Prop change

[jammer]

All the info is stamped on the prop... serial # and pitch... write it down and take it to a dealer...

[Mikey]

Well speaking of dummies, thats me when it comes to props. How are they rated/categorized what ever. Do they have #s, angles and what do they mean?

Martin,
props are a relatively simple mechanical concept that has a huge impact of the performance of your boat. Once you understand the basics of how a prop functions, you can begin to find the setup that works best with your rig. here are a variety of terms used to describe propeller characteristics as well as performance attributes. It is important that tboat owners have a good understanding of them.

Aluminum vs Stainless Steel:
Aluminum props are cheaper to buy and to repair, but are uncomparable from a performance standpoint when considering applications 50HP and higher. Smaller engines don't develop as much tourque therefore the flex of the aluminum prop is not highlighted. On a 50HP and higher, the aluminum prop will demonstrate more flex, causing a significant drop in performance through the entire performance band...hole shot, mid range and WOT (Wide Open Throttle) SS props demonstrate much better charcteristics when it comes to all of the above concerns versus aluminum, but are much more expensive to purchase and repair.

Prop diameter:

<img src="http://im1.shutterfly.com/procserv/47b5 ... aM2rFq4Yty">

Diameter is determined primarily by the RPM at which the propeller will be turning and the amount of power that will be delivered to the propeller through the shafts and gears. The degree to which the propeller may operate in a partially surfaced condition, as well as the intended forward velocity, will also play a role in determining the most desirable diameter. Within a given propeller line, the diameter usually increases for propellers used on slower boats and decreases for faster boats. If all other variables remain constant, diameter will increase as power increases; diameter will increase as propeller RPM decreases (slower powerhead or engine speed and/or more gear reduction); and diameter should increase as propeller surfacing increases. What this is essentially saying is that the faster the prop turns, the smaller the diameter becomes..

Pitch:

Pitch is the distance that a propeller would move in one revolution if it were moving through a soft solid, like a screw in wood
When a propeller is identified as 13 3/4 x 21, it has a 13 3/4" (35 cm) diameter with 21" (53 cm) of pitch. Theoretically, this propeller would move forward 21" in one revolution. This is the "ANGLE" you refer to....

Slip

Slip is the most misunderstood of all propeller terms, probably because it sounds like something undesirable. Slip is not a measure of propeller efficiency. Rather, slip is the difference between actual and theoretical travel resulting from a necessary propeller blade angle of attack. If the blade had no angle of attack, there would be no slip; but, of course, there would be no positive and negative pressure created on the blades and, therefore, there would be no thrust.

To create thrust there must be some angle of attack or slip. The objective of propeller design is to achieve the right amount of slip or angle of attack, which is around 40, give or take a degree . This is acheived by matching the right amount of blade diameter and blade area to the existing engine horsepower and propeller shaft RPM. Too much diameter and/or blade area will lower slip but will also lower propeller efficiency, resulting in reduced performance.

Efficiency
Although the average boater is not going to be able to calculate propeller efficiency, it is often confused with prop slip, a common misconception.
In simple terms, here's the basic formula. Hope this helps..

Prop efficiency % = Power out divided by power in X 100


All this to say that most boaters are unaware of how efficient their boats are performing. But if they were to take a little time to look at some of their boats numbers...and handling characteristics...they would see that their boat is not giving them the maximum performance potential it's capable of. In some cases, I've seen boats run up to 6- 8 mph faster from a simple prop change and engine height adjustment, not taking into consideration the the handling improvements. Case in hand: Nighttrollers Ranger. While fishing out of Rockland with my boat, Spinners and Nighttrollers, we did a bit of prop testing. I took my 25 Tempest Plus (Ref: Pitch above) and put it on Rich's boat (Running a 23 pitch) That prop got Rich another 4-5 mph and lifted the bow beautifully. This let the boat sit on the pad (First foot or so of the hull measuring forward from the bottom of the transom) of the hull, creating less resistance. The boat handled the extra speed well , with no chine walk (rocking from side to side). The down aspect were the rpm's the prop was too much pitch to let Richs engine get into it's max rpm range. it fell about 400 rpm's short. This would be easily remedied by going down 2 pitches, each pitch, up or down, will add or remove approx. 200 rpms. Hope this helps you get the basic concepts and get your boat running as well as it can.

Regards,
Mikey

[skooter]

Excellent read Mikey,

....I think I might check those numbers out come spring.....


Ah, .......I think I know why FFF may be a bit confused......

See, ...unfortunately, ....his "propeller",

...........hangs from a pole on the deck......

skooter

[Mikey]

hey...a props a prop hehehe!

[fishforfun]

Skooter I do have an inboard with a prop.
Mikey great read! Now I know a lot more, one of the reasons I asked was my summer fishing partner wacked his SS prop a couple of years ago. He's a farmer I'm a sailor, marina guy says I can fix it for this $$ or I can give you this one, aluminium, for your SS prop. Done deal were back fishing but the boat has lost her performance, forever adjusting trim tabs, doesn't come out of the hole the way it did etc. Maybe we should have asked more questions Thanks I'm gonna print that one and save it for reference.

[dh99]

Thanks Mikey

Great info, I am going to play around with my boat next year. Bought a used boat with a 50 HP, it had a spare prop with it. After looking at the manual I realized the spare prop is the original prop for the boat and had a larger diameter. I tried it out and it was terrible the boat would take forever to get on plane. KNow I understand why.

Thanks

DH

[Mikey]

Thanks Mikey

Great info, I am going to play around with my boat next year. Bought a used boat with a 50 HP, it had a spare prop with it. After looking at the manual I realized the spare prop is the original prop for the boat and had a larger diameter. I tried it out and it was terrible the boat would take forever to get on plane. KNow I understand why.

Thanks

DH

DH...you're last sentence is the most appreciated, glad I could help out in some way.