Prop size

[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.

[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.

[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]

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.

[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

[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]

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

[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

[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.

[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!

[Jerry Hammernik]

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....