Propping Yamaha C40tlrx

[1080tommy1]

Ok , I have a 2002 Alumaweld Xpress 1650 DB. I put a Yamaha C40 TLRX on her and need some help with the right prop. The current prop is a 12 1/4X9-G which I think is not the best prop because of the performance and rpms. The boat has an incredible hole shot but than when WOT the boat seems to be running at too high rpms and falling on it's face. I also noticed the fuel efficiency to be really bad. I do not know the exact rpms and just installed a Tiny Tach and will take the boat out tomrrow to find out actual WOT rpms are but if I am at a 9 pitch, how many degrees do I add to lower the rpms. WOT for this motor is 4500-5500.

I guess what I am actually asking is the relationship between adding pitch and the resulting rpms. I read that adding 1 degree of pitch will decrease rpms roughly 200 rpms. Is this correct? Also, how does diameter affect rpms. I noticed that this prop is the largest diameter with the flatest pitch listed for my motor and all of the other props listed are smaller diameter.

I am new to having a boat or an outboard motor so I hope this is the right format for the question and am sorry in advance if the question is not clear. Just don't want to over run the motor and do some damage. I will report what actual rpms are tomorrow if I can get out after work. Thanks in advance for any advice anybody could give a noob with some confusion on how to get the right prop. I am probably making this way more complicated than it has to be.

 

[steelespike]

Re: Propping Yamaha C40tlrx

We'll get you going.Once you get your present rpm and speed by gps we can tell better about your setup.
remind us off the prop size.
Basic prop selection is pretty straight forward. Subtleties refining performance or setups with strange numbers become more difficult.

 

[1080tommy1]

Re: Propping Yamaha C40tlrx

Thanks!!!! I was hoping to get her out and get the gps speed and rpms today but I forgot I have some commitments with the kids. Looks like tomorrow will be the day. I will get the reading and you especially seem to have the prop selection down to a science. It is difficult because I am so new to outboard motors. I am going out to the Mississippi next weekend and hope to have a better handle on this issue. Thanks again and I will re-post when I have more accurate numbers for you.

 

[pootnic]

Re: Propping Yamaha C40tlrx

I'm going to say your under proped(pitch wise).
A 40hp on a 1650 flat bottom,I'm thinking somewhere in the 13-14 pitch range maybe even 15 if the boats not loaded down.
Just saying so you don't go at WOT to long,it should be screaming with a 9 pitch.

 

[1080tommy1]

Re: Propping Yamaha C40tlrx

I am on the same page for sure. I can tell the engine was over reving just from the sound of it. I was carefull to only run at about half throttle for this reason. Also, if I go to wot, the boat just revs more but I do not gain any speed so it did not make sense to run over about half throttle. I did not know until last week that the prop was as flat as it is. I did install the tach and will finally have some concrete numbers to work with after tomorrow. Thanks again for all that advice, this makes a process that I was feeling a bit overwhelmed by much easier to work with.

 

[1080tommy1]

Re: Propping Yamaha C40tlrx

I decided to go with a 10 5/8X12 pitch as a testing prop. This is the prop on the Yamaha Website that they were putting on comprable boats. I figured even if I tached the 12 1/4X9, it would still be a crap shoot because of the drastic change that would need to be made. I am going to get the prop on Wednesday of this week and should be able to take her out on Thursday for Friday for a test run. I figure I still may be a need a little tweaking but this should be close and if I am still a little under propped I can use this as a spare. I will still buy a SS prop once I have the rpm and performance where I want it. Thanks for all the help on this website, it has made this process much easier to understand for someone that has not had a boat like this.

 

[Johny25]

Re: Propping Yamaha C40tlrx

Unless you are running some extraordinary amount of weight in your boat I would assume you are under propped with a 9 pitch. I am interested to see RPM reading. My guess is your turning close to 6k with that 9 at WOT.

 

[steelespike]

Re: Propping Yamaha C40tlrx

12" sure sounds reasonable and we should be able to tell better if another choice is necessary.

