4 blades or 3 blades props on a 58ft trader dual engines 2x435hp

[Patrick4002]

Our boat is a 58ft trader 2x 435hp current 3blade props, 26T displacement

Wat do you have your preference 4 blades or 3 blades props and why.

my 3 blades pros need to changes, they are broken due electrolyse.
the boat is 30 years old and there where no anodes on the prop shafts

i read a lot the speed is not so importend (runs fast if needed) fuel economy is important
but we do not travel so mutch so the impact is not so great. (i think)

is there a big difference at normal medium speed. or is it always ?

Best regards, Patrick

 

[SoWhat]

4 blades are much better at low speed, better economy and more bite so better response, (assuming you adjust pitch to get same rpms at WOT.)
I think price and availability are bigger factors. 4 blades are more expensive and may be special order.

 

[Patrick4002]

today, 4 blades are a little more expenxive then 3 blades but i got price for a 3 blades set for 5100 euro and a 4 blades set for 5600 euro so the price is not a big difference now.
thanks for the responce, we are not for the top speed (oure boat runs easy 12knots) and the handeling on low speed is importand

Best regards, Patrick

 

[tiltrider1]

Don’t worry about 3 blade vs 4 blade. Worry about the right prop. There is effectively no difference between the correct 3 blade prop for your boat and the correct 4 blade prop for your boat. Any claims of advantage will be so small that you won’t be able to tell the difference.

What you will notice is the wrong prop no matter how many blades.

 

[Comodave]

One reason to go to 4 blade is tip clearance. If you can’t get a large enough 3 blade prop due to tip clearance and to load the engines then by going to a 4 blade lets you get more prop surface and still maintain proper tip clearance.

Also if the props are toast due to corrosion you should probably check the shafts carefully for corrosion.

 

[tiltrider1]

One reason to go to 4 blade is tip clearance. If you can’t get a large enough 3 blade prop due to tip clearance and to load the engines then by going to a 4 blade lets you get more prop surface and still maintain proper tip clearance.

Also if the props are toast due to corrosion you should probably check the shafts carefully for corrosion.

Like Comodave said, if you don’t have the room for the correct 3 blade prop then the correct 4 blade prop will be the best decision.

 

[Patrick4002]

thanks all,i tink that i gone stay withe the 3 blade prop.

But still a lit confused one of the prop manufactures did some calculation and suggested to get a bigger pitch so the engine load would be better he proposed a 4 blade 30x30 instead the 3 blade 30x27

anny advice on this ?

the boat runs now pretty good the top speed is not so big but on channels he is almost to fast.

 

[Comodave]

What is the max recommended RPMs for your engine? Can you reach that RPM with the normal load in the boat?

 

[Irish Rambler]

4 blades are much better at low speed, better economy and more bite so better response, (assuming you adjust pitch to get same rpms at WOT.)
I think price and availability are bigger factors. 4 blades are more expensive and may be special order.

Just my 2 cents. For smoother running I was looking to change from a 3 to a 4 blade on our single engine 42' cruiser and had a prop builder make 2 for me, 1 for spare. I was told that 4 blades with a 4 cylinder engine would give sympathetic resonance. Actually not true because the engine/gearbox ratio eliminates that. I slightly over pitched them to help fuel economy and reduce engine noise, we never run WOT except in emergencies. Excellent results after 5 years.

 

[SoWhat]

thanks all,i tink that i gone stay withe the 3 blade prop.

But still a lit confused one of the prop manufactures did some calculation and suggested to get a bigger pitch so the engine load would be better he proposed a 4 blade 30x30 instead the 3 blade 30x27

anny advice on this ?

the boat runs now pretty good the top speed is not so big but on channels he is almost to fast.

Typically you would decrease pitch when switching to a 4 blade. If you are too fast at idle throttle then you might be over propped. Increasing pitch would result in even more thrust at low speed. You can decrease diameter or pitch on your 3 bladed props to achieve your desired performance to match your typical cruising style. The props have been changed in 30 years so who knows what was originally specified for the boat.

Prop calculations are designed to maximize performance., ie top speed of 10.2 kn at WOT of 2700 rpm. You might be more interesting in sacrificing top end performance for better control at lower speeds. Decrease pitch and you may see 9.9 kn at WOT. Increase pitch and you never reach WOT of 2700 rpm. Engine manufacturers specify a max rpm and recommend running at max rpm periodically to minimize carbon buildup but I don't believe a 10% variance is significant

I decreased pitch below prop calculations when I switched to 4 blades and now have far better control when maneuvering in marina fairways. Not much difference at my usual cruise settings of 1800rpm.

 

[Patrick4002]

thanks all for the feedback, i decided to stay with my current size of props. the boat runs fine get easy to 10 knots on 2200 rpm on one engine (withe the last test run the second engine dit go in alarm due to a faulty temp sensor) getting to our bird or in channel the boat runs almost to fast, the top speed is not the max but i want to get the best smooth ride. I take 3 blades as they are better for fuel consumption (the cats likes lots of fuel).

 

[MichaelD]

Not to cast further shade, but if the props are electrolyzed away, you must check all the running gear as there is likely more damage: shafts, struts, rudders, rudder boxes, shaft logs, etc. good luck.

 

[Patrick4002]

thanks, don it, im glad i did not find other problems but one ground bar was broken one on the bed propellor. i did not find any invoices of axel anodes in the backlog , i have all the invoices from delivery till the last 6 years before i got the boat.

 

[OldDan1943]

Don’t worry about 3 blade vs 4 blade. Worry about the right prop. There is effectively no difference between the correct 3 blade prop for your boat and the correct 4 blade prop for your boat. Any claims of advantage will be so small that you won’t be able to tell the difference.

What you will notice is the wrong prop no matter how many blades.

Visit a prop shop and discuss with them.
Remember, you want a prop for average conditions.

 

[Slider70]

My experience with my 38' trawler type hull drafting six feet (big hold) in changing from a three blade to a four blade are as follows:
- I was commercial fishing trolling for salmon. The fleets experience was a four blade prop was quieter, less vibration, than a three blade causing the boat to fish better and it did.
- The other changes I noted were when bucking into heavy seas, maybe something that most pleasure boaters try to avoid, it was a smoother ride.
- The only negative, other than price, I noted was docking. The four blade didn't grab the water in reverse as well as the three blade.