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Old 27 September 2020, 08:12   #1
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50hp 4 Blade Prop Recommendations

Our low hours 2006 Suzuki DF50 is in desperate need of a new prop and I temporarily straightened it out for our first outing on the new boat. The engine is fitted with a doel fin type hydrofoil.

Sadly we couldn't fully evaluate how it's working with the standard Suzuki 3 blade 11.3/8" x 14 because we were on a river and there were too many pesky fishermen hiding in bushes on the banks. They have no consideration for other river users lol. We were heavily loaded with four persons and gear, it briefly pulled 5800rpm at 22 knots.

I think I'd like to try a four blade prop, I never had one before. What would the equivalent be to what is already fitted, and when adding a blade do you go down a pitch or down an inch on diameter? Which is best? I had in mind to keep the hydrofoil fitted for now, I think the benefits outweigh disadvantages especially at semi-displacement speeds. Maybe the engine needs raising a hole on the transom.
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Old 27 September 2020, 08:31   #2
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Quote:
Originally Posted by Limecc View Post
Our low hours 2006 Suzuki DF50 is in desperate need of a new prop and I temporarily straightened it out for our first outing on the new boat. The engine is fitted with a doel fin type hydrofoil.

Sadly we couldn't fully evaluate how it's working with the standard Suzuki 3 blade 11.3/8" x 14 because we were on a river and there were too many pesky fishermen hiding in bushes on the banks. They have no consideration for other river users lol. We were heavily loaded with four persons and gear, it briefly pulled 5800rpm at 22 knots.

I think I'd like to try a four blade prop, I never had one before. What would the equivalent be to what is already fitted, and when adding a blade do you go down a pitch or down an inch on diameter? I had in mind to keep the hydrofoil fitted for now, I think the benefits outweigh disadvantages. Maybe the engine needs raising a hole on the transom.


4 blades tend to give more grip with less ventilation compared to a similar pitched 3 blade. The downside is higher fuel consumption. I usually drop an inch or 2 off the pitch when fitting a 4 blader, depending on where I am in the WOT range. Every RIB Iíve had has been improved by switching to a 4 blade prop. When I say improved, I mean better grip in rough conditions & turns. Iím not obsessed with top end speed.
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Old 27 September 2020, 08:39   #3
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Originally Posted by Pikey Dave View Post
4 blades tend to give more grip with less ventilation compared to a similar pitched 3 blade. The downside is higher fuel consumption. I usually drop an inch or 2 off the pitch when fitting a 4 blader, depending on where I am in the WOT range. Every RIB Iíve had has been improved by switching to a 4 blade prop. When I say improved, I mean better grip in rough conditions & turns. Iím not obsessed with top end speed.
So why go down on pitch and not diameter? Struggling to understand how the characteristics would change.
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Old 27 September 2020, 08:55   #4
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So why go down on pitch and not diameter? Struggling to understand how the characteristics would change.


You have more wetted area with a 4 blade (more friction) & less slip, so to keep the revs in a similar field to the 3 blade you drop a touch of pitch. I suppose you could also drop a tad off the diameter instead, it all depends on what you can find to fit. I just go down in pitch as itís always worked for me. Propping is very much a trial & error exercise, Iíve got a shed full of props
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Old 27 September 2020, 10:47   #5
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Going to a 4 blade gives more blade area as Dave said, going down in diameter at the same time would defeat the purpose of that. 3 blades are more efficient unless you are carrying a fair bit of weight
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Old 27 September 2020, 11:35   #6
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I think with genuine honda props the diameter drops when you go to four blade from 3 blade ,i know my 19 pitch 4 blade is smaller diameter than a genuine 19 pitch 3 blade , on the up to 100hp models ,i wonder if this is normal for other manufacturers ,maybe its to match rpm to pitch whether 3 or 4 blade ?
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Old 27 September 2020, 11:56   #7
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my 115 mariner came fitted with a 15x17p 3 blade mercury alloy prop and was hitting the limiter at 3/4 throttle so i changed to a 4 blade spitfire in 14x19p and it was much better still hitting 6k rpm but at full throttle but the thing that impressed most was the pickup from 3.5k to 4.0 k rpm to full throttle was instant great in a choppy sea .i have since tried a 14x21p four blade spitfire and to be honest I'm going back to the 19p as its no quicker still hitting 6k rpm but its lost the midrange snap and is down a knot or 2 top end
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Old 28 September 2020, 09:25   #8
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Originally Posted by Pikey Dave View Post
it all depends on what you can find to fit.
After looking what is available I realise how right you are PD.

It seems the Solas four blades are smaller diameter than their equivalent three blades like OrwellBoy pointed out is true of the Honda props.

So if I'm going down from 11.3/8" to 10.5" diameter should I also go down from 14" pitch to 13"? I calculated the slip to be quite high at 25%.

From [ http://boatpropellers.iboats.com/propeller-tutorials/blades/? ]
"The diameter can vary by up to 1/2" with no discernable change in performance."

Are we able to get Solas props from a UK supplier?
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Old 28 September 2020, 10:01   #9
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Originally Posted by Limecc View Post
After looking what is available I realise how right you are PD.



It seems the Solas four blades are smaller diameter than their equivalent three blades like OrwellBoy pointed out is true of the Honda props.



