prop pitch
 

have advantage 22 ft party cat with 496 ho 425 hp with bravo 1 the boat porposes and seems like it should go faster the prop pitch is 22 do you think changing the pitch will help with speed and handling and what do you recomend?

In most but not all cases porpoising is created by imbalance in the boat, normally the stern. If you can remove weight from the stern normally you will find the porpoising decreases to some degree. Another thing you can do is to get a propeller with a lot less rake than the one you are currently using. Rake in a propeller is designed to raise the bow of the boat and improve the holding power of the blades, but in a boat with porpoising issues all this does is exacerbate the porpoising. To decrease porpoising in a particular situation you want to have a propeller with cup in the trailing edge of the blades , so it raises the stern of the boat and not the bow. By raising the stern it removes a large amount of the imbalance that was exacerbated by the highly raked prop. Hope this helps steve.


H

OK, I'm going to stir the pot a little.

Porposing is caused by a variety of issues. Most often, inability of the propeller to carry the bow is the problem. Other reasons can be bottom imperfections such as a "hook" or a "rocker". Engine height also affects porpoising. Finally, really bad weight distribution can be the problem.

Carrying the bow is most often the best route to gain performance in boats of various designs. Indeed decreasing stern weight will decrease bow height and may reduce the porpoising. However, this will also flatten the boat attitude and increase wetted surface area as well as scrape speed and may hurt rough water ride characteristics.

If at all possible, I prefer to carry the bow higher rather than lower so as not to hurt the pre mentioned things. Cup can be used as rake angle and if used properly, will leverage the stern and hold the bow. Adding rake does not alway increase porpoising, on the contrary, it often helps it. If adjusting propeller geometry and depth don't fix the problem, I would have the boat bottom checked. Some boats have inherant issues no matter what.

Worse case senario, don't let the prop carry the boat at all and run it flat. Or, add weight where needed till it stops (also decreasing speed). Yes the boat will stop the hop.

Bret, I realize I don't have anywhere near the knowledge you do, and I know that cupping the blade tips will increase the theoretical rake as well as cupping the trailing edge will help lift the stern. I am not used to dealing with FAST boats with plenty of horsepower and I do agree having more of the boat out of the water does help speed. I am used to slower speeds and less horsepower that is eaten up by the cupping and the extra surface area of highly raked props. I want to thank you for the extra knowledge I have gained from your post. I can't ever seem to get enough knowledge about how to prop a boat when using all of the correct elements in a prop for that particular boat and motor combination. It is not really a black art, but the prop manufacturers make it seem that way by not having any specs on their props. I called Yamaha a couple of weeks ago inquiring about their different models and what were the differences in them, I asked what was the rake, were they progressive pitch, DAR and they knew absolutely NOTHING. I couldn't believe that so I got the number for Yamaha in Japan and when I called them I was informed that it was proprietary information. My next statement was, are you trying to tell me a prop shop can not take that prop and find everything they want to know just by putting it on the bench and going over it, that is when it got heated. LOL They keep us in the dark so they can sell more props. It is no different than them not having the information readily available on the torque and horsepower curves of their engines, we could make more informed decisions. In a hundred years they still don't have a STANDARD for CUP on a propeller, one manufacturers heavy cup can be another ones light cup. Let me get off my bandwagon, and I do thank you for the extra knowledge.


H

Your oh so right.

Yamaha has always been lack luster in their propeller design. Thier propellers have been made by Turbo/ ppc for years. The geometry and technology was and still is only know by a few internally at the plant in Indiana. I think you will be seeing their products improve in big steps now that they are under 1 roof. Yamaha bought Turbo a year ago.

You won't get straight answers from most propeller manf. because they don't know themselves. In most companies, the hydro engineers are unavailable to the general public.

The one place you used to be able to get straight answer right from the horses mouth was when Dennis was still alive and he and I ran the Mercury Racing propeller dept. People would ask any question and we would answer it. That was a service unique within the industry. The current times have changed that to some degree.

We will help you at BBLADES in any way we can, just ask.

Brett

Brett, thank you very much for your response. I have been studying propellers and the mathematics and geometry of them for many years and feel I have a good understanding of their airfoils. The parts that are hard to deal with is every boat is different and can have built in problems from so many variables. A perfectly made bottom can be ruined in a matter of a few years just by having the wrong trailer supports underneath it. I have seen boats on trailers with a good 14" hanging off the back of the trialer with a 150 on the transom. LOL But, it is interesting how props with totally different elements and geometry can produce the same speeds, but with extremely different handling characteristics of the boat itself when they are running at higher speeds. Maybe with a few more years of studying I will be able to transfer some of my knowledge as you do so willingly.



H