PDA

View Full Version : Relationship between prop dia and pitch?????



raritan
03-08-2005, 09:49 PM
Whats the effect of increasing diameter but not pitch and visa versa.

Lenny
03-09-2005, 01:52 AM
..just a stab here, BUT, I believe diameter creates lift/fall ( bow, transom) whereas pitch creates/diminishes available forward motion.

Cup and rake is a whole 'nuther thing. As is trailing edge of the blade.

:D good luck...Let me know if you find out. :)

smokediver
03-09-2005, 07:03 AM
If you only increase diameter , you will lose rpm's . about 200 for every 1/4 inch maybe 300 . You see a lot of people on here running turbo props instead of a mirage plus . The diameter of the mirage plus on a 23 pitch is 14 5/8 . Going to a turbo, where the diameter is 14 1/4 is where the speed/rpm gain is seen . There is a point to where blade size is needed in order to handle a lot of horsepower and any drop in rpm's is made up for in the slip percentages . I think :p

boldts
03-09-2005, 11:22 AM
I'm no expert and truthfully I'd talk to a prop shop to be sure. The folks at those shops who work their magic know a whole lot more than I.

As mentioned, if you increase the diameter, you also usually increase the blade area of the prop. This increases the effiency of the prop. (less slipage) which is why in most cases, you also lose rpms at the engine. The motor has to work harder to turn the prop in the water. Now, because you have more blade area and a loss of engine rpms, you will also probably have to decrease the pitch of the prop. Pitch is the distance the prop should move per 1 revelution if it were turning thru a solid. In the case of water, it is almost impossible to not have slippage due to it being a liquid and not a solid. If you decrease the pitch, the boat does not move as far in the water per turn and you usually get a loss of top end speed.

With that said, if you look at hydroplane race boats, you'll find that most are turning very small diameter props with tremendous prop pitch. Drag boats use this type of prop also. These props slip like crazy until they finally hook up to the water and then because of the big pitch and the small diameter, the speed is incredable!

Sometimes, a large diameter prop may also have adverse effects on the boat it is pushing. The prop grabs the water so well, it causes what is called chine walking or porposing of the bow of the boat. Everytime that big blade area of a large dia. prop grabs the water, it's trying to twist the boat from side to side. (chine walking) It also because the slip is less pushes the bow up and down as it grabs then lets go of the water.

I'm not going to argue the Turbo vs Mirage + debate. When I had a 22 Classic with a stock 454 MAG and Bravo 1 drive, I gained 3 mph running a Turbo 1 vs a Mirage that came with my boat. The Mirage caused ungodly porposing although I never experience chine walking. Heres the kicker. The Mirage was a 23" pitch and more dia. so when I ordered the smaller dia. Turbo 1, I thought lets start with a 23" pitch Turbo 1. After testing the Turbo 1, I found I actually lost rpms in comparison to the Mirage and the speed was almost exactly the same. I returned the 23" for a 22" Turbo 1 and hit the jackpot. Boat went from 65 to 68 mph and was turning 4950 rpms. Best of all the porposing problem was gone through all rpm ranges. With the Mirage, the only time I could get the porposing out of the ride was at full throttle. Didn't matter what trim the drive was at as more professional drivers than I drove the boat with the same results.

It's just a matter of getting your hands on different makes of props and trying them on your application. You may find you like the handling of a Hydromotive 4 blade over every 3 blade prop you try. You may find that a Turbo is faster than a Mirage +. The Mirage + may handle better than a Turbo on your boat. You need to know what exactly you want in your boat ride. Speed, Cruising speed, Handling, Out of the hole performance for watersking? Best thing to do is do some testing, but look for others who have tried props on a boat much like yours. They'll be the best source of information for you. You may find that 1 prop does not meet all your needs. Good Luck! Props are such a big research area in boating. Some work, others don't. You won't know until you do your own research on your boat.

