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Old 07-30-2015, 05:38 PM   #1
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Prop size and pitch

Lollygag is currently on the hard for some work. We pulled the props and and got less than good news from the prop shop. Our props are stamped 26X26 but are actually around 26X19! One has a crack right across the blade and the edge is paper thin. The other is OK but the manufacturer is out of business and no matching replacement is available.

Lollygag is a 1979 42' CHB Europa with twin FL120s and she weighs 17 tons. What size prop should she have?

Since there are so many options with people changing pitch and size to suit their own cruising style, what size props do you have on your similar, 42' semi displacement twin engine boat?
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Old 07-30-2015, 05:46 PM   #2
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The prop shop should be able to tell you that, and if they can't, I'd be suspicious of them. In the meantime, these guys are an excellent resource:

http://www.deepblueyachtsupply.com/propeller-sizing

There is also a good DIY prop selector on boatdiesel.com. By the way, one of the specs needed is what your transmission gear reduction ratio is.
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Old 07-30-2015, 06:30 PM   #3
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The prop shop is looking it up now after I gave them the gear reduction and WOT RPM, length and weight. What I'm looking for is what other people with my size boat are using. Obviously the PO had the props re-pitched at some point and I'm trying to figure out why. Lowering the pitch just doesn't make sense to me unless he got a deal on some props and made them work. CHBs didn't always get the maintenance they deserved. They're good boats but people bought them cheap (relatively) and then skimped on replacements and maintenance. Getting caught up is frustrating and expensive.
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Old 07-30-2015, 06:33 PM   #4
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You told the prop shop your gear ratio, it would help to tell us too!!!

Make sure you check both gears, not uncommon for FL with BW to have slightly different ratios P to S.

Also do you have rpm/speed data for both cruise and for full power? Those data are very helpful in selecting new props.
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Old 07-30-2015, 07:15 PM   #5
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Duh! The gear reduction is 2.5/1 I should have posted it earlier but having the haul out bill double kinda ruined my day. I don't have any speed/RPM data.


We bought the boat 14 months ago and she's been in the slip ever since. She has new Formica, mattress, toilets, interior paint, plumbing and a host of other things but she needed a haul out to fix some through hulls and, we thought, cutlass bearings. Turns out she didn't need the bearings but the props need to be replaced. Damn, I could afford the bearings.
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Old 07-30-2015, 08:47 PM   #6
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With 2.5:1 reduction 26x19 sounds close but perhaps a bit over propped. I have 2.0 reduction and 23x18's and I can just about make 2500 at WOT which is close to the max of 2600 for my Lehman 135's.

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Old 07-31-2015, 03:21 AM   #7
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A stock Grand Banks 42 with twin Cat 3208s had three-bladed props of 26" diameter, 30" pitch from the manufacturer. This is from the owners manual.

The manual for a later GB42 with the same engines but four-bladed props calls out 26" diameter and 27" pitch.
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Old 07-31-2015, 06:53 AM   #8
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From your description of the props I'd suggest you have your boat checked for stray current and/or cathodic protection.
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Old 07-31-2015, 11:13 AM   #9
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Propeller information from "similar" boats is pretty much worthless. The minimum information you need is displacement, waterline length, engine horsepower at maximum rated rpms and transmission gear ratio. Then you can start optimizing your props. The details of prop diameter and pitch will depend on the number of blades and their style as well as the disc area ratio of your chosen prop. There are a number of on-line calculators that will give you "approximate" prop dimensions based on displacement, LWL, engine power and rpms and transmission gear ratio. However, properly sizing your props requires a more detailed calculation based on the design of your hull and your desired operating characteristics.

One reason for decreasing pitch might have been to get a lower speed at idle for maneuvering during docking etc. Of course, decreasing pitch gives up top end speed which might not be all that important on a displacement type hull. (depending on installed engine power).
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Old 07-31-2015, 11:24 AM   #10
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When we thought we needed new props we contacted what was recommended to us as the best prop shop in Seattle. They said they needed the following information:

Boat make and model.
Type of engines.
Max hp of the engines.
Max rated rpm of the engines.
RPM of each engine in gear at max throttle with the boat underway.
Number of blades we wanted on the propellers.
And they wanted the existing props brought to their shop.

That information would enable them to determine the optimum prop configuration for our boat.
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Old 07-31-2015, 11:46 AM   #11
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If you buy the props suggested by the propeller shop from them and they do not turn out to be the right size you have some leverage with the shop. If you buy props of the size you suggest and the boat does not get to proper RPM the shop may not stand behind them. Ask the prop shop what they will do if the props they suggest do not perform. Some shops will have a no return policy as props are an art not a science.
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Old 07-31-2015, 04:40 PM   #12
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Agree with above, if the prop shop will not stand by his reccomendation, you don't need him. There are props available online at good savings also ebay and craigslist has many pairs. But you must give them the correct information they ask for, no guessing such as taking the weight off the documentation papers.
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Old 07-31-2015, 05:40 PM   #13
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Thanks, Kchace and Marin, for answering the question. I'm still waiting for the prop shop to get back to me. They are looking for used replacement props to help keep the cost down.


I've been around the block a few times and think I can deal with the prop shop without help. They are the shop recommended by the yard and though this is the first time I've worked with them I find them to be forthright and honest. If that turns out to not be true I'll give you all the details, and their name, here.


As for electrolysis, the boat was in salt water in some dodgy marinas for years. Now she is in fresh water and I'm changing the zincs to magnesium (magnesiums?) for the lower conductivity. I'll switch back in a year or two when we start cruising again.
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Old 08-01-2015, 09:29 AM   #14
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"and the boat does not get to proper RPM"

Towing water skiers , or a sport fish , is easy but for a cruising boat with the usual outsized engine , there are differences of opinion on "proper>.

Proper RPM is as much a religion as a science.

To Cruise or To Flank , that is the question.

If the vessel performs as the owner desires with no problems for the engine , thats "proper" enough to me..
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Old 08-03-2015, 10:13 AM   #15
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The prop calculator on boat diesel was spot on for me
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Old 08-03-2015, 10:37 AM   #16
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Three or four blade question usually is a matter of blade area and boat design .. not preference.

The blade area dosn't vary much from prop to prop so if you "want" a four blade prop and have a three blade you probably will have difficulty finding the four blade. The three blade is almost always better unless you have limited diameter space .. then you actually need the four blade to get the required blade area for the power. It's a matter of blade area and power.
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Old 08-04-2015, 06:41 AM   #17
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It makes the prop guesstimate much easier if your yard can weigh the boat the next haul out.
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Old 08-04-2015, 06:16 PM   #18
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Whew, found out the shocking evidence of not keeping up the zincs on your boat today when we hauled. Prairie 36 with 22x 20's new props are $1300 a piece. Make sure you get the right props and take care of 'em 'cause it ain't cheap to replace.

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Old 08-06-2015, 02:59 PM   #19
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Final update on pros and pitch... I hope. The prop shop (Delta Propellers in Lodi, CA) was great. I ended up buying new props from an internet site. When I called the prop shop to see if they could match the price they said no. So I asked what I owed them for cleaning my old props and researching the correct pitch. Nothing! Not a cent, just come pick up the old props. Might not owe anything but the tip will be good! I recommend this shop to anyone in the Delta region.
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Old 08-06-2015, 03:20 PM   #20
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Apop,
That's not unusual.
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