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Old 01-20-2016, 09:29 AM   #41
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The number of blades on your prop is or should not be a matter of emotion or likes and dislikes but a matter of what's right for your boat. It's an engineering question that deserves an engineering answer.

And if one wants to avoid engineering there's not much danger of making a bad choice by using the prop that came w the boat. Not your boat as it my be what a PO installed but the prop that usually is found on your boat model w the engine you have. But if you don't have a stock boat then some engineering research may be best.
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Old 01-20-2016, 04:59 PM   #42
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Isn't there a marine propeller expert/business owner on TF who could be brought into this thread? Maybe some member knows a prop expert who could be induced to join TF for this discussion... just might bring him some business! It would be good for all of us propeller-design-conjecturers to learn real numbers and experiences from a thread participant who previously-had or does-now actually live the prop-alteration game - hands on for years, that is.

Prop design is not rocket science - but far as I can tell - it's pretty darn close! LOL
Just buy Dave Gerr's book. The pdf is a taster... See p10 of the pdf. It is mostly very easy to read and a great reference to have on board.
http://mojaladja.com/upload/The-Prop...-Dave-Gerr.pdf
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Old 01-20-2016, 09:16 PM   #43
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Just buy Dave Gerr's book. The pdf is a taster... See p10 of the pdf. It is mostly very easy to read and a great reference to have on board.
http://mojaladja.com/upload/The-Prop...-Dave-Gerr.pdf
Thanks, Brian

I after skimming the link... can see that having the book aboard would be good and interesting too. Think I'll order one!

However, I have not time nor inclination to become a propeller expert, similar as no time to become a dental surgery expert... and, they have not time to become an expert in what I do... therefore I pay money to have them solve my needs, as they do pay me to solve their needs.

I repeat that it would be great to have a marine propeller expert join this thread.

Happy Prod-Design Daze! - Art
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Old 01-20-2016, 09:53 PM   #44
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Props are 87% black art.

Some folks say they never blink and roam around the woods at night foraging for wolves. Still others say their tears are adhesive and stairs confuse them. All I know is they tune props and know their Schitt.
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Old 01-20-2016, 10:18 PM   #45
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I never blink, I chase down wolves at night, kill them and eat them for breakfast, my tears are adhesive, I hate stairs, I've studied engineering.... and when it comes to running prop numbers, I usually miss the mark. Close, but no cookie. The guys that can get it right get my kudos, and dollars!! And those dudes are some of the strangest birds on the planet.

Black art, indeed.
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Old 01-20-2016, 10:42 PM   #46
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I never blink, I chase down wolves at night, kill them and eat them for breakfast, my tears are adhesive, I hate stairs, I've studied engineering.... and when it comes to running prop numbers, I usually miss the mark. Close, but no cookie. The guys that can get it right get my kudos, and dollars!! And those dudes are some of the strangest birds on the planet.

Black art, indeed.
I'm afraid of the dark.

At least on land.

A couple of years ago, Julie and I were visiting an old friend in Sonoma, CA.
Not more then a few blocks from the town center.

Right next to her apartment, they had converted an old train right of way into a running, walking track that went thru some trees.

After dinner, Julie and I decided to take a little walk along this path.

It was dark, but not later then 9 p.m.

We knew there was absolutely nothing to fear.

We start walking and within 5 minutes, we were passing thru the grove of trees.

We really didn't hear or see anything, but within minutes we were nervous and once nervous, forgetaboutit.

Within 30 seconds we went from two adults holding hands having a nice evening stroll to two babies, running back to the house as fast as we could.

We knew our imagination was running wild, and we were being ridiculous, but simply couldn't help it.

We laughed about it then and still laugh about it now.

So crossing an ocean for us is far easier than that walk in the woods.

And as for that prop, as Ski and Manyboats have explained so well, take it to someone who knows.
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Old 01-20-2016, 10:54 PM   #47
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Just buy Dave Gerr's book. The pdf is a taster... See p10 of the pdf. It is mostly very easy to read and a great reference to have on board.
http://mojaladja.com/upload/The-Prop...-Dave-Gerr.pdf
Brian... you got me going... Thanks for the gentle push.

I ordered the prop book... and, then I ordered - - > BOAT MECHANICAL SYSTEMS HANDBOOK - DAVE GERR

I am occasionally an easy sell! Looking forward to reviewing both. The second one may often get hands-on use. Both in hard cover!
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Old 01-21-2016, 04:00 AM   #48
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I've read Gerr's book and I still f'd up. There is a modicum of witchcraft involved. I still need to lower my pitch by 0.5" or so.

BTW: My tears are not adhesive, but I do have a full size tattoo of my face, on my face. And I can open a beer bottle with my testes.
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Old 01-21-2016, 08:02 AM   #49
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One thing to keep in mind is that if the blades line up with the strut, as in a 3 blade prop on a boat with a V strut or a 4 blade prop behind a full keel, smoothness can be compromised. I can look it up in Gerr's book if anyone is interested.
I had read that the V-strut angle should never match the prop blades or it will throb since the un-encumbered blades are pulling in laminar flow while the ones passing the struts are encumbered with turbulence...

So, the V-strut should not be 90 degrees for a 4 bladed prop, or a 120 degree V-strut for a 3 bladed prop.

This is not down to the molecular level or thermodynamics.
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Old 01-21-2016, 12:03 PM   #50
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"V struts turbulence" ........

If anybody was to have turbulence vibration from disturbances ahead of the prop it would be me. (see pic). V struts and I struts are probably almost ZIP in this regard.