joseph m. hahnl
03-09-2005, 05:48 PM
I believe you are mistaken on what chine walking is. the propeller is allways aligned in the water to to cavitation fin in the water in other words the propellor and the fin will allways ride parrellel to the surface of the water.chine walking comes from the trim being set to high. It pushes the hull higher up in the water or out of the water which makes the boat become un stable. the boat has a tendency to go faster because there is less resistance on the hull. it also creates the porposing because it's allways trying to point the bow upwards but looses to gravity. it will also cavitate in the waves. . the smaller the dia of the prop the more cavitation. the more cavitation the more rpm on the prop. you need a degree of cavitation to get the boat out of the hole . if you think of it like a high stall speed torque converter .or winding the clutch up. if you have high horsepower you need to use both larger dia and more pitch because it will cavitate to much " like if you dump the clutch the tire spin no traction" if you have low horse power you have to decrease the dia as you go up in pitch to maintain the right degree of cavition. a larger pitch with to much load that looses top rpm could go slower than if you had a smaller pitch with less load turning a higher rpm . if you dont Know pitch is in inches meaning
a 19" pitch .one revolution is equel to 19" of travel. At one to one a 17"prop turning At 1000rpm is about equal to a 19" prop turnig at 894 rpm.

BigGrizzly
03-09-2005, 06:10 PM
Sorry guys. You are all righ and all wrong at the same time. First the Turbo vs the Marage Plus thing. There is a majior difference in the progression of the pitch between the two props and the Diameter has very little to do with the performance beteen the two. If you actuaLLY MEASURE THE tURBO you will find it is bigger than 14 1/4 That number is a nonimal number The pitch numbers between two manufactures are dirived differently especially in this case, because of the progression. This I know because I have seen the numbers and graphs. BTW The smaller the pitch on a m plus have bigger diameter. Scott in your case the diameter had nothing to do with the performance. It was the hole package. not any single part. We have tested for years and on some lower units because of the design and hydrodynamics they may work better with bigger diameter props. There isn't a prop company that has the same prop with different diameters and the same pitch available. There is alwayds a big difference in the two props. We have had them custom made with different diameters to cope with ineffient gear cases. The biggest thing I have noticed is the torque steer with bigger diameter props. This is going 1/2 inch or bigger. Rake angle has much more to do with bow and stern lift tham diameter. Unless you test the boat the same day and same conditions you are just guessing. My test Marage vs the Turbo1 werw done the same day same conditions same boat same load and with n two galleons of duel of each test run. Turbo won on all counts. My turbo is 14 1/2 diameter. After all my years of boating I have learned that all prop theories have holes in them and all boats react different to prop designs. My Criterion hates Bravo 1 props weather they are labed or not .( I have 4 different bravo1 props on it, one from Merc famed and the late Dave Cavnaugh). M- plus was tolerable, and the Turbo1 was better. The lightning and TXP work about the same and I prefer them to the Turbo1 at higher speeds with a hevy load. In conclusion If it works well for you then use it.

mattyboy
03-09-2005, 06:22 PM
I thought I understood this stuff but now i'm totally lost

trim cannot be the only factor in chine walking, I have 2 volvo props Identical physical measurements pitch diameter but the blade shape is different
one ultra one elephant ear no chinewalk ultra, chinewalk round ear no trim on a volvo running the same settings??

is a 3 blade prop ever in alignment??? it is either Y , or >- or -< or V
l

if that makes sense ??? 1 blade on one side 2 on the other, those hydro's use a 2 blade or 4 blade prop where it is always the blades in direct opposition or in balance
doesn't the loading and unloading of the blade also have an torqueing and untorgueing effect on the boat???

and a 19 pitch prop in one revolution ( of the prop) should travel 19 inches in theory, in a vacuum not accounting for slippage or hydrodynamic short falls is what I thought, regardless of drive ratio the key is to prop the boat where you make the most HP??

