3 or 4 blade prop

On Beach Nut 29 ft with 50 hp Perkins. It has a 4 blade on it now. Was on it when I purchased it last summer. It also has a 3 blade on board. Can anyone tell me which would be better. Would like to get a little more speed if possible. A prop shop told me the 3 blade would give more speed. Any ideas. Right now I'm doing about 6 to 6.25 kts at 3000 rpm

Seems like your current maximum speed is "hull speed." Highly doubt any propeller change would make a difference. To go faster, you'll need a more powerful engine and perhaps a different hull configuration.

captbuddy said:

On Beach Nut 29 ft with 50 hp Perkins. It has a 4 blade on it now. Was on it when I purchased it last summer. It also has a 3 blade on board. Can anyone tell me which would be better. Would like to get a little more speed if possible. A prop shop told me the 3 blade would give more speed. Any ideas. Right now I'm doing about 6 to 6.25 kts at 3000 rpm

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Not an easy question. Fewer blades are more efficient. Theoretically, the most efficient propeller would have ONE blade (obviously would need a theoretical counterweight), because each blade's efficiency Is reduced by operating in the agitated fluid of the preceding blade.

But blade surface--actually, leading edge surface-- is also a factor in energy transmission, so one very long blade ( with a bigger pretend counterweight, of course) is the ideal, but we start adding blades because of the architectural limits of the boat ( or aircraft, which is my real background). A long response, but the answer is, "...it's complicated."

I shifted from a 2-blade to 3 on my airplane, because the manufacturer stopped supporting the original. It cost me about 3%in speed... but it looks faster.

As I recall the P29 has limited prop clearance so the 4 blade may be optimal.

are they the same diameter and pitch ?

captbuddy said:

On Beach Nut 29 ft with 50 hp Perkins. It has a 4 blade on it now. Was on it when I purchased it last summer. It also has a 3 blade on board. Can anyone tell me which would be better. Would like to get a little more speed if possible. A prop shop told me the 3 blade would give more speed. Any ideas. Right now I'm doing about 6 to 6.25 kts at 3000 rpm

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You need to find a reputable prop shop in your area, they will provide you with a sheet asking many pertinent questions, all of which need to be carefully answered to match your rig with the right propeller.
Your need for speed may be futile, as mentioned above.
If you are a diehard DIYrselfer, purchase a copy of Dave Gerr's "Propeller Handbook", its cheap and informative.

4 blade are far better for wake boarding if that's any help .

Your prop is either too fine a pitch, or too small in diameter. Your engine should not be able to reach 3000 rpm. Usually about 2300 rpm is max for Perkins engines at WOT. Consult your engine owners manual. The engine governor should also limit rpm. Have you checked the accuracy of your tach?

No I have not checked the tach. Not sure but I think the props are different pitch. Will go by a prop shop in Panama City. Just thought other 29 owners could tell me what they had. Must not be many with this little perkins

Buddy,
What is your gear ratio?
I had a 4-107 Perkins and replaced it w a Mitsubishi w the same bore and stroke. In a boat your Perky is a 36hp engine at 3000rpm. All older 30' Willard boats had the 107-108 Perkins engine.
My Willard has 2.57-1 gears and I got 2750rpm w the original prop (I'm going to change back to the original prop at the next haulout).
Original prop was 14x18. Only got 2750rpm. I had an inch of pitch taken out and then got 3000rpm. With the Mitsu I got about 3050rpm. Perfect for a 3000rpm rated engine. Changed props (another 3 blade) and have been down to 2925rpm since. I'm anxious to get the original prop back on and return to 3050rpm.

This should be applicable to you if you have 2.57 gears. And w a 13x18 prop you should get 3000rpm given those gears.

PS my post #4 was assuming you had a 6 cyl Perkins.
Rereading your OP I think you must have 2.57-1 gears and at that speed and rpm You will not do better.

kapnd said:

You need to find a reputable prop shop in your area, they will provide you with a sheet asking many pertinent questions, all of which need to be carefully answered to match your rig with the right propeller.
Your need for speed may be futile, as mentioned above.
If you are a diehard DIYrselfer, purchase a copy of Dave Gerr's "Propeller Handbook", its cheap and informative.

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What he said.

Does your Prairie still have the engraved plaque with RPM to MPH speed listed?

I doubt you will EVER get the boat above 8.5. Unless its fair current, or down hill.

You will be lucky to get the 50 HP to 7.

Period.

No change of wheel or pitch will help.

These old girls just only go so fast..... Slow.

I have a 4'236 85 HP. At 2000 RPM I do 6.3 all day long. 3/4 GPH.

But when I 'floor it' to 2400 RPM I hit a whopping 7.9. Period. But consumption flies to 5 GPH.

These Perkins engines are not 50hp.

