15 or 17 pitch on a 89' 125 HP Force?

Hi everybody, I have a '89 18' Wellcraft CC that has the original '89 125 HP Force motor, the motor came with a 13.5 x 17, I bought a new one aluminum prop, same size, but the motor stays at around 4500 RPM WOT and I read somewhere that 5k-5500 is the recommended RPM at WOT, should I change the prop to a 15 pitch? or I should just keep it that way, motor runs great and since it's an old motor I don't wanna put too much stress on it..any suggestions? thanks.

Daniel

Re: 15 or 17 pitch on a 89' 125 HP Force?

Too much stress is probably more of a concern with the 4500 than the 5500 . . . With that said, I don't think you are going to pick up 500 - 1000 RPM with a 2 inch change.

I am always concerned about the tachometer with these threads, so I'd like to know it is accurate. How long have you had the boat? Has it ever run faster before? How fast are you going? GPS?

Re: 15 or 17 pitch on a 89' 125 HP Force?

Dmas, if I knew the base weight of the boat I could be more helpful. And what RPM did it turn with the old 17" Pitch prop, and exactly which manufacturer and model of prop, diameter you just bought.

H

Re: 15 or 17 pitch on a 89' 125 HP Force?

I would go with droping your pitch to get the proper RPM, it isnt more stress for it to run at the rated RPM no mater the engines age. It is good for it, at WOT it should always run in those RPM. I prefer that mine runs near the higher end so that i am hitting the top of its power curve and getting the best power i can with out abusing the engine. Good Luck.

Also if the new prop and old prop are the same sizes but different blade count you will also see a differance in RPM.

Re: 15 or 17 pitch on a 89' 125 HP Force?

QC said:

Too much stress is probably more of a concern with the 4500 than the 5500 . . . With that said, I don't think you are going to pick up 500 - 1000 RPM with a 2 inch change.

I am always concerned about the tachometer with these threads, so I'd like to know it is accurate. How long have you had the boat? Has it ever run faster before? How fast are you going? GPS?

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thanks for your reply, I dont know the max speed before, since I replaced the propeller I soon as I got the boat because the old one was in really bad shape. and the speed by GPS is 33-34 MPH

Re: 15 or 17 pitch on a 89' 125 HP Force?

hwsiii said:

Dmas, if I knew the base weight of the boat I could be more helpful. And what RPM did it turn with the old 17" Pitch prop, and exactly which manufacturer and model of prop, diameter you just bought.

H

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the boat weights 2300 lbs dry, I don't know the RPM's with the old propeller since I replaced the propeller when I got the boat, I bought a brand new Michigan aluminun propeller from this site.

Re: 15 or 17 pitch on a 89' 125 HP Force?

solar7647 said:

I would go with droping your pitch to get the proper RPM, it isnt more stress for it to run at the rated RPM no mater the engines age. It is good for it, at WOT it should always run in those RPM. I prefer that mine runs near the higher end so that i am hitting the top of its power curve and getting the best power i can with out abusing the engine. Good Luck.

Also if the new prop and old prop are the same sizes but different blade count you will also see a differance in RPM.

Click to expand...

both propeller are exactly the same, I have had the boat since feb 2007 it's just that I dont know if I should get close to the factory RPM's or keep it this way..thanks to all for your advice.

Re: 15 or 17 pitch on a 89' 125 HP Force?

You NEED to be within the range, trimmed for maximum speed and loaded like you would normally use her. I'd verify the tach and then go to work to determine if the motor is doing what it is supposed to (compression check, tune-up etc.). If the motor and tach are right, you are over propped.

Re: 15 or 17 pitch on a 89' 125 HP Force?

Dmas, I show your motor with a 1.73 gear ratio and you said the base boat weight was 2,300 pounds. I used these numbers in my program to calculate prop pitch and speed as well as allup boat weight of about 3,200 pounds. If these are wrong my calculations will be wrong. You should be running a 14" Pitch Prop in the same manufacturer and model of prop you have now but at about 36 MPH and about 5,350 RPM. Here are the calculations I used. I used 5,375 RPM because ther3e were two motors with one at 5,250 RPM and the other at 5,500 RPM.

Boat and Motor Info



Prop Picker



Prop Change

Re: 15 or 17 pitch on a 89' 125 HP Force?

