Prop pitch for 3-blade vs. 4-blade for the same boat

[mjradk]

I want to replace the 3-blade prop on my OMC 4.3L I/O 1996 Sunbird with a 4-blade prop. The present 3-blade prop has a 21" pitch. When I go to different prop websites and use their "Prop selectors" they usually recommend a lower pitch.

I can understand recommending a lower pitch where there will be 4 blades instead of 3 since an additional blade would induce more of a load. However, I don't want to buy too low a pitch and have the rpms be too high,

Since there are 4-blade 21" pitch props available, should I continue with a 4-blade 21" pitch prop or go with the lower pitch recommendations from the "Prop selectors"? Thanks!

 

[jestor68]

If you use the same pitch 4 blade, expect the rpm to drop about 200 rpm.

It is recommended to drop one inch of pitch in order to maintain the same rpm when switching to a 4 blade.

So, whether you choose the same pitch or not depends on where your WOT rpm is/was with the 3 blade.

 

[KYsean]

If you go with a mercury Spitfire 4 blade. You would stay with the same pitch as long as your RPM's are in the correct range at WOT with the 3 blade.

 

[Starcraft5834]

MJ what do you want to switch for? if I may ask..

 

[mjradk]

Thanks for your responses. The reason I want to change to a 4-blade prop is because of planing problems. The boat is a v-bottom and I need to get up more speed and rpms than are reasonable to get the boat to plane. I put Doel-Fin hydrofoil planing stabilizers on a couple of years ago, and these helped a lot. Still, if I have 5 or 6 adults on board, the boat struggles to plane. I understand that 4-blade props help with planing problems as well. I've been thinking about trying one and when my wife asked me what I wanted for father's day, I saw an opportunity to spend a few dollars without getting into trouble.

 

[jestor68]

With the load you mention, a 19 pitch 3 blade would be recommended for a light load and a 17 pitch 4 blade to haul the 5-6 adults in a decent fashion.

You have to understand that it's rare for one prop size do it all.

Just putting two or three more adults in there can require a prop change in order to maintain WOT rpm.

 

[Mi duckdown]

and Your Present Rpm and Gps speeds,ARE?

 

[UncleWillie]

Before we start the 3 vs. 4 blade debate....

What size is the boat?
Are you saying you have a 4.3L engine that will not even get on plane with 5 people on board?
This sounds like an engine problem instead of a prop issue.

You should be on plane before 20mph.
When you are on plane what is the top speed? 35+? And the RPM? It should be in the upper 4000's!

If you are lugging along at under 20 mph, a 4 blade prop is not going to magically boost you near 40 mph!

 

[jestor68]

No Title

This is his boat?

Oops; picture didn't load.

 

[Starcraft5834]

Thanks for your responses. The reason I want to change to a 4-blade prop is because of planing problems. The boat is a v-bottom and I need to get up more speed and rpms than are reasonable to get the boat to plane. I put Doel-Fin hydrofoil planing stabilizers on a couple of years ago, and these helped a lot. Still, if I have 5 or 6 adults on board, the boat struggles to plane. I understand that 4-blade props help with planing problems as well. I've been thinking about trying one and when my wife asked me what I wanted for father's day, I saw an opportunity to spend a few dollars without getting into trouble.

The 4 blade will help for sure.. dont expect to increase your top end speed with it, you wont.. moves from 3 blade to 4 blade will costs you and everyone else anywhere from 1-3 mph. However, the benefits exceed that loss of a little top speed. 4.3 Sunbird? what's that 17-18 ft? assuming your present prop is giving you WOT #'s consistent with the recommended range. I'd suggest a 4 blade and go down, perhaps 18p. That's in the ballpark.. you should see a hole shot performance improvement with it

 

[Slip Away]

You need the following....

Michigan Wheel Vortex 4bld. Aluminum, part # 992204 14 x 20p

Hardly lost any RPM from my stock 14-1/4 x 21p 3-blade, and barely 2 mph at WOT, but jumps up on plane much quicker.

 

[mjradk]

Thanks for all your comments, and the prop suggestions. To answer some questions:

1. Present top end is 4500 rpm and 42 mph
2. The boat is a 1996 Sunfish Corsair 18.5 ft. in length. It's a bowrider. The boat is in really good shape which is why I continue to fool with it.
3. Right now, the boat planes at 15-17 mph with 2 people in it (my wife in the bow)

Before I put the Doel-fins on, the boat wouldn't plane with 5-6 adults in it unless I asked them all to move up toward the front of the boat as far as they could. Otherwise, the boat plowed thru the water with the nose in the air without coming close to planing .... even at full power. The Doel-fins fixed that (mostly) but it still takes a long time to plane with 5-6 adults, and a grandchild or two.

Once it has planed, it stays planed until it drops to 15 mph or so.

I'm not interested in going faster. I'm interested in getting the boat's stern out of the hole and up on plane ... the hole shot improvement mentioned above.

I make sure the outdrive is trimmed all the way down to start off. The engine has run very well for the years I've had it (except for a gas supply problem at the end of last season). At the beginning of last year, I had the local marina rebuild the carburetor, and fix or adjust anything they thought needed it to make sure I wasn't overlooking an engine problem.

This has all been very helpful.

 

[UncleWillie]

Dropping the pitch would be recommended. 4500 rpm is OK, but not at the upper limit.
I would Lose the DoelFins and would add Smart-Tabs to get the rear out of the water.
It is strange that a 4.3 will not push you right up with full down trim.
Something else is at play.

 

[mjradk]

I could be wrong but I always blamed the V-bottom. It's pretty pronounced. My brother's boat with an OMC outdrive similar to mine has a nearly flat bottom and planes almost immediately. The "V" seems to cut into the water without much lift.

When the boat is planed at a higher speed, the boat seems to lean to one side of the V. I have to shift people around to balance it. It's not so much a problem on a straight run but if I make a turn on the side the boat is leaning to, the turning lean of the boat is more pronounced.

I've thought about getting hydraulic trim tabs for it, but the kits tend to be pricey. The boating season is pretty short in the NY Adirondacks, so it influences what I spend money on.

 

[Tail_Gunner]

If you want to launch 4-5 people with a older 4.3 and a deep v think along the lines of 18/19 pitch and lose some top end.

 

[dingbat]

Tabs are not the solve all solution.If you need tabs to get out of the water, you have other issues to address first.

I have 200 HP and my boat is 4 feet longer and weighs almost # 2,000 more. No problem getting out of the hole whatsoever.

You start by selecting a prop with a lot of stern lift. A good stern lifting prop does wonders for getting out of the hole. I put a Stiletto SS prop on my last boat and it was day and night getting out of the hole. Best money I spent on that boat.

 

[MikDee]

I would go with a suggested Turning point 18" 4 blade prop, they're supposed to keep your speed up, while providing more torque, and are reasonably priced.