Boat diesl prop calc accuracy ?

jerseysportfisher

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I am aware formulas only take you so far, and real world is real world.but powders pro's post got me thinking on how accurate it is, i believe he takes an inch of pitch out of the final ? anyone else have similar methods ?
 

BillD

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I am aware formulas only take you so far, and real world is real world.but powders pro's post got me thinking on how accurate it is, i believe he takes an inch of pitch out of the final ? anyone else have similar methods ?

I used the boatdiesel calculator recently on my "virtual build".
I used "average planing hull" and figured the weight an extra 750 lbs.
The prop came out very close to what Larry came up with @ Accutech Propeller.

My plan is to use "two known" boats of the same hull and similar weight. Use the boats' KNOWN data for engine, gear, prop and speeds.

Then I'll look for a used prop to borrow or buy that is sized to under prop the boat based a propeller shop's "best guess" and the known data from the two boats.

Use the used prop for sea trials. If the boat is under propped a bit and you are happy with the performance keep the prop on. If not then buy and "tweak" a new prop to get the boat dialed in.

It's nice to have a spare prop if you can afford it.

FWIW,

Bill D
 

jerseysportfisher

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I used the boatdiesel calculator recently on my "virtual build".
I used "average planing hull" and figured the weight an extra 750 lbs.
The prop came out very close to what Larry came up with @ Accutech Propeller.

My plan is to use "two known" boats of the same hull and similar weight. Use the boats' KNOWN data for engine, gear, prop and speeds.

Then I'll look for a used prop to borrow or buy that is sized to under prop the boat based a propeller shop's "best guess" and the known data from the two boats.

Use the used prop for sea trials. If the boat is under propped a bit and you are happy with the performance keep the prop on. If not then buy and "tweak" a new prop to get the boat dialed in.

It's nice to have a spare prop if you can afford it.

FWIW,

Bill D

bill , you ever try it against your blackfin ?
 

BillD

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http://boatdiesel.com/Members/Calculators/PropCalc.cfm

The prop calculator cmae up with 22X24.7 props.

My Blackfin has 22X22.25 DQX blades and the 370s are propped to 3100

Real cruising speed is 23 knots @ 2500.

IMO a prop calculator gets you a "close guess".
Prop shops are better @ "guessing". Their databases have lots of "known data".
 

jerseysportfisher

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BillD

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according to BF forums, with cummins you should be 12.2K-12.5K 29 combi

31 Blackfin is weigh heavier than the 29.

12'2" beam.

average planing
30 feet on the water
13,000 lbs.
370 hp
3000 rated
2500 cruise (I cruise @ this rpm)
1:53 gear
22 diameter prop (I have) no cup

Calculator came up with 22X24.7
The props ARE 22X22.25 DQX

Accutech is my prop shop
 
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Powderpro

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Jersey- I have always used "average planning" for boats I have built, and the calculator's top speed is always close to actual top speed. My boats are usually a little faster (maybe 1 knot) than what the calculator says because I input a weight that is a little heavier than what I think the actual weight of the boat will be. So if I think my boat will be 17,000 lbs, I put in about 19,000 lbs. That way I'm not disappointed at sea trial.

Also, the calculator always recommends more pitch than what I really need. What I think is happening, is the calculator is assuming my prop will be slipping more than it really does. So to make up for that extra slip that it thinks will occur, it adds pitch. My prop slip has always been lower than what the calculator thinks it will be. In other words, my prop is more efficient in reality than the calculator assumes.

Sometimes the calculator has only suggested 1" extra pitch, but in some cases it has been as high as 3" extra pitch, this has been my experience, it may not be your experience. With the boats I build, I know from experience about what pitch should be appropriate, so I weigh that against what the calculator says. As you all know, correct propping can only be done once the boat is in the water full of fuel, gear, crew, etc; the calculator is just a close guess at what might work.

And every boat is different, so everything I just wrote may not apply to your boat. I did the prop calculator on that 38' Calvin Beal with the 575hp C9 Cat. It calculated a top speed of about 24.5 knots assuming a weight of 23,000 pounds. That's about exactly what I would have expected that boat to do, and with a 34" diameter prop, it was calling for like 36" of pitch. Well as we all know, the pitch on the boat was 34", it was a 34x34 prop. The calculator was correct on the top speed, but was suggesting 2" extra pitch, which again confirms my experience with the calculator.
 

Powderpro

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When I built my 34' Calvin, which was the first boat I built, the calculator suggested a 26x27.2 prop. This was assuming a weight of 16,000 lbs. So being the speed demon I am, and having very little experience in propping boats, I initially went with a 26x28. The calculator was also saying top speed of 27.0 knots, which made me smile.

At sea trials, the boat was very light, probably under 14,000 lbs (BillD could pull up the old thread off noreast.com and give us my original write up). But I'm going off memory here, so Bill can correct me if I'm wrong ;).

