MacGregorSailors.com

I am always disappointed with my speed. WOT is impossible as the rpms rocket out of control beyond about 6000rpm as if the prop is out of the water. It could be a slipping prop, but most likely cavitation. Adjusting the trim does not help. I figure it would be time to try a new one this summer. Is there an ideal prop out there?

Rob

What size are you currently using? Is it 3 blade or 4 blade? If you are topping out at 6000rpm with empty ballast that is not too bad. I can get up to 6000rpm on mine with empty ballst aand I use a Solas Alcup 4 X 11.4 X 10.75, you are not likely cavitating but more likely ventilating, pulling air down to the prop to cause excessive rpm.

Terry wrote:What size are you currently using? Is it 3 blade or 4 blade? If you are topping out at 6000rpm with empty ballast that is not too bad. I can get up to 6000rpm on mine with empty ballst aand I use a Solas Alcup 4 X 11.4 X 10.75, you are not likely cavitating but more likely ventilating, pulling air down to the prop to cause excessive rpm.

Good question I seem to remember 3 blades but not certain. I will have to check. Empty or full ballast, same result.

My BF50 came with a pair of 11-3/4" x 10" pitch 3-blade props. The engine runs at about 5500 rpm or a little higher, and the boat tops out at about 16 mph with no ballast (but with 3 men onboard), per my gps system. I even pull a wakeboarder with it, though it won't go that fast, as expected. I was thinking of trying a 4-blade with 9" pitch this coming season, just to try.

Here is an old post when I used to have the Honda 50. I found the 4 blades to work best.

http://www.macgregorsailors.com/forum/v ... op#p163194

2nd Erik's suggestion. We picked up 1 mph with the 58134-ZV5-009ah. Wakeboarding became an option with smaller kids and that prop.

Thanks guys!

Looks like I will be prop shopping soon.

Rob

vkmaynard wrote:2nd Erik's suggestion. We picked up 1 mph with the 58134-ZV5-009ah. Wakeboarding became an option with smaller kids and that prop.

That part number is a Honda number, and their site lists it as a "Solas Honda" part. Solas has a similar 4 x 11.8 x 9 prop with a standard rubber hub. I have to wonder if it's the same prop. It would stand to reason that it is, I would think.

$104 at iboats.com. Not bad.

http://boatpropellers.iboats.com/?searc ... d&x=39&y=6

Terry said;

you are not likely cavitating but more likely ventilating, pulling air down to the prop to cause excessive rpm.

Cavitation and ventilating are one and the same. The flat plate near the bottom of the motor, but above the propeller is called the anti cavitation plate, or anti ventilation plate.

Quoting the latest issue of Pennsylvania Angler & Boater; ““cavitation occurs when a high-speed propeller loses it's “bite" on the water, creating a partial vacuum, loss of thrust and excessive shaft speed; continued cavatation can result in excessive blade wear.""

The article goes on to state that cupping a prop “adds bite in the water and aids the prop in getting better “traction," particularly when the motor is tilted up while running in shallow water."

The last paragraph goes on to state that “if you are like many Pennsylvanians who fish in the morning, want power in the afternoon to pull a water skier and the best fuel economy while cruising in the evening, the best propeller is the one the engine boat manufacturer recommends. It won't give you top performance in any of these applications, but it will give you a good average performance all around. And, it won't damage your boat motor."

I mention this because I have 50HP Honda and use it under a variety of situations, one of which is WOT. I have had the prop replaced once, and it was the factory recomemded size. At WOT I am very close to 6000 RPM.

My suggestion to Rob is before going out and purchasing a prop, check to see what Honda recommends for the motor, and then see what is actually on there. You can always get a prop later, but first exercise good diligence. Cheking with this board is a good start. This is where I go first.

Ray

Rob with your engine running away at 6000rpm's the prop is ingesting air. Have you checked the cavitation plate position to see whether it is positioned too high at the stern? Ideally, I think it should be level, to a half inch or so below the bottom hull line.
After that, modify your prop to your hearts content!
Ross

NiceAft wrote:Terry said;

you are not likely cavitating but more likely ventilating, pulling air down to the prop to cause excessive rpm.

Cavitation and ventilating are one and the same. The flat plate near the bottom of the motor, but above the propeller is called the anti cavitation plate, or anti ventilation plate.

Quoting the latest issue of Pennsylvania Angler & Boater; ““cavitation occurs when a high-speed propeller loses it's “bite" on the water, creating a partial vacuum, loss of thrust and excessive shaft speed; continued cavatation can result in excessive blade wear.""

The article goes on to state that cupping a prop “adds bite in the water and aids the prop in getting better “traction," particularly when the motor is tilted up while running in shallow water."
Ray

I beg to differ Ray,
Cavitation is a different effect than ventilation and causes different effects on the propellar. Ventilation is when excessive air surrounds the propellar, causing excess spinning or higher revs, usually because the prop is not submerged deep enough in the water, cavitation results when there is a pressure change on the back or suction side of the prop and it causes pitting and scoring on the back of the propellar, which I believe but not totally sure is caused by the lack of bite. They are two different effects with two different properties caused by two different forces creating two different results to the prop. I read about this in some technical article somewhere and now I have to hunt it back down somehow.

Terry wrote:. . . cavitation results when there is a pressure change on the back or suction side of the prop and it causes pitting and scoring on the back of the propellar, which I believe but not totally sure is caused by the lack of bite.

Cavitation happens when the prop pulls enough of a vacuum to 'boil' the water on the back of the prop, which itself isn't bad, but when the bubbles collapse they chew the metal, which is very bad. This doesn't just happen to boat propellers - it happens in hydraulic systems and other fluid environments. Hydraulic oil can cavitate in pumps and motors, and even as it passes through some types of valves where large pressure drops occur, and can chew up precision parts in a hurry if it's bad enough.

I would guess cavitation happens in a boat prop primarily when it's overpowered, so it's moving water too fast for the incoming water to keep up with and the boat isn't moving fast enough to limit the pressure drop by supplying a fast enough incoming stream. Probably too small a diameter (a guess) and/or too steep a pitch for the boat speed (coupled with a small prop). Putting a small, deep pitch prop on a 50 hp mounted to a slow boat like an would, in my mind, be a recipe for cavitation. But that's just a guess.

Crikey wrote:Rob with your engine running away at 6000rpm's the prop is ingesting air. Have you checked the cavitation plate position to see whether it is positioned too high at the stern? Ideally, I think it should be level, to a half inch or so below the bottom hull line.
After that, modify your prop to your hearts content!
Ross

Cavitation plate? What is that? I will investigate.

Ventilation/cavitaion, not sure what happens to it is the same as if I am going over big waves and the prop comes out of the water. The engine rpms screem off the dial and speed dies. Sounds like a bigger prop is needed from what you guys are saying. I will check out what I have. Being -14c outside I am not sure if I will check it out today though. I just got back from spending almost 2 months in California and it feels worse than it should

Rob

Ok, researched what a cavitaion plate is. I found this too. Interesting. http://forums.iboats.com/showthread.php?t=157458

Rob, sorry to interrupt your post-Californian suntan, but is there any chance of a picture of your arrangement? The transom type of the hull in your supplied link may be more forgiving than the wider, and flatter MacGregor stern.
Mine's wrapped tighter than a fish's **** at present and I don't have anything suitable in my photobucket collection to show you.

Ross