Prop Selection

In the past I was blessed with pre-rigged Stratos XL18 with a Evinrude 150 & stainless prop. Upper 60s was fast enough. That rig is long gone.. I JUST bout a used 2000 Lund 1800 18'6" Fisherman w/2000 Merc Optimax 135. Currently installed is an Alum prop (no jackplate). Dont know the pitch ect- but it is my assumption that a stainless prop will gain me more speed.

Q: How would I deternime what stainless prop to buy to get the max speed ? Am I dreaming to hope for 50mph ?

Current Alum prop is ( very faint stamping on hub 415 21P)

The area I live in does not have an outlet for "try it & buy it" sales... Thanks for the advice.. Billfo

Re: Prop Selection

Hi Billfo-

I will try to help you out, but you are approaching this a bit bass ackwards....

First, you need to know where you are at before you can talk about prop selection. You need a base line with your current set up. Don't assume that this is rigged properly until you get a chance to run it at WOT to find out what your base line is. Prop purchases are initially made to get you to your peak RPM. Since you don't know what RPM the current set up is running at as a reference point, you can't go beyond this step.

Things that effect the RPM at full throttle include water conditions, prop pitch, engine height, and trim. Your Optimax NEEDS to run as close to 5400-5600 RPM's as you can get. Try to stay at or slightly under the 5600 RPM mark and absolutely no less than 5000 or you will be lugging the engine. Your Optimax is in its peak power band in the 5000-5600 RPM range and needs to be in the upper end of that to start to fine tune your prop selection to get the max speed at that RPM.

It is also helpful to let us know how you are going to use it. Just run & gun? Trolling? Pulling skiers or tubes?

Finding the perfect prop to wring out every last ounce of performance out of an engine takes some trial and error. But you need a base line RPM for us to even make suggestions to you for a prop as to where to go next.

There are several tutorials on line to help you with prop selection. Most of the prop manufacturers also have tutorials on line to help you learn more about prop selection.

A 1 inch reduction in propeller pitch will result an an increase of 150 to 200 RPM at full throttle.

If at full throttle, the engine RPM is too high, you will need to try a propeller with either a higher pitch or larger diameter. Conversely, if the engine RPM is too low at full throttle, try a propeller with a lower pitch or smaller diameter.

Due to the differences in hull designs, engines, and weight (and weight distribution) from one boat to another, finding the correct propeller for each boat will vary.

Finding the best propeller can only be accomplished by trial and error after determining what is most important to you (speed, handling, etc.) under specific water conditions (lake, ocean, rough seas, etc.)

Good luck!

Re: Prop Selection

Excellent reply.. Especially a few points.. Like me finding out "what Ive got & where Im at"

I found this statement most educating..."A 1 inch reduction in propeller pitch will result an an increase of 150 to 200 RPM at full throttle.

If at full throttle, the engine RPM is too high, you will need to try a propeller with either a higher pitch or larger diameter. Conversely, if the engine RPM is too low at full throttle, try a propeller with a lower pitch or smaller diameter.

Due to the differences in hull designs, engines, and weight (and weight distribution) from one boat to another, finding the correct propeller for each boat will vary.

Finding the best propeller can only be accomplished by trial and error after determining what is most important to you (speed, handling, etc.) under specific water conditions (lake, ocean, rough seas, etc.)
"

I will do as suggested. Sunday we will be heading out for a "discovery run". I will be sure to take some notes.. I will be mindful of the 5400-5600RPMs & see see what speed is reflected by GPS.
Thanks & I will report back later !

ziemann said:

Hi Billfo-

I will try to help you out, but you are approaching this a bit bass ackwards....

First, you need to know where you are at before you can talk about prop selection. You need a base line with your current set up. Don't assume that this is rigged properly until you get a chance to run it at WOT to find out what your base line is. Prop purchases are initially made to get you to your peak RPM. Since you don't know what RPM the current set up is running at as a reference point, you can't go beyond this step.

Things that effect the RPM at full throttle include water conditions, prop pitch, engine height, and trim. Your Optimax NEEDS to run as close to 5400-5600 RPM's as you can get. Try to stay at or slightly under the 5600 RPM mark and absolutely no less than 5000 or you will be lugging the engine. Your Optimax is in its peak power band in the 5000-5600 RPM range and needs to be in the upper end of that to start to fine tune your prop selection to get the max speed at that RPM.

It is also helpful to let us know how you are going to use it. Just run & gun? Trolling? Pulling skiers or tubes?

Finding the perfect prop to wring out every last ounce of performance out of an engine takes some trial and error. But you need a base line RPM for us to even make suggestions to you for a prop as to where to go next.

There are several tutorials on line to help you with prop selection. Most of the prop manufacturers also have tutorials on line to help you learn more about prop selection.

