4 blades v 3 blades on an Albin 27

I'm toying with the idea of replacing the OEM 3-blade prop on our Albin 27FC with a 4 blade prop. We have the Peugeot Lehman 4D61 engine, and I would be amazed, nay concerned, if we exceeded 9 knots, so speed is not a deciding factor.

We intend some longer cruising in Spring pf 2016, and I am wondering if a 4-blader would be smoother, less "revvy" and hence quieter than a 3-blader, for normal cruising, and if there would any effects on fuel consumption, SOG, handling at lower speeds and issues when running close to WOT (etc.)

Also: "Mazboot" is a 1984 27FC without a skeg under the prop. Has anyone retrofitted one of these? If so, any tips on best ways to do it would be welcomed. (Going to do some gunk-holing in the Bay, and would like the protection a skeg provides.)

If you have experience with these 2 modifications I would love to hear from you.

The three blade is better unless you don't have enough clearence for the propper amount of blade area. Do power, gear ratio/prop speed calculations and if there's enough vertical space for a good match w a three blade stay with the three blade prop. Go w the four blade only if the three blade runs out of blade area in the calculations. Avoid large prop diameter w/o enough pitch. Too much power will be lost just turning the excessive blade area. More pitch and less area would be more efficient in that case. See the pitch/dia optimum ratios to confirm what I've said. Just my opinion of course.

The general rule as it was explained to us by arguably the best prop shops in Seattle is that the fewer blades a prop has the more efficient it will be and the more blades a prop has the smoother it will run.

The most efficient prop would have just one blade but it would be a bitch to balance.

Our 1973 cabin cruiser was delivered from the manufacturer with a pair of three-bladed props, 24" diameter and 17" and 18" pitches.

A previous owner replaced these with a pair of four-bladed Michigan props, also of 24" diameter and 17" and 18" pitches. Turns out the props had been horribly set up when the boat was still in SFO Bay, so when we thought we might need new props the shop said they were okay, but needed work. So now they are both 23" diameter and 16" pitch.

As another general rule, the shop said a four-bladed prop should have an inch less pitch than the comparable three-bladed prop.

Another characteristic of a three bladed prop vs a four bladed prop is that the three-bladed prop will "back better" meaning that it will generate a bit less propwalk than a four bladed prop. This is an advantage in a single engine boat when one generally wants a minimum of propwalk and a disadvantage in a twin engine boat (assuming counter-rotating props) when one wants as much propwalk as possible.

Bottom line is that if efficiency is your main priority, a three-bladed prop will generally do better in this respect than a four-bladed prop. If smooth running, minimal or no vibration, is your main priority, a four-bladed prop may serve you better.

Faster, more powerful boats can benefit from even more blades---- our friend's 420 hp 36' custom lobsterboat, for example, runs a five-bladed prop for better speed and less vibration.

Marin said:

Another characteristic of a three bladed prop vs a four bladed prop is that the three-bladed prop will "back better" meaning that it will generate a bit less propwalk than a four bladed prop. This is an advantage in a single engine boat.

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Don't disagree with anything you said except that. I have never owned anything but single engine boats and have always appreciated a lot of prop walk as I find them more predictable and the "walk" very usable once you get the hang of it.

boatpoker said:

.... have always appreciated a lot of prop walk as I find them more predictable and the "walk" very usable once you get the hang of it.

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I agree. The narrowboats we run on the canals in England have massive amounts of propwalk and a totally flat bottom. The boats pivot under propwalk in reverse about halfway along their 60' length. While this can be very confusing to some boaters, once you get used to the propwalk characteristic and the inertia characteristics of these 20-ton boats, you can use both of them to put the boat wherever you want it or back it dead straight between rows of moored narrowboats for several hundred yards (assuming no adverse wind).

So yes, propwalk can be very useful in single engine boats as well as twins. But it seems a lot of single-engine boat operators don't know how to take advantage of it so just wish they didn't have any.

"The most efficient prop would have just one blade but it would be a bitch to balance."

