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-   -   Help picking a new prop (http://www.spearboard.com/showthread.php?t=42429)

Redneck7 03-07-2007 05:16 PM

Help picking a new prop
 
I repowered my boat and we've been running it through it's paces and today went for WFO...operating range is supposed to be 4400-4800...she screamed to 5000 before we knew which way was up and was only running 33mph.

Do I need a bigger prop, more pitch, or both?

How do I decide where to start?

bgbill 03-07-2007 05:21 PM

Re: Help picking a new prop
 
If you go up in pitch, for every one inch, you will drop 200 RPM.

What size prop are you running now?

Was the boat loaded as you normally run it?

kaborkian 03-07-2007 05:45 PM

Re: Help picking a new prop
 
More boat info would be nice.

What boat, what weight, what engine, and what prop is on it now?

Also, not all "15 x 19 pitch" (or whatever the size is) are created equal. The blade size, shape, overlap if any, and cup all make a huge difference.

Mariner 03-07-2007 05:45 PM

Re: Help picking a new prop
 
what size boat?
what kind/hp motor?
what size prop where you running?

Redneck7 03-07-2007 05:51 PM

Re: Help picking a new prop
 
Sorry, could have provided more info. 23' Fish Nautique, fuel wasn't full, no scuba gear, but there were a bunch of tools and more people than normal, so in the end I was probably close to normal load.

Went from a 240hp 351w to a 320hp 350. Full inboard.

I believe the current prop is a 14x11.

Anything else I missed?

bgbill 03-07-2007 05:53 PM

Re: Help picking a new prop
 
Quote:

Originally Posted by Redneck7
Sorry, could have provided more info. Fish Nautique, fuel wasn't full, no scuba gear, but there were a bunch of tools and more people than normal.

Went from a 240hp 351w to a 320hp 350. Full inboard.

I believe the current prop is a 14x11.

Anything else I missed?


Sounds like you are underpropped, try a 14x13, that should drop your WOT down by 400 RPM's.

kaborkian 03-07-2007 07:33 PM

Re: Help picking a new prop
 
Based on your 14 x 11, and an assumed loaded weight of 8500 lbs:



MAX PERFORMANCE
Available Power: 232.8 SAE hp ( 173.6 kW) @ 5000 RPM
Max Speed: 22.3 kts
Fuel: 14 US Gall/Hr (typical)
RECOMMENDED PROPELLER
Diameter: 14.4 in. Pitch: 10.7 in.
Blades: 3 Cupped: No

Increasing the transmission ratio will improve prop efficiency.
PROPELLER BASED ON INPUT DIAMETER
Diameter: 14.0
Pitch: 11.2
Blades: 3


MAX PERFORMANCE
Available Power: 310.4 SAE hp ( 231.5 kW) @ 5000 RPM
Max Speed: 25.8 kts
Fuel: 19 US Gall/Hr (typical)
RECOMMENDED PROPELLER
Diameter: 15.3 in. Pitch: 12.2 in.
Blades: 3 Cupped: No

Increasing the transmission ratio will improve prop efficiency.
PROPELLER BASED ON INPUT DIAMETER
Diameter: 14.0
Pitch: 13.7
Blades: 3




This says you should go from 14 x 11 to 14 x 13.7

You did say though that you're not sure of the prop size, so increasing by about 3" pitch from where you are should get you close.

Gotta love it when the predictions and the gut-feeling from knowing boats seem to match well.

Oh yea, the fuel GPH predictions are diesel...ignore them :rolleyes:

Nate Baker 03-07-2007 07:50 PM

Re: Help picking a new prop
 
You might also try a dedicated prop shop if there's one in your area. You'll pay a little more, but many of them will let you try a couple of props if you return them undamaged. Then you can dial it in yourself.

Is it safe to assume that with that much horsepower you're using a stainless prop?

Redneck7 03-07-2007 11:21 PM

Re: Help picking a new prop
 
Tomol: NiBrAl prop.

Kaborkian: Where'd those calculations come from? Seems like a nifty program. Weight might be on the high side, but I'd rather calculate from there just in case I start carrying concrete out to make AR's ;) .

kaborkian 03-08-2007 04:35 PM

Re: Help picking a new prop
 
Yea, I thought so too on the weight, but I varied the weight until it came out to the right RPM/prop size for 240 horsepower.

