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Posted: Sun Mar 06, 2011 1:30 pm
by Struts and Rudders
Get yourself a flash tach and find out how fast the driveshaft is turning.
Knowing the exact RPM of the shaft is key in landing on the right prop.
It is a simple device to use. You just put a piece of reflective tape on the shaft and aim the timing light at the tape as the shaft spins.
The company I work for has one...we got it on Amazon for less than $70.00
which is way cheaper than any trip I have ever taken to the prop shop!
I would almost bet the prop shop you are working with will have one that you could rent or borrow.
SRD
F36
Posted: Sun Mar 06, 2011 1:40 pm
by rossjo
prowlersfish wrote:rossjo wrote:Abster et al,
Always good to start with good data.
"Trans 72C Velvet Drive 1:1 ratio
At 3000 RPM running the 13/10 WOT speed is 27.6 mph"
is not possible. So you have erroneous tach, prop or GPS data to begin with.
GIGO
Hard to say WHY ??
Hard to say WHY?
WHAT?
3000RPM on 10 Pitch with 1:1 going 27.6mph is not physically/mathematically possible. That's WHY. So he has erroneous data to begin with.
Posted: Sun Mar 06, 2011 2:02 pm
by prowlersfish
Rossjo, We really do not know what he has as it is a cut and rebent prop . Thats why I say what I say . THATS WHY ! your asuming that his tach is off and or his gps is wrong , but you seen to take the prop numbers as fact , I don't not from a shop that cuts props down for a plaing hull , I still would like to se the photos of that prop
Again I agree the tach needs to be checked , but it still calls for a 12x10
BTW the data I use has nothing to do with the current prop/speed / etc
I used the boat info / engine info (hp /rpm )
Posted: Sun Mar 06, 2011 8:37 pm
by foofer b
I say he just straps a coupla 250 hp johnsons to the transom.
Posted: Sun Mar 06, 2011 10:44 pm
by captainmaniac
foofer b wrote:I say he just straps a coupla 250 hp johnsons to the transom.
A couple ? Why stop there ? Try this !!
http://graphics2.snopes.com/photos/boat ... ugboat.jpg
Posted: Tue Mar 08, 2011 6:30 am
by foofer b
Holy toledo Batman, that's a mess of motors!
Posted: Tue Mar 08, 2011 10:06 am
by rossjo
Midnight Express 1,750HP!
Posted: Tue Mar 08, 2011 12:17 pm
by jddens
For what it's worth, my 30' Sea Raider is the same length and probably has similar if not exact hull and weight is very close. I have 2 - 340hp mercs swinging 15X17 props. I'm not sure of the ratio but think it's 2:1. At 3000 rpm I'm running about 16-18 knots.....at 4400 about 28 knots and WOT 5200 around 34-35. Speeds are GSP with clean bottom and gear..........John
Posted: Tue Mar 08, 2011 1:03 pm
by prowlersfish
jddens
His boat is a single screw and a 1 to 1 gear . would be great is he had a 2 to 1
Posted: Tue Mar 08, 2011 5:22 pm
by rossjo
d claims to be going a mathematically impossible 27.6mph ... speedo is way off.
Posted: Wed Mar 09, 2011 2:52 pm
by AbsterMcgee
OK All, seems like everyone's right! Mystery Solved
Rossjo you got it, put a digital timing lite to it, the Tach is 1600 RPM off @ WOT, Imagine that! Brand New Ferili Tach & Dash, Ferili sounds Italian but I bet it's made in CHINA! Thanks for the Start with good Data prompted me to check it!
Prowler fish, when you ran the numbers for the 13" dia prop it was real close to what I am running now you suggested a 13/8 I'm running a 13/10
My WOT is 5160 ( No Load ) WOT under load 4600 RPM w/ 13/10 right where I should be, speed is 27.8 MPH minus a -2mph out going tide puts me @ 25-26 mph also right!
You guys are a Great help, Great People
Thanks Again
Abster / Gone Fish'in !!!!!!!