How did you get the hull weight? 2300 dry seems rather heavy for an 18 foot hull. My 21 foot cuddy only weighs 1600 pounds dry weight as specified by the factory and about 23-2500 with engine and load.

Before I went to a 15 pitch prop which may very well reduce your top speed, I would try raising the engine on the transom an inch. Are you trimming the engine up or out for maximum performace at cruising speed?

HOWEVER: I have seen 18 foot hulls with 125 Force engines that were not capable of turning anything more than a 15 pitch prop. Shame! That 1.73 lower really does not let the engine rev up.

Another hint: Pre Mercury Force engines respond very well to a lower unit change. If you get a 2 to 1 ratio gearcase from an 85 or 90 and keep the 17 pitch prop, you will most likely see a 4-500 RPM gain AND a couple of MPH more top speed. Turning the higher pitched prop slower results in less horsepower wasted in turning the prop and more horsepower delivered to moving the boat. And a used lower will not cost much more than a new prop.

Changing to a stainless prop will also yeild at least a couple of hundred RPM more if the prop is not cupped. If it is cupped, RPM will stay the same but it will hold the water better. You may be able ro trim out further which will raise the bow and may result in increased top speed.

Re: 15 or 17 pitch on a 89' 125 HP Force?

QC said:

You NEED to be within the range, trimmed for maximum speed and loaded like you would normally use her. I'd verify the tach and then go to work to determine if the motor is doing what it is supposed to (compression check, tune-up etc.). If the motor and tach are right, you are over propped.

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that could be the case, compression is fine and the motor was tuned up recently.

hwsiii said:

Dmas, I show your motor with a 1.73 gear ratio and you said the base boat weight was 2,300 pounds. I used these numbers in my program to calculate prop pitch and speed as well as allup boat weight of about 3,200 pounds. If these are wrong my calculations will be wrong. You should be running a 14" Pitch Prop in the same manufacturer and model of prop you have now but at about 36 MPH and about 5,350 RPM. Here are the calculations I used. I used 5,375 RPM because ther3e were two motors with one at 5,250 RPM and the other at 5,500 RPM.

Boat and Motor Info

Prop Picker

Prop Change

Click to expand...

thanks for the info, all the specs are about right, I may have to try a 13 or 15, because they don't make them in 14 for this motor.

Frank Acampora said:

How did you get the hull weight? 2300 dry seems rather heavy for an 18 foot hull. My 21 foot cuddy only weighs 1600 pounds dry weight as specified by the factory and about 23-2500 with engine and load.

Before I went to a 15 pitch prop which may very well reduce your top speed, I would try raising the engine on the transom an inch. Are you trimming the engine up or out for maximum performace at cruising speed?

HOWEVER: I have seen 18 foot hulls with 125 Force engines that were not capable of turning anything more than a 15 pitch prop. Shame! That 1.73 lower really does not let the engine rev up.

Another hint: Pre Mercury Force engines respond very well to a lower unit change. If you get a 2 to 1 ratio gearcase from an 85 or 90 and keep the 17 pitch prop, you will most likely see a 4-500 RPM gain AND a couple of MPH more top speed. Turning the higher pitched prop slower results in less horsepower wasted in turning the prop and more horsepower delivered to moving the boat. And a used lower will not cost much more than a new prop.

Changing to a stainless prop will also yeild at least a couple of hundred RPM more if the prop is not cupped. If it is cupped, RPM will stay the same but it will hold the water better. You may be able ro trim out further which will raise the bow and may result in increased top speed.

Click to expand...

I got the weight from a this flyer http://www.wellcraftv20.com/gallery/cat_1989/016_wellcraft from my friends at the V20 forum.

does any gearcase from a 85 or 90 force will work in my engine?

and I have tried to get a SS propeller but they are ridiculously expensive.

Re: 15 or 17 pitch on a 89' 125 HP Force?

Then you need to go with the 15' Pitch.

H

Re: 15 or 17 pitch on a 89' 125 HP Force?

thanks for your help H

Re: 15 or 17 pitch on a 89' 125 HP Force?

You are very welome Dmas. Please post your RPM and speed for my database.

H

Re: 15 or 17 pitch on a 89' 125 HP Force?