I decided to go with a 10 5/8X12 pitch as a testing prop. This is the prop on the Yamaha Website that they were putting on comprable boats. I figured even if I tached the 12 1/4X9, it would still be a crap shoot because of the drastic change that would need to be made. I am going to get the prop on Wednesday of this week and should be able to take her out on Thursday for Friday for a test run. I figure I still may be a need a little tweaking but this should be close and if I am still a little under propped I can use this as a spare. I will still buy a SS prop once I have the rpm and performance where I want it. Thanks for all the help on this website, it has made this process much easier to understand for someone that has not had a boat like this.

 

[1080tommy1]

Re: Propping Yamaha C40tlrx

After doing some research, the motor did come off of a big pontoon boat that was used as sort of a floating dock. That would explain the severe under propping of the motor. I figured with a 9 pitch even rpm readings would probably be tough to determine the next prop because propping from what I have seen is not an exact science. I figured with the information that 99yam40 presented to me on the Yamaha website, it seems I should be in the 12 to 13 range. Now I will have a use for the 9 pitch come hunting season because I will have a lot more possibly 350-400 lbs in the boat. I really can't say enough about websites like this because all of the posts have really helped me make what I think was an informed decision about what steps to take to get her right. I am getting the new prop today and will take her out tomorrow and have some readings for you guys off the Tiny Tach. I went with a 12 because if I am still slightly under propped I can just back off a little on the throttle and did not want to over prop and have a paper weight or try to sell a prop that I just bought.

 

[Johny25]

Re: Propping Yamaha C40tlrx

well your boat only weighs 400lbs so unless you have a bunch of mods like plywood floors and what not I don't think that 9 pitch will ever be able to be used. And I am doubting the 12 pitch would not be under propping also? I bet you could run that 12 with the extra 400lbs in the boat and easily hit your max RPM still. the 12 pitch just seems low to me for a 40HP motor on a light tin like you have. But that is just my opinion. A tach will tell you a lot and take the guessing out of it.

 

[1080tommy1]

Re: Propping Yamaha C40tlrx

Thanks johny and I agree. I will have at times a lot of extra weight in the boat. Fishing will be me and only one passenger. Hunting I will have two people plus a 125lb plus blind and at least another 100-150lbs of dekes. Hope the tach will tell me if this prop will work for hunting and if I am still under propped, I will get a 13-14 SS. Thanks again for all the feedback.

 

[1080tommy1]

Re: Propping Yamaha C40tlrx

Finally got the water test today!!!! Great results!!!

99 Yamaha C40 TLRX
Quicksilver Black Diamond Prop w/Flo Torque Hub 3 X10 5/8X 12
WOT Range 4500-5500
Actual WOT highest was 5720 @ 35.43mph with two gps one in my sonar and one in my phone and both were within .1 mph of eachother
2002 Xpress Modified V Loaded light with me 200llbs 4.5 gallons of gas and approximately 80lbs of fishing gear

First of all thanks so much for making this process easier by all the informative posts on props. I don't think I would have gotten this close without this site and the information presented in these posts.

Now onto to business, I will have at least 200-250 lbs more in the boat because I will rarely be alone for fishing. So, would it be a good idea to go up to a 13 or 14 pitch same diameter prop to get the rpm's within the acceptable range?? I was 200 rpm's over the range but I was loaded light. I do not want to over prop because I know that will make the prop useless. I think for hunting this prop will be ideal because I will have much more weight in the boat with all the hunting gear. I would like to get a SS prop and just want to be educated so I do not spend 250-300 dollars on a paperweight. I hope this will be enough info to make a good decision on a new prop for fishing. Thanks again!!!!

 

[Johny25]

Re: Propping Yamaha C40tlrx

Sounds like the 12 pitch will be ideal for hunting when you have extra weight. With a couple hundred pounds more in the boat you should be real close to 5500 rpm.

