12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

[115johnson78]

My boat is a 1978 Deep V 17' Invaider BR.
It is powerd by a 1981 Johnson 115hp, it has a 13"X19p three blade prop.

I have a large family and we pull a tube, the boat has great hole and mid prefomance and wot speeds.
The prop has a few dings and I can feel a little vibration at lower rpm's. I would like to change the prop with new.

Here is my Question's, I found (here on Iboats) a 12.5"X19p four blade prop, how close would that be to my 13"X19p three blade?
Is the four blade props that much better than three.
Should I just buy a replace ment 13X19 3-blade.
Thanks for all the help.
Go Iboats.

 

[Borchik]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

in theory a 3 blade is the most efficient however real world isnt theory. if i had to take an educated guess the 4 blade would help, i only say this because i have been on a similar boat and someone did the same thing, made a big difference.

 

[steelespike]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

Without knowing the details of your present performance we can only deal in generalities.Usually it is suggested you drop one inch in pitch when going to a 4 blade.That would depend on your present performance.A 4 blade will generally reduce your max rpm and speed thus the pitch reduction.
The 4blade will inprove hole shot,stay on plane with less throttle, feel smoother,
have better control on plane and at the dock.It will usually have lower max speed.
2 to 3 mph. Your actual performance comparison would depend on your present performance.

 

[115johnson78]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

My current wot rpm's is around 5,300( with all the family) and my spedo says 27 mph( I think its off a little).
I am happy with how the boat performs now, if the 4 blade can perfome some what the same and give better dock performance the that is great.


Thanks for the help..

Go I Boats.

 

[Aloysius]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

Theoretically, a 19" pitch prop moves 19" for each revolution. In real life, it 'slips" and doesn't move that far. The difference between 3 blade and 4 blade is blade AREA, and the corresponding "slip".

I would think in your case a 4 blade is the ticket. You're looking for bite (blade area) not speed. You may want to consider dropping a couple inches of pitch, though, as the additional bite and blade friction may reduce your top end rpm. You're a little overpropped at present already.

 

[2ndtry]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

The 4 blade is going to give you a better hole shot but hurt your top end. Not sure this is what you want, based on your OP.

But something's wrong with your current setup. A 19" prop turning 5,300 RPM has a theoretical top speed of 48 (1.86 gear ratio) or 51 (2.00 gear ratio) - I can't find the ratio on a 1981. Calculated slip for either is WAY to high, even if your speedo (27?) is a little off.

I think you need a new prop, but with the numbers you provided something is really wrong.

 

[steelespike]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

I agree there is something wrong with your numbers.5400 seems high for all the family. Especially with a 19.You may want to varify the tach.See if you can get a gps for speed.Also your speed seems low as suggested.
It appears the gear ratio is 2.00:1. I think a 17 would more likely be the right prop.

 

[trendsetter240]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

Yep that speedo must be way off or your prop is slipping about 43%.

With your setup a 12.75 x 17P Solas Amita 4-blade would be a good option. It will provide much stronger holeshot than your current 3 blade 19p and much better cruise and top end.

The 4-blade 17p will greatly reduce your prop slip so you should see far better performance in all areas.

They sell the Amita 4 blade here on iboats for about a hundred bucks.

Cheers

 

[115johnson78]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

Without knowing the details of your present performance we can only deal in generalities.Usually it is suggested you drop one inch in pitch when going to a 4 blade.That would depend on your present performance.A 4 blade will generally reduce your max rpm and speed thus the pitch reduction.
The 4blade will inprove hole shot,stay on plane with less throttle, feel smoother,
have better control on plane and at the dock.It will usually have lower max speed.
2 to 3 mph. Your actual performance comparison would depend on your present performance.

As for the preformance, when we are leaving the dock at idle speed, and get past the no wake zone I can give it wot and the tach jumps straight to 4,000 rpm's(the bow rises right up)for about 15-20 seconds then the tach starts climbing again( as the boat planes out) I ussually back off the throt a bit to keep the rpm's around 4500-4800.

I guess that a check of the Tach and new speedo is in order. Thanks for all the really great help.

 

[115johnson78]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

This past week I change my Tach.
How good are they calibrated from the factory?
It showed the same rpm as the old.

I was putting along and hit my prop.. (before i did the rest)

I had another boat (cause I couldn't find any family with a gps, and I didn't want to spend the money for 1) travel beside me and side by side clocked me doing 33-35 mph (trimed out) at 5,600 rpm.

It has been suggested that I get the 14X17 4blade.

I have found a 12.8"x17" (composite) blade prop.

Do you think that would work?

 

[mommicked]

Re: 12.5"X19P 4-Blade vs. 13"X19P 3-Blade.

get an alum.if you want a good cheap 4 blade prop.composites are troublesome from what ive read.