Prop Ventilation Issues

[trdprotruck]

Hi All,

I'm hoping some of you can give me some advice. Over the weekend I was at Dana Point testing out my new to me 2001 for the first time. I have been experiencing what I believe to be prop ventilation regardless if the ballast is full or empty. At 3000 rpm my speed was about 5 mph (GPS) and at 5000 rpm 7.5 mph. It seemed like the engine would be screaming and would not get any grip in the water. At times there would be a lot of rpm surging with a small following sea.

The boat was somewhat lightly loaded with mast and boom aboard, but not much else. My motor is a 2001 50 hp Johnson 2 stroke with a 4 bladed Vortex prop by Michigan Wheel. This prop is a 11.5 x 9 with a little cupping and big dog ears like was recommended by the posts I've read in the past. My motor is fairly low and I can only go down one more hole.

I tried calling Michigan wheel, but they seemed to believe the prop was correct and didn't know what to recommend except to buy another prop with 3 blades and more pitch.

Is this really the route I should take? It seems contrary to the posts that I have read.

I'm tempted to try out those composite Piranah propellors so that I can economically experiment with different pitches, but I'm really confused as to why the current setup is not working properly.

Does anyone have any recommendations or know of a dealer in southern california / near Orange County that will allow me to test several props?

Thanks for your advice in advance.

Randy

[Crikey]

Randy,

Even though you have only one hole adjustment remaining what is the current position of the cavitatiion plate above the blade, in relation to the bottom of the transom, when the motor is set for running? It should not be above this level.

Ross

[trdprotruck]

Randy,

Even though you have only one hole adjustment remaining what is the current position of the cavitatiion plate above the blade, in relation to the bottom of the transom, when the motor is set for running? It should not be above this level.

Ross

Hmm I just took some pictures. Do these look ok? It looks to be below the transom.

[aya16]

doesnt look like to much is in the way to cause cavitation, maybe try and take the fishfinder transducer off just to see if it makes a diff.
the motor cavitation plate looks good, but I would drop the motor all the way down any way.

your prop being a 4 blade has very little pitch, If all fails above, that I suggested, I would go with the 3 blade more pitch prop. The only other thing I can think of is the centerboard may be damaged and is disturbing the water so much that the prop is riding in a lot of air bubbles, make sure the center board is all the way up, or all the way down when you try diff. things to stop the ventilation.

I would do the free stuff first, transducer, lower the engine, center board all the way up. Your engine is already trimmed all the way down,
but you could wedge the top bolts out some and get the engine at more of a inward angle, that will make up for the mac bow heavy ride some,
that causes the engine to be tilted out a bit. On my boat the engine works best when trimmed all the way in.

if all that still doesn't solve the problem Then I would pop for the 3 bladed prop.

ohh one more thing, if that 4 blade prop you have now is out of whack some, (bent blade) it will cavitate.
Mike

[Crikey]

Totally concur with Mike here! Everything looks right with the depth of the engine but before I'd go to the total PITA of dropping it to the last hole I'd check the blade. In one of your pictures, one blade seems to reflect light different from the others. Is it possible to get air drawn in through the keel receptacle and trailing out along the centre of the hull bottom? That would definately do the dirty, but it would have to be pretty substantial. See if you can temporarily plug rope or bolt holes with some putty and see if that has any affect. Little dicey trying to hang over the side with a facemask!

Ross

[trdprotruck]

Thanks Mike and Ross!

I think the blade in the picture is just a lighting thing. The prop is brand new and looks ok. I'll try lowering the motor and removing the transducer. I'm sure I pulled the CB line as far as it will go, but it's an old line and I guess there could be some stretch. I guess I could dive under the boat and take a look next time. Might as well replace the line at the same time.

Could the fact that the leading edge of the blade is so close to the skeg also be a factor?

-Randy

[Crikey]

You might be on to something there. Here's a pic of mine that's a lot further away (Suz DF60a) but I can't speak for other manufacturers.



The spacing section on mine appears to be a shaft bushing with bolt heads that I would love to change to acorns. Can you check a manual for your exact setup?

