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  #31  
Old 03-17-2011
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Originally Posted by asdf38 View Post
Sailingdog, not to hyjack this thread, but what is a good prop shop in the greater Boston area? I'm in the market for a prop right now and am trying to figure out where to look.
What type of prop? For prop work it is very, very tough to beat Accutech Marine. They also sell props. I have not seen many shops that can balance a prop like Accutech or who understand props and prop issues better.
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  #32  
Old 04-26-2011
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The easiest way to do this is go to the Michigan Wheel website and fill in the info for them to do a prop calculation. Itís free.

The ultimate reference for doing you own calculations is Dan Spurrís ďPropeller HandbookĒ. Itís a bit technical but may be at your local library..

Basically your boat has a hull speed that you should reach when your propeller diameter and pitch is correct and your engine is fully loaded at max RPM. Diesels want to be run at about 85+% power. One of the biggest problems with sailboat diesels is they arenít run at full power enough. It looks youíre your prop may not be matched well. Letís just say your boat displaces 12,000 # loaded and the waterline is 26ft.. This gives you a Lb/HP of 480#HP. Your speed/length ratio is 1.25 so ďhull speedĒ is 1.2x√26 or 1.2x5 or 7 kts. So at 4 kts your propeller is off or your engine is not up to power, or you need to scrap the barnacles.

So what about 2 blades or three? Two blades are more efficient in driving the boat under power and have less drag under sail. The reason to go to three blades is usually because you donít have the room to fit the correct size 2 bladed propeller. You need a minimum amount of blade area to keep your prop from cavitation. You also need a minimum clearance between your prop tip and the hull. Thatís about 15% of the prop diameter. When a two bladed prop with enough blade area doesnít fit the space that you have you go to a three blade prop. More blades more area.

Three blades are also a bit smoother. But thatís mostly when their two blade counterparts donít have enough hull clearance.

In 1995 some grad students evaluated sailboat props in a test tank at MIT. Their paper has a lot of detail on how different props compare. They showed about a 50% drop in drag by freewheeling a non feathering prop instead of leaving it engaged to the engine. Reverse power with a 3 blade also looked better. But bear in mind this stopping power is hrust while the boat is still going ahead.

Easy answer, ask Michigan Wheel to make a calculation
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  #33  
Old 04-27-2011
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Thanks Maine Sail and Waltthesalt.

I guess I'm struggling with 3 things. First, how bad is my existing prop. There were a lot of barnacles on it when I pulled the boat out, however I don't recall there being a difference between early in the season and late in the season (I first test sailed the boat in early July and bought it in late July), the boat was always slow. I emailed the previous owner to see if he remembered cleaning off the prop over the previous winter for the off-chance that there were multiple years of growth that I've now cleaned off. Also, the prop seems as if it's mounted too close to the hull - there is very little space between the prop and the back of the keel and the keel is quite thick. Perhaps a better installation of the existing prop combined with the barnacle removal would mostly solve my problem.

The 2nd question is that assuming I want a new prop, what is the correct size. This one is strait forward to solve as there are plenty of resources online.

The 3rd question relates to my lack of experience in the marine world and is a reoccurring problem. If I decide to replace the prop and don't want to try it myself (I'm pretty sure about this one), how do I find a professional to do it? I'm not even sure what exactly to look up in the yellow pages. What's the best way to go about this? Of course I chat with people when I see them at marinas and so forth but in half a season I haven't build up many resources here. Thanks.
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  #34  
Old 04-27-2011
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Quote:
Originally Posted by sailingdog View Post
I was thinking the same thing.
If I'm not mistaken... if it is a true displacement boat.. that's actually impossible to go that fast for that size boat no matter how slick or how much power. No doubt the gps is giving boat + current speed.
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  #35  
Old 04-27-2011
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Quote:
Originally Posted by ByAirAndBySea View Post
If I'm not mistaken... if it is a true displacement boat.. that's actually impossible to go that fast for that size boat no matter how slick or how much power. No doubt the gps is giving boat + current speed.
Not impossible, just more difficult. There is nothing "magic" about theoretical hull speed. The commonly used formula (1.34 times the square-root of the waterline) is the speed at which the bow wave/wake is exactly the waterline length. Getting a boat to go faster than this means that the boat starts to "climb" its bow wave, or pitch up a bit. Since this takes a bit of energy, more power is required to move the boat. Think of how a planing motorboat behaves before it gets on to a plane: first it acts like a regular displacement hull, essentially sitting level as it moves forward; as the speed increases, it starts to pitch up (this is where it is a bit above hull speed and is "climbing" its bow wave); then, as it gets going faster it seems to "jump forward" and up onto a plane. A Catalina 27 with an A4 (nominally 30hp) has about three times as much power available as it needs to travel at hull speed with a clean bottom. So, if the engine was putting out about 1/3 to 1/2 of its rated hp (or about 10 to 15 hp of 30 hp), the boat would go at just about hull speed (about 6.3 kts for a Cat 27). The rest of the power output of the engine (15 to 20 hp) is needed to get it past hull speed and up to 7.5 kts.