Hi H

I just bought off ebay a 13-3/4 x 15 new michigan aluminum prop. very cheap, it will be here by next weekend, I will install it and try it out, as soon as I get the numbers I will let you know, thanks.

Re: 15 or 17 pitch on a 89' 125 HP Force?

Thank you very much.

H

Re: 15 or 17 pitch on a 89' 125 HP Force?

Hi hwsii

sorry it took me so long to answer you, but I run into some problems with the steering and had to replace helm and cable, I finally took the boat out on the water a couple of weeks ago using the 15 pitch propeller.

the RPM went up to from 4500 to 5000 at WOT, but the speed went down from 34 to 30 and also the boat didn't plane it felt sluggish, I got not idea why would this be, I'm going back to my old 17 pitch, thanks a lot guys for your help.

Re: 15 or 17 pitch on a 89' 125 HP Force?

Don't listen to Frank! Hate to say it, BUT: "I told you a 15 pitch might reduce your top speed." I pretty much knew it would, but didn't want to say it outright because there is always that one engine/boat that makes a liar out of me.

Early model Chrysler and Force 85 lowers were also 1.73 and only the 90s had 2 to 1 ratio. I believe the later model 85 and 90 were both 2 to 1. Just have the seller check before you buy--have him spin the drive shaft 1 turn. If the prop shaft goes more than 1/2 turn, then it is NOT 2 to 1 ratio.

If you go to the 2 to 1 and keep your 17 pitch prop, you will increase 500 RPM. AND, You SHOULD see a corresponding increase on MPH. If you go to a 19 pitch you will see the same RPM and probably gain 3-5 MPH. That, I would not guarantee though-- all I can say is that it worked for me. I have done this on so many Chrysler and Force engines--- I NEVER keep a 1.73 unit on any engine unless I don't have a replacement for it. It just strangles the performance.

Re: 15 or 17 pitch on a 89' 125 HP Force?

thanks Frank I will try that, I was looking for one of those 90hp lower units on ebay but people is just crazy they want from $500 to $700 for them, I could probably get a whole engine for that money.



and btw, I should've had listen to you!!!

Re: 15 or 17 pitch on a 89' 125 HP Force?

Dmas, thank you very much for posting your new numbers for me, I appreciate that.
Now let's talk about your new prop, there is something wrong with it. Whenever you change from a higher pitch to a lower pitch you should always have an increase in hole shot and the acceleration should also increase from a lower speed to a higher speed, this is the normal way props work. Frank I will agree with the theory that you can have a possible loss of top speed, but you should not lose anywhere near 4 MPH from 34 MPH with the same make and model of prop with a 2" lower pitch change and the commensurate increase in RPM , because the RPM increase makes up for most of theoretical speed loss from pitch.
Dmas, I ran my numbers with the 17" Prop and the 15" prop, you had a loss of 4 MPH, but you also went from a prop slip of 19% with the 17" prop to a prop slip with the new 15" prop of 27%. That is not normal, when you go down in pitch normally you have an increase in diameter, and we did in this case as well, we had in increase in diameter from 13.5" to 13.75" which I assume is the Vortex prop, is that what you had before or did you have the Michigan Match. When you have an increase in diameter that means there is more blade surface area on the new prop, which means that the prop slip should go DOWN not UP, as has happened in this case.
The reason I use mathematics to figure out what prop pitch changes are to be expected is because it is Not empirical evidence. What we see when this prop loses 4 MPH is not what should happen, and there is a reason for that.
There is something wrong with this particular prop, I don't know what it is but it is a defect of some kind, believe me. I certainly make errors, ONLY if I am breathing. LOL But, this should not have happened.
I have made many prop pitch recommendations here, and anytime anybody uses the same make and model of prop and decreases the pitch there is ALWAYS an increase in hole shot and acceleration as well as a DECREASE in prop slip. Yours is exactly the opposite of that by your statements, and that is not the way it should be, that is why I say there is a defect in the prop, as well as my numbers show what has actually happened.
My recommendation is to take the prop to a reputable prop shop and have them put it on the pitch block and find the problem, and if you can't do that then sell it on ebay and buy another prop in the same model as you had before in a 15" pitch.
Here are my numbers and why I say there is a problem with this prop. You should have lost less than 1 MPH.

Prop Slip




H