Changing Props for Performance and SS versus Aluminum

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
When you are looking to improve the performance of your boat you should have a better understanding of what to look for in a new prop versus your old prop, and the other changes that can increase your maximum speed and cost you nothing except your time and a little bit of labor. ALL boat motors should always be able to attain the minimum recommended RPM for the motor you have installed with your normal weight load, if you do not do this you can expect your motor to lose some or a lot of it's longevity, just depending on how low your maximum RPMs are and how bad this is Lugging your motor. When you cannot reach your manufacturers recommended upper RPM with the normal load you carry, then your motor is Lugging at ALL RPM levels, not just at WOT. Whereas, being able to attain the maximum recommended WOT RPM will increases motor longevity and fuel efficiency as well. With today's ethanol fuels lugging of your motor creates even more carbon deposits on the pistons and rings than in the past, which over a period of time creates coking of the pistons and rings, which eventually leads to massive engine failure.

1. Before you make any decisions about changing props, you need to know if your outboard motor is at the best height for maximum performance with any prop. Anytime you see water coming up the transom of the boat or the leg of the motor when you are planing means your motor is mounted too low on the transom. Your motor normally should be at least 1" above the bottom of the transom of the boat. Your prop WILL gain RPM and speed by raising the motor higher, until it starts ventilating. On inboard outboard boats you cannot change the height of the out drive, unless you have an old OMC stringer drive, so the following does not apply. Normally the anti ventilation plate should be at least 1? and preferably 2" or higher above the bottom of the boat for the best speeds and the least drag created by the motor. And if you change from an aluminum 3 blade prop to a 4 blade or SS prop you need to raise the motor one more notch again to see if you can gain more RPM and speed, because these props tend to hold the water better and thus can be raised higher in the water column.

Motor Transom Height
AntiventilationPlateStraightEdge-2.jpg


When the boat is running in the top 20% of your speed you should be able to see the anti ventilation plate very clearly at the top of the water and have water just splashing over the top of it.

Correct Anti Ventilation Height

correctheight.jpg


2. Then you need to know how much prop slip you have in order to determine if you are using the wrong prop pitch and/or blade geometry for your boat and motor combination. Just getting your motor to turn the correct RPM at WOT does NOT mean your boat is running efficiently and you are attaining good fuel efficiency and maximum speed from your motor. If your Prop Slip is over 15%, excluding heavy cruisers, when using a GPS, then you should consider changing to a prop with a different blade geometry than the prop you now have, to improve your performance, speed and fuel efficiency at all RPM.

Prop Slip
PropSlipFormula2-1.jpg


Checking the amount of prop slip you have, will give you a very good indication of how well the prop you are running is at converting Theoretical speed to Actual speed. Every time your prop slip is higher than 15% you should definitely consider changing prop design, but you need to make sure your speed numbers are correct, and that means using a GPS for speed and running the speed tests in 180 degree opposite directions to account for any wind or running water that influences the test results, add the two numbers together and divide by two for Actual speed results.

SolasAmita4PropSlip.jpg


3. An easy and inexpensive change for performance, if you are experiencing high prop slip, is to go to a 4 blade prop with more blade surface area. The things that need to be taken into account are how much difference there is in the blade geometry and how many RPM you will lose having the 4th blade versus the three blades. As a normal course of events if you have an aluminum 3 blade prop on your boat you can expect about a 200 to 300 drop in RPM, changing pitch for pitch, if you buy a Solas Amita aluminum 4 blade prop and they run about $ 120, here at Iboats. Solas aluminum props are also made with the newer squeeze casting pressure process, which makes for enhanced mechanical properties because of the fine grain structure caused by rapid solidification of the prop and the high pressures used in the process, as well as the blades can be made thinner and stronger because of this. This particular prop is known as more of a stern lifter and you would normally expect to lose about 1 MPH at WOT depending on how much prop slip you have, but if your prop slip is about 20% or higher you might even gain 2 or 3 MPH.

But, If you are running a stainless steel prop and still having high Prop Slip problems, then you would want to consult with an experienced prop man or a prop shop that will let you exchange props, as normally stainless props have much better blade geometry and don't have as high a Prop Slip as aluminum props do. But you can't just change Stainless props pitch for pitch the same way you can't just change all aluminum props pitch for pitch, because of blade geometry differences between models of props, even from the same manufacturers, you have to have the knowledge to know the difference between what you have and what you need for your particular use.

