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Old 11-09-2018, 11:16 PM   #21
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The first propellers were indeed a single blade and was first invented by a Frenchman from Honfluer, one of the prettiest seaports in France.
Later British warships experimented with an auger screw, unsuccessfully !
Sub props shapes are designed for stealth, sprint, son teaches u/w acoustics, we'll leave that subject alone on a public site.
The first parameter is the hull shape of your boat, after that comes blade tip clearance, engine power performance etc and its is a specialist subject in itself as can be seen by Dave Gerr's book.
I have a 'low wash' displacement hull and wanted my prop to be slightly 'over-propped' to make the most efficient use of engine torque and 4 blade for smooth running.
I couldn't buy exactly what I wanted so I designed my own.
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Old 11-10-2018, 11:01 AM   #22
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What does the keel look like? A three or five blade will have one blade at a time in the shadow of the keel. A four will have two in the shadow at a time.

Gut instinct is to go four if keel is not an issue, and if blade loading is moderate. In general 5b is used when blade load is high like a fast boat.

Most efficient would be a 3b, but there can be noise issues at blade passing frequency. With regards to the efficiency difference between 3b and 5b, dang that is a crapshoot to guess. I'd venture only a couple or three percent. Expensive to do a apples to apples test!!

Subs go with more blades due to noise and cavitation issues, efficiency is not a top concern there. And yep, I'm an ex submarine engineer!!
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Old 11-10-2018, 06:55 PM   #23
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What does the keel look like? A three or five blade will have one blade at a time in the shadow of the keel. A four will have two in the shadow at a time.

Gut instinct is to go four if keel is not an issue, and if blade loading is moderate. In general 5b is used when blade load is high like a fast boat.

Most efficient would be a 3b, but there can be noise issues at blade passing frequency. With regards to the efficiency difference between 3b and 5b, dang that is a crapshoot to guess. I'd venture only a couple or three percent. Expensive to do a apples to apples test!!

Subs go with more blades due to noise and cavitation issues, efficiency is not a top concern there. And yep, I'm an ex submarine engineer!!


The keel ends a good 3-4 feet ahead of the prop, so the prop should be well clear of any keel shadow/turbulence. At least thats the theory. But doesn't the same concern exist as the blades pass in front of the rudder?

The trouble, as you say, is the lack of any apples to apples comparison. Everyone either likes or dislikes what they have, but few can say what the other side of the fence looks like for the same boat. We can only guess. This is why I was hoping to dredge up one or two people who have changed props and have A-B experience on the same boat.
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Old 11-10-2018, 07:39 PM   #24
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Theory says 5 blades will be less efficient, but by how much? Yet 5 blades should be smoother, but by how much? It's subjective, but that's what I'm looking for. Other info is of course welcome, but I wanted to clarify the original query.


I recently got Gerr's book and will give a read through.
So Peter,
When you finish that book, give me a recommendation too.
PO put a 4 blade on.
I have the original 3 blade. I'm thinking of swapping at the next haul out.
Currently, I can't discern any prop noise over the engine noise, so maybe I would gain even more efficiency?
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Old 11-10-2018, 07:43 PM   #25
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https://en.wikipedia.org/wiki/Ducted_propeller
This may help you.
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Old 11-10-2018, 09:40 PM   #26
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OK twist,
Sorry about my “upgrade” comment.
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Old 11-16-2018, 01:34 PM   #27
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I can't speak much to hydrodynamics but I suspect that there are similarities between that discipline and mine, which is aerodynamics.

More blades were added to airplane propellers from the original two to now even six or seven to allow shorter blades because of the at higher rotational speeds available as engines became more powerful, the blade tip speeds were approaching the speed of sound or even exceeding it.

The prototype, and some of the earlier production P-51 Mustang fighters of WWII had a three-blade props but with the Rolls-Merlin engine the prop tips were creating all sorts of problems in the trans-sonic range. The fix was to add a fourth blade, make it fatter and square off the tips, resulting no Mach buffet, less drag and more thrust.

Modern turbo-prop airplanes can have up to seven blades with scimitar shapes and geared drives that spin the props at much lower rpm and a bunch of other refinements to improve efficiency and reduce noise in the cabin.

I suspect just adding blades to a boat's prop will create unintentional consequences without profession design and development. I do see multi-blade, scimitar shaped prop blades on military vessels however so maybe the civilian design will start to emulate that.
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Old 11-16-2018, 09:18 PM   #28
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I went from three to four on my 38' commercial troller. Drafted 6' with large hold. I did it to reduce vibration and noise. It did that and was smoother into a 'bucked' sea. The down side was less efficiency and the prop didn't 'grab' as quickly in reverse on docking. Would I do it again given the same conditions? Yes, but only because it improved the fishing. Wouldn't do it on the same boat if I was only using her for cruising as I would want the better mileage over any potential improvements in fishing or the mild improvement in vibration.
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