Propeller Pitch

E

El and Bill

New member
Might be good to put our collective ideas regarding prop pitch on one thread to summarize ideas for those who are buying boats.

There are a number of variables, such as the use of boat (pulling skiers would be different than fishing and both different than a 'heavy' boat of long-distance cruisers), altitude can make a difference (we knew a boater who carried a big box of props and changed them for every thousand feet of elevation change [he lived in Wyoming]), size (and number) of engines can be a variable, fresh water vs. salt water might be a variable, etc.

Size of prop might also be a factor, and how important is pitch with prop size? Should big props have a different pitch than small, all other variables being equal?

How important is pitch? Is it only at the extreme edge of variables that it is important? How much extra load does it REALLY put on an engine (how about real numbers, not just opinion). How much difference IS there in performance -- is it just a couple miles per hour at the top range and insignificant at lower speeds? (real numbers, guys, not just opinions!).

Should we carry extra props with different pitch and change them when conditions change, or is that not necessary (if pitch isn't really that important under 'normal' conditions.

So -- (if pitch is important), is it possible to summarize the best pitch for our various C-Dory types for each different variable. Or, as long as our use of boat stays much the same, should we get a prop with one size, one pitch, and live happily ever after ... ?
 
One thing that remains constant is that the motor should run in the maximum rpm range ie: 5,000 to 6,000 (this is an example, look in your owners manual for your motors max rpm range) when the throttle is wide open. If you cannot achieve the minimum rpm of the range, then your prop has too much pitch. If your motor revs above that range, then the pitch of your prop is too low.

So, load your boat with your normal load, take it out on flat water and test. Compare your rpm to the range specified in your owners manual. This give you a basis to choose another prop. If you can borrow some props for testing, that is great. If you only have one prop, test and record, then decide if you want to get another prop.

Talk to your dealer about your results and which prop to try next. Sometimes they will let you exchange a prop for another pitch if there is no visible damage to the prop.

To guestimate pitch, one inch of pitch change should result in 200-400 rpm change on WOT(wide open throttle).

As always, your results may vary. :teeth

If you take your boat to higher altitudes, you may want to have a prop with one or two inches less pitch. Motors loose 3% of HP for each 1,000 ft of altitude.

Owners of twins may want to have a lower pitch prop available for operating on one motor.

Dogon Dan is right about this topic having so many variables that it is nigh impossible to compare different boats, motors, and props.

'Propping out' a boat is an experimental process.
 
dogon dory":1r7gklpo said:
I was running a Honda BF90 on the old boat. It was turning a cupped aluminum 13.5x15 three blade and was running a couple of knots faster cruise speed at 4800 rpm. What's up with that? Smaller prop, less pitch, lower engine speed but running faster with more or less the same gear on the boat. Maybe Suzuki just sucks as suggested in another thread somewhere.
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Taller gears in the Suzuki. Engine RPM and output shaft RPM are not the same thing. I will stay completely out of the Suckin' Suzuki discussion.

There are other, more subtle variables in there too, aren't there? Like prop slippage, blade surface friction, and a bunch of NASAesque mathematical gibberish?

If we were staging CD races, one might want to get into power tuning propellers. But most of us want to use a common, quickly replaceable, off-the-shelf prop. Those seem to be pretty much standard within a small range of diameter/pitch ratios.

And here's a question you guys can help me with. The "pitch" is a theoretical distance through the water the prop will travel per revolution under ideal circumstances, is it not? Therefore, wouldn't a change in the diameter require an adjustment in the blade characteristics (i.e., cupping, angle, surface area, etc.) to achieve the same "pitch" as described above? Being a simple (minded?) put-'er-together-with-the-parts-you-got mechanic rather than a design engineer with the means to requisition the perfect parts, I am inclined too think that pitch is pitch, and through the water is where I want it to go. If we was going to the moon - different story.

Crap, I'm gonna be sorry I jumped into this one!!
 
Another unknown variable is the actual H.P. output of the engine. I have read where from the factory a plus or minus 10% variation in H.P. output from what is on the engine cover is acceptable. From the low side to the high side would yield very different results.
 
