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MX-15 Motor Height

H

Hidditt

New member
MX-15 70 Hp Johnson

Not meant for top end - just touring about at 3/4 speed - 25 to 30 MPH. Maybe some skiing.

Motor seems to sit high. Cavitation plate about 1/2" above bottom of the Vee on the transom. Will this cause cavitation? Motor Won't go lower - bracket hits the top of the transom motor mounting. Prop is standard 3 blate aluminum.

Should I notch the transom to get the cavitation plate in the water or give it a try first?
 
Give it a try first! even on none performance boats you set the cav plate even or just above the bottom of the boat. If you get too much water running over the top of the plate it will cause water to spray up from hitting the front of the center section.
 
Hidditt said:
MX-15 70 Hp Johnson

Not meant for top end - just touring about at 3/4 speed - 25 to 30 MPH. Maybe some skiing.

Motor seems to sit high. Cavitation plate about 1/2" above bottom of the Vee on the transom. Will this cause cavitation? Motor Won't go lower - bracket hits the top of the transom motor mounting. Prop is standard 3 blate aluminum.

Should I notch the transom to get the cavitation plate in the water or give it a try first?
Click to expand...

Post up a few pics and show us what you got . We can help from there .....
 
Thanks for the suggestions on motor height.

I was able to get the engine lowered a 1/4" further with a little persuation. I installed a set of Nauticus SX Smart Trim Tabs as well. The boats runs without cavitation and negligable bow lift and porposing. I can crank the tilt up quite a bit too (manually unfortunately). I am very pleased with the ride. Another MX-15 back in the water.

I will see what I can do about photos. I am having trouble posting an image despite the clear instructions. I did set my defaults as instructed. Hmmm.

The motor is a 1980 Johnson so it is white with orange and grey accents. The boat now has a white top, and orange bottom to match, and grey and white seats with a grey carpet. I used TSC Form Implement Enamel - kind of a silly thing to do, but it worked. Will see how long it lasts.

Speed is around 40 mph wide open, but bow still too low for maximum top end performance - which I can't really afford with the price of gas now anyway. Will play around with the tilt and tab settings to tune it up.

I'm pretty impressed with the trim tabs, which I picked up in a bargain bin a few years back. The boat stays on plane at a really low speed with minimal bow rise. No electrical or hydralic connections either. Should be a good boat for a family that likes to tube, and ski.

Water pours in the sides (joint between top and hull) on takeoff, tight low speed corners, etc. I will seal it up with some good clear silicone marine sealant.
 
Restored MX-15

DropBox
 
Texas MX-15 Restore - Update after a decade!


Ran the 70 Johnson until it developed an electrical issue. Replaced every electrical component, but still couldn't track down the problem and gave up. Speed about 39 MPH when it ran. Loved it when it ran.

Switched to a 1980 Evinrude 60 twin. Speed 36 MPH. Got rid of the trim tabs. Ran the Evinrude until the wiring disintegrated. It was noisy compared to the 70 triple.

The MX-15 got crushed by fallen trees in the May 2022 Derecho wind storm that went through southeastern Ontario. The windshield was destroyed and the top of the boat cracked across the front seats. The seats were wrecked too. We figured that was it for the MX-15.

After clearing the trees off the boat, I discovered the hull bottom was fine. I repaired the two cracks, added a monster plywood boombox in front of the motor well and a gauge radio in an attempt to cut the noise level of the old two stroke.

Its a long story, but I got the 60 Evinrude from my daughter. She had a very tired Grew SS-14 that I had painted Fuchsia for her Wakeboarding Company. It leaked, had a soft transom and floor, and wasn't getting the love it needed. So I made her a deal to paint the Checkmate Fuchsia if she donated her Evinrude 60 and her boat seats. Well, a can of tintable Home Hardware Rust Paint and a roller, and my daughters Cricut vinyl cutting machine turned the Checkmate into the "Barbie Boat" just in time for the 2023 Barbie Movie. Man, that boat turns heads now.

I mentioned the 60 Evinrude gave up the ghost. I replaced it with a brand new all white Yamaha 70. First motor with power trim. Unbelievable motor. Extremely quiet, fuel efficient, and a 44 mph ski boat. Great for evening cruises with the gauge radio playing Spotify jazz playlists. I don't miss the windshield at all. I've picked up new white seats for this year. Seems crazy to put on a high dollar outboard on a $200 Texas barn find, but I'd do it over in a heart beat.
 

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