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Old 03-30-2021, 07:09 AM   #1
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4 blade or 3 blade

On my 1978 29ft with 4.108 Perkins, it has a 4 blade prop on it since I purchased it several years back. It has a 3 blade sitting in back hatch. Can someone tell me the difference in performance of each and which one was standard on the 29 with 4.108 engine.
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Old 03-30-2021, 07:32 AM   #2
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The PO was looking to get 41 mpg vs 40 mpg from a diesel VW Rabbit. Not enough difference to worry about.
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Old 03-30-2021, 08:06 AM   #3
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Please provide diameter and pitch for each prop. Does you vessel achieve full rated RPM with current prop?
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Old 03-30-2021, 10:39 AM   #4
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Your performance will be better w the 3 blade unless you’re severely limited re prop diameter swinging room.
The only reason to go w a 4 blade on a typical sized trawler or smaller is if a 3 blade won’t provide the necessary prop blade area.
Your pitch should be about 70% of your prop dia. Like 18” dia. and 13-14” pitch. I have the same size as you (both engine and boat) and the builder put an 18” X 14” dia. Prop on. There is plenty of room for a larger dia. prop but they (Willard) chose the 18” dia.
The reasons are there. For one thing performance will be better if the prop chosen provides a broader range of good performance re speed and pitch. For example if you put a two blade prop on 24x10 (if you could) you’d have great max thrust ... at one speed/load combination. But poor performance at your cruise speed and even worse at top speed/power. You’d have too much blade area and loose too much power just spinning the blades .. re the friction of the water passing over the blades.
Short of it .... use the 3 blade prop.

Al from Ketchikan has a Prairie 29 but he often dosn’t use the ideal prop and now he has a bigger engine. Still Perkins but bigger. His prop choice may be dictated by his engine choice. I’m not throwing tomatoes at Al. He’s a great guy. He just has different priorities.
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Old 03-30-2021, 10:53 AM   #5
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Not much difference. I have both and have run them both. Can't really tell the difference

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Old 03-30-2021, 11:13 AM   #6
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As my reading and experience has indicated, the primary reason for a four-bladed prop versus a three bladed prop in a single engine installation is to smooth out or reduce the propeller vibration imparted to the hull. As an example, check out the four propeller installation on the Iowa class battleships. The outboard props are "in the breeze" clear of any keel or other hull structure ahead of them, and they are three-bladed. Contrarily, the inboard two propellers are immediately behind twin skegs of considerable size and are four-bladed. While wandering around in the dry dock under the Iowa while I was a crewmember, the chief engineer pointed out the differences in the props and informed us all that the reason was four blades behind the skegs had proven to reduce the vibration imparted to the hull. I think the diameters were the same for all four.

As a bit of an aside, for maneuvering purposes, we used engine orders as simply port or starboard engine ahead or astern to such and such a speed as that was how the engine order telegraph on the bridge was install. So ordering port engine ahead 1/3 and stbd engine astern 1/3 had both port props spinning at ahead 1/3 and both stbd props at astern 1/3. The only time this was ever not the case in my three years aboard was when I lost the outboard stbd engine while entering Guantanamo, Cuba and I had to conn at slow speed with a breeze coming from stbd. It was actually quite difficult to maintain course. Loosing an inboard engine would have not been as serious.

So now I own a boat with a single engine with a five-bladed prop. I guess Mainship figured that if a little bit was good, a whole lot was better, especially with a tunnel over the thing.
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Old 03-30-2021, 01:49 PM   #7
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Thanks

Thanks Guys. I might put the three blade on there when I get it hauled out for the next bottom job. I don't have any idea what the pitch or size of each prop is. It's had the 4 blade on it since I've had her.
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Old 03-30-2021, 04:30 PM   #8
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Rich,
As far as turbulence behind the keel goes most trawlers have a blunt end as far away from a tapered end as you can get. So the infeed water is so stired up as it enters the prop the “in the shadow of two of the four blade prop” theory is quite doubtful.
I’d like to hear a NA’s opinion on both.
Another angle on this is that it may be related to the reduced prop infeed flow speed re the drag of the keel probably reducing that speed.

