Prop size

Discussions about Cape Dory, Intrepid and Robinhood sailboats and how we use them. Got questions? Have answers? Provide them here.

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gordhat
Posts: 4
Joined: May 17th, '18, 07:27

Prop size

Post by gordhat »

My name is Gordon and I purchased a 1974 Cape Dory 28 hull#9 last year that was headed for the landfill. I fell in love with the boat and I am attempting to restore it over 3 years. The best feature about the boat was a really nice Yanmar 2gm20f that runs tip -top. I did get it in the water last year and found that above 2000 rpm s the stern would squat down to the point that water would come out the top of the rudder tube. Max speed 5 knots. Above that the engine would start to smoke and the stern would squat further. I figured the boat was overproped. The propeller is a 3 blade 15/12. Talked to one prop guy and he suggested a 16/8.
Does anyone else have a 2 cylinder Yanmar and if so what are you using for a Prop? Does 16/8 make sense? Will this stop the squatting problem?
Any advise, help or support will be greatly appreciated.
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tjr818
Posts: 1851
Joined: Oct 13th, '07, 13:42
Location: Previously owned 1980 CD 27 Slainte, Hull #185. NO.1257949

Re: Prop size

Post by tjr818 »

How did you measure the 5 knots? That is getting close to hull speed and that is when the stern will squat. How was she loaded, two people in the cockpit and nothing forward? You might want to move some of the weight around. Does she sit on her lines at the dock?...with crew on board?
The 28 is a wonderful boat, thank you for saving her.
Tim
Nonsuch 26 Ultra,
Previously, Sláinte a CD27
gordhat
Posts: 4
Joined: May 17th, '18, 07:27

Re: Prop size

Post by gordhat »

Thank you for the reply. I measured the speed with the GPS at slack water with no wind on recipical courses. 5 to 5.1 knots at 2000 rpms. I was hoping for closer to 6 knots for max speed.
gordhat
Posts: 4
Joined: May 17th, '18, 07:27

Re: Prop size

Post by gordhat »

tjr818,

Oh, yes 2 people in the cockpit, 8 gallons of fuel in the tank, one group 29 battery in the starboard locker near the engine but not much weight forward. She sits nicely at the dock but I might be admiring her beauty and not paying attention to her lines.
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Ben Thomas
Posts: 215
Joined: Feb 5th, '05, 12:17
Location: 82 CD30 Milagro Hull #248

Re: Prop size

Post by Ben Thomas »

Gordon,
some important key data to determine prop size:
1. displacement of vessel
2. LWL of vessel
3. Gear ratio of transmission
4. Horsepower rating of engine
5. Engine RPM rating (max rpms)
6. and on our boats Aperture size
The 15x12 and 16x8 seem to me way oversized.
If the transmission is a 2.21:1 a 14x9 3 blade would be in more appropriate.
Higher gear ratios in transmissions increase propeller size and decrease rotation speed.
Cheers, Ben
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GeorgeH
Posts: 89
Joined: Mar 23rd, '10, 13:01
Location: CD28,
s/v Spartina
Setauket Harbor, NY

Re: Prop size

Post by GeorgeH »

Gordon,
I have a 28 with a 2gm20F and use a 14/12-2 blade prop and do 5 knots at 3,000 rpm without any smoke or engine squat. When I repowered a few years ago I tried a 14/14 that would cause the engine to smoke at 3000 so I had it re pitched to 12 and it now runs smoke free.

I am also glad you saved your boat from the scrap heap. The 28 is a solid and capable sailboat.
gordhat
Posts: 4
Joined: May 17th, '18, 07:27

Re: Prop size

Post by gordhat »

GeorgeH,

Thank you for your reply.
I'm sure the 3 blade 15/12 is too much prop. I would think the speed should be closer to six knots at 3000 RPM's but I am no expert on this. Is 5 knots what you typically cruise at?
Regards,

Gordon
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GeorgeH
Posts: 89
Joined: Mar 23rd, '10, 13:01
Location: CD28,
s/v Spartina
Setauket Harbor, NY

Re: Prop size

Post by GeorgeH »

Gordon,
I tend to cruise at 5 and 5 1/2 knots under power because of currents. Of course a bit higher if the current is with me. As you know the hull speed for the 28 is around 6 knots.
GH
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Jerry Hammernik
Posts: 258
Joined: Feb 5th, '05, 15:02
Location: Lion's Paw CD 28 #341
Lake Michigan

Re: Prop size

Post by Jerry Hammernik »

I don't have a Yanmar, my 28 has a Volvo. But I went with a Campbell Sailor prop. I had a two blade before that and went to a 3. I picked up a knot of boat speed at the same rpms. And I don't feel like I've lost any speed under sail. The specs probably don't matter since the engines are different, but I found the folks at West by North very helpful in determining the correct prop.

Where I diverge from many others is in rpm. My engine has been very reliable but it's 35 years old. At 3000 rpm it sounds like the pistons are trying to swap holes. I generally don't exceed 2200 rpm which puts me in the 5 to 5.2 k range. I'm more interested in avoiding engine issues than I am in squeezing out the last bit of speed.

Just my $0.02 worth.
Jerry Hammernik

"Money can't buy happiness, but it sure can buy a lot of things that will make me happy."
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mgphl52
Posts: 1809
Joined: Feb 6th, '05, 14:15
Location: s/v KAYLA CD 28 #318
Contact:

Re: Prop size

Post by mgphl52 »

Jerry: I have a similar "retired" MD7A and put on a Campbell Sailor over a year ago. Would you please share the size & pitch you have as mine is not getting as much speed... (:

Thanks!
-michael & Toni CDSOA #789
s/v KAYLA CD28 #318
2012 FLSTC Heritage Classic
Niceville FL
+30° 30' 24.60", -86° 26' 32.10"
"Just because it worked, doesn't mean it works." -me
No shirt + No shorts = No problem!
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Jerry Hammernik
Posts: 258
Joined: Feb 5th, '05, 15:02
Location: Lion's Paw CD 28 #341
Lake Michigan

Re: Prop size

Post by Jerry Hammernik »

mgphl52 wrote:Jerry: I have a similar "retired" MD7A and put on a Campbell Sailor over a year ago. Would you please share the size & pitch you have as mine is not getting as much speed... (:

Thanks!
I won't be home for a week, but I can check when I get back. Mine is an MD7B. I think the B revs higher than the A....
Jerry Hammernik

"Money can't buy happiness, but it sure can buy a lot of things that will make me happy."
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