Prop Ventilation Issues

[trdprotruck]

It's maybe a goofy idea, but is it possible that the center hub has a loose or worn rubber insert piece and the shaft torque is just not being fully coupled to the prop and is slipping?

I know the standard story for this symptom is "in gear, engine running fine, but I'm not going anywhere".
Not the case here but it sounds close to it, kinda, sorta perhaps.

But then again, maybe this outboard uses a regular shear pin type design to couple the torque to the prop?

Thanks for the suggestions.

This outboard uses a splined shaft with a castle nut to couple the prop to the hub. The michigan wheel prop I have uses a hard polyurethane type material in the hub to cushion the load on the lower unit as well as make the same prop fit multiple outboards (I'm guessing). I checked the hub and it does not seem to be slipping (It better not since it's brand spaking new!) I'm thinking that maybe this "universal" prop for multiple outboards design is causing problems with my outboard. It seems that some other people have used the same prop with some success (Over 7 mph), but I'm wondering if the skeg might be too close to the prop's leading edge (see pics above) causing cavitation/ ventilation issues. I noticed in Ross's pic the actual blades are much farther away from the skeg, but that could also be because of my shallow pitch is 9" vs his 12".

The outboard seems to be at the right height with regards to the cavitation plate and the bottom of the hull, but I will try to lower it to the last hole.

[Crikey]

The Suzuki dealer that we bought the prop from let us try a test run for free, as long as we didn't scrape or damage it, and took only a few days. I thought this was the standard practice of someone who wanted to do business and was happy enough with my second choice even though I would relegate it to spare once I got the 4 blade adjustable composite that would fit on my shaft.

Prop hub slippage is possible but should be evident with the prop removed, and the motor would probably race at different loads (not exactly the same each time). I don't think a limiter would conceal the rpm.

Ross

[aya16]

not sure about the johnson having a rev limiter, but our suzuki's do. A rev limiter does just that, wont let the engine rev beyond
max rpm. Good way to tell if there's a limiter is to place the boat in water, and when the engine has been warmed up, keep the engine in neutral
rev the motor to max rpm for short bursts. If the motor will only rev's to 4000 or so and surges, it has a rev limiter. If it wants to rev to the moon don't let it, but there's no limiter.

I have had a prop spin its hub before, it doesn't have the same sound that cavitation does, and the rubber in the hub looks burned, and tore up.
although I wasn't able to spin it by hand and make it slip.

9mph is the key here, I think, If the brand new prop never worked right and, It has been cavitating since new somethings wrong.
The cavitating may not be cavitation at all, but just a prop that doesn't bite, revs up, and slips. cavitation has a unique sound and most times it happens if you back off on the throttle then push the throttle forward again the cavitation will stop. The exhaust tone changes too, as the exhaust will be dumping into bubbles and the sound is distinct, plus there's some vibration that goes along with it.

another sign of bad cavitation is paint on the prop will get blasted off at the edge of the blades, eventually, almost like a sand blaster would do.

Lowering the engine to give the prop the best bite it can get in clear water, and it still does the same thing all the time, its got to be a prop issue, defect, or plain too small and not enough pitch.

the only way to tell is get her in the water, with the lowered engine. Don't forget to post follow ups, nothing worse than dealing with an issue
and never hear about the outcome, like watching a movie and not seeing the end....
Mike

[Crikey]

Just a little 'late nite reading'....

http://americanhistory.si.edu/subs/angl ... ming2.html

Good luck with your testing and let us know!

Ross

[dennisneal]

Randy,

First a couple of differences: I own a 26M with an E-TEC 60 HP outboard.

That being said, I did compare your photos with my boat and the setups are very similar. The prop needs to be a little lower if the boat is in the ocean, because the waves add more verticle tipping motion to the hull. My boat's anti-cavitation plate is about one inch below the hull.

Also, I have a three bladed aluminum prop, (13.75 X 13), and the clearance between the prop and the motor's skeg also seem to be about the same as yours.