Think about what you carry out too though...4 divers with gear and tanks can easily hit 1600 lbs. Fuel adds another 1200 or more lbs. Ice, 400 more. That's 3200 in gear/fuel alone.

It's a web based program on boatdiesel.com, but you need a subscription to use it.

Redneck7 03-08-2007 09:40 PM

Re: Help picking a new prop
 
Prop shop today ran a calculation based solely on desired top speed, current horsepower and boat length. Came out with the exact prop I think is on the boat. Either I'm wrong or they are because 5000RPM at 35mph is not optimal. I'm dropping it off tomorrow for them to put an inch or two of pitch in it and see where it puts me.

Ran it with one of their guys who thought my transmission was slipping based on how fast it ran up to 5 grand.

Then he checked a few things out and came to the conclusion that I should either buy a new prop, or paint the boat yellow and put "Sea Tow" down the side.

dagodiver 03-09-2007 07:18 AM

Re: Help picking a new prop
 
I am running the same boat with the same power and with a 13 X 12 I can spin 4800 and run 40mph all day.

Stick with the NiBrAl wheel as the SS suck for this ap.
Make sure your tranny is staying at the proper running temp during normal operations. I have run these tranny's with 800hp with no problems but there are a few tricks to keeping them running good and not slipping.




Dago.

dagodiver 03-09-2007 07:49 AM

Re: Help picking a new prop
 
Quote:

Originally Posted by kaborkian
Based on your 14 x 11, and an assumed loaded weight of 8500 lbs:



MAX PERFORMANCE
Available Power: 232.8 SAE hp ( 173.6 kW) @ 5000 RPM
Max Speed: 22.3 kts
Fuel: 14 US Gall/Hr (typical)
RECOMMENDED PROPELLER
Diameter: 14.4 in. Pitch: 10.7 in.
Blades: 3 Cupped: No

Increasing the transmission ratio will improve prop efficiency.
PROPELLER BASED ON INPUT DIAMETER
Diameter: 14.0
Pitch: 11.2
Blades: 3


MAX PERFORMANCE
Available Power: 310.4 SAE hp ( 231.5 kW) @ 5000 RPM
Max Speed: 25.8 kts
Fuel: 19 US Gall/Hr (typical)
RECOMMENDED PROPELLER
Diameter: 15.3 in. Pitch: 12.2 in.
Blades: 3 Cupped: No

Increasing the transmission ratio will improve prop efficiency.
PROPELLER BASED ON INPUT DIAMETER
Diameter: 14.0
Pitch: 13.7
Blades: 3




This says you should go from 14 x 11 to 14 x 13.7

You did say though that you're not sure of the prop size, so increasing by about 3" pitch from where you are should get you close.

Gotta love it when the predictions and the gut-feeling from knowing boats seem to match well.

Oh yea, the fuel GPH predictions are diesel...ignore them :rolleyes:



While that is some cool program those numbers are WAY off.
The boat is more like 4500 maybe. With a stock 240hp I was running 34 all day.


Dago.

Kahuna 03-09-2007 09:21 AM

Re: Help picking a new prop
 
Mike call me I have a couple of three blades you can play with. If think thye were 15x13 or maybe 14.

kaborkian 03-09-2007 09:40 AM

Re: Help picking a new prop
 
Quote:

Originally Posted by dagodiver
While that is some cool program those numbers are WAY off.
The boat is more like 4500 maybe. With a stock 240hp I was running 34 all day.


Dago.

Yea, it's really made for a larger inboard diesel boat with big slippage numbers. That's why I ran it both ways, old and new, to get a relative change instead of just a suggested prop.

It matches very close for my boat (35' Luhrs, ~22,000 lbs), but quite often misses on weight or predicted speed for center console boats that have more like 90+% prop efficieny numbers instead of the 60's or 70's like full-cabin heavier boats.

I also didn't have all relevant info, so I made some assumptions to get the data, which again is why I ran it for both conditons.

Here's what it says if I use what I now percieve as correct info:

MAX PERFORMANCE
Available Power: 232.8 SAE hp ( 173.6 kW) @ 5000 RPM
Max Speed: 42.1 kts
Fuel: 14 US Gall/Hr (typical)
RECOMMENDED PROPELLER
Diameter: 14.4 in. Pitch: 18.9 in.
Blades: 3 Cupped: No

What's the gear ratio of the transmission? I used 1.5:1, which I found on website. That of course makes a huge difference...


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