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Old 05-23-2012, 04:50 AM   #21
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Another solution, which may not apply here for your boat, if prop clearance is limited can be to go to more blades. Carey's custom lobsterboat with 420hp could not make its speed target with a four bladed prop. Either the engine shop we all use or a prop shop recommended a five bladed prop. IIRC this enabled Carey to reach his boat's target speed.

Not sure this solution works with lower powered boats with low cruise speeds like ours. But for higher powered go-fast(er) boats it's apparently one way to get better performance if prop clearance is a limitation.
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Old 05-23-2012, 08:21 AM   #22
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Running with full tabs down I start backing off, that is tab up buttons, but even with just a slight adjustment tab up the speed starts to drop off. My limited experience with tabs on other boats and the reaction here leds me to believe the situation won't get any better in between full up and full down.

Bought a laser photo tach yesterday and going to check out tach. engine rpm today to help further define my prop situation. Considering we aren't fully loaded yet for fishing I am pretty sure I am overpropped, but how much I won't know till I get a crankshaft rpm reading.
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Old 05-24-2012, 11:51 AM   #23
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I have a 34 LRC with 3208 NA's and 2:1 gears. Just put rebuilt props on because the old ones dissolved over the 6 months I have owned the boat due to a lack of prop shaft zincs. The props are 23x23 3 blade. Full tanks and all the stuff that will fit on a boat I get to 2700 rpm on each engine and reach 20 knots. I cruise at 1400-1600 rpm and that is 8-14 knots depending on current. Adjusting the trim tabs impacts speed by about .5 knots max. I use them more to level the boat since with full fuel and water tanks and a 8kw westerbeke generator centerline aft the boat does squat in the aft.
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Old 05-24-2012, 02:31 PM   #24
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Assuming no current does the 1400-1600 give you about 10 knots?
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Old 05-25-2012, 12:34 AM   #25
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1600 rpm will get you 9-10 knots without current
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Old 03-12-2014, 12:52 AM   #26
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Mgtd...I see the light!!!
I bought my 34 LCR 6 months ago...starting to learn about her. Twin 3208NA on Cat trans 2:1, I have no idea what size props are on or shaft diameter. I would love to cruise at 12-14 and have a 18-20 WOT. Reality, 2400 WOT at 12-14 knts. Engines are in good condition, no smoke at all, a bit black smoke on WOT when bottom is not clean.
I"ll have props taken out next week to see what I have.
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Old 03-12-2014, 08:08 AM   #27
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Anybody with 4 blades props on a 34 with twin cats?
I think I saw somewhere that the factory spec was 22X24 4 blade
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Old 03-15-2014, 10:25 AM   #28
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I have no way of knowing if mine are original but I have 3 bladed props on my 3208 powered 34 LRC.
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Old 03-15-2014, 11:35 AM   #29
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Number of blades depends mostly on blade area. If your pitch is too low you're loosing efficiency from excess drag from too much blade area. If your pitch is too high you're loosing efficiency from too much water tumbling around the blade tips ("tip losses"). The correct amount of blade area will bring about the ideal amount of pitch. The aspect ratio and blade design are also involved.

For most trawlers a 3 blade prop should produce the best efficiency. The ones best suited to 4 blade props don't have enough clearance for a 3 blade w a good amount of pitch. So basically the four blade prop is only for boats that lack the vertical clearance to run a 3 blade. Not so much an issue w trawlers is blade loading. If you've got too much power for a properly pitched 3 blade you need more blade area and a bigger blade 3 blade prop or a 4 blade prop is the answer.

I suspect many trawlers have 4 blade props because owners think they will run smoother. If that's true from what I've heard the difference is so small blade design is probably a bigger factor. But a proper propeller on a trawler is probably least likely to cause vibration if it's pitched and balanced properly.
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Old 03-19-2014, 07:04 PM   #30
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Finally they are out. 23x21 on 1 1/2 shaft.
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Old 04-01-2014, 08:31 PM   #31
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Took the props to the local shop. Was advised props were in bad shape, but fixable. They are now 23X23 3 blade. Took her out yesterday afternoon, 15 knt headwind 16 knt wot at 2400rpm, cruise confortable 12 knts at 2100rpms. Same rpms as before but 4 knts faster, bottom could be better, full water, 3/4 diesel.
Very happy with the results, but a friend got a pair of 23X23 4 blade that i will try. Report on that later.
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Old 04-01-2014, 10:18 PM   #32
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Good to hear! I'll be very interested in learning the 4-blade results.