So if I'm going down from 11.3/8" to 10.5" diameter should I also go down from 14" pitch to 13"? I calculated the slip to be quite high at 25%.



From [ http://boatpropellers.iboats.com/pro...rials/blades/? ]

"The diameter can vary by up to 1/2" with no discernable change in performance."



Are we able to get Solas props from a UK supplier?


Steel developments are Solas stockists.

https://www.steeldevelopments.net/

Re changing the pitch, thatís the $64000 question. Where are you currently in the WOT range ?
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Old 28 September 2020, 10:57   #10
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Re changing the pitch, thatís the $64000 question. Where are you currently in the WOT range ?
100rpm outside @5800rpm heavily loaded.
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Old 28 September 2020, 13:53   #11
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This is a highly informative article regarding the effect of diameter moving to a four blade. In my case the difference is 5/8" smaller so I don't know whether to keep the same 14" pitch or not.
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This article hopes to explain how propeller diameter (not pitch) affects performance when searching for the perfect prop.

We bought a Solas Rubex L4 Blade Stainless 15.25" dia. X 20" pitch prop to replace a stock Suzuki 16 X 21 3-blade prop on our 2015 Suzuki DF200AP. This outboard has a 2.5:1 final gear ratio which explains why it can turn such a large prop.

Researching this further we found we needed to go down to a 20 inch pitch because thats what you do when you add a blade - drop the pitch an inch. Ok - easy enough. However the other variable (diameter) is harder to define. There isn't a lot of information out there on how changing diameter affects performance. Changing diameter has a definite impact - just like pitch does. Changing to a 4 blade and dropping pitch from 21P to 20P I expected similar wide open throttle (WOT) RPM between the 2 props. What we actually found was surprising.

The smaller (by .75 inch) diameter resulted in WOT being too high now which set off my engine overspeed alarm every time I tried to achieve full throttle. Compared to previous performance readings this prop now allowed the motor to rev up an additional 200-250 RPM. In other words, our 3 blade 25 mph cruise speed was achieved at 3900 RPM but now we have to run @ 4150 to achieve the same 25 mph cruise speed. Assuming the pitch change was correct (because we added a blade) this extra RPM must be the result of the Solas prop's smaller diameter.

Knowing what we know now we would have ordered the same 4 blade prop but in a higher pitch (22P) to make up for the smaller diameter.

All is not lost in our case as we are fortunate to have a qualified prop shop nearby. We brought the prop to them and explained our predicament. The prop guys first suggestion was to add more "cup" to the trailing edges of the 4 blades. We agreed and had the cupping done and the prop is back on the outboard ready for testing. We're hoping the extra cup brings the RPM's back down where we want them: 6000 RPM @ WOT without the overspeed alarm going off @ 6200 like it did without the cup added. UPDATE: We tested the 15.25 X 20 Solas prop with the added cup and...we're almost there! We hit 6150 with occasional forays into overspeed range but this was a definite improvement! We sent the prop back to the shop for the addition of an inch of pitch added to the 4 blades and returned to the boat with our 15.25 X 21P (with added cup which was equivalent to another inch of pitch) This turned out to be the perfect setup! We hit 6000 RPM with a light load of fuel and 2 persons on board. This is right where we wanted to be! Top speed was a respectable 45 mph and fuel flow was 17 GPH @ WOT. Finally!

In summary: when moving from a 3 blade 16 X 21.5P to a 4 blade prop WE SHOULD HAVE selected the 4 blade 15.25 X 22P in the first place as the .75" reduction in diameter had a dramatic effect on WOT RPM.
At $500+ and up there isn't much room for economical trial-and-error testing when you try new props in this size range. That's why we posted this info - so it might help others avoid the same issue and make prop selection more reliable.

On a positive note: the extra blade did provide more stern lift and a better hole shot as well as better directional control in reverse (yes!) on our single engine boat. These aspects of 4 blade props over 3 blade props proved to be accurate during our testing.

Morale of the story: When you change diameter - add opposite pitch. Smaller diameter? Add an inch of pitch to your selection. Larger diameter? Subtract an inch of pitch. Hopefully this helps boaters select the perfect prop for their boat. The one that comes with your outboard isn't always the perfect match for the boat it's installed on. Dealers/Manufacturers take a guess and many boaters just accept the choice made for them and miss out on the perfect prop.

Final note: All outboards have a rated horsepower at a specific RPM (i.e. 150 HP @ 6000 RPM - check the specs). This means a prop with too much pitch won’t allow full RPM to be developed - yielding less available horsepower. You bought an outboard and want every pony you paid for, right?

Our point is: you have the correct propeller pitch on your outboard when you’re able to hit this 6000 RPM mark when your outboard is at WOT (Wide Open Throttle) and the boat is loaded normally. You really only have to check this once if the speed worries you or your family. Besides, you have to “break-in” your new outboard before this check can be accomplished.
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Old 30 September 2020, 12:34   #12
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I called into Reddish Marine the Ranieri dealer yesterday and the tech guy recommended a Solas Amita4 which is 10.5x14 and when I called Steel Developments they didn't have that and recommended the Rubex C4 which is 11.5x13 over the standard Suzuki prop which is 11.325"x14.

$64M Q.
Strange that Rubex props have a bigger diameter than the equivalent pitch Amita series. I think each design adjusts the diameter to keep the same surface area.

Has anybody imported a prop? How much were the import duties?
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