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-   -   What size prop? (https://www.moyermarineforum.com/forums/showthread.php?t=6774)

smp 09-05-2012 11:22 AM

What size prop?
 
I don't know anything about props. I know that I have a two blade fixed on my C&C 32 and I think it's right hand drive and I think that because I read that right hand drive has prop walk to port and that's what I have! Lots. Right now backing up is really difficult. Would a folding prop make this harder or easier? Probably a tough question to answer as there are different designs. What size should I be looking for? There's a guy locally selling a folding prop it's 15" diameter with a 12" pitch and 1" shaft, is this right for my boat?

I want to go faster :)
And if it improves reversing that's great. If it makes it worse that's bad. If reverse remains tricky but the boat goes faster that's still a win.

Thanks for any and all input.
Steve

Mo 09-05-2012 11:32 AM

Folding prop is good for racing your 32....will have a considerably less performance than your two blade in reverse. I see you are in Toronto so docking might not be as horrendous there as here some days. I have a 3 blade and it's staying on just for the reverse....I'll take the hit on speed. Some days I can have 30 kts of wind on the stern coming into dock...I can lay her there.

Mine is 12/6 (pretty sure). The stamp is hard to read...either a 6 or a 9 but I don't think it's a 9....performs very well with my 30.

Hope that helps you out.

Dave Neptune 09-05-2012 11:56 AM

Walking!
 
Steve, the only way to really reduce prop walk is to get away from a 2 blade and go to 3. I did so for the same reason as Mo. Now I can back out of my slip and I have more power forward. I went with a small 3 blade and it actually has less drag than the 2 blade I removed. I went from a 13 X 7 to the Indigo and have been happier and faster ever since, that's sailing and motoring:D.
My boat is 35' and really works the lil' beastie below;).

Dave Neptune :cool:

sastanley 09-05-2012 12:48 PM

Steve, that prop is WAY TOO BIG for your boat, probably even with a reduction drive Atomic4. (edit, thanks Mo..I think you're right..15" too big even with reduction. ;))

One major difference between most Atomic 4's and their diesel counterparts is most diesels have transmissions that reduce the prop speed as compared to engine speed by typically 50% (2:1 reduction) - most Atomic4's are direct drive..the 'detent' you feel when shifting into forward locks the clutch plates so your 'transmission' (reversing gear) and prop spins the same speed as the engine...Slower prop speeds allow you to swing a bigger prop..faster prop speeds require a smaller prop. This presents some challenges for efficient motoring and it is easy to overprop an Atomic4.

You probably want a smaller prop..the Atomic4 does not make enough power at typical cruising RPMs to spin a big prop. The Indigo prop mentioned by Dave is a 10" diameter x 7.4" pitch 3 blade prop specifically designed for the Atomic 4.

As Dave & Mo commented a two blade will walk more than a 3-blade, generally speaking, and also generally have less sailing drag (Dave's huge old prop being an exception). Total blade area (i.e. drag when sailing) is a factor, and there is a trade-off...a better prop for motoring will result in some trade-off for sailing...a prop that is good for sailing (folding) will result in reduced motoring performance, as they typically don't back real well. A 2-blade folding or feathering prop would provide the least sailing drag. A fixed 2-blade prop can be stored vertically to hide it behind the keel/deadwood/strut to some degree..a 3-blade does none of the above, but probably performs the best under power.

There are other props for Atomic4's...i.e. the CDI prop..which has flexible blades that flatten out at higher RPMs to unload the motor...they are a type of plastic though, so conventional wisdom would indicate they are more prone to damage (I have no personal knowledge of this, my own theory). There are prop companies ($$) that will make a prop that you can adjust the pitch on so you can find the perfect match for your boat & sailing conditions..these may feather or fold, but are likely pretty expensive..a company like MaxProp or Kiwi Props comes to mind here.

One more thing about backing up with an Atomic 4 boat. The reversing gear assembly provides a slight reduction to create "reverse"...about 1.3:1...so 2,000 engine & prop RPM in forward (w/ direct drive) does not equal 2,000 prop RPM in reverse. I can run WOT about 2,250 RPM in forward, but spin the engine over 2,600 RPM in reverse at WOT...and it scares you at first because the reversing gear screams and howls, but I think the engine loves it since they generally run in a lugging condition under 2,000 RPM. :cool:

So, now you know a little more about props...don't buy that huge 15" prop unless you are certain you have enough tip clearance (distance between prop tip & hull), a 2:1 reduction drive on your Atomic 4, etc.. - The absolute largest prop I could spin on my C-30 is 12" diameter, due to tip clearance, unless I moved the strut aft and a longer shaft to get it away from the hull. I have the 10" Indigo and took the slight sailing performance hit, for the happier engine and motoring advantages. If I raced my boat, I'd have a reasonably priced Martec 12" x 6" 2-blade folding prop and deal with the middle-of-the-road motoring performance.

