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post #11 of 23 Old 07-25-2019 Thread Starter
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Re: Motoring RPM

I will check my boat speed and RPM at full throttle in the right conditions and report back. I assume my tach is accurate although I'm not certain how to verify this. Thanks.
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post #12 of 23 Old 07-25-2019
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Originally Posted by Jonah71 View Post
I will check my boat speed and RPM at full throttle in the right conditions and report back. I assume my tach is accurate although I'm not certain how to verify this. Thanks.
I checked the rpm on my engine using a piece of white tape on the front crank pulley and a digital laser-pointed rpm reader from Harbor Freight. It was under $30. as I recall. I wanted to verity that the tach in the engine panel was accurate, and it was.

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post #13 of 23 Old 07-25-2019
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Re: Motoring RPM

Second the motion to buy an inexpensive laser tach. I got mine on Amazon for under $30. Tachs are often wrong. 50-100 rpm difference may be expected. More is a bad tach or wiring.

Hull speed has nothing to do with a proper engine/prop operations. Think about it this way. If you put a lawn mower engine in your boat, it could be operating perfectly well, but never reach hull speed. Put one from a battleship in there and it could be coughing and sputtering and get you to hull speed.

You should be able to make max RPM, under load, at wide open throttle. If not, the problem is most often the prop or drive train. The next, of course, is the motor.

If WOT is really only getting you to 60% of rated RPM, the engine is grossly lugging, even if you're getting the speed you like. Just like going up hill in too high a bicycle gear. You might be moving along, but you're killing your legs. It does not do instant damage, but it does take a toll over the long term.


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post #14 of 23 Old 07-25-2019
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Re: Motoring RPM

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Originally Posted by Jonah71 View Post
1988 PSC 34 Yanmar 3hm35F 2 blade prop 17X10 RH
I understand that this engine is designed to run at a peak continuous rpm of 3200. My throttle pegs at 2400. It seems I am leaving a lot of power unused and therefore losing potential speed motoring. Is the solution a throttle adjustment? What role does the propeller play in this? I have sailed the boat since I purchased it 4 years ago typically motoring at 2000-2200 rpm because this seemed appropriate given that flat out is 2400.

Thoughts?
When we bought Irish Eyes in 2004 she had three propellers, all two blade, bronze, and solid. Installed on the boat was a 17x10, and in the locker were both a badly corroded 17x11 and a dirty 17x10. We have continued to use the 17x10 although with a clean hull and clean propeller the engine (Yanmar 3HM35F) can go over its maximum rpm (3400 rpm). With a few barnacles and with the alternator and the refrigeration compressor running (engine drive refrigeration), we come up to the red line with no smoke. I've thought about mounting the corroded 17x11 and giving it a try, but I've never gotten around to it.

According to the owner's manual, the standard 2 blade propeller is a 17x12. I figure that I have a slightly lowered pitch prop to allow the easy operation of the oversize 105A alternator and the refrigeration compressor. We motor on the ICW at 2400 rpm.

Make sure that the throttle on the engine itself (actually the governor) is fully open. Move it with your fingers and see. The only times we have not reached 3400 rpm flat out is with massive - like basketball size - amounts of growth on the hull and propeller.

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post #15 of 23 Old 07-26-2019 Thread Starter
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Re: Motoring RPM

It definitely sounds like there is something not quite right with my set up. I will check to see if the throttle on the engine is wide open when the lever at the helm is pegged - I realize I have never looked at this. I had thought that the the total distance the throttle lever at the helm can move the throttle cable is determined by the geometry of the mechanism but maybe the forward stop is adjustable. It is an Edson helm which I have assumed is standard equipment. The engine has always started immediately and has never smoked. Maybe it is time to have a good diesel mechanic go over the engine.
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post #16 of 23 Old 07-26-2019
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Re: Motoring RPM

Had this boat with this engine. Before blowing the bucks please check the throttle cable. Unscrew the four screws at the corners and lift the access panel in the cockpit sole out. Detach the linkage to the engine. Have someone push it stop to stop and make sure there’s full range of movement. With cable off move governor on the engine and make sure it has full range of movement. Then reattach cable and check for full range of movement. There’s stops on these things. They maybe set wrong. The cable maybe binding or kinked.
You report no smoke, easy starting and normal running. So would check the simple stuff first.
Loved this boat. Never liked the weird way you get full engine access.
BTW had the 3 bladed maxprop on this boat. Dramatically improved low speed handling. Especially backing up.
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post #17 of 23 Old 07-26-2019
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Cool Re: Motoring RPM

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Originally Posted by outbound View Post
Had this boat with this engine. Before blowing the bucks please check the throttle cable. Unscrew the four screws at the corners and lift the access panel in the cockpit sole out. Detach the linkage to the engine. Have someone push it stop to stop and make sure there’s full range of movement. With cable off move governor on the engine and make sure it has full range of movement. Then reattach cable and check for full range of movement. There’s stops on these things. They maybe set wrong. The cable maybe binding or kinked.
You report no smoke, easy starting and normal running. So would check the simple stuff first.
Loved this boat. Never liked the weird way you get full engine access.
BTW had the 3 bladed maxprop on this boat. Dramatically improved low speed handling. Especially backing up.
Occam's razor.... always the best way to shave!
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post #18 of 23 Old 07-26-2019
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Re: Motoring RPM

I've found that my PSC 34 cruises best at 2000 RPM. I did an RPM speed curve in flat water. But then after a few years of records looked at RPM and fuel consumption. 2000 RPM gave me over 5 knots burning 2 hours on one gallon of diesel.

I'm sure there are as many takes on this as there are sailors...

Bob Steneck
PSC 34 Alaria
Christmas Cove, Maine
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post #19 of 23 Old 08-08-2019 Thread Starter
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Re: Motoring RPM

Reporting back but not much to report. At max throttle in flat calm condition I hit 2500 rpm and 6 + knots boat speed. My hull is clean but not particularly fair. The nicks in the keeI from minor encounters with Maine ledge are from the previous owner (I swear). I have not yet had the opportunity to disconnect the throttle and check this set up. Thanks for all the comments.
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post #20 of 23 Old 08-08-2019
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Re: Motoring RPM

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Originally Posted by Damon Gannon View Post
Does the boat reach hull speed? If it does, then it won't go any faster, regardless of increasing engine RPM. Most likely explanation is that you are "over-propped" (either the pitch or diameter of the prop is too great), so the engine is unable to get up to full RPMs. If it is over-propped, you should consider an appropriately-sized prop because you are increasing the wear on the engine and transmission. Another simple explanation may be your fuel filters. Have you changed them? How does the engine run at idle? Does it run rough or seem like it is being starved of fuel?
Fuel filters, dirty prop, carbon clogged exhaust elbow is were I’d start.


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