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Old 22-07-2020, 09:13   #1
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Low boat speed at cruising RPM

I have a Yanmar 55HP (4JH2E model number) running my 1998 Hunter 410. Ever since I bought this boat about 4 months ago, this engine has pushed me along at about 5 - 5.3 kts on flat water. Against wind or current I quickly drop to 3.5-4 kts, with wind and/or current I can make 6-6.5 kts. This has been consistent performance since I've owned the boat, and seems pretty normal to me (although I'm the furthest thing from an expert).

However, I've met a few people recently with the same or very similar engines in similarly-sized boats. They have all claimed much faster cruising speeds than I have experienced (6.5-7kts is what they have claimed). Some have suggested I must have some kind of problem - the wrong propeller mounted, a majorly dirty bottom, power being lost somewhere along the way, etc.

Looking for anyone who can comment whether my experience seems reasonable, or if I should be hunting for a problem. I certainly wouldn't mind a bit better speed if I can get it, especially as I'm heading up to New York where I expect some killer currents.

Relevant details I can think to provide:
I cruise at 2400 RPM

My bottom is scraped in the water about once every 2 months. After a scraping I usually see on the order of 0.3 kts improvement at cruising RPM.

I have a fixed 3-blade prop. I don't know any of the specs on it

Standard 150-hour oil and filter change had no impact on cruising speed that I noticed.

I run with the boat pretty heavily loaded - 150 gal water, 65 gal diesel (including cans), dinghy on davits, and everything I own for living onboard. My guess is this is the biggest contributor to my slow speeds, but I'll let y'all tell me.
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Old 22-07-2020, 09:37   #2
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Re: Low boat speed at cruising RPM

Sounds like your prop is not pitched right for 2400 rpm. You should be a lot faster. What's your waterline length? My 44 (avatar) had nearly 39 ft water line and regularly did 8 knots in flat water at 2400+ rpm and a 51 HP engine.
How far up on RPM are you able to go? That will indicate what you are pitched for.
Heavy load has little to do with speed. But will effect how quickly you get up to speed.

With a nearly 38' waterline you should be able to make 8 knots in flat water. Repitch for your rpm.
In a perfect world take a look at your HP/RPM data and make sure you are using a big chunk of available HP. You dont need to get max rpm necessarily if you dont like sounding like an airplane. But look for the sweet spot for good HP without too much RPM. Then pitch to that.
That engine isn't a Turbo is it?
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Old 22-07-2020, 09:47   #3
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Re: Low boat speed at cruising RPM

Sounds like you are under propped.
If you go full throttle (in gear) do you exceed your max rpm for the motor? If so, you are under propped.
Do you know what size prop (dia. & pitch) you have on your boat now?(edit: I see you don't know this and you will need to find this info)
Your boat is fairly common, so others w/the same boat should be able to tell you their prop size.
If you call a prop shop to discuss, besides the engine info you should have the transmission gear ratios and approx. cruising weight of your boat.
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Old 22-07-2020, 09:57   #4
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Re: Low boat speed at cruising RPM

Quote:
Originally Posted by Bill O View Post
Sounds like you are under propped.
If you go full throttle (in gear) do you exceed your max rpm for the motor? If so, you are under propped.
Do you know what size prop (dia. & pitch) you have on your boat now?
Your boat is fairly common, so others w/the same boat should be able to tell you their prop size.
If you call a prop shop to discuss, besides the engine info you should have the transmission gear ratios and approx. cruising weight of your boat
I too think this is a propeller pitch issue. It happened to me. After years of getting up to 7.5 knots from our boat at 3200 RPM, we bought a new propeller but we were unsure of the pitch of our old one. Working with the propeller manufacturer we guessed, AND GUESSED WRONG! Now we are lucky to get 6.5 knots and if motoring was a bigger part of our cruising life we'd have to buy another new propeller, at least 2" more pitch.
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Old 22-07-2020, 10:14   #5
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Re: Low boat speed at cruising RPM

A few new questions based on what everyone is saying so far. Keep in mind, I know f***-all about transmissions, props, gear ratios, and really anything to do with diesels beyond the PMs and the bits I've learned along the way as stuff breaks...

How would I determine what the specs on my prop are? Diameter seems straightforward, but how do I measure pitch? Can I do it while it's in the water?

Cheechako - what do you mean by "take a look at your HP/RMP Data?" Am I taking data points of speeds at different RPMs by actually experimenting? Or something else?