Dogon Dan,

Yeah, there is a lot more to it than just the pitch. :wink My comments were for an owner that has a prop on the boat and is wondering if its right.

If you have a stock aluminium prop and replace it with the same model, different pitch, the diameter may be the same or slightly different, but there is usually no choice to the buyer.
 
dogon dory":jpxscjc0 said:
annular
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:shock: We'll have to check with Bill, but I don't think you're supposed to say that on here.


In my short time screwing around with these boat screws, I have seen the differences you speak of. I tried three different props on my CD22/Honda 75, all having diameters within 1/2" of each other. The 17" pitch that came with it was too much, so I went to 15" pitch. Three different manufacturers, three different WOT/top end results.

Same with the 130 Honda on my CD25. The 15" Michigan Wheel gave me about 5000 rpm max, not enough speed at cruise, and not enough reserve get-up-and-go for when I needed it to keep up with a swell. The 13" Honda aluminum made 6000 (and maybe a hair more) rpm quickly, with a speed that was too slow at cruise rpm. The 13" Honda branded stainless prop I have on there now (with the same dia. and pitch numbers as the aluminum) goes up to 5900 rpm, and provides an acceptable, albeit slower than I would like, cruise speed at 4500 rpm. My boat is heavier empty than is the new model CD25, and I have it loaded heavier than normal people, besides. I run about 14 kts at 4500. It will make 18 kts at 5000 rpm, but I don't like to run it there for long periods. The sweet spot, judging primarily by engine sound, is between 4500 and 4600, so I just let the other CDs pass me. It does have a good deal of oomph between cruise rpm and WOT if needed.

I have one prop yet to try. It is a Propco Marine brand 3x13.75x15 that I bought for the CD22. I was a little distrusting of the thing, because as shipped it wouldn't even go on the shaft. The base of the prop hub that fits into the skirt on the Honda had too large an OD, and was off center or out of round. Consequently, it made contact for about 120º of rotation. After I got no response from the outfit that sold it to me, I chucked it up and turned the OD at the base down about .050". I did try it for a while on the 22, and it was very smooth, and produced results comparable to the Honda prop with the same numbers. One of these days, I'll try it on the 130 - it could work as a spare, or it might be the best thing going.

What I think I need on my Honda 130 is a 14" pitch (and, of course, the ideal diameter). If I ever have any money, I might have a prop shop tweak one up for me. Or maybe give one of those ProPulse composite adjustable jobs a try. For the time being, I gots to run what I brung or stay home.
 
Mike, Ol' bud --

Some horses are gonna go just so fast, no matter what kind of saddle you put on them. Man. a cruise of 14-15 knots, up on easy plane, is way cool. At your weight (well, the boats total weight) about 200 hp would be ideal. Pair of 90 E-Tec jewels mebbe??

Be HAPPY,

Dusty
 
Sorry folks fer not havin the time to read all the prior-post here :crook consider:

Why in tarnation would you want to spin a prop faster in the water, to achieve the same hull-speed as a lower-geared outboard. There's energy wastin there.... not matter what formula is applied :idea: :!: the way of the future is obviously to spin a steeper pitch (slower turning) and gain "traction in the water toboot :!: Suzuki has a good explanation for this on there website (I think...if they still do)

Sorry bout the "quicky-message" and not payin attention to all that hi-tech discussin up there, but it's high-time the RedFox went fishin :smilep
 
Hmmmmmmmmm.......Fun to read all the technical information over and over again and do the calculations too, but what it all comes down to is trying out props on the boat under conditions you usually operate in with the boat loaded as you usually do.

As far as the CD-22 is concerned, about a 14-17 inch pitch range with a 13 to 13 5/8 or so diameter seems to be the agreed upon common ground unless you have a Suzuki 90 with a 2.60:1 gear ratio where a 19 inch pitch may be within range.

