How to choose proper pitch on prop?

[flash1977]

I am kind of new to the whole buying a prop game. I have a 94 Mercury 40 that I bought for $100 the other day that needs a prop. My boat is a 13.5' Kennedy Kraft and I don't know what size prop to order, as I am looking at the props here on iboats website to order (no mercury dealers within 100 miles of me.)

There is a ton of different pitch props and I don't know how to choose the correct pitch for my boat. Here is what I do know and I don't if it will make a difference or not. I have a 25 and 35 Evinrude, the 25 has a 9X10 and the 35 has a 10X13. The 25 actually runs faster than the 35 without a load, but will run about the same speed as the 35 with the load. So, should I get a smaller pitch prop for the 40 to get a better speed?

Thanks for the help.

Scott

 

[Rodneys]

Re: How to choose proper pitch on prop?

Read this.


http://sites.mercurymarine.com/port...26_49291:126_49299&_dad=portal&_schema=PORTAL

 

[flash1977]

Re: How to choose proper pitch on prop?

Thanks, I have read it and am trying to better understand it. I am only concerned that with a load, the powerhead won't get the rpm's needed to make the boat run properly, as is the case with the 35 Evinrude that I have.

Anyway, thanks and I appreciate the help.

 

[steelespike]

Re: How to choose proper pitch on prop?

If you can run it with the present prop and get us the info.
Prop size(usually on the barrel or under the prop nut.
Max rpm and speed. A tiny tach and a gps will work.
TinyTach is reasonable easy to install and will fit any similar motor
regardless of stator poles.If I was going to guess for that application I would go with a 12" pitch.It may turn out light but at least you could run it
and watch the rpm.You shouldn't make a prop selection without a tach to check the rpm.For its health and efficiency a motor needs to run within its rpm range.

 

[sschefer]

Re: How to choose proper pitch on prop?

I'm just curious.. what size engine is the boat rated for. 40hp seems like a lot of HP for a 14 ft boat. I put a 25 Evinrude on a 14ft (13'-9") Western and hit mid 30's with a 9-10 factory prop.

However, the starting point is with your engine manufactures recommend prop diameter.

A basic 94' 40 hp can handle up to a 11-3/5" diameter prop. Some folks claim that diameter means little but if you choose a specialty cup it can mean a lot. The middle of the road for you is 12" pitch. Personally, I would start with a smaller diameter (say, 10-3/8" x 12) with a standard cup design in aluminum. If that ends up being to small then keep it for high altitude runs and step up to a 15.

 

[hwsiii]

Re: How to choose proper pitch on prop?

Flash, , we will try to help you as much as we can, but there is certain information that is needed in order to help you. The following form has all of the information that is needed to help you with your problems finding the right prop. If you do not know all of the information that is asked for on the form please give us the most information you have available as I have a program I designed to find the right prop for any planning hull boat that needs this information, and anyone else who is trying to help you will need the same information. It requires time to find all this information when we have to look for it and in my defense I expect you to be willing to expend as much effort in finding the right prop as I do. We hope to be able to help you get the exact prop you need for your particular uses, and this information is required to do that.
It is not as simple to pick the right prop as most people coming to this forum think it is, that is why we ask so many questions, it is for your benefit we ask all these questions. And if you have a stainless steel prop or are changing to one the blade geometry of the prop can make a lot of difference in speed and RPM.

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor
8. Manufacturer?s recommended WOT range
9. Antiventilation Plate height above keel of boat if it is an outboard in inches
10. Make, model, diameter, pitch and whether SS or aluminum prop
11. WOT RPM and speed from each prop

H

 

[sschefer]

Re: How to choose proper pitch on prop?

hwsii - I think it's great that you've written the code for picking props. I also think that we often go to extremes when trying to select props because the shear number of choices is just plain overwhelming.

Just out of curiosity, does your software have a variable for elevation?

My recommendation is a 10 3/8 x 12, standard cup, aluminum as a starting point. It will be interesting to see what your software comes up with.

 

[SilverZuk]

Re: How to choose proper pitch on prop?

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor
8. Manufacturer?s recommended WOT range
9. Antiventilation Plate height above keel of boat if it is an outboard in inches
10. Make, model, diameter, pitch and whether SS or aluminum prop
11. WOT RPM and speed from each prop

H

I am in a similar situation. The prop came off my 50 HP evinrude. The boat is fairly new to me, and either broke/slung the retainer pin, or didn't have one.
So I have no idea what prop I need.

