Re: 87 Sea Ray 270 w/twin 4.3 (185HP) - the infamous best prop selection question LOL
PROPS
Having had prop issues myself and seeing that prop issues are common questions here, I decided to educate myself about them. The information here is based on research I have done, but mostly on a conversation with technicians at Marine Wheels, a local, professional prop shop (not one guy with a torch) here in Panama City Fl. This is not ment as an advertisement for them, but I simply wish to convey where I got my information from.
Prop sizing
Props are measured with 2 dimensions and are shown like this- 15 x 15.
The first number is the diameter of the prop and the second number is the pitch, or degree of angle, of the blades. So, the 15 x 15 prop is 15" in diameter and has 15 degrees of pitch relative to the props main body. You can find this number on the prop. Usually it is stamped into the main body of the prop at it's rear, or if not there, it can also be found occasionaly on the washer beneth the prop nut. If it can't be found in either location you can try to find the MFG part number and contact the MFG to get the size. If no numbers at all can be found than you will have to take it to a prop shop to get sized.
Effects of pitch on speed, power, and RPM
The pitch of your prop directly influences speed, power (hole shot), and your engine's RPM. Your engine has a specified RPM range stated by your engine's MFG. When running your engine at WOT (wide open throttle) you want to be within this range otherwise damage to your engine could occur. Let's take a look at how differant pitched props effect your boat. Basically, the more pitch you have the more power (hole shot) you have and your RPM's will drop due to your engine having to work harder to turn the prop. You will also loose some top speed (approx 3-5MPH per change in pitch). The less pitch you have, the less power (hole shot) you will have and the longer it will take to get on plane. Your RPM's will increase and you will gain some top speed (approx 3-5MPH per change in pitch).
Other factors effecting speed, power and RPM's
Other factors besides the prop have an effect on your boats performance. Weight of your loaded boat, distribution of the weight, shape of your boats hull all impact the boats performance. You can change the weight of your boat making it heavier or lighter and you can move weight around your boat affecting performance. You can't really do much as far as the shape or type of hull you have though trim tabs mounted to the transome and "Whales Tails or Dolphins Tails" mounted to the ventilation plate on the engine have a pronounced effect.
What's a pitch worth?
One degree of pitch is worth approx 500 RPM. So lets say you have a 15 x 15 prop and you increase the pitch to 17. The effect to your boat would be something allong the lines of an decrease of RPM's of about 1000 and a loss in speed of approx 6-10MPH and you would gain power (hole shot). If you decreased the pitch to 13 you would gain approx 1000 RPM and gain approx 6-10MPH, but you would lose power (hole shot) and it will take your longer to get on plane though you will have a higher top speed.
Cup. What is it and what's it worth
If you look at your prop blades and than look at the end of the blade you may notice that it seams to be bent a bit more than the pitch of your blades. This is called the cup. A cup is worth approx 250RPM and is used to tweek a blade to a more exact performance. You can have cup added, but you cannot have negative cup. It also effects the props "grip" on the water during a turn. A prop with no cup is more likely to cavatate, or "vent" in a high speed turn than a prop with cup. Cup has implications for creaping up on better performance as stated above and watersports where you make high speed turns (racing, pulling skiers, etc...).
Choosing the "Right" prop
Selecting the best prop for your boat depends on how you use your boat and how you want it to perform on the water. If your using your boat for watersports than you will likely want a prop that gives you a great holeshot and good turning performance (generally a "power" prop with higher pitch and cup but less RPM's yet still allows you to maintain good speed for pulling skiers, etc.... If your into top speed you will want a prop with less pitch and cup or no cup depending on how you use the boat and higher RPM's. Getting the "best prop" is often a trial and error process. If you will notice I use the word approx allot in this. Prop selection is not an exact science since there are many variables that effect boat performance. When choosing a prop, I suggest you select one that you feel will get you close to where you want to be and than have it "worked" by a prop shop to better close in on where you want to be. Many boaters choose to have more than one prop to better tailor their boats performance to what they are doing (a power prop for pulling skiers and a speed prop for top speed). Many try to find a prop size that will give them good performance overall. Carrying a spare prop on your boat is highly recomended. NOTE- A brand new cast prop and a worked on prop will not be or perform exactly the same.
Diameter
According to the research I've done a change of 1" in diameter up or down has little to no effect on the performance of the prop.
How many blades should I run?
Props come in 2, 3, or 4 bladed styles. Props with 2 blades are used on smaller engines and 3 or 4 bladed props are used on large engines. Vibration and balance are a concern and vibration decreses as you add more blades. 4 bladed props are mostly used in watersports applications as they provide a great holeshot, but not great top speed.
Prop Material
There is allot of discussion about this. Stainless Steel props are more consistant, provide better performance and withstand corrosion better, but cost allot more than aluminum props. There is also a concern that if you strike an underwater object the stainlees prop will transfer the energy of the strike and will be more likely to trash your lower unit. Aluminum props are seen as good performance, good price, and less likely to trash your lower incase of a strike. The new composite props are still to new and people are too divided to really give a recomendation one way or the other.
Prop Instalation
When installing a prop you should enshure that the shaft is clean and free of debris. Lightly coat the shaft with a marine grade greese, slide the thrust washer down, slide the prop down enshuring to line up the splines, install the top washer making sure to line up the splines, thread the propnut till it makes contact with the top washer and hand tighten snugly, than install the cotterpin in the hole in the shaft above the propnut. There should be little to no play in the prop (sliding fore and aft). Excessive play indicates a poor fitting prop or you forgot a washer and can damage the propshaft and propnut.
Conclusion
I hope this information has answered any questions and helped guide you in prop selection. Again, I reiterate, prop selection is not an exact science so all numbers given are approximate.
Art
