Which way to fix prop ventilation???

[frozenokie]

I went on my maiden voyage after all this rebuilding. I was disappointed to find that by using the existing mounting holes for the motor, that the prop gets to much ventilation. I checked the relevance of the cavitation plate to the bottom of the transom and the plate is only about 1/2" above the bottom of the transom and the motor is as far down as it can go without either redrilling holes in the transom and removing the new SS angles atop the transom and remounting. I'm afraid that in doing this, it might not be enough drop since it would only amount to around 1/2" total. I have thought about purchasing a small jackplate so that I can adjust it to wherever it needs, but I'm not sure if this 1976 Merc 115 is a compatible style for a jackplate mount.

Should the 1/2" drop fix this problem or would I be better off just to go the jackplate route?​

 

[j_martin]

Re: Which way to fix prop ventilation???

What's the boat, and what propeller are you using, and is it in good condition.

Some propellers are designed to pierce the surface. The advantage is that with as little lower unit in the water as possible, friction is reduced. The same boat-engine-prop with the engine lowered an inch or so could be a real dog.

Setup is critical. It needs the right prop, and such things as engine water pressure and temperature have to be closely monitored.

A surfacing setup is extremely sensitive to prop damage.

You might find more detailed help and knowledge on setup at a performance oriented board, like Scream and Fly. "screamandfly.com"

hope it helps
John

 

[JB]

Re: Which way to fix prop ventilation???

This is not an engine problem. Let's try the Prop forum.

 

[frozenokie]

Re: Which way to fix prop ventilation???

What's the boat, and what propeller are you using, and is it in good condition.

Some propellers are designed to pierce the surface. The advantage is that with as little lower unit in the water as possible, friction is reduced. The same boat-engine-prop with the engine lowered an inch or so could be a real dog.

Setup is critical. It needs the right prop, and such things as engine water pressure and temperature have to be closely monitored.

A surfacing setup is extremely sensitive to prop damage.

The boat is a 79 Laser Bass (pictured) and the prop is a SS 13 3/4 19P w/no damage or dings. Water pressure = great, temp - I can put my hand to the power head w/o burning - seems perfect on fresh rebuild. I'll check out the link when time permits. Thanks j_martin!

 

[steelespike]

Re: Which way to fix prop ventilation???

What is "too much ventilation" useless or just touchy in corners unable to trim etc?
It would seem you will need to lower the motor.Are you sure there isn't anything disturbing the water near the prop.Its possible only 1/4 inch lower will be enough stop the venting.If it will fit a jack plate you could really fine tune the height and the setback may allow the same or even more height.

 

[CharlieB]

Re: Which way to fix prop ventilation???

Are you SURE that the prop is ventilating?

Could the hub be slipping now that the engine makes power?

Before changing your set-up check that prop hub be marking a line across the hub and nut, test run the boat and see if your lines still line up, I'm almost ready to bet that they do not.

 

[Bondo]

Re: Which way to fix prop ventilation???

and the prop is a SS 13 3/4 19P w/no damage or dings.

Ayuh,......

What Style of Prop is this,.... Round Ear, Cleaver, Chopper,..??

With the Cav plate Only 1/2" above the keel,......

I'd take the prop to a Good Prop Shop,+ have them curl alittle Cup onto the trailing edges,......
That might be enough to lock it up with the water..... Especially if you're turning a Round Ear....

 

[frozenokie]

Re: Which way to fix prop ventilation???

Here is the best way I can describe how and when it ventilates.

Throttle up - up on plane - and throttle back to keep just on plane (about 30mph). I then start to trim up. After only 2 seconds of upward trim, the boat starts porpoising more and more, so I trim back down fully and give a little more throttle and try again. If I give too much throttle - prop gets too much air when I trim up. If I turn the boat left or right while still hooked up, it will also start to lose prop-to-water bite. The ONLY time that it does NOT ventilate is when I leave the motor trimmed all the way down with the keel pushing water at high speed, but as soon as I turn the boat, out she comes.

Hey Bond-o, the prop is round eared with smooth edge cups. I seriously doubt the prop is at fault. I changed it out with a little heavier cupped and polished SS prop that I have. It did the same thing.

The biggest sign I believe is that it porpoises prematurely in relation to how much upward trim that has been achieved. I have never had this problem with any of the four previous boats that I have owned except for when I simply have had the motors trimmed up too high, but none of them ever ventilated. What do yall think?

BTW - I'm having my boat man come by tomorrow and pick up the boat to fix this problem. So I wanted to get a little educated beforehand in order to tell him to jack plate it or not.

 

[frozenokie]

Re: Which way to fix prop ventilation???

Well, after having the boat mechanic look at this, he saw that I was about 5" high!!!! The cavitation plate was in line with the stepped bottom of the transom and not the keel bottom of the boat. Ahhh... I didn't realize that was the reference point vs. the transom bottom. He said whoever has owned this boat before you and used the same motor had the same problem because this boat should utilize a XL shaft (25") which this motor is a long shaft (20").

All is good though since I had a brand new jack plate w/ 5 1/2" set back, new controls, and new tilt/trim harness waiting on him to install. He assured me that the jack plate will fix the problem and I can fine tune it on the lake. Finally at the end of those curve balls I belive (hope)!!

 

[Dhadley]

Re: Which way to fix prop ventilation???

If your AV plate is in line with the step, you're about right. That's the whole idea of the step. But you'll need a pretty decent prop. Just remember, all 19" props are not created equal.

 

[frozenokie]

Re: Which way to fix prop ventilation???

Whoah... didn't expect that reply. So should I call my mechanic and discuss this? He assuredly pointed out that it should be lined up with the very bottom of the boat and I've done some checking up on him also and to find out that he has done this for about 25 years. But if he's wrong then I need to discuss the mounting arrangement of the jack plate with him before he begins.

My personal feeling on the mounting of this jack plate is that ALONE should fix the problems without so much emphasis on how close the AV plate is to the stepped area as long as it's close to begin with which it is. Am I incorrect about this?

 

[Dhadley]

Re: Which way to fix prop ventilation???

The whole idea of the step is to allow a higher X dimension which means less drag which means better efficiency.

So your set up looks something like this? --

 

[frozenokie]

Re: Which way to fix prop ventilation???

Please pardon me... I stand corrected

You are right and it was my misunderstanding in the conversation with my mechanic after discussing the issue with him again. Looking at your photo told me that I'm only a tiny bit high on the mount as it currently looks very close to the setup you show. My mechanics words were that the center of the nose or "bullet" of the gear housing should be in line with the bottom of the boat. Not the AV plate which was what I thought he was meaning. I couldn't have been more wrong. Hence my confusion on the whole matter.

We had decided to try lowering the motor w/o the use of the jack plate first since I'm pretty close to begin with. If that doesn't do the job, we'll use the jack plate. I'm also taking this route due to having the incorrect jack plate too. Since my motor has the "sailboat" mount, the bolt holes didn't line up which I didn't know. My mechanic said that there are other jack plates that would be suitable if the need is there. I decided to let him choose the appropriate one if it comes to that. He has more experience in this than I do anyway.

 

[Dhadley]

Re: Which way to fix prop ventilation???

Don't feel too bad about understanding him. It's pretty common to use the propshaft centerline instead of the AV plate. The "go fast" crowd usually uses the shaft, the "fishing" crowd usually uses the AV plate. Both are correct and say the same thing - as long as we're all on the same page. Sometimes it's hard to keep up.

As for your set up, I think you have a good plan. Max out what you have then make changes. As for the jackplate, decide if you want one before you buy a prop. That's a pretty good hull, you should have a lot of fun with it.