Hay Guy's is my thinking right

[jcsercsa]

Hay all I got a new to me 1969 merc 125, I posted this in my thread and thought I should post it here also for all you prop guys, any help will greatly be appreciated !!


OK guys when I was out last night [ what a great night ] I finally have a working tack .the one is the tron is broke, I took the one out of the hustler, That one works !! well she was only hitting just under 4000 , so I know I need a few new wires on her and I think I can get a little more out of the throttle cable !! I am hitting 42 with a 3 blade alum, on there , and also need to clean the carbs on her , or should I say I am going to run some sea foam threw the next few tanks !!

OK now I don't really want to get her where we top out , that is 5500 RPM I have to check , shes a 69 125 hp merc !! I am going to start looking for a prop like a 21 at least stainless with a 2 step up in pitch i should pick up 400 rmp at least

I have a two blade that came with her, but it has a couple chip I need to get fixed !! he was telling me when I got the motor that he was getting 45 with the 3 blade bronze prop I was getting 40, with the 3 blade alum I was getting 42, With the 2 blade he said he was getting 55 out of her , with the two blade i should get 50 at least !! I am going to look for a 3 blade stainless steel a 21 !! what do you guys think !! I should post this also in the prop forum !! John

Ps she is on a 15.6 glastron tri-hull 1977
My load in the boat is 2 6gals gas tanks and 500 lbs in people.
prop is 12 3/4 inch 19 pitch make not on prop 3 blade alum
3 blade bronze we were only getting 40 mph didnt have tack working then 18pitch
running 42 at 4000 rpms with the alum
the hole boat when new was 925, should be about that now she's all new !

 

[hwsiii]

Re: Hay Guy's is my thinking right

J, you are ertainly headed in the wrong diretion by getting a higher pith prop,you will LOSE 400 RPM not GAIN it.

Welcome to Iboats prop forum and we will try to help you as much as we can but without more information no one can help you, and I will tell you I need more information than anyone else. But if you would like my help fill this form out and I will help you as much as I can, if you would prefer to not go to this much trouble there are other people who will gladly help you.

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
2a.What is the maximum recommended HP for your 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. manufacturer and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape a c ross the prop and the lower unit and then cut it between the prop and the housing and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a board under the keel and stiking out to the anti ventilation plate for a referene
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H

 

[jcsercsa]

Re: Hay Guy's is my thinking right

Thanks there hwsiii Thats why I asked !! then I got to head to a 17 pitch !!

Ok if i go to a 17 it still should gain speed ???

I did find out WOT is 4800 to 5300 RPMs and she has a 2 to 1 ratio on the leg

Thanks again John

 

[hwsiii]

Re: Hay Guy's is my thinking right

J, you have a problem of some kind, you need to fill the form out so I an analyze what is going on and you are not getting 42 MPH at 4,000 RPM with a 2:1 gear ratio and an 18" pith prop. That would be a -23% prop slip and that is impossibe.

Prop Slip

H

 

[jcsercsa]

Re: Hay Guy's is my thinking right

ITS A 19 pitch I had on her !!

4000 is what the tach said and the speedo was at 42 !!

I just want all the speed i can get out of her !!

Ok Thanks for the help , I will go try to answer then all !! John

 

[hwsiii]

Re: Hay Guy's is my thinking right

Even with a 19" pith you are showing a -17% prop slip.


H

 

[jcsercsa]

Re: Hay Guy's is my thinking right

Even with a 19" pith you are showing a -17% prop slip.