A 1 inch reduction in propeller pitch will result an an increase of 150 to 200 RPM at full throttle.

If at full throttle, the engine RPM is too high, you will need to try a propeller with either a higher pitch or larger diameter. Conversely, if the engine RPM is too low at full throttle, try a propeller with a lower pitch or smaller diameter.

Due to the differences in hull designs, engines, and weight (and weight distribution) from one boat to another, finding the correct propeller for each boat will vary.

Finding the best propeller can only be accomplished by trial and error after determining what is most important to you (speed, handling, etc.) under specific water conditions (lake, ocean, rough seas, etc.)

Good luck!

Click to expand...

Re: Prop Selection

Fantastic- let us know what your base line is and we can go from there.

TIP: take a hand held GPS with you to pin down your speed. Watch your tach and make a mental note when you are properly trimmed running at full throttle. Take in to account what your average load will be (number of passengers, equipment, fuel) and if it is possible run as close to your "average" load as you can.

Example- if I am running solo and with a half tank of fuel (in an '05 Lund Mr. Pike 17), my RPM's can be 5350 with my Evinrude E-tec 90. With a full tank of fuel, and 2 people in the boat, I can drop 150 RPM's...

I will be excited to hear where you end up. Good luck!

Re: Prop Selection ( baseline info )

Re: Prop Selection ( baseline info )

Ok here we go.. First of all, I know some people are not fans of Stingray Hydofoil tails ( I have one on).. Personally I like them as I dont care for bowlift & I feel I stay up on plane at slower speeds. My main use for the motor is cruising bigger lakes "comfortably & hopefully at 50-55mph at 75% throttle". NOW for the baseline.....

At wide open throttle I am at 5400rmps & 50mph. Boat was loaded somewhat with two people & a full gas tank..

Sounds like I am as close. IF I went down one size in a stainless prop, I would be at about 5550 rpms (if I understand it correctly) not sure how much more mph would be gained... whats your thoughts now..

edit::: Do I want to run for long distances at WOT, will that damage the motor or just drain my gas mileage ?

ziemann said:

Hi Billfo-

I will try to help you out, but you are approaching this a bit bass ackwards....

First, you need to know where you are at before you can talk about prop selection. You need a base line with your current set up. Don't assume that this is rigged properly until you get a chance to run it at WOT to find out what your base line is. Prop purchases are initially made to get you to your peak RPM. Since you don't know what RPM the current set up is running at as a reference point, you can't go beyond this step.

Things that effect the RPM at full throttle include water conditions, prop pitch, engine height, and trim. Your Optimax NEEDS to run as close to 5400-5600 RPM's as you can get. Try to stay at or slightly under the 5600 RPM mark and absolutely no less than 5000 or you will be lugging the engine. Your Optimax is in its peak power band in the 5000-5600 RPM range and needs to be in the upper end of that to start to fine tune your prop selection to get the max speed at that RPM.

It is also helpful to let us know how you are going to use it. Just run & gun? Trolling? Pulling skiers or tubes?

Finding the perfect prop to wring out every last ounce of performance out of an engine takes some trial and error. But you need a base line RPM for us to even make suggestions to you for a prop as to where to go next.

There are several tutorials on line to help you with prop selection. Most of the prop manufacturers also have tutorials on line to help you learn more about prop selection.

A 1 inch reduction in propeller pitch will result an an increase of 150 to 200 RPM at full throttle.

If at full throttle, the engine RPM is too high, you will need to try a propeller with either a higher pitch or larger diameter. Conversely, if the engine RPM is too low at full throttle, try a propeller with a lower pitch or smaller diameter.

Due to the differences in hull designs, engines, and weight (and weight distribution) from one boat to another, finding the correct propeller for each boat will vary.

Finding the best propeller can only be accomplished by trial and error after determining what is most important to you (speed, handling, etc.) under specific water conditions (lake, ocean, rough seas, etc.)

Good luck!

Click to expand...

Re: Prop Selection

Billfo- sorry for the delay- been busy at work this week....

I agree with your assessment. You ARE close....but, you might be able to eek out just a bit more out of her....

If I was in your shoes, I would try the other prop to try to get to 5600...but I am a tinkerer...

I know you said that it is tough to get props to try.....where are you located? Maybe someone here knows of a prop shop in your region that will let you try a prop..... Try it before you buy it to make sure it improves your performance since you are close already....

Also keep in mind that you are currently running an aluminum prop- if you go to a stainless, there will not be as much give/ flex in the blades, so you may very well see a reasonable speed increase....

Keep in mind if you are running a lighter load (less passengers or fuel) your RPM's will increase somewhat....

It sounds like someone did a pretty good job rigging it before you got it....

As far as running WOT, yes you will burn more fuel.... but as long as you aren't seriously over revving the engine, she should purr along just fine at WOT....

