I have a CD30 Cutter with a Volvo MD7A engine and would like to go from a 2 blade to a 3 blade prop. The manual says maximum RPM should be 2600 RPM. I would like to know if anyone has added a 3 blade prop, and what they are using for diameter and pitch.
Also how much power can be gained motoring against strong winds, and also how much will this slow the boat down under sail.
Thanks in advance,
Jerry J Commisso
liasboat@aol.com
2 BLADE to 3 BLADE SWAP
Moderator: Jim Walsh
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Mike
Re: 2 BLADE to 3 BLADE SWAP
We had a C30 and I thought about switcing from a 2 to a 3 blade prop. After reading a lot of stuff and talking to a lot of people, it became pretty clear that the difference would be a neglible increase in backing performance, a negligible increase in oomph(!), a decrease in speed undersail. In short, no advantage worth even the time involved let alone the cost of the new prop and hauling costs to install. MikeJerry J Commisso wrote: I have a CD30 Cutter with a Volvo MD7A engine and would like to go from a 2 blade to a 3 blade prop. The manual says maximum RPM should be 2600 RPM. I would like to know if anyone has added a 3 blade prop, and what they are using for diameter and pitch.
Also how much power can be gained motoring against strong winds, and also how much will this slow the boat down under sail.
Thanks in advance,
Jerry J Commisso
michaelconniesmith@home.com
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Ken Cave
Re: 2 BLADE to 3 BLADE SWAP
You might try reading the past notes on three bladed props on this site-there is a wealth of information on this subject.
Also, there is a great site on the internet by Michigal Wheel Corp.
where you can post your boat, ask for the proper prop, and they will give you the info in about three days on what prop to use.
As I am on the hard putting in a new cutlass, shaft, etc. I am going with a three bladed prop-will let you know how it works
Ken Cave
Cape Dory 28 #227
bcave@whidbey.net
Also, there is a great site on the internet by Michigal Wheel Corp.
where you can post your boat, ask for the proper prop, and they will give you the info in about three days on what prop to use.
As I am on the hard putting in a new cutlass, shaft, etc. I am going with a three bladed prop-will let you know how it works
Ken Cave
Cape Dory 28 #227
bcave@whidbey.net
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John MacArthur
Re: 2 BLADE to 3 BLADE SWAP
Jerry--I've tried to get information on this subject and found it frustrating. Nobody's *sure* they're right. Michigan Wheel's site pops up lots of Java errors, and crashes. For the record, my CD-30 uses a 13 x 12 3bladed prop. It was pretty badly corroded when I got it, and I was looking for info on replacement. However, I was able to get a prop shop to weld and refair it, and now it works quite nicely. I'm pretty sure it was the original prop. Several people have told me they think it is much too aggressive, or hungry. However, I can run along a 2200-2300 at around 5 - 5.5 k, and cut through lots of chop. It will run up to 2600, but overheats pretty quickly, so that's for emergency only. It gets great mileage at 1800 and 4.5 or so k, if I'm not in too much of a hurry. I don't know how it compares to a 2-blade one, never having had one on the boat. She doesn't back up very well, but she will move when I goose the throttle. I think I like that setup, now that I'm getting used to her. Hope this helps a little.Jerry J Commisso wrote: I have a CD30 Cutter with a Volvo MD7A engine and would like to go from a 2 blade to a 3 blade prop. The manual says maximum RPM should be 2600 RPM. I would like to know if anyone has added a 3 blade prop, and what they are using for diameter and pitch.
Also how much power can be gained motoring against strong winds, and also how much will this slow the boat down under sail.
Thanks in advance,
Jerry J Commisso
jmac@laplaza.org
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Ryan Turner
Re: 2 BLADE to 3 BLADE SWAP
I believe the general rule of thumb is to drop 2 inches of pitch when going from a two blade to three blade prop. Leave the diameter the same. This is true if your existing two blade prop is the right size for your boat and motor. You should be able to come close to but not reach the max rpms for your motor when motoring on calm water with no wind. Your bottom and prop need to be clean to get it right.
rturnertec@aol.com
rturnertec@aol.com
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Andy Denmark
Re: 2 BLADE to 3 BLADE SWAP
Hi Jerry,
I went thru the 2 vs. 3-blade prop thing on several boats over the years. The most notable outcome was deterioration of sailing performance. No improvement in speed, power, efficiency or maneuverability. Some thought the 3-blade was a bit smoother but I could tell no difference. This seems to be typical of smaller (<30 ft) full-keel designs. Refer to the nomogram in Skene's _Elements of Yacht Design_ and that will get you into the ballpark with pitch. Diameter is pretty well determined by the aperture size. Efficiency is < 50%.
RPM's and exhaust will tell you whether pitch is correct. Failure to reach mfr's suggested max RPM's and/or black exhaust smoke means (among other things) that pitch is too aggressive, while revving higher than suggested max RPM's at full throttle means pitch is not aggressive enough. Any good prop shop can repitch your prop while you're hauled out.
I had the advantage of being able to haulout free so prop adjustments were easy. Without that luxury, I'd suggest looking at this as a long term process, adjusting pitch during annual haulouts (an inch at a time) unless you are seriously out of whack. Rarely does one get it "dead on" the first try. All this assumes a clean prop, bottom and correct trim.