damn now I got a head ache :)

joseph m. hahnl
03-09-2005, 06:52 PM
I thought I understood this stuff but now i'm totally lost

trim cannot be the only factor in chine walking, I have 2 volvo props Identical physical measurements pitch diameter but the blade shape is different
one ultra one elephant ear no chinewalk ultra, chinewalk round ear no trim on a volvo running the same settings??

is a 3 blade prop ever in alignment??? it is either Y , or >- or -< or V
l

if that makes sense ??? 1 blade on one side 2 on the other, those hydro's use a 2 blade or 4 blade prop where it is always the blades in direct opposition or in balance
doesn't the loading and unloading of the blade also have an torqueing and untorgueing effect on the boat???

and a 19 pitch prop in one revolution ( of the prop) should travel 19 inches in theory, in a vacuum not accounting for slippage or hydrodynamic short falls is what I thought, regardless of drive ratio the key is to prop the boat where you make the most HP??

damn now I got a head ache :)




well on my out board when i have the tilt all the way up it chine walks , it porposes, and goes super fast as in where's my tether cord fast. if i put it one down it goes away no chine walk no porpose . bot not as fast. this ocurrs with either my 17 or my 19. so is trim the same as tilt or are we talking about to diffrent things

Rootsy
03-09-2005, 06:56 PM
actually i always thought that the larger the diameter the better.. least that's waht the chicks all seem to say... and if it's just too small in diameter they say pitch it... that sound right to you matty???? maybe we need toona to do a graph for us...

JR - running what hydromotive told me would NOT work... :biggrin:

mattyboy
03-09-2005, 07:06 PM
well on my out board when i have the tilt all the way up it chine walks , it porposes, and goes super fast as in where's my tether cord fast. if i put it one down it goes away no chine walk no porpose . bot not as fast. this ocurrs with either my 17 or my 19. so is trim the same as tilt or are we talking about to diffrent things



my drive does not move it stays put no trim tilt whatever you want to call it .. 2 props one chine walks one doesn't so trim/tilt is not the only factor to chine walk

rootsy i'm irish so diameter is my best friend :)

joseph m. hahnl
03-09-2005, 07:27 PM
my drive does not move it stays put no trim tilt whatever you want to call it .. 2 props one chine walks one doesn't so trim/tilt is not the only factor to chine walk

rootsy i'm irish so diameter is my best friend :)

i have been told in the past that chine walk occurs by exceeding the boats hull speed.

but i sure your right that there are other factors involved.

i.ve also been told it's the motion of the ocean ,not the size of the vessel

gcarter
03-09-2005, 07:29 PM
This outdrive stuff is all new to me. I tend to agree w/Griz as a good prop design would be part of the prop/ drive package. Anytime you're working within 10% slip, the margins are so tiny the same prop couldn't possibly work as well on different shape drives/HP/loads. Imperical design considerations, and experience would reign supreme.
The props/gear ratios on these boats we love are transitional between a cruiser design/drive ratios and race boats since we all run ratios less than 1/1.
On a race boat you see step up ratios since the prop has to spin very fast indeed to go 150/200 MPH.
On cruisers (inboards), we want the lowest ratio, largest diameter, highest pitch, and largest blade area you have room for and horsepower to drive. Even then you can seldom expect less than 30% slip.
Only on work boats/ships do we see slips less than 10%. I guess we ought to be happy with that. :biggrin:

Fish boy
03-10-2005, 07:25 AM
well on my out board when i have the tilt all the way up it chine walks , it porposes, and goes super fast as in where's my tether cord fast. if i put it one down it goes away no chine walk no porpose . bot not as fast. this ocurrs with either my 17 or my 19. so is trim the same as tilt or are we talking about to diffrent things

just curious, what sort of outboard boat you running that "goes super fast as in where's my tether cord fast" with a 17-19 prop?

BTW, the stv that blew over last weekend (pics in another thread) was turning I beleive a 10 1/2x 22. the blades were so sharp that when we were recovering the boat, I guess I lightly grazed the blade a couple times. Did not feel anyting and thought I was being careful to avoid the prop, but I sliced my hand open in two places. Razor sharp!!

