UPDATE!!! Is there an accurate formula for determining the right prop? UPDATE!!!

[180Fisherman]

OK I found it. The prop says 14x17. It's aluminium and while old and pretty beat up at the edges there are no significant chunks taken out of it. Although judging by one of the reply posts I should already be over propped. Do props lose pitch over time from the forces of the water? Should I go up more? Pitch or diameter?

I have this boat (data taken from NADA site):

Boats and Personal Watercraft
2000 WELLCRAFT MARINE CORP
A GENMAR COMPANY
FISHERMAN 180(***)
Outboard Boats

Length: 18'
Model Name/Description: FISHERMAN 180(***)
Boat Type: Outboard Boats
Hull Material: Fiberglass
Beam: 7' 11"
Engine: N/A
Net Weight: 2780

(***)INCLUDES THE VALUE OF THE TRAILER. -- APPLY NOTE WITH * ONLY WHEN THERE IS AN ASTERISK(S) FOLLOWING THE MODEL NAME.

The boat's manual cites the weight the same but a 2800 net weight seem way high for an 18' center console. Might that include the trailer or engine?

Anyway it's powered by a 2000 Johnson 115 V4 looper that runs great and the performance is adequate but wot I'm getting 6K rpms which is higher than what I have found to be the quoted max rpm range. Speed I'd guess is about 40mph. I'd like to bring max rpm down closer to 5500 rpms and in doin so I'd hope for a little more cruising speed as say 4500 rpms as with a larger or higher pitch prop I should be pushing a bit more water. Unfortunately I have no idea what prop I have now.

Is this enough information to determine a good starting point for determining what size and pitch prop I need to achieve my goal?

 

[DaNinja]

Re: Is there an accurate formula for determining the right prop?

Re: Is there an accurate formula for determining the right prop?

I'll take a look. There was a long thread here awhile back that was like a running physics class. Great info though. Try a search of "Prop selection formula". That's where I'm starting.

This is not the thread I was recalling, but it's close.
http://forums.iboats.com/showthread.php?t=346693&highlight=prop+formula

 

[jelli]

Re: Is there an accurate formula for determining the right prop?

Re: Is there an accurate formula for determining the right prop?

google prop calculator...prop size should be on the prop by the castle nut

 

[180shabah]

Re: Is there an accurate formula for determining the right prop?

Re: Is there an accurate formula for determining the right prop?

Without info on the prop you have now, we have no baseline to start from.

 

[DaNinja]

Re: Is there an accurate formula for determining the right prop?

Re: Is there an accurate formula for determining the right prop?

I found the thread that I was thinking of.
http://forums.iboats.com/showthread.php?t=368380

All the formulas, physics, and math that you can stand.

 

[Dhadley]

Re: Is there an accurate formula for determining the right prop?

Re: Is there an accurate formula for determining the right prop?

Formulas rely on comparing things that are equal. Not all props of the same pitch are the same. They are built to do different things. Bow lift, stern lift or overall lift to mention a few. Props with progressive pitch, as most are today, aren't measured the same from manufacturer to manufacturer. Everybody throw the term "cup" around but there are many variations of cup - degree and placement to start with.

Not all hulls are created equal, even those that look the same. That affects the way a prop works on a given hull design even if the boats weigh the same.

Applications affect things too. There are many more options with outboards than stern drives.

One case comes to mind. We're working on a Talon tunnel boat now with a 300 Pro XS. Our baseline prop is a 26 Turbo TXP. Mercury Racing suggested a 27 Bravo I XS. The 27 Bravo turned 300 more rpm but was a bit over 1 mph slower. Merc Racing now suggests a Bravo I XS 29. Basically they're hoping a (another) $700+ prop in 29 pitch will be faster than a 26" $450 Turbo.

My 19' 6" Hewes is a pad bottom flats boat with a stepped transom. Our tournament boat is a 20' Lake & Bay, again a pad bottom boat with a stepped transom. The fastest and best all around prop we've found so far is a Turbo Ultima 4 on my Hewes. The L&B hates Ultima 4 props and, frankly, gets a bit weird handling with one. Both boats have hydraulic jackplates. Both boat bottoms look alike and there is only 6" difference in length.

Formulas are great but cannot replace testing.

 

[ExxWhy]

Re: Is there an accurate formula for determining the right prop?

Re: Is there an accurate formula for determining the right prop?

http://www.wellcraft.com/owners/infoguides/2000/WC_PIG_180_Fisherman_00.jpg

There is the product info guide from the Wellcraft site. 2780 is with the 115 OB.

Also says the original prop was a 13 3/4 X 15 OMC aluminum which is supposed to be 39.6 MPH top speed @ 5400 RPM.

I'm quite far from being any sort of expert on props, but I have read a lot of threads about them on this forum. It all adds up to what Dhadley (who is expert on props) says, "it depends" and if you want to squeeze every bit out, you'd have to try several. That said, pretty close is good enough for most people in most applications which is why most people buy some sort of generic prop which is engineered for most applications. I'd guess with that boat, you aren't all that worried about a MPH here or there, but are more wondering about why the extra RPM's and if they can be put to better use.

If you have the original 15p prop on there and are getting 40 MPH @ 6000 RPM, I'd wonder if the tach is right. However, I have no idea how reliable any manufacturers general claims are regarding speeds and RPM's when new. One would hope they would be pretty close. The standard rule of thumb seems to be 200 rpm per inch of pitch. Maybe there is a 13P prop on there now which was put on for heavy loads? Maybe the tach is off? Maybe the prop is worn out? Maybe you have a hot running 115 that can push a 17P to 5400 and higher speeds?

Surely there are some numbers on that prop someplace that one of the experts can decipher. Once you have that, then you can go from there.

This caught my eye since I have a 1989 180 Fisherman. They got faster in 11 years. But I only have a merc 100 on mine. (probably getting tired at that)

 

[180Fisherman]

Re: Is there an accurate formula for determining the right prop? UPDATE!!!

Re: Is there an accurate formula for determining the right prop? UPDATE!!!

OK I found it. The prop says 14x17. Although judging by the last post I should already be over propped. Should I go up more? Pitch or diameter? Or does the 14x17 prop I have now suggest to you I'm over revving for some other reason?

 

[180Fisherman]

Re: UPDATE!!! Is there an accurate formula for determining the right prop? UPDATE!!!

Anyone?

 

[ExxWhy]

Re: UPDATE!!! Is there an accurate formula for determining the right prop? UPDATE!!!

I guess I would still be suspicious of the tach. In theory, putting a 17 on there should have lowered the RPM from the factory spec, but it's higher than original. I know some have had surprising results with some props, but usually it's some high performance hull and/or prop.

May want to think about getting a tiny tach to cross check your tach. I'm assuming the prop is all there and looks OK. Also assuming it feels OK when running it?

The more I think I know about this, it becomes apparent the less I know! Having issues of my own after putting a 15p 4 blade on my boat yesterday, but that's for another thread. Maybe someone else can chime in with an opinion that would be more helpful in your case.