Another prop question- 3 to 4 blade to max accelleration/planing

[bumblebeeamigo]

I've spent too much time researching this and I'm going mad!

So if I can summarize it with some generalizations:
Every pitch decrease raises 200 RPM
Moving from Aluminum to Stainless drops 200-300 RPM
Moving from 3 to 4 blades drops 400 RPM

Suggested full throttle RPM is 4800, true redline is 5400

So that would mean if I am seeing 4800 RPM with a 3 blade Aluminum, and I wanted to plane faster and didn't care about top speed, I should:
Go to 4 blade stainless (minus 600 RPM) and drop the pitch 3" (Plus 600) to stay the same.

If I want to push the motor, I could do the same and drop the pitch 6" and I'd be pushing it.

Therefore in Summary, if my current blade is a 23" pitch Aluminum, I'd want a 19-21p 4 blade stainless.

I do understand cupping matters, blade size matters, and design matters, but I have to start somewhere. I also know this is debated in depth and the right answer is "try a million until you find the one that works for you", but I need a starting point

Thanks much!

 

[JASinIL2006]

I started with a 14"x23P three-bladed aluminum prop that kept me at about 4200-4300 RPM at WOT. I next went with a 4-bladed aluminum prop (20P) and it was about right for keeping me at WOT RPMs. Finally, I've moved to a cupped stainless steel 4-bladed prop, at 18P. It also keeps my WOT RPMs right where they should be. Hope this is useful...

 

[Sea Rider]

Have not specified which boat and HP engine is it ? What you're requesting it's not exact rocket science, lots of variables. You need to test current combo lightly loaded on flat calm waters at wot with current prop and from there start your costly prop experimentation. Will say that going one less pitch on a 3 blade alum prop will not equal + 150-200 revs, expect way moooooore if in pursuit to complete tad more rpm to be at exact max wot revs.

Happy Boating

 

[SkiDad]

Try a four blade solas aluminum prop. This is a cheap way to get a better baseline. I would just drop 2" in pitch. Maybe 3 if it's really struggling. Is this an I/O or outboard ?

 

[bumblebeeamigo]

Just a small 18' fiberglass with a I/O 4.3 l 190 hp v6 mercruiser.

Thanks for the pointers! Looks like dropping to a 18 or 19p stainless may be a better start point, especially with that extra 600 rpm to work with if need be.

 

[bumblebeeamigo]

Try a four blade solas aluminum prop. This is a cheap way to get a better baseline. I would just drop 2" in pitch. Maybe 3 if it's really struggling. Is this an I/O or outboard ?

Wouldn't it make more sense to use a stainless to get a baseline? One less variable to account for?

 

[batman99]

Try a four blade solas aluminum prop. This is a cheap way to get a better baseline. I would just drop 2" in pitch. Maybe 3 if it's really struggling. Is this an I/O or outboard ?

Agree.

My Johnson 115 V4 OB went from 17' 3-blade aluminum to 15' 4-blade aluminum - Solaris Amita 4 blade. Worked great for my 17ft Bayliner bow rider design. My boat (with its heavy weight stern) likes 4-blades better than 3-blades. Especially around the dock and for hole shots...

 

[SkiDad]

Oh you have an i/o. I would not drop much as you are already at the top of your rpm range. You do NOT want to run in the 5000's.

 

[littlerayray]

Check out my thread http://forums.iboats.com/forum/gene...p-questions-and-topics/10014302-merc-spitfire I really should look into becoming a merc rep :facepalm:

 

[littlerayray]

I must add your boat also isn't pushing enough hp to distort an aluminum prop. unless your looking for the bling factor. save yourself the cash

 

[bumblebeeamigo]

I always hear people say the stainless are more expensive, but I don't see it. I bought a new 3 blade stainless for $125 shipped in great shape. I figure if its not the right fit I can resell it for about the same price. There's a 4 blade I was looking at around 150. Some on craigslist are in the 80s/90s.

The Aluminums sell for 10-15% less than that. The aluminum ones on the used market seem to be dinged up a little more for my liking. If I buy new then I take a resell hit if its not a good fit, and pay the same as a stainless. But Maybe I will just go with a solas. Seems like a lot of people here are talking them up.

Good to know about not pushing redline on the I/Os. I race in the SCCA and I tend to look at redline as something to push, but its a different ballgame when your running at redline for extended periods. I'll try not to push it. I'm thinking 20-21" Stainless 4 blade, or if I go with the spitfire I'll go with the smaller diameter and maybe a 1" drop (22p). I'll look on the used market and if I don't have luck I'll look new from iboats. If I buy new I'll go aluminum.

Thanks for the recs

 

[bumblebeeamigo]

I've taken everyone's advice and went with a brand new 21p Aluminum 14" 4 blade prop. I'm going to give it a shot this weekend.

I also bought a stainless 3 blade 23p 14.25 before I went down this correct avenue. That should give me a good reliable "top speed" setup, although I might have to try a few options for both before I'm satisfied!

I'll report back once I can get back on the water.

 

[fishin98]

My 2000 Bayliner Capri 1802 Cuddy Cabin with a 90HP Merc came with a 3 blade 19p WOT 5800 rpm, depending on load 36-40 MPH, cruise about 3/4 throttle.
Tried a CompProp 4 blade 19p, good hole shot, problem was I was always running up against the rev limiter at WOT, did not like heavy loads (4 adults). Switched to a 17p Solas Amita 4 blade, GREAT hole shot, Back to 36-40 WOT, with 4 adults on board. Could maintain plane at just above 1/2 throttle, nice cruise just under 3/4...The best part......better fuel consumption...did not have to run the motor hard to do what I wanted to do...just my 2cts