Solas Squeeze Casting

SolasSqueeze.jpg


4 Blade props can also have thinner blades as each one of them only has to carry ? of the HP per blade whereas 3 blade props have to carry 1/3 of the HP on each blade, with a 300 HP motor a 4 blade prop only has 75 HP on each blade and a 3 blade prop has to handle 100 HP on each blade. Theoretically this means the blades on a 4 blade prop can be cast 25% thinner than a comparable 3 blade propeller. As well as with the less blade area on a 3 blade prop there is a higher chance for more prop slip on a 3 blade prop.

Most people with outboard or inboard/outboard motors use aluminum propellers to propel their boat. Most people also believe that aluminum props also flex a lot while running, and thus lose pitch and forward speed when propelling a boat at cruising speeds and above, versus a stainless steel prop, and that is because of the aluminum flexing.

The actual difference between most aluminum props and stainless props has nothing to do with flexing of the aluminum propeller, for speeds up to and above 45 MPH, that is why aluminum props are much thicker than stainless steel props. The differences are in the actual time and money spent in the blade geometry of the two props, as well as the cost in materials and that is why stainless props cost much more than aluminum props. There is a lot more engineering designed into stainless props, partly because aluminum can't be cast with the tolerances required for the blade geometry design and partly because the stainless props can be designed much thinner, which also helps lower prop friction.

Heavy progressive parabolic rake, flat rake, zero rake, progressive pitch, trailing edge cup or blade tip cup, more or less blade surface area and thinner blade sections are the things that can reduce prop slip and raise the boat for less wetted surface area, and thus increase speed when used in the right combinations for a particular hull form design and motor combination.

Picking a very good prop for your particular boat and motor combination requires knowledge, but it is not an exact science. I use mathematical software and physics to get what prop fits best in Pitch and Diameter for a certain boat and motor combination, but then that is where the knowledge of prop geometry starts and experience is required to pick a better prop that will conform better to your uses and needs.

Aluminum props are very economical and can produce very good results for most boats, but for the maximum performance from your boat normally you would buy a stainless prop, although you will probably have to test a few of them to get the BEST one for your boat and uses if you are looking for Maximum speed.

You might also try reading this to learn more about Prop Slip and Blade Surface Area.

http://forums.iboats.com/showthread.php?t=381820



H
?2009
 
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solar7647

Lieutenant Junior Grade
Joined
Apr 23, 2009
Messages
1,218
Re: Changing Props for Performance and SS versus Aluminum

This is great info...considering you seem to know alot on the subject...could you provide info on composit props vs stainless and aluminum as they seem to be becoming more popular...I my self use one and it really seems to achive the pros of a stainless prop with out the cost...I found in my own testing that I was able to see a considerable differance in speed and how the boat responded in high speed turns. I know that when they first started to come out they were mainly for small engines but I run a four bladed pro-pulse adjustable pitch on 180hp and have not had any issue....considering that they are made by injection molding my thought is they can hold a tight tolerance as a stainless does.....I would realy like to know your toughts.

Thanks again for the info..its like a one stop shop for prop info!
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Changing Props for Performance and SS versus Aluminum

Solar, thank you very much for the compliment, and I like the Prop Pulse prop because of the 4 blades and the adjustable pitch. In my opinion that is the ONLY composite prop I would buy. You hear very many stories on here about composite props, I acn't use one for the simple reason I use my flats boat in shallow water, and I mean shallow. I would break the blades regularly and they would have nciks in all the blades in less than a week.
If it was me I would throttle up without shoving the throttle to wide open beause that is when the most thrust is applied to a prop, so if you take off a little slower it will stress the prop blades less and with 4 blades there is less load on each blade. In my opinion the Pro Pulse will do good for you if you are always in deep water and don't get in shallow water very often.


