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Old 29-02-2020, 09:22   #46
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Re: Installing EGT Probe on Yanmar 4JH3

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Originally Posted by malbert73 View Post
Sure. But remember engine manufacturers assume boat operators are dumb and will pin the throttle every chance they get. So they are making guidelines that protect their engine from being quickly ruined via lugging. It’s the “decerebrate” approach and certainly safest.

Automobiles run with gears and no one runs at highway speed in second gear. A64 makes good points about under loading but I think the intermittent high load of acceleration is what makes them not suffer effects of under loading.

But I’m no expert - don’t take my word for it. But I read and spoke to a lot of experts and didn’t rely on dogma in my setup.

Read Nigel Calder, John Harries (Morgan’s cloud), and Steve Dashew’s approach. All have done the same thing or are proponents of it. Andy Schell did same on his repower and wrote about it.
Malbert,

I've already written that I believe you are overpropping the safe way with the installation of an EGT. But I also believe that you over did it with your Beta 50. Here's why I say that.

In Dashew's case, as far as I can remember, his concern was about the high revving Yanmar causing cavitation with his MaxProp when the shaft RPM exceeded some value because of excess tip speed. Which it often did due to the Yanmar's power curve. Yanmar was a real interrupter in the recreational marine engine market due to their stated marketing HP and small envelope. The stated gross HP was a marketing angle, yes they'll produce mid 50 HP but at what RPM, 3600 to 3800. Nobody runs there so it was foolish to size or pitch your prop to that gross HP RPM. So what Dashew reckoned was to over pitch his Yanmar down to the continuous HP rating and maybe a bit more to match the increments afforded by his MaxProp. No issue with this at all.

John Harries had the same cavitation issue with his MaxProp and during repower he was having an issue finding a perfect an engine that met all his criteria, again it's been years since I read his account. But I believed he settled on a normally aspirated Perkins (Sabre?) that had more HP than he needed or wanted so he over pitched his MaxProp to get the shaft RPM's down. He also installed an EGT to monitor temps because he was aware of the issue. I also remember in Harrie's account that the yard in Maine that did the repower was strongly against the overpropping. This is the same yard/diesel mechanics that Harrie states forgot more about diesel engines than he'll ever know. Which is why he travelled down to Maine from Lunenberg. In fact I sort of remember a throw away comment he made at the time for none of his readers to let the yard know he overpropped. But he didn't over prop by as big a margin as you seem to have.

Andy Schell, if I remember correctly, was all about matching your engine's output to the manufacturer's fuel map to attain the highest efficiency for the HP put into the water. He did that using a variable pitch Hundested propeller. The wind up of his article was to show how one could vary the pitch of the propeller AND rpm to get the EGT into the fuel maps sweet spot. You can't match that with a fixed pitch prop. You can match to one sweet spot but all the other RPM points are sub optimal. And the higher RPM's are now off limits to you.

I took the liberty of running the HP requirements for your Tartan 40, great boat BTW, and the estimate that my re-powering software provides is that your T40 (I assumed a laden displacement of 19,000 lbs.) requires 36 HP to drive your hull to 7.5 knots with a clean bottom and calm sea. From the Kubota power curve for your V2203, I see that engine produces 36HP at about 1900 RPM. So for you to reach hull speed at 2200 you have in reality turned your 50 HP/ 2800 RPM engine into a 36 HP / 2200 RPM engine with a rich fuel mixture throughout the 2200 RPM range. The reason your EGT goes north when you cross 2200 is because the prop power curve has intersected and gone above the engine's power curve and serious overloading has begun. Sailors without an EGT don't know this because most can only monitor cooling water temperature which doesn't measure engine load. It's not that engine manufacturers think boaters are ignorant, they know that most do not have the instrumentation necessary to run slightly overpropped.

Call me a heretic but I don't follow Nigel Calder.

