, the boat is rated for a maximum of a 20HP so hopefully the weight won’t be too much of an issue. Guessing motors weigh more these days though. On that thought, how would I know if it weighs too much? Would sit too low in the back, or have problems getting trimmed out at high speed?
 

Jimbeau

Active member
Joined
Aug 12, 2022
Messages
43
Reaction score
1
Location
Memphis, TN
Pretty good news I’d say. The motor isn’t too far off. Here’s a pic of a straight piece of 3/4” tubing held against the bottom of the boat. Also one of the the tubing laid across the anti-cavitation plate. It’s hard to get the motor’s tilt perfectly square for a perfect measurement, but looks like the motor is only sitting about 3/4” - 1” too low. I should be able to take care of that (if needed) by welding piece of flat aluminum bar across the top of the transom.
Sorry for the sideways pics.
 

Attachments

  • 47717AAA-C314-40FF-B404-FA7033473261.jpeg
    47717AAA-C314-40FF-B404-FA7033473261.jpeg
    2.5 MB · Views: 84
  • B02265F6-8F60-4D16-A590-FD5B770A1274.jpeg
    B02265F6-8F60-4D16-A590-FD5B770A1274.jpeg
    3.4 MB · Views: 83

FuzzyGrub

Well-known member
Joined
Jun 7, 2010
Messages
709
Reaction score
37
Location
Rural NY
I have seen mfg installation manuals that state 1" below, so I would try that and see how it performs for you. Then experiement by raising it as Pappy suggested. Just a block of wood below the motor, say 3/4" - 1" thick, as long as the clamps have good bite on the transom.

Test with load and distribution that you plan to use most of the time. Raising the motor will mainly benefit top-speed, but might also cause some cavitation issues.
 

Jimbeau

Active member
Joined
Aug 12, 2022
Messages
43
Reaction score
1
Location
Memphis, TN
FuzzyGrub said:
I have seen mfg installation manuals that state 1" below, so I would try that and see how it performs for you. Then experiement by raising it as Pappy suggested. Just a block of wood below the motor, say 3/4" - 1" thick, as long as the clamps have good bite on the transom.

Test with load and distribution that you plan to use most of the time. Raising the motor will mainly benefit top-speed, but might also cause some cavitation issues.


I’m Looking forward to doing exactly this! My dealer’s director of maintenance however strongly suggested that I not attempt to raise it for test runs unless the transom is properly altered. His concern is that I’ll damage the transom and loose the motor. Keep in mind that he hasn’t seen the boat, his advice was on a phone call only. My transom has been completely reworked and is extremely solid.
I purchased a brand new motor for it to perform, so I definitely want it rigged out right! Once I do the test runs I can have my welder tack on a flat plate to shim the difference. Then I can bolt her down and enjoy.
 

JL8Jeff

Well-known member
Joined
Oct 26, 2013
Messages
949
Reaction score
25
Location
Ewing, NJ
My motor sits around 2-1/2 to 3" above the transom to get the jet tucked in at the right height in the tunnel (and it's a short shaft jet) but it's bolted to the transom. I haven't had any problems in the 7 or 8 years I've had it. Does your motor mounting plate have holes to bolt it on or is it just a clamp on setup?
 

Attachments

  • lowe_trimmed1.jpg
    lowe_trimmed1.jpg
    442.1 KB · Views: 60

Jimbeau

Active member
Joined
Aug 12, 2022
Messages
43
Reaction score
1
Location
Memphis, TN
JL8Jeff said:
Does your motor mounting plate have holes to bolt it on or is it just a clamp on setup?

It has both. Typical clamps, plus bolt holes. It’s “required” to be bolted in addition to the clamps. According to my dealer it’s due to safety, and warranty concerns.
 

FuzzyGrub

Well-known member
Joined
Jun 7, 2010
Messages
709
Reaction score
37
Location
Rural NY
I think the dealer is being over cautious. Check what the mfg installation manual says. I have only used the clamps on outboards 40hp and less. Bolt through is definitely more secure, just want to make sure you are happy with performance before doing so.
 

