Perkins 4.108 + Flexofold Propeller Sizing - opinions sought!

[Columbia43]

G'day all, this is our first post

We are currently involved in an effort to apply some much needed TLC to a 1971 Columbia 43, powered by a Perkins 4.108 with a Walter Gear Drive (1.46:1 reduction).

The current propeller is a 15x10 LH fixed 3 blade, however it has some signs of corrosion so we are replacing with a new 3 blade flexofold.

Flexofold Denmark said they are having difficulties recommending the correct size due to the reduction ratio, and suggested that a 15x8 or 15x9 may be overpropped but quoted for those sizes anyway.

They also quoted for a 14x9.

We have punched the numbers into a few online prop size calculators, and all seem to suggest between 12" and 16" diameter and 6" to 9" pitch, depending on which value is used for max RPM.

Our understanding of the Perkins 4.108 is:

47 SHP @ 4000 RPM
Max continuous 3000 RPM
Ideal Cruising RPM ~ 1800 to 2400

Please can anyone offer any experience or opinions on which Flexofold prop size we should go with? Any help or pointers will be greatly appreciated!

Regrettably we have no data on existing prop performance, RPM or fuel burn rates etc.

Many thanks & warm regards,

Columbia43

[donradcliffe]

Don't even think about 4000 rpm. That is just a marketing gimmick for making a 37 hp engine sound like a 50 hp engine.

3000 will bring you a lot of noise and oil leaks. I would use 2400 and 35 hp to size the prop.

[skipperpete]

Would that mean you have a Borg Warner 1:1 reversing gear? If the boat is in the water, give the prop a clean and and take her for a run at full throttle to see what max rpm you achieve, that should settle the “overpitched or not” question.

[Columbia43]

Originally Posted by donradcliffe

Don't even think about 4000 rpm. That is just a marketing gimmick for making a 37 hp engine sound like a 50 hp engine.

3000 will bring you a lot of noise and oil leaks. I would use 2400 and 35 hp to size the prop.

Thank you for the reply, using 2400 and 35 hp gives the following 3 blade specs on various online calculators:

Michigan Wheel: 17 x 9
Vicprop: 15.1 x 8.6
Sailing n Cruising: 15 x 10
Gcalc: 7.9" pitch @ 30% prop slip to 9.2" pitch @ 40% prop slip

So perhaps the 15 x 9 quoted by Flexofold is the one to go with, which is the first one they listed in their reply... however their comment "having difficulties recommending the correct size" doesnt exactly convey much confidence!

[Columbia43]

Originally Posted by skipperpete

Would that mean you have a Borg Warner 1:1 reversing gear? If the boat is in the water, give the prop a clean and and take her for a run at full throttle to see what max rpm you achieve, that should settle the “overpitched or not” question.

Thank you for your reply. I do not know the answer re the Borg Warner so will need to check next week however the Walter is I believe a reversing gear (RV26).

Unfortunately the boat is now on hardstanding with the old prop removed...

[DougR]

Of course the reason FlexOFold is having a difficult time making a recommendation is that a 1.46:1 reduction ratio is an extremely poor ratio for a displacement vessel.

As Skipperpete alluded, does that mean you have a BW 1:1 feeding into the Walters?

It works like this.....When you have an engine mounted transmission feeding into a remote Vdrive, the overall final reduction ratio is the engine mounted transmission reduction ratio multiplied by the remote transmission ratio.

So if you have a 1:1 ratio on the engine and a 1:46 ratio on the remote, your overall ratio is 1 x 1.46=1.46 .
If, for example, the engine had a 2:1 reduction and the remote vdrive had a 1.46:1 ratio, then the final drive ratio would be 2 x 1.46= 2.92.

Of course, the above assumes you have a remote vdrive. If the Walters is close coupled to the transmission then the final reduction will be the same as the vdrive reduction.

No one can give a reasonable prop recommendation without knowing what the actual overall reduction ratio is.

[skipperpete]

Excellent explanation DougR. I’m still a bit fuzzy about the reversing Walter drive, I have a feeling that it simply reverses the output of the vee drive to suit the propeller, if he has a borg warner it can’t be run continuously in reverse if the prop is the wrong rotation so they do it in the Walter.

