Engine mounting height question

[rbiederwolf]

What is a good rule of thumb on where to mount enging heightwise?

Just had center tube added to boat. When they remounted the engine the mechanic said on most all pontoons the cavitation plate should be about 1 inch below the bottom of the tubes. Originally mine was mounted in the second hole from the top and he changed it to the top hole to lower the engine about 1 inch.

However, the new center toon projects about 1.5 feet farther out the rear from the boat than the other. And because there is a slight downward angle when the boat sits in the water (rear of boat heavier than front) The engine seems to sit considerably lower in the water. My biggest concern is that when I unload the boat from the trailer with the engine down, the water line while unloading (boat at ramp angle) is up on the cowling (cover,, right word??) It is still below the piss hole.

When in the water sitting flat it is not an issue. I boat in salt water and am concerned about the water getting in the engine??

Is this a real problem or not. I think if I unload the boat with the engine tilted up might make a difference.

I hope I explained this correctly

[BoatCop]

Rule of thumb is the cav plate should be at the level of the hull bottom. Any higher and you'll ventilate the prop on plane. Any lower and you risk getting water reversion at slow speeds.

If you're worried about it being too low due to the increased distance, you'll have to get in the habit of trimming it up when launching or retrieving.

I always have the engine trimmed up when launching on our patrol boats. I usually trim it so it's sitting level with the water surface and then trim it down once free of the trailer. I got into this habit when we had boats that the skeg was lower than the trailer and would drag on the ground if not trimmed up.

[Bryden24shp]

The cavitiation plate needs to gliding on top of the waters surface. Only way to have some one run the boat while you look over the stern to se where the cavition plate is riding. Unless you have cruise control and a few bungee cords to wrap around the steering wheel, but thats for going to the cooler, not engine height testing....

[rbiederwolf]

The cavitiation plate needs to gliding on top of the waters surface. Only way to have some one run the boat while you look over the stern to se where the cavition plate is riding. Unless you have cruise control and a few bungee cords to wrap around the steering wheel, but thats for going to the cooler, not engine height testing....

If I understand correctly when I am running with the engine trimmed properly, I should be able to see the cavitation plate "surfing or skimming" right at water surface level? I am visualizing the water being pushed or flowing just under the cavitation plate at planed speed - correct?.

I will see tomorrow. Currently I know engine is set so that cav. plate sits about 1 inch below bottom of center tube,when fully trimmed down

[Bryden24shp]

[If I understand correctly when I am running with the engine trimmed properly, I should be able to see the cavitation plate "surfing or skimming" right at water surface level? I am visualizing the water being pushed or flowing just under the cavitation plate at planed speed - correct?.

I will see tomorrow. Currently I know engine is set so that cav. plate sits about 1 inch below bottom of center tube,when fully trimmed down

Yep! You are right! Here is a picture of mine, set up correctly... This is with the outboard in the neutral trim position. The cavitation plate is just above the prop. I posted a video here awhile back, not sure of which post it was under, showing the difference with it in and out of the water.

[ronb]

oh jeez, if that's true, I am hanging about 4-5" way too low...

IMG-20120630-01641.jpg (38.73 KiB) Viewed 22687 times

The cavitation plate is the one right above the prop, not the second one above that, either way, it appears that I need to move it all up a few notches.
Here is a side shot..

IMG-20120531-01167.jpg (65.68 KiB) Viewed 22687 times

is there a defined procedure? Like do I level the pontoon on the trailer (using a level on the deck) and then bring the trim up until the cavitation plate is also level and then measure the difference between the cavitation plate and the transom floor and then raise it that much?



-ron

[rbiederwolf]

Rule of thumb is the cav plate should be at the level of the hull bottom. Any higher and you'll ventilate the prop on plane. Any lower and you risk getting water reversion at slow speeds.

When you say hull bottom. Do you mean the very bottom of the pontoons?? Can't tell exactly where mine is as it is in the water right now. However I think it sits lower. At slow speeds I have a loud vibration/cavitation noise. It goes away when I begin to push the boat, and gone at top speeds.

