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oldtimer57.
The two terms that come to mind are static thrust and dynamic velocity. Static thrust is something that is a function of diameter, blade area and pitch. As diameter goes up, pitch has to go down or else RPM goes down. Bigger props push a large load better.Smaller props produce less thrust at low speeds, but because of their increased pitch, they produce more velocity at higher speeds.One of my hobbies (besides high-performance boats) has been high-performance model airplanes. We have a form of radio-controlled racing called “quickie 500″ and the main thing we can tweak is the props, because the airplanes are fixed (the 500 comes from 500 square inches of wing area), the motors are fixed, so props are all we can change. And it is a complex issue. 9×6 (diameter vs pitch) vs 9×7, vs 8×7, etc. All fly at different speeds, accelerate out of the corners differently, etc.My fastest ever prop on my previous bass boat was a 28” chopper of the variety made for the older 115hp (small gear case) motors. It had a smaller diameter than the normal v6 choppers. The increased pitch would let it fly, and the reduced diameter let that motor turn the prop fast enough to fly. But the reduced diameter/area had a side-effect, in that at cruising speeds, slip was very high. If I added a passenger it became a dog. Etc.So, to answer your question, you will probably find a larger diameter provides more consistent performance across a variety of loads and weather conditions, but will be slower than a smaller diameter/steeper pitch prop.It is a wildly complicated issue. Racing teams spend hours and hours modeling props, running them in fluid tanks, etc. Many teams also cover their props between rounds to keep their trickery under wraps…Kind of a rambling answer, but that is a general description of what diameter does. One thing is for certain. For a specific boat, with a specific horsepower and torque curve, and a specific final gear ratio, there is an optimal diameter/pitch for a specific load and weather conditions. A basser is probably not going to haul around a couple of dozen props to handle the variables, and will choose something that is more general-purpose. There are just a few of us idiots that are interested in finding that “optimal setup” and are willing to invest the time and effort to do so. Not to mention the cursing and the money. 2008 Pantera Classic2014 Mercury Pro XS 200
Oldtimer57, You really shock me with your knowledge on motors and props. Its great having you here!
Thats what makes oldtimers “oldtimers”. Been there, done that, got the T-shirt _many_ times over. If you read a response from the BCB guys to me in another thread, theyve been down the same road as well. My problem, if you want to call it that, (and my wife does regularly BTW) is that I really am a scientist in real life. Computing is what I do day in and day out. And that leads me to drive people crazy with the “whys” (when I was young) or doing the research myself (today). But understanding what is going on is the first step to understanding how each change will affect things and why it is that way in the first place.A few years ago I spent a couple of miserable days on the water with a friend from LA. He brought a literal truckload of props (he was in the prop business somewhere down there). For example, he had one of every kind of chopper and cleaver from 24″ to 32″. He had big ear and small ear choppers. He had choppers that had been cut down a bit, choppers with thinned blades, choppers with more or less cupping, the list goes on and on. And we bolted nearly every one on his boat and ran up and down the river doing acceleration tests, top end tests, corner holding tests, etc. Would it surprise you if I told you that a prop wrench can eventually make a grown man puke? I learned a lot about diameter vs pitch, etc. And after we picked out a few favorites, we hit the river again with both of us in the boat and we discovered that our favorites were not so hot with an extra 200 pounds on board.About the best you can do for a bass boat is pick a good compromise that will get you to a reasonable speed, in a reasonable amount of time, in a reasonably pleasant to drive way. And not worry about the fact that you are very likely leaving 3-4-5 mph on the table by not carrying around a trailer full of props to optimize to the prevailing conditions. 2008 Pantera Classic2014 Mercury Pro XS 200
oldtimer – I enjoy your posts as well. They are very informative this one in particular.Jason Ferbrache – Edmond, OK 1995 Basscat Eyra
Thank-you oldtimer for the input. I have found that the 25P doesnt allow me to hit the rev limiter like the 25P Trophy. I was also running a 26P Trophy, and it would not hit the rev limiter, and it ran faster with tourney load, about 70, maybe 71, down current. But this 25P runs at about 5510 rpm and I am running around 68, give or take a few tenths either way. And I feel that I have a little better hole shot, but the big thing for me is the fact that the dealer had never had a blade break unlike the Trophy. They seem so much thicker. Thanks again for your input.Phil WilliamsBay Minette, AL
The Trophy+ has “small” hub diameter.What does a smaller hub diameter do/ not do?Thanks in advance.Tim
the hub matches up with the diameter of the gearcase. That gives a smooth water transition from gearcase to the hub. A smaller hub lets some exhaust flow over the hub, which can help ventilate the prop on take-off. It also creates some turbulence around the center of the blades which can be good or bad depending… And at extreme speeds (not so likely with these kinds of props) it helps create blowout, which if you have not experienced it, is an exciting thing to have happen at 80+. Rear end of the boat just suddenly darts right and you make a 90 degree “bat-turn” _right now_. And if you are lucky, you just get banged against the side of the boat. If you are less lucky, you get thrown out. If you are unlucky, …, well you get the picture there.Chopper/cleaver props use over-hub exhaust. They are usually extremely high pitches, and ventilation is critical or one could never get up on plane with em. Thats the idea behind the PVS plugs in fact. Get some exhaust into the prop arc to displace some of the water and make it easier to wind the prop up to get into the engines torque band quickly…There are so many possible variables one could twiddle with, it isnt funny. Pitch. Diameter. Hub. Cupping. Blade rake. progressive pitch. Blade width vs length. All props are a compromise in order to eliminate a lot of potential choices…2008 Pantera Classic2014 Mercury Pro XS 200
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