Prop choice confusion

I've visited every possible prop calculator site I can find, I've even read a book about choosing props, and I still can't figure it out, so I'm gonna dump some info and if you folks have any recommendations, lemme know

Boat is a 19' Baja SunSport I/O
Dry weight 2160, estimated 2500-ish with a person, cooler, and gas
Engine is Vortec 350, approximately 300 hp at about 4800 rpm
Alpha ratio is 1.48:1

I bought the boat with a non-running 305 and no prop, so I couldn't baseline any speeds or RPMs, but the only prop that came with it was a 26p wheel that (in my limited experience) seems to steep.

I want to buy a few cheap Ebay props in the neighborhood of the right pitch so that when I get to the lake I can try a few and hone in on the right RPM. I'm not looking for super high performance, I just want a good prop that will very occasionally pull a skier and doesn't easily ventilate. With 300-hp to an alpha, the last thing I need is to shock it at 5200-rpm after cavitating

-what diameter?
-3 or 4 blades? How much top end does a 4-blade give up?
-is that enough information that you could give me a range, like 21-23p and I could get one of each to try?

 

Sounds like you’re on the right track. Cheap prop until the right combination is found.

I can tell you that your configuration is almost identical as mine with the exception of the gear ratio which I am looking to change. I recently learned the Alpha One upper units are somewhat weak and especially with my current gear ratio and I have integrity concerns.

My 1998 19ft. Sea Ray bow rider weight is approximately 2,750 and has 8.7 hours on 350hp 5.7 liter vortec with 1.62 gear ratio and using a 23 pitch quicksilver stainless 3 bladed prop.

Currently the vessel takes about 3 seconds to plane and due to engine break in, I have only taken it up to 4400 RPM once so far. The boat speed was around 50mph at 4400.

As you know the best way to prop is trial and error for which it appears you are prepared to do. So many variables in selection that prop calculators don't take into consideration.

Also I borrowed a friends 4 bladed 21 pitch prop and boat responded well but not enough to justify the cost.


Good luck

 

Your 300 HP motor probably provides around 270 HP at the Prop, which calculates to 57MPH using George Crouch's Formula :

MPH = C ( HP\W )^ 0.5)

57 MPH @4800rpm with 1.48 gear and 10% slip, calculates to an "effective" pitch of 21.

 

See, I get totally different numbers. With 270 to the prop, 2500 lbs, and 10% slip, all the online calculators give me 67 mph. That gives me a pitch of 24.

And then, what diameter do I get?

I'm so confused.

 

It's all in the bottom Shape.

In order to get 67 Mph w. 10% slip, 270 Hp at the Prop your Hull Factor, C, would have to equal 204 or Higher, which equates to a deadrise at the Transom of less than 5 0r 6 deg.

According to George Crouch,

Average Runabout (14 - 18 deg deadrise) C = 172

Very High Speed runabouts ( less than 4 deg ) C= 218

Flat Bottom racers C = 240

Your bottom, probably has a 16 deg.deadrise. I would try a 13 3/4 or 14 x 21 prop. A 23 p. will drop max RPM by 400 and will only increase top end if the 21 P. is allowing the motor to run faster than it's max. rated rpm.

A four blade is actually less efficient, and only should be used during high loading were slip is excessive - multible skiers etc.

Hope this Helps, John

 

just to put my two cents in:
beech2000 has the best advice so far. for its been my experiance that two identical boats can use two differant props so what's good for peter may not be good for paul
the best prop will allow the boat to get into the max rpm range when trimmed out lite load one person half fuel high end of max rpm range full fuel and boat loaded bottom end of max prm range try a couple of props with what you know and have as you decrees pitch you gain rpm in most cases 200rpm per pitch but this again veries by hull design and i've have had as much as 500 rpm change
this is a good way to find propper pitch without to many headaches
if a 26p gets 4800rpm with a lite load and your hole shot is good your in the ball park if 26p gives you 1/2 sec hole shot and 5200 rpm more pitch if you have 5 second hole shot and 3800 rpm less pitch
you don't have to make it hard just try what you have first then make adjustments from what you find out
only cost you time on the water before you buy often try to rent or beg from you buddies for a test prop of a differant pitch

 

ps. as far as diameter if your not racing forget it and three vrs. four blades, vented or not vented props get close on rpm first, fine tuning comes later

 

Crouch's formula seems to work well.

Thanks Jango.

 

Ok, thanks for all the replies. Its making more sense to me now. I guess I was putting too much thought into it.

The speed calculator over at go-fast.com lists Coefficients like this:
Heavy V bottom Low drive height, cabin, side by side engines C=225 10 - 13 % slip
Fast V bottom High Drives, boxes, notched transom, good bottom C=250 8 - 9 % slip
Catamaran Pleasure type cat C=275 6 - 8 % slip
Fast Catamaran Late Skater full race, etc C=300 + 4 - 6% slip

Deadrise is the angle of the bottom from the keel to the chines right? A flat bottom boat would be 0 deadrise, right? Anyway, its not a typical comfort cruiser boat like you might see on a SeaRay or Cobalt, its a little more performance oriented than that. Let me see if I can find some photos and maybe someone with a trained eye can guess on my deadrise...





By the way... I have a full story about my first attempt at major boat repairs over at this link: http://www.teambajamarine.com/forum/showthread.php?t=9895 I took on floor and motor stringer repair, as well as going from a 305 with a pre-83 transom assembly and an MR drive to retrofitting a Vortec 350 with a Gen1 T/A and rebuilding the MR. I'm very mechanically inclined, but the learning curve was fast to say the least. All of what you see at the link above took place in 16 days

 

Curtis, I believe the "go-fast" info uses different values for shaft HP.i.e. 70-75% of BHP, which would provide similar results as Crouch.
The Crouch formula works perfectly for my 20' runabout - 360 shaft HP, 2400LB, 3 1/2 deg deadrise, 84 mph.

Your deadrise appears to be around 16 deg. which should make the bottom factor (172) correct, although strakes and flats at the chines help. In addition shaft HP is just an educated guess.

I believe I would try the 21P. first and go from there. Keep in mind lower pitch may give up some on top end, but you'll get better hole-shot and load carrying ability.

 

Great. I'll give it a shot at 21 and I'll keep my eyes peeled for a 20 and a 22 or 23 on the cheap to have on hand.

Thanks everyone.

 

curtis, its good to see that your doing this rebuild/restore right you should be very proud of it. you have an amazing story unfolding here so keep up the good work.
sorry still think with a 19'baja a strong 350 and the gear ratio of the drive more pitch but this is were i give an opinion and you get to slap me for being wrong
dam good on the project so far almost time to get her wet

 

Forgot to mention, About 10 years ago, I owned a 20' Baja, w. 270HP mercruiser(350 chev). Motor was tweeked with better cam & carb jets. Best I could get with the S.S. 21P. 3 blade was 4400 RPM - about 56 MPH.

Also a cupped 21P will act like a 22P or higher. (for calculating purposes)

My 20 footer was probably a couple of Hundred pounds heavier than a 19

 

I have a couple props gathering dust. I know one is a mercruiser SS prop and the other might be an OMC, but I'll see what I have and go from there.

Thanks for the help.