New prop selection

[riverkeg]

Okay, I've read through the sticky and I think I have what's needed to help me with a new prop for this year.

Performance objectives: to run within mfg WOT spec (4600-4800 rpm), lower planning speed (currently low 20s), decent holeshot and handling for tubing and skiing. Not too concerned with top speed but hitting 50 would be nice.

Current prop: Mercury Black Max 14-1/2" X 19 pitch 3 blade aluminum
Current WOT: 5000+ (I get scared and back off after that)
Current top speed: 44 - 48 mph (best guess by speedo)
Engine: 350 Magnum (270 hp), alpha 1, 1.5 gear ratio
Boat: 1991 Rinker Captiva 206 (20' 6"), 2700 dry weight (i think)

I honestly believe I'm a little under propped and a 3 blade 21 pitch would work but I'm intrigued by a 4 blade. The boat seems stern heavy and I'd like to flatten the wake for tubing and skiing. I'm also not sure I want to shell out the dough for stainless because I'm also considering getting Smart Tabs but I would if a stainless would make enough difference without adding tabs.

Thanks in advance for your help.

 

[steelespike]

Re: New prop selection

Generally a 4 blade close to the same diameter will improve hole shot,stay on plane at reduced throttle,handle better at speed and at the dock.Usually in the same pitch will reduce rpm and top speed slightly.might put you at 4,800.

 

[Tail_Gunner]

Re: New prop selection

Lets try a few things...Is the motor carbed or is it fuel injected....then there is tach accuracy. If it is mpi that will clear a few things up. Do not be afraid of top rpm its just a guide line...Actually 4800 or 4900 is no big deal for a 350, but thats where the cam and valve train quit running correctly so thats where one quits measuring performance and keeps yopu in mfg warranty...

As to a 4 blade it has slighty diffferent performance character than a 3 blade...better midrange lower planning speeds and it should be smoother..The cost of that is usually 2-5 mph loss of top end speed.

No prop will make a launch as effective as trim tabs...there hydraulic jacks sort of. Not all boats need them not all boaters like them on smaller boats that is. Larger boats well you cant go out without them.

 

[riverkeg]

Re: New prop selection

Tail_Gunner - The motor is carbed and I think the tach is fairly accurate. Early last year, the tach was bouncy but after cycling the cylinder switch a few times, it smoothed out nicely.

Any other suggestions/votes out there? In my mind, the choices are narrowed down to 3 blade, 21 pitch or a 4 blade, 20 pitch.

 

[Tail_Gunner]

Re: New prop selection

If you had mpi the engine would be rev limited to 4900 or 4800 but you dont, So lets look at your numbers...19p @ 5000 with a 1.5 nets 60 with 0% slip. Your actual numbers are 19 @ 5000 with 1.5 nets 48...Thats a 20% slip. Either your speedo is off or your tach...12% is bad with that much hp on a 20' boat.. I suspect both in this case, dont let that get you down it is simple enough to take the time to get the tach verified and do a gps run. Little frustrating at first but when done properly i have never seen a post that stated damm that was stupid.

You can order the tabs a get on with your journery but id do the above..a 350 mag in that boat you have should be a terror and setup correctly should put a big grin on your mug...A good 4 blade should make that combo pull like a tractor and dance @ Wot about 55+

Solas makes a great 4 blade alum
Stiletto Baypro 11 in a ss....it does not get any better.

 

[riverkeg]

Re: New prop selection

I really appreciate the help Tail_Gunner. There is no doubt the speedo is off being it's a pickup style and 20 years old. Do I just pull the tach and take it in to my local shop? I've never had a tach verified. I don't have a portable gps but I think I could borrow one.

I agree with your assessment about that combo and it's one of the reasons I bought it. The 350 mag was supposed to be a great power plant and I knew how well it was maintained. It really does run nice.

And I will check out the Solas props after I confirm my data (in about 2 months ). Thanks again!

 

[Tail_Gunner]

Re: New prop selection

Well as to the tach id leave it in the boat. Go to napa or your favorite auto store and find one you can hook up to your current engine...and check for variations...ANy suggestions out there..i get a bit carried away sometimes...