Author Topic: Prop information...  (Read 21233 times)

codertimt

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Prop information...
« on: February 07, 2010, 02:07:34 PM »
Before the forums got wiped, we had a really good discussion on props going on.  So I am now going to try and recreate a little of that information in a sticky post here.

First some basics. There are 3 main specification for a prop, number of blades, diameter and pitch. Below I'll give a quick overview of these specs and how they affect boat performance.

Number of Blades:
It wasn't long ago that it was quite rare to find anything other than a 3 blade prop and they are still the most common. In general when you add more blades, you end up with less prop slippage upon takeoff and thus better planing time and better pulling capabilities.  Kind of like knobby tires give a vehicle better traction in mud, the extra blade(s) give the prop more traction in the water.  However the tradeoff is that once you're planned, the extra blade just kind of gets in the way, causing more drag and a loss in efficiency.  That means slightly lower speeds and possible drop in fuel economy.  There's are other things to be taken into consideration also when changing the number of blades, such as blade design but in general expect more pulling power, slighly slower mph and a slight rpm drop when going to a higher number of blades.

Diameter:
The props diameter is mostly just a byproduct of it's design.  The blade design, number of blades, etc determine what the diameter ends up as.  I believe the general school of though is that a slightly larger diamter will help with planing as it should have blades with more surface area thus be able to push more water.  In general shop by number of blades and pitch for your particular application and the diameter will generally fall in line and be okay.

Pitch:
This is the the most often quoted prop specification.  It's refered to in inches, but isn't an actual physical measurement of the prop rather the distance the prop will move though the water in one rotation.  So a 23" prop will move forward 23" in the water and a 19" prop will move 19" in the water.  To use another car analogy, it's pretty much like your tire size.  A taller tire moves a vehicle further down the road in one rotation.  But that taller tire is more difficult to get moving, thus takeoff power is reduced.  The same thing with a prop, the lower pitch equates to better pulling power and lower speeds.  The higher the pitch the slower the takeoff, but faster topend is possible.  However, there are limitations, you can't just put a huge prop on any boat and expect it to get faster and faster.  The boat must have enough power to turn the prop efficiently.  Similarly if you choose to small a prop, the motor may be able to turn it too easily and you could overrev the motor at WOT.  Rule of thumb is a 200RPM change for every 1" change in pitch.

Prop Selection:
So now that we have the basic, what does that tell us in regards to how to select the correct prop.  Well unfortunately, it really doesn't tell us enough.  We actually have to have real world numbers to go by to see if a prop is appropriate for a certain setup.  So if you were starting with a new boat, technically the first prop choice would just be a best guess.  However, the factory has already done some testing and put a prop on you boat they feel is appropriate.  However, it might not be appropriate for your situation.

The general rule is that a boat should be propped so that it is able to reach the engines recommended RPM range at WOT, generally 4400-4800(possibly 4600 for the Volvos, but I see no reason it couldn't be run to 4800 too) RPM for a Mercruiser inboard/outboard.  However, this is greatly affected by the load in the boat.  So if you plan to use your boat fully loaded most of the time, then the factory provided prop is generally too large.  Also, if you only use your boat for watersports, you might choose to even underprop your boat for more pulling power, with the knowledge that it would go over the 4800RPMs if you allowed it.  You obviously have to be careful in that situation, but your slalom skiers might thank you.

I'll finish up here with an example from my specific boat, to show how it reacted to certian props and loads. Most of our boats here are fairly similar in construction, so will react similarly.  

My 2007 GT185 with 4.3 Mercruiser came with an aluminum 23" 3 blade prop from the dealer, diameter unknown.  It would run 54mph on the gps with only me aboard, I think at around 4600-4700 RPM.  When I loaded it down with my family, the rpms dropped and it would only manage around 47-48mph.  It was a struggle sometimes to for my 250lb self to get up on a slalom.  