edit - Steve, after my long dissertation on props, I have some questions for you.."you want to go faster"...motoring or sailing? The boat will only go to hull speed, and a larger sailboat (i.e. over 30' or so) will never get there with the size motor that is in them..you can get close, but there is simply too big a trade-off between HP & prop size to get a boat to theoretical hull speed under power. With a super clean bottom & clean prop and good running engine, I am in the 6.05-6.1 knot range at WOT in perfect conditions. One week's worth of bottom growth knocks this to 5.8-5.9, so the conditions have to be perfect all of the time..flat water, clean bottom, clean prop, strong engine, no wind, etc... It does not take long to knock the performance off, and as you can see, there are many factors. Therefore, all that being said, I like to cruise about 5-ish to 5.2 knots...That is about 1,850-1,950 RPM and the motor is happy and I am happy. Motor sailing helps a lot too, and I usually have the mainsail up in 95% of my "under way conditions" and it can give me anywhere from 0.1 knot of help to a knot & a half sometimes.

So - several questions for you - :D
1. What do you have now?
2. Do you want to motor or sail faster? (How much faster?)
3. Do you want to motor faster at the engine's expense?

Mo 09-05-2012 12:54 PM

Shawn is right. Over looked the size of the prop you were looking at....forget it even with a reduction drive...too much for her. The engine won't turn it and then you will get off the dock in a breeze and the boat can't get going fast enough....then you blow into someone.

There is sweet spot with props...go outside that range and boat control, in various situations, is gone out the window.

smp 09-10-2012 10:51 AM

Thanks for all the input guys, much appreciated.
To answer your question Sastanley, I want to go faster under sail and I would like to reverse better. I'm on the C&C mailing list and I was following a thread about folding props and I read a few comments about IMPROVED reversing performance with folding or feathering, can't remember which. Anyway, on the long list of projects that require large inputs of boat bucks I think that a new prop is very low on the list. You guys have shed light on the subject for me and I thank you :)

sastanley 09-10-2012 10:31 PM

Steve,

From my understanding, a feathering prop is better for motoring performance (especially in reverse compared to a folding), and will give you the best 'under sail' performance as well in terms of the least amount of drag. The compromise in this case is $$$, as feathering props tend to be expensive.

I know a guy on Sailing Anarchy, by the name of "RockHead"...he is a US dealer for a good feathering prop company. I think it is Flex-o-Fold.

Skywalker 09-10-2012 11:28 PM

Neptune

Did you really gain boat speed with the Indigo?

On my Tartan 27, I started with the original two blade 13 x 7, switched to the Indigo to deal with he very strong current in my harbor, then switched to a Vari prop two blade feather when racing became more important.

If I did not race, I'd stick with the Indigo. Unfortunately, I sold the Vari prop (long story), so now i have to chose between the two blade 13x7 and the Indigo.

I'm still racing...

I'm leaning to the two blade and hiding it behind the deadwood.

Dave Neptune 09-11-2012 08:43 AM

Choice
 
Skywalker, I did not loose any speed with the Indigo when changing from a 13 X 8", which has about the same "area" exposed. My prop runs in the open not behind anything. If you can tuck the 2 blade behind it is less drag. Remember the Indigo is quite small at 10" diameter and with the "seconds" you gain with a 3 blade rating may just offset the speed and make a gain for you, it did in my case.

Dave Neptune :cool:

Skywalker 09-11-2012 07:38 PM

Thanks for the feedback.

I'm torn. I love the performance of the Indigo but I get a little crazy with the racing.

I'm planning on entering the Around Long Island Regatta again, a 190 mile ocean race.

Weekly beer can, local regattas, too.

I don't know.

Loki9 09-11-2012 08:06 PM

Quote:

Originally Posted by Skywalker (Post 59136)
Thanks for the feedback.

I'm torn. I love the performance of the Indigo but I get a little crazy with the racing.

I'm planning on entering the Around Long Island Regatta again, a 190 mile ocean race.

Weekly beer can, local regattas, too.

I don't know.

I was in the same boat, so to speak, and I went with a Martec 12x7 folding prop. I'm very happy with it, easiest 1/2 knot you can get, and I don't feel like I gave up anything to get it (other than $$$).

Skywalker 09-11-2012 08:32 PM

Unfortunately, my T 27 has an aperture - no room for folding and don't have two grand for feather!

Loki9 09-11-2012 10:27 PM

Oh well, hiding a 2 blade is probably the best option then.

sastanley 09-11-2012 11:24 PM

Skywalker...do they give you a 6 second PHRF credit where you are for a fixed prop? :confused:

Skywalker 09-12-2012 06:02 AM

I've gotta dig out my old PHRF certificate. I think it's only 3 seconds.

sastanley 09-12-2012 08:35 AM

Hmmm..OK..I was just curious. :cool:

I only do one "race" per year in my boat, and it is a fun double-handed one. I use a 6 second credit here on the Chesapeake, but my boat hasn't had a real PHRF cert for 20+ years.

In reality, none of that matters much..6 seconds per mile is something like 2 boatlengths (too early to do the math).. :rolleyes:

New sails, a clean relatively smooth bottom, not carrying 500 lbs. of junk on board and no bad crew work is really what wins the race.


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