Theretics of my new prop

[olivialyn11]

Ok, So im new to the boat world, got a 19' galaxy bowrider and a 1975 evinrude 85hp 13tooth spline

Ive been looking into the origional prop that came on the motor and it seams that it was a 13-3/4x23 and a 13-3/4x21

Ive spent about 6 months rebuilding the entire boat, new floor, transom, seats, dash, paint, and now the motor. Ive been reading about props and motors and unlike a car, the RPMs of the boat are what determine the speed, so my two questions?

1: If the speed in wich the prop turns and the pitch determins the speed in wich the boat travels, why do people boast a higher HP motor when they are just adding weight?

2: Is there any place to find a 4 blad prop with the same specs? 13-3/4x23, my goal is to keep my speed and add bite to the prop.

Please inform me with what you can, as though money is not an option and no amout of work is too much.

 

[zagger]

Re: Theretics of my new prop

Think of a boat as a car driving up a high grade, constantly fighting gravity. With given HP the car will reach certain speed and no more (top speed = max attained wheel rpm). By installing bigger (taller) tire the same rpm will produce faster overall speed but will require more HP to keep them turning at said rpm. Adding a blade has similar effect as going up an even steeper hill (drag & slip) and increasing the pitch is like putting on a bigger (taller) tire. Best thing is to get the new set up wet, get the numbers as asked in "read this first" on top of page and report back.

 

[olivialyn11]

Re: Theretics of my new prop

Think of a boat as a car driving up a high grade, constantly fighting gravity. With given HP the car will reach certain speed and no more (top speed = max attained wheel rpm). By installing bigger (taller) tire the same rpm will produce faster overall speed but will require more HP to keep them turning at said rpm. Adding a blade has similar effect as going up an even steeper hill (drag & slip) and increasing the pitch is like putting on a bigger (taller) tire. Best thing is to get the new set up wet, get the numbers as asked in "read this first" on top of page and report back.

You cant compare the two, tires do not spin freely like a prop does in water. Your compairing two compleatly different topics, in a CAR when you add taller tires or climb a steeper grade more HP, is a bonus yes but its not required added hp allows you to tow a bigger object and allow you faster accelleration, but has no bearing on the top speed of the vehicle unless you add in the factor of gears, That same car will with a smaller engine will acheave the same top speed just over a longer period of time. Thats why a CAR has gears to compensate to allow for more speed or ability to move more weight.

Thats like compairing a bike that has 88hp vs a truck with 415hp in a race and the 88hp bike winnning when the truck obviously has taller tires and a bigger motor.

I understand that a bigger prop allows for more bite, to help get the boat in motion, same with use of a 4 blade vs a 3 blade. You have more surface to push the water to get it moving to full use of the RPMs, and the pitch is the over RPM rateing to multipluy it for speed. ( i read that "pitch" is the estimated space the boat will travel per rpm)

Nothings wet yet becase i have no prop for the motor or baseline specs on the motor, still have to drop in electrical wireing for ignition, builg pump, lights, tach, and other accs.

 

[steelespike]

Re: Theretics of my new prop

Your motor has a 2.00 gear ratio.The motor turns twice for each revolution of the prop.Very likely your props are too big.
A test run with the 21 once you get setup may tell us what prop you need.You'll be a little underpowered with an 85 but
the right prop will get you going pretty good.
Keep in mind as you increase pitch you take the motor closer to an overdrive like gear and its lack of power.
The key is a prop that gets you close to the wot rated rpm.probably about 5,500.

 

[sublauxation]

Re: Theretics of my new prop

If the prop just spun freely in the water none of us would get very far. As steele said, a 21 would likely be to much prop, and a 4 blade 23 pitch is definitely going to be to much. Since you don't own one yet find a shop that will let you test first, or I'd start with something like a 19 pitch prop, or even a 17. I know hull design plays a part but I have a 90 hp on a lighter AL hull with a 17 pitch AL prop works well. It's a longshot (it's a big world out there) but if you're near Minneapolis I have a 19 and a 17 pitch prop you could try.

 

[olivialyn11]

Re: Theretics of my new prop

to spin free, equivilent of 900hp mustang on wet roads, spinning the tires, its just a waist untill it finaly grabs.

so even though the motor came off a boat that was 17' with a stock 13-3/4x23 its to much of a reach for my boat? i dont mind buying a prop, but i just am trying to get the most out of my trial prop, no one seams to have on hand one for my old motor, 13spline so i have to fork up the cash to play around.... is there a kit i can get to change the lower unit to maybe a newer style of spline? or have a shaft machined to fit?

 

[sublauxation]

Re: Theretics of my new prop

What came "stock" on an outboard doesn't really mean anything, especially when you put it on a different boat.

The beauty of a prop is you can adjust the pitch or add cup or another blade to make it grab the water. That's also the downfall as you may find in trying to find the right one.

