Changing prop

[onegreatsteak]

I know this is an old often used topic but what I want is to just get a starting point.

Restoring an old Bayliner Nisqually. Its a 24 Ft cabin cruiser. 4400 pounds, 60 gallons of fuel 420 pounds , deep V , Flybridge. Guessing 4 crew at 600 pounds. supplies/whatever 300 pounds. For 5720 total pounds. Probably call it 6000 Pounds.

Original prop is a 15" 18 pitch for a 220 Hp Volvo Penta. Motor is completely rebuilt and modernized with newer heads, comp cams Marine cam rated for Idle to 4800 rpm, and carburetor. Comp cams Dyno chart on the cam showed it at 335 hp at 4800 rpm. That was dyno not real world. My guess is 290 to 320 which seems to be a good guess, since the original prop is doing about 4000 rpm at half carburetor opening measured at the shaft. The engine is new and I am not pushing it further till I get more hours on the engine. Did not even stay long there then.

I am after a starting guess on the prop. I would like to go to a 4 blade but the ones available for an AQ 270 are 14" 23 pitch one inch smaller then my 15" prop but much higher pitch. Losing the inch effectively negates some of the high pitch. But I am not sure if the inch loss would be a - for efficiency. Or if the 4th blade would make up for it.

The boat will spend most of its time at just over plane and trolling off shore at low speed which is why the super torkey cam. If I can get close I can have the prop repitched up or down 2 inches probably be on the money.

 

[Bondo]

Re: Changing prop

I know this is an old often used topic but what I want is to just get a starting point.

Restoring an old Bayliner Nisqually. Its a 24 Ft cabin cruiser. 4400 pounds, 60 gallons of fuel 420 pounds , deep V , Flybridge. Guessing 4 crew at 600 pounds. supplies/whatever 300 pounds. For 5720 total pounds. Probably call it 6000 Pounds.

Original prop is a 15" 18 pitch for a 220 Hp Volvo Penta. Motor is completely rebuilt and modernized with newer heads, comp cams Marine cam rated for Idle to 4800 rpm, and carburetor. Comp cams Dyno chart on the cam showed it at 335 hp at 4800 rpm. That was dyno not real world. My guess is 290 to 320 which seems to be a good guess, since the original prop is doing about 4000 rpm at half carburetor opening measured at the shaft. The engine is new and I am not pushing it further till I get more hours on the engine. Did not even stay long there then.

I am after a starting guess on the prop. I would like to go to a 4 blade but the ones available for an AQ 270 are 14" 23 pitch one inch smaller then my 15" prop but much higher pitch. Losing the inch effectively negates some of the high pitch. But I am not sure if the inch loss would be a - for efficiency. Or if the 4th blade would make up for it.

The boat will spend most of its time at just over plane and trolling off shore at low speed which is why the super torkey cam. If I can get close I can have the prop repitched up or down 2 inches probably be on the money.

Ayuh,.... Yer givin' the Diameter to much credit, Pitch is what matters,...

You'll Never turn a 23" pitch....

 

[onegreatsteak]

Re: Changing prop

Ayuh,.... Yer givin' the Diameter to much credit, Pitch is what matters,...

You'll Never turn a 23" pitch....

I don't know boat props that well but I was concerned about Diameter because on Aircraft It's better to have the biggest prop at a lower pitch than a smaller prop at a higher pitch. Assuming it"s not longer than the landing gear.

If one inch less means little I would not mind a little more clearance above the skeg.

What would you call a good starting point?

This engine has little to do with the one it replaces so I know it,s just best guess.

 

[steelespike]

Re: Changing prop

I can't help much with details but with that weight I think you want plenty of diameter and maybe a 4 blade as well.
There should be someone along with more details.

 

[Bondo]

Re: Changing prop

I don't know boat props that well but I was concerned about Diameter because on Aircraft It's better to have the biggest prop at a lower pitch than a smaller prop at a higher pitch. Assuming it"s not longer than the landing gear.

If one inch less means little I would not mind a little more clearance above the skeg.

What would you call a good starting point?

