Outboard Cavitation Plate Location...???

[GT1000000]

Hello Iboats,
Been here a bit but this is my first post in this section of the forum...
Decided to post this question here to try and get few different points of view...

Got a question as to where would be the best location for the cavitation plate of my outboard in relation to the particular design of the rear end of my hull...


OK, still not quite to this point yet, but soon...
I understand the cavitation plate of the outboard motor is usually in line with the bottom of the hull...
On my tub, the transom is built with a kick out or set back as illustrated below...
Which of the two lines, RED or GREEN would be the correct height placement for the cav plate on my set up...?


I have already received a couple of responses in my usual hang out at the restoration forum...
The basic consensus is that I will have to experiment to get the best placement...
I understand there is some adjustment available from the mounting bracket of my O/B and that I could also spend boo-koo bucks and install a jack plate...
My thing is, I would sure like, if possible, to eliminate both of the above and stick this thing in the right place, the first time...
Is that even possible or am I stuck with the trial and error method?

Thanks!
GT1M

 

[tpenfield]

Re: Outboard Cavitation Plate Location...???

Actually, you are referring to the ventilation plate. Looks to be set up for a long shaft 20" transom. I'm sure you will get some varying inputs could be set up for a 20 or a 25" based on your diagram

 

[jigngrub]

Re: Outboard Cavitation Plate Location...???

Have a look at page 3:

http://media.channelblade.com/boat_...s/dts/dealerlibraries/8441/copetipstricks.pdf

 

[Scott Danforth]

Re: Outboard Cavitation Plate Location...???

it is possible to nail it the first time ....... with a jack plate.

my experience is that the "optimum" location changes with load in the boat, hence the jack plate.

the stepped hull pocket allows the motor to sit a bit higher, however the jack plate is usually required. FWIW, I found this on Engine height with stepped hull at transom - before I screw things up (with photos) - The Hull Truth - Boating and Fishing Forum

 

[Bob_VT]

Re: Outboard Cavitation Plate Location...???

Before you spend the $$$$ on a jack plate I would strongly recommend that you contact Bob's Machine Shop and ask about the rigging The shop does performance work.

 

[GT1000000]

Re: Outboard Cavitation Plate Location...???

Alrighty then...

That just about covers it...

Thanks for all the links...

Been reading through all of them and slowly but surely getting a fair understanding of this...

So far I have figured I should contact a marine engineer, a spiritualist, a santero, a magician, a scientist, a witch, a hindu guru, a vulcan, a klingon, a witch doctor, a fairy, a rabbi and a priest...have them all get into a circle around the boat while holding hands, do some chanting, make a couple of sacrifices, do a voodoo dance, speak some incantations, throw some magic pixie dust at it, do some theoretical mathematics and perform an exorcism...

That should just about get it set in the right spot...

All kidding aside, I do have a much better understanding of what to do and look for...
I think from all the info presented, I am going to set it up where the anti vent plate is just above the red line, give or take an inch or two, and leave all the adjustments available on the motor mounting bracket to allow the motor to be raised up from there...



If after getting it up to the maximum setting I still have not reached the sweet spot, I will start saving up for a jack plate...

That all make sense???

Again, thanks guys...
GT1M

PS- Before I do make a final commitment to drill the mounting holes, I have sent an email to Bob's Machine for any advice they might be willing to share...

 

[Chris1956]

Re: Outboard Cavitation Plate Location...???

Gee, The plate in question is an "anti-ventilation" plate. It is designed to help stop air from being drawn into the prop. At cruise, the A-V plate should ride just at the surface or slightly above the surface on normal boats. Hi performance boats may want it to ride higher, but then could have ventilation issues. Special props can help with ventilation issues.

 

[gm280]

Re: Outboard Cavitation Plate Location...???

GT1M, I would think that you especially would be able to make your own jack plate using some 1/4" or even 1/2" thick 4" angle aluminum. It would only take a few short pieces. But the adjustments would be limitless and you could experiment until you hit that magic number. They really are not that difficult to make... And after seeing your work and abilities, it would be a snap for you to do! Think about it...

 

[GT1000000]

Re: Outboard Cavitation Plate Location...???

Gee, The plate in question is an "anti-ventilation" plate. It is designed to help stop air from being drawn into the prop. At cruise, the A-V plate should ride just at the surface or slightly above the surface on normal boats. Hi performance boats may want it to ride higher, but then could have ventilation issues. Special props can help with ventilation issues.

Thanks Chris, yeah, I have been reading and learning all about this...thanks to all the links provided, it is beginning to make some sense...

GT1M, I would think that you especially would be able to make your own jack plate using some 1/4" or even 1/2" thick 4" angle aluminum. It would only take a few short pieces. But the adjustments would be limitless and you could experiment until you hit that magic number. They really are not that difficult to make... And after seeing your work and abilities, it would be a snap for you to do! Think about it...

Thanks GM, already been brewing that idea in my noggin...might even give it a whirl from the get go...

 

[Scott Danforth]

Re: Outboard Cavitation Plate Location...???

with a manual jack plate at $150 why even attempt to make one. if you watch ebay and CL you can find hydraulic units used in the $300 range.

 

[GT1000000]

Re: Outboard Cavitation Plate Location...???

with a manual jack plate at $150 why even attempt to make one. if you watch ebay and CL you can find hydraulic units used in the $300 range.

That is a very good point, sir...
The search for the materials, time involved fabricating, etc...could easily cost that and more...

 

[Texasmark]

Re: Outboard Cavitation Plate Location...???

With a setback transom like you are showing, the water is higher with respect to the top of the transom where the propeller is located.

Think about it. Most boats plane with a bow high attitude meaning the stern, aka rear of the hull is the deepest in the water. Once the boat passes, the water returns to a level surface of sorts meaning it rises back up where it was with prop wash turbulence present and all. Setback allows the prop to be back from the hull where the water has less turbulence allowing for better grip and higher speeds.

With setback, the "board" on which the engine is mounted, aka transom, is behind the end of the hull that pushed the water aside and to prevent the engine from being too deep you have to raise it. Years ago on here there was a number they used that specified how much higher you had to raise the engine vs how much setback you had.....like for every 3 inches back from the rear of the hull, the engine had to be raised 1 inch sort of thing. Bass Boats usually incorporate setback to assist in pad riding and their ability to obtain minimum hull wetting (drag) and high speeds. Jack plates are usually used to find the sweet spot....correct trim and engine height to max out the mph.

Rake in the prop provides bow lift. If you ever run a high rake prop you will see just what bow lift is....but it takes the right kind of hull for it to work. A deep V cruiser doesn't respond to the benefit of high rake props.

Cupping of the trailing edge of the prop helps the prop to grab the water and not blow out in rough water or high speed turns. Usually the higher pitched SS props have high rake and cupping as they are usually used on high performance, high hp rigs like bass boats. Some include porting to assist in a reasonable hole shot.

HTH,
Mark