Turbulence from the hull is another matter. Mostly because it's mostly not turbulence causing the trouble it's the same thing as an angled prop shaft is causing. Different in-feed speeds caused by the drag of the hull. Willy is way ahead in that regard and she's a very smooth running boat. I could prove it by putting a two blade prop on and observe that vibration did indeed increase. Probably won't. But at my boat speed a two blade MP prop (that's what I have) would require more pitch. Enough pitch to make up for the loss of one blade. So instead of an 18X14 I'd need something like an 18X 9-10" prop. But I have the room for a 19" dia prop so I'd obviously go that route but for armchair comparison then it wouldn't be apples and apples.

I would, however go to a 2 blade prop on Willy before I'd get a 4 blade. A four blade would probably require a 10 or 11" pitch. That's almost half as much pitch as dia and that's generally considered to be a no no. It would be a 25% increase in blade area and hence parasite drag. I could get the thrust I had w the 3 blade BUT .. only if I was to magically get 5 free extra hp to turn that extra blade through the water.

Two blade props would be fine (I think) if the in-feed speed of the water in front of the prop was the same all around ... but w a strut type drive it's way far from that. Small airplanes use two blade props a lot and if they had inherent vibration just from running a two blade that wouldn't be true.

And because of what's on the prop market most boats really don't have a choice of how many blades to run. Unless your particular boat was on the upper and lower edge of the preferred best (dia pitch ratio) of a specific prop. Then you should be shopping for a slightly different style of blade so you can choose to be in the middle of the Pitch/dia ratio range. And if you want to simplify the issue just look for a prop w about .75% as much pitch as dia. But look it up because I'm just pull'in that .75 out what I may remember as the ideal ratio. The above mostly applies to boats operating fairly close to hull speed.
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Old 01-21-2016, 12:25 PM   #51
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I wonder how long it takes water to fill back in behind your keel. I forget who, but someone on TF was talking about having a keel skeg that stopped abruptly in front of the prop and they thought it was causing prop noise.

If it were me, I think I would ease the square edge a bit and encourage laminar flow before the prop gets it.

If you had a four blade prop, the two vertical blades would be in shadow of the skeg for a bit each rotation. With a three bladed prop, it alternates top and bottom as the prop rotates.

So much for my armchair analysis.
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Old 01-21-2016, 03:20 PM   #52
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Yes I've heard the 2 blade 4 blade behind the the keel cutoff should cause vibration. I'd love to try a 2 blade to see of there was a difference but for the time and money .....
Especially since Willy is quite smooth anyway.

Something to that comes to mind is the choped off backs of high performance cars and race cars shows chopped right off w a clean edge is best. One of the reasoms I haven't attempted to fair mine in. I know a Willard owner that did though but don't know if he experienced any difference.
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Old 01-21-2016, 03:31 PM   #53
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Yes I've heard the 2 blade 4 blade behind the the keel cutoff should cause vibration. I'd love to try a 2 blade to see of there was a difference but for the time and money .....
Especially since Willy is quite smooth anyway.

Something to that comes to mind is the choped off backs of high performance cars and race cars shows chopped right off w a clean edge is best. One of the reasoms I haven't attempted to fair mine in. I know a Willard owner that did though but don't know if he experienced any difference.
If it isn't broken, don't fix it?

I suspect your prop is far enough aft and you're traveling at hull speed or less, so the water has time to fill into the gap. But I'm not a fluid dynamics guy either.
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Old 01-23-2016, 06:57 AM   #54
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I don't know how efficient these are, but they sure are purdy!

Found on this Hatteras in Ft Lauderdale...
http://www.yachtworld.com/boats/2000...s#.VqLU4PkrJqM

I don't think that keel is offering much protection to the props though.
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Old 01-23-2016, 10:37 AM   #55
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Pushes big bow wave in one of the first picts. 6 blades... wholly shat man! Thy not 7??? I'd like to see the engineering behind # 6! lol
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Old 01-23-2016, 11:02 AM   #56
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Pushes big bow wave in one of the first picts. 6 blades... wholly shat man! Thy not 7??? I'd like to see the engineering behind # 6! lol
The helm settee looks like they swiped it out of a 1967 mustang back seat...

Lots to like, but not that settee...
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Old 01-23-2016, 12:03 PM   #57
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I don't know how efficient these are, but they sure are purdy!

Found on this Hatteras in Ft Lauderdale...
2000 Hatteras 75 Beautiful Condition! CPMY Power Boat For Sale - www.yachtworld.com

I don't think that keel is offering much protection to the props though.

Shallow water is the Capt's nightmare and a prop shops windfall.
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Old 01-23-2016, 12:14 PM   #58
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Hatt got on this kick a ways back where they put like 3:1 reduction, big engines, thick shafts and HUGE props. I guess trying to get speed out of boat that was simply too heavy to be fast.

What sucked was the hull previously had some decent grounding protection with the keel, now with those EXPENSIVE props, they hang below the keel. Did I mention those wheels are expensive?? Oh, and some prop guys can't easy tune them as the blades overlap too much to use their usual tools.
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Old 01-23-2016, 12:19 PM   #59
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I don't know how efficient these are, but they sure are purdy!

Found on this Hatteras in Ft Lauderdale...
2000 Hatteras 75 Beautiful Condition! CPMY Power Boat For Sale - www.yachtworld.com

I don't think that keel is offering much protection to the props though.
If these props are more efficient it's probably due to the higher blade aspect ratio. Stubby short blades do not get efficiency .. like aircraft wings. But the real reason for props like these is usually smoothness and quiet running. Submarines have similar props mostly for quiet running.

These props may be skewed toward good performance w limited prop clearence. I say that observing that the blade area seems to be concentrated closer to the blade tips than usual. That dosn't change the fact that typical props on trawlers will have better efficiency w fewer baldes. That's my take on it.
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Old 01-23-2016, 01:03 PM   #60
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Assuming clearance, shaft angle and draft considerations... plus a few more....
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