Prop Diameter vs Pitch

[wolfenzee]

The prop in the photo is a 16x9, at the time I the photo was taken I had removed all the old bottom paint and barrier coat, repaired the fiberglass where necessary (flexible shaft coupling broke, 15,000lbs of boat and no brakes...it was race day so the dock was empty, just cracked the glass a little) and applied 3 layers of epoxy barrier coat. This shows what the propeller configuration on my boat is like.

[Cheechako]

Two blades do not perform near as well in manuevering and docking and stopping in reverse.... if that matters to you.

[wolfenzee]

I have learned to take advantage of the heavy displacement in docking.....a little thrust here, a tad more....maybe a thrust in reverse....low windage docking is actually alot easier than in a light boat with high freeboard and low power engine. With the power I have I can change direction or stop on a dime around the docks and by useing the heavy displacement, prop walk and big rudder I can turn the boat on it's own axis. Throw the tiller over to one side in forward, just as it starts to move forward, throw the tiller to the other side in reverse, repeat this and you can turn just about anything on it's own axis. (the secound owner of my boat did it with a 100'+ schooner)

[Salted]

hi all,

seeking prop wisdom here.

I have a Shipman 28 SHIPMAN 28 sailboat specifications and details on sailboatdata.com and she's fitted with a 14" 3-blade fixed, have not measured pitch yet, but symptoms observed are vibration/not able to reach hull speed/and lousy reverse (as per MINIMUM PROP DIAMETER)

so I measure the beam@WL & draft(-keel) and they are 7.5'-ish & 2.4'-ish respectively, which gives me a 17.25" prop diameter.

I have a prop shaft to hull clearance of 11" & a prop shaft tip to rudder clearance of 15". So I suppose with reference to aperture clearances, a 17" prop still fits.

The engine is a Yanmar 2GM20, and gearbox ratio is 2.62 which according to PROP TABLE GM and 3JH series I should stick to a 15" prop unless I change to ratio 3.22, I can't use a 17" prop.

The conventional wisdom is get the biggest prop that fits the boat + engine. So does this mean I should go 3.22 for a 17" prop?

The other prop calculators advise a 15x9ish prop. Bigger is better?

Thanks in advance for any observations, wisdom and advice you share!

Regards

[westwinds]

The Shipman 28 originally had a one cylinder diesel Faryman 12 horsepower engine with a Hydromarine hydraulic drive. There was a hydraulic pump on the engine that drove a hydraulic motor on the propeller shaft. I am thinking your setup is now just an ordinary propeller shaft with stuffing box. Your Yanmar is going to have about 16 horsepower at 3400 RPM continuous output or 18 horsepower at 3600 at maximum power for a short period of time. Normally the 3600 RPM is how the propeller dimensions are calculated as you want maximum power in an emergency.

If you are getting vibration, then it is probably misalignment of the engine with the propeller shaft. Nigel Calder's Boatowner's Mechanical and Electrical Manual has information on engine alignment. There is plenty of propeller tip clearance so there will be no vibration from that. The diameter and pitch should be stamped on the propeller hub between where the blades attach to it. Since the engine is not original, a propeller with larger pitch and/or diameter I assume was fitted

[Salted]

Thx Westwinds, so you're saying I needn't go 17", just fix the alignment, get a better prop matching the engine setup?

[westwinds]

Originally Posted by Salted

Thx Westwinds, so you're saying I needn't go 17", just fix the alignment, get a better prop matching the engine setup?

Keep your 2.62 gearing and use a 16 inch propeller and a pitch of 12 inches or a 17 inch diameter and a pitch of 9 inches. I am not sure if a 17 inch diameter is available in that low a pitch. What is the diameter of the propeller shaft as you need to match that to the diameter the propeller has for the propeller shaft? I did these calculations using a hull speed of 6.1 knots, but it would be a lot better if I knew the weight of the sailboat loaded with crew and supplies including water and fuel. A good estimate of weight is to figure two thirds of weight of cargo (food), water, and fuel. Also add full weigh of crew and gear. The boat weighs 6614 pounds.

[Salted]

Originally Posted by westwinds

Keep your 2.62 gearing and use a 16 inch propeller and a pitch of 12 inches or a 17 inch diameter and a pitch of 9 inches. I am not sure if a 17 inch diameter is available in that low a pitch. What is the diameter of the propeller shaft as you need to match that to the diameter the propeller has for the propeller shaft? I did these calculations using a hull speed of 6.1 knots, but it would be a lot better if I knew the weight of the sailboat loaded with crew and supplies including water and fuel. A good estimate of weight is to figure two thirds of weight of cargo (food), water, and fuel. Also add full weigh of crew and gear. The boat weighs 6614 pounds.

