how much can you do with prop options

[limitout]

Re: how much can you do with prop options

heck yes... if you can get loaners that is perfect..... around here you have to buy em and as long as they don't get one single mark on em you can exchange..... I would say go ahead and test with the current prop 1: because you already have it so it costs you nothing except a few minutes of your time, and 2: so that you (and we) will know what and how much the improvement is.....

well I don't know if they still do that here but im assuming it is, the way it used to work is you buy the prop since they all cost the same then you start with an average rule of thumb pitch, then you swap it out up or down in pitch size until you find just the right pitch, but as you say, provided its not damaged in any way.

and just to be clear about it, for the prop I do select it should be ... what sized rake? 0? 15? 20? these are the only rake sizes I have seen mentioned so far on the prop websites I have seen.

I never new rake mattered and its hard as hell to find the rake listed on most props so how do I know what rake it is if it isn't listed any where?

I really want to know what rakes to avoid buying for it

 

[smokeonthewater]

Re: how much can you do with prop options

as I said before, my opinion is that you should get data w the current prop.hwsiii can probably suggest a particular make and model prop to test next time he checks in but numbers with the current prop will make it easier for him and get you a better starting point

 

[V153]

Re: how much can you do with prop options

I'm very fortunate to've cultivated a wonderful relationship with my local prop shop. We've evolved to a "you break it you buy it" agreement. Thank god I ain't broke one yet ...

All begins witha case'a donuts, and/or beer, etc ...

 

[limitout]

Re: how much can you do with prop options

as I said before, my opinion is that you should get data w the current prop.hwsiii can probably suggest a particular make and model prop to test next time he checks in but numbers with the current prop will make it easier for him and get you a better starting point

befor I get into actually selecting a new prop im trying to understand how and why I am making decisions about these prop charactoristics.

im just trying to wrap my head around this rake category and what it really means so I can find out if I am correct in assuming it only affects the boats trim, take off, and how it rides and not necessarily affect overall rpms or speed that much?

it would appear from what I can find out by searching internet blogs is that a 15* rake is basicly the "standard" or average rake for general application props so what does it really mean that I have a 20* rake?

is the 5* more rake in my prop a lot that makes a big difference like 5* of different pitch would make or is it not that dramatic?

 

[smokeonthewater]

Re: how much can you do with prop options

The rake effects the entire range of performance... EVERYTHING done with prop design is a tradeoff.... example your john boat and my 30' cruiser both benefit from stern lift and as such need minimal rake... at the top end this raked prop could be keeping you from trimming up as much as you could (before porpoising) with another prop and hurting your top speed... soooo if your next prop had the same pitch and less rake you might achieve more speed and thus more rpm.... While I have played around w props quite a bit I'm hesitant to suggest a specific because I think hwsiii and possibly a couple other guru's here will do better at it.... in fact I wouldn't buy one myself without double checking here

for what it's worth a few years ago when I wanted to understand some of the same questions you are asking now I did some google searching on boat propeller design.. I found quite a bit of silly fluff designed to sell props but also found some really good stuff

the guys here did help me quite a bit but some of the stuff requires hours of reading to understand and just isn't gonna get posted in a forum. if you care to google a little you can find enough to keep you busy with a slight headache for a week or so lol

Also here is a fair prop selector but don't take it as written in stone.... sometimes it is spot on and sometimes not so much

Prop Selector | Mercury Marine

 

[limitout]

Re: how much can you do with prop options

The rake effects the entire range of performance... I think hwsiii and possibly a couple other guru's here will do better at it.... in fact I wouldn't buy one myself without double checking here

ok, thanks

for what it's worth a few years ago when I wanted to understand some of the same questions you are asking now I did some google searching on boat propeller design.. I found quite a bit of silly fluff designed to sell props but also found some really good stuff

the guys here did help me quite a bit but some of the stuff requires hours of reading to understand and just isn't gonna get posted in a forum. if you care to google a little you can find enough to keep you busy with a slight headache for a week or so lol

yep, I been looking around and trying to ignore anything put out by prop makers because they are mostly "why ours is better" kinda stuff but theres not a lot of general info out there that isn't biased to sell props so I only found a few instructional things.

