[rbyham]

Well the next step in my refit is a new prop. The one currently on in water still has the three blades but due to somewhere around 10 years of no use and no bottom paint, each blade has only half a fin of metal remaining. It is interesting how corrosion has eaten each blade almost the exact same amount. I know this because I have actually put the trans in gear for testing and while it makes a very visible wake, there is little to no vibration. Anyway a complete replacement is in order.

My problem is I know little to nothing about the science behind props and wondered if I could come here for a little prop 101 discussion?

My questions are:

1. See attachments for appropriate manual pages. I have the 3qm30(H) engine/transmission. I also see the number 203 for reduction stamped on the KH trans. So I am assuming I need a 18RH12 3 blade. But I am confused about shaft size especially the taper. The other attachment reveal shaft dimensions so I see 30mm/1.18" as full shaft size with 1/10 taper. Is it as simple as that?

2. In my initial search for a used prop I have not seen any 18x12RH. So I am wondering what the impact would be of slightly increasing or decreasing either of those dimensions?

3. Lastly I am assuming RH because in other places the manual indicates prop rotation to be clockwise when viewed from stern. Correct?

4. When looking at a used prop, what should one look for in terms of condition? I would assume smooth edges and no pits but is there more to this?

5. Lastly can anyone suggest sources for used props? I am looking regularly at Ebay and CL as well as having contacted a few marine surplus places in FL. Will try Bacons in morning.

Thanks in advance...

 

[overbored]

Minneys yacht surplus has a lot of used props

 

[Faster]

Run your numbers here ; Vicprop - Prop calculator for Displacement and semi-displacement hulls (or any other online prop sizing site) to check to see if 18x12 is about right (common size, btw)

Generally tapers are pretty standard in North America, but beware any shafts built elsewhere.. our Brazilian built boat did not have the same taper as the Jeanneau our second hand Max prop came from, but I'm not certain which was 'standard'.. or if either was, for that matter.

 

[SVAuspicious]

Generally tapers are pretty standard in North America, but beware any shafts built elsewhere.. our Brazilian built boat did not have the same taper as the Jeanneau our second hand Max prop came from, but I'm not certain which was 'standard'.. or if either was, for that matter.

SAE tapers are 1:16 and 1:12 (generally smaller shafts use 1:16 and larger ones above 3" or so are 1:12). Metric tapers are 1:10 and 1:20.

 

[sony2000]

Here is a Max Prop 18" for a 30mm shaft. $950 USD.
Hélice Max-Prop 18" - Divers (un article) - Annonces | Les Puces Nautiques
Usually negotiable in price. And I think the 30mm diameter comes with the right taper.
The seller is locally.

 

[rbyham]

Thanks all. Very helpful. We got the shaft log, packing nut and flange all back on just yesterday. Was a bit of a bear to get shaft into flange in water. Out of water a few wacks with a 2x4 on outside would have driven shaft in. We had to get a bit creative and ultimately, cleaning the shaft extra good, putting a little Kroils on the inside of flange and dressing the key ever so slightly had her pretty much walk on. She was all bolted on and tested in gear last night. My plan is now to send divers to pull prop. That way I was thinking I could mic the shaft to confirm the manual figures of 30mm or 1.18" with a 1:10 taper. Being highly confident of those numbers I am on the hunt for a replacement prop. Being the cost effective (some would say cheap) type, my budget max is about $500. I was in Sailor Exchange a week ago in St. Augustine. They had 4 shelves of props but none exact. I am confident something will pop up. Thanks again.

 

[Maine Sail]

Thanks all. Very helpful. We got the shaft log, packing nut and flange all back on just yesterday. Was a bit of a bear to get shaft into flange in water. Out of water a few wacks with a 2x4 on outside would have driven shaft in. We had to get a bit creative and ultimately, cleaning the shaft extra good, putting a little Kroils on the inside of flange and dressing the key ever so slightly had her pretty much walk on. She was all bolted on and tested in gear last night. My plan is now to send divers to pull prop. That way I was thinking I could mic the shaft to confirm the manual figures of 30mm or 1.18" with a 1:10 taper. Being highly confident of those numbers I am on the hunt for a replacement prop. Being the cost effective (some would say cheap) type, my budget max is about $500. I was in Sailor Exchange a week ago in St. Augustine. They had 4 shelves of props but none exact. I am confident something will pop up. Thanks again.

The fit of the shaft to coupling & the key is supposed to be that snug. Removing shaft material may now give you an improper fit and improperly load the key, so keep an eye on it... BTW deal with your corrosion problem before you buy a new prop.

Keep in mind you want a 15% tip clearance on your prop to hull so what Yanmar recommends as "ideal" is rarely an ideal fit on all boats. This usually means down sizing prop diameter and increasing pitch. Having a prop shop who is experienced in doing a proper sizing for your boat, is well worth the money.

 

[Jaramaz]

18 inch dia is unusually large on sailboats with that size of boat & engine. As Maine Sails says, there should be a good clearance between prop and hull.
Otherwise the combo 18 x 12 is good, as pitch is limited. Very often one has to go down in dia and compensate with higher pitch. IIRC 1" dia corresponds to 3" in pitch, within a small range.

A prop should be carefully dimensioned for the boat. It is not difficult to do, there is no reason to just grab one from the shelf and try.