David O
03-10-2005, 09:05 AM
4 very good chapters on props.

http://www.mercurymarine.com/chapter_1_-_history__development

If I remember correctly the Solas props Big Grizz is getting for us old volvo outdrives are 15.5 inch Dia props. From my experience and prop calc numbers the 15.5 X 21 and the 23 are very efficient.
I had no chine walking. I had porpoising only when trimmed to high and at that same time my speed also would start to drop.
Steering torque was controlled totally by proper trim. I have internal single cable Teleflex steering and when trimmed properly I could steer the boat in any direction at any speed with my little finger of my left hand on a right handed person.
Big note to note: when trimmed properly I had the greatest speed for a given rpm and no steering torque, uhmmm? Steering torque must translate into some form of DRAG. So you guys with the high dollar hydraulic steering, do what you can to make sure you are not concealing torque with the power of hydraulics. It will help your overall speed to eliminate the torque with proper setup not by adding power steering.
Power steering has many great benefits but hiding a poor setup is not one of them.

David

Lenny
03-10-2005, 09:33 AM
David, any chance you could snap a pic of your steering cable set-up and anchor point at the rear inside of your boat for me?

David O
03-10-2005, 09:39 AM
Yep, will get it at lunch today.

Lenny
03-10-2005, 06:11 PM
Only diff I see is that I use the exhaust manifold on port to anchor my steering support. Also a bad angle of the dangle.

Hmm...I like the way yours is set up.

I can "still" say that no one can turn this thing under full power one direction, no way period....

more hmmm....

MOP
03-10-2005, 06:39 PM
Only diff I see is that I use the exhaust manifold on port to anchor my steering support. Also a bad angle of the dangle.

Hmm...I like the way yours is set up.

I can "still" say that no one can turn this thing under full power one direction, no way period....

more hmmm....

Lenny having mounted a ton of the Volvo anchors the main thing to getting it set up the best you can is to get the cable set at the half travel position then locate the anchor so it is level with the tiller. If you have an upward or downward slope to the setup it is harder to steer, and even harder at one end of the travel depending on prop rotation. You anchor looks to low!

Phil

David O
03-10-2005, 06:50 PM
Lenny
Have you tried kicking the foot back/up or does you drive even have a means for changing the trim of the foot?
Have you checked all the bearings, shims, bushings, etc. in the out drive assembly related to steering?
Gene Schmidt also has an older volvo unit without trim, using an ultra prop and his boat drives fine but yet he does have single cylinder hydraulic steering so maybe he too has a lot of trim hidden by the hydraulics therefore we think it drives fine. it's an illusion!!!
David

MOP
03-10-2005, 07:03 PM
A little more to chew on! I was a Volvo tech for a long time, when they first brought out the Elephant ear props they looked weird. One thing they did do was help with tourque steer along with adding low end. The only input I have on chine walking is my experiance with outboards, to reduce and in many times eliminate CW was to raise the motor up. No amount of trimming or tab adjustment did much to help the problem. A good jack plate is the best solution as many times the engine needs to go up so much that the hole shot is GONE and it is a bugger to get them to plane! I have seen 10+ mile an hour gains with a jack plate or raising way high. Example one client had a G3 with a 90 Merc before the JP he ran 62 on a measured mile with the plate he ran 67 and handling improved greatly. As it has been said before no two boats are the same, outboards are a snap to setup compared to an I/O. Most everyone that I have spoken with that raised the X or gone to a shorty liked the improved handling but bitched about the hole shot. We need someome to come up with an articulating box with half shafts so we can raise and lower our drives X at will without screwing up trim angle like a Stern Jack which stressing the crap out of the gimble.

Phil

Lenny
03-10-2005, 07:13 PM
Phil. I like the responses. David, (as well as Phil) I agree, there is an angle to the dangle, (about 15 degrees) and I have three props, 2 of other rotation, and it just reverses the problem. The drive is relatively "tight". Not the issue I am told by local Volvo dude.