As for an SS prop, I wouldn't recommend any prop from Cabels's.....lol. If you read the last few posts in my thread you will know why. I am expecting my 10X14 Solas SS prop on Monday and should be running it by Wed or Thurs so I will give you info on the Solas. My guess is a 13 SS would be perfect for your normal running conditions and you could probably even turn 5300 RPM loaded for hunting which is still on the higher end of your RPM specs. I bought the Solas off of this site for $168 + shipping so the price is not bad.

 

[Johny25]

Re: Propping Yamaha C40tlrx

Hey something doesn't seem right? Your gear ration is 2.00:1 correct? That means at 35.43 mph, 12 pitch, 5720RPM you are getting a -1 slip which is not even theoretically possible. Something is wrong with these numbers I think?

 

[1080tommy1]

Re: Propping Yamaha C40tlrx

Man I have no idea about the numbers and how accurate they r. I did have to remove some of the wire off the tiny tach because I was reading too high but when I did remove some of the wire it read 5270 and 35.43 on my phone and 35.33 on my sonar. I did try several times to run WOT and the numbers were always the same. The speed shoul b accurate because it was gps x2. Maybe tiny tach was a little off???? I don't know but the engine did sound much better and from the sound was definitely running a little too high. Could the fact I was so light have anything to do with it????

 

[Johny25]

Re: Propping Yamaha C40tlrx

Well a prop usually has 7% to 12% slip average from what I gather. In order to run 35mph with a 12 pitch and a 2:1 gear ratio you would have to turn 6700-7000 RPMs. That's 6700 at 7% slip and over 7000 at 12% slip. Maybe steelspike will get on here and confirm my numbers? But something is not right. Your MPH was probably correct being you had 2 different instruments reading same numbers. My guess is you are over revving and your tach is not reading correctly? To hit 35 with a 12 pitch is almost impossible I believe unless you hit almost 7k RPM

 

[1080tommy1]

Re: Propping Yamaha C40tlrx

Now I am confused again on the prop. I was getting a reading in the 6800 to 7k range but thought it was because I had too many wraps of wire on the spark plug wire from the Tiny Tach. It said in the directions that 3 to 5 wraps was normal and if wrapped too many times, the tach could read high. I reduced the wraps from 5 to 3 and then got the 5720 reading. It seemed like the engine was way more powerful throughout the application of throttle but from about 5/8 throttle through full wot, there did not seem to be any speed gain but the engine could be heard to rev much higher. Maybe I will take her out again tomorrow if there are not the storms we are forecast to get and double check my readings. If I think about it and I was really at 7K I would need to come down 1500 rpm to be in range and that would be something like 8 degrees of pitch and I do not think this baby can turn a 22 pitch. Thanks for checking the numbers for me and I will report any new readings when I have them.

 

[Johny25]

Re: Propping Yamaha C40tlrx

Well I believe that when under propped an engine can have a runaway effect do to the lack of pitch. Meaning at a certain speed the prop starts to spin freely like there is little to no load on it. I believe I read this somewhere else. And when I had that 10 pitch stainless on my 30hp I backed off when I hit 6300 RPM, I believe if I would have held it there at WOT for another 10 seconds I could have hit 6500 or more? But at 6300 I said that's enough and let off. So no you can't run a 22 but it is odd that I calculated your funny RPM reading by doing the prop slip calculations. This would backup what I thought was happening. I just would hate to see you wreck your motor

The only way to hit 35mph with your gearcase at 5720 RPM is a 14 or 15 pitch with 7% and 13% slip.

 

[Johny25]

Re: Propping Yamaha C40tlrx

The 5/8 to full throttle RPM rev and no speed gain or feeling it pushing concerns me

 

[steelespike]

Re: Propping Yamaha C40tlrx

I think the ratio is 1.85 still not right though. The rpm must be higher than indicated assuming everything else is accurate
it may be turning as much as 6000.It appears a 14" may be in order.