prop question...

[serious3]

i'm running a 15p, 10.25" dia stainless ballistic prop on my zapcat with a 40hp mariner, all up the whole package weighs about 170kg, its running at 46mph flat out. i ca get the prop tweaked/worked as in upping the pitch to 16/16.5 what effect will this have on my speed? i'm trying to crack 50mph with it! i'm ordering some reeds from chris carson and ii've lightened the flywheel by about 2lbs. is it worth working my prop or just look for another?

[rekcuftar]

All things being equal you will gain speed with increased pitched. But this assumes:

RPM's don't drop off.
Slip does not increase
You can still get on plane

The extent of changes to the other factors will modify your gains. In a perfect world you could borrow props from friends and see what changes benefit your setup the most. Then apply that to your rig.

Classic weasel response promise nothing.

[zul8tr]

provide rpm and gear ratio and some assesment can be made.

was the 46 mph by gps or?

[serious3]

46mph was with the gps, ratio not sure either 2.0:1 or 1.85:1(depends if its a merc or yami mariner!)revs i'll have to assume that its WOT 5600rpm, no tach but she's absolutely singing!!

[zul8tr]

46mph was with the gps, ratio not sure either 2.0:1 or 1.85:1(depends if its a merc or yami mariner!)revs i'll have to assume that its WOT 5600rpm, no tach but she's absolutely singing!!

Well going with the 46 mph and 1.85:1 and 5600 and using this calculator

http://continuouswave.com/cgi-bin/propcalc.pl

at 5600 rpm slip = - 7% not possible, so your rpms are higher

for zero slip then rpm = 5990

assume 10% slip then rpm = 6657

to go 50 with 10% slip at 6000 rpm pitch = 18"

I think you do not have the hp to turn this pitch.

You can play with this when you get the actual rpm and calculate the slip then do what ifs

Note that the calculation says nothing about diameter as this is strictly helix theory like a screw turning into a solid medium. Slip is what makes it a fluid medium. All props need slip or they cannot provide the lift to operate. The zero slip calc is just to provide a number for a theoretical maximum for the parameters provided. Note that pitch is the bugger to get right because it is a function of prop style and measured different ways by different gages and people. The pitch number provided by the manufacturer is usually at one location on the blade at a specific radius.

So the only way to get at optimum is to test and take good notes.

[JohnsonM50]

[QUOTE=zul8tr;118828]Well going with the 46 mph and 1.85:1 and 5600 and using this calculator

http://continuouswave.com/cgi-bin/propcalc.pl

at 5600 rpm slip = - 7% not possible, so your rpms are higher

for zero slip then rpm = 5990

assume 10% slip then rpm = 6657

to go 50 with 10% slip at 6000 rpm pitch = 18"

I think you do not have the hp to turn this pitch.

You can play with this when you get the actual rpm and calculate the slip then do what ifs

Note that the calculation says nothing about diameter as this is strictly helix theory like a screw turning into a solid medium. Slip is what makes it a fluid medium. All props need slip or they cannot provide the lift to operate. The zero slip calc is just to provide a number for a theoretical maximum for the parameters provided. Note that pitch is the bugger to get right because it is a function of prop style and measured different ways by different gages and people. The pitch number provided by the manufacturer is usually at one location on the blade at a specific radius.

So the only way to get at optimum is to test and take good notes.[
/QUOTE] I'll add that the notes work. You need speed & rpm's as accurate as possible, if possible. Speed is the most important, not just for brag rights but since varied rpm's per change don't automatically mean your going faster or not -could be the slip that's changing. When you make changes you should go 1 at a time so you know & note for sure what results. When trying a new prop try it at different heights & angles to be sure you get the best it has to offer. Good Luck

[fs5]

what model mariner is it mate?one up with a decent prop you should be able to hit 50mph easy enough.
ballistic props are pretty average / crap on these hulls.i wouldn't bother getting it repitched ,sell it or use it for a general purpose fun prop.
to run 50mph you'l need atleast an 18p prop,one of rons little 18p choppers would be a good start.
don't about forget engine height ,thats where you'l gain heaps of speed with the right prop.a good staring point is to have the centre of the propshaft atleast 70mm above the bottom of the highjackers and just keep raising it bit by bit until it stops sucking water ,then lower it 10mm .
if you can show us a pic of it mounted on your transom

[serious3]

its a 6E9 mariner twin, i'd love one of rons choppers! i know it would be fast in a straight line but what about turning?, i reckon i'll have to see if anyone has a 17p trad prop that will fit and see what happens with that one first, i also need to know what revs i'm doing so ill have to get a tiny tach as well. thanks for the replies lads! oh and what would be considered a "good" prop for these insane bits of kit?

[FrenchPhil]

Most of the speed records in your category were done with semi-cleavers, so I'd try a 10ish x16 & take it from there.

[FOGO]

Hi Mate,
I have a Thundercat with a Mercury 40HP (3 cyl, 698 cc. Tohatsu made it in Japan for Mercury)
I use a Yamaha SS 10.25" x 15" propeller standard and hit 80 Km/h (48.45 miles) at 6600 RPM.

I like to BUY a 10" x 18" "Chopper" style prop, but our rules says Exhaust Through 3 blade prop.
Any one Knows if sombady builds Choppers with Exhaust Through propellers???

Regards,

FOGO