Motor height. How much should I raise it.

My 90 ELPTO is mounted on the 2nd hole from the top. I went out and measured today and my AV plate is 1/2 inch below the lowest point on my keel. The motor doesnt have the slotted bolt holes so I need to have a shop raise the motor so I am only trying to pay once. I was thinking I only need to go up 1 hole and that should bring the AV plate to just above the keel. Wondering what you guys think? I have done some reading where people say the AV plate should be 1 inch above the keel and that would mean raising it up 2 holes.

I did the straight edge measurement with the yard stick but forgot my phone so was not able to get a picture.

You can adjust your motor height with you tongue jack! put blocks/wood under your motor raise the jack to take pressure off the bolts, pull out top bolts and loosen the bottom bolts. adjust the height by cranking the jack then re insert bolts at the desired location… play with motor height at the ramp see what works the best with your set up! don’t forget to silicone the bolts when your done!

p.s. don’t forget to unhook the truck and chalk the tires on the trailer!

“The motor doesnt have the slotted bolt holes”

oops didn’t read your post well enough guess that wont work

Guess I should mention. I changed my prop to a 13.25×19 turning point huster and brought the motor up 1 hole. I think I need to go 1 more higher. Still have some porpising issues however and feel like it is keeping me from hitting the 40mph mark

Here are some pics of my boat running wot, 5400 RPM (max is 5500) and 38.5 mph

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It doesn’t look like you need to raise it at all. Your cavitation plate is already on top.

SuperDave1959 wrote:

It doesn’t look like you need to raise it at all. Your cavitation plate is already on top.

X2………

You should only change one thing at a time, so you know what each change does. Positive or negative. My guess is the porpoise problem is the motor is too high already. What size boat are you running that you want to hit 40mph with a 90hp?

To start with, the boat I purchased 65hp 16ft under powered by 10hp. No porpising issues but lagged in top end. I lowered the motor 2 notchess
gained 4mph and a more level ride and this is without ability to trim.
I believe lifting the motor forces the bow down lowerinng the motor lifts the transim. I think you need to lift the transum.
just a boat owner like you so my assumptions may be wrong. My ego says different.

Does a disclaimer satisfy Biggill.

Mudshark wrote:

<div class=”d4p-bbt-quote-title”>SuperDave1959 wrote:</div>
It doesn’t look like you need to raise it at all. Your cavitation plate is already on top.

X2………

it’s not my AV plate (yellow arrow)but rather the splash guard (white arrow) that can be seen in the picture.

mojogunter wrote:

You should only change one thing at a time, so you know what each change does. Positive or negative. My guess is the porpoise problem is the motor is too high already. What size boat are you running that you want to hit 40mph with a 90hp?

It’s an Alumacraft tournament pro 170cs. 16’10 long, 81″ wide, 940# before the outboard and other equipment.

mojogunter wrote:

You should only change one thing at a time, so you know what each change does. Positive or negative. My guess is the porpoise problem is the motor is too high already. What size boat are you running that you want to hit 40mph with a 90hp?

I did change things 1 at a time. I put the Hustler prop on and it did help some with my porpise problem but my AV plate was not visible at WOT on plane. So I raised the motor 1 hole, gained RPM, some speed, but my trim window is definetly smaller.

hl&sinker wrote:

To start with, the boat I purchased 65hp 16ft under powered by 10hp. No porpising issues but lagged in top end. I lowered the motor 2 notchess
gained 4mph and a more level ride and this is without ability to trim.
I believe lifting the motor forces the bow down lowerinng the motor lifts the transim. I think you need to lift the transum.
just a boat owner like you so my assumptions may be wrong. My ego says different.

Does a disclaimer satisfy Biggill.

Interesting. The difficult thing about a porpoise issue is nobody agrees why it happens. I have read so many people claim it’s because the bow isn’t being held up so more bow lift is needed and the opposite of less bow lift more stern lift.

I have been told I have too much weight on the transom. But I only have a series 24 battery and a 90 on there. Boat is rated for a 115. It also seemed as if the other day it was a bit less prone to porpoise when the rear livewell was filled.