I decided to go with a 4 blade 21" prop.  Thinking I would loose about 200RPM adding a blade and gain about 400RPM going to the smaller pitch.  I expected to be at around 4900RPM with a better hole shot when it was done.  I did end up with a bit better hole shot, but my rpms ended up not really changing.  I ended up at around 49MPH@4600rpm with just me on board, but only dropped a mph or two when the whole family was aboard, so ended up with about the same max speed fully loaded.  The reason I didn't get the "expected" rpm change probably had to do with the fact the new prop had completely different designed blades.  So there were too many variables changed(number of bladed, design of blades, pitch, diameter) to properly calculate.  It became a trial and error thing.  I'm fairly happy with the change, but if I had it to do over I would probably drop down to a 19" for more pulling power and just watch the rpms up top if I needed to.  

Anyway, hope this is helpful information for those perusing.  Feel free to ask any specific questions and I'll try to help out as much as I can.
« Last Edit: December 31, 1969, 06:00:00 PM by J&J »

Adrift

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Prop information...
« Reply #1 on: February 07, 2010, 09:59:44 PM »
Good Stuff.  I'll chip in with my experience...

I have the Volvo 4.3 in an 08 GT185 and it also came with an aluminum 23 pitch 3 blade.  I use the boat to tow (mainly tubes) and after reading a bit decided I needed more of a "tow" prop.  I selected an aluminum 21" 4 blade Solas Amitas (?) for about $100.  On the old thread I had all the revs and speeds.  I don't remember it all but coders numbers sound correct.  IIRC dropping to the 21 4 blade improved hole shot, had minimal impact on revs and dropped top speed slightly.  Unfortunately, the 21 was destroyed on my 3rd or 4th outing.  I replaced it with a 19" 4 blade aluminum Solas Amitas.

The 19 had much better hole shot than the 23 and better than the 21.  However, as expected, top end speed was reduced significantly.  The 19 will over rev my 4.3 so that has to be watched.   I also noticed that I have very little trim before the boat starts to porpoise.  For my purposes, towing is my main desire not top end so the cons were acceptable.

If I trashed the 19 I would probably buy another for my main purpose.  However, If I knew I was just cruising for the day the original 23 (now my spare) would be the better choice with the 21 being more of a dual purpose use.

HTH
2008 GT185 Black XL Bimini 4.3 Volvo

jt185

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Prop information...
« Reply #2 on: February 16, 2010, 09:38:33 PM »
I appreciate the info! Nice of you all doing the research and trial and error for me:) I'll be making these decisions this summer.
JT

2010 GT185XL Black w/4.3 Merc

J&J

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Prop information...
« Reply #3 on: March 30, 2010, 12:06:10 PM »
Thought I would post my experience.  Our boat orginally came with the 14.25x23 aluminum 3 blade prop.  It was fine for me and the misses for cruising, but sucked when having a boat full of people and skiing.  This prop however never really got the rpms to the high end of the engine (4800).  Talked to the dealer and they swapped out the 23 for a 21.  The 21 was much better, got the boat to 4800 rpms and better hole shot.  Worked better for skiing so I was pretty happy.  Then I wanted a spare prop and after a bunch of reading, I thought I would try a 4 blade prop.  I figured if I could get even more of a hole shot without losing too much top end and keep the engine in the right rpm range then I would have the perfert prop!  I chose the 14x20 Michigan Wheel aluminum prop for I read good reviews about their prop.  I also chose aluminum for I could not justify spending $300+ dollars for a prop that might find a submerged object.  I could not be happier with this prop!  Not only did I get more pull and keep the engine with the correct rpms (only lost about 3 mph), I noticed better control of the boat at idle speeds.  The boat does not seems to wander as much while idling through no wake channels.  Also, I noticed the boat seemed smoother (not as much vibration felt through the boat) with the 4 blade.  That being said, if I had to do it over again, I think I would go to stainless prop with either 4 blades or possibly 5 (the merc High 5 is supposed to be the best prop for those that do a lot of water sports!)  I wish there was any easier way to get the right prop without having to go through so much trial and error.  Oh well, hope this helps someone.
2005 Glastron GX205
Volvo Penta 5.0GXi

SDDSMAN

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Re: Prop information...
« Reply #4 on: March 18, 2011, 09:20:38 PM »
Thanks for all the info guys.  I boat in the Illinois river and I go through at least one prop a season.  I'm getting better, but the river hides a ton of rocks.

