Three Blade versus four blade propeller

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motion30

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Thinking changing my props to a little larger pitch. I can get a good deal on a set of three blade props. The current ones I have are four blade. You think there is the big difference between the three and four blade props at 1300 RPM and approximately 8 9 miles an hour?
 
1. the pitch is completely different from 4 to 3 blade props. A prop shop should be able to tell you the estimated pitch for the 3 blade prop .
2. 4 blade props run smoother.
3 4 blade props are an upgraded prop. You will be down grading.
4 . 4 blade prop cavate less.
 
1. the pitch is completely different from 4 to 3 blade props. A prop shop should be able to tell you the estimated pitch for the 3 blade prop .
2. 4 blade props run smoother.
3 4 blade props are an upgraded prop. You will be down grading.
4 . 4 blade prop cavate less.
4 blades aren't automatically better. If the boat doesn't need the extra blade area available with 4 blade designs (in other words prop diameter isn't limiting) then a 3 blade is likely to be slightly more efficient, although if it's a high blade area 3 blade design vs a 4 blade with higher aspect ratio blades that may not be the case. Comparisons like that are rare though, typically the 4 blade will have comparable blade aspect ratio and more blade area. The 4 blade will typically be smoother although this depends on the boat and isn't always significant. And cavitation goes along with blade area. Some applications are highly loaded and will benefit from more blade area while others don't and don't have cavitation problems to start with.
 
I have not noticed any significant difference between 3 blade and 4 blade on my 22" props. I went with 4 blade because they had what I wanted in stock. In theory, with equiv. pitch, 3 blade should be a bit faster, but in practice I am unable to detect a difference.
 
The internet says....
If you need improved low-speed handling or frequently boat in rough water conditions, a 4-blade propeller may be the best choice. A 3-blade propeller may be the better option if you prioritize top-end speed and efficiency.

I have 4 blade and agree .
 
Thinking changing my props to a little larger pitch. I can get a good deal on a set of three blade props. The current ones I have are four blade. You think there is the big difference between the three and four blade props at 1300 RPM and approximately 8 9 miles an hour?
The 3 blade ones will be better at high speed but the four blades will give better economy in a heavy boat at lower speeds. Depending upon your vessels characteristics you may not see much difference. If you over pitch your boat you may see a decrease in top speed. I would advise you to contact the people that made your boat and ask them first rather that take what looks like a good deal only to find out like i have in the past that I made the wrong decision. ..
 
I run this boat exclusively at low speed no more than 1400 RPM. I am just trying to get a little more heat in the motors at this speed and if it could increase fuel economy a bit that would be great
 
I run this boat exclusively at low speed no more than 1400 RPM. I am just trying to get a little more heat in the motors at this speed and if it could increase fuel economy a bit that would be great
Are you saying you wish the engine temp to be higher than it currently is at 1400rpm? If that's the case then you may wish to use a different thermostat. All this can be calculated for any given hull design displacement and speed. Even location and sea conditions can cause dramatic changes I've often spent money with little if any improvement especially in a displacement hull. ..
If there is nothing wrong with the props you have and they are in the range of OEM specs for your vessel save the bucks for an upgrade your sure will be an upgrade not an if.
 
Would like to learn. Been told in the past an odd number of blades does have some advantages compared to an even number. Is there any justification for that opinion? Know some sailors have gone to 5 blade maxprops with that in mind?
Is that only true when the prop is close behind a keel or skedge?
 
Would like to learn. Been told in the past an odd number of blades does have some advantages compared to an even number. Is there any justification for that opinion? Know some sailors have gone to 5 blade maxprops with that in mind?
Is that only true when the prop is close behind a keel or skedge?
I'm pretty sure that advantage is mostly related to vibration when you have a big keel or skeg in front of the prop causing dead spots in the water flow. But it'll depend on the details of each boat, there's no hard rule for what works best.
 
Best to ask your prop shop. Get your current props scanned and balanced. You can have them add pitch or sell you new props. Whether four five or three blades are best depends on your engine configuration, gear and prop aperature. Many times three blades are better for v6 engines four blades for inline 6’s. It’s a bit of science and voodoo.
 
I run this boat exclusively at low speed no more than 1400 RPM. I am just trying to get a little more heat in the motors at this speed and if it could increase fuel economy a bit that would be great
It is advisable to run the engines UNDER LOAD at full throttle for 5 minutes or so on every days run. This increases engine temp somewhat but more importantly increases cylinder pressures which is important for getting the piston rings to seat properly against the cylinder walls. Not doing so can, over time, cause the rings to not seat properly and create compression losses and increased oil consumption, which exacerbates the ring seating problem.
 
I run this boat exclusively at low speed no more than 1400 RPM. I am just trying to get a little more heat in the motors at this speed and if it could increase fuel economy a bit that would be great

I have swapped between 3 and 4 blade a couple of times. Have one of each for my current single engine boat. At that speed/throttle range I doubt there will be any appreciable difference, all other things equal. Either will do the job. Typically a 3 blade will have more pitch than 4 blade in the same application.

We've talked lots here about over propping, which is what I think you're describing as your goal. My experience is that it's not going to materially change fuel economy. IMO the best action for top economy is making sure whatever props you choose are in perfect condition.

I agree with recommendations to consult with a prop shop. They could scan your existing props, and rework either them or the replacements to achieve your goals and understand/quantify the differences between two different sets of props.
 
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The internet says....
If you need improved low-speed handling or frequently boat in rough water conditions, a 4-blade propeller may be the best choice. A 3-blade propeller may be the better option if you prioritize top-end speed and efficiency.

I have 4 blade and agree .
We are a full displacement not high speed boat. I switched from three to four blades specifically for the low speed handling. It is quite noticeable on Puffin and was also evident on Ebbtide. Made no difference on top speed but the additional volume of water over the rudder makes the stern move around quite nicely.
 
When I bought this boat it had 24 diameter by 26 pitch props. I do not remember if they were three or four blade. The previous owner had died so I had no chance to talk to him. But they had done the Great Loop so I would have to assume they were looking for economy . The motors topped out at that 2,400 RPM. After they're going through all the prop calculators I wound up buying a 24x 20p 4 blade props. They are perfect RPM wise. But like I said I never run it over 1500 RPMs the motors should top out at 2800. Sounds like the consensus here is there's nothing to gain by going to a larger pitch prop. I welcome your comments and thanks to those who have commented already
 
O by the way I am in the Florida Keys and a friend of mine was taking his sailboat to Guatemala. I roughly calculated my fuel cost to be about $6,700 round trip at the current price in the keys about $5 a gallon. It just doesn't seem worth it to me. Of course I might have a different opinion after hurricane season passes. It is forecast to be a bad year
 
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