I would expect a much higher top-end speed for your boat.

Is there any place in the driveline where "slippage" can occur, so that even though the motor is revved, the prop is not turning as fast as it should?

[trdprotruck]

Thanks for all the suggestions everyone! I was able to obtain a different prop and I will try that as well as some of the other suggestions such as cav plate height etc. I will report back with the results.

Thanks,

-Randy

[Wrudd3]

Randy,

Kinda late in the game for my advice, may I suggest do not lower the motor unless (as was mentioned) you are going in the ocean.

My first Mac was the 26x I had the same cavatation plate setting as you have.
I had a 50 HP Yamaha 4 stroke Hi trust with a 14 dia. x 10 pitch no problem.

Our heavy Macs do not have a good hull design for planeing. A 11x9 prop will not be able to push that weight onto a simi plane. More RPMs will only give you what you have . . . more cavatation.
A 13 dia. x 12 or 13 pitch should do well with pontoon style blades (large).

Will

[trdprotruck]

Hi All,

Finally got to do some testing. I was able to try a 12.5 x 13 3 blade prop. I didn't change anything else and I was able to get to 12.8 mph @ 5500 rpm with full ballast. Felt much better than with 11.5 x 9 x 4 blade. I looked at the cav plate while running and it looked ok to possibly being too low. I will try to post pictures later. Does this seem to be the best my motor / prop can do?

Still baffled by the other prop.

Thanks,

Randy

[aya16]

that sounds like your in the money as far as rpm and speed. you should get about 14-15 without ballast.
sounds like your problem is solved.
Mike

[bscott]

I raised my cavitation plate parallel to the bottom and picked up 200 rpm. and am a believer in the Power Thruster.

Bob

[trdprotruck]

Hi All,

Here are some pics of the motor and wake while running. Do you guys think I could raise the motor and gain some mph? I am going to be using the for the most part crossing to Catalina Island. The pics can be clicked for a close up.

Thanks,

Randy

[aya16]

Your pix on the first post show that all the motor mount nuts are on the outside, so all you have to do is take em off without spinning the bolts and raise the engine. You will still need to
5200 seal the washers back on, any break in the 5200 by spinning the bolt and that bolt will need new sealant or it may leak.

Now by owning a mac it automatically puts you in the "I want to go faster club", so how do I get more power out of my 50? You raise the engine you might get a couple hundred rpm more
not really going to give you much more speed. (but hey, you own a mac) Now if you raise the engine, and you are in some swells, going to Catalina, your prop may start cavitation
and your back to square one. Its really no big deal to lower it back, so why not?

right now you can cross to Catalina, from long beach in nice seas in a little less than 2 hours, not to bad, if you raise the engine, you can cross in a little less than 2 hours.
That old 50 two stroke johnson, can be replaced with a nice used 2 stroke 70 johnson when you wear the 50 out, and you wont have to change a thing, cables wiring, and such.
Then you can cross in a little over an hour, in kind seas.

all that above is called the mac bug,

enjoy, Mike

[aya16]

I guess I didnt answer your question, It looks like the cavitation plate is a little low in the water and doesnt look like a clean break in the water. But I dont know if you are plaining or not,
at 12mph, I dont think so. run without ballast and see what you get. Then look at the plate.

Whats the working red line of your motor? if its 5500, you are already there, may need to have the 13 inch prop pitched some. Im not sure if your johnson could handle a 14 inch prop
you raise the motor, and dump the ballast, you might be spinning at 6000 rpm, if your motor is rated for 5500, that extra 500 rpm is not good if you have it pinned for long periods of time.

If you dump the ballast, that might raise the engine an inch or so, when you go full throttle.

keep us up on what you do
Mike

[Tomfoolery]

Is your centerboard all the way up?

[bscott]

If you are getting any splash into the motor well, then you plate is too low--it does not look like you are planing yet.

Bob