Do you see much change in your idle speed?
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Old 04-02-2014, 12:25 AM   #33
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Quote:
Originally Posted by orvals View Post
Took the props to the local shop. Was advised props were in bad shape, but fixable. They are now 23X23 3 blade. Took her out yesterday afternoon, 15 knt headwind 16 knt wot at 2400rpm, cruise confortable 12 knts at 2100rpms. Same rpms as before but 4 knts faster, bottom could be better, full water, 3/4 diesel.
Very happy with the results, but a friend got a pair of 23X23 4 blade that i will try. Report on that later.
What is rated WOT on your 3208s? Seems that you are several 100 RPM short.
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Old 04-02-2014, 06:34 AM   #34
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Fw, yes, there is. Big difference, now when I get into the Marina, I cannot keep it in gear, I would say at least 1 1/2 knot faster at 800 rpm.
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Old 04-02-2014, 06:40 AM   #35
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Sc, WOT is 2800 rpm. I did notice that. I did all these runs with my mechanic, we will check on that, but with the original props that i got (23x21), wot stayed the same. But that was very underproped. Im getting the same wot rpm but with 2" more in pitch and 4 knts faster.
I get the WOT with clear exhaust, no smoke at all.
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Old 04-02-2014, 09:04 AM   #36
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Fw, yes, there is. Big difference, now when I get into the Marina, I cannot keep it in gear, I would say at least 1 1/2 knot faster at 800 rpm.
Three things:
-- it appears you have installed a bigger prop than you had previous
-- verify your RPM with a photo tach
-- what is your verified WOT out of gear
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Old 04-02-2014, 02:42 PM   #37
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I don't know if it's the same as on my Perkins, but my high pressure fuel pumps are marked with the WOT RPM. Maybe you can look to see if your pumps are marked 2400 RPM.
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Old 04-02-2014, 05:17 PM   #38
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Fixed props

These are the repitched 23x23...
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Old 04-02-2014, 05:28 PM   #39
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I just ordered an optical Tach to check the ones installed, needed to do that anyways to make sure they are accurate, but yes, I do have 2" more pitch on them. Rated WOT on the 3208NA is 2800RPM, they get there easy at no load. However, running the 23x21 they reach 2400RPM, just as they do now with the 23x23, but running 4 knts faster. The botom is being cleaned as I write this, next test run on saturday. One thing I did change and forgot to mention, my engines had a ridiculously small air filter on them, I did all these runs without a filter on while I search for a bigger filter that will fit them.
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Old 04-03-2014, 01:13 AM   #40
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Mine is not an exact duplicate of the 34LRC 3208 Cat powered boat of the OP. But after reading and holding my opinion to myself I think my experience might be applicable to a few readers here. I have a 34LRC with twin 200HP Perkins. Similar HP but somewhat lighter than the Cats. Up until late last year I had a pair of computer scanned 21x21 3 blades. They ran very well with a wot 0f 2450 rpm's at 23 MPH (statute). I was happy until I bumped the bottom on a couple occasions. The last one was rock. Who was it that said muddy Mississippi, they must have been in a different place). I ran across a pair of 21x19 4 blade Nibral props on Ebay for $900. I bought them and installed them just before haul out last fall. I reasoned that they might be under pitched but were close enough to be repitched if necessary over the winter. They looked tarnished but good when I bought them. After mounting them I could tell the difference immediately when I put the boat in gear pulling out from the yard. The boat moved right now when shifted. The props must have been reworked and not tweaked by the hard afterward, before I bought them. They were smooth, probably smoother than the 3 blades I had after they were rebuilt and scanned. WOT was the same 2450 RPM, but the speed increased about 1 MPH. At idle the boat is significantly faster. I have to keep it out of gear a lot more to maintain slow speed control. This is an advantage because if I shift to the opposite direction it stops right now. I suspect that the measured pitch has a bit of measurement tolerance. The 4 blades have more blade area and thus less slip compared to the three blades. This can explain the difference in speeds between the two. I couldn't be happier with the results, and this is made even sweeter with the cost.
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