SS to Composite

[obsessed]

So recently I touched down my stainless prop (14x19 Laser II) on takeoff. Boat had some horrible vibrations after. Decided I would go with a composite Piranha prop. I MIGHT be having issues, not sure. All MPH is using boat GPS, RPM is w/boat tach.

Boat is a 1986 19' Mark Twain Bow Rider. Weighs 4400 with single axle venture trailer. It is a deep V and feels like a heavy boat. Motor is a Merc 260, Outdrive is an Alpha One.Stock QJet carb.

Originally with the Stainless, I could get up above 4000 RPM (never really paid attention) and could for sure get 45MPH. Decent Hole shot. Cruising speed at ~28-30mph and ~3000RPM

With the Piranha (14x19) composite the hole shot feels similar but my RPMs have dropped at all speeds. Cruising speed ~28-30 the rpm are now in the 2700-2800 range. WOT I can only get 40MPH and the engine will not go beyond 3600 RPM.

The trim also feels less responsive. I am not sure where I go from here, I guess if I increase pitch, my hole shot gets worse, but I gain top speed? Is that true? What would diameter increase do for me? Is four blades a better choice?

Realistically I do not need to go much faster than 40 ever. Would it be detrimental to the engine to run it at WOT and have that be 3600?

Just looking for some advice. . .

 

[Texasmark]

Re: SS to Composite

So recently I touched down my stainless prop (14x19 Laser II) on takeoff. Boat had some horrible vibrations after. Decided I would go with a composite Piranha prop. I MIGHT be having issues, not sure. All MPH is using boat GPS, RPM is w/boat tach.

Boat is a 1986 19' Mark Twain Bow Rider. Weighs 4400 with single axle venture trailer. It is a deep V and feels like a heavy boat. Motor is a Merc 260, Outdrive is an Alpha One.Stock QJet carb.

Originally with the Stainless, I could get up above 4000 RPM (never really paid attention) and could for sure get 45MPH. Decent Hole shot. Cruising speed at ~28-30mph and ~3000RPM

With the Piranha (14x19) composite the hole shot feels similar but my RPMs have dropped at all speeds. Cruising speed ~28-30 the rpm are now in the 2700-2800 range. WOT I can only get 40MPH and the engine will not go beyond 3600 RPM.

The trim also feels less responsive. I am not sure where I go from here, I guess if I increase pitch, my hole shot gets worse, but I gain top speed? Is that true? What would diameter increase do for me? Is four blades a better choice?

Realistically I do not need to go much faster than 40 ever. Would it be detrimental to the engine to run it at WOT and have that be 3600?

Just looking for some advice. . .

Never ran a Pirhana. My experience with props other than SS is that they are of inferior performance..for several reasons usually reserved to the designers of SS props and the material's capability over others. So my off shoulder comment about your performance is that it is to be expected.

HTH.

Mark

 

[elkhunter338]

Re: SS to Composite

Make sure you have an accurate tach. I had a tach that was about 600-800 rpms off. Showed 4k when the engine was at 4.7k.
Blade design makes a difference. I would check to make sure you got the 19 pitch blades, if you do have the 19 pitch blades I would order some blades with less pitch.
I run a variable pitch prop and just one setting adjustment changes rpms by about 300 rpms.
I run a pro-pulse adjustable 4 blade prop. You drop pitch and gain rpms, you gain pitch and you loose rpms. You want your engine running at WOT in the rpm range your engine is suppose to run in.

 

[obsessed]

Re: SS to Composite

tach is accurate, checked it tonight. should I consider dropping to a 17?

Make sure you have an accurate tach. I had a tach that was about 600-800 rpms off. Showed 4k when the engine was at 4.7k.
Blade design makes a difference. I would check to make sure you got the 19 pitch blades, if you do have the 19 pitch blades I would order some blades with less pitch.
I run a variable pitch prop and just one setting adjustment changes rpms by about 300 rpms.
I run a pro-pulse adjustable 4 blade prop. You drop pitch and gain rpms, you gain pitch and you loose rpms. You want your engine running at WOT in the rpm range your engine is suppose to run in.

 

[mpsyamaha]

Re: SS to Composite

yes, you need to drop pitch. you are lugging the motor running it at such low rpms.

 

[obsessed]

Re: SS to Composite

Drop pitch meaning go to a 17? Should I expect to drop my top speed as well? Or will that allow me to get more RPM and therefore more speed?

yes, you need to drop pitch. you are lugging the motor running it at such low rpms.

 

[Texasmark]

Re: SS to Composite

tach is accurate, checked it tonight. should I consider dropping to a 17?

Probably a wise idea. Unloading the engine and allowing it to rev back up where it was may get you back closer to your original 45 mph, but don't be surprised if you don't. The Laser II has all the gimmics in the design to make it a hot dog prop.

I ran a 24P Laser I back in '89 on a 17' Ranger Fisherman with a Merc 115 and the speedo topped out at 55. It was my first outing with the boat and even though I was able to make that unbelievable speed, it was obvious that the prop was too much and I traded down to a 21P which it kept the remainder of it's life. In the end, you may wind up going back to another Laser II.....Having been where you were, you may never be satisfied till you can get those numbers again.

HTH

Mark

Mark

 

[elkhunter338]

Re: SS to Composite

You said you checked the tach. Did you check it under wot?
The reason I ask is because my tach read real close at idle and slightly higher running it on the garden hose.
I could not sell if the tach was really off until I got the engine up in rpms.

 

[elkhunter338]

Re: SS to Composite

I thought I read some where that one change in pitch = 300 rpms, so a 19 to 17 would give you about 4200 rpms wot, do some research on the web and verify what I am telling you because I am not 100% sure.
If you are in no hurry I just changed my prop from 15 pitch (4700 rpms) to a 17 pitch (? rpms)
I plan to run the boat in a few weeks.

 

[NSD]

Re: SS to Composite

@obsessed

Are you sure you got the 14x19A blades and not perhaps the 15x19A from Piranha?

When I switched from the S/S to the Piranha to test out how this spare prop would run, I found that whilst i lost top end speed, my rpm's increased by at least 400rpm compared to the S/S I was running before (BRP Viper 19) and I switched to the Piranha 15x19 blades.