Ross

[aya16]

could be, but if you lower the motor, which looks easy as all the bolts are outside the hull, trim the engine all the way down,
and take off slower you might fix the problem. Maybe you can ask some one to lend you a prop to try. But if all that free stuff doesn't work
you will need to try a new prop, 3 blade as much pitch as the motor will take and still be close to high rpm at full throttle, (ballast empty) oh also get a cupped prop. Im not a fan of four blade props, especially on a boat like ours, although the four blade grabs great, with the thin pitch we have to run, I don't think it can grab well.

what size motor is that, you have?

Mike

[aya16]

It sounds like your running a 25-30 hp engine with the speeds your saying?
If that's the case, then your speeds are about right, maybe should be a little faster.

I wouldn't mess with props till you get rid of the cavitation, then I would start with a three blade cupped, 10 or 11 inch pitch
but that really depends on the hp of your motor.
Mike

[trdprotruck]

Whoops I forgot to mention that my motor is a 50 hp Johnson. I was expecting a top speed of anywhere from 15-20 mph. After looking at Ross's prop, I'm wondering if the ventilation/cavitation problem may be related to a combination of the prop and the outboard. The blades are definately farther back from the skeg.

Ross, what pitch are you running?

Thanks,

Randy

It sounds like your running a 25-30 hp engine with the speeds your saying?
If that's the case, then your speeds are about right, maybe should be a little faster.

I wouldn't mess with props till you get rid of the cavitation, then I would start with a three blade cupped, 10 or 11 inch pitch
but that really depends on the hp of your motor.
Mike

[Crikey]

Randy

I'm running a 11 5/8' X 12 Suzuki aluminum 3 blade. Have so far given up my quest for a four bladed prop. I've had to return both Pirrhana and Propulse props, and both companies hadn't figured out the model change even by now. I have enough space for a 13".
Probably a top end near 20 on a good day without burning the motor. Composite quality was good and light weight - too bad I will have to wait.

R

[aya16]

oh wow, a 50 should get way better performance than 7mph. My guess is you have a prop problem then. I would still lower the engine all the way down.
my guess now is the 9 inch pitch and the small 4 blade isnt able to bite well and just spins. I would pop for this or something like it, http://www.overtons.com/modperl/product ... id&i=32570

Or this one, doesn't seem to be a 12 inch pitch available, http://www.overtons.com/modperl/product ... id&i=32571

If you buy the prop from a local dealer, you might be able to try the 13 see how it goes, if it doesn't reach full rpm (without ballast) maybe he would exchange it for the 11.
overtons might even exchange the props as long as it wasn't beat up. But I would try the 13 first. Your Johnson 2 stroke might be able to turn that 13 just fine. your speeds should go up to
13-17 mph, maybe more on a lightly loaded mac, If there is such a thing?????

It also sounds like what you have going on with your 4 blade is, its blowing past the top rpm, and there might be a rev limiter kicking in, so it makes us think its running at the right rpm
when in fact it trying to go way past that.

More pitch, bigger prop may be the answer, what size pitch, I don't know, but I would try the 13 first. for every inch in pitch, its only going to drop or raise the rpm by about 300 rpm.
so the deciding factor on the pitch would be if you like the hole shot, an 11 will give you better get up and go, but the 13 will keep the engine from screaming to much once you get going.
remember there is only going to be a 600 rpm diff. between the two, at top speed.

I don't think you can factor in the 4 blade, its so far off.
Mike

[aya16]

You cant go by what ross is using, he has a 4 stroke suzuki, with a diff gear ratio than you have with your 2 stroke. If you buy the prop I suggested, it has a 13 inch blade with a 13 pitch
way bigger than ross's 11 5/8x 12. (not knowing your gear ratio) I would guess its a lot lower than Ross's.
Mike

[Hardcrab]

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?

[Andre Emmenegger]

A general rule of thumb is to put the cavitation plate even and in line with the bottom surface of the hull. If the prop is stock, they usually come from the factory with too great a pitch for a Macgregor. The suggestion to see if the prop has a loose rubber insert is the place to start though, as mentioned above. The simplest way to do this is to try to borrow another prop and see what happens.