Height of cavitation plate Mariner 75hp?

[mwmotorsports]

Hi. My question for the day is I have been told by my boat shop that my engines cavitation plate should be 1 to 2 inches above the bottom of the hull for best performance and least drag. Is this true or is there a correct answer on where to position the motor? To give you all the details I have a 18 foot 1985 Sea Raider fiberglass fish and ski with the same year Mariner 75hp 4 cyl. I've been trying out a few different props and my dealer asked me about motor height before I get too crazy buying different sizes to try. I also have a CMC tilt and trim unit between the motor and transom which sets the motor back about 6 inches. Currently the cavitation plate is approx 1 1/2 inches below the bottom of the hull.

He told me they raised the motor to 1 to 2 inches above the bottom of the transom on most outboard boats for best performance. Any thoughts on this before I start throwing wrenches to it? Thanks!

 

[crb478]

Re: Height of cavitation plate Mariner 75hp?

I have always had them one to two inches above the bottom of the boat like he recommends. I would try it out in the water, and I believe that you will notice a improvement right away. I am sure some of our more experienced experts will weigh in here, but I would try it with the motor raised and then try to prop for the desired rpm if needed

 

[j442w30]

Re: Height of cavitation plate Mariner 75hp?

I would highly recommend raising the engine so the cavitation plate is at least an inch or so above the bottom of the boat. I had mine set so the cavitation plate was below the bottom of the boat and I had terrible porpoising problems. After I raised the engine up the porpoise went away and my top speed increased. Before you go trying props out try raising the engine up first, hopefully you don't have to drill any new holes.

 

[mwmotorsports]

Re: Height of cavitation plate Mariner 75hp?

Thanks. I have plenty of adjustment to go up with the motor but only one hole to drop it. When I installed the trim/tilt unit I mounted the motor to it using the same holes as it was to the transom but I always wondered about the height after the install. It just seemed to look a bit low to me.

Any more tips about it, keep it coming. I was gonna just let it be but I feel like the boat can do better than what it is currently. I don't expect drastic results but I just want to try and tweak things to find the best performance for the craft.

Thanks for all the suggestions and info.

 

[kenmyfam]

Re: Height of cavitation plate Mariner 75hp?

1 inch above bottom of hull is a good starting point but make sure the motor is still drawing cooling water or you will be in big trouble real quick.

 

[Texasmark]

Re: Height of cavitation plate Mariner 75hp?

Go and dig up archive files from Dhadley and Walleyehed. They talk a lot about setup. Also, according to them, you are headed in the right direction to get the setup right and then clean up whats left with the prop.

And yes, you are way too low even without your setback.

Mark

 

[studlymandingo]

Re: Height of cavitation plate Mariner 75hp?

Having the setback will generally allow you to raise the motor more; on the transom, the A/V plate below the bottom of the boat is too low, with a setback, it's way too low. Move that baby up, you're gonna notice a huge difference in performance! Keep an eye on water pressure and cooling, you want to make sure the outboard is still getting water.​

 

[BillP]

Re: Height of cavitation plate Mariner 75hp?

Your dealer is right for speed and your cavitation plate is a minimun of 2" too low...and probably closer to 2-1/2" too low for best speed. Raise it until the motor starts cavitating in mild turns or when running fast and hard in choppy water. This will happen way before cooling water pickup poses a problem. Drop it a notch when the cavitation is too sensitive.

bp

 

[mwmotorsports]

Re: Height of cavitation plate Mariner 75hp?

Thanks for all the tips! I did a measurement last night to be exact as I can and the cavitation plate is acually about 1/2 below the bottom of the hull. I will raise the motor one hole for now and see what happens to start off with. We are going on a long vacation the end of next week so I will not make any drastic adjustments until I return to home waters. I feel the one notch up will not pose any issues at all. I don't want to go too far and not be able to change it at the lake. We're going into the deep woods of Ontario, Canada and there is no way I could adjust the motor up there once we're there. Even with a friend we couldn't lift it into place to change it, so just the one hole for now.

I appreciate all the responses. Looks like my "boat guy" at the local shop is honest...so far.

 

[mwmotorsports]

Re: Height of cavitation plate Mariner 75hp?

I had some free time this evening that I didn't expect so I went ahead and moved the motor so it came out 1 inch above the bottom of the hull. Grabbed the wife and took it to the river for a few quick laps and all I can say is WOW!!!

The boat picked up 8 mph which was amazing! It had all the standard equipment I usually have in it and an almost full 12 gallon tank. I never imagined I'd get those results. I usually got about 35 to 38mph from the old girl but she did a sweet46+ in calm waters and I now realize I can "prop up" to the next size. Who knows what she'll do then. I ran my spare 13x19 and I have been using a 12 3/4 x 21 which did about 40 before the change. When I get the 21 back from repair( I chipped it last weekend) I expect some great top end speed that I would never have expected from the boat.

I can't thank you guys enough for the tips. I may even be able to go one more hole up without any issues but that will wait til after the vacation at the lake. Even the steering was more responsive and the very strange spray out the back of the boat is gone. Looks and runs like a boat should. Even the wife was excited. I found new life for the old girl and it has sparked a whole new interest in the boat. We're gonna have so much fun next week. I'll keep you informed of any new results after we get back home and play with it some more.

THANK YOU THANK YOU THANK YOU!!!!

 

[studlymandingo]

Re: Height of cavitation plate Mariner 75hp?

Really glad to hear! That's what this forum is for; so many boats are set up incorrectly, and 90+ percent of boat owners don't know about set-up. I've learned tons on this forum, and I continue to tweak my rig all of the time (big fun for me). I've helped a couple of friends with their boats as well. I have one buddy who was going to sell his boat because it just wasn't performing the way he wanted; we moved the outboard up as high as it would go. Same result as you, he picked up close to 10 MPH and the steering load was cut in half; now he's in love with the boat!

I'm guessing with the setback, you can go up another hole; keep us informed.​