H

Would a crack in one of the blades make it slip that much. its really not all that big maybe 3/4 , its still flat and together ? John

 

[kick bass]

Re: Hay Guy's is my thinking right

John,
What are you using to check speed. If you are using a speedometer, they are wrong most of the time. The best thing they are for is to stop up a hole in the dash. You need to use a GPS. As for the tach, I don't understand what you said. Believe it or not a tach is one of the best investments you can make. Contact Iboats and they can help you. A tach is not a tach. each engine set up uses different configurations. (my new big word for today) What Mr. H is saying is, the numbers don't match. That old Merc is a good engine. she will run the 5500, BUT DO NOT GO ABOVE THAT! Take it from a guy who has blown his share of engines! Anyhow, If you will order a new tach from iBoats, borrow or buy a gps, then you can get the numbers to figure what prop you need. It will be cheaper than buying several props on a guess, and the Merc will last a lot longer when you don't over rev her or lug her, so the tach may save an engine! I am almost afraid to say what I am about to, because of the back lash. But in the hot summer your engine may run as much as 15% slower than it will in the fall when the weather cools to 60 degrees. So, when you buy a prop realize this.

 

[jcsercsa]

Re: Hay Guy's is my thinking right

John,
What are you using to check speed. If you are using a speedometer, they are wrong most of the time. The best thing they are for is to stop up a hole in the dash. You need to use a GPS. As for the tach, I don't understand what you said. Believe it or not a tach is one of the best investments you can make. Contact Iboats and they can help you. A tach is not a tach. each engine set up uses different configurations. (my new big word for today) What Mr. H is saying is, the numbers don't match. That old Merc is a good engine. she will run the 5500, BUT DO NOT GO ABOVE THAT! Take it from a guy who has blown his share of engines! Anyhow, If you will order a new tach from iBoats, borrow or buy a gps, then you can get the numbers to figure what prop you need. It will be cheaper than buying several props on a guess, and the Merc will last a lot longer when you don't over rev her or lug her, so the tach may save an engine! I am almost afraid to say what I am about to, because of the back lash. But in the hot summer your engine may run as much as 15% slower than it will in the fall when the weather cools to 60 degrees. So, when you buy a prop realize this.

thanks there Kick Bass, yep it is a speedometer i was getting my speed from, and she is hooked up to a tach, thats how I was getting the revs ! I will have to get me a gps !! thanks again for the help !! John

 

[kick bass]

Re: Hay Guy's is my thinking right

Thanks for the kind words. I see you want to tweek it. I ask the question again, Is the tach the correct one for your engine?
Hang in there You'll make it. But please fill out Mr. H's questions before you buy a Prop.

 

[jcsercsa]

Re: Hay Guy's is my thinking right

Thanks for the kind words. I see you want to tweek it. I ask the question again, Is the tach the correct one for your engine?
Hang in there You'll make it. But please fill out Mr. H's questions before you buy a Prop.

Yea I would hope its the right one !! I took it off the boat that it was on !! I had one on my tron but couldnt get it to work at all !!

OOHh yea she will make it, been 2 years now working on her !! just got to get a gps for her now !! John

 

[jcsercsa]

Re: Hay Guy's is my thinking right

Ok guys I was out yesterday and had a GPS and my speedo was dead on !! all up the dial !!

I had a bronz 18 pitch 3 blade . I was only getting 3300 rpms out of her !! and we were at 40 mph on the gps and the speedo !! she needs a tune up, she dont want to start out very well , have to slowly give it throttle till she kicks in then she goes !!!!!

Now I dont know if the tach is wrong or right it came with the motor !! it seams right when I am throttled down it runs about 250 rpms , and when it wot it doesnt sound like she wound out at 3300 with that prop on !!

With a 19 pitch on there alum i am getting 4000 rpms and 42 mph on there !!

I am running with 2 - 6 gal tanks and 3 guys about 700 lbs the boat is 825 lbs . I havent checked but the manual says she has a 2 to 1 leg on her !!

Do you still think there is some thing wrong , think he could have switched the leg on her !!

With that bronz prop on her I lost 700 rpms , it has to weigh 3 times what the alum weighs !! but I only lost 2 mph it does have a lot heaver cup in the bronz !!it has a lip allmost on the edge !! and if my thinking is right here I would have to go with a 13 pitch , If i take the numbers off the bronz !!

Now if I go off the alum 19 pitch, at 4000 rpms . to get to the 4800 to 5300 max rpms I could go with a 15 pitch !! I'm I right ??? lol John