Rpm talk

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
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JAMES
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Rpm talk

Postby JAMES » Sun Jun 25, 2017 10:24 pm

I Cruise deep with 3 adults 25-26 mph at 3100 ish skinny 28-3 ish at 23-24 mph now me having my motor back at 5200 which by my dyno im making right at 500/490 numbers im making over 500 at 5400 now if i turn motor 56-5800 am i gaing anything other then nothing ?? Ive been told i should up my rpms WHY ! Why run more if not needed
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Gary S
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Re: Rpm talk

Postby Gary S » Mon Jun 26, 2017 7:17 am

whats your dyno say your making at 56- 5800?

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Re: Rpm talk

Postby kwanjangnihm » Mon Jun 26, 2017 7:31 am

JAMES wrote:As some of u guys no im building another carmotor boat i had the engne dynoed /dyno tuned with a quick fuel ss carb 750 dp it made impressive numbers for A 383 junk azz car engine !! 503 hp and right at 490 fpt @5600 rpms we pulled it down to 5300 rpms it made over 480 hp and around 480 foot pounds of torque I believe I can't remember offhand ill have to lok to verifie . Im running -4 cc forged piston I cut the compression back from 10.5-1 to lil less im not a fan of high comrsdion on this pump gas so im good how it is should make the freedom ride !! The motor is all forged with alum heads .


James if you set the prop on B mark, do a WOT run up, what is your max rpm?

According to WW, the EX PROP PERFORMANCE RPM RANGE 2000 – 2500

5600/2.68 = 2089
5300/2.68 = 1977
5200/2.68 = 1940
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JAMES
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Re: Rpm talk

Postby JAMES » Mon Jun 26, 2017 9:00 am

Gary S wrote:whats your dyno say your making at 56- 5800?

Starts dropping 504 The big question is running it up higher than my best numbers am I gaining anything
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Deano
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Re: Rpm talk

Postby Deano » Mon Jun 26, 2017 9:29 am

JAMES wrote:I Cruise deep with 3 adults 25-26 mph at 3100 ish skinny 28-3 ish at 23-24 mph . . . Why run more if not needed
If are satisfied with how it performs across the board, there is no reason (or need) to turn it higher. Personally, in most cases, I prefer not to wind everything up as tight as it will go.
It all depends how you use your boat most of the time, and to the degree of trade offs you are willing to make in order to best maximize that use.

To site two completely opposite extremes for the sake of example:

Scenario One: If your main use/requirement is to tote the biggest load possible and/or haul ass everywhere you go, every RPM you can squeeze out of it (till max hp) will allow you to use the most HP it is capable of. This approach will leave nothing on the table power wise, but will use more fuel, and possibly have a shorter service life before needing to be refreshed. In this case, the grin/fun factor is likely the most it can be.

Scenario Two: If your main use/requirement is to run with your regular load at a reasonable speed with the desire to maximize the efficiency and longevity of the engine, your focus would be at peak torque with little regard to whatever the peak HP may be. This approach may in fact leave some power on the table so to speak, but will use less fuel and most likely live a longer life before needing refreshed. The grin factor here would be for your wallet; money saved in fuel and replacement expenses.

Everyone that builds and/or has/runs a boat, has to make trade offs between these two extremes. There is simply no way around it. Naturally, parts selection (engine, gear, prop) will determine how successful those choices were to attaining the desired end.

Needless to say, not every build (and none of mine) starts with an unlimited budget and a blank sheet of paper. When there are already parts/pieces on hand to be used, the trade offs begin. Whether that amounts to an engine, a gearbox or a prop, the other parameters can (or maybe need to) be adjusted such that it helps negate whatever less than optimum consequences the preexisting parts bring to attainment of the desired result.

Your focus seems to be on your Max HP dyno number, but have not said anything about where your peak Torque is occurring. If your desired results is more like the second scenario than the first, that is the number that quite possibly may be of more importance.

With everything being optimal for scenario one, the engine rpm would be at peak HP and the and the gearing would put the prop at its most efficient rpm at the same time. Cruise speed would be what it is.

With everything being optimal for scenario two, the engine rpm would be near peak Torque at desired cruise speed and the gearing would put the prop at its most efficient rpm. Top speed would be what it is.

If your current gearing places both your cruise and wot rpms in the prop's working window that's great. If it does not, then you start tilting the rpms more toward your desired use preference. It is quite possible (if not likely) that a 2.38 gear would come closer to meeting both ends.

JAMES wrote:. . . Starts dropping 504 The big question is running it up higher than my best numbers am I gaining anything
NO. Not any reason to turn more rpms past peak hp. Doing so will only use more fuel and increase wear and tear.
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan

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JAMES
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Re: Rpm talk

Postby JAMES » Mon Jun 26, 2017 10:23 am

Deano wrote:
JAMES wrote:I Cruise deep with 3 adults 25-26 mph at 3100 ish skinny 28-3 ish at 23-24 mph . . . Why run more if not needed
If are satisfied with how it performs across the board, there is no reason (or need) to turn it higher. Personally, in most cases, I prefer not to wind everything up as tight as it will go.
It all depends how you use your boat most of the time, and to the degree of trade offs you are willing to make in order to best maximize that use.

