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Old 07-02-2013, 11:05 PM   #1
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Prop recommendations

Again I thank you for your time and wisdom....currently our boat has a bronze 34” diameter x 26” pitch RH, 4-bladed propeller. As you may know from the description she used to be a commercial trawler so I am guessing the choice of prop was based around her needs as a shrimper. I went to the Vicprops website and according to thier calculator she needs :

Number of blades
Diameter (inches)

Pitch (inches)
2 Blade
38.7
X
36.9
3 Blade
36.9
X
36.6
4 Blade
34.7
X
35.8

That seems like a big pitch change, would you guys agree with the 35" suggestion ?
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Old 07-03-2013, 01:54 AM   #2
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Mate,
Have you put the figures thru the boatdiesel.com calculator.
425.00 for membership or one of us , with all the info can do it for you,water line length, beam, displacement, gear box ratio, engine HP at wot and RPM etc.
PM me if you wish.
Cheers
Benn
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Old 07-03-2013, 05:50 AM   #3
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If the prop has the required surface area fewer blades is more efficient .

The 3 blade will be cheaper and if you have a 15% of diameter clearence should be nice and quiet.

Big blade area is required to put big HP in the water , you will probably cruise with 60-80HP just being a cruiser,small HP.

The big pitch change is good as it will lower your cruise RPM .
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Old 07-03-2013, 06:52 AM   #4
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A couple of considerations;

The high power small block diesels should get rated WOT and that will dictate prop size and pitch. If this is not important; disregard.

The prop calculator on boatdiesel.com has not been accurate for me in the past. There are other more accurate calculators out there, but remember garbage in - garbage out.
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Old 07-03-2013, 08:12 AM   #5
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Which calculators would you suggest ?
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Old 07-03-2013, 08:39 AM   #6
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Quote:
Originally Posted by AussieCraig View Post
Which calculators would you suggest ?
I can't suggest any. I did a search and found some, don't remember where.

If you're prop guy is suggesting a certain prop then he is on the hook to ensure you get what you expected.
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Old 07-03-2013, 10:15 AM   #7
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Well, first do you really need a new prop?

What engine and what is its rated rpm at maximum hp? What is your wot rpm in neutral? What is your wot rpm in gear and at what speed? What speed do you want to cruise at?

Answer these questions and then we can determine how close your prop is now.

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Old 07-03-2013, 10:26 AM   #8
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This is the info I plugged into the Vicprop calculator -

Length - 50'
Beam - 15'
Draft - 4'
HP - 150 (single 671 DD)
RPM - 1700
Gear ratio - 3 to 1
Hull speed - 9.48knts
Desired speed - 11 knts (definitely negotiable)
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Old 07-03-2013, 10:41 AM   #9
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Your present prop is sized properly. If the boat is already going at hull speed a bigger prop is not going to change the speed by by much. How do you figure 11 knots? Caluclated hull speed is 7 to 9 knots.

If you want to increase the rpm and maybe speed, the pitch needs to go de rease, not increase.
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Old 07-03-2013, 11:11 AM   #10
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I too am confused...the boat will only do 7-8 knots (which is truly fine if that's all she is capable of ), the calculator asked for a desired speed before it told me what hull speed was so I just entered 11...so the prop is OK as is ?
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Old 07-03-2013, 11:27 AM   #11
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Originally Posted by AussieCraig View Post
I too am confused...the boat will only do 7-8 knots (which is truly fine if that's all she is capable of ), the calculator asked for a desired speed before it told me what hull speed was so I just entered 11...so the prop is OK as is ?
Hull speed is calculates as the square root of the length on the water. In your case its 7, and max is 1.3 times 7 or max of 9. I run the Eagle as 1500 rpm and let her run. Some times is 6 some times its 10 depending on tide and wind.
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Old 07-03-2013, 01:22 PM   #12
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Craig:

The only way to know if your prop is ok as is, is to answer the questions I posed above plus estimated displacement if we are going to try to spec a new prop for you.

You have given us some data to try to spec a prop from scratch, but it a lot easier to start with a known boat, prop and performance data. The prop calculator (the boatdiesel one is what I would use, it has more input parameters) can then be calibrated to your specific hull type.

And having said all of the above, none of us is going to give you a better answer than a good prop shop. They will recommend a prop to suit your needs and then repitch it for you if it doesn't work out right.

But there is no point talking to a prop shop until you determine that you need a new prop, right?

My guess is that your current prop is underpitched by about 6". You can probably get close enough by adding 3-4" (probably the maximum feasible) to your existing prop. It won't let you go any faster but might improve fuel economy a tad.

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