I cruise at 2400 RPM simply because that's what the PO did. Is there a method of determining if this is correct? Maybe I'd be better off cruising higher?

I don't know what RPM I hit at full throttle, but I'll try tomorrow when I'm out for a daysail and report back.

Where else might I be losing/wasting power? Could this be a transmission issue of some kind or something? I seem to remember the PO claiming 6.5kts under engine, but I had put that down to him trying to sell the boat (during sea trial there was a good deal of current, so hard to judge if his claim was real).
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Old 22-07-2020, 10:30   #6
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Re: Low boat speed at cruising RPM

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Originally Posted by wingssail View Post
I too think this is a propeller pitch issue. It happened to me. After years of getting up to 7.5 knots from our boat at 3200 RPM, we bought a new propeller but we were unsure of the pitch of our old one. Working with the propeller manufacturer we guessed, AND GUESSED WRONG! Now we are lucky to get 6.5 knots and if motoring was a bigger part of our cruising life we'd have to buy another new propeller, at least 2" more pitch.

Ahhh, the nice thing of having a feathering prop is you can adjust the pitch! We generally sail much more than we motor, so less drag w/the feathering prop is better.

Initially, we pitched ours slightly heavier than needed as we too run lower rpm while out. It ran fine, but seemed to have a bit of a blade chop so we went w/less pitch (next setting up) and reduced the chopping we noticed previously.
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Old 22-07-2020, 10:44   #7
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Re: Low boat speed at cruising RPM

Quote:
Originally Posted by JebLostInSpace View Post
A few new questions based on what everyone is saying so far. Keep in mind, I know f***-all about transmissions, props, gear ratios, and really anything to do with diesels beyond the PMs and the bits I've learned along the way as stuff breaks...

How would I determine what the specs on my prop are? Diameter seems straightforward, but how do I measure pitch? Can I do it while it's in the water?

Cheechako - what do you mean by "take a look at your HP/RMP Data?" Am I taking data points of speeds at different RPMs by actually experimenting? Or something else?

I cruise at 2400 RPM simply because that's what the PO did. Is there a method of determining if this is correct? Maybe I'd be better off cruising higher?

I don't know what RPM I hit at full throttle, but I'll try tomorrow when I'm out for a daysail and report back.

Where else might I be losing/wasting power? Could this be a transmission issue of some kind or something? I seem to remember the PO claiming 6.5kts under engine, but I had put that down to him trying to sell the boat (during sea trial there was a good deal of current, so hard to judge if his claim was real).

To find the gearing, info look at your transmission and find the identification plate. The gear info will be there for both fwd and reverse.

The prop size (dia X pitch) should be stamped into the center hub of your prop. Do you know if the PO had purchased a new prop? Possibly in the box of receipts/records you may find the prop size.

In your engine manual or online brochure/specs for your motor, you should be able to find the power curve, torque and fuel consumption graphs for your motor (each compared to rpm).
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Old 22-07-2020, 10:54   #8
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Re: Low boat speed at cruising RPM

Quote:
Originally Posted by JebLostInSpace View Post
A few new questions based on what everyone is saying so far. Keep in mind, I know f***-all about transmissions, props, gear ratios, and really anything to do with diesels beyond the PMs and the bits I've learned along the way as stuff breaks...

How would I determine what the specs on my prop are? Diameter seems straightforward, but how do I measure pitch? Can I do it while it's in the water?

Cheechako - what do you mean by "take a look at your HP/RMP Data?" Am I taking data points of speeds at different RPMs by actually experimenting? Or something else?

I cruise at 2400 RPM simply because that's what the PO did. Is there a method of determining if this is correct? Maybe I'd be better off cruising higher?

I don't know what RPM I hit at full throttle, but I'll try tomorrow when I'm out for a daysail and report back.

Where else might I be losing/wasting power? Could this be a transmission issue of some kind or something? I seem to remember the PO claiming 6.5kts under engine, but I had put that down to him trying to sell the boat (during sea trial there was a good deal of current, so hard to judge if his claim was real).
-The prop hub will be metal stamped with diameter and pitch. Your bottom cleaner could look.