In a British taxi cab they were 50hp but they were rated at 4000rpm.
Never heard of one in a boat going over 3000rpm. Most of these old 4-107 and 4-108 engines came from the 70's. The 107 had wet replaceable liners whereas the 108 did not. Never heard of anybody replacing the liners but the seals gave trouble when they got old so consider it a blessing if you have a 108. My seals were leaking ... (a 107) one of the reasons I repowered.

These Perkins engines gave 36hp at 3000rpm. But in the 70's it was vogue to over-prop and most of those engines only made 36hp (approx) in their over-propped state. At 2700 to 2800rpm.

Any Prarie getting an honest cruise speed over 6 knots w this engine should be happy. No significant improvement is possible.

Cappy,
Somebody put an 80hp Cummins in a Willard and claimed to get 8 knots. I would think the Prarie would do a bit better w it's SD hull.

When researching prior to purchasing my vessel I noted that the claimed specs for 3 bladed props vs four there was a difference in stated fuel economy (with the same engine installed). I get a fuel burn of .42 gph at 1850 rpm's and about 14 mpg with my Yanmar, the four bladed prop didn't fare as well...

AKDoug said:

When researching prior to purchasing my vessel I noted that the claimed specs for 3 bladed props vs four there was a difference in stated fuel economy (with the same engine installed). I get a fuel burn of .42 gph at 1850 rpm's and about 14 mpg with my Yanmar, the four bladed prop didn't fare as well...

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Absolutely the same experience with my former boat, Boomarang (a Fales Willard clone) and it's 56HP Yanmar. Switched from a 17" 4-blade to an 18" 3-blade and made a world of positive difference. WOT RPM was the same, but mid to upper end performance was better. My cruise speed of 6.75kt was achieved about 125 fewer RPM. Much better 'bite', same in reverse. The blade design really comes into play here. Between Dave Gerr and Michigan Wheel I found all the information I needed to make the right propeller selection.

It's a complicated process, in my Broom 42' I use a 4 blade equipoise 4 blade for smooth running with no reduction in speed from the old 3 blade.
At first I worried that I might get sympathetic resonance with 4 cylinders and 4 blades but the gearbox reduction eliminated that.


Google Vicprop, enter all your crafts details in the online form and they'll give you the best prop spec.

The 4 blade 3 blade thing is not up for debate. It's a fact that normally or w all things equal a 3 blade is more efficient.

But Rambler has a point in the real world there are many variables. The biggest is the pitch/dia ratio. And then probably blade design. But the real reason to choose a 4 blade on a trawler is if the 3 blade prop lacks blade area for the amount of power the boat has. But some props have lower aspect ratio blades. They have more blade area within a given dia and can absorb more power. But they are less efficient than the higher aspect ratio blade. Especially at lower speeds. So it's easy to get a high blade area 3 blade prop that will absorb a bit more power but may not be better than a 4 blade w normal aspect ratio blades efficiency wise.

So a boat can benefit from a 4 blade prop but it's generally only if the prop dia is too small or the power applied is too great or some combination of the two.

As to smoothness I don't think there's enough difference to talk about. There's so many variables in vibration that there hardly exists an apples to apples situation for comparison. You may switch to a 3 blade and experience less vibration and the guy in the next slip may switch to a 4 blade and experience less vibration. But the difference (very small) probably has more to to w the variables than the number of blades on the props.

I have noticed all the vessels with big horsepower also have props with four or more blades, the Nordic Tugs with their turbo Cummins being a good example.

My preference is for 3b unless for some reason a 3b does not work or if vibes and noise are objectionable, or if tip clearance forces a 4b.

Also I would not want to run a 4108 steady state at 3000rpm. I would rather overprop it a bit to get cruise down to a lower rpm, say 2500. I don't worry about that engine being a bit overloaded at cruise, unless OP has a habit of running it on the pins. Reduced noise level a big plus, and maybe a bit less fuel burn.

If OP can, post dia and pitch of both the 3b and 4b.

We had several problems when we bought our boat s/h.
I wanted to utilize the maximum useable torque of the engine to reduce fuel consumption on the long trips we did over the last few years (you can only use road diesel in a pleasure cruiser in Europe).
We needed maximum power at sea and for punching the current up the river Rhine in Germany and that played a part.
I was getting some prop rumble at higher revs and needed to decrease the blade diameter by 1'' = 1/2'' between the tips of the blades and the hull.
Engine misalignment and shaft bearings also need to be perfect so you start with a mechanically clean sheet.
I designed my own 4 b prop with what I felt as the correct pitch and blade ratio bearing in mind the load factors and had it built to try it out, fortunately it worked well but as Nomad Willy correctly states there are so many variables between different boats/hulls and even the way you load and use your boat, all these factors can have an effect which is why fine tuning becomes a hot topic.
I'm the first to admit that I'm a finicky person when it comes to things mechanical, my thinking is that it must work perfectly, or be made too.
That just my point of view of course and everyone is entitled to their own views.