Online prop sizing calculators

Breakin Away · April 17, 2019, 07:43:52 PM

For reasons mentioned elsewhere here, I need to buy a new prop right now, a year earlier than planned. I had hoped to do some full throttle testing on my old prop this season, but that's no longer possible, so I have to go with the data I have available. If I can get some confidence in the specs, I may go with Flexofold instead of fixed prop. I know this has been discussed widely here, and we have somewhat different motors and transmissions. For this thread I'd like to focus on modeling results.

I spoke to one prop shop who took a few numbers from me and suggested I go with 15" x 10" 3-blade. I also spoke to CatalinaDirect and they took a much more extensive range of data and will get back to me tomorrow with their results.

I did find a couple of online calculators that gave me results ranging from 17" x 11" (Michigan Wheel calculator at https://www.miwheel.com/inboard-propellers/prop-it-now/ ) to 15" x 9" (https://www.vicprop.com/displacement_size.php). That's a huge range that just increases my confusion. I am curious whether any of you have any experience with these calculators and how they match up with your own experience.

Also, I found this article that warns of the dangers of over-propping, and makes me tempted to "leave well enough alone" with my current pitch: http://www.oceannavigator.com/January-February-2017/Are-you-over-propped/

FWIW, 15" x 9" 3-blade Michigan Sailer is what I currently have, and was thinking I'm a little under-pitched based on my max RPM of around 3400. But as I study this, I learned that there's more to this than just pitch, and things like E.A.R. can have an effect. My original Sailer design has very low E.A.R. (skinny blades) and is now discontinued and replaced by more skewed models that have significantly higher E.A.R. that could deliver more thrust (and more sailing drag) at the same pitch. That just complicates things more.

I'm open to your comments on the two models linked above, and any other online calculators that you're aware of. Many of them seem to be for powerboats, so one must select carefully.

Gregory M · April 17, 2019, 09:11:47 PM

To probably confuse you even more (sorry!). I'm planing to replace my prop and therefore, looking..... leaning towards "EWOL" super easy to adjust pitch! Just browse their website. Seams like very taught through design.
http://www.ewoltech.it/index.php?route=product/category&path=60_61

Ron Hill · April 18, 2019, 04:29:46 PM

Breaking : The boat builder dictates the prop diameter - to prevent "buffeting" from the blades against the hull. Very Important!!

BTW, the 3 bladed Michigan Wheel "sailor" props that I have seen, have much large blades (blade cord) than the Catalina 3 bladed "sailor" props!!!!

You need to read my Mainsheet tech note article on "Prop pitch/size and Boat Speed!!

A few thoughts

Breakin Away · April 18, 2019, 08:08:57 PM

Quote from: Ron Hill on April 18, 2019, 04:29:46 PM
Breaking : The boat builder dictates the prop diameter - to prevent "buffeting" from the blades against the hull. Very Important!!

BTW, the 3 bladed Michigan Wheel "sailor" props that I have seen, have much large blades (blade cord) than the Catalina 3 bladed "sailor" props!!!!

You need to read my Mainsheet tech note article on "Prop pitch/size and Boat Speed!!

A few thoughts

I have no intention of ever getting a different diameter prop than 15". I was just citing the crazy 17"x11" result that Michigan Wheel's calculator spit out.

I've read your Tech Note several times over the last few months. Very helpful, but there are no absolute answers, especially since there are so many variants on motor power and gear ratio. Also, there are some people with different goals. I see a lot of comments from people who are only able to get 2800 RPM at full throttle, but are thrilled with their performance at cruising speed. I see some who increased pitch and worked fine at first, but lost speed and/or gained heat as the season wore on. I'm not willing to accept full throttle speed under 3000 RPM (I want that reserve power if I ever encounter really strong headwinds), so I'll probably stick with 15"x9" for my next prop. I get decent speed at 2200-2400 RPM all season long, and great fuel economy, so I'm not going to risk higher pitch.

I agree that Michigan Wheel has changed the design of their Sailer props. The really early ones like came on our boats have very skinny blades (low E.A.R.), but the newer ones have fatter blades (higher E.A.R.). There are a few pictures on their website showing the thinner blades, but all the product listings show the new props, so I assume the older ones are either discontinued or were a special design specifically for Catalina. A prop of a given pitch but higher E.A.R. will get more thrust and thus act as if it has higher pitch.