Due to the nature of the CD hulls, we don't need high tech or exotic propellers such as the Raker, Stilletto, Rapture, or the ever popular Screaming Screw. Some of us do like the lesser vibration and better anti-ventilation characteristics of the 4-bladed props, and find they work very well.

I currently have 15 and 17 inch pitch Yamaha 3-blade OEM props, 4-blade Comprops of 15 and 17 inch pitch, and a 15 inch pitch Solas 4-blade prop. I'll let you know how they work out if I can ever get around to trying them all out!

Even part of the propeller selection process is subjective, so numbers don't always mean everything. Still helps to understand all the theory when trying to find the best fit, however. Joe.
 
dogon dory":19l2b995 said:
Stop by WalMart and get yourself a file. Then you got choices on diameter too.

Hey RedFox, we're finally getting there. I found out that my Suckzuki will run Yamaha props.
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Amentathat 8) :lol: A hammer to :idea had to reform a few props over the years bangin-around out in the Sound :smileo :teeth

Hey, my fave is the stock 17 or 19 pitch OEM alum' Yamaha! smooth and don't cavitate as much as others :thup I have never bent a shaft to date, in all my "groundings" you know what all the old-salts say: "If ya haven't run-aground---ya haven't been-around" :wink

Absolute minimum-pitch prop for a C-Dory: 17-pitch :!: :idea: anything less in my book leaves your bow-falling over the wave-tops too fast :idea: (thats experience talking there folks :shock: ) making that "flat-bottom C-Dory ride even worse :!:
 
I heard that ... :) :lol: If the companies would just put in reduction-gears (a does Suzy) the smaller motors could turn a nice steep prop :!:
My Yami will sling a 19 no problemo, but I get overloaded, and like the snap of the 17 pitch :thup :P
 
Crap, I'm gonna be sorry I dredged this back up!

I was looking back through this discussion because I just put on a different prop to try (haven't got it wet yet), and I came across this very good question that I must have somehow missed previously:
Why in tarnation would you want to spin a prop faster in the water, to achieve the same hull-speed as a lower-geared outboard?
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Certainly Dogon Dan or Dadgum Joe could explain it better, or at the very least use fancier words, but here's a mechanic's take on that. The higher the final drive ratio, the broader the useful range of prop RPM. Assuming the functional rpm range of the motor is between 1000 and 5000, an outboard with a 2.00:1 gear case ratio will turn the prop from 500 to 2500 rpm. An outboard with a 2.60:1 ratio will turn the prop from 385 to 1923 rpm. With the higher geared 2:1 drive, you get a useful prop rpm range of 2000, whereas the lower geared unit has a useful range of 1538 at the prop. That's a difference of nearly 25%, and will allow more flexibility in tuning the hull performance to sea conditions, boat loading, and engine efficiency. It's the difference between having a three speed on the column versus 4-on-the-floor. (We'll be able to spot the youngsters in the crowd here by watching who asks what a three on the column is.) Of course, with the lower gears, you will be able to turn a larger propeller which has benefits in itself. It really depends on what your desired use is. Kind of the same reason John Deere tractors have granny gears with huge drive wheels while Corvettes have little tires and overdrive.

I really don't have a logic based opinion on which brand of OB is superior, but I am betting that Honda didn't become the most popular and most dependable 4-stroke manufacturer in the world because it took so long for the other guys to figure out that Honda got the gears wrong. (Sorry RF, but I had a bad day.) I bought a Dodge 3500 with 4.10 gears to tow the TyBoo, and Larry got a Dodge 3500 with 3.73 gears to tow the Helen O. He's going to get better fuel economy running empty than I will, but I can beat him to the top of any fair sized hill if we're both pulling a CD25. (Neither of us race anymore, so the top end speed on the flats doesn't come into play.) So who has the better truck? Certainly not some nerd driving a Ford! Comparing our use of the trucks, knowing I drive many more miles empty than Larry does, his higher gears would actually suit my use better. But he likes silver paint and dual rear wheels, while I like my blue and singles. And since both of us got the deal we wanted on the truck we wanted when we wanted it, we took what we got. Did one of us make a mistake? (If I did, don't tell my wife, because we paid close to 35 grand for it.) Since both of us are driving Dodges, I should think our choices were ideal.

