4.3 Cobra Prop

[mthieme]

I have an '89 4.3 with a Cobra O/D in a '69 Searay SRV190. Prop is 21P. I was happy with this until I read another thread and it seems my boat is second slowest compared to everyone else's. She does 34 knots turning 4400rpm.
What do you think a 23P will do to my top end?
(AChris, will your program handle this? what other into do you need?)
Do you think a 4 bladed will do anything for me?

 

[642mx]

Re: 4.3 Cobra Prop

Sounds like you have too much prop already...... If your only spinning a 21 pitch at 4400 rpms, then you need to try a 19 out. 4.3's are suppose to top out at 4800 rpms.

 

[mthieme]

Re: 4.3 Cobra Prop

I tried a 19P last weekend, lost about 3 knots at 3000 rpm.
But she got up on plane quicker.

 

[642mx]

Re: 4.3 Cobra Prop

I tried a 19P last weekend, lost about 3 knots at 3000 rpm.
But she got up on plane quicker.

Let me get this straight....

You can spin a 21 at 4400 rpm, but you can only spin a 19 at 3000 rpm?...

Something is not right here. A 19p should spin more rpms than a 21p. Also, can we drop the knots thing? Every boat I've seen has a speedometer, not a knot-meter...

 

[mthieme]

Re: 4.3 Cobra Prop

No.
The measurements are just at different rpm. No problem hitting WOT.
If it helps, with the 19P prop at 3000 rpm she does 18-19 knots.
With the 21P prop at 3000 rpm she does about 22 knots.
I just don't have a WOT speed, but it is noticeably slower.
Hence the inquiry about a 23P prop for speed.

 

[642mx]

Re: 4.3 Cobra Prop

No.
The measurements are just at different rpm. No problem hitting WOT.
If it helps, with the 19P prop at 3000 rpm she does 18-19 knots.
With the 21P prop at 3000 rpm she does about 22 knots.
I just don't have a WOT speed, but it is noticeably slower.
Hence the inquiry about a 23P prop for speed.

A 23 won't do you any good if you max out at 4400 with a 21. You need to find your max speed and rpm with the 19 to make an accurate decision.

Of course your going to get more speed per rpm with the 21 vs. the 19, but there comes a point where the engine won't have enough power to turn the bigger prop, then you have to go back down in size until the engine will run the correct WOT rpms.

So my best advice is, try the 19 again, hold it wide open and see what it does. I'm guessing that it will rev close to 4700 rpm with a higher top speed than the 21.

 

[mthieme]

Re: 4.3 Cobra Prop

In the other post, some of the guys had a 23P.
I assume the motor has the power to spin it.
But you don't think a 23P will improve my top end?

 

[642mx]

Re: 4.3 Cobra Prop

In the other post, some of the guys had a 23P.
I assume the motor has the power to spin it.
But you don't think a 23P will improve my top end?

Some 4.3 powered boats can spin a 23p, but it a lot of factors play into it...like gear ratio, hull design, weight, ect, ect... My Baja will do it, but its a light boat.

The higher the pitch, the lower your WOT rpms will be. So if you spin a 21 at 4400, a 23 would probably only spin 4000-4100, which is not close to the target rpms for WOT and your top speed will probably drop because you can't turn it fast enough.

Try the 19 again.