[Columbia43]

Originally Posted by DougR

Of course the reason FlexOFold is having a difficult time making a recommendation is that a 1.46:1 reduction ratio is an extremely poor ratio for a displacement vessel.

As Skipperpete alluded, does that mean you have a BW 1:1 feeding into the Walters?

It works like this.....When you have an engine mounted transmission feeding into a remote Vdrive, the overall final reduction ratio is the engine mounted transmission reduction ratio multiplied by the remote transmission ratio.

So if you have a 1:1 ratio on the engine and a 1:46 ratio on the remote, your overall ratio is 1 x 1.46=1.46 .
If, for example, the engine had a 2:1 reduction and the remote vdrive had a 1.46:1 ratio, then the final drive ratio would be 2 x 1.46= 2.92.

Of course, the above assumes you have a remote vdrive. If the Walters is close coupled to the transmission then the final reduction will be the same as the vdrive reduction.

No one can give a reasonable prop recommendation without knowing what the actual overall reduction ratio is.

Thank you for the detailed reply, we were unaware of the possibility of there being an additional reduction gear in the drivetrain! Your explanation is very helpful and makes sense, thank you, we will need to further explore all components and will report back in due course...

[skipperpete]

What you will need to establish is the direction of propeller rotation in ahead.

[Columbia43]

Originally Posted by skipperpete

What you will need to establish is the direction of propeller rotation in ahead.

It's a Left Handed prop; anticlockwise rotation in ahead when viewed from astern looking forward.

We did motor a short distance with the old prop prior to being lifted onto the hard, however we were unable to note any rpm values due to non functioning instruments (other than oil temp). Just another of the endless tasks ahead!

Many thanks for your input, much appreciated, it will be a week or so before we can investigate the potential Borg Warner/additional reduction gear, but will update this thread when able.

[Mickeyrouse]

Originally Posted by Columbia43

Just another of the endless tasks ahead!

BOAT OWNERSHIP: a long-term cure for idleness.

[FPNC]

As stated by others and Flexifold. If you are close coupled with a 1.46:1 reduction Walter V-drive, this is a terrible choice for reduction ratio in this application. You can live with this error or change it. The RV26 can be converted to a 2.05:1 or 2.56:1 and you will not have to change any of the current engine mounting. Which can be a daunting task.

However, you would need to make sure that you have enough room for the larger diameter propeller that will be required with a lower gear reduction. The 4.108 should turn a 17" dia prop. 16" dia as minimum.

Call Walter Gear about the potential reduction change. Unless you can provide the model number and serial number from the Walter gear, they will not have much to tell you.

[Cheechako]

Originally Posted by donradcliffe

Don't even think about 4000 rpm. That is just a marketing gimmick for making a 37 hp engine sound like a 50 hp engine.

3000 will bring you a lot of noise and oil leaks. I would use 2400 and 35 hp to size the prop.

I agree, for a 44 ft boat (my avatar) my Perkins 4-108 was very happy at 2450 rpm and set up to max 2700 rpm or less. Plenty of HP too and great hull speed on very little fuel.

I highly recommend a Max Prop.

[NorthCoastJoe]

Look at J-prop. You can change the pitch while in the water.

[Mickeyrouse]

Originally Posted by Cheechako

I agree, for a 44 ft boat (my avatar) my Perkins 4-108 was very happy at 2450 rpm and set up to max 2700 rpm or less. Plenty of HP too and great hull speed on very little fuel.

I highly recommend a Max Prop.

It’s been a long time since I had a Perkins-Westerbeke 107 (almost identical) so I don’t remember the output curve. Is 2400-2700 about the peak of the HP output?
I played with the pitch on my Max Prop for a couple years. I was pretty proud that I could pitch it to get close to max speed at 1300 RPM, until it dawned on me that I wasn’t getting max HP at that engine speed, and that it was almost impossible to punch into any kind of chop. And it overheated.
I never felt like I got much improvement under sail- if any- but the reversing pitch in reverse was really appreciated on those “rare” occasions when I got in the mud.
Because haul-outs are inaccessible where we live, I couldn’t keep barnacles off the blades like it needed. You can’t imagine the sequence of events that transpired as a result.