/J

 

[sony2000]

''Very often one has to go down in dia and compensate with higher pitch. IIRC 1" dia corresponds to 3" in pitch, within a small range. ''

Isn't pitch, pitch? Irrelevant of diameter.
12'' of pitch goes thru 12'' of whatever, with every rotation.

 

[Stumble]

Sony,

Both pitch and diameter effect prop loading and act to limit engine rpm. The general 'ideal' is that the prop should allow just past redline with the boat in light ship trim. So by the time you add gear to it the RPM just drops back to redline.

And I know is ticky tacky,but there is supposed to be 17% tip clearance as a minimum not 15%. 20% is prefered. So on a 18" prop you are supposed to have 21.6" between the hull and the center of the shaft. As a minimum, more is better.

I know it is out of budget, but I would highly recomend looking for the extra money for a feathering prop. They make a massive difference in both propulsion, and under sail. Next to new sails it may be the best sailing upgrade you can make, and for light air sailing even more important than sails are.

It is incredible how much better a boat sails when it isn't dragging a massive bucket thru the water.

 

[capt jgwinks]

Not exactly. If it was a screw going into wood it would be. Since water is a fluid there is some inefficiency called "slip". The larger, heavier the boat, dirtier the bottom, head wind, etc, the more slip there would be. A larger prop would have less slip so there's the difference. A larger prop with the same pitch would have more thrust than a smaller one, because it is trying to push more water, and it would require more horsepower to do so.

 

[Maine Sail]

And I know is ticky tacky,but there is supposed to be 17% tip clearance as a minimum not 15%. 20% is prefered. So on a 18" prop you are supposed to have 21.6" between the hull and the center of the shaft. As a minimum, more is better.

Dave Gerr Propeller Hand Book:

"Generally, there should be a tip clearance of at least 15% of the overall propeller diameter between the blade tips and the hull. The ideal tip clearance is 20 percent or more; however, additional tip clearance is usually found at the cost of overall propeller diameter. Since smaller diameters mean lower efficiency, you are faced with a trade-off between the increase in efficiency from larger diameter and the increase in efficiency from improved water flow to the propeller and reduced vibration from greater tip clearance.

Actually the slower the shaft RPM and the lower the boat speed, the lower the minimum tip clearances may be.

Minimum Tip Clearance Table 7-1 below gives minimum tip clearances at varying RPM.

RPM
200-500 SL Ratio under 1.2 = 8%
300-1800 SL Ratio 1.2 - 1.5 = 10%
1000 > SL Ratio over 2.5 = 15%
High speed planing craft > SL Ratio over 3.0 = 20% "

 

[Stumble]

Main, that is a great resource! Thanks for posting it and correcting me.

 

[Maine Sail]

Main, that is a great resource! Thanks for posting it and correcting me.

Not correcting you just giving a more in-depth look into something that does not always fit into a box.

Generally speaking most sailboat props are designed for 15% tip clearance but some could actually go lower and get away with it.. As tip clearance goes lower we tend to run into vibration, tip-burn etc... Bottom line is to swing the biggest wheel you can, diameter wise, while remaining in or around the 15% mark....

 

[sony2000]

To find a used prop, start by looking for anything RH and 30mm.
Two or three blades. Folding is preferred.
Next you want a 12 P (pitch), in your selection. But you can buy 1''-2'' larger or smaller, with the intention to repitch it, at a prop shop for under $100.
Now diameter. Maximum 18'' and at a stretch 19''.
Jut a guideline for a ""novice shopper"" to get you started. Then confer with another 3GM owner, in your weight class.
FleaBay will show you what is out there!

 

[SVAuspicious]

Keep in mind you want a 15% tip clearance on your prop to hull so what Yanmar recommends as "ideal" is rarely an ideal fit on all boats.

Absolutely correct. The issue is cavitation due to pressure gradients. If you get the tip too close to the boat there will be material loss on both the blades and the hull, and the vibration will make you crazy.

 

[Jaramaz]

To find a used prop, start by looking for anything RH and 30mm.
Two or three blades. Folding is preferred.
Next you want a 12 P (pitch), in your selection. But you can buy 1''-2'' larger or smaller, with the intention to repitch it, at a prop shop for under $100.
Now diameter. Maximum 18'' and at a stretch 19''.
Jut a guideline for a ""novice shopper"" to get you started. Then confer with another 3GM owner, in your weight class.
FleaBay will show you what is out there!

Sony, are you the same one who posted this

Isn't pitch, pitch? Irrelevant of diameter.
12'' of pitch goes thru 12'' of whatever, with every rotation.

?

at the same time give such strong advises ...?

/J

 

[sony2000]

"IIRC 1" dia corresponds to 3" in pitch, within a small range. " was in your posting.

If this is true, then 18X12 is the same as 17X15 or 19X9. I strongly disagree.

It prompted me to relate the technique I used, to find a prop for a new, specific application. Anyways what ever he chooses, it will only last 3 years at that marina, with the same type of anode.

 

[capt jgwinks]

Right there, find and fix whatever made it corrode like that before putting a new prop on. Also any other under water metals like through hulls. There's another can of worms. At least if you can't get the boat out of the slip because there's no propeller, it's not going to kill you when the through hulls let go.

 

[travlin-easy]

A neat tip I learned from one of our forum members was that a highly polished stainless prop does not need to be painted and does not grow sea critters. He has been using the same prop for 4 years and nothing has grown on it while living in South Carolina behind the barrier islands.

Gary