Some early aircraft actually used single a blade prop, a small counter weight did the balance.

The larger diameter required was not a problem.

For most the 3 blade is a good compromise as a 2 blade requires more diameter than is found under some hulls.

With a speedy boat the blade count may need to be high to get area to absorb the high power .

Usually on a non speed boat a 4 blade is the sign the hull is too thin above the prop and is flexing and making noise with only 3 blades.

With as large an engine as you have you night consider a "cruising prop" , which would match the HP available at lower RPM with the power needed at your normal cruise speeds.

Much of the ditch is speed limited to about 5K , 6MPH , 10 klicks so cruising at low quiet RPM is a nice way to travel.

Our 32' GB has a 4 blade and our cruising friends 32' GB has a 3 blade. There is quite a difference in low speed maneuverability as the 4 blade is much more responsive. At WOT the difference in speed is negligible but the 4 blade is smoother. We both cruise at 1700rpm and the fuel burn is comparable.

Bob Cofer said:

Our 32' GB has a 4 blade and our cruising friends 32' GB has a 3 blade.

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Is the Ida Mae a GB32? Crossed paths Tuesday in Carquinez Strait. If I overheard a vhf radio transmission correctly, she was on her way to Half Moon Bay to watch the whales. (The boat's height seems quite high, at least as high as the Coot's sailing rig. Ida seems to have long/powerful antennas.)

That's a GB36 like Marin's.

Xsbank said:

That's a GB36 like Marin's.

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NOAA agrees!

Amazing that two bedrooms can be fitted in that short length.

"We both cruise at 1700rpm and the fuel burn is comparable."

This is because most folks do not learn the fuel consumption yo a few %.

The difference in fuel burn between 4 and 3 blades would only be about 5% , so it wont show up,, or do much with low fuel burn rates.

If you ran 40GPH the difference would be visible , but at 4GPH , hard to spot..

I run a 22" X 16" X 4 LH wheel, smoother and "bites" better when slow speed maneuvering it seems, prop walk (to starboard) is strong and I like it that way.

3 blade would be more efficient but I can't imagine that it would be very noticeable in my case. We burn 1.4-1.6 gph at 7-7.5 kph.

Marin is correct drop an inch in pitch for 4 blades. Swing as large a diameter as you can while maintaining 15-17% of wheel diameter from the hull. Limit shaft rpm to <1000 to prevent cavitation.

Some "trawler" prop stuff.
http://www.fao.org/docrep/x0487e/x0487e04.htm

I suspect the 4 blade 3 blade responsiveness difference may be explained more by how one is propped. If you rev the engine up and are over propped more water will be moved by the prop and a lot more engine rpm may be needed when underpropped. The difference in efficiency between 3 and 4 blade props is very small. It's probably a bit like the synthetic v/s dino oil question. And the over/under prop thing is far from black and white. I'm overpropped 50 to 100 rpm and don't like it but it's fly stuff and I'm not going to haul the boat just for that. Hardly anybody is right on but it's possible. With my other prop I can run her right up to 3000 and there she sits. But I could go through several props before I get that close again.

As far as manerverability goes reverse thrust is probably the #1 variable. My symmetrical prop blade shape is reported to generate more reverse thrust but it still takes a bit to stop the 8 ton 30' Willard w 37hp and an 18" prop. If I had 3-1 reduction, a bigger prop and SD level power stopping would be much quicker. So I think there's many variables re thrust response that may or could be more influential than blade count.

markpierce said:

Amazing that two bedrooms can be fitted in that short length.

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The aft cabin of our boat has a queen berth to starboard and a large drawers to port. There is also a head and shower and a large hanging closet.

The forward cabin has a V-berth with each berth being almost seven feet long. There is also a small head and closet in that cabin, too. Post 1988 GB36s are a bit larger all around as that's when they retired the original molds and started using new ones. The forward head got larger and incorporated a shower and the aft cabin acquired a separate shower stall in addition to the head.

Photos are the starboard and port sides of the aft cabin.