3 blade to 4 blade prop conversion chart

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 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.

Boat name: Highland Bluewater

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

Originally Posted by Last Tango

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

Originally Posted by Iron Dials

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.

Boat name: Highland Bluewater

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.

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.

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........

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

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

Attached Thumbnails

Boat name: Highland Bluewater

Not a rhetorical question......

What are you trying to achieve/resolve by changing your prop?

Remember the vented prop options (Laser & Tempest are vented) this give the best of both worlds hole-shot and top end. Down side is semi-displacement speed revs a bit higher than a non vented equivalent but it's probably not where you'd tend to run anyway.

Cavitation is the issue, particularly as boat weight increases.
Other than that it’s all about mid-range performance.

I’m going to buy a prop anyway, so that I have a spare. This of course gives me choices....
The opportunity to try something different to improve current performance lead to the question.

Originally Posted by Iron Dials

Cavitation is the issue, particularly as boat weight increases.....

Boat name: Highland Bluewater

Originally Posted by jwalker

Are you running the standard Mercury prop with blades like teddy bear's ears? And it breaks free in a sharp turn and won't regain a grip until you slow down? If so, you simply need a decent prop.

Rake & Cup.......a decent stainless prop. (unless the engine is mounted too high?)

I do have big difference 3 blade versus 3 blade - both steel

VIPER 19" - very nervous, boat reacts on any throttle move (easy to lost plane)
REBEL 17" - bigger diameter, bigger blade surface

Much better grip on REBEL and better fuel consumption at MID (the same speed). VIPER is "bow lifter", REBEL very good for throttle manipulation (smoothly change speed). REBEL is much better for me for offshore, choppy Sea

JW- It's a Mercury Black Max, so nothing too fancy
LT- Engine is mounted fine, see line above!
Matt- V interesting the difference you describe.

Anyway, after all the constructive input I've decided to go to 5 blades!
Previous boat came with a Mercury High5, not in it's first flush of youth. That boat was overpropped with it but was v solid in the water.
Pitch is 20 but diameter is 13 and a bit, so thinking if getting it refurbed and either it works or I sell it. And then I'll probably buy a good 3 blade SS anyway.

Thanks for all the replies.

Boat name: Highland Bluewater

Originally Posted by Iron Dials

JW- It's a Mercury Black Max, so nothing too fancy
LT- Engine is mounted fine, see line above!
Matt- V interesting the difference you describe.

Anyway, after all the constructive input I've decided to go to 5 blades!
Previous boat came with a Mercury High5, not in it's first flush of youth. That boat was overpropped with it but was v solid in the water.
Pitch is 20 but diameter is 13 and a bit, so thinking if getting it refurbed and either it works or I sell it. And then I'll probably buy a good 3 blade SS anyway.

Thanks for all the replies.

Wow, you're good!
I just checked and you're dead right.
Thanks v much.
Back to new prop so.....

Boat name: Highland Bluewater

There was an example of a 6.5m Zodiac on Mercury's site hitting 53mph with a 17" Enertia at 5880rpm (over revving slightly) and I'm getting 50mph with a 19" Tempest plus at 5650rpm both 3 blade with Merc F150.

On the basis you expect to run a bit "heavy" you're probably not going to make 53mph....so drop 10% speed and revs your probably going to be "in scope" with either of these props.
Prior to the re-engine mine would pull two skiers with the BF130 Honda but the difference in the hole-shot now is incredible with the F150's bottom end torque and I reckon you'd get away with the vented 19" Tempest if you're "economy or range" inclined or go for the 17" if your thinking more "workhorse"

Boat name: Clyde adventurer

Engine: Twin Merc 150 4str

If your having ventilation issues have you checked the plastic bore sleeves between prop and gearbox are present and in good condition?

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