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Old 16-11-2014, 07:31   #1
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Overpropp'd or not?

When I bought the boat two years ago, I was told it had a 12RH13 prop. When I cleaned her up before splash, didn't see the. actual numbers stamped on the hub. Two seasons of sailing and dealing. with (what I assume was severe) prop walk and 2.3KT at idle in fwd gear.
Now on thehard for the Winter, I see the prop is actually stamped 12HR9 !
Still have a bit of doubt about this pitch; so hoping to get some actual data on propping from others similarly equipped..

'80 Watkins 27 @ roughly 8k# loaded, 23.7' LWL, 10' beam. .
Repowered by PO w/a new (then), approx100 hrs Beta20, developing (book) 16HP. at Hi cruse, 20hp. at WOT of 3000rpm for short,bursts. Idle set. @ 1250 (warm) so it won't .stall out when engaged. Will drop to 1150 tied to dock and in fwd.. Will make WOT in neutral. hull clean, prop fine and on a calm day I can. barely get over (estimated) 2300 before she starts plowing at 5..5kt. Couldn't check actuals underway due to using a photo sensor tach single handed
Calculated hull speed is 6.6kt.

Should I be looking into a lesser prop?
Enuff info?

TIA,
Paul
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Old 17-11-2014, 06:40   #2
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Re: Overpropp'd or not?

Do you motor sail?
Your overpropped, but if you don't run her so she is blowing black smoke I believe you won't hurt her.
My boat is overpropped, my max RPM is 2500, should be 3600, I'm giving up a lot of performance as to keep from hurting the engine, I won't run it continuously over 2000 RPM, but I burn very little fuel and it's great for motorsailing, I just don't like motorsailing myself.
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Old 17-11-2014, 07:49   #3
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Re: Overpropp'd or not?

Quote:
Originally Posted by deltaten View Post
<snip>

'80 Watkins 27 @ roughly 8k# loaded, 23.7' LWL, 10' beam. .
Repowered by PO w/a new (then), approx100 hrs Beta20, developing (book) 16HP. at Hi cruse, 20hp. at WOT of 3000rpm for short,bursts.

Idle set. @ 1250 (warm) so it won't .stall out when engaged. Will drop to 1150 tied to dock and in fwd.. Will make WOT in neutral. hull clean, prop fine and on a calm day I can. barely get over (estimated) 2300 before she starts plowing at 5..5kt. Couldn't check actuals underway due to using a photo sensor tach single handed
Calculated hull speed is 6.6kt.

Should I be looking into a lesser prop?
Enuff info?

TIA,
Paul
Your post is not clear to me - I separated your paragraph into 2 above. Are you saying 3,000 RPM at WOT is what the book recommends?

Then you are saying it reaches 2300 and starts plowing? I don't know what plowing is.

If you are indicating that the RPM peaks at 2300 RPM and 5.5 knots, then yes you are overpropped. Especially if you are getting any smoke and especially if 2300 RPM is at WOT.

I personally like a little overpropped vs. undepropped so that you get hull speed at a little bit lower RPM, say 26-2700 RPM in flat water. Depending on the torque curve this gives a little throttle reserve for chop and wind against current.

BTW - 20hp should easily push your boat at hull speed.
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Old 17-11-2014, 08:02   #4
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Re: Overpropp'd or not?

A64pilot;
Idon't necessarily MS, simply overlap sometimes
I *was* hoping to match max cruising speed toRPS in the range. recommended by Beta; mostly to get best efficiency from the RPM/torque curve.

Looking about t. yard, I see much larger craft with smaller (in comparison) and lesser pitched props. ie:35+ footers with 13x7 , 2 blades. How the heck do they even move? . Guess they on. use as moving slip to fairway?

Fuel economy is fine as is. Best estimate is near 1qt./hr.; calm, flat at 4.KT. Might run to 1.5 Q(!)PH at 5.5kt. Right tough to get over that as she starts to drive under and makes not much bettter speed, an the.engine speed. Had her up to 6+KT once under perfect surface conditions; but developed some black smoke and seemed to be pushing it, so I keep her well under that.
Thanks for the input
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Old 17-11-2014, 08:13   #5
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Re: Overpropp'd or not?

Overpropping overloads an engine often resulting in burnt valves, a cracked cylinder head, broken piston rings and a shortened engine life.
It is important to remember that, with a diesel engine, it is the load and not the revs that determines fuel consumption. Therefore, continuous overloaded operation also results in unnecessarily high fuel consumption and increased maintenance costs.
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Old 17-11-2014, 08:30   #6
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Re: Overpropp'd or not?

Maybe this is an older version of the Beta 20 but new ones are rated at 3600 rpm. Are you certain this one is spec'd only to 3000?

Regardless, you should be propped so your engine can reach about 90% of max. Like trying to drive your car 30 miles an hour up a steep hill in 4th gear. Puts a big strain on the engine and over time is not good for it.

If you never push to hull speed (which I calculate to be more like 6.2 kts), never motor a long distance against waves or strong winds at hull speed and generally baby the engine you'll probably be OK.
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Old 17-11-2014, 09:50   #7
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Re: Overpropp'd or not?

As far as comparing other similar sized boats to get a feel of what size prop you have, you have to know the gearbox reduction ratio also, A large reduction ratio will turn a big, high pitch prop and do so well as it's at a lower RPM, a lower reduction ratio will turn a smaller, flatter pitched prop at a much higher RPM, but both will deliver the same speed.

I've always been taught that on lightly loaded boat, clean hull and smooth water, that if run at WOT, you should reach the engines max RPM, if you don't get there, you can never use the full power the engine can deliver as you not at that RPM at which that max power is made.
"Lugging" an engine will result in high exhaust gas temperature, which can cause the problems Gord cautions about, but if you baby an engine that is overpropped, it will last a long time, just your only getting to use about half the rated power.

Only good I can see from it is motorsailing, you can keep engine RPM down, but still get some thrust
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Old 17-11-2014, 10:29   #8
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Re: Overpropp'd or not?

All;

Good info. Thanks
Ex-calif;
"plowing' might not be the right (nautical) term.. My intent was to describe the tendency to push the bow down an into the surface/wave. Probably more a function of hull shape and prop shaft angle.
Older version of Beta20? Probably, as it was installed sometime before 2004? A 722D block, which has been subplanted y newer versions. IIRC, the manual says that a WOT of 3k is only for short duration bursts of power. I can probably make that number; but, I ttry to keep itdown to more sedate levels. That is distinctly for the reasons replied; fuel and longevity.

Now that I'm used to it, I down;t mind being a bit over-prop'd.. I actually like the idea of being. able to climb trees and pull stumps; to use a 4WD comparison! Just attempting to gain a bit more efficiency and be able to use a ttad more of the power band that's supposed to be available. The local prop. guy. can reduce the pitch by two inches. The price bandied about was near to $200.. For all the above info and opinion, it doesn't;t. seem worth it. Perhaps I'll just. keep my eyes peeled for a lesser prop. at the right price?

Thanks again,
Paul
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