Help Bigshrimpin! 14 3/4 x 17 viper prop

[gofastsandman]

Hey Big . I found a deal on that prop. Whatcha think for my `89 20 sf w/ a `96 ocean pro 150?

[Fr. Frank]

I dunno what he thinks, but the Viper is designed to create lots of bow lift. The SeaCraft hull needs stern lift. If you want extra control, acceleration and maneuverability, go with a 4 blade. You lose top end, though. If you strictly want top end, go with a thru-hub cleaver. If you want a good balance, with good economy, go with a PowerTech Offshore series 3 or 4 blade, or a Mercury Mirage Plus (yes they can be had for Johnnyrudes, just use the appropriate Flo-Torq hub set)
Just for the record, the PowerTech in a satin is $471, the Mirage Plus is $480.

FWIW, I run a PowerTech custom 15x16.5P on a Merc 150 on a Seafari

[gofastsandman]

Boy this is confusing. I called Seacraft and they got the best performance numbers with a viper 14 3/4 x 17. 45 mph @ 5600 w/ a 150 on a 20 footer. Why don`t they understand the dynamics of the hulls they make?

I`ve been talking w/ Powertech and they have been great. What pitch and diameter Powertech/ Mirage plus would you use w/ a 150 ocean pro on a 20 sf? I have a 40 gal livewell going under the, soon to be made by a friend , leaning post. dual bats under the console. I`m on a budget. Ha, I can`t believe I just said that.

Thank you Father Frank.

p.s. How do different props achieve bow / stern lift?

[gofastsandman]

just went to the merc site. 15.380 x18. for a 2001 ficht 150. They wont let you go back farther. I have about 1" of clearance on my vent / cav plate. Is this a concern? are the years different as far as cav plate clearance? I feel like I`m chasing windmills.

[eggsuckindog]

A 20 sf with a 150 shouldn't need any stern lift IMHO, I would think that bow lift would be nice get baby out the water and flying.

[Bigshrimpin]

I try not to listen to father frank b/c he's usually right!! I run a Mirage Plus on my boat. The stern lifting (low rake) props are nice on a 20 Seacraft . . . You'll see when/if you try a mirage plus. Anyways . . . You'll pickup 400 rpm and speed switching to the viper from the rapture (same pitch). Some folks here like the Laser II 19 pitch on their seacraft . . . which is a fast bow lifting prop. I don't think your motor will turn a 19 laser in the recommended RPM range and they don't make a 17 laser. Viper will get your RPM's up . . . but I agree with frank that the mirage plus is a better seacraft prop.

[Fr. Frank]

Two things. First some technical stuff as to why a stern lifting prop is better, then my credentials for saying so.

A technical note about the difference between stern lift and bow lift in props. A bow lifting prop produces more of it's thrust out toward the tips of the blades. A stern-lifting prop produces more toward the hub. The effect of a stern lifter is actually to lessen "P" factor, or sideways thrust (which torques the hull to the left), and to lift both bow and stern together. (That's why the best stern-lifters are cleaver-type blades.) It also affects the attitude of the hull on plane. More rake + more "P" factor= greater angle of attack of the planing surfaces.
Because the SeaCraft Hull has no "pad" upon which to ride, "P" factor is always an important consideration, except with counter-rotating twins. And because it is a deep V, the wetted area at the stern is always protruding below the surface tension of the water rather than riding upon it. The goal is to get the minimum amount of hull depth, at a minimum angle of attack, especially at the stern. To do that, you must lift the stern.

How I know what I know
(1)I have owned SeaCraft boats for 23 years.
(2)I have rigged many as I once was the Dockmaster at a SeaCraft dealer.
(3)I was active in powerboat racing from 1968 through 1986, from driving B outboard hydroplanes, to throttleman on 22' and 30' Velocity offshore powerboats.
(4)In '87-'88, I was part of the team that selected oem propeller choices for Wellcraft boats performance division: Scarab, Excaliber, Eclipse, Nova, and Nova III boats. We tested a lot of props during the development of these boats.

Finally, I own a 20' '72 Seafari, with a '92/'02 Merc 150, that I have had up to 55 mph while selecting a prop for it. That's with bottom paint, too.
After having the engine remanufactured, I tested the following props: Rapture, Mirage Plus, Stilletto Bay II 4 blade, Quicksilver 4 blade Cleaver II (over-hub), Yamaha SWS, and PowerTech. I was looking for the best top speed with easy planing capability. I also had a nose-cone on my lower.

• The Mirage Plus 17P planed the hull in about 3 seconds flat, with a top end of 46 mph at 5600 rpm
  The SWS and Stilletto Bay, both 17P, planed in about 4 seconds, but the Stilletto only gave me 42 mph, and the SWS about 44 mph, both at about 5700 rpm.
  The 19P 4 blade Cleaver II gave me a top end of 55 mph at 6100 rpms, but planed in a very long 8-10 seconds.
  The Rapture 18P planed in 4-5 seconds, with a top speed of 45 mph at 5400 rpms.
  
  Finally, I called the engineers at Powertech. I gave the all the info on the hull and weight, and after some computer modeling, they custom-designed and manufactured a 15"X16.5P 3 blade based upon the stern-lifting Offshore series.
  I planed in 4 seconds, with a top speed of 52-53 mph at 6150 rpms.

The prop cost me $700. It was worth every penny.

[gofastsandman]

In a word, WOW!

I was told I have a rev limiter at 6100. I`m not looking for all out top end , but good cruise and overall performance. Any thoughts about merc`s avenger 4 blade . Merc said 14 1/2 x 16.

For the mirage plus, Merc`s prop selector spec was 15 1/2 x 17. With the current 14 1/2 x 17 I have about 1" of clearance to the vent plate . Is that a concern? What are your thoughts. My typing skills suc#. Would you like to call me this evening? Talk through the message as we screen calls due to massive medical bills. 561-655-5080

Thanks for the help and patience w/ this newbie

[Bigshrimpin]

I'll double check the clearance on my mirage plus . . . I don't think I have much more than 1/2" . . . which is going to be about the same as what you'll have. Unmolested 17P Mirage Plus Props are less than $200 all year long on ebay. You could always go to a merc prop dealer and ask to try one out first or see if someone on the site might loan you one for the day.

[gofastsandman]

Tanks Bigshrimpin and bless you Father Frank for delivering me from evil.

Father Frank , What speed does the powertech give you @ 5600?
Thx,
GFS