The hustler prop is known for rake and has more bow lift than my Michigan wheel vortex. I may toss the vortex on there and see what it does.

So many things I need to try out. Sadly my outboard is not notched so I cant easily raise and lower it myself.

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When you say you have porpoising,
at what speed(s)
and what throttle settings?
and is this during acceleration, deceleration, or constant throttle?
Have you tried adjusting the trim to alleviate the issue?
and if so, what adjustments were made and when?

weight forward helps in hole shot
weight rearward aids in top speed

eyekatcher wrote:

When you say you have porpoising,
at what speed(s) 37-38MPH
and what throttle settings?Close to and at WOT
and is this during acceleration, deceleration, or constant throttle? Acceleration and constant throttle at WOT
Have you tried adjusting the trim to alleviate the issue?Yes and it does help but at a decent loss of speed and also plows the boat
and if so, what adjustments were made and when?

weight forward helps in hole shot
weight rearward aids in top speed

found it here http://boatinfo.no/lib/mercury/manuals/75-90.html#/58

The 2001 manual for 90HP ELPTO states

3. Install the outboard so that the anti-ventilation plate is
in-line with or less than 1 inch below the bottom of the boat.

I have a 2011 90HP 4 stroke on my Lund Impact 1675 and the AV plate level with the bottom of my boat. I have a PowerTech SS prop and at 6000 RPM (the max) I top out at 43 MPH . (There is a performance test results for the prop elsewhere on this site)
My motor has 5 mounting holes and is mounted in the center hole.
There is a note in my manual to keep the exhaust relief hole above the water line to prevent exhaust restriction and poor performance at idle.

P.S. the ELPTO manual also shows two lifting rings under the cowl cover for your motor

I have the manual for the 98 elpto all it says is to make sure the mounting height does not exceed 25 inches

Went out and tested all 3 of my props today. I also brought along another series 27 battery that weighed 55 pounds according to my bathroom scale.

1: 13.25×17 Quicksilver black diamond (mercury black max)

Max Speed: 36.6 MPH @ 5500 RPM ( could easly rev motor to 5850+RPM)

No porpoise issues until AV plate was out of the water. RPM/speed would not matter boat would porpoise if the AV plate was out of the water. Trim motor down porpoise issue would go away but would plow more water than the next 2 props. Add weight to bow and porpoise issue did not change. Add weight to rear of boat and did not experience any porpoising issues until again the AV plate was out of the water.

2. 13×19 Michigan wheel vortex.

Max Speed 38.0 MPH @ 5500 RPM

This prop was easier to obtain max speed and RPM than the Black Diamond prop only because I did not have to worry about over revving the motor. Boat would porpoise with AV plate coming out of the water. Weight up front made it worse. Weight in rear made it better but would still porpoise with AV plate out of the water. Prop would also blow out during turns and have to be trimmed down. When the boat was running in the sweet spot spray was exiting the hull right at the console.

3. 13.25×19 Turning Point Huster.

Max Speed 39.5 @ 5400 RPM

With no weight in the boat I could get the boat to porpoise but was harder to get the boat to porpoise than the other 2 props. Weight up front made the porpoise issue worse. Weight in the back and i could not get the boat to porpoise before blowing the prop out. The prop would blow out with the AV plate out of the water. I could trim the motor up higher it seemed but could not get the AV plate out of the water without getting blow out and speed loss from the prop. This prop would also have spray exiting the hull about 1-2 feet behind the console and forsure got more hull out of the water than any of the other 2 props

Thoughts? To me it seems as if I was more happy with overall performance with the Hustler prop. With the weight in the rear I could not make the boat porpoise unless I was blowing the prop out. I am however curious how a black diamond (black max) would do in a 19 pitch.

Seems to me I need to get some more weight in the rear of the boat. I do have a livewell in the rear I could keep filling. I only have a series 24 cranking battery perhaps I should upgrade to a larger size and maybe 2 of them?

I have someone near me selling a mercury vengance 13×18 stainless prop for a really good deal. Would it be worth it to check that prop out or should I go get a 19p black max?