This engine has little to do with the one it replaces so I know it,s just best guess.

Ayuh,.... For 1 thing, forget about the Diameter, you have little to no control of it anyways...
Diameter is basically a function of blade Design, 'n is determined by the Engineer that designs it...
This ain't yer airplane....
Generally speakin',... The taller the pitch, the smaller the diameter, 'n vice versa...

I'd start with yer basic 21" prop that fits yer drive, even if it ain't Right, it'll be a good back-up/ spare...

That'll give ya some usable numbers to dial it in, possibly a bigger prop, or more blades, or Whatever...

That was dyno not real world. My guess is 290 to 320 which seems to be a good guess, since the original prop is doing about 4000 rpm at half carburetor opening measured at the shaft.

 

[onegreatsteak]

Re: Changing prop

Ayuh,.... For 1 thing, forget about the Diameter, you have little to no control of it anyways...
Diameter is basically a function of blade Design, 'n is determined by the Engineer that designs it...
This ain't yer airplane....
Generally speakin',... The taller the pitch, the smaller the diameter, 'n vice versa...

I'd start with yer basic 21" prop that fits yer drive, even if it ain't Right, it'll be a good back-up/ spare...

That'll give ya some usable numbers to dial it in, possibly a bigger prop, or more blades, or Whatever...

Looking for around 4000 to 4800 Top rpm. In the middle about 4400 would probably be ideal. The motor will not see that rpm often. It will be mostly cruse just over plane which happens 3000 to 3200, depending on water smoothness. Getting the cruse rpm down will help economy to some degree but I don't want to lug the engine. I figure trolling rpm will take care if itself.

These values can vary depending on what I learn, nothing is firm.

 

[Bondo]

Re: Changing prop

Looking for around 4000 to 4800 Top rpm. In the middle about 4400 would probably be ideal. The motor will not see that rpm often. It will be mostly cruse just over plane which happens 3000 to 3200, depending on water smoothness. Getting the cruse rpm down will help economy to some degree but I don't want to lug the engine. I figure trolling rpm will take care if itself.

These values can vary depending on what I learn, nothing is firm.

Ayuh,.... 4 grand is too low,.... Shoot for 4600/ 4800 rpms at Wot...

Prop pitch changes it, 'bout 200 rpms per inch change, Roughly...

 

[onegreatsteak]

Re: Changing prop

Ayuh,.... 4 grand is too low,.... Shoot for 4600/ 4800 rpms at Wot...

Prop pitch changes it, 'bout 200 rpms per inch change, Roughly...

Looking at a 21 pitch 14 inch four blade.
Boat is at 4000 half throttle " still breaking in motor " with a 18 pitch 15 inch 3 blade
I figure if its close I can re-pitch it 2 inch's up or down.

 

[Bondo]

Re: Changing prop

Ayuh,.... That sounds reasonable,....

Btw,.... 1/2 throttle is pretty much meaningless....

The 2nd 1/2 doesn't equal the 1st 1/2....

 

[onegreatsteak]

Re: Changing prop

Figured it would not be.

Just looking at it as an indication. Guessing it would gain 1000 to 1500 on the second half. Probably would take twice to three times maybe even four times the hoarse power to go 50% faster. The flying bridge blocks a lot of air, not even considering water resistance.

If it did it would probably be doing 80. Not going to happen. Would be scary too.

It's already faster than my old 14Ft Seabrease. Got to get the Garmin on it next time.

 

[smitty477]

Re: Changing prop

"Restoring an old Bayliner Nisqually. Its a 24 Ft cabin cruiser. 4400 pounds, 60 gallons of fuel 420 pounds , deep V , Flybridge. Guessing 4 crew at 600 pounds. supplies/whatever 300 pounds. For 5720 total pounds. Probably call it 6000 Pounds."

FWIW - there are numerous owners and other folks very famiiar with the Bayliner Nisqually and the VP drives on the Bayliner owners club.
Perhaps try asking this question there to get a first hand account of these combinations.

Baylinerownersclub(dot)org

Hope this helps