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Old 06-24-2019, 02:03 PM   #1
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Prop size and pitch

I am looking at a boat that is getting 5.2knots at 2200 rpm. 35 ft trawler type. Prop is 23x17...3 blade...Perkins 135 hp..does this sound right or could it be under proper??
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Old 06-24-2019, 02:56 PM   #2
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Asuming the 5.2 kts at 2,200 rpm is at full wot throttle, then something is wrong. That engine should turn 2,600 at wot. Even if overpropped by 400 rpm I would have thought most single engine trawlers would go much faster, closer to 7 kts.

The easiest thing to check and maybe the most likely is that the throttle isn't opening all of the way and it should be a simple adjustment to fix it.

The other possibility is a very dirty bottom and/or a fouled prop. The exhaust riser could be plugged with scale and soot which will limit power and rpms. Also the filters could be clogged which restricts fuel.

Check all of these things before you think about tackling the prop.

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Old 06-24-2019, 03:49 PM   #3
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Prop size and pitch

I understand that the engine revs up to 2800 rpm and there is a little left in the throttle. That’s why a suspect that it is under proped..everything I read shows around 1800 revs gives about 7 knots with the boats and similar hp engine..
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Old 06-24-2019, 04:58 PM   #4
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If it is turning 2800 in gear at full throttle it may be a bit under pitched but not that much to only get 5.2 kts at 2200. I suspect there is something else like was mentioned before maybe a fouled bottom.
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Old 06-24-2019, 05:08 PM   #5
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Also what is the boat. Single Perkins are found in up to 34' trawlers usually. Those should easily get up to 7+ kts with the Perkins running in the low to mid 2000s. What is your wot rpm and speed?


Still sounds like a fouled bottom to me.



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Old 06-24-2019, 05:15 PM   #6
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My 36 with Perkins 135 goes 6.8 knots at 1650 RPM with 25"D X 22"P 3 blade. Yes it is over propped. Even with 1/2" of barnacles it only cuts that down to 6.3 knots.

Something is seriously wrong at 5.2 knots at 2200 rpm.
What is the transmission ratio?

BTW a Perkins 135 is a 2800 WOT engine.
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Old 06-24-2019, 05:31 PM   #7
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I own a 36' Grand Mariner with a Ford Lehman 120 HP with a 24"x 18" with a 1" cup. She runs at 1750RPM at a steady 7 knots burns about 6 litres/hr.
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Old 06-24-2019, 05:34 PM   #8
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Tach could be off too. Borrow/buy/steal a phototach.

Second others, sure sounds like dirty bottom/prop.

Post gear ratio too.
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Old 06-24-2019, 05:42 PM   #9
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At could also be that tach is reading too high. It is driven by the alternator so things that effect the alt speed, also reflect in the tach rpm. Worn belts, too small alt pulley or maybe someone tweeked the tach adjustment screw.
One quick look at the waterline will tell the dirty bottom story.
Cruise RPM is too high AND cruise speed too low!!!
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Old 06-24-2019, 06:12 PM   #10
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Is it smoking when warmed up at cruise? I'd expect black smoke if a bottom issue or over propping or some other cause of overloading.
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Old 06-24-2019, 07:15 PM   #11
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I ordered a laser tach. It will be awhile before I get to see it but will post the findings. Thanks
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Old 06-24-2019, 07:21 PM   #12
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Buy a digital tachometer; https://www.amazon.com/Digital-Photo.../dp/B001N4QY66
Calculate your propeller size; https://www.vicprop.com/calculator.htm

Doubt you are underpropped. Verify actual rpms on pulley and prop shaft first. Could be slipping transmission? Build a graph of rpm v. gps speed at slack tide. Should be linear until you start approaching WOT.
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Old 06-24-2019, 07:45 PM   #13
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Just ordered the exact one on amazon will take your advice. Thanks cliff
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Old 06-30-2019, 07:31 PM   #14
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A quick check you need to do is to run the motor up to full throttle in neutral, and note the indicated rpms.
If it won’t make 2800 and likely a little more, a tach check is the next step.
Exhaust gas temperature measurement can be very helpful in propeller tuning too.
Clean bottom and running gear, chart all available info at 200 rpm intervals, then run your numbers on the prop calculator mentioned above.
I would not be surprised to find that it’s overpropped, it’s common throughout the pleasure craft industry to do so.
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Old 06-30-2019, 07:47 PM   #15
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I am a little confused..if the engine is running at 2200 rpm and only getting 5.6nm wouldn’t that indicate it could be underpropped not overpropped? In any event I bought and received laser tac and we will see next week what is going on. I will use my gps phone to see the true speed and the tac to see the true rpm. Will also verify wot and note rpm and speed. Also verify shaft rpm and trans gear ratio. With all of this might be able to figure out something.
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Old 06-30-2019, 09:19 PM   #16
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If you are not reaching the proper max RPM then you are overpropped if everything else is correct, bottom not dirty, etc. if you were under propped then you would be turning more than the proper full RPMs.
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Old 06-30-2019, 09:33 PM   #17
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I also agree, low rpm's not high enough then an excellent chance your boat is over propped. My old boat came over propped, the last owner pretty much bragged about it, instructed me not to take the boat to WOT as it would stress the motor.

Why people over prop is a mystery to me, I don't think they understand what over propping can do to the motor.
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Old 07-01-2019, 05:00 AM   #18
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As one of my earlier post states the owner says he has had it up to 2800 rpm so might be underpropped... we will see...
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Old 07-05-2019, 04:32 PM   #19
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My Apollo 32 with 6-354 Perkins and 22 X 24 prop

My Apollo 32, with a clean bottom, and new Michigan Wheel 4 bladed 22" X 24 pitch pushes along at 4.2 knots at 550 RPM / idle and 8 knots runs at 1,100 RPM. My 1982 Perkins T6-354 manual says that only short runs under an hour at 2,250 RPM. Given the semi displacement hull, I can't imagine getting the boat to reasonably achieve that engine speed, let alone 2,600.


I went through numbers with three different prop shops and their calculators. All returned very close similar numbers to what we settled on for speed, engine power and economy. 2,200 RPM is no where near what works for an old Perkins.
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Old 07-05-2019, 05:21 PM   #20
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Quote:
Originally Posted by Tiffany View Post
Given the semi displacement hull, I can't imagine getting the boat to reasonably achieve that engine speed, let alone 2,600.
Perkins made many different engines. It may be capable of 2800 WOT intermittently and 2400 continuous. I suspect OP has T6-354-4 but he doesn't list the model.

I believe the OP's goal is to make a simple graph of rpm and speed. If he can reach 2800 rpm at idle, but can't reach 2800 rpm under way, then it might be an indication that he is over propped.
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