Your boat your call, but I believe you have excessively overpropped and would be well served to dial it back to 2650-2700 RPM.
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Old 29-02-2020, 10:38   #47
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Re: Installing EGT Probe on Yanmar 4JH3

I seriously doubt these people that claim they are cavitating a sailboat propellor are actually doing so, especially when they claim the prop isn’t big enough to absorb the mighty sailboat diesel or tip speed etc. I guess you could cavitate a really poorly designed prop

Take a look at a 350 HP outboard’s propellor, it doesn’t cavitate and isn’t as big as my prop on my 40 HP sailboat, and I can assure you it’s turning a higher RPM than a sailboat prop too.
If you are really cavitating your propellor, you will know from the damage it does to the propellor, if there is no damage, your not cavitating your prop.

http://brennen.caltech.edu/fluidbook...tionDamage.pdf
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Old 29-02-2020, 10:50   #48
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Re: Installing EGT Probe on Yanmar 4JH3

The fuel savings from overpropping is minimal at best, the fuel savings comes from the lower RPM of the motor and the reduced friction from it, it exists, but really isn’t all that much.
You overprop for reduced noise and vibration, if you overprop for fuel savings and cruise the same speed, you will be disappointed, however many that do overprop end up cruising at a slower speed due the lower HP output of the engine, and they do see considerable fuel savings, but it’s from the boat speed reduction.
As was stated previously it’s simple thermodynamics, and overpropping an engine doesn’t significantly increase efficiency, and therefore doesn’t save much fuel.
But if you do overprop, you need to realize and live with the fact that your 40 HP rated motor is now a 25 or so HP motor, cause the HP formula is tq X RPM /5252. Reduce the RPM and you reduce the HP.
You have also enabled the ability to overload the engine, so you have to ensure you don’t, normally an engine can’t be overloaded, but you have removed that safety.
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Old 29-02-2020, 11:08   #49
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Re: Installing EGT Probe on Yanmar 4JH3

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I seriously doubt these people that claim they are cavitating a sailboat propellor are actually doing so, especially when they claim the prop isn’t big enough to absorb the mighty sailboat diesel or tip speed etc. I guess you could cavitate a really poorly designed prop

Take a look at a 350 HP outboard’s propellor, it doesn’t cavitate and isn’t as big as my prop on my 40 HP sailboat, and I can assure you it’s turning a higher RPM than a sailboat prop too.
If you are really cavitating your propellor, you will know from the damage it does to the propellor, if there is no damage, your not cavitating your prop.

http://brennen.caltech.edu/fluidbook...tionDamage.pdf
a64pilot,

I wasn't saying they were experiencing cavitation or not. I said that Steve Dashew claimed that flat bladed props like MaxProp suffer from cavitation if the tip speed increases above a certain level.

Whether that's true or not is not my issue, but it was the stated issue why Dashew was overpropping his Yanmars...to reduce tip speed and set his prop to the Continuous HP RPM and not the max HP rpm.

I wonder how your outboard would perform if you put a flat bladed prop on it
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Old 29-02-2020, 12:32   #50
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Re: Installing EGT Probe on Yanmar 4JH3

I believe but am not sure but think a boats propeller speed limit is a function of the differential pressure of the two faces of the prop, and not tip speed. If the pressure differential exceeds a certain amount then of course the lower pressure will cause the water to boil and you get cavitation, so surely blade pitch and RPM will both contribute.
A “normal” prop’s pitch is washed out as it gets towards the tip, this is true for helicopter rotor blades, airplane propellors and boats as well, in the aero world it’s done to attempt to distribute the load across the entire blade and to prevent tip stall.
In the Marine world approaching tip stall would I’m sure cause cavitation.
So yes, a flat blade or planar prop would be much more susceptible to cavitation as it has no washout.
Planar props are what flat blades are called, I think, and they are notoriously inefficient. They work, but not very well. It’s a sacrifice made of course to gain a feathering prop with less resistance, although a prop with proper blade twist and an aero profile has a low drag when feathered.
Pure trivia, but one of the biggest reasons the Wright Brothers flew and others didn’t was they had a proper “prop”, they had figured out the propellor and others hadn’t, they often had planar props.
They were literally decades ahead of their times with the propellor, if you look, they had a modern propellor in 1903
The Wright Brothers and Airplane Propeller Design - Hartzell Propeller
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Old 06-03-2020, 13:16   #51
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Re: Installing EGT Probe on Yanmar 4JH3

Here’s a pic of my EGT install on a Beta 50. Probe is circled in blue

Click image for larger version

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Old 06-03-2020, 13:20   #52
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Re: Installing EGT Probe on Yanmar 4JH3

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Here’s a pic of my EGT install on a Beta 50. Probe is circled in blue

Attachment 210170
Thank you. Much appreciated.
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