Props, 3 blade vs 4 blade

rcwass · Posted: Mon Feb 13, 2006 6:11 pm Post subject: Props, 3 blade vs 4 blade

I operate in high current waters. My 22' cruiser is all over the place in this current. I currently have a 1990 70HP Johnson but will be re-powering in the early spring. Most boaters I know have 3 blade props. My current prop is a 3 blade. I'm told that a 4 blade prop would give me more control. Does anyone have knowledge of what a 4 blade prop would give me over a 3 blade prop? Thanks for your help.
Dick

starcrafttom · Posted: Mon Feb 13, 2006 7:01 pm Post subject:

This has been talked about before here at the site. try a search. also the whole compsite over alum or steel prop. I have run a comprop and now have a 4 blade alum one and I really like it.

rogerbum · Posted: Mon Feb 13, 2006 9:03 pm Post subject:

Here's a couple of links from elsewhere on the site

http://www.c-brats.com/viewtopic.php?t=1276&highlight=prop++blade
http://www.c-brats.com/viewtopic.php?t=837&highlight=prop++blade

You could
1) Look at these links - which discuss it a lot w/o really coming to a conclusion
2) Do your own search to find even more links which also discuss it without really coming to a conclusion
3) Do whatever feels right to you or
4) Wait for DoD or BC to reply since there's nothing they like better than talking about props Laughing

(and then do 3)...

Roger on the SeaDNA

Chuck S · Posted: Wed Feb 15, 2006 7:47 am Post subject:

When I bought a spare propeller for my 16 Cruiser/Honda 50 the dealer recommended the 4 blade version. I installed it to test and haven't taken it off, the original 3 blade is the spare. No scientific test data but the 4 blade wheel seems to provide a bit more control in rough condidions (for some reason).

-- Chuck

Pat Anderson · Posted: Wed Feb 15, 2006 8:49 am Post subject:

OK, here is a comparison, including MPH and MPG of two identical CD25s, both with Honda BF150s. Pat (Daydream) is running a 4 blade ProPulse and David (Anna Leigh) MUST be running the stock 3 blade stainless, based on reported speeds (I was getting exactly the same speeds when I ran the 3 blade). If anything, Daydream is loaded heavier (sorry I can't do real tables here - is there a way?).

RPM MPH..............MPG
.......Pat.David....Pat.David
2500 8.3 6.2.......5.5 4.8
3000 10.3 8.0.....3.0 3.5
3500 13.5 9.6.....3.1 2.5
4000 17.0 12.5....3.2 3.1
4500 20.7 15.5....3.6 3.2
5000 23.8 19..... 3.5 3.1

I am sure we will both do a lot more checking with the Navmans (Navmen?) but one thing you can see right off the bat is the 4 blade is giving higher speeds at lower RPMs, which I presume would translate into less engine wear - but it seems like we both have our "sweet spot" for fuel economy somewhere around 4500 RPMs, and 4 blade is giving a little bit better fuel economy.

Papillon · Posted: Wed Feb 15, 2006 11:58 am Post subject:

Pat here is you helping hand....Prop Chart.

Sea Wolf · Posted: Wed Feb 15, 2006 12:59 pm Post subject:

Pat-

Pretty interesting!

Assuming David's 3-blade is the same 15 inch pitch as yours, it would seem that the results support my theory that heavy boats like the C-Dory benefit more from the extra area of a 4 -blade prop than they can from the speed accomodation of a three-blade.

Lighter boats can often make better use of three blade props, which have a greater range of speed accomodation with their lesser surface area and other design differences in rake, cup, etc.

Big boats need more bite to efficiently transfer energy from the propshaft into forward thrust.

Joe.

journey on · Posted: Wed Feb 15, 2006 4:20 pm Post subject:

OK, I've plotted them, as usual. And I hope they appear below. Either that or they're posted on the Journey On album.

