1998 Mackinaw Cummins Stock Prop

Tuloko77

Well-Known Member
Supporting Captain
Sep 12, 2020
87
13
Mammoth Lakes
Ok, I’m at the point where I’m ready for a new prop. Currently have a Michigan 17x17. Engine is a Cummins 210 with a 1:1.22 gear, and I’m only able to get 2,500 plus RPMs rather the the 2,600 the engine is rated for. I am shooting for 2,700… in older posts, I’ve read that the mackinaw come stock with a 16x17 and 1:1 gear. Can anyone comment on that, and can anyone with a 1:1.2 comment? Would that 16x17 still be a good fit for my gear? In addition to getting my rpm’s into spec, I would love a little slower trolling speed. Not too concerned about slight fuel economy effects or top end. Currently I can hit 28 mph but don’t really need to. Lucky if I can cruise at 20 most of the time, in our waters. Also, please confirm I am reading my gear correctly! Thanks.
 

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Mistress

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Staff member
Supporting Captain
Jan 22, 2005
7,530
245
San Jose, CA
Nothing wrong with your reading of the data plate. Mine is BW 1:1 and came with 16x17 I believe. What's on it is I think a 15p (used, never scanned). Idle (turned down a bit) is about 4.5 knots.

Do you achieve your 2600+ in neutral?

I think Ship put about that gear in his with the repower.

I'd figure 20% more gear would want 20% more prop.
 

Tuloko77

Well-Known Member
Supporting Captain
Sep 12, 2020
87
13
Mammoth Lakes
As always Mistress, thanks for chiming in. Yep, I can achieve 2600+ in neutral. Do you think the 17 x 17 is 20% more than the 16 x 17, and should be adequate?
 

Mistress

Moderator
Staff member
Supporting Captain
Jan 22, 2005
7,530
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San Jose, CA
My math says it's not 20% more. Dunno why you aren't getting the rpm other than IME, Shamrock kinda over-propped from the factory and boats get slower and heavier over time.

That said, if you aren't against the pins I wouldn't worry about hurting that motor. It's got so much torque and it comes in so early much is "wasted" anyways. If you're typically running < 2400, that engine isn't laboring.
 

Tuloko77

Well-Known Member
Supporting Captain
Sep 12, 2020
87
13
Mammoth Lakes
Yeah, I’m never running against the pins. Generally, I’ll cruise around 2,000 if conditions permit, occasionally bumping it up for short periods. Really, the engine runs great but I just want to get it into spec one of these days, and get a slower idle speed.

I’ve got a new laser tach coming for one last check. The one I bought from Harbor Freight is just a POS- readings are all over the place.
 

fiddler

Member
May 13, 2004
10
0
Shark River, NJ
I had the same setup Cummins 210 & Hurth1:1.22 on a 1996 Mackinaw, boat came with a 17x17 Michigan.
Would get to 2640 WOT.
Eventually replaced it with an ACME 17x15 Medium cup.
ACME props have more surface area for the same pitch, so you downsize a bit.
Runs very smoothly and spun up to around 2680.
Lost a bit of top end but very nice midrange, where it matters.
Have since repowered the engine to a 270BTA and repitched the prop to 17x16 heavy cup.
Runs great, spins up over 2800, will replace with a 17x17 ACME next season.
Should get RPM down a bit and give around a 3+ knot boost over the 210 at 2000 RPM.
 

Tuloko77

Well-Known Member
Supporting Captain
Sep 12, 2020
87
13
Mammoth Lakes
Fiddler- Thanks so much for the information. To start, it looks like I have the stock prop, a 17 x 17 Michigan. It's got some decent pitting though and I wonder if that has affected my max RPMs. Since I purchased the boat, I also stripped the very light layer of paint on the bottom and added a hard (epoxy based?) paint and a few layers of as I keep the boat in the water year round. That could have an effect too. We'll see what the new laser tach tells me, but I might look at switching to an Acme 17 x 15.5. It doesn't look like they make the 17 x 15 anymore, but I'll check with my prop shop. That should at least get me closer to 2600 rpms. As mentioned earlier, I don't run the engine too hard so I'd probably be fine with just achieving 2600. Did you lose much top end or fuel economy going from the 17 to 15 pitch?
 

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fiddler

Member
May 13, 2004
10
0
Shark River, NJ
The ACME I have is three blade.
Its been a while since I had the Michigan on the boat, may have lost 1-2 Knots at WOT.
Same issue on my end, bottom was a bit rougher due to paint build up and flaking. Had sanded and repainted with water based paint since then.
The improvement with the ACME is at the low and midrange RPMs. Boat gets on plane easier and can run steady. Very nice when its a bit sloppy. You can have the prope tweaked once you run it as is. Adjusting the cup or pitch a bit up or down can get it dialed in nicely.
They are a bit of a pain to pull, though, since there is less room between the blades.
Needed to use a diffrent style puller.
 


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