The boat consistently hit 33 mph with no tide or wind aiding the numbers, which is 28.6 knots. And I think my engine load was 95 or 96% at WOT. So I was pitched perfectly at sea trial, but I knew the boat was lighter than it would be in the real world. So if I added 3,000 lbs to the boat, to get it to real world weight, I would have had to take out 1" or maybe 2" of pitch. And at real world weight, the boat would have probably topped out at 27.5 knots. So in the case with my 34' Calvin Beal, the prop calculator was basically right on the money.
 

Powderpro

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I forgot to mention, the reason I think boatdiesel's prop calculator was so accurate on my 34' Calvin Beal was because I did have more prop slip on that boat than I should have. So the calculator automatically assume's a certain % of prop slip, and that assumption was correct on that particular boat. Here is why I feel that boat had more prop slip than it could have:

1. I went with a 1.75:1 reduction because I was told I could only fit a 26" diameter prop under the boat. A 26" prop is not as efficient at transfering 500hp to the water as a 28" prop would have been. So I had to go with a less efficient reduction gear and a less efficient prop, which meant more slip. My shaft speed at WOT was 1,486 rpms. While this was not extremely high, my preference would be a more efficient 1,200 - 1,300 rpm shaft speed at WOT.

After I got the boat, I measured the apeture, and I could have fit a 28" diameter prop. But it was too late, I already had the engine/trans, prop, etc. For 500hp, I think a 26" prop is smaller than ideal, but in a lighter weight application, it is definitely sufficient.
 

jerseysportfisher

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Brian, i understand what your saying about every boat being different. most the 33 bertram guys are running 3 blades 20x23 #5 cup. I'm one of a few running a 4 blade. The problem i face is the old wheels are shot, dinged up and out of balance. I know what the stamping is on them, but thats a crapshoot for accuracy. I have the old prop spec sheet for the boat when it had 260hp diesels. I know what she ran last year, we hit 34.5 on the pins with horrendous vibration. Since then i have blueprinted the hull abit, removed weight, removed a shit load of appendages off the boat. i est she's around 18500 - 20000. atleast in that ballpark. old props were around 22x20 on a 1.53 reduction. Larry from acc thinks i should run 20x23. I'm convinced 21x21 with #3 cup is a good start. The BD calc wants me to run a 19 inch wheel 25 pitch. when i force 21 dia it goes to 22.5 pitch with cup. I was just curious, but i'm hell bent on my own calc on 21x21 med cup. F it



thanks

Jim
 

hardcore1

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i have found the calculator to be pretty close. the weight of the boat has a large factor in it. i believe the 31 blackfin is closer to 15-17000lbs. my 29 was around 13.5-14000. when doing the harris 28, used a weight of 13000, she was a heavy boat. most weights i would say are under estimated. boats get heavy through the years, and don t forget 250galons just about 1800lbs on your weight. i believe the 31 bf holds 350 so thats about 2500lbs on the manufacturers weight. then the water tank, gear, etc. 31 bf sitting in the slip ready to fish is probebly every bit of 20000lb. when you are trying to prop the boat correctly it should be at the weight you use the boat at.
 

hardcore1

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jersey if your old wheels were 22/20 4blade, you should be able to run 21/22 with the same cup and achieve the same rpm. if they were 3blade, you prob can run 21/21 or 21/20 4blade. i would not add more cup as it will eat up your rpm and more or less not give you any speed. the bertram hull doesn t need the heavy cup just a little. how close are the wheels currently to the hull? not enough clearence will cause vibration.
 

jerseysportfisher

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i have found the calculator to be pretty close. the weight of the boat has a large factor in it. i believe the 31 blackfin is closer to 15-17000lbs. my 29 was around 13.5-14000. when doing the harris 28, used a weight of 13000, she was a heavy boat. most weights i would say are under estimated. boats get heavy through the years, and don t forget 250galons just about 1800lbs on your weight. i believe the 31 bf holds 350 so thats about 2500lbs on the manufacturers weight. then the water tank, gear, etc. 31 bf sitting in the slip ready to fish is probebly every bit of 20000lb. when you are trying to prop the boat correctly it should be at the weight you use the boat at.


Good to see ya over here, still got the tourney ? what wheels did you have on that, i figure with the yanmars you got to be close 2:1 2.5:1 reduction
 

jerseysportfisher

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jersey if your old wheels were 22/20 4blade, you should be able to run 21/22 with the same cup and achieve the same rpm. if they were 3blade, you prob can run 21/21 or 21/20 4blade. i would not add more cup as it will eat up your rpm and more or less not give you any speed. the bertram hull doesn t need the heavy cup just a little. how close are the wheels currently to the hull? not enough clearence will cause vibration.


for some reason the other guys like heavy cup,and it performs for them on their 3 blades.

we just re shafted and re aligned, i have an extra 1/4 inch of clearance, i feel with my reduction 22 inch wheel is abit to big, we got some cavitation on the tips.
 

hardcore1

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2:1 gear, running 19/23 4blade and may take out an inch as i would like to add more fuel to the boat. if you are getting cavitation on the tips the wheels are more than likeley too close to the hull and also the cause of your vibration.
 
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