bertram31.com

As I draw closer to the time I will actually launch Dreamsicle I thought props might be a logical step. I need from the Faithful two things, unless someone has a pair of suitable props. First, hull speed. Second, maximum diameter of props. From the edge of the shaft at the strut, measuring 90 degrees from the shaft it is 11" to the hull. I recall an ancient post saying the minimum distance from the edge of the prop to the hull should be 1 1/2" and the prop should be no further than 1 1/2" from the strut. Correct?
In other words, what do I do from here? With 6BTA 300's, what now? What info am I leaving out?
Thanks guys

Mikey,
You should be able to run a 21x22. My boat is a lot heavier and I run 20x23. Max hull speed for me is around 31 knts with 1/4 tank of fuel.
You should fly with 300's in that light little Dreamsicle.

Mikey,

You need to supply transmission ratio, whether you'll be going with 3 blade or 4 blade.

Clearance should be no less than 10% of prop diameter. A 20" diameter prop needs 2" of clearance. 20" is the max diameter that Bruce & I will install on a B31 with 15º shaft angle. There are some folks using 21" diameter...

You can acheive 21" performance with a medium cup on the blades without getting into your 2" safety margin...

Br,

Patrick

What are the safety issues when going into the less then 10% wheel to hull clearance?

I thought it was related only to performance, cavitation or something along those lines.

Thanks,
Carl

Carl,

Too close to the hull and the force of the prop wash will start eating away the hull....

Br,

Patrick

Patrick,
Thanks. Transmissions 1.5/1. No props at all so trying to get the prosective. Back to you, three blade or four?
Speed is good but trolling at less than five knots for rockfish is important. Why can't diesels be slowed down like gassers?

Mikey - AJ has 21x22 three blades with no cup thru 1.5 gears. I've got about 2" of tip clearance. The 6BTA 250s idle at 650 out of gear and a little over 600 in gear...5.3 kis on both, 3.7 on one, and one is fine if you have Partick's big rudders. Four blades will give you a higher idle speed and a little less top end due to the added drag of the fours. 20x22 threes is where I'd start since the CPL 970s cannot be overloaded even a smigeon or they won't last. You gotta be able to reach 2900+ WOT fully loaded or you are asking for trouble.

Well Faithful, its been a balmy 104 in Baton Rouge the last two days and not a cloud in the sky. All time records for the dates, and my electric bill is gonna make me gasp. Al Gore, call off the dogs of summer.

UV

Hey guys, I'd like to get in on this discussion............I have a pair of the CPL 970's that I am currently rebuilding for DeNada. I purchased them from Dan Diamond, one of Brewsters friends.

Dan told me that the engines had 5400 hours on them.

My question is what makes the 970's so tender.........what exactly fails when they are not wheeled correctly????

When I took the port engine apart (the engine that I was told had a dead cylinder) I found obviously a bad #6 piston, a cracked cylinder head and the cam had some minor flaking on a couple of the lobes. The head was cracked from the injector hole (7 MM dia.....NOT the 9 MM) into the exhaust ports.

Harry Babb

Harry, as I understand it (from boatdiesel.com, Tony Athens, and some other folks) the problem with the CPL 970s was increasing the rpms from 2600 on the 210s and 250s to 2800 on the 300s. The 300 hp CPL 0970 was produced in large numbers, as all the 6Bs were, mostly off road truck use, but a lot of marines.

The problem is that the heads crack (as you saw, much worse with the 9 mm injectors, but will do it with the 7s), and the pistons melt from the excessive exhaust gas temps from running the 970s overloaded. EGT has to stay not more than 900-925 at WOT fully loaded, in the mid to high 800s is a lot better. Cummins fumbled the ball taking the CPL 1247 250s and raising the rated rpms to 2800 on the 0970s from 2600 without doing any beef-ups. Tony Athens (who I consider to be the guru of 6B Cummins) says that a well maintained 0970 will last as long as a 210 or 250 if it is operated as if its a 250.....2400 rpm max cruise, but prop it to make 2850 or 2900 WOT.

UV

Mikey,

I'm running the 330 Cummins with the ZF220A, 1:5 to 1 ratio transmissions. I installed nibral, 3 blade, 21X23 props with a medium pitch. The props were used and given to me by a buddy. They measured slightly less 21" diameter becasue of some prior reconditionings. Everything seems to be OK. Of course I haven't been able to see about the prop wash erosion issue until I pull her at the end of the season. Fully loaded I'm turning 2950 rpm which is 100 rpm over the top end rating. SO I don't think I am over propped. Performance is great! About 22 knots at 2000 rpm and linear from there (i.e., 24 kts at 2200 rpm, etc.) Hope this helps!

Vince (Eiremar repower)

Mikey,

I strongly recommend you consider 4 blade props. Better bite, higher efficiency, and smoother.

I prefer them very much. I have used them when I had my gas engines, and now am running 4 blades with my diesels.

Dug

Mikey,
With 1.5" shafts you don't want to go over 20" diameter. There's a formula that I forgot but it basically says 20 is safe and 21 not so safe.
We run Cummins 330 with ZF 220a 1.5-1 gears.
Props are Michigan 3 blade 20x25 with #5 cup. Cruise at 2500 rpm is close to 28 kts. Wide open throttle is 2890 full fuel, full water, 3 guys. Top speed is 32 ish.
In gear 2 engines is 5.8 kts and 3.2 with one. We do have trolling valves but have not needed them yet. Good Luck

I believe that the recommended (not to exceed) prop diameter to shaft diameter ratio is 15:1 for normal applications.........This does not apply to High Performance applications.

Now for my OPINION.........when a propeller pitch exceeds the propeller diameter you are getting into high performance.

I would appreciate feedback on "MY opinion"

Harry Babb