26 ft Cummins 6bt

Tuloko77

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Supporting Captain
Sep 12, 2020
87
13
Mammoth Lakes
What's it reach in neutral?
I want to say around 750… I know it idles less than 800. I’d love to know what that tells you. I’ll confirm this weekend. And I checked it with a shitty RPM gauge and it still seemed just under 2,600, but not sure how much I trusted that gauge. I generally cruise around 2,000 so if I am overpropped I feel like I’m not pushing too hard at all.
 

seatux

Well-Known Member
Aug 19, 2006
3,091
172
Port Charlotte Florida
I'm getting about 10 psi boost so i dont think that is the problem
I don't know what the specs are on a cummins, but on my Yanmar boost is over 30 PSI. I would guess your boost to be way low, check out the specs.





I think that's about right for Cummins 6BT. They aren't wastegated.

One of the reasons these engines are rated for such high duty cycles (on the pins for 8 of 10 hours with small reduction for the remaining 2 hours IIRC) as well.

Take a look here OP:

 
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Mistress

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Jan 22, 2005
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San Jose, CA
Better late than never…. Wide open in neutral I get just under 2800 RPMs on the tachometer. Now I need to shoot it with a photo tach…

I'm guessing the tach is right and would indicate your throttle linkage/governor is in spec. But never hurts to check with a phototach.

Do you know what prop you have? Gear? Weren't you playing with some props, or was that someone else?
 

Tuloko77

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Supporting Captain
Sep 12, 2020
87
13
Mammoth Lakes
I'm guessing the tach is right and would indicate your throttle linkage/governor is in spec. But never hurts to check with a phototach.

Do you know what prop you have? Gear? Weren't you playing with some props, or was that someone else?
 

Tuloko77

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Sep 12, 2020
87
13
Mammoth Lakes
I have a 4 blade, 17 x 17 Michigan Dyna Quad. I haven’t begun looking at new props yet, but eventually need to. I had some galvanic corrosion on my prop this spring (see photo). Before I went to get a new one I wanted to make sure I resolved the source of corrosion, which I have. My gear is a Hurth 1.22.
 

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Mistress

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Jan 22, 2005
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Sure that wasn't cavitation?

What I've been shown on props is them turning pink and soft and rather uniform.

I've seen aluminum and even SS props pit from cavitation.

Bronze or Nibral?

I have a couple 3 blade props of lesser pitch you could try. You'd prolly have to get them scanned to find out what they really are though. I think stock was 16x17 on 1:1 trans. 17 x 17 sounds about right for a reduced gear. But IME Shamrock was a bit optimistic. In ocean conditions I couldn't get smooth enough water to really tach out.

I de-propped a bit for other reasons.
 

Tuloko77

Well-Known Member
Supporting Captain
Sep 12, 2020
87
13
Mammoth Lakes
Sure that wasn't cavitation?

What I've been shown on props is them turning pink and soft and rather uniform.

I've seen aluminum and even SS props pit from cavitation.

Bronze or Nibral?

I have a couple 3 blade props of lesser pitch you could try. You'd prolly have to get them scanned to find out what they really are though. I think stock was 16x17 on 1:1 trans. 17 x 17 sounds about right for a reduced gear. But IME Shamrock was a bit optimistic. In ocean conditions I couldn't get smooth enough water to really tach out.

I de-propped a bit for other reasons.
I initially thought that could be a culprit but Valley Propeller in Ventura says it is galvanic corrosion. Looks comparable to this: https://propspeed.com/media/news/news-item/2020/12/09/corrosion-a-boat-owner's-greatest-fear

It is a nibral prop… I appreciate the offer of trying yours and maybe will take you up on it in the future when I’m ready to pull it and do some testing. I just don’t have the time right now… and I would love to deprop anyway… at idle, I’m lucky if I can move less than 6mph.
 


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