newbie Prop help..

[dedge522]

I have a 1992 Arriva 2050 with a 5.7 merc and alpha one outdrive. The boat came with a SS 3 blade 21" prop that took forever to get on plane. I swapped it out for a 4 blade 14.5X18 prop that helped the hole shot tremendously however now the engine is at 5K RPM and I haven't even trimmed it out yet going about 45mph. MAX WOT on this engine is 4200-4600. I also just changed the Rochester carb to an edelbrock marine carb 1409 which really woke up the engine and added amazing responsiveness. Looking for advice on whether to go with a 14.5 or 14 diameter prop with 20" pitch or whatever else someone might recommend. I don't want to lose too much hole shot since I do a lot of wakeboarding but would like to even out the rpm in the correct range for WOT. I am worried I'm going to go to a 20-21" 4 blade prop and lose my hole shot. Any advice would be greatly appreciated. I am also thinking of adding smart tabs as I heard they help with holeshot and add some drag which will reduce rpms.

 

[dedge522]

Forgot to add I have through hull exhaust..

 

[MikDee]

I would take the 4 bld prop to a prop shop, and have them put a bit more pitch in it, if possible?

 

[MikDee]

Forgot to add I have through hull exhaust..

I'll bring my ear plugs,,,:lol:

 

[dedge522]

Lol..it can be a little loud! Dont have a decent prop shop to take it to in the area. Looking to buy another prop. That way I have a spare as well.

 

[Chris1956]

Gee, from your description, the original 21" P prop sounds like it was correct. The rule of thumb is that every 2" of pitch changes the RPM by about 400. If you can get 5K+ RPM with the 18P prop, the 21"P prop would normally drop your RPM by 500, which would be 4500+ RPM. Can you retest with it?

 

[dedge522]

Yes, I can throw it back on and retest. The only problem was with the 3 blade 21" it took forever to get on plane and you had no chance of pulling a skiier out of the water. However once it got on plane it took off and definitely had a high top end speed. Thinking for the wakeboarding I will still need a 4 blade to get the hole shot. I will try to give the 3 blade a test this weekend and report back on that. May just bite the bullet and order a 4 blade 21" to test as well.

 

[Starcraft5834]

some things you cant get around.. bite the bullet and get the 4 blade with more pitch...

 

[dedge522]

Thanks.. probably will do that. Any recommendations on 14.5 or 14 diameter?

 

[Starcraft5834]

.5. Not gonna matter a whole lot...I'd go smaller..better to be a speck high on rpms snd throttle down....

 

[MikDee]

If your tach is reading right, then I would suggest a 4blade 21" pitch prop myself.

 

[Chris1956]

I do not know much about 4 blade props, but Mike begs the question: "Why would a 4 blade 21P Prop plane the boat better than a 3 blade 21P Prop?". If the 3 blade prop is a dog, I would guess the 4 blade of the same pitch would be a dog, no?

 

[steelespike]

I think you need to verify your tach. First we need a wot rpm and gps speed You can over rev briefly to get the numbers as you might rev a car through the gears. 45 at 5000 guessing a 1.47 ratio indicates 22% slip about double of typical. Usually high slip is the result of inaccurate speed measurement or a tach that reads high.What was the wot rpm and gps speed with the 21" prop? If your not comfortable doing a wot with the 18" put the 21" on and get us the numbers.

 

[MikDee]

I do not know much about 4 blade props, but Mike begs the question: "Why would a 4 blade 21P Prop plane the boat better than a 3 blade 21P Prop?". If the 3 blade prop is a dog, I would guess the 4 blade of the same pitch would be a dog, no?

The 3bld prop is no dog, it just has a bit more trouble getting out of the hole. The 4bld prop by design, frees up flow between the blades giving you a quicker windup to top rpm, while losing a bit of top end. Apparently a Mercury Spitfire 4 bld is so efficient from what I read here, that you could use the same exact pitch as your 3 bld! I have no idea of the price of these though?