My Cal 2-27 has about the same LWL as a Catalina 27, and it will go just a bit faster than hull speed with her Yanmar 2gm20 wide open (or at about 16 hp of engine output). If I had twice the power available, she could undoubtedly go another knot or so. Neither boat is particularly fast, specially my the standards of modern sport boats. But, similar to what used to be said about the F-4 Phantom ("Proof that with enough horsepower, even a brick can fly."), with a big enough engine a boat's displacement hull speed can be overcome, a bit. No magic, just fluid dynamics.
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Last edited by SlowButSteady; 04-27-2011 at 03:07 PM.
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  #36  
Old 04-27-2011
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Use 1.4 rather than 1.34 and you'll get closer to your real hull speed. as for climbing the wave....physics won't allow a displacement vessel to leave the wave trough. The reason motorboats can is because they ride on top of the water more or less. Unless u r a centerboard boat u will not be riding on the water. (Most centerboards at least)
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  #37  
Old 04-27-2011
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Quote:
Originally Posted by ByAirAndBySea View Post
Use 1.4 rather than 1.34 and you'll get closer to your real hull speed. as for climbing the wave....physics won't allow a displacement vessel to leave the wave trough. The reason motorboats can is because they ride on top of the water more or less. Unless u r a centerboard boat u will not be riding on the water. (Most centerboards at least)
The 1.34 "factor" is actually derived from a combination of a number of factors, including the density, viscosity, and surface tension of the water (which all vary with temperature and salinity), wavelength (lambda), and gravity (g). As it turns out, if we set everything but lambda to reasonable constants (and discount surface tension, which is too small a factor to worry about at the relevant scale), the whole thing boils down to the common 1.34 times SQRTlambda. However, it will vary a bit with temperature and salinity.

Some folks, apparently you included, often add a bit to that 1.34 factor. This isn't terribly unreasonable, as the LWL is measured with the boat in a static condition and the dynamic waterline will increase a bit as the boat speeds up. However, there is a dramatic increase in the power required to propel a boat as it gets into the 1.34 to 1.5 times SQRTlambda range. There is no actual inflection point in the curve, so we can debate as to exactly which factor is most appropriate. However, a displacement boat (any displacement boat) can "overcome" its hull speed with a large enough engine. This isn't to say that a Catalina 27 with an A4 running at full throttle will plane, but one look at the bow wave will demonstrate that the NEXT crest of that wave will be beyond the stern of the boat (i.e., the wave generated has a longer lambda than the LWL of the boat). In this condition, the hull will be pitched slightly up, since the stern will be sitting in the trough of the bow wave and the bow will be perched (or "climbing up") that wave.

Note how the boat in the following pic is traveling a bit above hull speed (the stern is forward of the following crest), AND the hull appears to be pitched up slightly:

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Last edited by SlowButSteady; 04-27-2011 at 10:42 PM.
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  #38  
Old 04-27-2011
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Since we last addressed the propeller topic, I have purchased the Propeller Handbook by Dave Gerr. Crouch’s Propeller Method works well for sailboats, as there is no need to get every last bit of efficiency as one would with a tugboat for instance. The Bp-δ method is exact. The Bp-δ method also allows calculation for cavitation. Let me look at the book and do a few preliminary calculations. I will eventually need a speed and RPM to calculate slip by your making a run in two directions and taking the average GPS speed reading. Do you have a clean propeller? If not I can estimate. Do the run with little wind and tidal influence. Tide is less important than wind. I could use the 4 knots @ 1500 rpm, but speed at two places would be nice, like 4.1 knots or something like that for instance at 1500 rpm.

