4 blade props

[dingbat]

Does a 4 blade prop act differently at high RPM than a 3 blade? I have a 4 blade Renegade (14 ? x 17) on the 22’ walkaround with a 200 HP Ocean Pro

. I certainly can’t complain about the holeshot. For a boat as heavy as it is it gets out of the hole nicely and cruises well at 26-27 knots @ 4300-4500 rpm but I have a weird phenomenon that only seems to happen at elevated RPM. Once I get up around 30 knots I can dump the throttle and I get what I can best describe as slipping clutch type fell. Its doen't sound like it venting.

It only occurs for brief second and I don’t get a noticeable increase in RPM but its there none the less. Thought about a slipping hub but I would think that a slipping hub would show it’s face during a hole shot.

Using the prop calculator I’m coming up with 18% slip (39 MPH @ 5500 RPM) if it means anything.

 

[Silvertip]

Re: 4 blade props

Its easy to check for hub slip. Mark the outer part of the prop and place another mark on the hub directly in line with the first mark. Now run the boat until the problem occurs. Check the marks. If they are still lined up there is no slip. If they are misaligned, the hub is spun.

 

[achris]

Re: 4 blade props

The difference between 3 blades and 4 blades is quite a bit. You will find that the 4 blades are each smaller that the 3 blades, to make the total area about the same. The 4 blades are less efficient than 3 blades. You will find that your top speed will suffer with 4 blades, but the 4 blades props are a lot smoother. Once you've been running the 4 blade for a while, put the 3 blade back on, you can actually feel each blade. 4 blade props usually have a better bowlift than 3 blades also. So if your boat needs help keeping the nose up, a 4 blades prop will definitely help there. But be careful with that. My boat hates a prop that gives bowlift, When I have a big diameter prop or a 4 blade on, the boat tends to 'slam' coming off the crest of a wave. The prop is holding the bow up for as long as possible, eventually gravity wins the battle and down it all comes. A 3 blade allows the boat to ride naturally over the wave and gives a much softer ride. But that is just my boat, every boat has different hull characteristics and different props perform differently on them.

The only other thing I would check on your boat is that the engine is installed at the correct height. With the engine in the vertical position and the anti-cav plate parallel to the hull, the anti-cav plate should be level with the bottom of the hull. Outboards are really easy to change height. Just support the engine weight, remove the 4 mounting bolts and lower or raise the engine to the desired position. Re fit the bolts and go boating.

Chris................

 

[Texasmark]

Re: 4 blade props

On my last boat you could get her setup for max speed (trim just under speed loss position) and in that configuration you could hear the I6 purring and the 3 blades smacking the water....made a humming noise. You could also look at the engine and see it vibrating slightly. I suppose 4 would have been quieter and surely smoother.

I think the 4 blade came about to support the Bass Boat industry with their 3 batteries, livewells, and large fuel tanks, immediately in front of the transom, in addition to setback hulls (for top end) which were anything but flat. They were/are hard to get up and going. Then there is a guy down the road with a 5 blade on a BB. But folks that know indicate that the fewer the blades (over 1) the more efficient at top end (with the correspoding vibration increase) so you have to give and take.

Course other stern heavy boats could benefit also.

So what's this blabbing all about. Dunno. Just sitting here on Christmas morning listening to the family playing with their santa toys and yakin on the computer.

Merry Christmas and Happy New Year

Mark

 

[kennyk]

Re: 4 blade props

Hi all, I had two props for my 20' Procraft bass boat 200 Mariner which was a dual console, very deep design, and heavy. One was a three blade that was wanting out of the hole but did give a few more mph top end compared to my second prop a High Five (5 blades). Both were sst. Once I put the high five on I never wanted to go back to the 3 blade. I was up on plane in a matter of seconds and never lost any water unless the boat was up in the air. It would probably throw you out of the boat on a sharp turn if I was nuts enough. It was great for a deep chop where as the 3 blade would ventilate under the same conditions. I always said if I was to buy another prop I would try the 4 blade cause it seemed like the best of both worlds. Merry Christmas TO ALL. Ken

 

[seafox 257]

Re: 4 blade props

I have a 4 blade 24p on my checkmate would never go back to a 3 blade i lost about 4 mph on top end but who cares it comes out of the hole like a rocket and turns on a dime! It sounds to me like it is a cavitation problem

 

[dingbat]

Re: 4 blade props

Thanks All.
My gut feeling says it?s a problem with the prop. Maybe not, but I guess the easiest thing to do at this point is to get a new 4 blade prop and see if it exhibits the same high speed characteristics.

Cavitation is a possibly but I?m wondering why it only slips when I get on it. If I let it run on up at a progressive rate it doesn?t seem to do it.

How does 18% slip figure into the equation? Is 18% excessive for a 22? boat that weighs roughly 5500 lbs?

 

[Texasmark]

Re: 4 blade props

dingbat.

When you get on it, you are rapidly moving water away from the rear of the blade just like a jet aircraft pushes air away from the rear of the engine (thrust). This water has to be replaced as does the air. Air is easy to replace due to it's weight so the jet engine is a piece of cake, but water isn't.

So when you hammer down on it you create a vortex (hole that has to be filled with something) in front of the prop which is a vacuum attempting to get water back in front of the blades where you just removed it. The shortest distance to something much lighter and easier to move (air) is straight up.....hence the reason for the antiventilation plate.

When you come up at a reasonable thrust level, the water has time to flow back into the prop and there is not the void that you had otherwise, so no ventilation.....my theory anyway.

Not the prop expert and slip is caused by many things. I find that the lighter the boat and flatter the bottom in contact with the water, and depending upon speed, the smaller the surface, the lower the slip. My boat is light and fast with a small flat pad for high speed running and my slip goes from about 40% just above plane to around 10% at WOT when you add the cup on the prop into the pitch equation.

Mark