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Old 12-01-2018, 09:21 AM   #1
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Proper Props Californian 34LRC

Greetings from Michigan!
Knot salted is up in heated storage again. 2018 was good season with no issues, but not too many salmon either. We are not too serious, so it is always fun to try! No big trips this year.

Considering a three week cruise from Muskegon up to the North Channel next summer. I am wondering about my props as they have not been inspected since before we purchased KS.

No real problems in general cruising, the props sing nicely once in awhile when backing off throttles and when the water is colder, -indicating efficiency?- No vibrations or evident problems, but cannot get over 16-17kts and 2250 rpm most of the time, and it may be a power issue as the Perkins 6.354 turbos have 4300 hrs or so... with 10,000+ to go, I am not looking to do anything major with the engines, as there are no real issues other than stbd is a rough idler at 500-600 rpm (trolling speed) Have cracked the lines and replaced one injector with no change in performance. Both engines are smooth at all rpm under power, no smoke to speak of, start easy and never hesitate.

We are in fresh water, so the hull runs deeper and slower than she would in salt...

Also please don't get me wrong, 1050 to 1150 rpm and 7 kts depending on wind is where we are most happy and spend most of our time. This boat is made to get up and go when needed.

Boat new is 18,000 lbs dry.
Now is likely 21,000 lbs with gear and tools etc.. plus 250 gallons of fuel, so approx 23,000lbs

Bottom is clean as a whistle and smooth.

Max rated engine rpm is 2400, and max sustained cruise rpm is 2250 as I understand it. I can never get to 2400 and don't want to, but again, as I understand it, the boat should be able to do this if needed.

Trannys are velvets at 1.58:1

So, back to the props, a prop servicing facility took my specs verbally and said that my three blade 21x22 (cupping depth unknown to me) are correct for the boat. I have access to a set of 4 bladed 21x22 light cups and they stated that I would need to back them off to 21x21 or 21x21.5. Another place suggested backing my present three blades back to 21x21.

I am not looking to just increase rpm at given speeds, but want proper (pun) efficiency.

FlyWright, I bet you weigh in on this one..

I am not always online so will ping back when I can. Thanks All!
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Old 12-01-2018, 10:55 AM   #2
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Sometimes a prop scan shop tuning of your props can add speed and fuel efficiency at any speed with modern computer based methods. Props that look fine may not have proper blade shape. Scanning is often free and you get a print out of the props shape. You already know that you are over propped. Allowing the engine to reach proper loaded RPM wont cost any more fuel.
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Old 12-01-2018, 11:05 AM   #3
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I took my props to Holland Propellers last week to get them scanned. The scan is free and you only pay if you have work done. My props each had a blade that was out quite a bit so I left them for rebuild.
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Old 12-01-2018, 11:18 AM   #4
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Yes, Holland is where I got most of my info. I will plan to have them scanned there.

Thank you!
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Old 12-01-2018, 12:56 PM   #5
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KS, I wish I knew enough about props to help. All I know is what my Prop Scan sheet tells me for my little Perkins 4.236 engines. I did have mine scanned and tuned. That resulted in less vibes and a slightly higher (0.5 KT) WOT speed.

An added benefit of the Prop Scan is that since I now have the profile of each prop, any future single-prop repairs can be made without pulling both props to match them up.

Less pitch might allow a slightly higher and smoother trolling RPM. 17 Kts sounds like a good WOT speed to me, but what do I know? I putt along at 7.5 Kts and WOT 10.5 Kts. I do know of a 34 LRC with Detroit 250s that will do 23 Kts at WOT. What are you hoping for?

Have you verified your tach accuracy with a digital tach? I bought a cheap one for under $20 and confirmed mine years ago.

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Old 12-01-2018, 01:02 PM   #6
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As Flywright stated if you have not verified your tach accuracy with a hand held you don't really know where you are. You might already be at 2400 rpm.
Do you have any black smoke at WOT?
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Old 12-01-2018, 02:54 PM   #7
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Thank you all for the replies - The tachs are digital onboard, so I presume reasonably accurate - No visible smoke under WOT or any cruising speed. Slight at idle.

Really appreciate the prop scanning info as I have not looked into it before. Basic knowledge for most I am sure.. Will pursue.
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Old 12-01-2018, 02:59 PM   #8
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Quote:
Originally Posted by Knot Salted View Post
Thank you all for the replies - The tachs are digital onboard, so I presume reasonably accurate - No visible smoke under WOT or any cruising speed. Slight at idle.
.
Presume = assume and you know that breaks down.
I would still check them if it were my boat.
Generally speaking, if you have no black smoke you are not badly over propped if at all.
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Old 12-01-2018, 03:10 PM   #9
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Holland is going to repitch my props from 24X24.3 to 24X23.5 to get a few more RPMs at WOT. I have a vibration at higher RPMs on the starboard side that I hope the prop scan and repairs will help mitigate. Time and money will tell...
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Old 12-01-2018, 03:18 PM   #10
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Dash mounted digital tachs take either a signal from a magnetic sensor that outputs a pulse based on ring gear teeth passing it or (much worse for accuracy) pulses from the unrectified output of your alternators stator.


The alternator is subject to belt wear affecting the ratio between the alternator and crankshaft. Both are subject the the tach installer inputting the correct number of teeth for the ring gear or the correct ratio for the alternator.


A photach counts pulses directly from a piece of reflective tape put on the engine damper or pulley connected to the crankshaft. So you know the input is as good as it gets. The only source of errors would be a bad internal algorithm or frequency reference which would be quickly found out.


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Old 12-02-2018, 08:51 AM   #11
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Quote:
Originally Posted by djmarchand View Post
Dash mounted digital tachs take either a signal from a magnetic sensor that outputs a pulse based on ring gear teeth passing it or (much worse for accuracy) pulses from the unrectified output of your alternators stator.


The alternator is subject to belt wear affecting the ratio between the alternator and crankshaft. Both are subject the the tach installer inputting the correct number of teeth for the ring gear or the correct ratio for the alternator.


A photach counts pulses directly from a piece of reflective tape put on the engine damper or pulley connected to the crankshaft. So you know the input is as good as it gets. The only source of errors would be a bad internal algorithm or frequency reference which would be quickly found out.


David

Thanks David, Mine appear to be mechanical, so most likely fairly consistent. Photachs sound interesting as they could probably also be wireless....
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Old 12-02-2018, 10:24 AM   #12
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Originally Posted by Knot Salted View Post
Thanks David, Mine appear to be mechanical, so most likely fairly consistent. Photachs sound interesting as they could probably also be wireless....

Worst kind for accuracy!


All of the phototachs I have seen are handheld and are designed to be used to check the accuracy of dash mounted tachs. Buy one for $25 or so, put a piece of tape on your crankshaft damper/pulley, start the engine and shoot the tape so it counts pulses.


Run the engine up to about 2,000 or 3,000 and hold it steady at that value using the dash tach as the reference. Then shoot the pulley. Add or subtract the difference to the dash tach and use that percentage to adjust other speeds.


As noted above, until you accurately know your rpm, talking about props is worthless.



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Old 01-22-2019, 08:23 PM   #13
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I will be attending proper prop behavior late winter.
Regarless of tach readings, it is obvious when engines are in sync.
To dial them in with regard to pitch and cup can only be a good thing -I think.
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