J42 - Cruising Speed Under Power

[James Ashton]

Hello,

Can any other J42 owners share their typical cruising speed at a cruising rpm in mild conditions - little wind or current, no sails up.

Just wondering about my performance under power.

I'm have a 47HP 3JH2 - turbo and cruising RPM is about 2700 .

Thanks,
James

Arcadia, Hull #17

[Bill Bowers]

7kn with our flexofold

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On Jul 2, 2024, at 7:20 PM, James Ashton <jfelto...@gmail.com> wrote:

Hello,

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[Tom Keffer]

Same engine, 3-blade 18" Max-prop, we usually do about 6.8 kn at 2400 rpm. Get about 0.8 gph at that speed.

I pitch it at 18º. I will note that I have trouble reaching max RPM of 3600. Usually 3200 is about the best I can do.

[Wayne Cassady]

Max Prop 3-blade, J40, Yanmar 40hp, clean bottom, lightship, flat conditions about 3000 rpm, easily makes 6.5 knots on Garmin and B&G.

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On Jul 2, 2024, at 7:47 PM, Bill Bowers <wfb0...@gmail.com> wrote:

7kn with our flexofold

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[Todd Stevens]

About 7 knots. PO had the max prop adjusted iirc to 21 degrees just before I bought it. He thought it was a noticeable improvement 

Todd Stevens

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On Jul 2, 2024, at 16:57, Wayne Cassady <wayne.c...@gmail.com> wrote:

Max Prop 3-blade, J40, Yanmar 40hp, clean bottom, lightship, flat conditions about 3000 rpm, easily makes 6.5 knots on Garmin and B&G.

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[Tom Keffer]

... and you can reach 3600 with a 21º pitch? No way I could.

What diameter is the Max-Prop? And, which engine?

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[rers...@netscape.net]

56hp 4 cylinder Yanmar, shoal draft 2002 J42, 3 blade Maxprop 2400 rpm  7-7.5 kts. with 5000 lbs cruising load, and .8 gph fuel consumption.

[Todd Stevens]

Tom, was that directed to me?  

Engine 3JH2TE. Prop. 20” according to survey.

Reportedly, the pitch decision was made after consulting this forum.  To be honest, I haven’t tested WOT deliberately.  I have run at 3000-3200 a couple of times but fuel consumption gets crazy.  On return to HR, right after the prop was adjusted, an uncontrolled gybe ripped the control handle off, in the sluiceway on approach to Bonneville.  We were kind of panicking, but I *think* it was going at >3500, before we decided to shut down the engine.  Engine was roaring, not lugging.  Sailing up to the Bonneville lock was “interesting” but a different story.

As it happens, I did take a short video of the prop yesterday, for a different reason, but cannot see the pitch mark. The Schooner Creek invoice says only “increase pitch by 2 degrees.”  I think 21 is the max.    

Tomorrow, I’ll make the final visit to the boat until October. Time in the storage yard is limited to 30 minutes, and I already have a list of things to do...

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[Tom Keffer]

My prop is smaller and with a lower pitch. Don't know why I can't get full RPM, but I'm not going to sweat it. 

Welcome back to HR. Just in time for our first heat wave of the season!

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[James Ashton]

Thanks everyone. 

Has anyone in this group re-powered their J42?   

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[Gary Hooper]

We generally cruise at 26-2800 RPM, 6.4 knots, .58 gph.

Gary Hooper

J/42 Raven

USA 93486

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On Jul 2, 2024, at 7:47 PM, Bill Bowers <wfb0...@gmail.com> wrote:

7kn with our flexofold

To view this discussion on the web visit https://groups.google.com/d/msgid/j4x-owners-group/B156C8CA-A316-470B-A91B-760D70466E40%40gmail.com.

[Jay PA]

This is the speed vs rpm curve for my 2001 J/42 #47 with the Yanmar 4JH3E 56hp with a KM3A 1:2.64 marine gear turning a new Flexofold 3-blade 19x12-3R prop:

4.3kn @1500rpm

5.15kn @1700

6.1kn @2000

6.75kn @2250

7.15kn @2500

7.65kn @2730

7.9kn @3000

8.0kn @3200

8.1kn @3600

Wide Open Throttle is 3800rpm

Hull speed, 1.34sqrt(LWL) is around 7.94kn.