Also here is a fair prop selector but don't take it as written in stone.... sometimes it is spot on and sometimes not so much

Prop Selector | Mercury Marine

I tried it but it says I need a 15.9 pitch prop and I should be getting 32mph @ 5650 rpms with my boat (im kinda surprised about that)

im surprised it said I need a 16p although I hadn't expected needing something that high but maybe that is what I need if that calculator is somewhat accurate. if that's true then that 20* rake must be sucking up more power then I had imagined since im using a 13p now.

if I got 30-32 mph that sounds like a more reasonable top end speed for what I consider to be no load at all with a flatboat to push around and what I would be very happy to get

I was expecting it to give me a prop rake recommendation but I guess its just a pitch and speed calculator

 

[smokeonthewater]

Re: how much can you do with prop options

I actually ran through it for your boat and kept getting something to the effect of sorry no props for your application at the bottom of the page..... pissed me off a little because it has always recommended props for me in the past... I guess mercury marine just doesn't make any that work for your needs

keep in mind those numbers are theoretical and could be off.... Also notice that they were shooting for a lower rpm than you are currently running 5500... MIGHT be a good thing with that motor, I'm not certain

 

[hwsiii]

Re: how much can you do with prop options

limitout, as smoke mentioned "EVERYTHING done with prop design is a tradeoff ", since you appear to really want to understand more about what prop rake is and its affects on particular boat and motor combinations I will try to make it easier for you to have a better understanding of exactly what it is, what it does and what it actually looks like, so you can actually identify the different types of rakes in any prop that you are actually holding in your hand. I have downloaded some pictures from the internet, (for educational use only), that will actually let you see the differences in rakes, and where the rake is positioned on props. These pictures represent all of the different types of rake and where rake is actually located on a prop.



When I did a preview of the pictures it lost some of them so evidently there is a limitation on how many pictures you can use in 1 post so I am dividing this into different postings. The first ones will be the pictures and then I will better explain the theories behind the differences. As I believe the pictures themselves help show a tremendous amount of information.

H

 

[hwsiii]

Re: how much can you do with prop options

I will also try to better explain why there can be significant differences in prop diameter as well as why you have to know the reasons behind the differences in hub sizes in relationship to diameter and why that can be extremely important when you are making decisions about purchasing a new prop as gunner, another very knowledgeable person, found out recently. Most over hub props have a smaller diameter base where the blades attach and thus will have a smaller overall diameter than other props, but can actually have the same blade surface area of a larger diameter prop. I would venture to say that over 90% of props used for recreational boats have the exhaust coming out of the center of the hub and that is called thru hub exhaust, another is Over and thru hub exhaust, the next one is over hub exhaust and finally there is non thru hub exhaust. All of these different props have their own purposes and are designed for specific problems and advantages. I believe that the pictures give you a very good idea of why there can be such differences, because of what the designer is trying to accomplish, and how he is accomplishing his goal.

Thru Hub exhaust

Over and thru hub exhaust

Over hub exhaust

Non thru hub exhaust

I have to go to a barbecue now so I will come back online tonight and post explanations of the advantages and disadvantages of certain rakes and hubs for particular hull designs and motor combinations.

