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  Topic Review (Newest First)
3 Weeks Ago 04:18 PM
travlin-easy
Re: 2 blade or 3 blade propeller

For me, with a full keel boat, there are significant advantages of using a 2-blade prop over a 3-blade. The biggest advantage is in reverse, while trying to control the boat and backing up into the slip. The 3-blade prop does not have sufficient thrust in reverse because of the close proximity of the 6-inch thick keel, which is only a few inches away from the back of the prop. Much of the thrust is applied directly against the back of the keel, which is flat, thus it has no effect in backing up the boat. Additionally, the thrust generated against the keel is deflected outward, thereby interrupting what little thrust is toward the bow of the boat.

With a similar size 2-blade prop, which will be significantly longer, more of the prop projects beyond the margins of the keel, thereby allowing more water to be thrust toward the boat while backing up.

I will be installing a 2 blade prop later this season, when the cash flow improves a bit. I was going to buy one this week, but after noticing the car had four bald tires, the tire store got the prop money, plus a lot more this morning.

Now, I noticed the OP has a wing keel, so there really should not be a significant difference when motoring, either forward or reverse.

Good luck,

Gary
4 Weeks Ago 12:01 PM
overbored
Re: 2 blade or 3 blade propeller

Quote:
Originally Posted by Captin G View Post
How about RPM's verses changing prop. I have two issues. I can not seem to correct the vibration of my two blade. I removed and balanced. Then Aligned shaft. Still can't keep my cutlass bearing from loosening. The screws back out. I tried a little epoxy on a new cutlass and screws. lasted a few months. I thought a three blade may have less vibration.
Also, with a little head wind and current on the nose I can only get 2 or 2.5 knots motoring. I have a 35 hp perkins 3 cylinder parama M35 in a Heritage West Indies 36 (17500 displacement). I have a manual and have asked around however I can't seem to get a straight answer on RPM's. I try not to go over 2000, 2100 max. I would like more speed and less vibration. Can I rev her up to 2500 rpms? Anyone?
it's not because it is a two blade that it is vibrating. there are hundreds of thousands of two blade props that don't vibrate. you have a problem in the prop drive system. sounds like bent shaft or prop. you have excessive runout of some component. maybe corroded flang surface?
how did you do the alignment? Did you check the runout on the output flange? did you check prop shaft runout with a dial indicator? did you use a set of feeler gauges on the flange?
Your engine should cruise 2800 to 3200 rpm
4 Weeks Ago 09:51 AM
Minnewaska
Re: 2 blade or 3 blade propeller

For under $20, you can buy a digital laser tachometer that reads RPM off whatever you apply a reflective piece of tape upon. Good idea to use one to check the accuracy of your hardwired tachometer.
4 Weeks Ago 09:49 AM
Tanski
Re: 2 blade or 3 blade propeller

Does it vibrate through out the rpm range or just where you are trying to run it?
4 Weeks Ago 09:23 AM
colemj
Re: 2 blade or 3 blade propeller

The governed rpm on the Perama 35 is 3600. Your cruising rpm should be ~2800, and there would be no worries running continuously at 3200.

Always operating the engine at 2000rpm is bad for it.

Mark
4 Weeks Ago 08:16 AM
SanderO
Re: 2 blade or 3 blade propeller

I have had a fixed 2 blade prop on Shiva since I got her 32 years ago. I have no basis for comparison. But it would be interesting to try a season with a 3 blade or even a folding or feathering prop... since I am quite familiar with performance on this boat with a 2 blade fixed.

I don't race... fractions of a knot are usually of no interest to me... I know this can add up on a long passage to actual time. But really... does it matter? I sail to escape the constraints of time and the clock and appointments and so on... I sail to enjoy the boat, the motion, the wind, the waves... the solitude... the wonder and so on.

The two blade pushes the hull at hull speed, backs well... and seems to not have noticeable drag. It's simple. The prop seems to align itself in a vertical position when sailing.... I don't know why... maybe the orientation has the least drag???


I am not motivated to spend thousands for an advanced prop.
4 Weeks Ago 08:02 AM
Minnewaska
Re: 2 blade or 3 blade propeller

75% of max rated rpm is usually a rough sweet spot. You need to know what your motors max rated rpm is.

However, I don't think RPM is the cause of your vibration. Even if more seems to smooth it out, you should still figure out why its vibrating so much. You said you aligned your shaft, which is the first thing I would do. Were the engine mounts in good shape or could they be changing after you started the motor.
4 Weeks Ago 11:51 PM
Captin G
Re: 2 blade or 3 blade propeller

How about RPM's verses changing prop. I have two issues. I can not seem to correct the vibration of my two blade. I removed and balanced. Then Aligned shaft. Still can't keep my cutlass bearing from loosening. The screws back out. I tried a little epoxy on a new cutlass and screws. lasted a few months. I thought a three blade may have less vibration.
Also, with a little head wind and current on the nose I can only get 2 or 2.5 knots motoring. I have a 35 hp perkins 3 cylinder parama M35 in a Heritage West Indies 36 (17500 displacement). I have a manual and have asked around however I can't seem to get a straight answer on RPM's. I try not to go over 2000, 2100 max. I would like more speed and less vibration. Can I rev her up to 2500 rpms? Anyone?
10-11-2012 09:00 PM
HMoll
Re: 2 blade or 3 blade propeller

I'll tel you what's dragging: this thread! For cruising in heavy currents, on a small boat: 3 blade-folding. I went from feathering 2-blade maxprop to 3-blade folding flexofold. Gained a full knot motoring and maybe lost .2kts sailing, which is unmeasureable, 'cause I probably "lose" more than that just chillin' and enjoying the ride! Get 3 blades. No brainer.
10-11-2012 08:27 PM
sailingfool
Re: 2 blade or 3 blade propeller

Quote:
Originally Posted by Capesailor View Post
Hello,

I'm a new member. I'm buying a new Marshall Sanderling and am deciding on a two blade vs three blade prop for the 10 hp Nanni 2.10 diesel. Is there more drag with the 3 blade? Is the three blade quieter with less vibration? I am interested in getting max thrust in a tidal outlet to the Atlantic.

Capesailor
That's a small diesel, it certainly will work fine with a suitable size 2 blade. I would doubt they make a 3 blade to deliver so little thrust...
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