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Need help mounting 40 hp 20 inch shaft to my transom, transom has been infilled and is level across the transom. My question is how low should the cavitation plate be below the bottom of the boat, is their any publication or diagram to refer to. Boat overheats and wot, but is fine with three people in it.

Very frustrated

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The cavitation plate should be at or above the bottom of the hull.

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Depending on which model engine you have, you probably only have 4 holes in the engine brackets to adjust the height.

The mounting bolt holes are as follows:
1. Bolts in top hole means the engine is mounted all the way down
2. Bolts in second hole from top means engine is mounted one hole up
3. Bolts in third hole from top means engine is mounted two holes up
4. Bolts in bottom holes means the engine is mounted as high as it will go or all the way up.

Each bolt hole is 3/4 of an inch apart so you can raise or lower the engine 3/4 of an inch per bolt hole.

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The transom on that hull "slopes" forward when viewed from top to bottom. the top of the transom is farther aft than the bottom where it meets the hull. That means the water will travel almost a foot from when it leaves the bottom of the hull before it gets to the engine. (I know, the water isn't moving, it's the boat, but you know what I mean) The water will be coming up to meet the engine, so the engine can be mounted with the anticav plate an inch or more above the level of the bottom of the boat. (at static trim). That is where mine is, and it works great.
Mike

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moose,

And what holes are the bolts in????

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Joe,
That's something that didn't stick in my mind. My old brain only has room for a few things. The important matter for me was where the plate wound up. That's where I bolted it and don't know which hole it's in. I can check for you later if it is needed info.
Mike

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My bolts are in the top holes. I wiil send a picture

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bigut,

So that means your engine is mounted all the way down and is probably too low as moose and jeff mentions.

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My 25 4 stroke Merc is mounted on a 6" setback bracket, and the front of the prop is about 24" behind the bottom of the transom. The cavitation plate is 2 1/2" above the bottom, and on plane it rides on the surface.

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You are surely too low. If the transom is level then it is not even at tall as a factory 20" transom (which was slightly crowned) and the motor will need to be raised several holes.

Remember, you do NOT want the cavitaton plate below the hull. You do not even ant it even with the hull, you want it to be ABOVE the bottom of the hull.

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I wish I could post a picture in this thread of how the motor should be mounted. As said in above posts, the cavitation plate should be either level or higher than the bottom of the boat. While on a plane you should be able to clearly see your cavitation plate.

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Look at the pictures in my personal page to see how my motor is mounted now:

http://whalercentral.com/infusions/pe...er_id=8344

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Yeah, go ahead and raise the motor a hole or two.

Again, I have to disagree with Jeff and Cliff. You do NOT want the cavitation plate even with the bottom of the hull. You want it to be ABOVE the bottom of the hull.

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So getting back to your original post, what is it that frustrates you? Are you saying your motor overheats? If so it has nothing to do with how your motor is mounted.

It also appears that your motor is already mounted, so are you asking how or are are you asking about something else.

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I feel the motor lose pressure when I go faster, the pickup is either in the bubbles or out of the water and thats when it heats up.

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Can you give a better description of losing pressure?
Mike

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Yes, Water pressure, the engine will lose hp and then gain it back, like it is losing grip with the water.

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When you say it overheats at WOT, is that by an alarm or a temp gauge?

And does the overheat happen at the same time as the loss in WOT rpm?

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From looking at your PP, it appears that you may want to raise your engine up one hole to start. Your cavitation plate appears to be level or slightly below the bottom of your hull.

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It does on the temp gauge, I bought a surface mount gauge it would read 200 f at the start, when I had the problem it went to 240. I since have purchased a internal sender and the gauge reads at 170 in the driveway, I am going to raise the engine 1 hole and watch the gauge.

Thanks

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Cliff, can you post a picture on your personal page showing your engine position I would appreciate it

Thanks

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bigut,
I just looked at your personal page. I don't think raising it one hole is enough. I would consider 3 holes to start. And if the temp is really going up the engine, being too low shouldn't cause that. At first I thought you might have a spun hub, but that won't cause the temp to rise either. You might need a new inpellor or may have a water flow restriction somewhere. Also, what prop is on there? It looks like it has a lot of pitch.
Mike

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I wouldn't raise the engine at all until you figure out why it is overheating and the engine is revving up and down.

The way your engine is mounted now is not causing your overheat problems.

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I added a picture to my personal page. The level is sitting on top of the cavitation plate and it is about a inch below the hull.

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I agree, I think the two issues are unrelated. As engine load increases, so does internal temp. When was the last time your water pump was changed out and thermostats checked?

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bigut. It looks to me that you have your engine mounted all the way down. If you PM your email address to me, I can send you a picture of what mine looks like later. I think you need to raise your motor at least one hole. This however, will not help you with your cooling problem and will only help in your performance.

Cooling is a whole other issue......take a look at your impeller. I'm replacing mine today after work. It's been about two years since I've put one in so it's time. I try to do it about every two years whether it needs it or not.