4-Blade Disappointment

[FreeBeeTony]

I switched from a 3 blade 17" pitch to a 4 blade 16" pitch prop.

From what I read I thought I was going to get better performance a mid throttle and possibly loose some WOT.

I was running 30MPH at 3200 RPM with the 17" prop, yesterday was my first test with the 16" 4 blade and I lost ~2MPH at mid throttle. Now I am getting ~28MPH at 3400RPM.........was still able to reach my WOT at 4500.

The boat is a '84 24' GradyWhite Offshore with a 350 Mag (vortec) mercriuser.

Any thoughts? I am inclined to go back to the 17"..........

 

[Texasmark]

Re: 4-Blade Disappointment

Well sir, the guys I read say 200-250 rpm per inch and loss on a 4 is higher than a 3. With less pitch you were able to make up for the loss of the 4th blade at WOT. Probably would not have been the case had you stayed with the 17P in the 4 blade. That pretty well sums up your condition.

What you didn't say is what do you like about the 4 and can't you give it more mid range throttle if the 30 mph speed is important? How's your hole shot now. How does it perform right at/above planing speed? Does it hold plane without dropping back or surging ahead, better than with the 3 blade? Can you put more people or things in the boat and maintain your speed? Can you drop back in rough water and hold your speed easily without continuously playing with the throttle?

Mark

 

[FreeBeeTony]

Re: 4-Blade Disappointment

I haven't done much of an eval as of yet...

My original post were my first impresson(s)..........

I will keep it for now and see how it perfroms......

 

[Tail_Gunner]

Re: 4-Blade Disappointment

Is this boat a 5000lb boat with a 8 1/2' beam and a 20 degree v? and a I/O i take it..

 

[FreeBeeTony]

Re: 4-Blade Disappointment

its 4000lbs, 8' beam and, deadrise (?) and an I/O...........

 

[sabastianunf]

Re: 4-Blade Disappointment

by better midrange performance they mean better throttle responce - has nothing to do with how many rpm's you are running. 4 blades will also get you a better hole shot, less slippage, better handling in rough water and will plane at a lower speed. It sounds like you have a fishing boat - not sure why you would want a 4 blade unless you go offshore in rough water alot.

 

[FreeBeeTony]

Re: 4-Blade Disappointment

yes, it is basically a fishing boat...........i was hoping to go up in soeed at the same RPM's as the 3 blade............I do go offshore and the inlet can get a bit nasty at times...........will have to give it more time to evaluate...........my first impression was based on MPH vs. RPM's.

 

[Tail_Gunner]

Re: 4-Blade Disappointment

What type of prop did you put on aka a ss or alum, what brand type. And your orignal prop was it ss or alum and brand type? How' about your leg or drive, is it Omc, merc ,volvo ??

 

[FreeBeeTony]

Re: 4-Blade Disappointment

Both props are aluminum on a merc outdrive.

 

[Tail_Gunner]

Re: 4-Blade Disappointment

Tony plz post the drive type,aka bravo 1 or 2 alpa 1 gen 1 or 11

 

[FreeBeeTony]

Re: 4-Blade Disappointment

Alpha 1 gen 1

 

[Tail_Gunner]

Re: 4-Blade Disappointment

Do i have this right, 1984 350 vortec with a alpha one drive
17 pitch alum unknown speed @ wot
16 pitch alum unkown speed @ 4500
24' I/O with a weight of 4000lbs (dry?)

When switching to a 16 pitch four blade alum the boat turned 400 higher rpm @ lower speed's in the 3200 rpm range. If so the prop you have does not have much cupping or a bad design or not setup for that type of rig. Is it possible for you to post your speed's at wot and the name brand of the props' you are using, and are they new prop's?

 

[FreeBeeTony]

Re: 4-Blade Disappointment

Went out tonight for more of an eval..........

With the 3 blade I was going 30MPH at 3200RPM's...........now in order to get to 30MPH I had to increase the RPM's to 3800.

Sounds to me like this is not the right prop for me..........

I am going to switch back to the 17" 3 blade and try again.

NOTE: the 3 blade was a 14 1/2 X 17 and the 4 blade is 15 X 16.........any signifigance?

 

[Tredeb]

Re: 4-Blade Disappointment

You went down in pitch so your RPM's should be up from before. As mentioned earlier, a 4 blade is generally for better grip in the water not for speed. Most people change to a 4 or 5 blade for watersports to get more holeshot through less prop slippage. They usually forego the 2 or 3 mph top end for the better bottom end

 

[Tail_Gunner]

Re: 4-Blade Disappointment

Alpha One Models 225S, 233, 255TRS
All Years
200-320 HP

Alpha One Models 260, 280TRS, 330TRS



There are a few model's of alpha one drive's, could you plz be a bit more spefic. If you can post your speed @ wot, it would clear up a few thing's here such as the slip your experienceing with that 4 blade,

 

[Jdeagro]

Re: 4-Blade Disappointment

I just came accross this thread and felt the need to pop in. I have a question for Free Bee Tony - What are you trying to accomplish with the prop change? What improvements would you like to see?

Increasing the RPM's throughout the full range will only increase your fuel bill.

 

[FreeBeeTony]

Re: 4-Blade Disappointment

Not looking to increase the RPM's.............

Quite the opposite.............

I was hoping (from what I read) that I would get better MPH for the same (or lower) RPM's.

I might have jumped too fast............I found an old post from myself last year and I was getting 30MPH @ 3600RPM's...........not 3200RPM's like I had thought.

I already changed back to the 17" last night...........might take some data (MPH vs. RPM) and switch back again to compare.

 

[walleyehed]

Re: 4-Blade Disappointment

I just came accross this thread and felt the need to pop in. I have a question for Free Bee Tony - What are you trying to accomplish with the prop change? What improvements would you like to see?

Increasing the RPM's throughout the full range will only increase your fuel bill.

Increasing RPM does not always increase fuel consumption.
If the engine is propped to reach max recommended with a normal load and the rpm ends up higher at a given speed, it still may be working less and consuming less fuel, in fact, I proven this in several prop tests.
On the other hand, a poor blade design can go the other way, and the only way to find the right design for each rig is testing....everybody loads differently and some designs will work on 1 rig and not another of exact design...there's alot more to the equation than rpm.

 

[Tredeb]

Re: 4-Blade Disappointment

I was hoping (from what I read) that I would get better MPH for the same (or lower) RPM's.

You need to go up in pitch for that.

The only time you rally need to be concerned with RPM is at the redline. You want to run your engine at the correct RPM at WOT. Once you have found the pitch that gives you the desired results, you can go from there to fine tune for more holeshot.

Remember, Going up 2" of pitch will usually drop you 400 rpm. and vice versa.

It doesn't really matter how many rpm you turn at 30mph. If you are lugging your engine at low rpm, you will probably get less mileage than if you were running a higher rpm.

 

[sabastianunf]

Re: 4-Blade Disappointment

It sounds like the blades on your new prop may be cupped. I just bought a cupped 4 blade Vortec 1 pitch lower than my non-cupped 3 blade. Since the 4 blade is cupped I am not hitting the same rpm's as I was with the 3 blade - down @150-200 rpm's. It turns out I should have gone down 2" to hit the same wot rpms I was with the 3 blade. However, I still get a better hole shot even though I am not hitting my ideal rpm range at wot. Also not all 4 blades are the same - some are for watersports and some are for high hp offshore applications. I would contact the manufacturer to see what yours was designed for - it may not be the correct prop for your application.