To site two completely opposite extremes for the sake of example:

Scenario One: If your main use/requirement is to tote the biggest load possible and/or haul ass everywhere you go, every RPM you can squeeze out of it (till max hp) will allow you to use the most HP it is capable of. This approach will leave nothing on the table power wise, but will use more fuel, and possibly have a shorter service life before needing to be refreshed. In this case, the grin/fun factor is likely the most it can be.

Scenario Two: If your main use/requirement is to run with your regular load at a reasonable speed with the desire to maximize the efficiency and longevity of the engine, your focus would be at peak torque with little regard to whatever the peak HP may be. This approach may in fact leave some power on the table so to speak, but will use less fuel and most likely live a longer life before needing refreshed. The grin factor here would be for your wallet; money saved in fuel and replacement expenses.

Everyone that builds and/or has/runs a boat, has to make trade offs between these two extremes. There is simply no way around it. Naturally, parts selection (engine, gear, prop) will determine how successful those choices were to attaining the desired end.

Needless to say, not every build (and none of mine) starts with an unlimited budget and a blank sheet of paper. When there are already parts/pieces on hand to be used, the trade offs begin. Whether that amounts to an engine, a gearbox or a prop, the other parameters can (or maybe need to) be adjusted such that it helps negate whatever less than optimum consequences the preexisting parts bring to attainment of the desired result.

Your focus seems to be on your Max HP dyno number, but have not said anything about where your peak Torque is occurring. If your desired results is more like the second scenario than the first, that is the number that quite possibly may be of more importance.

With everything being optimal for scenario one, the engine rpm would be at peak HP and the and the gearing would put the prop at its most efficient rpm at the same time. Cruise speed would be what it is.

With everything being optimal for scenario two, the engine rpm would be near peak Torque at desired cruise speed and the gearing would put the prop at its most efficient rpm. Top speed would be what it is.

If your current gearing places both your cruise and wot rpms in the prop's working window that's great. If it does not, then you start tilting the rpms more toward your desired use preference. It is quite possible (if not likely) that a 2.38 gear would come closer to meeting both ends.

JAMES wrote:. . . Starts dropping 504 The big question is running it up higher than my best numbers am I gaining anything
NO. Not any reason to turn more rpms past peak hp. Doing so will only use more fuel and increase wear and tear.

Basically what I'm looking for is to get the most out of my engine but safely people tells me I need to turn it up past peak power I don't understand why it's making the most at about 54 to 5500 I see it pointless to spend the engine harder then your best numbers maybe I'm missing something but I've had a shit load of boats and never understood that Theroy and heard it million times
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Deano
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Re: Rpm talk

Postby Deano » Mon Jun 26, 2017 11:25 am

JAMES wrote:. . . Basically what I'm looking for is to get the most out of my engine, but safely. People tell me I need to turn it up past peak power, I don't understand why . . . I see it pointless to spin the engine harder than your best numbers . . . maybe I'm missing something . . .
I don't think you are missing anything. I think you have more figured out than they do.

To me, and many others, spinning an engine beyond peak HP is not only
counter-productive, but readily compares with attempting to piss up a rope.

When or If, there were a situation where it even looked necessary, that is NOT the proper
or the preferred means of dealing with it . . . even if it is the quickest or easiest or cheapest.

I would be inclined to NOT take any advice from someone that claimed otherwise.
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan

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JAMES
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Re: Rpm talk

Postby JAMES » Mon Jun 26, 2017 11:53 am

Deano wrote:
JAMES wrote:. . . Basically what I'm looking for is to get the most out of my engine, but safely. People tell me I need to turn it up past peak power, I don't understand why . . . I see it pointless to spin the engine harder than your best numbers . . . maybe I'm missing something . . .
I don't think you are missing anything. I think you have more figured out than they do.

To me, and many others, spinning an engine beyond peak HP is not only
counter-productive, but readily compares with attempting to piss up a rope.

When or If, there were a situation where it even looked necessary, that is NOT the proper
or the preferred means of dealing with it . . . even if it is the quickest or easiest or cheapest.

I would be inclined to NOT take any advice from someone that claimed otherwise.

Agreed
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Re: Rpm talk

Postby Deano » Mon Jun 26, 2017 12:19 pm

:thumbleft:

We both know . . . but perhaps I should have clarified
anyway, that there will likely be exceptions to every rule.

In this case, the tail end end of 400' or the back straight comes to mind. :D
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan

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Re: Rpm talk

Postby twb119 » Mon Jun 26, 2017 2:04 pm

Take the tools you need to change your pitch and go to the marsh lake swamp etc and try different pitches, rpm etc. don't get hung up on dyno numbers they don't push your boat. More pitch less rpm better cruse, economy and less snap for most, take out pitch and the opposite. Don't forget that your hull will play a huge part on your cruse number.

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Re: Rpm talk

Postby Swamp Thang » Fri Jul 21, 2017 8:56 am

Deano wrote::thumbleft:

We both know . . . but perhaps I should have clarified
anyway, that there will likely be exceptions to every rule.

In this case, the tail end end of 400' or the back straight comes to mind. :D


Yes if your boat is light and free there might be a extra few mph on the big end if you push it past your peak #'s but to me on a ride boat it just doesn't make sense to turn it that hard, I would much rather ride than work on them.


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