Prop size

Rob Gillaspie

Member
Oct 26, 2018
23
1
OK guys, I need your help, I need to know what the largest prop I can hang on this old boat....

26 Open with 5.9 cummins, we think it is 260 HP... I think it has a 18 x21 4 blade on it now, its old and hard to read... I can turn this engine up to 3500 rpms and I am only running about 27 mph... so with all of you guys out there, how big can I go, I understand its gonna hang past the keel, I don't mind that...


Rob
 

[email protected]

Well-Known Member
Aug 4, 2015
718
83
Pacific Northwest
Sounds like you're already turning a heckuva screw, but you need to know your engine's rated WOT RPM. Size your prop to make sure you can still hit that target WOT RPM. If you can't hit it, you're over-propped. You will likely have to go by trial and error, since it seems every one of our configurations are a bit different: motors; loads/weight; transmission ratios; and of course prop sizes and blade configurations.
27 MPH sounds pretty darn good to me actually. But I don't think any/many of us are looking to beat any speed records with our boats and greatly favor the reliability and economy of our diesel power over speed. But having said that last year I re-propped to from a 3 blade 17" to a 4 blade 18" and gained about 20% (from 21 to 25 MPH) in speed at WOT (2,600 RPM) for my Cummins 5.9 (6BT=210HP). Depending upon sea conditions I'm generally running @ 2100-2400 rpm when cruising in fair-good conditions to the fishing grounds and burning 3-4 GPH.
 

Damon

Well-Known Member
Supporting Captain
Jan 26, 2003
3,260
101
San Diego, CA
Ron, what gear ratio do you have with your transmission?
OK guys, I need your help, I need to know what the largest prop I can hang on this old boat....

26 Open with 5.9 cummins, we think it is 260 HP... I think it has a 18 x21 4 blade on it now, its old and hard to read... I can turn this engine up to 3500 rpms and I am only running about 27 mph... so with all of you guys out there, how big can I go, I understand its gonna hang past the keel, I don't mind that...


Rob

Ron,

You really need to indicate the HP of the Cummins (BT vs. BTA), and the gear ratio on the transmission, to determine the correct prop. Bigger is not always better. These came factory with the 210 HP Cummins BT, 1:1 ratio Velvet Drive transmission, and a 16 x 17 3-blade prop (I believe this was slightly over-propped). Guy's here have opted for the 1.21 or 1.22 to swing a 17" 4-blade, and I think one member went with a 1.52:1 transmission to swing an 18" 4-blade. From my understanding, it's very critical to reach the maximum rated RPMs on the diesels.

I read your previous posts and it seems that you do not know for sure what you have. I'd reach out to somebody to determine exactly what you have. This is important!
 

Rob Gillaspie

Member
Oct 26, 2018
23
1
ok mine is 6bta, because it has a aftercooler, or that is what I am told... the engine does not have a serial number, or I can't find it... but I have been sent the right parts for it from sbrmarine I don't know what the tranmission is... maybe I should post pictures, I will do that next week... pretty much all the issues I can find have been cared for, I am about ready to install the new baitwell, and take this baby for a ride... plan is to launch a Ft Lauderdale and cruse the Bahamas for a few days... prop has a few dings, I am gonna have it worked, however I think I can go bigger because I can turn it up to 3500 rpms with this one on it..
 

Nay

Well-Known Member
Dec 1, 2007
577
80
Suffolk, VA
Rob,
Check out the thread listed below, especially at comment on post #8. It discusses the Rules of Thumb (ROT) that the factory would use in determining the initial propeller size for a given displacement and disc loading. Be especially mindful of the max diameter and disc loading calcs.

http://www.shamrockboatownersclub.c...tip-distance-from-the-hull.32762/#post-277712

Using this method should get you VERY close to the correct prop. May need to fine tune the pitch an inch or two (or cup substitute) after initial trials.

BTW, just noticed that the old thread has an example prop diameter of 14 FEET which is obvisously a typo. The example calcs used 14 INCHES. Sorry.

Nay
 

Tuloko77

Well-Known Member
Supporting Captain
Sep 12, 2020
87
13
Mammoth Lakes
Yes, you're right Damon, good catch!!!
Joel,
It sounds like we have a similar setup... My 1998 Mack with a Cummins 210 and a 1.22:1 gear has a 17x17, 4 blade. I feel I am a little overpropped however, as I can only achieve 2,500 RPMS WOT. If not overpropped, then I'm likely having a fuel delivery issue... What is the pitch on your 17"? And what kind of RPMs can you achieve at WOT? Thank you.
 


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