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Old 01-29-2017, 10:47 AM   #381
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It's the ratio of pitch to diameter that counts and dictates if you've got too many or too few blades. If MV Content had a 30" X 19" prop better performance would probably be had w the 28X23 props. So as Eamonn says the prop calculation proved correct on the Content. A fourth blade would have required a big reduction in pitch to load the four blades correctly. That would mean the resulting prop would be about 28 X 18 .. just a guess. Huge difference in the pitch/dia ratio. Not pushing much water but eating up much power swinging that fourth blade around. If you keep decreasing pitch and adding blades eventially you'll wind up w a flat disk. Could be the same dia but probably will be more dia. All the power of the engine/s will be consumed turning the blades through the water and no thrust created. And of course if you go the other extreme w too little dia and too much pitch eventially you'll get 90 degrees to a feathered prop. Blades aligned w the prop shaft. With a certian blade size and dia all the power will be required to reach rated rpm and absolutely no thrust will be generated.

So either extreme will consume all the power and produce no thrust. So for every blade dia and area and blade design and speed through the water there is a perfect or better put optimal pitch and dia. The number of blades is almost meaningless but if you can't get enough dia to be in the optimal zone w 3 blades you then need four. So usually the factor that dictates 3 or 4 blades is swinging room for the propeller .. the maximum dia that is practical. There is a small range or zone that either 3 or 4 blades will produce the same efficiency. Then it matters not if you have three or four blades. The four blade just costs more money. So the basic rule of thumb is "three is best unless you need four".

And this "hole shot" talk has no place on TF. On outboards four blades are used to get blade area. Just like trawlers but in the case of OB's much more limited space is available.
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Old 01-29-2017, 11:12 AM   #382
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I started a new tread "Pitch Dia. Ratios for Props".

Back to bottoms.
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Old 10-13-2017, 09:11 AM   #383
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Originally Posted by Boatgm View Post
I saw this new boat on a news feed. A simple hull

SeaPiper 35 - Modern Compact Trawler - SeaPiper

SeaPiper 35 Specifications

• LOA: 35ft / 10.90 m
• Beam: 8ft-6in / 2.60 m
• Draft: 2ft-10in / 0.85 m
• Displacement: 16,300 lbs / 7400 kg
• Ballast: 2,600 lbs / 1200 kg
• Prismatic: 0.66
I see hull #1 is in the water and undergoing trials. Videos from just yesterday online now. Pretty neat boat.
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Old 10-13-2017, 09:22 AM   #384
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Hull Shapes----Show us your girl's bottom

Sea trial video:

https://youtu.be/kO-TEHNBr3o

Sure hope there is a door for the transom. With the cabin door at deck level it seems easy to get pooped and have water in the cabin.

Spiffy looking boat IMO.
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Old 10-13-2017, 09:40 AM   #385
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Sea trial video:

https://youtu.be/kO-TEHNBr3o

Sure hope there is a door for the transom. With the cabin door at deck level it seems easy to get pooped and have water in the cabin.

Spiffy looking boat IMO.
Digging big hole in water w/ quite the wake. Fuel use stats??
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Old 10-13-2017, 10:02 AM   #386
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I'm pleasantly supprised at the Sandpiper. Looks like she's going about 9 knots and riding very level for that speed. The bow wave waves are breaking. There may be a reason they show her at only that speed. I was thinking at over 7 knots fwd visability would be limited w the far aft wheelhouse. Would like to see more. The aft wheel house is an ideal location given a light boat and fairly flat/shallow V hull. Perhaps the boat has a hooked stern. Haven't seen hull pics.
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Old 10-13-2017, 10:25 AM   #387
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Prop 4 blades vs 3 blades

One sample of choice between 4 and 3 blades

For consumption our best compromise is at 7 kts for just under 1,5 usg per hour
And our last trial 120 nm with a customer @ 8,5 kts 3,28 usg per hour

At 7 kts it could be better if we had not the "brake" of the unused 27" 4 blades propeller
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Old 01-07-2018, 08:40 AM   #388
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My Girl's Bottom Exposing her 4 blades!!
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Old 01-07-2018, 08:52 AM   #389
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Here she is in the Bahamas for a bottom paint.

Sigh... would love to know how to rotate pics.
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Old 01-07-2018, 10:01 AM   #390
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Here you Go
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Old 01-07-2018, 10:20 AM   #391
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Hmmm, data?
Sorry no data, but
The number of blades affects propeller efficiency. The less blades in the propeller the more effective it is. The efficiency is due to the fact that each blade faces in the water an obstruction that slows down the rotation of the propeller and thus the speed. A single individual propeller would be the most powerful possible. In practice, however, it is impossible, as the vibration caused by it would be completely uncontrollable, not the components of the engine, and none / no one else could withstand it. A three-ply propeller is a compromise with blades. The number of blades also has an impact on propeller thrust properties.

The more blades propeller has a greater pushing effect on the water than with a lesser-rounded. Thanks to good thrust performance, multipurpose propellers are mostly used in heavy boats and in water jets. Larger (4-5) pivoted propellers are more economical at three speeds at mid / trip speeds, but at high speeds, the blade resistance increases dramatically and reduces efficiency for a 3-blade advantage

Here would be a good way to improve the propeller's benefit ratio, the miracle that you have not used a pleasure boat to enhance your safety and efficiency?

