Guys,
I remember reading some frustratrations about switching from Aluminum to S/S props. I think Jgroves had issues that I would like to avoid. His string indicated that the aluminum-to-S/S RPM numbers werent the same as would be expected. So here goes:
I have an FS-17 that fully loaded (engine, fuel, wells, coolers, 2 fat guys etc.) propably grosses at 1300 pounds. It's pushed by a Yamaha High thrust 4-stroke 60 hp. It currently has a 13-1/2 x 15 pitch aluminum prop on it. GPS max speed is 29 kts. Max RPM achievable is 5400. (Mfg suggested band is 5000-6000 rpm)
This thing comes out of the hole like a tractor on steroids! I think I should be able trade a little bottom end traction for some top end speed. Any suggestions?
KRE
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KRE as I recall your FS17 is pretty customized so I am adding a hundred+ pounds to your weight. Here is my breakdown
Boat 600
Motor, wet 260
fuel, 120
People 400
Misc. 300(1) 1 includes 100 for bait well water.
Total 1420
Even with this weight you should be getting mid 30's according to my Crouch calculator.
Now looking at the prop numbers the calculator comes up right on with what you are getting with the aluminum.
Do you see any cup in your aluminum prop. What brand is it, Yamaha? Where do you get up on plane. Where can you keep it on plane.
It looks like what we need to do is start at a Yamaha Black Painted SS 17". This should put you at mid 30's at 5800 rpms. Or if you want the PowerTec equivalent. What we want to do is since you have a large gear ratio is make use of it by using a large amount of propellor without a large amount of energy sapping rake or cup. We saw this with Larry's 2.23 ratio and his low torque. While you don't have the lower torque you don't have as much as other 4st's.
Tom
Boat 600
Motor, wet 260
fuel, 120
People 400
Misc. 300(1) 1 includes 100 for bait well water.
Total 1420
Even with this weight you should be getting mid 30's according to my Crouch calculator.
Now looking at the prop numbers the calculator comes up right on with what you are getting with the aluminum.
Do you see any cup in your aluminum prop. What brand is it, Yamaha? Where do you get up on plane. Where can you keep it on plane.
It looks like what we need to do is start at a Yamaha Black Painted SS 17". This should put you at mid 30's at 5800 rpms. Or if you want the PowerTec equivalent. What we want to do is since you have a large gear ratio is make use of it by using a large amount of propellor without a large amount of energy sapping rake or cup. We saw this with Larry's 2.23 ratio and his low torque. While you don't have the lower torque you don't have as much as other 4st's.
Tom
Tom,
I agree with your weight calculation. At 5300 rpm with the 15 pitch prop I can only get 5300 rpm. It seems to me that if I increase pitch, that logically, I shoud decrease diameter if I want to at least maintain or increase RPM. The 15X13-1/2" prop is yamaha OEM. I dont think it's cupped. I'm a little sketchy on your idea of increasing pitch without any other changes. Reading back on past strings, somebody else (Jgroves where are you?) lost a couple hundred RPM switching like for like.
I havent check actual kts for minimum plane, but I would guess it to be near 14-18 kts. My top speed of 29.6 kts is actually 33.35 mph. I like the idea of a black painted SS prop. Less bling is better.
I agree with your weight calculation. At 5300 rpm with the 15 pitch prop I can only get 5300 rpm. It seems to me that if I increase pitch, that logically, I shoud decrease diameter if I want to at least maintain or increase RPM. The 15X13-1/2" prop is yamaha OEM. I dont think it's cupped. I'm a little sketchy on your idea of increasing pitch without any other changes. Reading back on past strings, somebody else (Jgroves where are you?) lost a couple hundred RPM switching like for like.
I havent check actual kts for minimum plane, but I would guess it to be near 14-18 kts. My top speed of 29.6 kts is actually 33.35 mph. I like the idea of a black painted SS prop. Less bling is better.
KRE my apologies missed the Kts. To used to mph on here. Please double check my figures here as I am getting some strange calculations:
Top Speed 33.5 mph
Rpm's 5400
2.33 gearcase
13.5" x 15 pitch
Per your question on going to the 17" painted it is only 13" in diameter so we pick up 3-400 rpm's there. Also the slip of an aluminum prop is usually 3-6 points higher than a SS propeller. Slip for a SS will be from 6-11 or 12, while slip for an aluminum prop usually is around 13-15.
Tom
Top Speed 33.5 mph
Rpm's 5400
2.33 gearcase
13.5" x 15 pitch
Per your question on going to the 17" painted it is only 13" in diameter so we pick up 3-400 rpm's there. Also the slip of an aluminum prop is usually 3-6 points higher than a SS propeller. Slip for a SS will be from 6-11 or 12, while slip for an aluminum prop usually is around 13-15.
Tom
Last edited by TomW on Sun Mar 22, 2009 6:55 pm, edited 1 time in total.
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K, Im just an aluminum fan because where we are, you want that prop to be the frangible link. Banging a prop in the river can mean a 2-6mph downriver cruise until you can get yourself to shore, or possibly head out to sea if we are out in that area. Grenading a prop isn't so bad because we can whip a new one on in the water, where a stainless one tends to make mush out of everything in the lower unit and the drive splines on the main shaft.
If you think its soft enough to whack and not cause a problem, and you are having a power transmission issue with alu props, I can see some of the reason to switch.
I haven't seen too many smaller motors that get a staggering difference between identical alu and SS props. Under 100hp the engines just aren't strong enough to have large scale blade deflection. On my neighbor's bass boat with the big new Zook, he definitely needs it. Turned out to be something like 6mph difference, which is huge in tournaments.
E
If you think its soft enough to whack and not cause a problem, and you are having a power transmission issue with alu props, I can see some of the reason to switch.
I haven't seen too many smaller motors that get a staggering difference between identical alu and SS props. Under 100hp the engines just aren't strong enough to have large scale blade deflection. On my neighbor's bass boat with the big new Zook, he definitely needs it. Turned out to be something like 6mph difference, which is huge in tournaments.
E
KRE, Tom is right, with an aluminum 15" and no cup mathematically you can not get 33.5 MPH. My theory is that you are getting that extra two inches of pitch with cupping. But, you will gain the extra RPM's Tom is talking about with the SS Painted and a very good increase in speed as well. I would recommend you go with the 17 Painted as well, and I am sure you will be very happy with the results.
H
H
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Sorry, I have to disagree with that. It may have been true 30 years ago, but todays engines are designed to swing a SS prop, even smaller ones. If your lower unit tears up before the friction hub in the prop slips it was a POS to start with. I've been running stainless props 40 years and have never torn up a gearcase or stripped a shaft. I have prop strikes every trip out. Where we run boats an aluminum prop won't last a week. And I've never seen a boat that a SS prop didn't improve the performance, even on a 25 HP.where a stainless one tends to make mush out of everything in the lower unit and the drive splines on the main shaft.
OK, back to the drawing board. If Cups are at the trailing edge of the prop, then I dont have any. The contour of this prop is smooth (less the oyster scars). So If I increase pitch to 17, I loose 300-400 RPM, but I get it back with the of 1/2" diameter reduction. So now I'm back to zero RPM loss (in my mind), but the slipage will be less, therefore more HP required and I ultimately loose some RPM...Am I missing something here? At 5300 max RPM it seems as though I cant afford much. I would have never guessed a 17" prop would turn fast enough. Do they make a 17X12-3/4?
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