Propellers. 3 or 4 blade? What is the difference?

[Jsta_Rebel]

Good afternoon. I currently have a 26" X 24 pitch 3 blade propeller. I Need to install a new prop. There is a 24"x 26 pitch 4 blade prop available locally. I'm not a propeller expert and wonder if this propeller would be ok to use, or if I'm asking for trouble.

Boat is 35,000 lbs. 40' overall, 38 at the waterline, aprox. 12'. 5' beam at the waterline. 130 hp, 2500 max RPM, and a 2.45: 1 velvet drive. She's a motorsailer and I use her as such. Normal cruising RPM is 1800 or less. Hull speed is 8.5 knots, and I never run her at full RPM.

So are there any prop guys on the board that can offer up their opinions?

[Jsta_Rebel]

I'm in SXM. I lost a blade off of my prop and we limped on into Margot. Mike over at Shrimpy's has the prop I spoke about in my first post. We're side saddling up the dinghy this afternoon to go inside and post up to haul out tomorrow. Overdue for a haul out anyway, so I may as well get new bottom paint and a new cutless bearing as well as check the shaft rather than changing the prop in the water and calling things good. However, I'm a boatie and don't sail around on a 400,000 dollar boat so saving $1,500.00 is a big deal to me..

I was thinking that out of all of the posters on this board, that a couple of people on here must be fairly intelligent when it comes to props, so PLEASE.. If someone could help me out with what may be a possible substitute for my prop and save me some money, I'd appreciate it. If I'm stuck paying $2,200 for a new prop, so be it, but being I'm not top shelf when it comes to propulsion, I was hoping someone on here was.. Thanks..

[Phoenician]

My understanding is that more blades = less noise, and generally, greater thrust at a given rpm

You also gain the benefit of a smaller diameter.

MaxProp, whose classic 3 blade design equips thousands of boats around the world, is now promoting their 5 blade design, for precisely the reason above.

As to whether this justifies the extra cost, I guess only you can be the judge of that!

[Jsta_Rebel]

It's not extra cost. The 4 blade would save me $1,500. plus freight and time if it would work out. I just don't know what 2 " smaller diameter and going to 26 pitch from 24 would do which is why I asked the question. Anybody know what I could expect from my boat if using the four blade prop??

[DougR]

Without knowing a bit more about the engine performance (RPM) with the existing prop, a quick and dirty answer is that the 3 blade prop that you currently have doesnt seem optimum for your vessel, and the 4 blade wont be optimum either. That said, it will probably work satisfactorily, but your sweet spot for cruising might come at a slightly different RPM than with the 3 blade.

Want to explore a bit more?.... tell me the max RPM achievable under engine alone, and the max RPM achievable when motor sailing with a fair breeze.


Couple of other comments about props that might be interesting:
- A three blade prop is more efficient than a four blade prop if the diameter is not constrained.....ie. if the diameter is large enough that the blade area can absorb all the HP available. The reason is that there is less inter-blade interference on a three blade prop.
- Most modern powerboats run four or even five blade props for a couple of reasons:
-smoother....the blade tip pulses on the hull are not as severe as on the three blade props
- designers are trying for flatter shaft angles for more efficiency, and flatter shaft angles result in smaller prop diameters... hence they have to go to more blades to have enough blade area to absorb higher HP

-Sometimes the use of a four blade prop located close behind a wide and poorly faired deadwood can result in more vibration than a three blade prop behind the same deadwood. This is because there is an area behind the deadwood where there is little water flow as compared to the free stream flow to the side of the deadwood. A three blade prop hits this "stagnant" water area one blade at a time, whereas a four blade prop hits it two blades at a time. This two blade impact is sometimes the cause of a vibration, but its difficult to predict.

DougR

[Stumble]

There is no single answer, it depends on the boat and the available size for the prop, boat writ, engine HP, designed speed, wave conditions, wind conditions, and a few other things.

By preference you generally first go to the largest prop that can fit the aperture. Then you add as many blades as necessary to convert the available torque to propulsive force. While matching the pitch to the boats speed, engine rpm, and slippage.

There are a number of online calculators that can help, and many prop manufacturers can select the idea prop of their line for your boat.

Right now you seem to be over pitches pretty substantially. If you want to go to a 24" four blade prop the ideal pitch is closer to 17.3. But you should be using a 26 3 blade at 17.7. I am curious why you are currently so out of whack to be honest.

[Jsta_Rebel]

Originally Posted by DougR

Without knowing a bit more about the engine performance (RPM) with the existing prop, a quick and dirty answer is that the 3 blade prop that you currently have doesnt seem optimum for your vessel, and the 4 blade wont be optimum either. That said, it will probably work satisfactorily, but your sweet spot for cruising might come at a slightly different RPM than with the 3 blade.

Want to explore a bit more?.... tell me the max RPM achievable under engine alone, and the max RPM achievable when motor sailing with a fair breeze.


