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Old 21-11-2012, 03:58   #16
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The boat is very "slippery" for such a heavy displacement and has been at 9 kts with the original 2 blade prop at 2400rpm with a clean bottom.
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Old 21-11-2012, 07:11   #17
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Re: Prop Diameter vs Pitch

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The boat is very "slippery" for such a heavy displacement and has been at 9 kts with the original 2 blade prop at 2400rpm with a clean bottom.
That explains the problems I am having with Crouch's Propeller Method where assumptions on an average boat are used. This means I have to use Bp-d
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Old 21-11-2012, 19:28   #18
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Re: Prop Diameter vs Pitch

To use the Bp-d method, I have to work with the actual characteristics of your boat. I need:

Waterline beam in feet.

Hull draft, excluding keel, skeg or deadwood, in feet.

Displacement in pounds that includes crew and passengers, normal ship’s stores and gear, two thirds of all fuel and water, two thirds of all cargo, and of course the weight of the boat.

I already have waterline length of 39 feet.
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Old 21-11-2012, 19:41   #19
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Re: Prop diameter vs pitch

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Originally Posted by NCboatrx View Post
I think that you are going about this the wrong way. If you want the correct propeller for your boat, I suggest that instead of trying to buy a cheap one on ebay, that you contact a reputable propeller shop and let them work out what prop you need. Having someone who knows what he is doing, because it is his business, is often cheaper than trying to reinvent the wheel yourself.
The prop company will want the following information;
weight of the vessel
waterline
HP of the engine and max RPM
transmission ratio.

My company works out propeller calculations for our customers but always recommends that you let the prop company make the final recommendation. This way you only buy one prop that works instead of the hassel of a few incorrect ones.
+1.
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Old 21-11-2012, 20:37   #20
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Re: Prop Diameter vs Pitch

If you don't understand prop tech and try to figure it out with this forums help, you will never get the right prop. Also don't buy props from ebay. Call a prop shop and buy whatever they tell you to.
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Old 21-11-2012, 22:05   #21
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Re: Prop Diameter vs Pitch

The Bp-d Method allows us to reduce the diameter of the propeller while increasing the pitch, to vary the number of blades, two, three of four, all of this while checking for cavitation. Another benefit is increased accuracy in matching the propeller to a particular engine and gearbox with calculations that take into consideration the hull for this particular sailboat. A new propeller is going to cost about $2000 while an ebay buy should be considerably cheaper and doable if there is some flexibility in choice of diameter, pitch, and blade count.
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Old 22-11-2012, 04:26   #22
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Re: Prop Diameter vs Pitch

3 blade 26X18 File No
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Old 22-11-2012, 10:54   #23
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Re: Prop Diameter vs Pitch

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3 blade 26X18 File No
By the way, one would intuitively think that adding an extra blade to a propeller would mean having to increase the size of the engine, that it would make a big difference; however, the propeller size would only drop to 25 inches and the pitch would effectively increase by one percent or .12 inch. What you are getting with the extra blade is the ability to put more power to the propeller without having cavitation. This propeller would decrease the maximum RPM by approximately 450 at full power, which would mean a decrease in maximum horsepower of about 18%. The above are just from rules of thumb and it would not be a very good idea to actually specify a propeller with these rules. If the diameter of the three blade 25 inch propeller were decreased to 24 inches the pitch would have to be increased by 2 or 3 inches. Again though this sailboat must have a water line beam that is more narrow than most sailboats making the rules of thumb even more inexact. A 26 inch three blade with 12 inch pitch would not be the best choice because of decreased maximum horsepower available; however, I do not think there would be an engine overload. This would cause the oil temperature to go too high if there was an overload which can be a problem with controllable pitch propellers.
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Old 22-11-2012, 11:29   #24
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Re: Prop Diameter vs Pitch

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... A 26 inch three blade with 12 inch pitch would not be the best choice because of decreased maximum horsepower available; however, I do not think there would be an engine overload. This would cause the oil temperature to go too high if there was an overload which can be a problem with controllable pitch propellers.
This should be A 26 inch three blade with 18 inch pitch would not be the best choice because of decreased maximum horsepower available; however,
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Old 22-11-2012, 11:35   #25
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Re: Prop Diameter vs Pitch

You can try Michigan Wheel in Grand Rapids Michigan. Lot of props sold under many names were built by them. They can also repitch and repair, used them back when I had power boats.
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Old 22-11-2012, 11:59   #26
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Re: Prop Diameter vs Pitch

I am currently dealing with a similar issue, but on a powerboat that I have recently repowered. Many of the same principles apply here.

