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#84396 - 12/18/08 09:06 PM
propping your boat...
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Captain
Registered: 11/10/08
Posts: 955
Loc: cape may nj
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ther's a thread in the "boats" section,it's titled toatl weight...it pertains to propping a boat. there's really only one way to prop your boat,load the boat as you normally use it,fill the fuel tank,fill the water tank,and duplicate the people you take and the weight you carry-be as clsoe as possible.take the boat out and open the throttle,read what you get as top rpm,compare that number to what the engine mfg reccomends-you want to be at the top rpm.by doing this,the boat will plane quicker and it will consume less fuel.the common reply here is,"i never run the boat more than 3,500 rpm,"doesn't matter...if the engine can't reach it's reccoemended rpm,it's over loaded,end of story,running an engie overloaded,it severley reduces engine life.the exhaust temp rises and the end result is detonation-not good huh  ? do yourself a favor,prop your boat correctly... remember 1 " in pitch equals 150 rpm... prop work... if you need prop work,i can tell you truly from first hand experience,do not fall for the advertising ploys of some prop repair companies-there's 2 that make computer repair claims-you want to avoid these guys-trust me,these companies are smoke and mirrors...there's a few real good prop shops in our area,one is in tuckerton,the other is in ocean view nj-south jersey-the shop in ocean view,use a hale mri machine,their work is perfect,you get a full report of wha tthe prop was,and what the prop is after repair.i promise you,if you tell these guys what you need and what you get for rpm,when you get the prop back,you're done-i promise you,these guys are that good...the shop in tuckerton,i've never had repair work done by them,but,i'm aware of the good work they do.trust me,avoid the guys in maryland and mt laurel nj...you're paying way too much...
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jim anderson
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#84425 - 12/19/08 11:04 AM
Re: propping your boat...
[Re: jawz]
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Captain
Registered: 10/28/06
Posts: 1577
Loc: Island Heights, NJ
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Re: Pitch and RPMs... While it is a "rule of thumb" that 1" pitch = 150 RPMs ( some say 200) it is no way "linear", i.e. 4 " pitch not necessarily = 600 RPMs. The RPMs that an engine gets is related to the torque curve and to the load. The torque curve is no way linear, and the load varies with prop pitch, diameter and the hull characteristics...all very non linear. I've changed prop pitch by 2 " thinking I'd get 300 to 400 RPM change and had the change be more like 600 RPM.
The real issue is not if you reach the specified MAX RPM, but if you are within the recommended operating range. For example, on my I/O ( a 5.7L, 260 HP MERC MIE (2bbl) in front of a Volvo 280 drive), the MERC factory manual says... operating range 4200 to 4600. Just so long as I am within the range, the engine will be happy. However, within that range, there are differences in operating characteristics. If for example, a 19 pitch prop would get me 4550 RPM and a 21 would get me 4250. Then I would be able to get a bit more top speed and a bit better fuel consumption.
Why is this... Assuming a 1.66:1 reduction and 14% slip at 4250 and 15% slip at 4550, the 19 pitch at 4550 would get me about 42.4 MPH, while the 21 at 4250 would get me about 42.3 MPH.
So where does the improvement in fuel comsumption come from?
On 4 stroke engines there is a parameter called (MEP) Mean Effective Pressure ( internal cylinder). As the MEP increases, the engine is working more efficiently and fuel economy improves somewhat. Increasing the prop pitch to 21 causes the engine to "work" harder ( again within design limits) and the MEP increases.
Some folks think that the prop that gets them the highest RPMS within limits is the best...not always so. Sometimes, top speed actually drops at the higher pitch with the engine still within operating range, then you get to choose betwen a bit more speed and a bit better efficiency.
The non linearity of the amount of "push" needed to move a hull under various conditions, and the non linearity of the engine's torque curve (how much "twist force" an engine generates at a given RPM) is what makes prop selection a bit of a "trial by error" :-) procedure.
BTW... From what one prop shop told me, changing a prop's pitch by one inch or so is about all you can do.
_________________________
Capt Bob
1969 Chris Craft 23 ft Lancer "Retriever"
260HP 5.7L MERC w/Volvo outdrive
Atlantic Salt Water Flyrodders, Seaside Park, NJ
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#84478 - 12/19/08 05:44 PM
Re: propping your boat...
[Re: Retriever]
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Captain
Registered: 11/10/08
Posts: 955
Loc: cape may nj
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Re: Pitch and RPMs... While it is a "rule of thumb" that 1" pitch = 150 RPMs ( some say 200) it is no way "linear", i.e. 4 " pitch not necessarily = 600 RPMs. The RPMs that an engine gets is related to the torque curve and to the load. The torque curve is no way linear, and the load varies with prop pitch, diameter and the hull characteristics...all very non linear. I've changed prop pitch by 2 " thinking I'd get 300 to 400 RPM change and had the change be more like 600 RPM.
The real issue is not if you reach the specified MAX RPM, but if you are within the recommended operating range. For example, on my I/O ( a 5.7L, 260 HP MERC MIE (2bbl) in front of a Volvo 280 drive), the MERC factory manual says... operating range 4200 to 4600. Just so long as I am within the range, the engine will be happy. However, within that range, there are differences in operating characteristics. If for example, a 19 pitch prop would get me 4550 RPM and a 21 would get me 4250. Then I would be able to get a bit more top speed and a bit better fuel consumption.
