Choosing a prop

[iamslotman]

I have a 2001 Logic 15' boat with a 40 HP Johnson outboard. Upon takeoff, the engine seems weak until we put all the weight up front and it takes about 2 minutes to finally plane out and then it is very fast. It has a stock prop and I was wondering if a new prop would give it better take off power? I see them for sale in S/S and aluminum and they have many degree choices. Can anyone tell me what type will do best?

 

[wilde1j]

Re: Choosing a prop

What pitch prop are you using now?

 

[Silvertip]

Re: Choosing a prop

There is no such thing as a "stock" prop. In fact the engine is shipped from the manufacturer without one since they have no idea what boat the engine will be installed on. A V-hull, pontoon, work boat, barge all take different props. My guess is the current prop is a 12-1/4 x 15P. But with absolutely zero performance data from you we cannot help. We need wide open throttle rpm with what you consider an average load. If you don't have a tach you need to install one otherwise you are probably wasting money on props. The correct prop is one that allows that engine to rev at 5500 (even 5800) wide open with what you consider an average load. If the engine can't reach that rpm you need to decrease pitch. If it exceeds that rpm you need to drop back in pitch. Just so you know, pitch is measured in "inches" -- not "degrees".

 

[iamslotman]

Re: Choosing a prop

I don't know the difference between decreasing pitch and dropping the pitch but tuesday we will see what it does at full speed rpm and I will get back to you. I will find the additional date also by then. Thanks for the fast replies.

 

[steelespike]

Re: Choosing a prop

Welcome,
You will find the prop size on the barrel or under the prop nut.
We will need the details on your boat aluminum?fiberglass? dry weight if possible.
higher pitch numbers are harder to turn but if the motor can reach rpm it may be faster with poor hole shot but more speed.Lower numbers are easier to turn better hole shot perhaps less speed.Just the right prop will produce good hole shot and speed. Though with some comprimise.Its like trying to find a single gear in a car to do everything.If you read the stickies at the top of the page and the helps on the prop stire page it will help you better understand prop selection.We are here to help you along.

 

[hooked422]

Re: Choosing a prop

sorry to hi jack your thread. but i have a question on the same subjet. I have a 1973 6hp evinrude it has a plastic prop and it broke today. I was wondering what prop to get the old one is 7 and a quater pitch metal if it is available. Its on a 13 aluminum lowe
jon boat it

 

[iamslotman]

Re: Choosing a prop

OK here are the stats from todays trip....
Max speed was 24 MPH
Max RPM was 4000
Boat was at max weight of passengers (600#)
Max outboard size is listed as 60HP
I alone am nearly 300 so I need smaller friends
Prop on the outboard now says Solas OC3X12x17 (aluminum 3 blade)
Spare used prop that came as extra with the boat says T10666 11 3/4x17 (also aluminum 3 blade)
The boat takes about 8-10 minutes at full throttle to hit max speed. If you live on Long Island, we left West Islip Marina and never got it going much until after we went under the Robert Moses Bridge and were heading toward the Coast Guard Station. Until then if felt like it was dragging a heavy load (me?) Coming back we hit many 2'+ waves and it prevented us from ever reaching the speed we had on the way out.
Possibly related is the fact that raising the fast idle lever on the control box makes almost no increase in RPM's. Maybe it goes up to 1800 from 1000 so I am wondering if my problem may be partially that the engine has linkage problems too. This I think is a different cable than the throttle and may be a seperate issue. Thanks guys. I would love to see this boat hit 30+ and plane out.

 

[wilde1j]

Re: Choosing a prop

Fast idle shouldn't have anything to do with what you describe.

1st, your motor is marginal HP for your boat. 50 to 60 would perform way better.

RPM should be ~ 5500 @ WOT and you're not close.

You've got a 12x17 prop. I would start with a 12x13 prop (the first number is diameter, the second pitch in inches). That should up the RPM ~ 800 rpm and get your boat on plane way faster. If you can find an 11" pitch, get that. But first, take the moto0r cover off and remove the air silencer so you can observe the throttle plates. With the motor not running, have someone rotate the prop while you advance the throttle to max. The plates should be horizontal. If they are, close everything up and get a much lower pitch prop on your motor.

 

[iamslotman]

Re: Choosing a prop

I did the plate test you recommended and the plates actually were slightly tilted past horizontal until I backed off the throttle just a drop. This matches the performance we had on the water when I got the max RPM when I tapped the throttle back just a tad from full.
Another thought is that the boat came with a bolted on "whale tail" black hard plastic fin mounted horizontal just an inch above the top of the prop. Someone at the marina told me today that I should remove it because it causes the boat to lose top end speed. Can I have another opinion on that? Other than going faster, which is everyones wish, I am hoping this little boat can pull a grandchild on a tube if we get the right combination.
Also, I removed the control cable box and found a loose 3/8 nut behind the pivot for the fast idle lever and once tightened caused a much bigger throw on the cable.

 

[wilde1j]

Re: Choosing a prop

The "whale tail" will lose a few MPH at the top end for sure BC of added drag.

A much lower pitch prop is what you need to get more go power out of the hole. You won't pull anything well with your current prop.