The prop it replaced was a feathering Maxprop set to 14" (20 degree) pitch.

- 1500 rpm, 4.7 kn

- 2000 rpm, 5.8 kn

- 2200 rpm, 6.4 kn

- 2350 rpm, 6.8 kn

- 2450 rpm 7.0 kn

- 2500 rpm, 7.1 kn

The documentation that I have state that it consumes 0.8-0.9 gph at 2400 rpm. 

I usually cruise under power at 2200 to 2500 rpm, or motor-sail around 1700-2000 rpm.

The J/42s before hull number 47 (my boat) used the Yanmar 3JH2.  Here are the continuous output HP vs. RPM for the two engines, from the docs that came with my boat, fyi.

RPM         4JH3E         3JH2 TBE

1400         2.78             2.96

1600         4.16             4.42

1800         5.92             6.29

2000         8.12             8.62

2200          10.81          11.48

2400           14.03          14.9

2600           17.84          18.95

2800           22.28           23.66

3000           27.40           29.11

3200           33.26           35.32

3400           39.89           42.37

3600           47.35           50.30

3800           49.35           n/a

3800           55.69           n/a

The 3JH2 puts out slightly more hp at a given rpm than the 4JH3E.  The 4JH3 puts out a little more hp at an extended rpm range.

Jay

J/42 Cirrus

[James Ashton]

Jay ,

This is very helpful. Thanks to you and everyone else who responded.  

James F. Ashton

J42, Arvada #17

On Jul 6, 2024, at 4:12 PM, Jay PA <svh...@gmail.com> wrote:



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[smruff01]

Jay,

This is great information. Thank you!

We are looking into a Flexofold as a potential replacement for our Maxprop.  We are the new owners of Cayenne, J42 #65, with the same engine/reduction as Cirrus.

Some questions if you have the time:

- When you replaced the Maxprop with Flexofold, was there enough thread on the shaft to use the existing shaft or did you replace it?  I ask because our boatyard says they often have to trim the end of the shaft to get a Maxprop to properly fit.

- Flexofold said the max RPM with the 19-12-3R would be 3650 RPM. What max RPM are you able to achieve?

Any other observations or recommendations based on your experience would be most welcome.

Much obliged!

Scott Ruff

Cayenne, J42 #65

[Jay Pasco-Anderson]

Hi Scott,

Yes, my shaft had enough thread when I switched to the Flexofold.  I was present when it was mounted on the unmodified original shaft.  BTW they did deepen the indent for the set screw on the coupling at the inboard end of the shaft.

That speed vs. rpm curve is what I actually measured with a clean bottom on flat water.  So I was able to run at 3600 rpm and measured 8.1 knots (beyond hull speed) with  my 4JH3E, KM3A and Flexofold 19x12-3R.  I can't recall if I tried or was able to push it to WOT at 3800 rpm.  That prop was the one recommended to me by Flexofold for the J42.

I haven't had any issues or changes in five seasons of use.  I enjoy the simple design which requires zero maintenance.

Congratulations and good sailing.

Jay

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[jeff thayer]

Scott, I have a proposal for you:  We have a J/42 (#46) w/ a Yanmar 4JH3E engine and a three blade Flexofold prop that we are thinking of replacing for an appropriate MaxProp.   Why?   We love the forward drive efficiency, but the reverse efficiency is not so great.   I would sacrifice some forward efficiency for better stopping power.  Wanna trade?   

Jeff Thayer

SV Allez (#46)

Currently lying at RCR in Buffalo NY.  

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[smruff01]

Hi Jeff,

Thanks for the proposal!  I PM'd you.

Cheers,

Scott

[Sue Mikulski]

3years ago Paul changed our prop to a fixed prop.  We had some vibration in the transmission which had been rebuilt previously, as our slip is narrow.  We were given a slip at a local club in Oxford that involved backing in with 4 turns, and Paul nailed it with that prop.  He really felt it was a big improvement.  Of course if you’re racing you want that extra 6 sec.   We always find J boats back up so efficiently. No need to respond, just some interesting observations from my husband.

Sue

Sue Mikulski

410-212-0495

ssmik...@gmail.com

On Oct 14, 2025, at 8:43 AM, smruff01 <smru...@gmail.com> wrote:

Hi Jeff,

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[smruff01]

Thanks for the added insight and experience, Sue!

My best to you and Paul,

Scott