My '06 Etec is a 13 spline shaft, pretty sure it takes the same prop as your '75, so props shouldn't be that hard to find. Check around on EBAY, but be sure to check serial numbers to make sure it fits. Turning Point makes what is supposed to be a decent and cheap AL prop and may be a good starting point if you want to buy new. They do tend to act like a slightly bigger pitch than they actually are.

Just to give something slightly comparable, My boat hull weighs maybe 1100 lbs. Assuming yours is glass and a foot longer likely weighs a lot more. Like your boat, mine is arguably "underpowered." I'm actually using a 15 pitch cupped stainless on my 90 when skiing or when I have the whole family on board. My 17 pitch Turning Point acts just like the 19 pitch AL that came "stock" with my ETEC. Both are too much pitch unless I'm alone. Others are more experienced than me but I'd be surprised if your end results wouldn't at least be similar.

 

[Frank Acampora]

Re: Theretics of my new prop

You completely misunderstand the theoretical prop design parameters. Yes in one respect you can not compare it to autos but then again, you definitely can. The engine can only produce so much horsepower. Now put bigger tires or a higher rear end on a car and it will not have a faster top speed because it takes so much horsepower to overcome rolling resistance and wind resistance.

You can not put on a "bigger" prop (in this case, more pitch)and expect more speed because it takes so much horsepower to move the boat at its top speed. However, the available horsepower is set for the engine. The BEST you can hope for with a bigger prop is to equal the top speed and in many cases, it will indeed be lower. Modify the engine to make more hp (just as with cars) and it can now swing a higher pitched prop

The only time you can expect more speed from a "bigger" prop is if the existing prop is too low pitched and the engine revs up past its power band.--Just like autos, torque and horsepower curves cross at a certain RPM and past this horsepower starts to decline.

A propeller screws itself through the water and just as with an airplane, it takes a given amount of horsepower to turn the prop

 

[zagger]

Re: Theretics of my new prop

You cant compare the two, tires do not spin freely like a prop does in water. Your compairing two compleatly different topics, in a CAR when you add taller tires or climb a steeper grade more HP, is a bonus yes but its not required added hp allows you to tow a bigger object and allow you faster accelleration, but has no bearing on the top speed of the vehicle unless you add in the factor of gears, That same car will with a smaller engine will acheave the same top speed just over a longer period of time. Thats why a CAR has gears to compensate to allow for more speed or ability to move more weight.

Can't argue with that logic. Sounds like you have it figured out, good luck.

 

[steelespike]

Re: Theretics of my new prop

If you have the 21" prop you mentioned when the time comes do some test runs and get us as much info as possible
from " read this first" at the top of the page, gps speed is best.if you need a tach a Tinytach is easy to install and can be used on almost any motor.With the info we can tell if your setup is working and get an idea what prop will be close to what you need.

 

[sublauxation]

Re: Theretics of my new prop

If you have the 21" prop you mentioned when the time comes do some test runs and get us as much info as possible
from " read this first" at the top of the page, gps speed is best.if you need a tach a Tinytach is easy to install and can be used on almost any motor.With the info we can tell if your setup is working and get an idea what prop will be close to what you need.

If I understand right, he doesn't have a prop yet and is wondering where to start.

 

[olivialyn11]

Re: Theretics of my new prop

Yes in one respect you can not compare it to autos but then again, you definitely can. The engine can only produce so much horsepower. Now put bigger tires or a higher rear end on a car and it will not have a faster top speed because it takes so much horsepower to overcome rolling resistance and wind resistance.

Please stop compareing cars to boats because its obvious that you have no clue what HP truely is for a car. The more ponines you have under the hood allow for faster acceleration, and with a major difference in HP you can set up your gears in the trany with different ratios to allow for more speed or torque at different RPMs. Purpose built race motors have cams that kick in at different RPMs to allow better use of timing, valve placement, and pressure on the fule system.

Your comparison would state that the best motor for a boat would be a 400HP diesel motor that spins only 3k rRPMs vs a 112hp gas motor capable of turning over 15k RPMs

If I understand right, he doesn't have a prop yet and is wondering where to start.

That is correct, im so glad you've actually read this thread. The prop i have is compleatly destroyed and can not be used for any kind of testing except maybe suitable anchor replacements.

The prop on the motor is a 23" Pitch and i can get a stock replacement online in a 1" smaller size, but im looking for a 4 blade version i cant find anything except for 19" 4 blades to fit a 13 spline shaft

http://www.marineengine.com/newparts/part_details.php?pnum=OMC0384703&ptype=&Engine=&Model=

or if anyone knows a way to convert the 13splines into a different shaft, via new rods in the lower unit i would be happy to entertain the idea

 

[Frank Acampora]

Re: Theretics of my new prop

Uh! OK Since you know all about horsepower and cars, I will shut up--You don't need my knowledge or help.