 

[steelespike]

Re: Choosing a prop

I have a feeling your max speed of 24 was in error if it does turn out to be right it is exceptional considering the total weight and prop size. I agree with the initial prop size suggested by wilde1 I would add that removing the foil will likely slow hole shot but weighht distribution and motor trim will help.Some sort of trim tabs are a much better choice.The antivent plate just above the prop should be visable on plane.
you should raise the motor till visable.In the extreme this could net 2 to 3 mph.

 

[iamslotman]

Re: Choosing a prop

the speed of 24mph was based solely on the speedometer on the boat which I hear is unreliable. That was after we chugged along for nearly 10 minutes at full throttle and had everyone except the driver way up front. We had a gps on the droid phone but we were getting way too wet to try using it. It was as if someone was tossing bucket after bucket of water at us every 5 seconds. Once we hit top speed the water spray stopped. I think I will leave the spoiler on until after I change the prop and see how it reacts, then I will remove it and see the difference. I am guessing the 13" pitch is the way to go. I don't want to go "overboard" and buy one with too extreme a difference. Thanks for all this help. Now I have to jump to a boat forum and see about this mysterious door in the floor that leads to the hull but seems useless for storage because it runs under the floor and anything in there will slide into the rear where you cannot retrieve it and it gets wet.

 

[wilde1j]

Re: Choosing a prop

If you can find an 11" pitch, I would opt for that. If you don't get the RPM up 5500+, you risk long term engine damage due to 'lugging' the motor. You need to pick up 1500 RPM minimum, with ~ 200 RPM/in. of pitch, that calculates to ~ 8" of pitch less you need, but they don't make many props pitched that low. You're running "extreme" now ... extremely high pitch for your motor/boat combo!

Your basic problem is the motor HP is too low for your boat, a common problem with pontoons, since most buyers are focused on price, and a too small motor is very easy way for the dealer to accommodate.

 

[iamslotman]

Re: Choosing a prop

Just when I thought I had all the answers.... I seem to have trouble finding a 12" prop with an 11" pitch. I went to look at my old prop to see if a 12 1/2 would fit (it wouldn't) and I noticed that all 3 blades of my current prop have a slight "ding" in the exact same place as if it hit something that slightly bent each blade. The dent is on the outboard edge about halfway up the blade. Maybe this is causing the low rpm's and I should try a less radical change in pitch? I still have the extra prop that is a 11 1/2 size with a 17" pitch, but I was hesitant to put that one on and waste another day on the water bogging. I did see a 10 1/2" pitch in the 12" size but I don't want to lose too much top end and overrev the motor. Sorry to be redundant.

 

[wilde1j]

Re: Choosing a prop

iboats has props down to 8" pitch ... I assumed motor is a 2001 like the boat. I don't know where the 12" diameter is from ... all the iboats props are 13' dia. for your motor in 11" pitch.

see here: http://boatpropellers.iboats.com/Ev...andard&**********=400864395&*******=117646989

There no way an 11" pitch will "overrev" your motor (arithmetic: 7"x200rpm/in.= 1400 is the most increase you can expect, probably less. This will out you in the 5100 to 5400 range max. You motor can easily tolerate 5800 or more w/o any issues.

 

[iamslotman]

Re: Choosing a prop

the motor it came with is a rebuilt 89 Johnson 40 HP. It has a 12" prop on it now and my index finger just fits snugly between the top of the prop blade and the piece of the lower unit just above it. Also the oiler was disconnected when it was overhauled and I have been mixing 50-1 as instructed by the previous owner

 

[iamslotman]

Re: Choosing a prop

thank you all again for the assistance choosing a prop. I had no luck online finding a 12x11 or 12x13 propo and then I found a store on Long Island that only sells props (Bossler and Sweeney) and they had an
11-7/8"x13" prop in stock. Michigan Vortex. According to their books it was the correct diameter. They also said if it didn't perform perfectly, they would exchange it for another which impressed me. Yesterday it was running at 5000 WOT with the engine completely down and would raise to 5500-6000when I would tilt it up. This was with about 80# less than a normal crew. Top speed according to the boat speedo was 24MPH but I want to test it with a gps next time because the boat speedo doesn't register anything until 20 MPH and it seemed like we were cruising much faster than 24. It planes out really fast and now we can cruise without tasting every wave we encounter. Next trip I will remove that "whale tail" looking spoiler and see if the top speed improves. Thanks again for helping.

 

[sethjon]

Re: Choosing a prop

Without knowing what you have its hard to say. Remove the one you have and see what the pitch is. I would drop 2" on pitch, keep the same diameter and an aluminum will be fine and I think you'll be a happy camper.

 

[SRC8778]

Re: Choosing a prop

FYI, 200 rpm per inch of pitch is merely a very loose generalization. That will change greatly with different pitches and rpm ranges. Changing the pitch of a prop is like changing the gearing in a car. You are dealing with ratios not set amounts. As stated already, a lower pitch will give you more power out of the hole and a higher pitch, until you run out of power, will give you a higher top speed.

In theory, and not accounting for the horsepower rising with the rpms, 4000 rpm with a 17 pitch equates to 5500 rpm with a 12.36 pitch.

4000/5500x17=12.36

Based on that, I personally would have have tried a 13 pitch just as you did and appearantly you made a good choice. Glad to hear it is working out for you.



Hooked422, if you were happy with the performance of the old prop, I would replace it with one of the same pitch. If it was sluggish, go with a slightly lower pitch and if it was overreving, go with a slightly higher pitch.