Can't get up on cav plate

[mcmike]

I just put on a 25hp 1971 long shaft outboard on my 4816 mv jon boat. Great comp. 125-127 rebuilt carb, timing........motor starts and runs great.
I have a 20" transom and the motor is 23" from the bracket to the top of the plate, 9x10 prop. The plate is running 1-2" under the water, what
a drag.(sorry) I was running a 15hp 1975 ( both motors are Johnson?s ) with the same 23" length and the plate would ride up at 3/4 throttle.
I am running faster....not a ton...but I?m dragging the plate. It's a tiller handle and a riser with ANY set-back would be a pain in the neck. really.
I have tried all the pin trim holes, shifted weight around but it' just 2 men and late gear, 450 475lbs MAX. Remember the 15hp was fine!!!
Motor runs full rpm at wot
any and help is VERY WELCOME

 

[ondarvr]

Re: Can't get up on cav plate

Have you tried raising the motor up on the tramsom? Not tilting it, raising it.

 

[Mi duckdown]

Re: Can't get up on cav plate

Pics would help alot.??

 

[mcmike]

Re: Can't get up on cav plate

Raising the motor up 2" ( this would make the plate 1" under bottom ) should work the best, BUT that there was NO PROBLEM with the 1976 15hp of the same lenght is what has me stumped!! A boater i met at the dock said that the 15s slant type power head profile, less of it behind the boat, and lighter over all weight may have made the difference. Another boater said " the 15hp 1975 has the exhaust through the prop and this is like a jet so you get more power" ??????? So........i came here for some real advice
thanks
mcmike

 

[ondarvr]

Re: Can't get up on cav plate

Forget everything those boaters told you.

There is no way the AV plate on the old motor would be above the water and the new one 3" under the water if they are at exactly the same height unless the 25 is set back further from the transom due to the transom bracket having more setback. Even then 3" inches is a big difference.

Did you measure both of them?

The AV plate may be at different heights relative to the prop on each motor, so while the prop is at about the same level, the AV plate may be lower on the 25.

 

[mcmike]

Re: Can't get up on cav plate

both motors are 23 inches from bracket to plate, 100% sure ! i had the 15 for 20+ years and i am also sure the plate skimed on the
top of the water at WOT to 3/4 of WOT. As for the prop being at different heights i don't know. The 75 15hp was a through prop exhaust and the 71 25hp is not.
I no longer have the 15....bummer
My point about the "boaters" was some of the advice was at best laughable. sorry if it did not come across that way in my post.
i wish i had a way to measure the two as in center prop to plate but i don't
thanks
mcmike

 

[ondarvr]

Re: Can't get up on cav plate

I’m not sure what to tell you. If the old motor was the exact same length it means it would have been three inches below the transom too, and there is no way to get an AV plate that is three inches below the transom to ride above the surface. There must be more to the story.

Pics will be needed.

 

[mcmike]

Re: Can't get up on cav plate

OK i let my fishing buddy of 35 years read the post and he said my explaination was poor..not exactly the word he used....so here is his take
With the 15 at WOT if you look down off the back of the boat you can see the plate with little or no water passing over the top. It is NOT really "on top" of the water With the 25 at WOT when you look down you see maybe 2" of water over the plate. The boat does go alittle faster with the 25 Me again; the trim pin on the 15 was in the 2d from the transom. The boat has no set back. Both motors are mounted ON the transom My friend has run his boat tons of times and was with me for both the measuring of both motors and the running. I know this sounds crazzy!!!!! But this is accurate Could this old 25 just be heavier and not as much power as i think a 25 should be. Or maybe to many news yeras cookies i ate lol
hope this makes sense to someone
thanks again
mcmike

 

[Texasmark]

