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Guys, with all the prop info available on this site my brain is getting bogged down and I’m finding myself thinking in circles. Trying to order wheels,and after pricing, I’m definitely wanting to get it right or close the first time. 300hp 6BTA’s 970 cpl to be treated as 250hp don’t exceed 2500rpm cruise, WOT, tip speed, keep e’m square, pitch, 4 blade, 3blade, cup, bronze, nibral, weight, cruise speed, fuel economy, trolling speed, load on engines, throttle just touching governor. Do I need to keep going? The only givings are the motors and hp rating, 12 degree shaft angles, motors moved back 3” from factory install, 1 1/2” Aqualoy shafts, 1.5 to 1 220ZF trans. I have read on thread where Uncle Vic was running 21x22 three blades with same setup but he states he was under propped. Tony says he’s running 20x23” with some cup? But I can go up to 22” with some pitch? OK, how much cup and how much pitch? Not trying to sound testy here but it’s a bit overwhelming for the way I’m programmed. At this point I’m leaning towards a 21x23 4 blade nibral with medium cup? I can have diameter reduced if need be without having to purchase new wheels. Correct? As mentioned before I am not a speed demon just looking for 24-26 knot cruise with the most fuel efficiency I can get even if sea conditions mean pulling her back to 20 knots. Help. Thanks

At 22 inches you are pushing it for hull clearance. If you go 22 watch the hull for prop burn from the prop wash.

The 250/270 Cummins top out around 2650 room's. The torque curves are differnt than the 300hp.

3 blade props give you a lower cruise and a higher top end speed, 4 blade a higher cruise and lower top end speed. Nibtals give you a staiffer blade so less slip.

Now Bob took his to a whole other extreme. He took a prop blank that would be used for a bigger prop, then took it down to 21 inch diameter. This gave him more blade area which increases the efficiancy of the prop. Depending on the boat that could mean an extra 2 knots.

For what you are trying to accomplish, by running the engines at lower rpm than how they are rated, you really need a good prop shop. I think your situation makes it a little harder than copying what we have all done.

If the engines were run as designed, what rpm are they rated for at the top end? Matching my prop and running at 2300 rpm might not get you a 23 to 24 knot cruise due to the torque curve difference especially if your engines are rated at a higher rpm.

Hopefully Carl comments on this since he is better at the prop issues than I am, but I think you will have to play with it for a bit.

Tony- I have never been anything more than the middle man between customer n prop shops, some very good and some less expensive ones.

With that gathered a basic understanding and rules of thumb…nothing more.


My 02, search this board for members with your setup, eliminate the guys getting dialed in with high dollar Acme props n custom wheels. Stick to the mass of members running conservatively with tried n true performance. Look for a set of wheels in that range, even used wheels to just get a baseline to your performance after a major refit.
Being non storm block motors you r going to want to be slightly under propped, that is hit your WOT rpm plus at least 50-100rpm.

Once you have performance numbers for your setup to work with you can dial her in. Tweak your props or spend more for better props if you want.

I think my point, shoot for what is tried n true

Got it ,thanks

CPL 970, like Carl said but take it on the high side. 100 RPM OVER the rated RPM of the engines at WOT...NO LESS!!!

That means you need to get 100 RPM over Rated RPM at your normal cruise weight / configuration. Including fuel, people, etc etc...

Even underpropped like that you are still going to cruise 26 to 27 knots and maybe even more. It will be like you are running 250s - 270s

Those are good motors as long as you run them kindly. The original welded aftercoolers are a problem and hopefully yours have been upgraded to the newer style "serviceable" aftercooler

Another data point for you. Sounds like I have a similar setup as you. I have 300 HP 6BTA M2 CPL 970 with 7mm injectors. 1.5.:1 gears. I swing 20x23 Nibral Michigan Hytorq 4 blade propellers with medium cup . I feel that I am slightly under propped but I'm fine with that. I have a flybridge curiser, but probably lighter than most since I ripped out my restroom and kitchen area and have a laminated foam cockpit. No generator. I cruise at 23 - 26 knots depending on load at 2200 rpm. Top speed is 33 knots at 2900 rpm with no load.

There should be enough data points on here to make a good guess. Ive compared a lot of them myself, and they're all really similar, if you account for the the difference of rpm, gear ratio, and tip speed.
There's a lot of "magic" surrounding props because there are so many variables.
I also believe "correct" is a range to shoot for, not an exact point.
Just remember, better is the enemy of good, props are expensive, and boats are supposed to be fun.

Let us know what you decide on, then we'll tell you what you should have done instead.

Geebert wrote: ↑Sep 5th, '23, 11:57 There should be enough data points on here to make a good guess. Ive compared a lot of them myself, and they're all really similar, if you account for the the difference of rpm, gear ratio, and tip speed.
There's a lot of "magic" surrounding props because there are so many variables.
I also believe "correct" is a range to shoot for, not an exact point.
Just remember, better is the enemy of good, props are expensive, and boats are supposed to be fun.

Let us know what you decide on, then we'll tell you what you should have done instead.

Great post! Spot on

Geebert wrote: ↑Sep 5th, '23, 11:57 Let us know what you decide on, then we'll tell you what you should have done instead.

Geebert wrote: ↑Sep 5th, '23, 11:57 Let us know what you decide on, then we'll tell you what you should have done instead.

That is so good it had to be quoted at least twice...lol

Oh Boy. That would have been something UV would have said.

OK guys, you can pile on now, props came in today. Ordered 20X23 4 blade nibral with medium cup used. Got them from a fellow who was referred to me by a Captain friend of mine that runs a 72’ Viking. Service was great and sales seemed very knowledgeable. FYI, guys name is Brian with Wildcat Propeller out of Virginia. Props were gone thru, balanced etc., and sent with bar graph data sheet that’s “above my learn’n”. Just wondering, how can you tell the difference between bronze and nibral?

Wildcat is a great shop! You should be good with that size propeller. My buddy runs that same size with 330’s

The easiest way to tell is when it says “Nibral” or “bronze” on the hub of the prop.

Aside from that you can tap the blade with a piece of metal, like ringing a bell. The tone of Nibral is considerably different than bronze. It’s higher n rings a bit longer. The downside is you need to be familiar with the sound of both to distinguish the difference.

Good luck with the wheels

You should be fine with those props. At most you might need to adjust the cup a little bit, but that is the same size I run with the 270 cummins.