Maximising Propulsion effeciency

NBs
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Old 01-07-2018, 11:56 AM   #392
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It’s not really a question of the number of blades but a question of the blade aspect ratio.

If you have a given amount of power and prop speed if you increase the number of blades the blade area will need to be reduced per blade. That will automatically increase the aspect ratio (width of blade compared to the length of blade). Small high aspect ratio blade are generally not available. Most boats have too much power for such a prop. So going to more blades means increased diameter .. usually. But also usually there is no room to swing a larger dia prop. That is almost always the reason to increase the number of blades. The three blade lacks the blade area for the amount of power. That is almost always the reason for a 4 blade prop on a rec trawler.

But I’m just babbling about something I only know a little about. A good prop man will be able to tell us how many blades are best. And variables like power, dia space, shaft speed and boat speed determine the prop and the number of blades is the result of these variables. Not the other way around. The variables choose the prop. And if the boat owner says he wants a 4 blade instead of a 3 blade the result will likely be a prop without enough pitch. This will result in too much parasite drag from too much surface area. Lots of dia is thought of as being more efficient but if if the pitch/dia ratio is not in the “best” zone/range and efficiency will be less than optimum. For most trawlers three blade props are best. The prop man (engineer preferably) should decide the number blades. Not a boat owner.
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Old 01-07-2018, 12:49 PM   #393
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Thanks for the link... Items therein make me ponder...

Would the drag ratio created by thin keels on each side of twin screw planning hull be offset by Kort Marine Nozzle improved efficiency??

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Old 01-07-2018, 12:58 PM   #394
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Thank you johnP
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Old 01-07-2018, 01:35 PM   #395
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thought I posted this before...courtesy of RickB

5 attempts at rotating, and saving failed...enjoy it sideways....
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Old 01-07-2018, 03:13 PM   #396
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Art,
I would think the in-efficiency is largely a product of the prop blade to nozzle clearence and the drag of the entire nozzle. The prop blade tips to the nozzle clearence needs to be really small to have any gain at all.
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Old 01-07-2018, 03:56 PM   #397
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Art,
I would think the in-efficiency is largely a product of the prop blade to nozzle clearence and the drag of the entire nozzle. The prop blade tips to the nozzle clearence needs to be really small to have any gain at all.
Hey Eric - Thanks for input.

My understanding may be incorrect re what is mentioned on the link Maximising Propulsion efficiency Seems to me that efficiency increases up to around 15 knots. My misunderstanding may be that the mentioned efficiency increase only represents he difference between two models of nozzle... and does not represent an increase over open props that are using no nozzle?? See bold paragraph about 1/2 way down.

Quotes from link:

"Originally a nozzle was not considered practical for vessels operating above 10 knots, simply because it required more power than an open propeller. Today, Rice Speed Nozzle systems are in use for vessel speeds up to 15 knots. To better understand this technology, we first need to revisit the concepts of how propulsion is achieved, thereby allowing the maximizing of these to our advantage."

and

"Running open propellers when a nozzle will save 22% fuel or more on most trawlers, fishing and tuna vessels up to 15 knots."

"HOW PROPELLERS WORK - The High / Low Pressures of Propulsion
Let us freeze a propeller in motion. With right hand rotation, the propeller blades will rotate clockwise viewed from behind. As the blades push down, water is forced down and back, as is done by your hand when swimming. Because this motion has created a space, water must rush back simultaneously to fill the void resulting in a pressure difference between the two sides of the blade. There is a positive pressure or pushing effect on the driving face of the blade, and a negative pressure or pulling effect on the front face of the blade. As the propeller rotates, it draws water from its front through an imaginary tapered cylinder a little larger than the propeller diameter at the front. As the water passes through this imaginary cylinder, the water accelerates as it passes through the narrower end of the cylinder. This acceleration of water results in the force that we know as thrust.

HOW A NOZZLE WORKS
To obtain the most thrust, a propeller must move as much water as possible in a given time. A nozzle will assist the propeller in doing this, especially when a high thrust is needed at a low ship speed. As we already know, as the propeller blades rotate in the water, they generate high-pressure areas behind the each blade and low pressure areas in front, and it is this pressure differential that provides the force to drive the vessel. However, losses occur at the tip of each blade as water escapes from the high pressure side of the blade to the low pressure side, resulting in little benefit in terms of pushing the vessel forward. The presence of a close fitting duct around the propeller reduces these loses by restricting water flow to the propeller tips."

There are additional important items on the link. I need to read it carefully again!

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Old 01-22-2018, 01:54 PM   #398
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Bottoms up

Here are some photos from the my survey haul out and during price negotiations. 2 years and still a work in progress but she is coming along nicely. 1150 miles in 2017 in SF bay and delta. I wish that I had more time and $$.
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Old 01-22-2018, 02:18 PM   #399
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Here are some photos from the my survey haul out and during price negotiations. 2 years and still a work in progress but she is coming along nicely. 1150 miles in 2017 in SF bay and delta. I wish that I had more time and $$.
She looks nice... Enjoy!!
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Old 01-22-2018, 04:29 PM   #400
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Salt Wife Bares All!

44 Endeavour Trawlercat
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