I assume that both props are pitched at 15" My 4 blade is pitched at 16", and I'll try to get a RPM curve this weekend. The flowmeter comes later (much later, according to Judy.)

Note that the 4 blade runs slower (lower RPMs at same speed,geared lower) that the 3 blade. The fuel difference at a 15 mph cruise isn't so great, but once you go off 4000 rpm the 4 blade does give better mileage. And the CompProp is half the price of the !@#$ stainless 3 blade.

Boris

tpbrady · Posted: Wed Feb 15, 2006 11:14 pm Post subject:

I didn't see any speed changes on my 22 between a 3 bladed 15 pitch and a 4 bladed 15 pitch. What I did see was better acceleration. With the 4 blade I could feel myself being pushed back in the seat but never felt that with the 3 blade.

Sea Skipper · Posted: Thu Feb 16, 2006 12:15 am Post subject:

Pat, what is your maximum Wide Open Throttle (WOT) with the Pro Pulse prop set @ 15" pitch?

Hank (Sea Skipper)

dogon dory · Posted: Thu Feb 16, 2006 12:39 am Post subject:

Sorry, Folks - Post Deleted By Author

Pat Anderson · Posted: Thu Feb 16, 2006 9:06 am Post subject:

OK, we finally did run her up to WOT - 5600 RPM, about 26 mph, and probably climbing - we just wanted to see where the RPMs would go...

Casey

I've spent some time collecting performance data for the Suzuki 90 with a ProPulse (4-blade) prop set at 17P. I'm fairly satisfied with the ProPulse, (atleast performance seems somewhat better than with the factory 14x17P 3-blade).

Piranha Prop ( www.piranha.com ), another composite prop manufacturer, has a hub/blade configuration that is 14" diameter x 18P (whereas the ProPulse is only 13.5 diameter) that may be my next experiment. (Still trying to get up a reliable 4mpg at 4000-4200rpm cruise.)

With the ProPulse (17P) I got 5600@WOT which is high for the Suzuki (WOT: 4500-5500rpm). I figure the Piranha's greater diameter + 18P may bring WOT down to about 4800 with greater thrust (and hopefully increased MPG). We'll see.

The sea trials are kind of fun, but challenging since you'd got to keep test parameters reasonably standardized. Oh well, when I get the Piranha prop, I guess we'll just have to go back to the Keys for more testing. ...a guys gotta do, what a guys gotta do!

I'd post the Excel spreadsheet data, but can't figure-out how to do it here(...even read the instructions for doing Attachments!). Oh Well. If anyone want's a copy of the spreadsheet, email me. (I know how to do that!)

Casey
C-Dory Naknek
lorencasebeer@aol.com

journey on · Posted: Mon Feb 20, 2006 8:10 pm Post subject:

OK, here's what I found out today in San Diego.

First, Daydream, I assume when you said MPH, you meant MPH, not Knots?
That's a question.

For Journey On, two data points:

3500 RPM 16.0 MPH (13.9 Knts)
4000 RPM 19.6 MPH (17.0 Knts)

Fast enough for Judy, and the boat was loaded for the week end. CompProp at 16 in.

Boris

Pat Anderson · Posted: Mon Feb 20, 2006 9:19 pm Post subject:

Yes, it is mph not knots. When you say 16 in., I assume you mean 16 pitch. I have my ProPulse at 15 pitch. The big question at 16 pitch is whether at WOT you can hit the bottom of the full-throttle range, which on the BF150 is 5000 - 6000 RPM (page 104 of the manual). I originally had it set at 16 pitch and couldn't exceed 4500 RPMs, so I ratched it back to 15 pitch, which is as low as it goes. I did run it up to 5000 RPM just to check that I could get there, and that was nearly 24 mph, but did not keep going to see what RPM or speed WOT might ultimately reach - I am guessing maybe 5500 RPM. For me, the 4500 RPM speed of 20+ mph is about as fast as I am likely to go anyway.