H
 

solar7647

Lieutenant Junior Grade
Joined
Apr 23, 2009
Messages
1,218
Re: Changing Props for Performance and SS versus Aluminum

Thanks for the tip....I am one lake Erie and the Maumee River so I am in water that is more the 6 foot when I am running. I to realy like the adjustable pitch its like 5 props in 1. I luv I can change the pitch on the fly when when I go from cursing to pulling tubers to fishing. I got the the prop for that reason plus..we have a flood problem and the river gets a lot of logs washed into it that stay part submerged...I found one with an Aluminum prop and it bent the hell out of it....didnt bother the drive shaft put the prop was junk...did the same with the pro-pulse and if completely broke of one blade but since the blades can be changed it only cost $30 and a short trip to West Marine to get back on the water instead of 90-150 for a new prop..I keep an extra on board but its seen better days and its only there in case we are stuck and need to get to shore..it is in no shape to be used for regular use...its all chipped and damaged so it looses bite bad....but tt does its job..

Thanks for the info (again) I am sure i will reference this thread often.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Changing Props for Performance and SS versus Aluminum

Sounds like it works great for you, and I do like the adjustable pitch and 4th blade on the Pro Pulse, and I am glad you like the information in the thread.


H
 

turkk9

Cadet
Joined
Mar 7, 2010
Messages
11
Re: Changing Props for Performance and SS versus Aluminum

this post is for HWSiii
i dont know how to get up with you so i am trying to reply to this post. ok so i have researched you and your advice. i have heard alot of wind blowing from alot of lips aver the winter but by all means i have read all and i mean all your post. i have come to the conclution that u may be the only person on the face of the planet who can help me.
ok here goes my real email is 1013photo.com@gmail.com
let me start by telling you what i have as a problem. ok ever try and pull a log out of water? that is kinda like my hole shot. not to to bad but noticable. next is the boat rpm. now eather my guage is really bad off or i am spinning a prop like mad. next is boat speed. at 3/4 throttle maybe a little better the boat is at 46mph (gps) then when i gun it it jumps from 5g to 5600but boat goes no faster.
now i have spoken to several ppl and they say no way my setup will spin a prop that fast must have a rpm guage off. now i can see 2 or 3 mayyyybe even 400 rpm but 6plus???????? dont sound right. pluss i have a pro street drag bike so i am use to hearing rpms and it sounds above the recomended 4800 i beleave. the motor was a 305 spun bearing and now has a 350 with 305 heads and thats all. no rev limiter. keep in mind i would like to stay with a alumin....prop as i am on a budgett at this point. new to boats.

now for the setup i will try and answer all your questions in order
1) 1990 regal valanti 200 bowrider (open bow)

2) 20 foot 8 foot beam 2400 pounds

2a) max hp is 300

3) 3ppl sometimes 4 (if son has a friend over) not sure on gal of gas but allways stay around 1/2 tank 25 gal

4) ride the water with family (tube and ski behind 2 jetskis)

5) it is a v but dont think it is a deep v

6) 350 block 1969 heads 305 1990 (was all i had) stock 2bbl carb

7) alpha 1 outdrive 1990 stock not sure of gear but if i am right there r only one for those older years 300hp max

8) 4800 wot max maybe 5g but i think it is 4800 u will know better than i

9)n/a

10)n/a

11) has a wale tail but will remove on your recomindation if needed but will leave on eather way. doesnt matter 2 me

12) michigan ballistic 23 pitch 3 blade is all i got came with boat

13)wot rpm as of saturday 5600 5500 me and wife 330punds 2 life vest 3/4 tank gas gps 46 mph

14) lake james nc elev 1200 i think

15) motor new

16) not sure how long it has been on boat i bought boat at end of season last year only used about 8 times then winter hit

17) prop looks great and if u suggest a new one i will sell this one cheep

18) i think the boat is a little sluggish @ hole shot and it has to many rpm (if tack i right ) also top speed is it right at 46mph? boat planes at just over 3/4 ish throttle at 4800 ish rpm then when u give it wot rpm rais but speed doesnt

19) shew kinda hard this one but it cruises at 38 to 3900 rpm at 20 to 24mph any lower rpm and the boat comes off plane maybe i can get 3600 befor breaks off plane. 44 4500 rpm is about 40 then wot 54 to 5600 46 mph

email me if u have any other questions
i am afraid i am gonna hurt the alpha 1 at those rpms but not sure also am thinking of a turning point legacy do to the vent holes to give me that hole shot just in case i ever wanted to pull up a sing ski not sure i ever would but @ least i think the holes would help me get that log of a boat out on plane. but i will listen to any prop name and type u sugest

thank you for any help
joey
 
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