However, before I do leave you, HORSEPOWER is defined as 550 Foot pounds per second. That is, It takes one horsepower to move one pound 550 feet in one second. Or 550 pounds one foot in one second etc. It doesn't matter if you are trying to move a boat, car, plane, or train. So yes, they all can be compared. That's it BYE!

 

[olivialyn11]

Re: Theretics of my new prop

Uh! OK Since you know all about horsepower and cars, I will shut up--You don't need my knowledge or help.

Honestly i dont mean any disrespect but i build and race cars for drifting and drag, so i know more then just put gas in it and go.

My consern is to find a prop with some numbers that im looking for, and if its not made what i can do to get something to close.

The same motor is enough to move my cousins 20' but hes running a 11x19" 3blade, and tops around 45mph.

Im in search of a 13-3/4x23" 4blade, on my last post i showed a link where i can get a 12x23" 3blade but im just trying to find a equal prop to what im looking for or.... something close

 

[sublauxation]

Re: Theretics of my new prop

Honestly i dont mean any disrespect but i build and race cars for drifting and drag, so i know more then just put gas in it and go.

My consern is to find a prop with some numbers that im looking for, and if its not made what i can do to get something to close.

The same motor is enough to move my cousins 20' but hes running a 11x19" 3blade, and tops around 45mph.

Im in search of a 13-3/4x23" 4blade, on my last post i showed a link where i can get a 12x23" 3blade but im just trying to find a equal prop to what im looking for or.... something close

You're conversing here with some of the most knowledgeable boating people out there. They are trying to help explain that your take on how a prop works isn't quite right. Discounting what they say wont get you what you need. If you simply want a 23 pitch 13 spline prop repost asking for one. If you want the right prop, then I'd listen to what they're saying. You need RPMs with a prop to get a baseline. Personally I recommend stealing your cousins 19 pitch prop because he could probably use a 15 pitch anyways with an 85 on a 20ft glass boat. The props you are looking for will be to much. But as I said, if that's what you want, repost that simple and specific question and somebody will help you find one. I have a hard time believing they are hard to find since my 6 year old Evinrude has a 13 spline shaft.

 

[steelespike]

Re: Theretics of my new prop

I'll try one more time.A 23" prop absolutely will not work right on your boat not even close.
I just checked new Evinrude 90 hp same gear ratio on 4 different glass boats 1200lbs to 1800lbs.
3 used 15" pitch,one a 16" speeds range from 33 to 35 4875 rpm to 5450rpm
put a 23" prop on any of those boats will subtract about 1600 rpm
Your cousins boat doesn't do 45 and the 19" prop is at least 3 or 4" too big.
I don't pretend to know all there is about props but I do know what I've seen here on boats and elsewhere
Please note the number of my posts.

 

[sublauxation]

Re: Theretics of my new prop

The prop on the motor is a 23" Pitch and i can get a stock replacement online in a 1" smaller size, but im looking for a 4 blade version i cant find anything except for 19" 4 blades to fit a 13 spline shaft

Picking the proper sized prop is largely based on RPMs. If you put on too big a prop RPMs drop. Running it with RPMs too low is bad.

I think the reason you can't find what you're looking for is because 13 spline shafts only went on certain sized outboards. Prop makers are pretty smart, they would never make a 23 pitch 4 blade prop for a 90 hp outboard because there is absolutely no proper application for it.

I think what you need is likely a 17 pitch give or take depending on the specific properties of the prop you choose.

 

[topglock]

Re: Theretics of my new prop

Weight, hull design and available power are going to determine which prop you "need" for the application. Since you have no hard numbers on what works on your boat and you seem to be set on a 4 blade prop, Solas offers a 19p AMITA for under $100. They also offer a 17p for the same price. My "guess" would be the 17p, but I confess to being relatively new to prop design. What I do know is, when I needed help finding the right prop for my application, these same guys pretty much hit it right on the nose. If you want your rig to perform at its best, listen to what these guys are saying...

 

[olivialyn11]

Re: Theretics of my new prop

However, before I do leave you, HORSEPOWER is defined as 550 Foot pounds per second. That is, It takes one horsepower to move one pound 550 feet in one second. Or 550 pounds one foot in one second etc. It doesn't matter if you are trying to move a boat, car, plane, or train. So yes, they all can be compared. That's it BYE!

Im fully aware what horse power is but the same reason my ford ranger can out pull full size pickups when im running 340hp vs their 410hp because of my gearing, or the same reason a civic with 200hp can out run a car with 320hp, they are geared to do so, raw hp at the crank is not the same as hp at the wheels, the same way with a boat, after the ratio takes place in the unit that effects how fast it spins.
hp in a car is bennificial when your talking about accelleration, more torque is a benifit when your towing more... HP is not everything. The same reason a mac truck can pull 80,000lbs wether it has 650hp vs 800hp because the trany is geared for the diofference to compensate with torque.

 

[olivialyn11]

Re: Theretics of my new prop

thanks topglock, ill look further into the amita prop.