H

 

[limitout]

Re: how much can you do with prop options

I actually ran through it for your boat and kept getting something to the effect of sorry no props for your application at the bottom of the page..... pissed me off a little because it has always recommended props for me in the past... I guess mercury marine just doesn't make any that work for your needs

keep in mind those numbers are theoretical and could be off.... Also notice that they were shooting for a lower rpm than you are currently running 5500... MIGHT be a good thing with that motor, I'm not certain

I used the aluminum tiller version as it best fit my application because I felt it fit better the the picture of the aluminum console version and got 15.9 pitch but when tried with the console version it shows 16.3 pitch

it worked fine for me, I ran the calculator with 2002 Suzuki 3cylinde 4 stroke, 40hp, 2.27 gears, (then I used weight to determine the size) 500lb boat & motor, and assumed 2 person 500lb load plus a tank of gas so 1034lbs weight total I think it was. I assumed I was supposed to represent a loaded boat so I went with the average full load condition

 

[limitout]

Re: how much can you do with prop options

limitout, as smoke mentioned "EVERYTHING done with prop design is a tradeoff ", since you appear to really want to understand more about what prop rake is and its affects on particular boat and motor combinations I will try to make it easier for you to have a better understanding of exactly what it is, what it does and what it actually looks like, so you can actually identify the different types of rakes in any prop that you are actually holding in your hand. I have downloaded some pictures from the internet, (for educational use only), that will actually let you see the differences in rakes, and where the rake is positioned on props. These pictures represent all of the different types of rake and where rake is actually located on a prop.



When I did a preview of the pictures it lost some of them so evidently there is a limitation on how many pictures you can use in 1 post so I am dividing this into different postings. The first ones will be the pictures and then I will better explain the theories behind the differences. As I believe the pictures themselves help show a tremendous amount of information.

H

I have to go to a barbecue now so I will come back online tonight and post explanations of the advantages and disadvantages of certain rakes and hubs for particular hull designs and motor combinations.

H

yes, thank you

this whole thread is about wanting to know why im doing something and to understand why I am making a decision to buy this one over that one and what tells me one is better then the other.

I always thought pitch was the only decisions you had to make with props and now I understand why and how easy it is to get it wrong.

I am sorta upset that its just left to average guy like me (who in most cases doesn't have a clue about it) to do all by themselves when you buy an outboard engine.

I have been using the wrong prop for 11 years now and for you guys, as soon as you saw 20* rake you instantly knew it was wrong for my boat yet my dealer at the time mounted the motor to that very boat and said it was listed as the right prop for my boat, I am upset by that and for it to be so obvious to you guys too.

I hope it hasn't hurt my engine over all these years even with only 300 hours on it. thank goodness the prop is still in perfect condition without even a scratch on it so i'll have to try to sell it once I get the correct prop on my boat.

one final question is in a couple of weeks I am upgrading just the flatboat alone to a 17ft flatboat from my 15ft boat and moving my motor onto it (its already ordered just waiting on delivery in 2-4 weeks). should I wait until I put the motor on the new boat to ask for help in selecting a new prop for it or would it be ok to get the test readings with the 15ft boat I have now?

 

[hwsiii]

Re: how much can you do with prop options

Limitout. I am running sooo late for my barbecue, but I had to enlarge the hub pictures or they were useless, so I will be back later tonight to update the analysis on rake and hub sizes.

" I always thought pitch was the only decisions you had to make with props and with it being this easy to get it wrong I am sorta upset that its left to average guy who doesn't have a clue about it, to do all by themselves when you buy an outboard engine. " The PROP manufacturers make lot more MONEY this way, as they SELL many more props.

" I would think the dealers should have a chart telling them the correct prop for each application so pitch would be the only thing to decide on. " There is no chart that could be made to cover all of the different combinations of boats, motors and boat weights as well hull designs, and their different uses by each person.

BUT, prop manufacturers should be REQUIRED to post the Diameter, pitch, hub size and airflow directions, rake degree and direction as well as progressive - flat - or 0 degree, and whether it is cupped or not and exactly where the cup is on the prop. With this information a person who has the correct knowledge could pick a very good prop very quickly. I cannot tell you how many manufacturers I have called, even overseas at Solas, and they can't or won't tell me what is designed into any particular prop they sell. And this is NOT because it is a trade secret, because any good prop repair shop can get this information directly from the prop with the equipment he uses to repair props.