Here is what I know:
1. 1983 Sea Nypmph Bass Attacker (small aluminum fishing boat with 2 fishing seats, 2 riding seats, one live well, and two 6 gal fuel tanks.
2. 16' length, 5' width.
3. 2 adults, say 400#
4. fishing
5. Flat bottom
6. 1982 evinrude 50 HP, electric start, no power tilt/trim
7. 50 HP, not sure about gear ratio.
8. No clue on the WOT range
9. Not sure what an Antiventilation Plate height is
10. No clue about the prop other than it is aluminum.
11. No clue pm tje WOT RPM and speed from each prop

I know I am not much help, and therefore you guys probably can't help much. The only thing I can say is the previous prop was pitched about right, though it probably could have stood a bigger pitch. It came out of the water good, and the speed didn't seem to increase from 3/4 throttle to WOT. I could hear the engine pick up RPM, but the speed change was not noticeable.

 

[hwsiii]

Re: How to choose proper pitch on prop?

Silver, I started a new thread in your name with my choice of prop pitch for your boat. here is the thread http://forums.iboats.com/showthread.php?t=328756 as I didn't want to disrupt Flash's thread.

Flash if you will give me as much information as you have on your boat I will be glad to help you. I need a good approximate base boat weight with no motor just boat alone, and it appears to me the gear ratio is 2:1 for your motor please verify this. Try looking for a plate at the back of the boat with this information.

Sschefer, my software does take into effect elevation by lowering the HP, as a loss of HP is what happens at altitude.

H

 

[sschefer]

Re: How to choose proper pitch on prop?

Silver, I started a new thread in your name with my choice of prop pitch for your boat. here is the thread http://forums.iboats.com/showthread.php?t=328756 as I didn't want to disrupt Flash's thread.

Flash if you will give me as much information as you have on your boat I will be glad to help you. I need a good approximate base boat weight with no motor just boat alone, and it appears to me the gear ratio is 2:1 for your motor please verify this. Try looking for a plate at the back of the boat with this information.

Sschefer, my software does take into effect elevation by lowering the HP, as a loss of HP is what happens at altitude.

H

Thats a nice piece of software. I've used the drop and inch every 2000 feet rule for years but until recently I didn't realized the lower HP is not the only prop problem.

Most of us came to believe that the only problem incurred by high altitude is lower oxygen levels and fuel density. That is certainly the reason for lowered HP but the lower atmospheric pressure also causes props to ventilate/cavitate (slip) easier. Proper cup selection becomes a relative issue at high altitude. Have you considered adding it to your calc's?

I know that most manufacturers now consider it a very important feature of the props at various altitudes. Unfortunately, they guard the secret closely and it's not an obvious selection number on the box.

 

[hwsiii]

Re: How to choose proper pitch on prop?

Thank you for the compliment very much. I do know for about every 5,000' above sea level the boiling point of water reduces about 9 degrees because the vapor pressure has to be equal to the atmospheric pressure surrounding it in order for water to boil. And that theoretically means that a prop can start cavitation at about 194 degrees instead of 212 degrees at an elevation of 10,000 feet, so yes I can understand that blade geometry makes a larger difference in higher elevations in that respect. A boat also requires quite a pitch change to compensate for the loss in HP at high elevations and a commensurate loss of speed as well.
It will be easy to add a formula for elevation so far as elevation is concerned with horsepower and that would take care of the pitch as well, but I believe that most well designed cupped props with good blade geometry will handle the cavitaion and ventilation. I will see about writing a formula for elevation as well. Thank you for the suggestion, that is a good idea.

H

 

[sschefer]

Re: How to choose proper pitch on prop?

Thank you for the compliment very much. I do know for about every 5,000' above sea level the boiling point of water reduces about 9 degrees because the vapor pressure has to be equal to the atmospheric pressure surrounding it in order for water to boil. And that theoretically means that a prop can start cavitation at about 194 degrees instead of 212 degrees at an elevation of 10,000 feet, so yes I can understand that blade geometry makes a larger difference in higher elevations in that respect. A boat also requires quite a pitch change to compensate for the loss in HP at high elevations and a commensurate loss of speed as well.
It will be easy to add a formula for elevation so far as elevation is concerned with horsepower and that would take care of the pitch as well, but I believe that most well designed cupped props with good blade geometry will handle the cavitaion and ventilation. I will see about writing a formula for elevation as well. Thank you for the suggestion, that is a good idea.

H

The elevation problem has been plauging us traveling fisherman for years. The newer engines compensate to a much greater degree then older ones but the need for prop changes remains. It would be great to have a better method of selecting in this arena.