Couple of other comments about props that might be interesting:
- A three blade prop is more efficient than a four blade prop if the diameter is not constrained.....ie. if the diameter is large enough that the blade area can absorb all the HP available. The reason is that there is less inter-blade interference on a three blade prop.
- Most modern powerboats run four or even five blade props for a couple of reasons:
-smoother....the blade tip pulses on the hull are not as severe as on the three blade props
- designers are trying for flatter shaft angles for more efficiency, and flatter shaft angles result in smaller prop diameters... hence they have to go to more blades to have enough blade area to absorb higher HP

-Sometimes the use of a four blade prop located close behind a wide and poorly faired deadwood can result in more vibration than a three blade prop behind the same deadwood. This is because there is an area behind the deadwood where there is little water flow as compared to the free stream flow to the side of the deadwood. A three blade prop hits this "stagnant" water area one blade at a time, whereas a four blade prop hits it two blades at a time. This two blade impact is sometimes the cause of a vibration, but its difficult to predict.

DougR

Thanks for the reply Doug. The engine base hp Is 120. RPM is 2500 max. speed @max hp is 8.4 Knotts. I usually run the engine between 1500 and 1800 RPM motorsailing. 1800 just motor gives me aprox. 7 knots with just motor. I included much info including gear ratio in my original post when starting the thread. Again, apreciate all the help I can get on this subject..

[DougR]

Want to explore a bit more?.... tell me the max RPM achievable under engine alone, and the max RPM achievable when motor sailing with a fair breeze.

Looking at the 3 blade propeller that you are currently using, and also the proposed 4 blade prop, both Stumble and I suggest that you are over propped by about 6 inches of pitch on both props. This means that you should not be able to attain the 2500 rated RPM of the engine, and the engine wont produce the power that it should. It is possible however, that the extra thrust provided by the sails when motor sailing will unload the prop somewhat and allow additional RPM. Thats why I asked for the RPMs that the engine can actually make, both under engine alone and also when motor sailing.

However, to simplify a bit, consider the following:

- optimum three blade 26 inch prop for 8.5 kts and your HP/RPM/gear ratio combination is a 26 X 18 X 3 which gives 40% efficiency and 2190 lbs of thrust

- optimum four blade 24 inch prop for 8.5 kts and your HP/RPM/gear ratio combination is a 24 X 19.5 X 4 which gives 38% efficiency and 2120 lbs. of thrust

So.....both of the props under consideration will perform in a similar fashion, but both could perform better with less pitch.

DougR

[Jsta_Rebel]

Thank you for your input Doug. I entered data to an online prop calculator and it gave me similar results as are in your post. The 26 x 24 three blade prop is what every IT 40 came with from Marine Trader. Interesting enough, this same Prop configuration is also on the IT 46 which is considerably heavier and wider at the beam than the 40 which got me thinking about it. FYI, the engine can make 2500 RPM with the 26 x 24. and speed is 8.4- 8.6 knots. I've never ran max RPM with all sails , so I couldn't answer the other question honestly. I just have never had or felt the need to run her that hard.

Thanks again for your posts..

Patrick.

[DougR]

Maybe the actual gear ratio is more like 3:1 ....? That would make sense..

Anyway, good luck with it!

DougR

[Jsta_Rebel]

My grandest apologies to both posters. The prop information I was offering up was from the maintenance logs/ mechanical info. that that came with the boat some 12 years ago. I got the prop and cutless bearing housing off today, Scraped it off, and the reality is that you gentlemen are spot on. The prop is indeed a 26 x 18 pitch. The shaft information was incorrect as well and is in fact 1 3/4" measuring the large bore. Call me what you will, but I'm a fairly straight flying individual and wanted to post openly that your information was absolutely spot on. I'm running over to the Dutch side in the morning to speak with Steve who is a salvage diver who says he has something close that could get me by until I'm able to get a proper replacement. I'd like to feel free to run the dimensions by you if he doesn't have a perfect fit if you gentlemen wouldn't mind.

[DougR]

Patrick,
No apologies needed! Its a common situation....the literature says one thing and the reality is something else.

I will be happy to comment on the available selection of props, but I will be unavailable tomorrow (Wednesday) and Thurday, returning on Friday.

Meanwhile, you have an idea of the diameter/ pitch range where your prop should be. If you stray too far, efficiency will suffer.

Regards,
DougR

[Jsta_Rebel]

Originally Posted by DougR

Patrick,
No apologies needed! Its a common situation....the literature says one thing and the reality is something else.

I will be happy to comment on the available selection of props, but I will be unavailable tomorrow (Wednesday) and Thurday, returning on Friday.

Meanwhile, you have an idea of the diameter/ pitch range where your prop should be. If you stray too far, efficiency will suffer.

Regards,
DougR

Thanks Doug. Yeah, I have a much better idea now. Have a great.couple of days. I'll let.you know how I ended up. Time honestly because of work is my issue btw. I sourced a new prop from the states today, but I've got to go to St. Kitts and this prop probably won't arrive until Tuesday next week at the earliest. I don't like leaving my boat anywhere while working. Especially this time of year.

Thanks again..