Folding prop question

[Compass790]

Recently we threw a blade off our 2 blade folding prop due to electrolysis & procrastination. Our prop was a left hand prop. We have been given a free identical prop except it's right hand. Is there any reason we cant use it & make reverse forward & vice-versa? The w/shop manual says the gear ratios are the same. Has anyone out there done this? Engine is a Yanmar YSM8 with a Kanzaki gearbox.
Thanks if anyone can help

[billknny]

If you had a feathering prop, for most brands the answer would be LH and RH blades are the same, it's the internal settings that control it.

But a folding prop, the twist in the blade that makes it a LH or RH is different and they can not be interchanged. There might be a brand where they can, but I am having a hard time imagining the geometry that would allow it.

[Jim Cate]

Some trannies can be used "backwards" as you describe, others not so well. I'd suggest contacting Yanmar and asking them rather than internet experts.

Jim

[Compass790]

Originally Posted by Jim Cate

Some trannies can be used "backwards" as you describe, others not so well. I'd suggest contacting Yanmar and asking them rather than internet experts.

Jim

You are probably right but maybe someone has done it. Besides I may have burnt my boats with Yanmar after abusing them for their parts prices.Guess i could use another email tho.Cant see anything in the gearbox that would prevent it

[Wotname]

Well the prop doesn't know what is driving it, the clutch doesn't know which way the shaft is turning, the ratios (fwd & astern) is the same so as you suggest, it can only be a problem (if any) for the gearbox.

I can't see anything in the transmission that could be upset and perhaps, it could even be better. There is probably little wear of the astern components that will now be getting much more use. It's worth a trial, especially as the you already have the second prop.

[Compass790]

Originally Posted by Wotname

Well the prop doesn't know what is driving it, the clutch doesn't know which way the shaft is turning, the ratios (fwd & astern) is the same so as you suggest, it can only be a problem (if any) for the gearbox.

I can't see anything in the transmission that could be upset and perhaps, it could even be better. There is probably little wear of the astern components that will now be getting much more use. It's worth a trial, especially as the you already have the second prop.

Glad to hear someone else is thinking along the same lines & said it a lot better. Certainly I will trial it. The gearbox is massive for the horsepower. Thanks

[DougR]

There are a couple of things to consider here:

- clutch - if it's a cone clutch there probably won't be a problem with the clutch using "reverse" as the primary drive gear as normally the cone surface area is the same on both the forward and reverse side of the cone. If it's a plate clutch it's important to know that the torque handling capacity is the same for both forward and reverse clutch packs, i.e.. Same number of plates on both packs. An example of this is that Borg Warner " Velvet Drive " gearboxes have smaller capacity reverse clutch packs and cannot be run continuously in reverse.

- Reverse mechanism- many transmissions that use horizontal input and output shafts lying parallel to each other obtain their reverse function by directing the power across an idler gear. Frequently this idler gear and it's shaft are not as robust as the main shafts in the transmission, so the manufacturer places a limitation on the amount of power that can be used continuously in the " reverse" gear position. Common examples of this are the mechanical Hurth/ZF gears.

As you point out "the gearbox is massive for the power" so it may be that running in "reverse gear" for forward propulsion will be OK......but it can't hurt to inquire.

One other thing to consider is that many naval architects purposely offset the prop shaft relative to vessel centerline to counter the effects of prop walk and steering torque while underway in forward gear. To change prop rotation in such a case might result in heavily loaded steering while underway.

DougR

[billknny]

Originally Posted by Wotname

Well the prop doesn't know what is driving it, the clutch doesn't know which way the shaft is turning, the ratios (fwd & astern) is the same so as you suggest, it can only be a problem (if any) for the gearbox.

I can't see anything in the transmission that could be upset and perhaps, it could even be better. There is probably little wear of the astern components that will now be getting much more use. It's worth a trial, especially as the you already have the second prop.

Just because you "can't see anything" doesn't mean it's not there.

It is absolutely 100% NOT true that forward and reverse gear ratios are ALWAYS the same. Many of the common Hurth transmissions for example run 30% slower one direction that the other. That would have a serious impact on propeller choice...

There are gear boxes that do not care which way they turn, there are some that are rated differently if they turn "backwards" and there are some that are "one way only."

The idea that you can run it backwards MIGHT be true... but you need to check the specifics before you try that.

[billknny]

Originally Posted by DougR

One other thing to consider is that many naval architects purposely offset the prop shaft relative to vessel centerline to counter the effects of prop walk and steering torque while underway in forward gear. To change prop rotation in such a case might result in heavily loaded steering while underway.

DougR

Baloney.

Another thing you hear a lot that is NOT true. Prop shafts are offset not to counter prop walk, but so they can be pulled out clear of the rudder. If they were installed straight, you would have to remove the rudder to pull the shaft. Of course lots of boat builders ignore this...

[DougR]

No, I don't think it's baloney....

I used to own a 30 foot S&S design which had the forward end of the prop shaft offset toward port side, and it ran a RH prop. The prop was nowhere near the rudder and the shaft could be pulled completely free of the boat well before it got to the rudder. This same design feature is used on most of the S&S designs, and I think their pedigree carries a bit of weight.

