Propeller sizing question (diameter/pitch tradeoffs) for upgrade to folding

[RichW9603]

Hi all,
I have a 1989 Hunter 33.5. From what I have been able to research, the OEM propeller is a 16x11 (1" shaft), but the engine struggles with that load and when researching I read one owner had luck downsizing to a 15x11. I am trying to buy a used folding propeller to upgrade to when I have the boat out for some maintenance in the next month or two, but 15x11's are hard to find. Anyone know if there is a way to determine how trading off to a larger diameter but smaller pitch changes the load as seen by the engine? For example there are a few 16x10 props available which looks nice, and also an 18x8 but not sure if that would have the blade tips impact the bottom of the hull. Any thoughts?
Thanks.

 

[wufibugs]

Call support at Flexofold. They will give you the right folding prop specs for your boat.

 

[RichW9603]

They said 15x11. But there is only one of those on the used market and it doesn't look right (on ebay), one of the blades looks fairly longer than the other as if the prop was damaged or it isn't folding correctly. So I'm trying to find what other diameter/pitch combos would have the same load as seen by the engine. I think 16x10 would work but before I bought one, thought I would check. I don't race so the upgrade isn't worth spending $1,300 for a new prop, but one in good used condition for $300-$500 would be worth it.

 

[Jeff]

The previous owner put a 16" two blade feathering prop on our 1993 H33.5. It has an adjustable pitch for forward and another for reverse. Minimal prop walk in reverse is nice. It's over twenty years old and still works great.

 

[RichW9603]

I'd love to get a feathering prop but that's an entirely different level of sophistication. Kudos on having one Jeff!
Jeff (or anyone) on your H33.5, how much clearance is there between the tip of the blade and the bottom of your hull? There's a nice 18"x8" folding prop for sale but that would be an inch closer to the bottom. Prop walk would also probably be worse with a larger diameter.

 

[fstbttms]

They said 15x11. But there is only one of those on the used market and it doesn't look right (on ebay), one of the blades looks fairly longer than the other as if the prop was damaged or it isn't folding correctly.

I'm looking at that prop right now. It has certainly been around the block a time or two, but there doesn't appear to be anything wrong with it.

http://www.ebay.com/itm/2-Blade-Bra...ash=item46525715ab:g:JdQAAOSwX~dWrpWk&vxp=mtr

 

[RichW9603]

Unfortunately that one is for a 7/8" shaft, otherwise it would work well. I need the 1" shaft.
The Flexofold one I found which looks like there is something wrong with it is this one (I was wrong, it is a 14" not 15"):
http://www.ebay.com/itm/Flexofold-1...ash=item2cb090a2c6:g:I6AAAOSwZtJW-JgH&vxp=mtr

It's only a 14", but my boat only has a baseline 2GM (13-15HP) so I don't mind the 1" smaller diameter since the 15" diameter is better for a 2GM20. But as you can see from some of the pictures, one blade looks out of whack.

This here 16x10 might show the same load on an engine as a 15x11, but not sure.
http://www.ebay.com/itm/Martec-16D-...ash=item23704f0cab:g:8yYAAOSwrnNXQcg-&vxp=mtr

This one here is an 18x8, so two more inches in diameter offset by 3 fewer inches in pitch, and it is the latest generation of the Martec which I think works better in reverse. But I'm not sure 1) how an 18x8 would stack up load-wise as seen by the engine to a 15x11, and 2) if the 18" would even clear the hull. http://www.ebay.com/itm/Folding-Pro...ash=item3aca2eab9e:g:wkMAAOSwHoFXsKvs&vxp=mtr

 

[fstbttms]

You're right about the Flex-O-Fold. Looks like mismatched blades. Weird.

 

[RichW9603]

OK I'm very glad it wasn't just me wondering about that. I bet one blade was damaged, he found another Flexofold blade from a different prop and just put it in and is hoping whoever buys it won't figure out something is wrong until down the line.

