Another Prop question (20 sf)

[fly4navy]

I have taken my new 74 20 sf out for three sea trials in the last ten days. I am running my new 115 hp E-TEC, full fuel (45 gals), two people,T-top. Prop is a new SS Viper 17 provided by the dealer.
Best WOT I can achieve is 5000 rpm (even when trimmed up) and it seems a little slow out of the hole. Top end is about 28kts. Dealer is replacing the prop tomorrow with a 15.
Do those in the community think this should help or are there other options I should be thinking about?

Thanks for your input!

Go Navy!!

[slowJEEP]

Sounds over-propped. Know what the gearcase ratio is?

[Mikem8560]

just put a 200 hp on my 20 a prop specialist shop recommended a 17p prop for me t top 2 big guys I saw 5600o rpms and 33 knots so id think a 17p for a 115 hp big my old evinrude 150 hp was running a 15p

[FishStretcher]

17/15*5000= 5600-5700 RPM. A change to a 15 P prop is probably a good starting point, plus you will probably make a tiny bit more power at that RPM, so you might pull a bit more than 5700 at WOT, if everything is just right. Holeshot should be lots better, I would think. I did this same think on my F100 on a 20 and it is much better. Only about 1/2 to 1 knot faster, but much better acceleration with the 15P. I pull about 28 knots that way with a slightly lighter boat and less power (20 gallon tank, no frills).

[fly4navy]

Sorry guys. Turns out the dealer supplied prop is 12 7/8 X 15.
Don't think I have many choices from here. The hull is moving plenty fast for me at about 29 kts (wouldn't feel good going any faster)..just want to make sure I am doing justice to my new power plant running it at about 4500 rpm (on average)

[McGillicuddy]

Just one suckers opinion but, if you can't get max target RPM, at WOT with a load -swap the prop. If you're slow out of the hole, swap the prop. Wrong prop for this hull. Viper is a speed prop. Too much long sweeping blade and probably too much rake. Get a stern-lifting design, 14 x 15 or so. There are many options - but what you have sounds suspect.

You have a top notch motor, don't cheat yourself by settling for the wrong prop. I'd take it back to the dealer and let him know you'd like to try some other options. Like the old SST 13-3/4 x 15....I'd be willing to bet you get better performance from a $100 Merc Aluminum Black Max than the Viper. I'd move away from the BRP props altogether. You can do much better. PowerTechs OST3R15 is like the old SST and a respected stern-lifter. New SST has double cup but it might be too much. You should be able to get nearly 40 mph with a fresh etec 115 on a 20 with the right prop and set-up.

Check your motor height, too...it will perform best with the anti-cav plate at least an inch above the keel.

[fly4navy]

Thanks,


That is great information for someone like me that doesn't have the background on these things.
I really appreciate it!

[slowJEEP]

Let us know how the 15 goes.

[Bushwacker]

Gillie's right, the optimum rpm on your motor is 5500-5750, so you'll get much better performance and durability if you prop it to rev to that level with the maximum load you intend to carry. http://s157.photobucket.com/user/LIQ...tml?sort=6&o=5

First make sure motor is mounted high enough that the AV plate is clearly visible when you're up on plane. A 13 3/4 x 15p SST prop on the 1975 vintage 115 I originally had on my boat was a perfect match. However your E-Tec is at least 20 hp stronger at the prop than that motor and your CC is a couple hundred pounds lighter than the Seafari, so you might be able to turn one with a little more pitch or diameter than that.

[fly4navy]

Thanks Mcgillacuddy and Bushwacker,

I took your comments up to my E-TEC dealer and he gave me a new 13 7/8 X 15 SST to try. I am hopeful and fully expect this to help. That VIPER looked nice but I am convinced it was just mashing up water instead of delivering thrust.
Weather isn't looking all that great until Monday so will not get to test it till then AND will report back to you.

Really want to thank you for your inoput and help.

Geoff