Prop Pitch & Cup

[RobS]

When we surveyed TOY-RIFIC last year it was discovered that the dash tach was off approx. 200rpms. The dash tach showed over 3900rpm's but actual WOT was approx. 3750rpm as measured by the photo-tach. Therefore, it was close enough to be accept that there was no gross over loading, etc. in her first 438hrs of prior-owner life.

Well now that Mr. Meticulous is her proud owner, close enough is no longer good enough. Engine Life vs. Engine Loading, proper propping, etc. is the hottest topic over on Boatdiesel.com

Went out last night with 3 guys, full fuel, full water and full gear. Turning 3725rpm at WOT. I need to get back a good 150rpms.

I had the prop scanned over the winter so I would know what I was working with. I have a LH 19x19 4-blade with a 6 cup. I can give up pitch or cup to get the rpm’s I need.

Proper cup is needed at cruise speed for the leading and trailing edges of the prop to cut cleanly through the water without causing cavitation. 6 cup is substantial. I’d rather play with cup than pitch as it’s less work on the prop but I do not want to create any cavitation, vibration, etc. as she is silky smooth now.

My prop shop feels he can do either and get the results I need and he is researching it further. I gave him all of my specs: boat, engine, tranny, rpm’s, etc.

I also called H&H Propeller who was a supplier and outfitter for Albin. I was told that the 28TE Engine box should be outfitted with a LH 19x18 4-blade with a 5 cup.

So I am heavy in both pitch & cup.

JackK I remember you had your prop scanned a couple seasons back and you had emailed me the report but I do not have it anymore.

In theory a Flush Deck and Engine Box could/should have different cup as more or less cup, and whether or not the cup runs the full length of the blade (from the root to the full dia) will effect stern or bow lift.

Anyone have prop scan reports or know what pitch & cup they are turning?

[Russell]

Rob: I have heard that you should get 50 - 100 rpm above the max horsepower rating at WOT. Does that agree with Boatdiesel? I have similar numbers to yours on my Cummins rated at 2600 RPM. The mechanic said he got 2600 RPM at WOT with his phototach and he was satisfied. But that was with empty tanks and I recently purchased my own phototach and the best I can get is 2575 and sometimes as low as 2525. Do you think this needs correcting?

[RobS]

They say more like 100-200 over rated but 100 is certainly fine so I would say you should look to get 2700. Since prop adjusting is not a big deal it would be worth it.

Both of our boats are propped close to being right - we are certainly not in danger of catastrophic failures but why not make it as “ideal” as possible within our control for a reasonable effort.

Also, Diesels, by design, will govern themselves to reach the intended rpm by introducing more fuel, so a heavier loaded engine at 2200 rpms is using more fuel than a lighter loaded engine at that same rpm.

Here is an article from Tony Athens, the specialist at Boatdiesel.

[RobS]

Rob: I have heard that you should get 50 - 100 rpm

BTW, 100+ is safer as 50 can be easily lost, ie. dirty bottom, addition of gear/eqpt, etc.

[Russell]

Rob: Thanks for the reply. The article scared me enough to call a prop shop and the guy said as long as I was within 50 RPM of the peak I would not be hurting the engine. But next time I do a haul out I might take it in to get an inch or so of pitch taken out. He estimated that it would cost around $200 with cleaning and rebalancing.

[RobS]

Definetly no cause for alarm. But with winter approaching and the boats hitting the hard it's time to think about something like a small prop adjustment. It's more crucial on the high speed 3800 6LP's than your 2600 Cummins.

I just spoke with my prop shop and he is recommending we leave the cup alone and taking a half inch of pitch out and that will bring it to mid 3800's. I may decide to go slightly more.

$200 was his exact price but that was discounted because he did a scan and recon on my prop last winter. So you're $200 is a good price.

[JackK]

Rob -

I'll send you the PDF file from my scan again. I was trying to post it here for all who may want it but no luck.

My scan is from 2006 on my 2003 28TE Flushdeck prop.

Jack

[RobS]

Thanks Jack. Attached is your report and mine. BTW, you may have had trouble attaching the PDF as the file size was too big.

[Pitou]

I ended up having a prop scan done as well. Finishing up my second season with the reconditioned prop and RPM's are still on. Still trying to track the paperwork down to post a PDF attachment. Photo tachs at 600 RPM for idle and 3000 RPM at WOT & amazingly matches the tach on the dash / matching the specs on a dime for my Cummins. The symptoms leading up to the "tune-up" .... a shimmy / vibration in the deck so I decided to tune the prop along with replacing the cutlass bearing which had some play. The vibration is gone and all is well.

[RobS]

Kevin,
That's really turning for a 6BTA. Is that 3000rpm "fully" loaded? Usually you see the 6BTA's targeted at 2900-2950...

[Pitou]

Kevin,
That's really turning for a 6BTA. Is that 3000rpm "fully" loaded? Usually you see the 6BTA's targeted at 2900-2950...

Rob,
The 3k is "fully" loaded / water tank and lots of gear 2-4 bodies. Light she turns just under 3200. Manual calls for WOT RPM at 3000 and not to run more than 1 hour in 8 at 3000. My per usual is 2200- 2400 w/an occasional 2600 and a rare spin of 3000 if out running a thunder boomer.

I got my numbers for tuning from the manual and Cummins NE. I thought I got it right .... ????

[RobS]

I was interested in a 6BTA 330 boat last year and he was well over propped, don't recall what he was turning, I think 25 to 2600 or so. I had spent some time on Boatdiesel then and remembered 2900 to 2950, that's all.

Knowing you, especially that I was fortunate enought this summer to get the Pitou tour, I'm sure it's right on!

[Pitou]

Rob,

Thanks for checking up .... always happy to hear another point of view or opinion.

Thanks ......... Kevin