Re: Prop Selection

if you go to a stainless, there will not be as much give/ flex in the blades, so you may very well see a reasonable speed increase....
Im like hearing that !!! LOL...
If like to have a 2nd prop just in case- so getting a stainless is what I would like to do..... IF I want to gain 150 rmp, should I reduce one size in diameter or reduce 1" of pitch ? Thanks again for the rep[lies... no rush, its snowing here anyway...

ziemann said:

Billfo- sorry for the delay- been busy at work this week....

I agree with your assessment. You ARE close....but, you might be able to eek out just a bit more out of her....

If I was in your shoes, I would try the other prop to try to get to 5600...but I am a tinkerer...

I know you said that it is tough to get props to try.....where are you located? Maybe someone here knows of a prop shop in your region that will let you try a prop..... Try it before you buy it to make sure it improves your performance since you are close already....

Also keep in mind that you are currently running an aluminum prop- if you go to a stainless, there will not be as much give/ flex in the blades, so you may very well see a reasonable speed increase....

Keep in mind if you are running a lighter load (less passengers or fuel) your RPM's will increase somewhat....

It sounds like someone did a pretty good job rigging it before you got it....

As far as running WOT, yes you will burn more fuel.... but as long as you aren't seriously over revving the engine, she should purr along just fine at WOT....

Click to expand...

Re: Prop Selection

There is a very fine line here.... in my opinion, your boat was already rigged pretty good. So we are again splitting hairs here.

I THINK....in my opinion...etc...

If you bought a stainless prop in the same pitch as your existing aluminum, you would see a drop in RPM's slightly since there is less blade flex in the stainless. You would likely gain a slight bit of speed.

Ultimately, I think you are going to end up going down in pitch to try to increase in RPM's, but the stainless blade will pretty much cancel out the pitch difference. I THINK, a stainless prop will increase performance, and maybe slightly increase your RPM's (but not as drastically as you would think.

I am hoping that someone that routinely swaps back and forth between stainless and aluminum can chime in here.

You honestly need to try the props out if you can before you buy them. Sometimes, things don't work quite as expected...some times they work better than expected. Stainless is expensive and these rules of thumb can be cancelled out based upon design of the prop etc........

Sorry if I sound wishy-washy here.... We are getting down to splitting hairs...

Re: Prop Selection

no wishy washy at all.. I understand what your saying. Reality is- if I cant get another 5-8 mph.... screw it ! Probably not worth the money. Perhaps next season when I get to the bigger waters down south (into Missouri) I may find a boat shop willing to let me try before I buy.. BUT like I said- I want noticeable speed.....without buying a new motor.,... dont we all.. LOL !!!!

ziemann said:

There is a very fine line here.... in my opinion, your boat was already rigged pretty good. So we are again splitting hairs here.

I THINK....in my opinion...etc...

If you bought a stainless prop in the same pitch as your existing aluminum, you would see a drop in RPM's slightly since there is less blade flex in the stainless. You would likely gain a slight bit of speed.

Ultimately, I think you are going to end up going down in pitch to try to increase in RPM's, but the stainless blade will pretty much cancel out the pitch difference. I THINK, a stainless prop will increase performance, and maybe slightly increase your RPM's (but not as drastically as you would think.

I am hoping that someone that routinely swaps back and forth between stainless and aluminum can chime in here.

You honestly need to try the props out if you can before you buy them. Sometimes, things don't work quite as expected...some times they work better than expected. Stainless is expensive and these rules of thumb can be cancelled out based upon design of the prop etc........

Sorry if I sound wishy-washy here.... We are getting down to splitting hairs...

Click to expand...

Re: Prop Selection

I think your expectations are very realistic. We all want to get the best performance we can and not break the bank.

I think you have a sound plan. You definately want to be able to do some trial and error and if it can wait until next spring, it will give you some more time to research prop shops in the areas that you are going to be fishing.

If you ever make it to Minnesota to fish, there are prop shops all over that cater to folks like you that will allow you to fine tune your performance instead of living with a compromise.

Good luck!

Re: Prop Selection

Still learning, Q: Could I gain my needed 200-350 rpms by switching to a non cupped stainless steel propeller ?

Also I read someplace that changing the prop "diameter" 1/2 inch up or down has little effect on rpms.. Is that true ?

ziemann said:

I think your expectations are very realistic. We all want to get the best performance we can and not break the bank.

I think you have a sound plan. You definately want to be able to do some trial and error and if it can wait until next spring, it will give you some more time to research prop shops in the areas that you are going to be fishing.

If you ever make it to Minnesota to fish, there are prop shops all over that cater to folks like you that will allow you to fine tune your performance instead of living with a compromise.

Good luck!

Click to expand...