Why is this... Assuming a 1.66:1 reduction and 14% slip at 4250 and 15% slip at 4550, the 19 pitch at 4550 would get me about 42.4 MPH, while the 21 at 4250 would get me about 42.3 MPH.
So where does the improvement in fuel comsumption come from?
On 4 stroke engines there is a parameter called (MEP) Mean Effective Pressure ( internal cylinder). As the MEP increases, the engine is working more efficiently and fuel economy improves somewhat. Increasing the prop pitch to 21 causes the engine to "work" harder ( again within design limits) and the MEP increases.
Some folks think that the prop that gets them the highest RPMS within limits is the best...not always so. Sometimes, top speed actually drops at the higher pitch with the engine still within operating range, then you get to choose betwen a bit more speed and a bit better efficiency.
The non linearity of the amount of "push" needed to move a hull under various conditions, and the non linearity of the engine's torque curve (how much "twist force" an engine generates at a given RPM) is what makes prop selection a bit of a "trial by error" :-) procedure.
BTW... From what one prop shop told me, changing a prop's pitch by one inch or so is about all you can do.
interesting... this is why i explained to joe,a little is alot.if the engine is allowed to have more turns,more of the hull is gonna be out of the water,meaning the rpms will climb.this is the reason that 2" of pitch you changed gave you 600rpms-it's all about load... it's like this there's a thing called "load",if that engine is allowed to run up to it's full rpm,the load on that engine is minimal...take the same engine,load it,so it will not reach it's full operating rpm,the load is much greater,right ? the engine is working harder,common sense,right ?i believe that's what you mean...i prop boats to run at the top of the reccomended range,i do that for a reason,it's referred to as "propping lite",by doing this the boat will plane at a lower rpm,the load on the engine is greatly reduced and you're gonna burn less fuel...as i explained to joe earlier,most of the boats i see,they're overwheeled. there's other variables that come into play with propping a boat,and it is trial and error,there's no exact science to this. stainless outboard and sterndrive props,that i 1 inch thing is correct.inboard props can usually be moved ahead or backwards 2" in pitch,meaning,if a prop starts out at 17" pitch it can be changed to a 15" pitch and it can be changed to a 19" pitch...
Edited by jawz (12/19/08 05:48 PM)
_________________________
jim anderson
signature edited...
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#84522 - 12/20/08 08:39 AM
Re: propping your boat...
[Re: Retriever]
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Captain
Registered: 11/10/08
Posts: 955
Loc: cape may nj
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Re:Pitch and RPMs.. on my I/O ( a 5.7L, 260 HP MERC MIE (2bbl) in front of a Volvo 280 drive), the MERC factory manual says... operating range 4200 to 4600. Just so long as I am within the range, the engine will be happy. However, within that range, there are differences in operating characteristics. If for example, a 19 pitch prop would get me 4550 RPM and a 21 would get me 4250. Then I would be able to get a bit more top speed and a bit better fuel consumption.
Why is this... Assuming a 1.66:1 reduction and 14% slip at 4250 and 15% slip at 4550, the 19 pitch at 4550 would get me about 42.4 MPH, while the 21 at 4250 would get me about 42.3 MPH.
So where does the improvement in fuel comsumption come from?
On 4 stroke engines there is a parameter called (MEP) Mean Effective Pressure ( internal cylinder). As the MEP increases, the engine is working more efficiently and fuel economy improves somewhat. Increasing the prop pitch to 21 causes the engine to "work" harder ( again within design limits) and the MEP increases.
Some folks think that the prop that gets them the highest RPMS within limits is the best...not always so. Sometimes, top speed actually drops at the higher pitch with the engine still within operating range, then you get to choose betwen a bit more speed and a bit better efficiency. i just reread this... it's true concerning the rpm range,however,there's a thing called "load" again...you need to have the boat loaded as you normally use it-propping your boat with only a few gallons of fuel in the tank,and just you on board is wrong,true,you're in the operating range,but what happens when you load the boat to go fishing ? rpms drop and the boat will not plane properly-this is why a boat is propped loaded... higher rpms and higher pitcha re not the same,bigger pitch drops rpm,smaller pitch increases rpm..."pitch",is the distance a prop will travel in one 360' revolution,measured in inches... propping a boat correctly is the difference in a well rigged boat,and one that's not...
_________________________
jim anderson
signature edited...
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#92130 - 04/27/09 05:47 PM
Re: propping your boat...
[Re: Jersey Joe]
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Captain
Registered: 11/10/08
Posts: 955
Loc: cape may nj
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joe,i hear ya !!
it's a different boat when it's propped correctly,i try and explain this,but,some guys still don't get it...
the prop shop,i gotta give this guy a plug again,pat whelen,eastern prop...pat and his son are the best,trust me,these guys have a hale mri machine,the prop comes back perfect.
_________________________
jim anderson
signature edited...
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#92151 - 04/27/09 09:32 PM
Re: propping your boat...
[Re: Sean]
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Captain
Registered: 11/10/08
Posts: 955
Loc: cape may nj
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Mckee Craft 185 18.5' long. They mentioned that adjusting the trim tabs properly can help with getting on plane and being more fuel efficient.
As I was doing the tutorial it would tell you if you were too "bow high" or "bow low" and it instructed you to move the tabs up or down. i think you may be confused between power trim and trim tabs...do you know the difference ? does your boat have trim tabs ? it may be easier for you to call me...609-214-7556,you can call me till 10:30 tonite...
_________________________
jim anderson
signature edited...
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