Cavitation plate location, what is normal?

[ronboonville]

I've been searching the forums and can't find an answer. Where should the cavitation plate be in relation to the bottom of the boat? Flush in the same plane? or below that point. This is an old gale 35 hp on a 1980's sea nymph travler, 16' aluminum jon. gale was converted to a long shaft and transom is 21" and plate is about 24" down.

 

[Frank Acampora]

Re: Cavitation plate location, what is normal?

Typically the anti-ventilation plate (cavitation plate) would start even or flush with the bottom of the boat Then, by experimentation, you raise it until you have a ventilation problem then lower it slightly. Hulls vary in shape and running characteristics and each design requires a different height. Also, engines on setback jackplates are generally run higher than engine on the transom.

SO: The only "Normal" setting is not below the hull bottom.

 

[joelybob]

Re: Cavitation plate location, what is normal?

my motor cavitation plate is even with the bottom of the v. but that is just mine. dont know if there is a set distance.

 

[roscoe]

Re: Cavitation plate location, what is normal?

Start even or slightly above, and go up from there.

If you are using a jack plate that sets the engine back from the transom, you will need to raise the motor even more.

 

[kfa4303]

Re: Cavitation plate location, what is normal?

+1. You usually want it level, or slightly higher than the keel such that it just barely breaks the surface while on plane at speed. Here's a pretty good pic.

 

[Texasmark]

Re: Cavitation plate location, what is normal?

Go to old omc de site which is operated by a guy in Germany. In the sites are his collection of old sales brochures which have a lot of pictures of omc engines hanging on boats while on a trailer.

If you will notice, every one of the pictures shows the antivent plate (we used to call it anticavitation plate too till coming to this site and realizing that is prevents ventilation, not cavitation) is below the bottom of the boat by approximately 1".

My not professional answer to your question for the older omc engines is that due to the way the water pickup was situated, both behind the prop and on the side of the LU just above the plate on the port side, you need those engines mounted lower on the transom than engines of today's designs with the water pickups mounted lower on the sides of the LU.

So, with that said, I'm going to throw my hat in the ring for 1" below.

For the record, I am running a 90 hp Merc, '02 vintage at 1 1/2" above.

HTH,
Mark

 

[Outsider]

Re: Cavitation plate location, what is normal?

Adjust the height so the 'plate' is just above the level of the water when on plane, but not 'in' the water. It can be run higher, to a point, but any lower and the drag curve goes up. If you need a starting point, then use 'level' ...

 

[Texasmark]

Re: Cavitation plate location, what is normal?

I hit this today (bumped) looking for a knowledgeable OMC guy from the '50-'60 era that knows of the water pick up requirements for those engines.

So if you are out there, let's hear from you.

Is there any need to keep the ACP below the bottom of the boat to satisfy the cooling requirements of those engines?

Or was it a thing with boat mfgrs. that made short transoms because possibly the engine consortiums hadn't sorted just how long a 15" transom really was and they wanted their props in the water. Or was it because the engines of the day weren't all that powerful and at 25 mph you really weren't all that interested in drag?

Mark

 

[kfa4303]

Re: Cavitation plate location, what is normal?

Hi Texasmark. I have a '66 20hp johnson and I run the anti-vent plate level with the keel of the boat and it works fine. Because OMC motors of this era have kind of an odd leg length of 18'' it makes them a little long on a standard 15'' transom and a little too short for a 20'' transom. I built a little transom riser out of some scrap aluminum. It's nothing pretty, but the price was right (free), and it gets the job done. I'm not sure raising the motor made me any faster per se, but steering is lighter, I get less stern squat and the draft is shallower which is handy in the skinny water. I think I top out at a little over 20mph and she hops right up on plane. Here are some pics of the riser and a vid of me running around at a local beach.






(here's a shot of how she sits in the water)


(vid at a local beach)
Wakulla Beach, FL movie.wmv - YouTube

 

[Texasmark]

Re: Cavitation plate location, what is normal?

Hi Texasmark. Because OMC motors of this era have kind of an odd leg length of 18''

Thanks for the reply. The part of your answer I highlighted is exactly my question. Why? Was it because back then BIA may have been young if existing at all, were there a lot of maverick boat builders with no set guidelines, or was it a suspicion of mine as mentioned: The engine had to be lower for the water p/u to function properly?

The answer does me absolutely no good at this time, but I did own a handfull of OMC's of the era and might have then. Just an idle curiousity, like any curiousity of things you experienced in yesteryear.

I guess if water p/u was the issue you would be having overheating issues with your engine. Are you?

Mark

 

[kfa4303]

Re: Cavitation plate location, what is normal?

I think the odd leg lengths were due to a general lack of building standards (no HIN plates, USCG ratings, flotation standards, etc...). One company's "standard" transom might have been 15" while another might be closer to 20". I guess OMC decided to split the difference and make their legs around 18" with the optional extension kit for most models. I haven't had any overheating issues yet. The water uptake is 1"-1.5" below the water at all times and a bit more if I'm actually in the boat with my weight aft. I did try to add a tell-tale to the motor by tapping into the t-stat housing, just to make checking the water flow easier while underway. It worked well, but I don't think the motor liked it much, so I took it off. Back to the stock set up on a transom riser, with a PVC tiller extender and PVC gear shift push/pull lever so I don't have to reach around to change gears anymore.

