yet another 3 vs 4 blade question

[ddennis]

2002 merc 75 2 stroke classsic, 17.5 foot nitro bass boat. Current prop is a 23 pitch (unsure of diameter, think 12.5) I have an opportunity to get a 4 blade 21p stainless that is 13.75 Other than gaining a few RPM, what kind of performance will I see? I currently run out at about 40 on the gps at full fishing load.

 

[Silvertip]

Re: yet another 3 vs 4 blade question

Should gain hole shot. Other than that, it's hard to tell what performance change you will see since we don't know what type of prop (brand, style, and material) you have now and what your current wide open throttle RPM is as well as what brand, style and material the 21P is. But -- running some numbers in the prop calculator, your current prop shows somewhere between 18 and 22% slip depending on WOT rpm (5250 and 5500 respectively). Running the 21P shows a theoretical boat speed of 40.8 MPH at 5500 RPM with 13% slip. Again, since we don't know what your WOT rpm is with an average load thats about all I can do for your. Walleyehed will be along I'm sure and can shed more light on this for you.

 

[ddennis]

Re: yet another 3 vs 4 blade question

The 3 blade is merc stock aluminum. 4 blade is stainless trophy plus. Rpm on 3 blade is 5200

 

[Silvertip]

Re: yet another 3 vs 4 blade question

OK - that helps. But what are you expecting to achieve by going to a four blade? So many people see a boat with a four blade prop (or even a five blade) and feel they need one too, but have no idea why other than it looks cool and more blades must be faster. Your engine is currently running where it should with the current prop. If you are looking for more performance you might play with engine height to optimize the setup. That alone can pick up some RPM. The four blade will likely give you much better hole shot since you are dropping pitch and also going four blade. Your existing prop shows 19% slip using your numbers. The 21P will have less slip so using a 13% slip figure, 5300 rpm will net about 39.8 MPH. If you pick up some rpm that speed will come up a little but remember, you are already 50 RPM over the max recommended WOT rpm for that motor. Picking up even 200 rpm will put you at 5500. Will it damage the motor -- probably not since the 90HP version of that block is rated at 5500. The calculator says at 5500 RPM you will still be at 40.8 MPH so you see, adding blades does not make it a race boat.

 

[ddennis]

Re: yet another 3 vs 4 blade question

Thanks for the info, my main goal is a better holeshot, it is rather sluggish getting out of the hole and onto the pad. I was hoping that this would help, and from what you are saying it should do what I am looking for. I do realize that I will need to watch the rpm's a bit closer than I do now.

Hmm just a thought..is there anyway to limit the throw of the throttle lever? poor mans way of limiting rpms.....

 

[walleyehed]

Re: yet another 3 vs 4 blade question

23 alum 3-bld to a 21 SS 4-bld....I say your rpm will remain the same if not drop a tad.
What engine is the Trophy off of?
Sounds like a V6, or large gearcase prop...Do you have a 13 or 15 spline prop-shaft???

 

[ddennis]

Re: yet another 3 vs 4 blade question

I have the merc torq flow II setup on mine, the trophy came off a 2004 merc 125. I have a 2002 merc 75.. I just played with the merc prop selector, they reccomended a 24 pitch, 13inch 4 blade.?? NOW I am really confused....ARRUUUGGHHH

 

[Campylobacter]

Re: yet another 3 vs 4 blade question

You will definitely have a much better hole shot with 21P SS 4 blade. The question is how much (if any) you will lose at the top. The lower pitch and extra blade are working against you, but the stiffer SS should help. If you are already bumping up against your redline you may not be happy.

 

[Silvertip]

Re: yet another 3 vs 4 blade question

Limiting the throw of the throttle lever is not a wise idea either and why we preach pick a prop that puts the engine at or very near the max receommended rpm. . If you are complaining about hole shot now, limiting the movement of the throttle at WOT will limit hole shot as well because you obviously are limiting power.

 

[ddennis]

Re: yet another 3 vs 4 blade question

arruggh... I am still confused.. Just talked to an offshore racer who has a prop business,, He reccomends 13.75 21pitch stainless 3 blade Or a 4 blade 19 or 20 pitch 13 inch, not the 4 blade 21 pitch stainless..

thoughts?

 

[MikDee]

Re: yet another 3 vs 4 blade question

IMO, going to a 4 blade prop, you would normally need to drop the pitch some, then going to a stainless from an aluminum prop, also means drop the pitch again. Hence going from a 3-blade 23" Pitch, 2 changes down in pitch, I'd say a 4-blade 19" Pitch would be pretty close.

 

[walleyehed]

Re: yet another 3 vs 4 blade question

23 alum 3-bld to a 21 SS 4-bld....I say your rpm will remain the same if not drop a tad.
What engine is the Trophy off of?
Sounds like a V6, or large gearcase prop...Do you have a 13 or 15 spline prop-shaft???

I'm staying with the first part of my above post.
You are not going to gain any rpm with the 21P Trophy.
The Race dude is making an assumption based on the numbers you give him on the 3-bld alum. I'd say he's real close in saying to use a 21P in a good quality 3-bld SS...it'll do the same if not more than your 23 alum., and the rpm will be about the same, speed may be slightly higher.

 

[ddennis]

Re: yet another 3 vs 4 blade question

Ok.. I did it. I bought it and its on the way. Hopefully will be here friday so I can test out on saturday and let you fine gentlemen know what happens. I will run my original prop and note all performance info that I can, and then do the same thing for the new prop. As some say in Lousyanna and Arkansas...HOLD MY BEER WATCH THIS! LOL

 

[ddennis]

Re: yet another 3 vs 4 blade question

OK, got the prop in yesterday and tried it out today.

Took about 15 - 20 seconds to get on plane. RPM was only about 5200 (redline is 5250) MPH was only 39 with just me in the boat. 38 with me, my son and fishing buddy. Had 2 PVS plugs out, 1 meduim and 1 small. Going to get some more PVS plugs and try it again next weekend... Did not get a change to run the aluminum prop to compare against.... I am kinda thinking a stainless, 3 blade 21 pitch may be the one I need...That should get me the lost rpm's back.. Thoughts?

 

[lrdchaos]

Re: yet another 3 vs 4 blade question

I would say your doing pretty well...if your boat planes that quick and it's running 50 rpms shy of redline. What more do you want, topend?

 

[ddennis]

Re: yet another 3 vs 4 blade question

was looking for better holeshot..Think I may have it though.. Did more reading on the pvs plugs. 2 are out, 1 meduim and one small in. I am going to get several packs of plugs this week and go play agian next weekend.
I am suspecting that I can get a bit better holeshot... by working on the plugs a bit.

 

[walleyehed]

Re: yet another 3 vs 4 blade question

Which prop did you buy and run?
Was the 5200 with just you or with your son and your buddy on board?
Keep in mind the OAT....when it gets warmer with more humidity, performance will drop even more.

 

[ddennis]

Re: yet another 3 vs 4 blade question

RPM was with me and son in boat.. I was using the 21p 4blade trophy plus. I did think about that..it was rather cool out when we ran it. Looking to trade it out on another website for a 21p 3 blade stainless...