Noah · April 18, 2019, 09:11:04 PM

If it were me and I had the "advantage" of being able to pull my boat out every season, I would try the higher 10 in. pitch, see how it preformed. If it proved unsatisfactory, then at the end of the season, take it off and have the prop shop re-pitch it to 9 in. Won't know unless you try.

Ron Hill · April 19, 2019, 03:06:48 PM

Guys : I have tried to get a logical explanation to "no avail", of why Catalina's OEM 3 bladed prop is a 15"x 9" for a 21hp engine C34 and the SAME for a 35hp engine C34??? It's like HP is not a factor?!?

Maybe it's the same logic as a linear gage reading for an asymmetrical fuel tank??

As Noah said, "If you don't try another pitch you'll never know"!!

A few thoughts

Redwing · April 19, 2019, 05:34:16 PM

I went with the 15" 10 degree max prop feathering and have been very happy with it. It's currently set at 10* but I can't get maximum rpm from the engine (M35, 30 Hp at 3200 rpm)

Breakin Away · April 19, 2019, 06:11:33 PM

Noah and I had a good talk on the phone today. His experience is very helpful.

Quote from: Ron Hill on April 19, 2019, 03:06:48 PM
Guys : I have tried to get a logical explanation to "no avail", of why Catalina's OEM 3 bladed prop is a 15"x 9" for a 21hp engine C34 and the SAME for a 35hp engine C34??? It's like HP is not a factor?!?

Here's a logical explanation why lower pitch may be a better fit for the more powerful M35B motor, although it admittedly fails to explain why Catalina didn't use a higher pitch on the older boats (which seem to be working very well with 15"x10"):

Horsepower is a big factor, and that's why the 35 HP M35B/HBW100 has a more aggressive gear ratio (1.79:1) than the earlier 21 and 23 HP motors (2.05:1). At a given motor RPM, the M35B's transmission uses the motor's greater torque to rotate the prop 15% faster (1.79:1 reduction vs. 2.05:1 reduction). Since our boats all have the same 29.8' LWL and therefore the same 7.3 kt theoretical hull speed, a prop of a given diameter and blade shape that's spinning 15% faster with the M35B/HBW100 will "screw" the same distance/speed through the water with a 15% lower blade pitch. So the M35B powertrain is properly matched to the boat's hull speed with a 15% lower pitch prop. Attempts to put a more aggressively pitched prop on the M35B powertrain will accomplish little other than preventing attainment of full spec RPMs because the boat cannot plane - it will just dig its stern in the water as it tries to "climb out of the hole". So all you guys with older, less powerful motors can stop scratching your heads over this. Just because your 15"x10" or even 15"x11" prop works well for your powertrain does NOT mean that it will work on the MkII boats with more power motor and more aggressive gearbox. We spin our props a lot faster. As a first guess, I'd take any prop that the older motors are spinning and reduce its pitch by 15% for the M35B/HBW100.

I've read as many comments on this as I can, and can see that many older boats are seeing success with the higher pitched props. Some MkII owners are also happy with 15"x10" props. But it looks to me that among those who are not happy with 15"x10" props, virtually all seem to be with the M35B/HBW100 powertrain. I believe that my above explanation is why. I don't have time to go back and re-read everything I've seen over the last several days, but here is one example:

John Langford, MkII boat: https://c34.org/bbs/index.php/topic,8715.msg62337.html#msg62337

QuoteWhen I used the 15x10 configuration on my new flexofold the stern submerged at my normal cruising revs of 2200 rpm and I couldn't get to 2900 at all. For the MK II at least, 15x9 is the right combo. It's a great prop in forward and reverse.

I think there's another example that just posted while I was writing this.

There is a lot more to a prop's design than just the blade pitch. The Flexofold props, in particular, have been shown to generate significantly greater speed than other props at a given RPM (see the famous Yachting Monthly tests). https://www.yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807 That just doesn't happen by magic. If a prop is giving more speed at the same RPM, it is going to have more resistance against the shaft's torque, and result in lower RPMs at full throttle. So in my case, where I'm planning to replace a "skinny blade" original Sailer 15"x9" prop with a Flexofold 15"x9", I think the Flexofold's blade design is going to lower my maximum RPMs even if I stay at 15"x9" blade pitch. I do believe that 15"x10" might fail to achieve full throttle RPM of 3000 per Universal spec, so I am gaining confidence in my choice.