King Cobra prop 3, 4 or 5 blade help

[bigike]

Out at canyon lake (TX) with the new (to me) 3 blade stainless 19p, got a new top speed of 49 loaded in my 95' Horizon 240 with the 7.4 KC. The lake ended up biting me again, 2 years ago it took my 3 blade aluminium 17p prop. Now my Balisitc 19p requires a visit to the prop shop.

What i'm wondering is this, can i go up in pitch and blade # to equal a hole shot of that 3x17p and get the top end of the 3x19p. Top end is not really a big deal, i'd rather have hole shot and a quick plane. Any of this make sense? We pull tubes and swim, no drag racing, might put a wake tower on it some day.

Right now the RPM's are not reading right, with the 17p they where 6k and the 19p is 5.8k.

I'm looking at a 4x21p aluminum (cheap), would that be more hole shot or less than a 3x19p or 17p? Same for a 5x21p (not cheap).

Thanks for y'alls time.

 

[RogersJetboat454]

Re: King Cobra prop 3, 4 or 5 blade help

IMHO, you need accurate RPM readings before you can make an informed decision.
Current RPM figures would be over-revving a stocker BBC.

 

[bigike]

Re: King Cobra prop 3, 4 or 5 blade help

She is a hoss. And it had a 17p running WOT, i don't think any of the other props are going to rev higher than the 3x17p.

Take the boat out of the equation. You lose top end with a 4 and 5 blade but you gain hole shot, drop the plaining speed. What i want to do is lose as little top end as possible but gain a lot in hole shoot. follow?

 

[Levinz11]

Re: King Cobra prop 3, 4 or 5 blade help

IMHO, you need accurate RPM readings before you can make an informed decision.
Current RPM figures would be over-revving a stocker BBC.

^^^This^^^ Get a 4 blade whatever for hole shot and an accurate tach.

 

[bigike]

Re: King Cobra prop 3, 4 or 5 blade help

Yeah I understand about rpm needing to be accurate, but i'm not worried about that. Forget my boat, on your boat if you wanted to get on plane faster and keep a good top end would swapping to a 4 or 5 blade work? Also if coming from a 19p 3 blade to a 21p 4 blade would you gain hole shot? Again forget my boat.

 

[jestor68]

Re: King Cobra prop 3, 4 or 5 blade help

A 4 blade of the same pitch would improve hole shot while losing minimal top speed.

Typically a 4 blade will run about 100 rpm less than the same pitch 3 blade.

 

[bigike]

Re: King Cobra prop 3, 4 or 5 blade help

I really want a good water sports prop, with the 17 she would explode out of the water, the 19 isn't quit as nice, was thinking maybe a 21 in 4 blade or even 5.

Think a 21 would 4 or 5 would plain better than a 3x19p?

Next time out i'm going to remember the snapon timing light and have someone look at the RPM i think it is 1000 off. She sounds good a WOT, not windows to be made in this block.

 

[jestor68]

Re: King Cobra prop 3, 4 or 5 blade help

Since the max engine speed for that motor is 4400 or 4600 rpm(depending on where you look), you really need an accurate tachometer for a couple of reasons; to keep from ruining your motor, and to choose the correct propeller.

 

[bigike]

Re: King Cobra prop 3, 4 or 5 blade help

Jestor/guys, i agree whole heartily. I was attempting to pose the question in a matter where my boat rpm didn't matter. I know that if i slap a 4 or 5 blade 19p prop she will be fine rpm wise.

My RPM is good with the 3x19, it was good with the 3x17. I'm sure i could put a 23p on it and she would still scream.

What i want is to get out of the water hard, like with the 17... but have the top end of say the 19 or a 21. I understand that we gain hole shot on plane with larger number of blades, but trade top end. Maybe this would be a good way to put it by contrasting 4 and 5 blades to 3 blades....

4x20 on plane like a 3x?? and top end of 3x??
4x23 on plane like a 3x?? and top end of 3x??
5x21 on plane like a 3x?? and top end of 3x??
5x23 on plane like a 3x?? and top end of 3x??

Does that make sense? I'm old Navy but relavitely new to boating... While offshore i have many ideas that require answers... haha boredom does that.

 

[jestor68]

Re: King Cobra prop 3, 4 or 5 blade help

What is the gear ratio? I've run two different ratios through the selector and get a 19 or 21 pitch, depending on the ratio. Interestingly, both ratios/pitches came out with like 49.8 mph at 4400 rpm(estimated).

The one "rule" to understand is; a 4 blade prop will run about 100 rpm slower than the same pitch 3 blade prop.

If you want the same(or close to it) rpm, you reduce 1 inch of pitch going from 3 to 4.

If you use the same pitch, expect to lose about 100 rpm going from 3 to 4 blades.

 

[bigike]

Re: King Cobra prop 3, 4 or 5 blade help

1.42

so is the RPM and speed relative from a 3 to 4 blade?

What about plane time? Is there a similarity? Like, a 4x21 planes like a 3x19? I know that if i had a cold beer in my hand face to face i could pose this question in a way that wouldn't be so confusing...

 

[jestor68]

Re: King Cobra prop 3, 4 or 5 blade help

1.42

so is the RPM and speed relative from a 3 to 4 blade?

What about plane time? Is there a similarity? Like, a 4x21 planes like a 3x19? I know that if i had a cold beer in my hand face to face i could pose this question in a way that wouldn't be so confusing...

With similar pitch, rpm and top speed will be a little lower; while out of the hole acceleration will be noticeably stronger with the 4 blade, as well as cruising performance.

The down side of the 4 blade doesn't manifest itself until the upper speed range. The increased blade area(of the 4 blade); while good at take off and cruise speed(more thrust per revolution), causes more drag at higher speeds; resulting in that slight lose of WOT rpm and a couple mph off the top speed.

Everything's a compromise. For top speed, the 3 blade is almost always best. But for better acceleration and cruising performance, the 4/5 blade rules. They are simply the kings of acceleration.

If you are turning good rpm with a 3 blade, I would just put a 4/5 blade of the same pitch on it for improved acceleration for water sports.

 

[jestor68]

Re: King Cobra prop 3, 4 or 5 blade help

How many splines on your prop shaft; 15 or 19? Just curious.

 

[bigike]

Re: King Cobra prop 3, 4 or 5 blade help

Think it's 15, the OMC 800 prop fits. On an oilrig right now... Chopper in 30 minutes though!!!