Thanks WW! typically she's used as a daysailer, so probably won't see anything over 7000# usually; if we go on a long weekend cruise, unlikely more than 8000#s?

the shaft is 1" standard; I forgot to mention that with a clean hull at about 2100 rpm, with the current+ light winds, she was making 6 knts; against current & light winds <5 knts. observations on the same morning sail.

Thanks!

[westwinds]

Originally Posted by Salted

Thanks WW! typically she's used as a daysailer, so probably won't see anything over 7000# usually; if we go on a long weekend cruise, unlikely more than 8000#s?

the shaft is 1" standard; I forgot to mention that with a clean hull at about 2100 rpm, with the current+ light winds, she was making 6 knts; against current & light winds <5 knts. observations on the same morning sail.

Thanks!

The best way to approach the propeller diameter and pitch problem is to get some numbers from an existing setup. The numbers are specifically for your boat, not estimates for all boats. You need a calm day and no current. Do a run in both directions and keep RPM constant. Since speed is not linear with respect to horsepower, it takes a lot of horsepower for a little more speed, averaging speed over ground is problematic or maybe I should say speed over the bottom is problematic. If the current is really constant, not tidal, then you can do an average speed. Must be a calm day though. Speed from a GPS is over the ground. A timed course between two stationary markers could also be done instead. The bottom of the boat and the propeller should be clean. The pitch for your 14 inch propeller must be up there around 18 inches, just a very rough estimate on that. Maybe cavitation could be a problem. Does Your Prop Have Secret Cavitation? | BoatTEST.com

[Cheechako]

With a one inch shaft I'd be tempted to stay down at maybe 15" diameter. My 44 ft cutter had a 17" prop and performed fine in all conditions.

[westwinds]

The formula for diameter of a propeller shaft is the cube root of all of the following (321,000 X Shaft Horsepower X SafetyFactor3ForYachts) Divided by(TorsionalShearYieldStrength X RPM)

It may seem counter intuitive, but diameter of the propeller is not in the equation. Tobin Bronze and stainless steel 304 are the weakest materials for a propeller shaft with a yield strength in torsional Shear of 20,000 PSI, but even with that, a one inch propeller shaft at an RPM of 1300 will handle 27 horsepower. The Yanmar has 18 horsepower at 3600 RPM and a gear box of 2.62 to reduce the propeller RPM to 1300.

[westwinds]

Originally Posted by westwinds

Keep your 2.62 gearing and use a 16 inch propeller and a pitch of 12 inches or a 17 inch diameter and a pitch of 9 inches. I am not sure if a 17 inch diameter is available in that low a pitch. What is the diameter of the propeller shaft as you need to match that to the diameter the propeller has for the propeller shaft? I did these calculations using a hull speed of 6.1 knots, but it would be a lot better if I knew the weight of the sailboat loaded with crew and supplies including water and fuel. A good estimate of weight is to figure two thirds of weight of cargo (food), water, and fuel. Also add full weigh of crew and gear. The boat weighs 6614 pounds.

There is some guess work in getting the correct pitch and/or diameter. It is best to have a place to buy a propeller that will let you try several to get exactly the one with the right pitch. Another thing you could do is get a propeller with pitch that can be changed with the added advantage that you have a propeller that has low drag that either folds or that the propeller feathers so that the blades orient with the direction of the water flow presenting very low drag. Your speed will be about 15% faster, especially in low wind conditions than with a fixed propeller. http://flexofold.com/upload_dir/docs...onthly_low.pdf

You probably do not want a folding propeller as they do not open easily if you are going forwards and back to get into a slip, or to do a very tight turn where you go forward and back. The Autoprop is interesting because it matches the engine load to the propeller so that you get better economy. The Maxprop by PYI is the oldest manufacturer. You can set only one pitch. Your Yanmar's gearbox has a lower reduction in reverse than forward, but the Autoprop will take a different pitch in reverse. Not sure if the prices here are competitive at getaprop.com, but you can see a variety all at once. There are many manufactures of these low drag propellers so maybe one of the others is a better fit for your boat. Before you buy a particular propeller of this type, do Google to see what folks are saying.

[Salted]

Thanks WW, I was actually eyeing flexofold. Will check out the others!