Trim is important with Alberg designs. Too many are seen with all kinds of crap hung on the stern and around the cockpit, cockpit lockers jammed with heavy stuff, and the boat squatting. This stern-down configuration requires a lot more power (from the engine or from the sail plan) to move the boat through the water. Boat is slow (especially under sail), has too much weather helm, decks drain improperly, etc. Trim her out before changing anything as this makes a significant difference.
If you aren't somewhere near benchmark hull speed at cruising RPM (+/- 80% of max) then the answer is not in prop diameter or pitch but in increasing horsepower.
Right behind fuel filters on my spares list is a spare prop. It's no fun to be stuck for a weeks on the hard in a strange boatyard miles from home waiting for a new prop. Especially bad if you are in another country when duties and high shipping rates kick in. Makes the intitial purchase $$$ seem a bargain. (BTW, the spare prop is stowed in the bilge along with Luke storm anchor, chain, zincs, shackles, etc. -- not in the lazarette). Hang on to that "old" prop - it's good, cheap insurance.
Just one old guy's opinion. Hope it helps.
Andy Denmark
CD-27 "Rhiannon"
trekker@coastalnet.com
I went thru the 2 vs. 3-blade prop thing on several boats over the years. The most notable outcome was deterioration of sailing performance. No improvement in speed, power, efficiency or maneuverability. Some thought the 3-blade was a bit smoother but I could tell no difference. This seems to be typical of smaller (<30 ft) full-keel designs. Refer to the nomogram in Skene's _Elements of Yacht Design_ and that will get you into the ballpark with pitch. Diameter is pretty well determined by the aperture size. Efficiency is < 50%.
RPM's and exhaust will tell you whether pitch is correct. Failure to reach mfr's suggested max RPM's and/or black exhaust smoke means (among other things) that pitch is too aggressive, while revving higher than suggested max RPM's at full throttle means pitch is not aggressive enough. Any good prop shop can repitch your prop while you're hauled out.
I had the advantage of being able to haulout free so prop adjustments were easy. Without that luxury, I'd suggest looking at this as a long term process, adjusting pitch during annual haulouts (an inch at a time) unless you are seriously out of whack. Rarely does one get it "dead on" the first try. All this assumes a clean prop, bottom and correct trim.
Trim is important with Alberg designs. Too many are seen with all kinds of crap hung on the stern and around the cockpit, cockpit lockers jammed with heavy stuff, and the boat squatting. This stern-down configuration requires a lot more power (from the engine or from the sail plan) to move the boat through the water. Boat is slow (especially under sail), has too much weather helm, decks drain improperly, etc. Trim her out before changing anything as this makes a significant difference.
If you aren't somewhere near benchmark hull speed at cruising RPM (+/- 80% of max) then the answer is not in prop diameter or pitch but in increasing horsepower.
Right behind fuel filters on my spares list is a spare prop. It's no fun to be stuck for a weeks on the hard in a strange boatyard miles from home waiting for a new prop. Especially bad if you are in another country when duties and high shipping rates kick in. Makes the intitial purchase $$$ seem a bargain. (BTW, the spare prop is stowed in the bilge along with Luke storm anchor, chain, zincs, shackles, etc. -- not in the lazarette). Hang on to that "old" prop - it's good, cheap insurance.
Just one old guy's opinion. Hope it helps.
Andy Denmark
CD-27 "Rhiannon"
trekker@coastalnet.com
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John MacArthur
Re: 2 BLADE to 3 BLADE SWAP
(BTW, the spare prop is stowed in the bilge along with Luke storm anchor, chain, zincs, shackles, etc. -- not in the lazarette).
jmac@laplaza.org
Andy--Is the bilge in the 27 similar to that in the 30 -- with a tiny trap door, and a plastic liner in it? If so, how do you get *anything* in it? Have you removed the liner? I have considered doing this, but I don't want to tear out the sole yet. Might be worth it, as it seems to have quite a bit of unavailable space in it.Andy Denmark wrote: Andy Denmark
CD-27 "Rhiannon"
jmac@laplaza.org
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Andy Denmark
Re: 2 BLADE to 3 BLADE SWAP
John,
Interesting you should ask. My 1984 27' has the small hatch just forward of the engine compartment for access to the deepest part of the bilge but no plastic liner or box. The space underneath the cabin sole extends beyond the shower grate forward on top of the ballast. Getting the lengthy items in is not easy -- Luke anchor stock, spare prop shaft (actually the old bronze one -- worn but still serviceable), etc. For better access I'm going to put in a longer hatch midway between the existing hatch and the main bulkhead. Hoping this can be accomplished by simply cutting out the sole and reinforcing/trimming like the existing hatch. If this can't be done and maintain strength I'll rip out the entire sole and reinstall the whole thing. In either case I'll install fiberglass panels athwartships to compartmentalize the bilge area to keep stuff from going adrift down there.
trekker@coastalnet.com
Interesting you should ask. My 1984 27' has the small hatch just forward of the engine compartment for access to the deepest part of the bilge but no plastic liner or box. The space underneath the cabin sole extends beyond the shower grate forward on top of the ballast. Getting the lengthy items in is not easy -- Luke anchor stock, spare prop shaft (actually the old bronze one -- worn but still serviceable), etc. For better access I'm going to put in a longer hatch midway between the existing hatch and the main bulkhead. Hoping this can be accomplished by simply cutting out the sole and reinforcing/trimming like the existing hatch. If this can't be done and maintain strength I'll rip out the entire sole and reinstall the whole thing. In either case I'll install fiberglass panels athwartships to compartmentalize the bilge area to keep stuff from going adrift down there.
trekker@coastalnet.com