4-Blade Prop Choices or go to a 3-Blade?

[jmbfin]

I have been trying to prop my '05 Sailfish 1900 Bay Boat with a Yamaha F115 correctly. The two 4-blade props I have are a Solas aluminum 12.75 X 17 and a Powertech Stainless 12.75 X 17. Both have a good hole shot but have difficulty maintaining speed when turning hard.

The Solas maintains 25 mph at 4000 rpm and 39 mph at 5600 rpm at WOT. The Powertech maintains 29 mph at 4000 rpm and 40 mph at 5300 rpm at WOT. These figures are with one person, 50 gallons of fuel, and Yamaha Digital Speedometer and Tach, therefore actual speed may be different. The top end speed is very satisfactory considering I was pushing 3,000 lbs.

Based upon what I think I understand:

1. A Powertech 4-blade 13.25 X 15 should increase my rpm's at WOT to about 5700 rpm's and top speed would probably drop to maybe 39 mph. These figures would be very similar to my current Solas aluminum prop.

2. If the Solas 4-blade prop were replaced with a 13.25 X 15 Solas 4-blade prop, my WOT rpm's would probably be at the top of my outboard's recommended operating range though at a reduction in top speed.

Powertech offers a 3-Blade Stainless prop in the following sizes for my application. 14 X 13, 13.25 X 15, and 13.25 X 17. Based upon your experience, which of these would most likely increase my rpm's to the top of recommended operating range, maintain top speed, and provide a comparable hole shot?

The Solas aluminum prop performs very well in my non-expert opinion. What would you recommend, reducing pitch, going to a 3-blade prop, or keeping the current aluminum prop in the Solas line of aluminum props?

 

[evin268fs]

Re: 4-Blade Prop Choices or go to a 3-Blade?

when turning hard, you must trim in to aviod cavitating. You will lose less speed trimming in, and will not have to wait for the blades to reach clean water before it bites to accelerate out; once you straighten out just trim back up. Even if you have to back off the trottle a little, you will not stall out like when the prop loses bite and cavitiates. A three blade is likely to cavitate worse in hard turns, unless it is a high quality stainless, typically you will always have to trim no matter what you are running.

 

[jmbfin]

Re: 4-Blade Prop Choices or go to a 3-Blade?

evin268fs, thanks for the info. I am new to boating and am seeking all the information I can get.

I want to maintain my hole shot, keep top end speed around 40mph, and increase WOT rpm's. Based upon my previous post, what pitch and/or number of blades should I be seeking to help me achieve these goals? Anyone have any advice for this rookie? Thanks

 

[evin268fs]

Re: 4-Blade Prop Choices or go to a 3-Blade?

Not a prop expert, but the 13.25 x 15 Powertech 3 blade,
By dropping pitch you should at least maintain holeshot of the four blade propellor, the 3 blade should put you at the top of the range, I don't think it will drop much since you are coming from a 4-blade and running too low in the rpm range

with either of the 4 blade choices you will probably see a noticable drop in top speed, 2-3 mph, but your holeshot will increase noticably.

 

[jmbfin]

Re: 4-Blade Prop Choices or go to a 3-Blade?

Thanks. I will try the Powertech 3-blade 13.25 X 15 stainless steel prop. This should get me close to 6000 rpms.

 

[jmbfin]

Re: 4-Blade Prop Choices or go to a 3-Blade?

Well, the Powertech Technician recommended the 15 inch pitch 4-blade prop. That is the one I will try. However, I am skeptical of gaining 700 rpm with only a 2 inch drop in pitch.

 

[jmbfin]

Re: 4-Blade Prop Choices or go to a 3-Blade?

Well, here are the final numbers I have from the 4-blade props.

Solas Aluminum 17" pitch: cruising 4000 rpm @ 25 mph
WOT 5600 rpm @ 39 mph

Powertech SS 17" pitch: cruising 4000 rpm @ 29 mph
WOT 5300 rpm @ 40 mph

Powertech SS 15" pitch: cruising 4000 rpm @ 25 mph
WOT 5800 rpm @ 40 mph

I included the mph at 4000 rpm because according to Yamaha performance bulletins that is near the most economical operating rpm range of the Yamaha F115. However, everything I have read disregards speed at given rpm ranges. The only concern seems to be rpm and speed at WOT.

I am considering the 17" and 15" pitch 3-blade Powertech stainless props. Generally speaking, how would the above performance numbers be affected? Should my only concern be getting the motor to operate at 6000 rpm at WOT or should cruising performance be considered?

Any suggestions for this rookie from the prop experts? I am keeping the Solas aluminum prop as a back-up prop. I exchanged the 17" Powertech for the 15" Powertech at a cost of $50. Would it be worthwhile to exchange it again for a 3-blade?

 

[Silvertip]

Re: 4-Blade Prop Choices or go to a 3-Blade?

I see no discussion here about engine height. Where does the AV plate site in relation to the bottom of the hull? You might want to play with engine height which can gain some RPM before spending money on props. Be sure water pressure is maintained.

 

[jmbfin]

Re: 4-Blade Prop Choices or go to a 3-Blade?

I see no discussion here about engine height. Where does the AV plate site in relation to the bottom of the hull? You might want to play with engine height which can gain some RPM before spending money on props. Be sure water pressure is maintained.

Plate is even or a little above the bottom of the hull.