Yes, I am about 2-3" low on my "up-thrust" against the tiller arm. Also, it is attached to the stern end of the exhaust manifold. I am also running in the 3rd hole, (cuz I like it there and it is "flighty" which makes me smile :D ) and makes you think.

I have no trim.

Those might be good ideas to move the tiller up cuz mine is nothing like Davids. He is on an even plane.

Oh, and Phil, there is a Bravo transom assembly that lets you raise and lower the drive like a Jack-Plate only I am sworn to secrecy about it. I won't tell anyone, offline or on, now either. It comes from Poodleland but not from him.

mattyboy
03-10-2005, 07:17 PM
Lenny,
i have the same setup and have had all the same problems
bet ya i can turn it
the prop has alot to do with it and some slop can cause the drive to load one way under high power or make it osiclate from side to side ask Cliff :eek:
does the cable bind when at idle or not under power??
mine was fixed with a prop a rubber washer in the helmut and a tweak of the torque tab drives like it has power steering
drop me a line when i can get to the boat I'll take pics

BUIZILLA
03-10-2005, 08:07 PM
osiclateWTH kinda word is this?? :cistineb: :biggrin: :umbrella:
JH

mattyboy
03-10-2005, 08:15 PM
that's a 25 cent word ;)

ok wiggle back and forth

probably spelled it wrong

sounds like
a sil ate

joseph m. hahnl
03-10-2005, 08:26 PM
[QUOTE=Fish boy]just curious, what sort of outboard boat you running that "goes super fast as in where's my tether cord fast" with a 17-19 prop?


13" whaler with a vro johnson 50 go 45Mph with the tilt all the way up but is really scarey were you have to sit really low and in the center of the boat.

penbroke
03-10-2005, 09:04 PM
....I am also running in the 3rd hole, (cuz I like it there and it is "flighty" which makes me smile :D ) and makes you think....


Trimming the drive that high is going to take a LOT of correction from the fin. I normally run mine in the middle hole with easy steering. I made a set of bushings that allow me to set it between the middle and third hole (which is about where I think it should be) and it is quite manageable with some fin adjustment. I have run it in the third hole a few times but have not had the time to tweak it.

Keep in mind that any adjustments that work for one prop and trim setting may not work for another combination.

Frank

Lenny
03-11-2005, 02:43 AM
joseph m. hahnl

one of my FIRST boats in life was an 11' Boston Whaler Sport , (first with a 50 Hp Merc and then an 80, 4 cylinder both, stood forward to get it it to plane )

I am aware of what to expect even 30 years later.

...just that,... I want to be able to steer it :D

I've had 11' , 15' , 17' ...whalers

Fish boy
03-11-2005, 07:05 AM
[QUOTE=Fish boy]just curious, what sort of outboard boat you running that "goes super fast as in where's my tether cord fast" with a 17-19 prop?


13" whaler with a vro johnson 50 go 45Mph with the tilt all the way up but is really scarey were you have to sit really low and in the center of the boat.

I am with you on the whaler, one of my first boats was a 15' with a 70hp. Thinking back, this boat and the way I drove it, belongs in the "stupid things I did as a kid thread." :D Then again, I still drive boats that are fast and low to the water, so aparently I have a limitless supply of stupidity :rlol:

Fish boy
03-11-2005, 07:08 AM
BTW, here is a link to prop testing being done in fla I posted in the tech thread. Prop testing (http://www.donzi.net/forums/showthread.php?t=38854)

Unfortunately, it is the same day as dora, but I really could use the prop testing on both boats.

joseph m. hahnl
03-11-2005, 03:12 PM
yeeh fnn haaa!!!!!!! i got my minx home yesterday from florida. just needs a little TLC. but winter won't let go up here in the north it's snowing again today and again on sunday. any way the guy i bought it from said he had it going 72Mph
but when i,m looking at the specs on the other donzi owner it doesn't seem to jive. it has a 21 mirage with the chevy 260hp merc cruiser. seems like from what i,ve been seeing. it will only go 55mph. madpoodle you been tracking all the specs what would your best educated guess be?

