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Old 24 May 2018, 09:11   #1
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Changing from 3 blade to 4 blade prop???

I'm going to try a 4 blade prop on my Mercury 150 4stroke.

Currently I have a 3 blade aluminium Mercury Black Max 141/2 x 19 and WOT is about 5400 @ about 41 kts with a nearly clean hull and light load.

So, while I know I need to put any prop on the boat to see how it behaves I was hoping to learn from other people's experience. We don't really have try before you buy options locally for props I'm afraid.

Solas do a 14 1/4 x 19 aluminium 4 blade.
Will that pitch work successfully or do I need to go down to 17? If I go to Stainless do I need to drop pitch?
What likely benefits will I see from the 4 blade?
The Mercury manual says I'll lose 150-200 rpm on the WOT by adding a blade to equivalent pitch. Is that people's experience?

Thanks in advance.
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Old 24 May 2018, 09:27   #2
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I went from a 3 blade 21p stainless to a 19p 4 blade stainless for the same revs on my DF140. 4 blade will use slightly more fuel, but will grip better in the rough. I get zero cavitation with the 4 blade, where the 3 blade used to blow out.
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Old 24 May 2018, 09:38   #3
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I've got the same engine on my 6.5mtr Coastline.

Two up, about 90ltrs fuel, 43.2kn GPS.(Flat calm fresh water) 5650 RPM. (During "break in").

Getting about 23knots at 3200RPM

Three blade 19" Tempest plus. (and the hole shot is awesome)

Check out about 7mins 30sec

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Old 24 May 2018, 11:08   #4
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Quote:
Originally Posted by Last Tango View Post
I've got the same engine on my 6.5mtr Coastline.

Two up, about 90ltrs fuel, 43.2kn GPS.(Flat calm fresh water) 5650 RPM. (During "break in").

Getting about 23knots at 3200RPM

Three blade 19" Tempest plus. (and the hole shot is awesome)

Check out about 7mins 30sec

it didn't think it was that dark when we got back from colonsay
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Old 24 May 2018, 11:12   #5
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Quote:
Originally Posted by Iron Dials View Post
I'm going to try a 4 blade prop on my Mercury 150 4stroke.



Currently I have a 3 blade aluminium Mercury Black Max 141/2 x 19 and WOT is about 5400 @ about 41 kts with a nearly clean hull and light load.



So, while I know I need to put any prop on the boat to see how it behaves I was hoping to learn from other people's experience. We don't really have try before you buy options locally for props I'm afraid.



Solas do a 14 1/4 x 19 aluminium 4 blade.

Will that pitch work successfully or do I need to go down to 17? If I go to Stainless do I need to drop pitch?

What likely benefits will I see from the 4 blade?

The Mercury manual says I'll lose 150-200 rpm on the WOT by adding a blade to equivalent pitch. Is that people's experience?



Thanks in advance.


I had Mercury Rev4s on the last boat, very impressed but they won't fit the DF. I would have them again if they fitted. I have a Solas one on the DF.
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Old 24 May 2018, 11:35   #6
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You might already have seen this, but it gives some useful comparisons ....

https://www.mercurymarine.com/en-gb/...on+Redirection

I don't have any issues with cavitation/ventilation so I'm not sure I'd gain anything from four blades. There is certainly a big difference between different props that goes way beyond just pitch and diameter and some of them defy the calculations (mines did, when I do the revs/speed/ratio calcs comparing with my previous engine/prop the "speed to revs" should be exactly the same.....but it's not. I'm speed matching at about 400rpm lower.

Best thing really is to try and borrow before comiting.
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Old 24 May 2018, 13:14   #7
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You can't simply take the notion that you're only adding a blade and then make meaningful deductions from that. This would only be true if the blades of your 4 blader were identical to the blades on your 3 blader. If that were the case then adding a blade will give more lift per revolution at the expense of efficiency. The fourth blade would add drag so there's your loss of efficiency, but you'd have more blade area so an improved grip on the water. Because there's more grip, it will take more energy to turn the prop. If the present 3 blade prop is correct in diameter and pitch, then adding another blade will overload the engine so a drop in pitch will be required. Dropping a pitch will improve the grip by reducing the load on each blade so it's a win win situation grip wise. But the reduced efficiency will effect fuel consumption and top speed.

Once you change prop blade characteristics then you can't make direct comparisons 3 blade to 4 blade.
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Old 24 May 2018, 14:39   #8
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Thanks folks.

PD- I'm interested to read that fuel consumption increases, as some (Solas HR4) claim better mid range perf/economy. although it makes sense. Which variety of Solas did you buy?

LT- you're certainly not hanging around in the vid, looks like a lovely trip. I suspect I'll be running a bit heavier than you- occasionally up to 4 people and up to 200 ltrs plus "stuff", so a lower pitch might suit me better.

JW- yes I agree with the science of it- backs up PDs experience of fuel consumption.

It appears there are few regrets from going to 4 blade, if done correctly.
As I want to have spare prop the option is open to me as I upgrade.
If I had 3 blade SS that worked well I'd probably just buy aluminium and leave it in the box as a spare.
Decisions, decisions........
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Old 24 May 2018, 14:55   #9
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I have the HR4-D, I initially went with the 21" on the assumption that the reduction in diameter would compensate for the extra blade, but I was wrong. I was overpropped by about 500rpm. I bought a 19" & its bang on the money. I'm hitting 6000rpm & hole shot is much improved, if anything the economy has improved with the smaller prop, probably due to the engine being free to rev.
I have the 21" for sale if anyone is interested
https://www.steeldevelopments.net/50...-d-series.html
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Old 24 May 2018, 15:38   #10
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Prop 19"

I would stay with a 19" 3 plade SS as the attached. Worked very well for us on a 6.1 rib 150hp 46kn
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