I once put a 5 blade prop on Willy but it had too much blade area. Couldn’t even get close to rated engine speed. So I didn’t learn anything about 3 blade v/s 4 blade except that blade area is way up there.

But like Pete says not much difference. That is if both examples are pitched to reach rated rpm or a bit more.
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Old 03-30-2021, 04:52 PM   #9
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I wonder... If you had a 3 blade and 4 blade of equal diameter and blade area, would the 4 possibly be more efficient due to having higher aspect ratio blades?
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Old 03-30-2021, 06:37 PM   #10
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I wonder... If you had a 3 blade and 4 blade of equal diameter and blade area, would the 4 possibly be more efficient due to having higher aspect ratio blades?
Rslifkin. sorry but you are speaking way above my pay grade. Lol. I have no idea. We didn't have a prop class in captain school.
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Old 03-30-2021, 06:52 PM   #11
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Originally Posted by rslifkin View Post
I wonder... If you had a 3 blade and 4 blade of equal diameter and blade area, would the 4 possibly be more efficient due to having higher aspect ratio blades?
No IMO because the 4 blade would require too little pitch. Too little pitch would cause high surface drag on the blades.
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Old 03-30-2021, 06:55 PM   #12
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Originally Posted by captbuddy View Post
Rslifkin. sorry but you are speaking way above my pay grade. Lol. I have no idea. We didn't have a prop class in captain school.
HaHa Captbuddy that was/is taken care of in boat design school.
Captains pilot the boat/ship and leave the design work to NAs.
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Old 03-30-2021, 07:31 PM   #13
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All I know is I'm only getting about 5.2 knots out of her at 2500 rpm
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Old 03-30-2021, 07:38 PM   #14
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I’m still think’in bout the tripple screw ..... I just re-read and see it was a quad.

That’s different. I don’t see how turbulence from the keel (assumed to be on CL) could cause excessive vibration on the inb’d props of a quad ... or a tripple.

I was going to say re the triple that perhaps the center engine was different, as in more powerful and the extra blade absorbed that power.

Also flank speed is a very significant part of the performance of a battleship. Perhaps the inb’d props were as we say “overpropped” during normal cruising speeds. But at flank speed these props would provide very high flank speeds.
However the three blade would then be underloaded .. not producing max.

IMO my tripple screw ideas fly better than for the quad. And the two inb’d engines could have been larger. That allows for the “more power more blade area” rule to handily explain the blade count difference. Only say’in it fits fairly well.

And if we were to consider this further more possible and plausible explanations would very likely appear.
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Old 03-30-2021, 08:15 PM   #15
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Originally Posted by Nomad Willy View Post
No IMO because the 4 blade would require too little pitch. Too little pitch would cause high surface drag on the blades.
I'd only expect a pitch reduction if there's more blade area, right?
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Old 03-30-2021, 09:40 PM   #16
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rslifkin,
If you’ve got one prop dialed in whereas it’s got 75% as much pitch as dia. in inches and wanted your other prop to be a 4 blade you’d need to have the 4 blade w an extremely low pitch .. because you’ve got to turn an extra blade. Most of the power would be consumed just turning those four blades in the water. OK for bollard pull but if you want to keep your boat moving AND provide reasonable amount thrust to do so you’ll need to reduce pitch or blade area.
Unfortunately most propellers don’t vary much in blade area per blade. Putting a high aspect ratio blade on a prop w small area would allow us to use more 4 blade props. But off the shelf props usually don’t have high aspect ratio blades.

So most all of us should just buck up and run 3 blade props.

Here's a five blade I tried on Willy some time ago.
It had high aspect ratio blades but too much pitch. Only got 2000rpm.
It was a Michigan "Star" propeller.
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