-Sounds like you really need to look at boat speed with the throttle wide open.. and rpm. Maybe you are just running it too slow! Many Yanmars are pitched for 3200 or more! It's a Yanmar thing.. I dont like it ...but that's possibly your only issue. Your PO may have been fine going slow speed. Prop pitch is often set at maybe 10% below rated max rpm.. So if the engine is 3600 max rpm, you could pitch the prop at maybe 3250. If you have plenty of HP for your boat, you could pitch it lower if you like running lower. By "pitch it" I mean you select a pitch that keeps the engine from going much higher rpm than you want to cruise at. If it's a Turbo engine I have heard you want to run at rated rpm for turbo cooling, but others may have info on that.

-The owner's manual or on line should have a "power curve" chart for that engine. But this is not a big thing to worry about. Other owners, or prop shops may know what prop you need IF you find more RPM doesn't cure your speed.

-You should be doing 8 knots when you want.
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Old 22-07-2020, 11:25   #9
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Re: Low boat speed at cruising RPM

I have an older Yanmar 4JHE - but basically the same engine.
The maximum rated speed is 3600 rpm, and the engine is rated to run continuously at 3500
Most sources say that 75% of max is a good place to run the engine - so 2700 rpm would be ideal.
Running a diesel too slow, continuously, is not a good idea, as it leads to glazing of the cylinder walls. An occasional blast at full throttle is actually recommended.
Having said all that - it does sound like you may be under-propped. At wide open throttle, you should hit nearly max rpm. At around 3200, you should hit hull speed, of around 8 kn. If you can't hit max rpm you're possibly over-propped. IF you don't hit 8 kn, you're possibly underpropped.
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Old 22-07-2020, 11:34   #10
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Re: Low boat speed at cruising RPM

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Originally Posted by pilott View Post
I have an older Yanmar 4JHE - but basically the same engine.
The maximum rated speed is 3600 rpm, and the engine is rated to run continuously at 3500
Most sources say that 75% of max is a good place to run the engine - so 2700 rpm would be ideal.
Running a diesel too slow, continuously, is not a good idea, as it leads to glazing of the cylinder walls. An occasional blast at full throttle is actually recommended.
Having said all that - it does sound like you may be under-propped. At wide open throttle, you should hit nearly max rpm. At around 3200, you should hit hull speed, of around 8 kn. If you can't hit max rpm you're possibly over-propped. IF you don't hit 8 kn, you're possibly underpropped.
Wow, awesome info here. It helps a dummy like me a ton to get it all in black and white with numbers appropriate for my specific boat! I'll take it out tomorrow at slack tide and run at 24, 27, 32, and max throttle to see what happens. It does sound like I should just be running higher as a matter of course, so maybe this is one of those rare rare boat problems that is easier to solve than expected...

I'm continuously amazed at how quickly and thoroughly I can get information on these forums. You all are brilliant and amazing and thank you.
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Old 22-07-2020, 11:41   #11
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Re: Low boat speed at cruising RPM

Jeb,

You are welcome.
Here's some good data sources

Sail Calculator Pro v3.54 - 3200+ boats
select your boat, and it will show hull speed

https://www.vicprop.com/displacement_size.php
fill in details, and it will select the correct prop dia & pitch

If you look on line, you can find a pdf copy of your engine manual. If not, PM me and I can send you it
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Old 22-07-2020, 11:42   #12
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Re: Low boat speed at cruising RPM

Jeb,had the same problem,similar boat,Bene 423,yanmar 54.Checked my engine speed with a non contact tachometer,Amazon $30,found engine was running 20% slower than instrument panel tachometer was showing,at all rpms. Of course if you think you are turning 3000rpm but really only turning 2400 you will be going slower than expected.
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Old 22-07-2020, 11:57   #13
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Re: Low boat speed at cruising RPM

Wow, that would never have occurred to me... So now if I find my wide open throttle fails to get up to max RPM, I also have to wonder if the readout is lying to me. Hmmmm
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Old 22-07-2020, 12:06   #14
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Re: Low boat speed at cruising RPM

Boat is new to me.PO had a balmar alt installed,tach reads off of the alternator,which I believe is the source of the discrepancy
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Old 22-07-2020, 12:16   #15
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Re: Low boat speed at cruising RPM

Hopefully, your tacho is reading correctly - on the 4jhe engine, the rpm sensor is on the engine. You could check using a strobelight app downloaded to your cellphone.

Here's a website for the engine manual
chrome-extension://oemmndcbldboiebfnladdacbdfmadadm/http://www.endeavourowners.com/dscsn/info/manuals/4JH2E%20Service%20Manual.pdf
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