Today I own a Cal39, Bill Lapworth design, which runs a LH prop and the forward end of the prop shaft is offset toward starboard.....funny coincidence, no?

If you would like a more in depth explanation from a well respected naval architect I suggest you slide on over to boatdesign.net and look in the propulsion area under "offset single prop". There you will find a good description of why this is done, written by the well known designer/ naval architect Eric Sponberg. If we are lucky, and Eric isn't out cruising, he may comment here as he also frequents this forum.

DougR

[billknny]

Originally Posted by DougR

No, I don't think it's baloney

So what your saying is all boats with centerline props and shafts motor in circles?

[a64pilot]

Originally Posted by billknny

So what your saying is all boats with centerline props and shafts motor in circles?

No of course not, it’s offset so that you don’t have to counter the torque with opposite rudder which would induce drag.
Aircraft I used to build was a single engine turbine, it’s engine mount was offset down and right, the down countered somewhat the nose pitching moment when adding power and the right offset the torque, enough so that on most aircraft built the rudder trim tab was centered, meaning that no opposite rudder was required, which as I said reduces drag.
I’d assume at least in this instance that aircraft and boats are similar in this instance.

[Wotname]

Originally Posted by billknny

Just because you "can't see anything" doesn't mean it's not there.

It is absolutely 100% NOT true that forward and reverse gear ratios are ALWAYS the same. Many of the common Hurth transmissions for example run 30% slower one direction that the other. That would have a serious impact on propeller choice...

There are gear boxes that do not care which way they turn, there are some that are rated differently if they turn "backwards" and there are some that are "one way only."

The idea that you can run it backwards MIGHT be true... but you need to check the specifics before you try that.

Hmm... Good Morning billknny.

Let's start at the start; the OP stated his engine type (YSM 8) and gearbox manufacturer (Kanzaki), thus I was referencing my statements directly to his equipment.

This combination came with two different ratios (1.95 & 2.93), in both instances the forward and reverse ratios are the same. If you need to check, the manuals can be found on the net.

As to "can't see anything" can also mean "I had a look and I can't see anything" which is the situation in this instance. To expand, before posting previously, I went to the shed and had a close look at my Kanzaki gearbox (same model as the O/Ps) to confirm my understanding of what the manual states. The I/P and O/P shafts are in parallel with a forward and reverse gears along with forward and reverse idler gears. The clutch is a single plate wet mechanical type. There was nothing to suggest the OP will have a problem but you will note, I qualified that assertion with "it's worth a trial..."

I think the specifics have been checked - thank you.

[nhschneider]

Gori folding props have advertised that the reverse attitude has a different pitch than the forward attitude, however, you can now order a Gori prop that has the same pitch in each direction.

Fair winds and calm seas.

[Compass790]

Originally Posted by DougR

There are a couple of things to consider here:

- clutch - if it's a cone clutch there probably won't be a problem with the clutch using "reverse" as the primary drive gear as normally the cone surface area is the same on both the forward and reverse side of the cone. If it's a plate clutch it's important to know that the torque handling capacity is the same for both forward and reverse clutch packs, i.e.. Same number of plates on both packs. An example of this is that Borg Warner " Velvet Drive " gearboxes have smaller capacity reverse clutch packs and cannot be run continuously in reverse.

- Reverse mechanism- many transmissions that use horizontal input and output shafts lying parallel to each other obtain their reverse function by directing the power across an idler gear. Frequently this idler gear and it's shaft are not as robust as the main shafts in the transmission, so the manufacturer places a limitation on the amount of power that can be used continuously in the " reverse" gear position. Common examples of this are the mechanical Hurth/ZF gears.

As you point out "the gearbox is massive for the power" so it may be that running in "reverse gear" for forward propulsion will be OK......but it can't hurt to inquire.

One other thing to consider is that many naval architects purposely offset the prop shaft relative to vessel centerline to counter the effects of prop walk and steering torque while underway in forward gear. To change prop rotation in such a case might result in heavily loaded steering while underway.

DougR

Thanks a lot for taking the time out to write your comprehensive analysis Doug.Our prop is on centreline & yes we would have to remove rudder or engine to get shaft out. I'm now hoping to just remove the prop blade pivot pin & swap the blades but that is dependent on how corroded the rest of the prop assembly is. Clutch is identical for forward or reverse as is gear ratios. I haven't bothered to try Yanmar as the chances of getting someone who knows ( or cares about a 1980 engine/transmission) are slim to none. I have had the gearbox apart &, from memory, ( not to be trusted) I could see no reason for it not to be fine using reverse as forward at the time & inspection of the gearbox diagrams tends to confirm this.
What I was hoping to find was someone who had actually swapped fwd & reverse due to fitting an opposite hand prop but I guess it was a slim to none chance that someone with the specific gearbox would have done that.
Bottom line is I will try it & report back. Of course you'd need a 35 year trial but I doubt I'll live that long