Pappy

Well-known member
TinBoats Supporter
Joined
May 15, 2010
Messages
3,362
Reaction score
26
Location
Central Florida
Very true on safety.
Until you establish your optimum mount height you would be wise to avoid sharp corners with a higher throttle angle. That is the condition where engines twist (sideways) up and off the transom. Tie a line around the midsection through the gap between the exhaust housing and steering tube then secure it to the boat. Make sure the line is longer than you think it should be. If you are really serious about optimizing you will eventually get into propeller modifications. Make this decision now before you drill your transom as modified props allow you to run higher than production props will. Some aftermarket propellers have this capability already. Start doing your research into what afermarket prop roughly corresponds to your production prop RPM wise.
 

Jimbeau

Active member
Joined
Aug 12, 2022
Messages
43
Reaction score
1
Location
Memphis, TN
Excellent points. Thank you. I made a 3/4’ wooden shim to raise the motor up to proper anti-cav height. I’m starting to become concerned about the motor coming off during the testing though, with it raised up on the transom that is. I’ll definitely rope it down well but still, the thought of it raising up is scary. I saw the video with a guy turning, which caused the engine to come off. Definitely will be careful if I actually to this.
Thanks for the tip about props! I certainly didn’t think about that. Yes, I’d be interested in the performance but have no idea about prop options.
With all this in mind, would it be best to have some sort of a jack pad that would allow me to make safe height adjustments during testing, prop changes, etc? Even if it’s a simple manual adjustment type with bolts. That would be better than incorrectly drilling my transom. I know they make jack pads for larger boats, but what about smaller applications that could work for a 20HP?

Thanks again.
 

Pappy

Well-known member
TinBoats Supporter
Joined
May 15, 2010
Messages
3,362
Reaction score
26
Location
Central Florida
A jack plate is a good way to do what you want to do. Manual plates, however will place your engine back away from the transom and your tiller arm and throttle move back as well. Sit in the boat and grip your tiller arm back around 6" from where you normally grab the throttle and see if that will work.
Shift moves back as well, so......
 

Jimbeau

Active member
Joined
Aug 12, 2022
Messages
43
Reaction score
1
Location
Memphis, TN
Actually, moving the motor back a little would be a welcomed improvement in regards to comfort. I noticed some plates only move the motor back 4 inches but that might not be the proper type for my boat. Is there any other benefit, or consequences involved in moving the motor aft on a 14’ Jon? Surly it affects performance in some way.
Any suggestions for a Jack plate that would work well for this setup? I’m definitely getting into unfamiliar territory here. I’ve always had a boat of some sort but never one that was setup properly. Certainly nothing with a Jack pad!
 

Jimbeau

Active member
Joined
Aug 12, 2022
Messages
43
Reaction score
1
Location
Memphis, TN
Well, I took it to the lake to knock out two hours of the break in procedure. The motor worked well except for the top end. Sounds like it’s hitting the rev limiter at about the last 1/8” - 1/4 “ of throttle twist. Doesn’t seem to make any difference when the tilt angle is adjusted either way.
Does anyone know why a new motor would do this? Seems like it might be a prop pitch issue allowing the motor to hit the limit. I figured the manufacturer would put a more conservative prop on so that this couldn’t happen?
 

Zum

Well-known member
Joined
Apr 13, 2008
Messages
3,669
Reaction score
0
Location
Nova Scotia
Without a tach cant say for sure if its a prop issue...in saying that it very well could be. They make cheap induction tachs that could help you dial that in. Maybe I missed it but put the boat size shape and prop pitch you currently have and someone may have a similar setup to help. For me, its better to hit the limiter then lugging a motor...add another person or some more weight and you might have the right prop...also different tilt postions, where the weight is,how high, etc...can also effect that.
 
Top