The cavitiation plate needs to gliding on top of the waters surface. Only way to have some one run the boat while you look over the stern to se where the cavition plate is riding. Unless you have cruise control and a few bungee cords to wrap around the steering wheel, but thats for going to the cooler, not engine height testing....

At cruising speed , trimmed, I can see the top of a bolt that is on top of the cavitation plate, but not the plate itself??

[Bryden24shp]

Raise her up until the whole cavitation plate is out of the water. You'll notice the reduced drag as you pull it up on the transom.

[BoatCop]

Rule of thumb is the cav plate should be at the level of the hull bottom. Any higher and you'll ventilate the prop on plane. Any lower and you risk getting water reversion at slow speeds.

When you say hull bottom. Do you mean the very bottom of the pontoons?? Can't tell exactly where mine is as it is in the water right now. However I think it sits lower. At slow speeds I have a loud vibration/cavitation noise. It goes away when I begin to push the boat, and gone at top speeds.

The cavitiation plate needs to gliding on top of the waters surface. Only way to have some one run the boat while you look over the stern to se where the cavition plate is riding. Unless you have cruise control and a few bungee cords to wrap around the steering wheel, but thats for going to the cooler, not engine height testing....

At cruising speed , trimmed, I can see the top of a bolt that is on top of the cavitation plate, but not the plate itself??

It should be at the level of the bottom of the hull piece directly in front of it. That piece that the engine is mounted to. (I don't know the proper name.)

NOT the level of the outer pontoons. On a tri-toon, it should be at the level of the bottom of the center pontoon.

You should be able to see the cav plate sliding on the water surface. Not above it or below it

[rbiederwolf]

Boatcop and Bryden24shp =Much appreciation.

Thanks, this site is the best. Where else can you get info like this. Tomorrow I will pull the boat and adjust accordingly. If I had to guess I am at least 1 to 2 inches low. Since I have a tritoon I will raise the engine so thst the cav, plate is equal to the bottom of the center tube.

DO you think this will increase my top speed at all????? Hopefully the vibration will go away

Regards,

Rick

[rancherlee]

I've found on traditional 2 tube toons that you want the motor as low as it will go. Raising it up makes little difference in drag as the transom/motor pod creates 90% of the drag by crashing into the wake between the tubes. Triple tubes (with built in transom) setup just like normal boat, cavitation plate level or 1" higher than the lowest point of the transom. Older 2.5 tube toons are a different breed, it really depends how close the motor pod is to the end of the center tube.

[rancherlee]

Boatcop and Bryden24shp =Much appreciation.

Thanks, this site is the best. Where else can you get info like this. Tomorrow I will pull the boat and adjust accordingly. If I had to guess I am at least 1 to 2 inches low. Since I have a tritoon I will raise the engine so thst the cav, plate is equal to the bottom of the center tube.

DO you think this will increase my top speed at all????? Hopefully the vibration will go away

Regards,

Rick

If it is 1-2" low I could see you picking up another 1mph. The issue I had going to 3 tubes is the wakes between the tubes get forced up high and catch the cross memebers slowing me up some (still gained 5+ mph on this old toon). I just finished my rebuild and am currently under skinning the back half to keep the wakes from catching the cross memebers.

[ronb]

I read this article shortly after I joined here, it seemed to cover what I needed.

This link was provided by lakebilly
Propping correctly & how to test props.....
http://www.veradoclub.com/smf/index.php?topic=137.0

Now if only they would come to my house and help me raise the motor...
-ron

[Bryden24shp]

ronb... Exactly! But the twin tubes are different, looks like you are set about right. I don't thing you will gain much, but for as easy as it is to raise it, could sure try.
rbiederwolf... I gained about 4 mph, biggest difference was the ride when re-entering the water, after jumping waves. With it down you would almost go through the winshield it drug so bad. Now it re-enters smooth. Also, fuel economy on my SmartCraft went up by about a half a MPG. As far as the vibration... I am not sure. All you can do is try it!