If the boat is out of the water so that you cannot do a speed run, then I need the distance from waterline to bottom of boat, not including the keel, how far the boat sinks into the water and also the beam at the waterline. When you clean the propeller, try to get the number off the hub face, which is where the propeller shaft enters the propeller. The number is sometimes stamped on the hub where the blades attach to it. The number will start with the diameter, 17, then an "X" followed by another number. That other number is the pitch. You may have to remove the sacrificial zinc if it is bolted to the propeller shaft just behind the skeg to see the number, but you will need to replace the zinc anyway. The zinc could be attached to the propeller where the nut is that holds the propeller on the propeller shaft. Propeller sizing on this boat could best be done with the Bp-δ method because she is so wide, the assumptions needed to use Crouch’s Propeller Method could throw it off.

You will need a puller suitable for a two or three blade propeller. Most pullers are for three blades. Pullers are easy to use, just attach to the propeller and turn the bolt to pull the propeller. Sometimes you have to tighten quite a bit and the propeller will come off with a good pop sound. Loosen the nut holding the propeller on, but leave on loose to prevent the prop from falling off the propeller shaft onto the ground. If you have this done do not let them use a slide hammer to remove the propeller! Use a puller. A slide hammer can destroy the transmission with immediate failure of the case, or damage internally that causes failure at sea such as a bearing. Also while you are at it, you should check the cutless bearing. You could even take the flange loose at the transmission and pull the propeller shaft out some to check for crevice corrosion and pitting if you have a stainless steel propeller shaft.

The minimum distance from the skeg to the 17 inch propeller is 30% of the diameter or slightly more than five inches. For tip clearance to hull, a 15 percent value is suitable for sailboats because of the slower turning propellers. Take 15 percent of 17 inches and you get two and a half inches. By the way, no matter what the propeller size, the clearance must be at least 2 inches. For a propeller in an aperture, the value is 12 percent, but I believe the Bombay Clipper does not have an aperture. The distance from the propeller to the rudder must be at least fifteen percent of the diameter, which is again two and a half inches.

Last edited by LakeSuperiorGeezer; 04-28-2011 at 12:13 PM.
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  #39  
Old 04-28-2011
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If you are really concerned about boat speed under sail and reverse thrust is important, you might consider a feathering prop, like a MaxProp, that has no blade "cup". It will be less efficient motoring forward, but the reverse thrust is hugely more than with a fixed prop.

A MaxProp has another advantage: you can "reprop" by changing the pitch, -- you just don't want to do it with the boat in the water. The downside is that they are expensive.
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  #40  
Old 04-28-2011
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Question 1,2
Do I need to replace the propeller? Sailboat propellers don’t wear out like on high speed outboards. Sailboat props only need to be replaced if damaged and vibrate. Even then a prop shop can usually straighten it out. Some people change from a fixed prop to a folding or feathering to get better speed under sail. But under power fixed props perform as well or better than feathering of folding ones.

If the diameter and pitch of the prop is correct for your boat and engine you don’t need to replace the propeller. You can find this out by checking what’s stamped on the prop hub when you pull the boat for painting. Check this against calculations done for your boat/motor. If your boat is a class-type boat say an Erickson 31, you can ask that boat owners’ association or Sailnet what prop pitch and size they use. Prop shops can measure pitch from the prop and may be able to make some adjustment to solve the problem . If your engine is mechanically OK but can’t get up to rpm the prop is overloading your engine and you have too much pitch. A prop shop may be able to reduce the pitch. If the engine gets up to RPM but isn’t pushing you to speed which I think is your problem The engine is under loaded and maybe you have too little pitch and they may be able to increase the pitch.

Note on a gas engine a too little prop causes the engine to overspeed as the throttle directly controls the gas to the engine. Diesels are different as the throttle controls a governor which controls the flow of fuel to the engine. So at full throttle on a diesel you just go up to full rpm regardless of how hard the engine is pushing.

Cavitations is something else. It’s when the prop loses its bite on the water. Like spinning your car wheels. A heavily barnacled prop can case this. There are antifoling spray-can paints for propellers. There’s a recent Sailnet tread on this. Also a prop that’s too small can cause cavitation… not enough blade area. As mentioned before a larger diameter and/or more blades give your more area.

Having the prop too close to the keel as you mention can also reduce it’s effectiveness. Usually the designer has this figured out and unless the shaft has been shortened or rearranged it should be OK. Anyway the thumb rule is that the the distance between the prop and keel or skeg should be at least 30% of the propeller at the middle of the propeller blade. So about 6 in for you. I’m not talking about a strut which is pretty streamlined we’re talking about a solid keel in front of the prop. Another point is that the aft edge of that keel should be fared and not blunt or square edged.

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Ask around the marina or boatyard many boat mechanics don’t have the best of reputations. People will tell you if they’ve been burned.
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