And a correction to an earlier post on this thread, where I said:
The 13" Honda aluminum made 6000 (and maybe a hair more) rpm quickly, with a speed that was too slow at cruise rpm. The 13" Honda branded stainless prop I have on there now (with the same dia. and pitch numbers as the aluminum) goes up to 5900 rpm, and provides an acceptable, albeit slower than I would like, cruise speed at 4500 rpm.
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I looked at the back of the stainless prop tonight when I pulled it, and it is 13" diameter (13 x 13 x 3). The aluminum prop was 3/4" larger in diameter (13 3/4x13x3). The prop I just stuck on is 13 3/4x15x3. The last 15" pitch I tried was a Michigan wheel at 13.5" diameter, and it made no more than 5000 WOT. This is some odd brand that I am trying next, and it has some different characteristics to it than the Honda brands. Any guess as to what the WOT and the cruise speed at 1000 under WOT will be?
 
Mike-

Sounds like you had a bad day, and are still mad about missing Blakely!

Good discussion of gearing in outboards!!! Like you say everything in gear and prop selection involves a series of trade-offs in trying to find the optimum solution for a specific application, as well as the fact that different people have different preferences that are also major factors in the decision making process.

Not much point in re-hashing most of what you've said, as it's all true except for the sideways pot-shots at Ford, the Nerd, other outboard brands, Red Fox, and Mother Nature.

You do have a very good point about the higher geared drive ratios have a greater range of rpm operation at the prop. This is very good, but also has its downside, in that the higher prop rpms lead to greater frictional losses, but this is relatively minor.

Gear ratio is like pitch, diameter, number of blades, blade design, and several other factors: each element in the interactive whole has to have its point of greatest return traded off away from its optimum to accommodate all the other factors which in turn must do the same as they cannot all be optimized absolutely at the same time. What we're seeking, then, is to find the combination of factors that give us the best possible performance we can get that suits our needs and style preferences.

To get the best efficiency in terms of energy and momentum exchange, we'd use a low gear ratio (2.3-2.6:1), turning the prop slowly to reduce frictional losses, use a large diameter prop, a higher pitch, and more blades. This would lead to less friction, less slip, and a more efficient push on the water with less losses to blade surface friction and push the boat through the water with less energy loss and better momentum exchange, just like the long oar, long slow stoke analogy. The problem here is that there isn't much room for error or accommodation. It's just like your truck analogy with the gears set up for highway freewheeling. Very little accommodation for various speeds and loads, etc. This might work if we had lots of extra torque and power.


We could go the other way and go for maximum accommodation:
High gear ratio (2-2.3:1), thereby speeding up the prop, using a smaller prop, with fewer blades, and less pitch. We would be able to turn the prop far easier, have more slip, acceleration, and more accommodation, but would suffer higher frictional losses on the blade surfaces and not as efficiently push the boat forward through the water just like with the short oar and fast, furious stroke analogy.

This is a bit over-simplified, but it does point out that we can go too far in either direction. The problem is to find the optimum middle ground on a whole bunch of variables, and we haven't even considered the boat, the loading, and sea condition variables. Add in elevation, temperature, and humidity changes, and the search for the universal prop becomes increasingly difficult!!

Your point is to have a gear ratio that works like a good middle range accomodating transmission in a truck, and it's a very good one. Mr. Fox has lots of extra power and torque and is more concerned with limiting prop ventilation in choppy water. Please don't point out to him he has a 2.07:1 ratio in that big Yammy!!!

Got to go to other fun things, including a dental appt.! Joe.
 
Sorry to be the know-nothing amateur here but here's my take on it....

I bought my boat with twin Honda's and whatever props they put on them at the factory - don't know what they are, don't care, have a spare in a box.

Boat goes fast enough (27kts under ideal conditions), boat goes slow enough (1.8 kts on a single engine):wink: .

Roger on the SeaDNA
 
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