I have a Volvo 3.0 and want to experiment with a 4 blade prop.  The boat is fine with just me in it (43 mph) and when my family (3 of us) joins in I'm at 38 mph.  If I add a few more people it's hard to plane.  I'm hoping a different prop might help me plane better.

I'll let ya know how it goes.
Steve / Colleen / Bryan

GT 185 (2008) --  Lil Babe
Batavia, Illinois

orion1335

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Re: Prop information...
« Reply #5 on: March 30, 2011, 03:54:40 AM »
I just finished reading the prop discussion, and I myself am considering the Michigan Wheel Prop. I have also read good things about the cupping of the blades. My stock prop, bought from The Boat Brokers in Lake Havasu, Az., delivers good low end, and I, with my wife in the boat with me, reached 63 mph on glass, early one morning. I verified it with a gps, and the boat was trimmed right on the edge. It began to chine walk, so I backed down since my Wifes eyes were so big I figured she had had enough. I would like to not loose that speed, but I do need better low speed manuverabillity. I think I will research the Michigan Wheel further, and perhaps make a phone call to get some specifics. Thank You All for the help. This has helped me tremendously. C/C

Bilito

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Re: Prop information...
« Reply #6 on: April 20, 2011, 09:55:33 AM »
I have an SX-175 with a Volvo 3.0, the prop is a SS 14 1/2 X 20, 3 blade, cupped. I get a good hole shot, she gets up on a plane fast and will take the wife and me to 45 mph GPS in smooth water. It just hate the way it walks all over the place when idleing out the very long Black Point channel to the bay. I have had it up to almost 40 with 5 aboard
I never checked the RPM at WOT I was having too much fun at the time

prawnhead

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Re: Prop information...
« Reply #7 on: June 16, 2012, 03:54:02 AM »
Anyone have any experience with the Volvo Duo Prop?  I have a 5.0 Injected Volvo, with a single ally prop which has a few dings and was wondering when I change the prop is there any advantage in going to a Duoprop leg.
2008 GS 249

camdelle

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Re: Prop information...
« Reply #8 on: June 16, 2014, 09:08:01 PM »

2008 glastron GT 185 New to Me! Over reving.
So I bought this boat on Friday and used it for the first time yesterday, This is our first boat But I have been driving boats for 35 years.

Here is the issue. On Sunday I went WOT a couple times for a minute or so. It reved up to 5100 rpm and that is to much for the volvo penta 4.3GL.

I would like to re-prop the boat.

Right now I am using a 14.3 x 19p 3 blade that owner put on.

I have a 14.5 x 21p that was original to the boat.

I also have a mercruiser vensura. 48-825900 19p 4 blade SS

As I can Hit 5100 rpm(by the dashboard gauge...(how accurate is that?) and I should be around 4200-4600..... what should I be doing to get these numbers in line?

It looks like original equipment should have been a 23p???

thank you.

codertimt

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Re: Prop information...
« Reply #9 on: July 22, 2015, 03:29:26 PM »

2008 glastron GT 185 New to Me! Over reving.
So I bought this boat on Friday and used it for the first time yesterday, This is our first boat But I have been driving boats for 35 years.

Here is the issue. On Sunday I went WOT a couple times for a minute or so. It reved up to 5100 rpm and that is to much for the volvo penta 4.3GL.

I would like to re-prop the boat.

Right now I am using a 14.3 x 19p 3 blade that owner put on.

I have a 14.5 x 21p that was original to the boat.

I also have a mercruiser vensura. 48-825900 19p 4 blade SS

As I can Hit 5100 rpm(by the dashboard gauge...(how accurate is that?) and I should be around 4200-4600..... what should I be doing to get these numbers in line?

