What's wrong with this picture

[Subway Sailor]

Originally Posted by Canuck Sailor

Subway:

Not to disagree with all the some of the other replies but having been a Yanmar rep for over 15 years and sailboat propeller business for another 15 years, I have to put my two cents in.

Your Yanmar 2GM20F in a 28' boat should push that boat along at over 4 knots at those RPM, especially with a newly painted bottom. You should have the reserve to WOT to fight into head winds and seas at approximately the same speed.

You don't mention the propeller at all, number of blades, diameter, pitch or transmission ratio. This will, like they say make a difference to your wide open throttle rpm.

In neutral at the dock you should be able to reach 3800 - 4,000 rpm to show you have throttle settings correct and no restrictions. Scary to do but best way to check the linkage.

Underway at WOT you should as they suggested be able reach 3400 - 3600 rpm. Naturally it would be preferable to do it after the engine is warmed up. If it goes over the 3600 your pitch/diameter is probably too low and could be increased. That would also give you higher boat speed at your optimum running rpm that you are presently using. If it only reaches 3200 or less your problem is not likely to be resolved with the propeller as it may be a transmission or engine power problem since you aren't getting the speed at low rpm with an over pitched prop.

At any rate running at the 2000-2500 rpm range is not a problem but you should run it up over 3000 rpm for at least 10 - 20 minutes each time you use it to heat up the exhaust and blow out the carbon and it also can help to keep your hull clean. The water mixing exhaust elbow needs this particularly as it can close up and restrict exhaust flow and cause low power problems in your engine.

I am assuming that this is not a new installation and it has had a previous owner.

Another point never use your engine for charging battery by idling it at 1200 - 1500 rpm as it is not loaded up, as others have suggested and it does need to work. Put the engine in reverse at anchor if that is when you need to charge or at the dock in gear. At the dock better yet, plug in and use shore power to charge. Or even better change anchorages and motor over charging at the same time.

More than two cents, I know but once started hard to stop.

I have to tell you that i don't have a temp gauge...not sure why...but thats what i got on the boat, id like to monitor this type of stuff so ld like a solution to that.

In regards to my prop, i only learned about prop pitch in this discussion and will have to look at haul out in the fall, but i can tell you that it is a three blade! Lol. Take a peek at my haul out video and let me know what you think.

Since tropical storm ana didn't let me go check my WOT performance or my increased rpm speed i will have to wait till monday/tuesday


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[Cheechako]

Re: Battery charging: at 1500 rpm that engine produces maybe 5HP. A 100amp alternator will be loading it with maybe +/- 2hp. Sounds like the engine is loaded to me.

[Adelie]

Per psychosnail's implementation of Gerr's formulas an SC28 (20.17' LWL) in lightship condition (8500lb) should use
Hp Kt
3.7 3.0
5.1 3.6
6.5 4.2
8.5 4.8
12.3 5.4
18.0 6.0 (hull speed)

http://www.psychosnail.com/boatspeedcalculator.aspx




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[Subway Sailor]

Originally Posted by Adelie

Per psychosnail's implementation of Gerr's formulas an SC28 (20.17' LWL) in lightship condition (8500lb) should use
Hp Kt
3.7 3.0
5.1 3.6
6.5 4.2
8.5 4.8
12.3 5.4
18.0 6.0 (hull speed)

http://www.psychosnail.com/boatspeedcalculator.aspx




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Now match an rpm column


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Subway Sailor is on YouTube

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[Adelie]

Originally Posted by Subway Sailor

Now match an rpm column


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I'm leaving that as an exercise for the reader.

Also I'm working off my phone and don't want to read off the graphs on it


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[hellosailor]

Battery charging & alternators:
You never know if the right alternator & right size alternator pulley are on an old engine. In theory, the pulley must be sixed correctly so that the alternator is putting out "some" power at idle speed, and FULL power at cruising speeds, but without over-revving the alternator is case you are at max rpm fighting headwinds, etc. for hours. And of course, yes, you will need to steal about 1 hp for every 50 watts of alternator output, but that's not the most likely issue here, just one to add to the punch list in future. Correctly sizing the alternator & pulley can make a huge difference in battery charging time. Like, halve or double it.

[SkiprJohn]

If you can't hold your hand in the exhaust stream without burning it then it's too hot. Run you engine up to a little less temp than that.

[Wotname]

Originally Posted by Adelie

Per psychosnail's implementation of Gerr's formulas an SC28 (20.17' LWL) in lightship condition (8500lb) should use
Hp Kt
3.7 3.0
5.1 3.6
6.5 4.2
8.5 4.8
12.3 5.4
18.0 6.0 (hull speed)

Boat Speed Calculator

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Originally Posted by Subway Sailor

Now match an rpm column


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Subway Sailor is on YouTube

Originally Posted by Adelie

I'm leaving that as an exercise for the reader.

Also I'm working off my phone and don't want to read off the graphs on it

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Let me give you one easy data point
18 hp; 6 kts (hull speed); 3600 rpm

[bailsout]

Wotname, thanks for jumping in and making me feel a little better about how I have been operating my engine.

[Subway Sailor]

Originally Posted by SkiprJohn

If you can't hold your hand in the exhaust stream without burning it then it's too hot. Run you engine up to a little less temp than that.

I assume that is safe then?
Are you the youtube guy with all the tips?


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[DougR]

Originally Posted by Shawn67

"At 2000RPM that works out at only about 3HP, at 2500RPM it is about 6HP."

That seems low.

HP = (Torque x 5252) / RPM
Torque = ( 5252 * HP) / RPM

Diesels have pretty flat torque curve and peak torque is probably not at max RPM. At 3600 RPM if it is making 18hp it is making just over 26 ft/lbs of torque. If you use 26 ft/pounds at 2000 the engine is putting out almost 10hp and a little over 12hp at 2500 rpm.

Shawn

The formula shown here for HP is incorrect.....should be:
HP= (torque x RPM) / 5252
DougR

[Subway Sailor]

So guys.. Took the boat this week as per this picture.. Had the alternator belt snap on me and get the water pump">raw water pump belt loosen up several times...but based on your instructions. I got up 5-6 knots with rpms from 34-3600! So i feel at ease with the performance and fuel consumption.

I did realize that the belts have all gotten loose again after 12 hrs of motoring in the rain with no wind..is this normal? What can i do to avoid this?


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Subway Sailor is on YouTube

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[Wotname]

Originally Posted by Subway Sailor

So guys.....
I did realize that the belts have all gotten loose again after 12 hrs of motoring in the rain with no wind..is this normal? What can i do to avoid this?


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Certainly not normal, should be getting 100 or 200 hours before adjusting and will be adjusting for wear & stretch. New belts will need adjustment after a few hours and maybe again after say 20 or 50 hours.

Presumably something is coming loose or wearing excessively. Any belt dust floating around?