I used numerous prop calculators including boatdiesel.com and vicprops.com. I even used David Geer's forumula from his book on propellers. I had similar results using both calculators and David Geer's formula. However, real world sea trials proved completely different performance.

Even though there were plenty of opportunities to factor variables such as my specific hull type, beam, engine max RPM, etc., the calculations were so incredibly off base I am having to purchase and try several propellers. I have made progress, but it has been only through trying numerous propellers.

In my case, a 16X10 propeller was recommended. I installed it and couldn't even attain hull speed. I took a drastic step and purchased a 15X15, thinking it would stress my engine enough to give me a base line and allow me to re-pitch properly. The pitch was still too low. I now purchased a 15.3X19 and have yet to install and sea trial it. If this is too low, I will need to re-gear.

With respect to your unique situation, reliance on a propeller calculator to give you the maximum allowable pitch has very profound limits, as I have discovered. There are so many variables that are unique only to your boat; and I have learned propeller selection is something very specific to your boat. That might especially be the case for you since your boat is not a production boat with a plethora of sea trial data specific to your hull.

Many propeller shops will provide "loaner" propellers if you agree to purchase from them. That might be a viable path for you -- as you will have access to a lot of choices. Going from a two blade propeller to a three blade is a large step up, which will undoubtedly involve a pitch reduction. And on a side note, you will notice a considerable amount of stability in performance. Engine vibrations will be reduced and your drive system will have a longer life.

Best of luck to you and welcome to the forum!
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Old 22-11-2012, 12:35   #27
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Many thanks for your words of advice. I have contacted Deep Blue props and asked for a recommendation based on my inputs. All prop calcs so far agree with my learned forum colleagues but unfortunately my boat is in Hawaii so no chance of a loaner. My last purchase ended up being the wrong hand so I still have the option of the transmission adjustment from LH to RH.
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Old 22-11-2012, 12:53   #28
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Re: Prop Diameter vs Pitch

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Thankyou all so far. I will try to answer most questions. Boat is 39' WL 48000lbs displacement full keel. Engine is a Ford 80hp @2600rpm with 2.9:1 Velvet Drive which I know is not desirable and gives a slow prop speed although achieving 8 kts is easy. Original prop was 26x18 2 blade but is badly corroded hence my decision for 3 blade. Rotation is LH. I have input to a couple of prop calculators with similar results. Clearance to tip appears ok at 2-3". More suggestions appreciated.
So it sounds like the origial prop was fine, so your question is "what is the equivalent 3 blade prop to match my original 26x18 2 blade" .... right?
Jus call a prop shop and ask them and ask how much the prop is even if you dont buy it. That should get you closer than any calculator.... my guess is you're talking 26 x 16 or so. (BTW: 15% tip clearance would be 3.9")
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Old 22-11-2012, 13:03   #29
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Re: Prop Diameter vs Pitch

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Originally Posted by Astral Blue View Post
I am currently dealing with a similar issue, but on a powerboat that I have recently repowered. Many of the same principles apply here.

I used numerous prop calculators including boatdiesel.com and vicprops.com. I even used David Geer's forumula from his book on propellers. I had similar results using both calculators and David Geer's formula. However, real world sea trials proved completely different performance...
On the Dave Gerr formulas, did you use Crouch's Propeller Method, or the the Bp-d formulas? By the way, I am currious, what changes did you make with your new engine, power and weigh wise? You must have gone to more power where the engine went to full governed RPM, but you did not have enough diameter and pitch to take all torque the new engine put out?
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Old 22-11-2012, 14:33   #30
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Re: Prop Diameter vs Pitch

Quote:
Originally Posted by Astral Blue View Post
I am currently dealing with a similar issue, but on a powerboat that I have recently repowered. Many of the same principles apply here.

I used numerous prop calculators including boatdiesel.com and vicprops.com. I even used David Geer's forumula from his book on propellers. I had similar results using both calculators and David Geer's formula. However, real world sea trials proved completely different performance.

Even though there were plenty of opportunities to factor variables such as my specific hull type, beam, engine max RPM, etc., the calculations were so incredibly off base I am having to purchase and try several propellers. I have made progress, but it has been only through trying numerous propellers.

In my case, a 16X10 propeller was recommended. I installed it and couldn't even attain hull speed. I took a drastic step and purchased a 15X15, thinking it would stress my engine enough to give me a base line and allow me to re-pitch properly. The pitch was still too low. I now purchased a 15.3X19 and have yet to install and sea trial it. If this is too low, I will need to re-gear...
There might be a cavitation problem. What was the original propeller diameter? Use that diameter and increase the number of blades if possible to decrease blade loading? Need to do calculations to determine blade loading.
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