Cavitation plate depth. Does this look right?

[maryhannaj]

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The boat is wot.

 

[roscoe]

Re: Cavitation plate depth. Does this look right?

looks too low to me.

 

[maryhannaj]

Re: Cavitation plate depth. Does this look right?

Thanks roscoe. Im tearing it all apart to do a full restore on it real soon and needed to know how to re mount. I think I will be getting a jackplate too. Thanks for the reply. I had it out on the water for the last time this year today and took a video of it running that im going to try and post as soon as I figure it out. Thanks again!

 

[chriscraft254]

Re: Cavitation plate depth. Does this look right?

Yeah, looks a little low. I'd try raising it if you can before installing or even spending money on a jack plate.

 

[Frank Acampora]

Re: Cavitation plate depth. Does this look right?

The so called "cavitation plate" is really an ANTI-VENTILATION plate. Its purpose is to prevent surface air from being drawn into the low pressure area of the propeller since entrained air will cause loss of thrust and possible cavitation--thus the misnomer.

As an anti ventilation aid, it should ride on the surface of the water or very slightly below it. IF you are not experiencing ventilation when traveling straight, the height is either correct or too low. When set correctly, the engine should have a slight bit of ventilation in tight turns. This is corrected by tucking in the power trim if the engine has it.

 

[roscoe]

Re: Cavitation plate depth. Does this look right?

If you add a jack plate, it will set the engine back further, and the motor can/will need to be raised even more.

 

[Sea Rider]

Does this look right?

Does this look right?

For proper tech answer, data on the following is needed :

Which HP is your engine ? Is it a short or long shaft one ? Is it a mechanical trim & tilt ? Does your engine have clamps or it's fixed & bolted to transom ?

If at wot your AV plate is exposed as in bottom example, AV plate is riding too close to water flow. Need to lower tail 2” for water flow to pass slightly under upper water deflector plate, must have a well ballanced and trimmed combo as in video.- Tohatsu 20hp w/ 3/4" shim and proper trim - YouTube

Happy Boating

 

[maryhannaj]

Re: Does this look right?

Re: Does this look right?

It is a 1982 Johnson 115 on a Hpv165. WOT will actually give me closer to 6000 rpm now since de carb. I only achieve the 6000 rpm through ventilation I believe. If trimmed almost full out, it ventilates and only runs at about 37 mph. If I trim slightly back in, I loose the rooster tail, it sounds like I am digging better, my rpm drop to 5500 and hit max 44 mph. It is hydraulic tilt/trim, steel 12 3/4 21 pitch prop, 4750 feet of elevation and about 60 degrees on average here this last week.
The motor is coming off for a transom replace and a restore. The motor is already mounted pretty high which is why I mentioned a jackplate. Does anyone know a good jackplate that isn't super expensive and what set back would be recommended for this boat? And since I dont have a tree big enough to act as a hoist point, can anyone tell me where to find the specs on how to build a hoist out of lumber from a home improvement store that will hold up to the weight of this motor. 320 lbs. Thanks.

 

[roscoe]

Re: Does this look right?

Re: Does this look right?

It is a 1982 Johnson 115 on a Hpv165. WOT will actually give me closer to 6000 rpm now since de carb. I only achieve the 6000 rpm through ventilation I believe. If trimmed almost full out, it ventilates and only runs at about 37 mph. If I trim slightly back in, I loose the rooster tail, it sounds like I am digging better, my rpm drop to 5500 and hit max 44 mph. It is hydraulic tilt/trim, steel 12 3/4 21 pitch prop, 4750 feet of elevation and about 60 degrees on average here this last week.
The motor is coming off for a transom replace and a restore. The motor is already mounted pretty high which is why I mentioned a jackplate. Does anyone know a good jackplate that isn't super expensive and what set back would be recommended for this boat? And since I dont have a tree big enough to act as a hoist point, can anyone tell me where to find the specs on how to build a hoist out of lumber from a home improvement store that will hold up to the weight of this motor. 320 lbs. Thanks.


View attachment 214200View attachment 214201

Given these new photos, and your description above, I think your motor is mounted at the correct height.

Photo in the first post is hard to see, given the angle of the photo.

I would not add a jack plate.

Sounds like it performs as it should.

With the mounting holes where they are, you can lower the motor if you like.


Rent or buy an engine hoist like this:

1 Ton Engine Motor Hoist Cherry Picker Shop Crane Lift Foldable | eBay


Or build an a-frame, like a giant sawhorse, out of 2x6, and use a small chain hoist like this.


New 2000lb 1 Ton Chain Hoist Winch Engine Lift System Rigging Puller Block Fall | eBay

 

[maryhannaj]

Re: Cavitation plate depth. Does this look right?

Thank you very much. It just seems like I am wasting rpm. Would re propping help gain more top end and keep the extra rpm?

 

[roscoe]

Re: Cavitation plate depth. Does this look right?

5500 rpm and 44 mph is pretty good.

Not sure what that hull is capable of.

Sure you could re-prop to a 19p or 17 p and get your rpm up.

But I doubt you would see any appreciable speed increase.

You'd be turning more rpm, burning more gas, but spinning a shorter pitch prop.

The only reason I'd re-prop, is if your holeshot was poor.

 

[maryhannaj]

Re: Cavitation plate depth. Does this look right?

Thanks again.

 

[maryhannaj]

Re: Cavitation plate depth. Does this look right?

 

[Chris1956]

Re: Cavitation plate depth. Does this look right?

Roscoe, Due to the proximity to halloween, you should be recommending that Mary build a Hangman's scaffold. She can lift/lower the motor with it. When the end of the month comes, she can toss a couple of hangman's nooses over it. labeled with names of her favorite people. It can be a big hit with the neighbors, and your kids...

 

[maryhannaj]

Re: Cavitation plate depth. Does this look right?

Yeah, that is a great idea. Maybe he( but you would'nt know that with a screen name like mine) could do just that. Only in my neighborhood someone may steal it. Or maybe I'm just misssing something here?

 

[maryhannaj]

Re: Cavitation plate depth. Does this look right?

Here is a video of a run out on the bay. Maybe? I dont know why it wont just show a video wthout the rest of the B.S.

http://www.youtube.com/watch?v=hZzUk_6q6hM

 

[metriccrescentwrench]

Re: Cavitation plate depth. Does this look right?

IMO it looks a little low. Normally you'd expect the AV plate to skim the surface of the water, any lower and you're getting extra drag.

If you're inclined to add a jack plate I'd go for it. You can find used manual ones on ebay for not all that much. I'm thinking a 4, 5, or 6 inch setback would be good. The theory with adding setback (be it a jackplate, gil bracket, or whatever) is that the prop will be running in 'cleaner' water and therefore be able to operate more efficiently (moving the prop back works for pusher airplanes, same general theory as for a boat). And naturally with a jackplate you'll be able to very easily experiment with engine height 'till you find what works for your boat.

ONE thing to watch out for when adding a jackplate is going to be steering cable routing. Depending on how yours is installed and how long it is you may or may not have a pain in the rear getting it installed.

 

[maryhannaj]

Re: Cavitation plate depth. Does this look right?

Thanks for the info metriccrescentwrench!