Propellers

captainlarry84

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Propellers are the most important part of your vessels arsenal. It is pretty much a known fact that three bladed propellers are faster that four bladed propellers.
It is also true that New England boats run better & faster with four bladed propellers because of keel inference. The reason they work better is that no matter where the propeller is during any time of rotation there is always the same amount of blade area on both sides of the keel, which gives the smoothness and efficiency needed to defeat keel inference.
I think this photo of the titanic’s propellers stated it all in nut shell. She was designed well before her time and was doing 20 knots on her maiden voyage. To get the speed & efficiency the wing motors had three bladed propellers and the main had a four bladed propeller. Image the casting of these wheels back in the day. In addition they had to get the four bladed & three bladed wheels to work in sync…. And that they did @ 20 knots.
Just though I would share a little history & propeller reasoning

1285284602-titanicpropellers[1].jpg
 
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El Mar

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captainlarry84

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Cup is similar to pitch only it is just on the edge of the blade. Cup is added in numbers from 1-9. A good propeller shop when they recon will usually stamp the wheel such as 22 x 22 #4 meaning a four cup.

Cupping is good to help quiet a wheel down if there is close bottom clearance or if the wheel needs efficiency help to hold the water onto the blade longer.

Cup is not free and it cost you load which slows down RPMs.

Many times in tuning a wheel to get adjusted RPMs cup is added rather than re-pitching the wheel.

The less you re-pitch the better and cupping is a simpler and easier way to adjust RPMs.

If you want to lose RPMs add cup to gain RPMs reduce cup.

A good propeller shop should be able to sell you the wheel you need within the 1 Once done you can tune a little with cupping.

On my JC31 they hit the wheel right on with a 23 X 21 from the factory. Once seatested and loaded I had a slight over turn of about 50 RPMs. Rather that fool with pitch a #3 cup was added and it was spot on.

The photo is my Class I nibral 23 x 21 #3. You can barely see the cup on the edge but it is there. This is what a wheel should look like the start of every season.

fit.jpg

Picture 098.jpg
 

tunaorlater

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Larry whats your feeling on wether or not you loose speed with cup? Meaning if you add a #4 cup you cold probably add an inch of pitch with the increased pitch would have more foward movement. On my NB the added cup brougt the bow higher out of the water, not sure if that is normal or not but both my 36's acted the same way.
 

captainlarry84

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Cupping is for a 30 to 50 RPM problem. Pitching is for a load problem in excess of 100 RPMs or more. You do not correct a 100 RPM problem with cup, that is for pitch to do. A 100 RPM or better load factor must be carried by the entire blade and not just the edge with some cup. If you cup to correct 100 RPM the load on that edge will weaken and start to flatten out the cup over time.

Cupping does increase bow raise on N/E boats. The reason being is that without cup the water flying off the boat hits the bottom hard and gives upward thrust which lowers the bow. With cup the water is thrown to the side and it allows the stern to sink into the void caused by the water loss.

Back to your N/B what size wheel & how much cup? A #4 cup is about 1/3 of pitch. Give up your numbers and we will kick this around.

N/B I think is a 28 wheel, pitch is nextwhat is yours?
 

tunaorlater

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26x24 #3 cup i believe might be a #4. 480hp volvo 1.73 to 1 24.5 knots wot full load. Turning 2650rpm

The old NB was 1.53 to 1 610hp volvo 28x27 #4 cup. 28 knots at 2250rpm

We added cup to the old boat for lift and added to this one because it worked the first time and I didn't want to redo the prop again.

The difference in the 2 engines is I could have run that 122p from the ocean through ocean beach back into the bay and the rpm would not change the whole time! lol that motor was a beast!
 

JimRP31

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Would a 3 versus 4 blade prop be the same on an outboard?
 

Tunascapes

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Would a 3 versus 4 blade prop be the same on an outboard?
3 blade vs 4 blade is definitely not the same on an outboard. You need to drop down in diameter and usually pitch when you go to a 4 blade. A four blade is great if you are pushing a load or want more consistent numbers at cruising speeds.
 

JimRP31

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3 blade vs 4 blade is definitely not the same on an outboard. You need to drop down in diameter and usually pitch when you go to a 4 blade. A four blade is great if you are pushing a load or want more consistent numbers at cruising speeds.
Thanks for your reply. I have a SISU 22 with a 115 OB and I am turning too many RPM's and having trouble getting the bow down. I am thinking tabs or trying a different pitch propeller.
 

Tunascapes

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I would tweak the prop and get it right with the same number of blades as you are currently running and then think about switching. Props really can be confusing some times and you need to have one that works right so you know how to change it.

I have always run a seaway boat that is similar to your sisu and tabs are a must. You will be amazed how they help.
 

JimRP31

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Thank again for the insight. You are on a long list of people who told me tabs are a must.
 

CaptDave

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Time for a new prop on my Eastern 31, it is a John Deere 6076 AFM 30 rated at 300 HP with a TD 1.77:1 Max RPM is 2400. Hoping to get a little speed ie: honest 15 knts cruise with full charter. Currently with the deck and foamed in tank out, I believe we will reduce the weight of the cockpit by about 800 LBS. We are going with Fir 2 X 4's as stringers and 3/4" Bluewater Nautical 26 Coosa Board. The fuel tank is out being fabricated now along with the new Wynne windows. Lots of changes and very excited about the upgrades.

Dave
 

captainlarry84

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The fun part...

Dave,

I lot going on I see. 800 pounds out? Sounds like a lot of weigh. If that weight is correct she may be a little bow down at rest. New tank placement may need to be researched to make up the trim.
What size propeller are you spinning currently? That will help figure this out.

On a guess.. Your current max shaft speed will be 1336 RPMs @ 300 HP and a Cruising RPM of 2200 with a shaft speed of 1242. My wild guess would be a 23 X 22.

This should give you about 75% wheel efficiency with the following numbers:

WOT @ 2400 ROMs 18 knots
Cruising RPMs @ 2200 16.6 knots

If you drop to a 23 X 21 with reduced pitch you lose a lot. If you keep the pitch & drop the Diameter to a 22 you will lose your 75% efficiency and lose more speed.

With the 23 X 21
WOT @ 2400 ROMs 17.3 knots
Cruising RPMs @ 2200 16.0 knots

This is my best guess without have your current information. Lastly before you start to wheel your boat always photo tach the motor, check the high speed governor with the photo tach at no load & change all fuel filters for maximum fuel flow performance before sizing the wheel.
 

CaptDave

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Larry, I will let you know what we currently have, no choice but to have the fuel tanks where they are with placement. Yes the 800 LBS is a lot but the boat has been changed dramtically over the years with the removal of the Flying Bridge and the Galley changed to a Seatee. Whoever buys this will get a good boat just like yours and still can not believe yours is still there as it is the best 31 out there and the low price is a steal.

Dave
 

captainlarry84

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Thanks DaveShe is turn key & ready to go. I am selling her in her prime and hope the new owner keeper her in the condition that she is used to.

If you give me your old wheel information and we already have the weight loss I think I can compute your wheel pretty close. I enjoy doing the math with my back yard method. In fact I think I am within 1 already.

I like the idea of the cold tar coating it looks pretty easy. What is the plan for securing the tanks as it seems like foaming is the swan song for metal tanks.
 

F/V First Team

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Larry,
Your nuts are on backwards.
The half nut is supposed to be against the propeller, the full nut tightens up against the half nut and takes the pressure off the threads of the half nut.
Just a heads up
 

captainlarry84

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LOL............. I know they are. No comment on how that happened I was waiting for some to spot the Opps. The good news is that all of my other nuts are not......
 
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