Re: Can't get up on cav plate

OK i let my fishing buddy of 35 years read the post and he said my explaination was poor..not exactly the word he used....so here is his take
With the 15 at WOT if you look down off the back of the boat you can see the plate with little or no water passing over the top. It is NOT really "on top" of the water With the 25 at WOT when you look down you see maybe 2" of water over the plate. The boat does go alittle faster with the 25 Me again; the trim pin on the 15 was in the 2d from the transom. The boat has no set back. Both motors are mounted ON the transom My friend has run his boat tons of times and was with me for both the measuring of both motors and the running. I know this sounds crazzy!!!!! But this is accurate Could this old 25 just be heavier and not as much power as i think a 25 should be. Or maybe to many news yeras cookies i ate lol
hope this makes sense to someone
thanks again
mcmike

You provided the required info when you said what you said about looking over the transom at the water and the plate. The thru prop exhaust is neat and added performance is true (I guess, makes sense) but I wouldn't expect a mph on that sized engine in improvement....other things would swamp it out.

You would be amazed at what a difference it makes to get the engine up and out of the water.....it's like dragging a baseball bat behind the boat (vertically) and a machete.

Get the engine up off the transom 2" at least even if you have to extend the transom with some home made extensions; on that hp engine it shouldn't be rocket science.....however, you need to pay attention to your water pickups. If low on the lower unit, like below the anti vent plate, and a series of holes vertically going down the LU, great....if you have the old style OMC with some p/u above the avp and some on the leading edge of the exhaust snout, then you may need to leave the LU low in the water to get adequate cooling at WOT.....Personally, I think OMC to name one, deliberately built their housings a couple of inches too long to accommodate their archaic water pickup scheme. Look at some "Old OMC" www sites where they show sales brochures of the '60's and 70's and pay attention to the plate location vs the transom.....all are a couple of inches (seemly) below the bottom of the boat.

My 2c.

Mark

 

[mcmike]

Re: Can't get up on cav plate

lskdkdf I would like to lift the motor up 1 ? to 2 inches if it will cause no harm and give me better performance. Naturally I do not to cause any overheating at WOT. I hope these pictures can clear this up. With this type of water pick-up should the cav plat be 1" under transom or should i leave it where it is....3" under transom and and cav plate 2" under water at WOT.
thanks to all
mcmike

 

[ondarvr]

Re: Can't get up on cav plate

You need to back up and take more pics, these don't show enough to help. Take a pic so we can see the AV plate and how it lines up with the transom.

 

[Texasmark]

Re: Can't get up on cav plate

Those pictures show exactly what I was talking about. Go back and look at the sales brochures I told you about for OMC engines with this kind of water pickup.

They (OEM and dealers) aren't about to tell you, but it doesn't take a rocket scientist to figure out that they make the mid section 3" longer than it is supposed to be to accommodate their style of water pick up.

You will probably shoot yourself in the foot if you try to elevate engines with these types of cooling water inlets.....Had you bought Mercury, you wouldn't have the 3" too long mid section and the drag problem you are experiencing.

This water pickup scheme dates back to the 1950's when the biggest engine they produced was 25 hp. Fastest speed was 25-27 mph. It worked for them since the speed was so low that too much lower unit in the water (drag) wasn't known about at the time and at those speeds, you didn't know what you were loosing, and it might have made a mile or two difference.

My comments are based upon conjecture but you PROVE ME WRONG!!!!!!!! Doubt you can do that. Trying to help....OEM's and dealers don't tell you the whole story for their own reasons. I tell it the way I have learned to see it as I don't have to kiss up to anyone/anybody/anything.

Mark

 

[oldman570]

Re: Can't get up on cav plate

View attachment 128811lskdkdfView attachment 128807 I would like to lift the motor up 1 ? to 2 inches if it will cause no harm and give me better performance. Naturally I do not to cause any overheating at WOT. I hope these pictures can clear this up. With this type of water pick-up should the cav plat be 1" under transom or should i leave it where it is....3" under transom and and cav plate 2" under water at WOT.
thanks to all
mcmike

I would leave it under the water because the water pickup holes are above the cav plate. Raising the motor would cause a water flow shortage to cool the motor and cause overheating if rasied to far up. As for speed, rasing the motor will only get you about 1 mile faster at best on that older style motor. Not worth taking the chance of overheating the motor in my neck of the woods.