I did find this one, it is a 15x12, so the right diameter but 1" more in pitch. Not sure how the engine would favor this compared to a 16x10.
http://www.ebay.com/itm/Folding-Gori-Propeller-15-x-12-RH/122083926317

 

[fstbttms]

I really recommend you get some expert prop sizing advice. There are online calculators but better yet, you could call Martec or the Gori or Flex-O-Fold dealers. This is not an area where you should be taking shots in the dark, IMHO.

 

[RichW9603]

I spoke with the FlexoFold people but you can imagine they weren't interested in discussing anything aside from which size prop I should order them new. They weren't interested in answering questions about what kind of tradeoffs would be acceptable buying a used prop, as you can imagine. I doubt Martec or Gori would put much effort into helping someone buy a used prop as well, but maybe I'll check with Martec anyway.

The online calculators I found also just spit out a recommended diameter and pitch, I couldn't find one where I would set the diameter and it would calculate the corresponding pitch. Hence my post, I wanted to see if anyone had experience in these kinds of tradeoffs.

 

[fstbttms]

I spoke with the FlexoFold people but you can imagine they weren't interested in discussing anything aside from which size prop I should order them new. They weren't interested in answering questions about what kind of tradeoffs would be acceptable buying a used prop, as you can imagine. I doubt Martec or Gori would put much effort into helping someone buy a used prop as well, but maybe I'll check with Martec anyway.

The online calculators I found also just spit out a recommended diameter and pitch, I couldn't find one where I would set the diameter and it would calculate the corresponding pitch. Hence my post, I wanted to see if anyone had experience in these kinds of tradeoffs.

Why tell them you are looking for a used prop? The correct prop specs are the correct prop specs. You aren't going to help yourself or your engine by buying something that is "sort of close" to the correct size.

 

[RichW9603]

I know the correct prop specs - 15x11. The point of my post was that those exact dimensions aren't always available, so what other diameter/pitch combinations would also provide a similar load as seen by the engine. The manufacturers have no reason to entertain discussion beyond telling me what they think is the prop spec I should order from them, so you can imagine the lack of response when I asked them if a 16x10 would also work.

I just emailed the Martec folks, will let you know what they say.

 

[fstbttms]

I know the correct prop specs - 15x11. The point of my post was that those exact dimensions aren't always available, so what other diameter/pitch combinations would also provide a similar load as seen by the engine. The manufacturers have no reason to entertain discussion beyond telling me what they think is the prop spec I should order from them, so you can imagine the lack of response when I asked them if a 16x10 would also work.

I just emailed the Martec folks, will let you know what they say.

I get it, you are considering buying a used prop that isn't the correct size for your boat. I'm telling you that is a mistake. Buy new or used, but buy the correct size.

 

[Jeff]

I'd love to get a feathering prop but that's an entirely different level of sophistication. Kudos on having one Jeff!
Jeff (or anyone) on your H33.5, how much clearance is there between the tip of the blade and the bottom of your hull? There's a nice 18"x8" folding prop for sale but that would be an inch closer to the bottom. Prop walk would also probably be worse with a larger diameter.

I have 2-3 inches between the blade and hull. Here's a pic right after the last haul out. The prop had been in for two years. Wish I'd taken a picture after cleaning it up.

 

[Richard Bryer]

Prop sizing is complicated. One size does not apply to the various types of props. The correct size is made up of the correct combination of all the factors- dia, pitch, number of blades, cupping etc. I changed from a 15X12 2 blade to a 3 blade Campbell Sailor that is 14X9. I found it to be over propped- I couldnt get over 2000rpm. Had it re- pitched to 14X7 and is a bit better- now can cruise at 2400.

 

[RichW9603]

Thanks Richard. I take it your 15x12 was on your Hunter 34? Did that have the 2GM20? Could you hit the full ~3400 RPM on it?

 

[Richard Bryer]

Thanks Richard. I take it your 15x12 was on your Hunter 34? Did that have the 2GM20? Could you hit the full ~3400 RPM on it?

The engine is a Yanmar 3GMF- it appears my boat was built in the transition from Westerbeke to Yanmar as that prop size- 15X12 is the size for the Westerbeke. The Yanmar should have a 15X15. I could hit 3300- cruising at just under 7 knts was 2800. The tachs on these are not that accurate.