It looks like original equipment should have been a 23p???

thank you.

I know this post is over a year old, but I thought I would reply anyway for folks that might happen to be reading in the future...and maybe the OP might drop back by at some point with feedback on what direction they decided to go...

I have the same basic boat as mentioned in the original post...and I personally think the answer is "it depends on how you plan to use the boat"...

If you plan to regularly have 4 adults on board and try to do water sports, then the 19p prop is probably a good choice...even if it can overrev at WOT...  I would personally love to have  that 19p 4 blade SS prop...  That would be the perfect prop for me and my use cases...
 
I think the 23p that mine was originally equipped with was too much prop for anything that cruising while lightly loaded.  The 4 blade 21p I have now does okay with me, my wife and 3 kids...  Once we get other adults on board though...I need more prop to pull slalom skiers well. 

Now, as for the accuracy of the tach...don't know...supposed to be within 2% or something like that, but with age who knows....  I just tested my boat again after a long dormancy and with just me and perfect water I hit 50mph@4900 with my 21p 4 blade.  That's more rpm that I have tracked from the past...but it's also just a titch faster...so I'm not sure if the tach is reading off now or not...  I personally wouldn't worry too badly about it...  You aren't going to be cruising around at 4000+ RPM most of the time... Prop it to where it performs like you need it to when loaded and then just watch yourself when it comes to extended times around WOT...

GuyStron

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Re: Prop information...
« Reply #10 on: September 22, 2015, 09:28:57 PM »
Good info!  Question:  I'd like to get a new prop, but how do you determine the pitch of the e9xisting prop?

Also, what might you recommend for the boat I just picked up? It's a '96 GS 160 with a 115 hp Johnson outboard.  I'm converting it from a ski to a fishing boat.  I'll be getting a trolling motor and I drift a lot, so looking for more speed to reach destinations more quickly. Just had it out this weekend and gps said I was only going 36 mph with 2 of us in the boat. Motor runs good and did not trim up when cruising, as it seemed to plane out nicely.

Thanks, in advance!

Aussie Dave

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Re: Prop information...
« Reply #11 on: August 01, 2016, 02:55:38 AM »
I have recently purchased a 2004 Glastron 5.2m with a 4.3 Volvo penta in it. Can anyone advise me on what the best prop would be for use with water sports? It currently has a 3 blade 14.3 x 21 prop on it. I would appreciate it if some one could point me in the right direction with this. Thanks and have a great day.

Regards

Aussie Dave

Aussie Dave

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Re: Prop information...
« Reply #12 on: August 01, 2016, 02:56:55 AM »
It is a GT 185

Regards

Aussie Dave

The Canadian Guy

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Re: Prop information...
« Reply #13 on: December 09, 2017, 01:52:51 AM »
Pirhanna Prop

I bought a Pirhanna Prop with changeable blades for my Glastron SX 175 w/ 135hp inboard.  It's made from carbon fibre and I get about 5 km an hour more speed out of the boat.  With interchangeable blades it's a breeze to change a blade on the prop without having to take the boat out of the water.

I think it's a great investment.  Cost me 350 at the Ottawa Boat Show.
2006 SX 175
Lives In Ottawa, Canada

MrPix

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Re: Prop information...
« Reply #14 on: January 28, 2018, 11:24:35 AM »
What prop change might people suggest for cruising at idle?

I have a Volvo Penta 5.0 GXi into the DuoProp with a 1.95 gear ratio. My cruising style is either showing off to friends, which means getting her lifted but throttling back to around 2800 rpm to get the best fuel economy (about 20% of my time) OR puttering along in idle or just above (1000 rpm or less) where I consume about 1.4 GPH and make about 4-6 mph.

Looking ahead, I'm considering the Great Loop. If a prop change (90% of the time I'd be 10 mph or less) would save me more than its cost in fuel, I'd do it. I just have to consider that it being a DPS-A my propeller costs are doubled.

What do you think?