I am sooooooooooo late for the cookout. WAIT

H

 

[limitout]

Re: how much can you do with prop options

I am sooooooooooo late for the cookout. WAIT

H

do your thing, its the weekend

you can edumacate me later when you have more time.

I called brother inlaw and told him im taking him fishing tomorrow so he can bring his gps faster and I can get me some speed readings of a fully max loaded boat with 2 people in it.

whats the best way to get test readings? a fully max loaded condition or normal light load conditions?

I would say im max load (2 people) about 25% of the time and 75% of the time im at the light load conditions (just me and fishing poles) so the only variable then is if I catch fish which might add around 30-40lbs more weight

 

[smokeonthewater]

Re: how much can you do with prop options

get numbers with as many variables as possible... IE both loaded heavy with both tanks full and light with one tank half full


the ultimate goal is to be just under red line when light and still within suggested rpm range when fully loaded

you have NOT hurt your engine by running this prop as you have been within the suggested range....

The right prop will be different but not THAT different... don't sweat it

Absolutely wait till you get the new boat.... kinda like sending your smaller friend to try on shoes that you're gonna wear.... the MIGHT be OK for you but you get a better fit trying your own shoes

 

[hwsiii]

Re: how much can you do with prop options

limit, sorry it has taken me so long to get back with you, but I went fishing with my brother today and had a great time catching rat reds and speckled trout.

The following dialog refers to the pictures in my post number 28 in this thread.



Lets start with picture number 1. If you will notice the rake on a prop starts at the hub and extends to the outside edges of the prop. Rake is the number of degrees that the blades angle either forward or backwards from a perpendicular line relevant to the hub.



Then we have picture number 2 which shows that by adding cup to the blade tips it actually increases the amount of rake that is designed into any prop.



Starting with picture number 3 we really get into rake and the first drawing shows a prop with Forward rake, which is used to hold the bow of a boat down and as a normal course of events has a very small number of degrees compared to most of the aft raked props.
0 degree rake is used to help lift the stern of the boat and the blades are 90 degrees perpendicular to the hub,
Aft rake is used on almost all stainless steel props and is used to raise the bow of the boat as well as it helps control ventilation on props that are used closer to the surface of the water and these props can have as much as 35 degrees of rake.
Parabolic rake is just a way to design the curvature of any particular raked prop, it is very noticeable by its concave looks.



Picture 4 shows excellent pictures of how rake is measured for the different types of rake designed into props.

This particular picture also is the only one that shows progressive rake and Progressive rake increases the amount of rake on a prop the further it is from the hub.


H

 

[limitout]

Re: how much can you do with prop options

limit, sorry it has taken me so long to get back with you, but I went fishing with my brother today and had a great time catching rat reds and speckled trout.

H

we had a tough time yesterday with a really slow bite but managed to get one red and 20 specks averaging 14-16". it was unbelievable because we had a good falling tide and could see schools of 100 count shrimp flowing thru the canals but few fish were active so it was frustrating to see that and not have schools of fish there but it was still a good day.

back on topic: rake

to help me understand the affect a change in rake has on the motor (this is the thing I am most confused about), what forces is it best described as? the change in pitch is like shifting to a different sprocket size on a bike so what is a change in rake going to be like? or will it act the same as a change in pitch only give a different reactive behavior to the way the boat moves in the water? IE: take offs, turning, and planning.

 

[smokeonthewater]

Re: how much can you do with prop options

the rake will change the thrust vector... it won't (directly) change load on the engine but rather will effect leverage on the hull

imagine it like the choke on a shotgun

 

[limitout]

Re: how much can you do with prop options

the rake will change the thrust vector... it won't (directly) change load on the engine but rather will effect leverage on the hull

imagine it like the choke on a shotgun

im not a hunter so that doesn't clarify anything for me but are you saying its mainly related to how it lifts the hull?

where im getting confused "mainly" is that both of you knew instantly that 20* rakes was the wrong prop for my boat but when I asked about what a standard or average rake a prop for my boat should have I got no answer other then all applications are different. so you guys know some form of "rule of thumb" that allowed you to instantly know it was wrong for my boat yet that hasn't been relayed to me yet as to how you instantly knew it wasn't the right prop.

