Spun hub or ventilation?

[kdp89]

Hey guys, I tried to research as much as I could but I think I need some input.

I recently put a 'new to me' 1976 Johnson 135 on my 18' Fiberglass runabout fishing boat. I took it out for its initial test and am having some problems. The motor idles great and runs well up till 12mph then it revs ways up and slows right down. I know this sounds like a spun hub, but the part that confuses me is that the prop is still spinning and kicks up a small rooster tail when this happens. If I bring it back to idle, then accelerate again it will move, but if I just lay off he throttle a little it keeps spinning. Also, i have to keep the motor trimmed all he way down, or this happens as soon as i hit the throttle.

So my question is, is my motor mounted too high causing ventilation? Or do i have a spun hub?

Thanks a lot guys.

 

[BonairII]

Re: Spun hub or ventilation?

The hub may be grabbing just enough to throw water...but not enough to propel the boat.

Scribe a line thru the hub of the prop. If the line is broken after your next run...it's a spun prop.

Mount your motor so that vent plate is even with the keel. It's a good starting point anyway. Cav plate should run on the surface of the water at WOT.

 

[kdp89]

Re: Spun hub or ventilation?

I noticed you mentioned vent plate and cav plate. There is the bottom 'cav' plate and one more smaller a couple inches above, is that the vent? Sorry, bit of a rookie to this.

 

[Mi duckdown]

Re: Spun hub or ventilation?

vent plate is the one just above the prop. there no such thing as a cav plate. the vent plate is sometimes called cav plate, which is wrong.

 

[Frank Acampora]

Re: Spun hub or ventilation?

Well, If you want to be a stickler, it really is an ANTI-Ventilation plate. Since ventilation frequently leads to cavitation, the plate is most usually colloquially known as a "cavitation Plate." Use either term: Most people will know what you mean.

 

[foodfisher]

Re: Spun hub or ventilation?

[QUOTE
So my question is, is my motor mounted too high causing ventilation? Or do i have a spun hub?

Thanks a lot guys.[/QUOTE]

Yes or yes. Antiventilation plate within 1
' of the bottim of the hull or do the mark across the hub test.

 

[BonairII]

Re: Spun hub or ventilation?

I noticed you mentioned vent plate and cav plate. There is the bottom 'cav' plate and one more smaller a couple inches above, is that the vent? Sorry, bit of a rookie to this.

My bad. I used both terms in the same post.

Yup...Cav and vent plate are one and the same. Most refer to it as the cavitation plate, but as FA has explained....anti-ventilation plate is actually the correct name for it.

 

[steelespike]

Re: Spun hub or ventilation?

Sounds to me like the motor is too high.

 

[kdp89]

Re: Spun hub or ventilation?

Ok, thanks for the tip on the Vent not Cav.

Here are some pictures of my transom/motor, let me know if you think this is too high. This it trimmed all the way down.

 

[jestor68]

Re: Spun hub or ventilation?

Your motor is not mounted too high.

Replace your old OMC prop with a nice Solas or TP aluminum prop of the appropriate pitch.

 

[kdp89]

Re: Spun hub or ventilation?

Ok, there's no visible markings on he prop. And this is a new boat for this motor. Any ideas on prop selection?

 

[kdp89]

Re: Spun hub or ventilation?

I know it this is very vague, but any ideas on a prop to start out with? Just to get me rolling until I can do some actually testing? I realize you can't give me the ideal prop without numbers, but I can't give numbers without a prop. Assume the boat, loaded, weighs 1800 lbs.

 

[Outsider]

Re: Spun hub or ventilation?

Here are some pictures of my transom/motor, let me know if you think this is too high.

Where the motor sits on the trailer is mostly irrelevant other than a starting point, it's the location of the plate when at high cruise speed that determines high, low, or just right. It should be very slightly above the surface, in the flow is too low, higher is a narrow range of 'maybe'. Take the prop to a prop shop and let them measure it, then have the hub replaced. Run it WOT with a tach to determine if RPMs are within the upper limits. Until you know the height is correct and the RPM it yields, you're just guessing if your set-up is OK or what direction to change anything ...

 

[kdp89]

Re: Spun hub or ventilation?

Ok, that makes sense. Ill have the prop re-hubbed and go from there. Trying to save from spending money twice, but i guess its always good to have a spare anyway. Thanks for all the help guys, ill report back once I get back on the water.

 

[kdp89]

Re: Spun hub or ventilation?

Back to square one.

I put a new 13x19 prop on today to sole all my problems, and it didn't. The original prop was in bad shape, blades out of pitch, etc, along with the hub, so I figured I would be all set with the new one. The good news is that the new prop got me upago 16 mph before ventilating, instead of 11, but the prop still breaks loose at about 16mph and 2500 rpms. I verified this by watching the lower end this time and the vent plate got above the water right as I broke loose.

So I guess it's time to get the motor lower into the water. Going to have to cut down my transom to do this. I am thinking of cutting it down 2" as I have room to spare with a huge splash well.


Any other ideas before I go cutting my boat apart?

 

[jestor68]

Re: Spun hub or ventilation?

Is the motor trimmed down as low as it'll go? If I draw an imaginary line from the keel past the lower unit, it appears that the motor is not down far enough. In fact, it appears that as the boats comes on plane, the prop blade tips would be breaking the surface.

It could be that your transom does not have enough angle built into it. There are wedges available that go between the motor and transom to increase that angle and permit the motor to trim in further.

The optimum transom angle is about 15-16 degrees. If you have much less than that, a wedge may be needed to allow the motor to come in further toward the transom.

Here's a picture of a transom wedge. Most are 5 degrees of positive angle.

 

[kdp89]

Re: Spun hub or ventilation?

Yes, this is happening with the motor tucked in all the way. And yes, the blade tips come out of the water as I am approaching plane.

I'll have a look at the angle and see if I need the shims, but I think dropping the transom is what im going to have to do.

I'll be tackling it this weekend hopefully.

 

[batman99]

Re: Spun hub or ventilation?

.

Is the "1976 Johnson 135 HP" motor factory for your fishing boat? Or, is it a replacement motor?. Using Google search, I noticed this engine has shaft length of 20" or 25". Instead of cutting the transom to lower the engine, perhaps it might be better (simplier) to measure motor's shaft. And if current 20", convert to 25" shaft length. Perhaps this better solution (to get motor's prop deeper into the water)...

 

[steelespike]

Re: Spun hub or ventilation?

There may be holes in the mounting bracket that will let you lower the motor while leaving the bracket mounted as it is.
A 5" extension will be too much forcing you to raise the motor.

 

[Mi duckdown]

Re: Spun hub or ventilation?

I was not trying to be a no it all. When kdp89 asked about vent and cav plate, i did not want him to think there there two different plates.
just trying help out.