Re-Prop Mistake?

[Micah Yeung]

Thank you in advance, sorry about all the info but want to give as much data as possible for the best results!

Hi all
I know this topic has been written about a bunch of times but I’m looking for some help to I can get this right after an expensive first (and failed) attempt at a repower and new “recommended” props.
I have used a bunch of prop calculators but want to give actual data vs theoretical as I have already wasted $1000’s on a set of props from the recommendations of the “experts” at the prop shop.

Background: I repowered 42′ Mickelson from twin DD 8.2 (fuel pincher) 220 HP to 330HP 6BTAs due to poor reliability of the DDs.
The project has gone well overall except for I believe I’m over propped.

My wish list:
1. Want to make 2850 RPMs at WOT (to be a little under propped and not over work the engines)
can only make (P)2500 & (S)2400 WOT (note can make over 3K at the dock with no load)
2. Increased top speed at WOT given I have more power and more rpms
Note: I use to be able to make 20Knts max at WOT

2 Questions:
1. What Props should I be getting???
My current thinking…
The boat is over propped and down on speed because of the combination of
(A) The 4th blade causing so much more drag/resistance (vs the original 3 blade)
(B) Top Speed would always be down because the pitch is too low with the RPMs and gear ratios I have?
e.g.,
– Old running gear RPM/Reduction ratios X Prop Pitch = (3200/2)x22.5 = 36,000″ travel per minute
– NEW running gear RPM/Reduction ratios X Prop Pitch = (2800/1.5)x16″ = 29,866″ travel per minute

Thus, given the above and the data/specs below what props would be recommended?
And…. given the specs is it worth getting new props vs just going back to the old ones for normal cruising speed of 17knts and potential top speeds?

2. Thoughts on the starboard turbo boost reading low?

DATA and SPECIFICATIONS

The Boat
1990 42′ Mickelson
LOA 42′
Beam 13′
Draft 3′
Weight (fully loaded, fuel, water, people food drinks etc) 31,000 pounds or 14,000KGs
Modified V hull.

OLD Running Gear
Engines: Twin DD 8.2lt (fuel Pincher) 220 hp, Max RPM 3200, 468Lb.ft (624NM@1700rpm)
Gears: Borg Warner V drive 2:1 ratio
Props: 3 blade 21.5″ X 22.5″ (On 1.5″ shafts)
Single Skeg Bearing

New Running Gear
Engines: Twin Cummins 6BTA 330 hp, Max RPM 2800, 722Lb.ft (979NM@2400rpm)
Gears: ZF 220V-LD 1.527:1 Ratio
Props: 4 blade 21.0″ X 16.0″ (On 1.5″ shafts)
Single Skeg Bearing

Measurements
Please see attached Files

Topic

[Rob Schepis]

Photo-tach the engines to verify your tach gauges.
Photo-tach your shafts to verify your gear ratios.
Run some prop calculators using the engine output specs I posted and be 100% sure all of the data you input reflects YOUR boat as it sits today and loaded up for how you use it.

Rob
"Luck is the residue of good design."

[Micah Yeung]

Any other advice please??

Just wondering if anyone has any other advice from the info given???

Thanks.

[Micah Yeung]

Thanks Rob

Originally I just provided the specs of the boat and engines etc and asked the prop guy to provide their recommendation for the boat/engines combination.

After fitting when I went back to them their response was I must have given them the wrong gear ratios….. to which I sent them photos of the manufactures plate.

Since then, they have gone very quiet…. and I have started to do more homework.

Hence coming back to this forum (Brains trust) on what props should be on the boat?
I should have stated that the data provided in the attachments is from the boat as she is running now taken as averages over 4 runs in calm waters/weather.

Thanks for stating the right RPM target for correct propping.

[Rob Schepis]

2850 is not under-propped. You want 2800 + 3%-5% when the boat is loaded up.

What did you tell your prop guy?

He should have been told: 315 bhp at 2900-2925.

Rob
"Luck is the residue of good design."

[Author]

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