well I know all applications are different but im thinking it all depends on the load on the motor so there must be a "rule of thumb" based on that load. a light weight flatboat like mine, where no matter how many variables you apply to it, its still going to be a little to no weight load on it so there "should be" a standard rake that is best for applications like mine because it is so well defined unlike other boats that come in all shapes, sizes, and weights.

there should also be IMO a general use (most popular) rake "rule of thumb" used on the greatest number of generic boats but still with the understanding that all applications are different.

with no clue how right or wrong these guesses might be and assuming rake is all about size of boat and weight load involving the kind of lift it needs, I have something imagined in my mind along the lines of:

portable outboards do better with 0* raked style props
small or very light weight boats with outboards do better with a 15* raked style props
larger or medium to heavy boats with outboards do better with a 20* raked style props
very heavy or very large boats with outboards do better with 30* raked style props

so if I am thinking about this correctly, rake is affecting performance but more about the hull lift or direction water is pushed out the back of the engine , that being upward angle, downward angle, or straight back and the rake determines that angle, or am I completely lost here?

I will start a new thread for my request to help in finding a new prop for the motor since this thread is just about talking about what rake is vs pitch and solving reasons for the prop selection

find the prop selection request thread here: http://forums.iboats.com/prop-questions-topics/finding-right-flatboat-prop-631655.html

 

[hwsiii]

Re: how much can you do with prop options

limit, I guess I am not very good at explaining complex prop theory, " so you guys know some form of "rule of thumb" that allowed you to instantly know it was wrong for my boat yet that hasn't been relayed to me yet as to how you instantly knew it wasn't the right prop. "

I had a very spirited discussion on here one time with 45Auto , who is extremely knowledgeable in the field of fluid dynamics, which is a subject that is beyond my base of knowledge. So I decided that I was going to put a lot of effort into learning what I needed to understand in order to use these complex equations, so I could utilize them to have a better knowledge in water flow on props and boats. After about 3 months reading theory and working with equations I finally figured out that I would have to spend years in order to gain the knowledge required to actually use them in any manner that would be useful in my applications, so I gave up.

" with no clue how right or wrong these guesses might be and assuming rake is all about size of boat and weight load involving the kind of lift it needs, I have something imagined in my mind along the lines of: "

What you are not taking into account with your guesses is the hull design of the boat as well as the horsepower and whether it is on the low end of what is required or is it at the maximum that can be used on that particular hull, because some prop designs require the maximum horse power and torque in order to maximize the advantages of their designs on a particular boat.

In your boat and the motor you have it is better to raise the stern and that is definitely NOT what you get with a 20 degree aft rake prop that raises the bow of the boat and not the stern," its a 15 foot aluminum jon boat with a Suzuki df40 outboard motor on it and the boat and motor combined only weight 500-550lbs. it would stand straight up on take off (literally the transom was within an inch of going under) ", as you found out to your chagrin.

Hopefully that answers your question.

H

 

[limitout]

Re: how much can you do with prop options

In your boat and the motor you have it is better to raise the stern and that is definitely NOT what you get with a 20 degree aft rake prop that raises the bow of the boat and not the stern," its a 15 foot aluminum jon boat with a Suzuki df40 outboard motor on it and the boat and motor combined only weight 500-550lbs. it would stand straight up on take off (literally the transom was within an inch of going under) ", as you found out to your chagrin.

Hopefully that answers your question.

H

almost but not quite, you still haven't told me yet what rake I "should" be trying to find for my boat.

I understand I don't want 20* rake and im certain I don't want 30* rake which would be worse but I don't know what I "am" looking for

so am I looking for a 15* rake prop or a 0* rake prop? and if I find both which is the better choice to try?

and please look here to find the thread with the speed and rpm numbers you requested: http://forums.iboats.com/prop-questi...op-631655.html