Best of Boating
Oldman570

 

[Chinewalker]

Re: Can't get up on cav plate

There is no cooling issue with raising the AV plate level a bit. The design is such that the prop force feeds a good stream into the intake at speed, even if the small plate above the AV plate is out of the water. We used to run a '56 Johnson 30hp short shaft on a racing hull at 19-1/2 inches on the transom with the AV plate a full 3-1/2 inches above the bottom of the boat - and it peed like a racehorse. That was as high as we could go before we blew out the prop. If you set yours up with the plate a half inch to an inch below the bottom you'll be fine. That'll keep your prop out of any keel turbulence.

The boat is NOT supposed to ride on the AV plate - what you were seeing is either an illusion or one of your measurements is off. Measurements must be made plane to plane. Being off even a little on the vertical can add or subtract to your motor's perceived transom height a fair bit. If both motors are actually at 23" then both motors would ride with the plate at the same depth, so I suspect something is off on the measurements...

 

[Willyclay]

Re: Can't get up on cav plate

I would leave it under the water because the water pickup holes are above the cav plate. Oldman570

I believe this describes the "by-pass water inlet" which allows water to be drawn into the water pump when the motor is in neutral or reverse gear. The primary water inlet is in the leading edge of the triangular exhaust projection below the aft end of the AVP. Good luck!

 

[Texasmark]

Re: Can't get up on cav plate

I believe this describes the "by-pass water inlet" which allows water to be drawn into the water pump when the motor is in neutral or reverse gear. The primary water inlet is in the leading edge of the triangular exhaust projection below the aft end of the AVP. Good luck!

Yes, that's what it is. So, back to the inlet located on the front of the exhaust deflector. If the pressurized water from the prop forces water into that inlet and it does not bleed back through the bypass inlet, since there is essentially no water coming from there, then I agree with you. Course Chine walker said he ran a vintage engine (era I referred to) 3 1/2" inches out of the water which pretty much says it all. Obviously he knows....I was just applying what I thought to be common sense about the problem the bypass inlet could cause. So with all this said, I still don't understand why OMC ran such long midsections.

Mark

 

[Chinewalker]

Re: Can't get up on cav plate

They made them a little on the long side due to lack of an industry standard in regards to boat builders. Some boat hulls have deeper transoms than others and rather than have to trim transoms, OMC simply made their motors able to fit a broader range of transoms within the standard "short" and "long" ranges. For boat builders, some of that variance may be due to keels, bottom strakes, etc. that could lead to turbulence in the propwash.

 

[steelespike]

Re: Can't get up on cav plate

My 55 Bucaneer 5hp measures 17". I checked a couple of old manuals. They discuss anti "cavitation" plates
and also suggest tapering 12-18" of the keel down to the transom to reduce turbulance out the back
and this was when most outboards were 25hp or less.

 

[Texasmark]

Re: Can't get up on cav plate

My 55 Bucaneer 5hp measures 17". I checked a couple of old manuals. They discuss anti "cavitation" plates
and also suggest tapering 12-18" of the keel down to the transom to reduce turbulance out the back
and this was when most outboards were 25hp or less.

The boating industry where I grew up used the misnomer all the time. Wasn't till I started visiting this site that "the error of my ways...er ah sayins" was corrected. Once you think about it, it's pretty easy to see that it is ventilation and cavitation is entirely different and not surface (air water interface) related.

Mark

 

[Texasmark]

Re: Can't get up on cav plate

They made them a little on the long side due to lack of an industry standard in regards to boat builders. Some boat hulls have deeper transoms than others and rather than have to trim transoms, OMC simply made their motors able to fit a broader range of transoms within the standard "short" and "long" ranges. For boat builders, some of that variance may be due to keels, bottom strakes, etc. that could lead to turbulence in the propwash.

Well that makes since as even in the sales brochures I mentioned it was obvious that there were variances as a function of boat style/hull shape. Butttt since they didn't go very fast they got away with it.

Mark