 

[Texasmark]

Re: Cavitation plate location, what is normal?

I think the odd leg lengths were due to a general lack of building standards (no HIN plates, USCG ratings, flotation standards, etc...). One company's "standard" transom might have been 15" while another might be closer to 20". I guess OMC decided to split the difference and make their legs around 18" with the optional extension kit for most models. I haven't had any overheating issues yet. The water uptake is 1"-1.5" below the water at all times and a bit more if I'm actually in the boat with my weight aft. I did try to add a tell-tale to the motor by tapping into the t-stat housing, just to make checking the water flow easier while underway. It worked well, but I don't think the motor liked it much, so I took it off. Back to the stock set up on a transom riser, with a PVC tiller extender and PVC gear shift push/pull lever so I don't have to reach around to change gears anymore.

You answered my question. Thanks

 

[Outsider]

Re: Cavitation plate location, what is normal?

Look at you motor, water intake ports will be easy to determine, be sure to look under the cave plate. If there is one located there, height becomes a little more critical ...

 

[Texasmark]

Re: Cavitation plate location, what is normal?

Look at you motor, water intake ports will be easy to determine, be sure to look under the cave plate. If there is one located there, height becomes a little more critical ...

As I recall I had a '70's era Merc 7 1/2 and it had a/the pickup under the AC plate. Course on that engine 20 mph was sizzling so one didn't have to worry much about it being out of the water.

Mark

 

[james__12345]

Re: Cavitation plate location, what is normal?

...., or was it a suspicion of mine as mentioned: The engine had to be lower for the water p/u to function properly?

The answer does me absolutely no good at this time, but I did own a handfull of OMC's of the era and might have then. Just an idle curiousity, like any curiousity of things you experienced in yesteryear.

I guess if water p/u was the issue you would be having overheating issues with your engine. Are you?

Mark

Its kinda funny that I found this, because I have been wondering the exact same thing my self with my current set up and have been reading cavitation plate location threads for days on end trying to find it mentioned and had about given up and was getting ready to post it my self. In some other threads about my motor it has been suggested to raise the motor up a little on the transom to get the plate more level with the bottom of the hull because of the longer length of the shaft. When that came up, I started to wonder this exact same thing. It would seem that with this style of motor, the cav plate running right at the top of the water, would put the plate for flushing the motor out, above the water and could cause some problems.

kfa4303 So you're saying you have the cav plate even with the bottom of the hull, but that its still running low enough to keep the holes submerged instead of the cav plate skimming the surface like is usually suggested?

 

[kfa4303]

Re: Cavitation plate location, what is normal?

On vintage OMC motors there are actually 2 water intake ports. One is found behind a little metal plate just above the anti-vent plate itself and is used at idle/test tank conditions. The second intake is found beneath the anti-vent plate behind the prop and usually has a screened opening. This is the uptake used while underway. Both the pressure from the prop and forward motion of the boat force water into the opening and up towards the impeller which then sends it up to the power head. Here's a pic that may help.

 

[Texasmark]

Re: Cavitation plate location, what is normal?

On vintage OMC motors there are actually 2 water intake ports. One is found behind a little metal plate just above the anti-vent plate itself and is used at idle/test tank conditions. The second intake is found beneath the anti-vent plate behind the prop and usually has a screened opening. This is the uptake used while underway. Both the pressure from the prop and forward motion of the boat force water into the opening and up towards the impeller which then sends it up to the power head. Here's a pic that may help.

View attachment 194215View attachment 194216View attachment 194215View attachment 194216

Excellent pic with explanation. Always wondered about the plate on the side.

Wife's dad had that 35. I had the same year in 18 hp.

Thanks, Mark

 

[james__12345]

Re: Cavitation plate location, what is normal?

I know there's an intake at the bottom too, but I was concerned that if the upper plate was out of the water, it might draw air and cause the system to not pump right.

 

[ctswf]

Re: Cavitation plate location, what is normal?

good info here, I had not given thought to this, I have an 18' deep V hull I'm about to mount my motor to. The cavitation plate is going to be a few inches Higher than the bottom. I don't have a choice without buying a motor mount. What would I look for in performance issues if it is indeed to high?

 

[Texasmark]

Re: Cavitation plate location, what is normal?

good info here, I had not given thought to this, I have an 18' deep V hull I'm about to mount my motor to. The cavitation plate is going to be a few inches Higher than the bottom. I don't have a choice without buying a motor mount. What would I look for in performance issues if it is indeed to high?

Where are your water pickups on the lower unit? Picture???? A few inches is a lot. A few inches could not only be a pickup problem for the water pump but could also put an inch or so of your prop out of the water, if you could get on plane and had enough power to get reasonable speed. But there are two problems and both have to do with ventilation. You probably can't do a hole shot for excessive ventilation and if up and running you would ventilate and over rev and loose mph.

You need to rethink your strategy.

Mark