HallJ
03-11-2005, 03:42 PM
yeeh fnn haaa!!!!!!! i got my minx home yesterday from florida. just needs a little TLC. but winter won't let go up here in the north it's snowing again today and again on sunday. any way the guy i bought it from said he had it going 72Mph
but when i,m looking at the specs on the other donzi owner it doesn't seem to jive. it has a 21 mirage with the chevy 260hp merc cruiser. seems like from what i,ve been seeing. it will only go 55mph. madpoodle you been tracking all the specs what would your best educated guess be?

I've got a Minx with a 350/Mag 270 and 23p prop.
I can get the speedo to read 65 at about 4700 RPM In some good chop.

If that helps at all?

Jeff

BUIZILLA
03-11-2005, 05:32 PM
I have spent mucho time and 4 props playing with mine, a 350-270. I have a pretty strong engine, for a stock engine. On GPS it runs 58-60 any given day, best of 62 in cool weather, with light aired chop. In summer it's 58-59.. tops. 65 indicated on the factory VDO speedo is an error for sure. When I was looking for one, one seller told me his would go 70, I told him if it didn't he had to refund $1000 for every mph it didn't do, as he stated. He never called me back to confirm that deal...

JH :umbrella:

gcarter
03-11-2005, 05:38 PM
Mine has a new Vortec engine w/ less than three hours on it. So far, w/ a 21" cleaver, at 4000 RPM (not WOT) it did 53 MPH on my Gaffrig liquid filled monster speedo and confirmed w/ Catch 22's GPS running next to me. I may have an additional 50-80 HP than the original engine. I'm looking forward to the adventure. :biggrin:

Lenny
03-11-2005, 05:56 PM
Mine has a new Vortec engine w/ less than three hours on it. So far, w/ a 21" cleaver, at 4000 RPM (not WOT) it did 53 MPH on my Gaffrig liquid filled monster speedo and confirmed w/ Catch 22's GPS running next to me. I may have an additional 50-80 HP than the original engine. I'm looking forward to the adventure. :biggrin:

ASSUMING a 1:50 drive

Ahh, George, with 0% slip you should see 53.03MPH at that RPM. Seems to me that that prop has hooked up as best as can EVER be expected.

That last 1200 or so RPM should be interesting. 15.90 MPH is still out there if it stayes hooked up.

gcarter
03-11-2005, 06:19 PM
Well, actually I know nothing aqbout the prop. It's number indicates a 23" pitch, but it has been struck through the "3" and a "1" stamped in its place. I have Jim Collins much newer 23" to try, we'll see how that does.

Air 22
03-12-2005, 07:24 AM
Chine walk can be caused by too much trim, poor hull design, strakes that run too far back, loose steering, crappy prop... I'm sure there's more, I'm just too tired to think...



adding to your list...and.....The Driver.... :rolleyes: :banghead: :bonk: :biggrin.:

Air 22
03-12-2005, 08:02 AM
Gosh J.R., I was trying to keep your name out of the discussion, but since you volunteered :biggrin.::biggrin.::wink::moped:


Isn't a rooster tail kool...... :rolleyes: :banghead: :D :D

HallJ
03-14-2005, 11:53 AM
I have spent mucho time and 4 props playing with mine, a 350-270. I have a pretty strong engine, for a stock engine. On GPS it runs 58-60 any given day, best of 62 in cool weather, with light aired chop. In summer it's 58-59.. tops. 65 indicated on the factory VDO speedo is an error for sure. When I was looking for one, one seller told me his would go 70, I told him if it didn't he had to refund $1000 for every mph it didn't do, as he stated. He never called me back to confirm that deal...

JH :umbrella:

Is there a calibration that can be done to the VDO gauge to get it a little closer to being accurate?

mattyboy
03-14-2005, 01:14 PM
yes remove the gauge and replace with a gps speedo ;)

no not really