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Old 07-06-2016, 02:30 AM   #1
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Unhappy Prop too large

Hi,

First let me thank you for reading this thread.
I am working on a new wooden fishing boat with the ff: It's a 68 footer with 400hp engine @ 1800rpm, gear ratio 4:1, Displacement of about 150000Ib. I have used two prop calculators and they all recommend 50x45x4 propeller. The problem is a 50inch prop is too large to fit, a 40 inches would fit. I am wondering if a 5 blade is the answer. Your recommendations, PLEASE!!!
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Old 07-06-2016, 05:59 AM   #2
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All the math , and graphs and any compromises you might need are in Skenes , at your local library.

The required blade area to absorb the HP might or might not require more blades.

The diameter and pitch can be jiggled to produce the thrust available at what ever shaft speed you need.

Smaller diameter will loose some efficiency.

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Old 07-06-2016, 05:15 PM   #3
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Prop too large

Thanks FF, but the problem is I am in Ghana, therefore, I do not have access to this information. It is not in our library.
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Old 07-06-2016, 06:04 PM   #4
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Great to hear from you nabb and it sounds as though your project is moving forward. I'll send you my new email address in a private message so if you can send me some pictures I can post them for you. Assuming the jungle of Ghana still has limited bandwidth for you.

Sounds like you gave up on the Kort nozzle plan. Can only imagine the difficulty of being an engineer with an indigenous tribal workforce. They are great craftsmen though so you've got that going for you. Good luck.
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Old 07-06-2016, 06:06 PM   #5
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I think when all is said and done, although I don't know your LWL , intended cruise speed, or type of fishing; you will need a 3:1 transmission to get the proper sized propeller under her. Maybe even 2.75:1.

5 blades won't help.
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Old 07-06-2016, 08:22 PM   #6
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Have you already aquired the 4:1 trans? A smaller prop can be reasonably efficient if spun a little faster. Better than a too small prop spun slow.
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Old 07-06-2016, 11:24 PM   #7
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The eagle is a full 38 ton displacement powered by a single 165 hp 671, 3 to 1 trans with 38" 26" 4 blade prop.

So a couple of things.

150000 displacement is heavy for a 68 ft trawler. 400 hp is double required and 4 to 1 is fast turning. So it's not surprising with the results.
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Old 07-07-2016, 06:41 AM   #8
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"the problem is I am in Ghana, therefore, I do not have access to this information. It is not in our library"

What time frame are you working on.

Air mail is expensive but ,,,,,with a credit card.

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Old 07-07-2016, 09:55 AM   #9
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After thinking about it, 400 hp would be required to drag nets around. Some of the commercial trawler repowered because the DD 671 165 hp was not powerful enough. A 68 ft full displacement speed is 8 to 10. I think the 4 to 1 tran is not sized right?

Might try less hp at hull speed to see what you get. Also work back wards starting with 40" prop which is a given.
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Old 07-07-2016, 01:25 PM   #10
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This boat is brand new never launched yet Phil so a test run is not possible. The OP is trying to get in the right ballpark prior to launch as there's no launch/haul out facilities in his part of Africa as we know them here. They're going to slip it off the beach if I recall. No prop shops either, whatever they don't make onsite gets shipped in from China, India or the Middle East.

No naval architect is involved either that I'm aware of.
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Old 07-07-2016, 03:10 PM   #11
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Hi
I guess I'll put my oar of confusion into the pot

First your max diameter is 40", use that.
Pitch; I would likely use about 26" to 28" max.
Blades- 3, but would not rule out 4

Reasoning:
You will not use all that horsepower,
I have 390 hp @ 1225 rpm, 3 to 1 reduction, displacement 150,000 lbs
(without fuel and water), length 57 ft, draft 8 1/2 ft, prop is 53" dia.
don't know the pitch.
I cruise at 7 to 7 1/2 knots at 800 to 850 rpm.
Idle rpm is 475 rpm, still faster than I like for slow manoeuvring around
the docks.

My previous boat had 78 hp @ 2150 rpm, 4.48 to 1 reduction.
Prop was 40" x 26" ( switched between a 3 and 4 blade)
Boat was 36', displaced approx. 40 to 45,000 lbs. loaded
Cruised at 7 knots at 1750 rpm using 1 gph (metered)
Still used neutral to control docking speed but not nearly as much as the
boat I have now.

Can I make this any more confusing?

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Old 07-07-2016, 04:38 PM   #12
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I need to answer some questions.
1. The boat is intended for purse seine fishing.
2. Speed: 9-10 knots
3. Lwl - 63ft.

The reason I went for 4:1 ratio is because of the transfer capacity of the engine. In this case its hp/rpm, which gave me 0.22. Now the gearbox with that transfer capacity has a ratio of 4:1, 4.53 and 5.12. My other reason is I want a push power of 40KN. So this gearbox fits the bill. My only concern is that it significantly reduces the shaft rpm to 450rpm with five blade. I am looking for a shaft rpm of 600.

I agree with Ted, since I am not going to use all that power I may as well use 4 blade instead of 5. That might help increase the shaft speed. The DAR has to 65%.

Thanks.
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Old 07-07-2016, 07:13 PM   #13
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Finally somebody that actually needs to get a bigger prop. But the size of a prop is actually the blade area .. not the dia.

And the usual route to "bigger" prop is more blades. But the # of blades is not the question. Blade area is. You may be able to get props w the same or even fewer blades but wider (fatter) as my wife says that will provide the necessary blade area. I have a Michigan wheel (MP) that has a workboat equivalent prop that will handle considerably more power. I'ts more heavily built too.

Call Michigan Wheel and have a conversation w them laced w all your facts and numbers.
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Old 07-08-2016, 07:03 AM   #14
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A good rule of thumb is an inch of pitch with zero slip will give 1K at 1000Shaft RPM

With the common displacement 50% slip and your engine numbers you should be able to get in the ballpark.

Towing a net the slip will be much higher , I would look on line at the intl fisheries trawler building sites for guidance.
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