Cruising Speed vs WOT RPMs

[72SideWinderSS]

Re: Cruising Speed vs WOT RPMs

Way to many variables for MPG to be accurate, such as:
Current
Wind
Weight
Altitude
Air Density
Water Temp
Humidy
Quailty of fuel
....

the list goes on and on.

As stated by Silvertip for this reason the Avation Industry calculates range on Lbs/HR or GPH.

An old pilot adage:
A headwind hurts you more than a tailwind helps you.

 

[QC]

Re: Cruising Speed vs WOT RPMs

He is missing a heading at the end which is NM Range. Also, MPG is always important, think about it. Imagine with the example above (tommays data), the next fuel is 130 miles away (no current, no wind, but it is still the most efficient speed). What speed are you going to run? It's crystal clear to me . . .

 

[72SideWinderSS]

Re: Cruising Speed vs WOT RPMs

What, does your speedometer have an odometer in it, Mine don't. Nor have I ever seen one on a boat. Here is another variable.

If it works for you to calculate by MPG that is fine with me. I know my GPH rate at all my crusing speeds (rpms) and can calculate my range very accurately.

Besides, I don't know of any lakes, around here, that are 130 miles long, or that have no wind.

 

[Silvertip]

Re: Cruising Speed vs WOT RPMs

Exactly. The problem with MPG is that it's an "average" not an absolute like burn rate is. You may be happy as a clam to know that you had 6 mpg on the last outing but after you've gone through half your fuel load on the current trip you do some mental calculation only to conclude something is amiss and you are now getting only 5. With a fuel flow monitor you know within 200 yards from the dock what your fuel burn rate is. If its drastically different than the last trip you know immediately something is wrong (whether it's wind, water conditions, load, or may the engine needs some work) you can adjust your trip or cancel it if necessary. Knowing the absolute worst case burn rate leaves you with an absolute worst case "time" to destination at WOT based on fuel load. With a flow monitor you tell it the amount of fuel you added. It doesn't get any more absolute than that. MPG is certainly a nice to know measurement if it makes you feel good. But its an after the fact measurement unless you have an on-board fuel computer linked to GPS so and flow monitor so you can measure distance, time, speed and fuel used to average the consumption. You can certainly determine distance using the speedometer and your watch but that too is not a real accurate measure since you have no idea what your MPG is or was "exactly". Not much help to know that you got xx MPG only to find out you needed to get xx+y to reach your destination this day. Although I'm in the midwest where one would think you don't have opportunity to cover much water, the Mississippi and St Croix Rivers and many of our bigger lakes provide more than adequate water to test the fuel use of any boat. I run from the cabin, downriver, 5.5 miles to a large lake. I can put on 50 miles before breakfast. There are two gas pumps on the entire body of water so I need to know how I stand on fuel at all times. This is tiring. I'm outa here.

 

[QC]

Re: Cruising Speed vs WOT RPMs

What, does your speedometer have an odometer in it,

Well it's called a GPS or a map, and whether you have one of those or not the Marina with the fuel is a specific number of miles away. Yes, current matters, but without being in the middle of that current right now I must assume none . . . Also, if there is a current against me I may not make it to the Marina I imagined even at 25 MPH, but I can guarantee I will not make it to that Marina @ 40 MPH . . .

With a fuel flow monitor you know within 200 yards from the dock what your fuel burn rate is.

It doesn't mean carp if you run out before you get there . . . You have to know your range. Without knowing how far you are going on that same number of gallons, GPH is meaningless. This discussion cracks me up, and this is not the first time we've had it. GPH is wonderful IF you know your speed. Without it, have a fuel use party because that is all that info is good for.

I just thought of a rule of thumb for you guys. GPH will tell you when you are going to run out of fuel not where . . .

 

[Silvertip]

Re: Cruising Speed vs WOT RPMs

So how do you know your range before you leave the dock? You may have an idea based on previous trips and you can certainly average that. But there is no guarantee you will have that range this day, tomorrow, or the next. On the other hand If you know your absolute worst case flow rate you know before you leave the dock what your range is under the worst case scenario. Under way you know immediately what the current situtation is. Either way you still need to know speed. I do have GPS, the pitot speedo, and the locator speedo so I do have a fair idea how fast I'm going. Instant and average flow rate is availble at the touch of a button as is exact fuel remaining. And in 20 seconds I can determine the "sweet spot" if I really want to "be a penney pincher" rather than let it all hang out and get to where I want to be. That takes 20 seconds because I have the refersh rate set for 10 seconds and I want at least two cycles to determine the flow rate at the sweet spot. Time from my watch and knowledge of the water I'm on gives me a very precise idea of range at any given moment during the trip. Could you tell instantly if your flow rate suddenly went from say 8 g/h ro 10 g/hr? That might signal a leak at the engine yet the engine might be performing normally. I would know that instantly and can abort, stop and investigate and return if necessary. Guess I'm going to have to alert the aviation iindustry as they've aparently had it wrong all these years. d Anyway, once again we are getting away from the original topic which was what's more economical, cruise or wide open. No doubt that the sweet spot is more economical. Is it as much as its cracked up to be? The answer is "it depends". If increases in fuel consumption amounting to 7 or 8% to as much as 15% or in some cases 20% (depending on the boat, load, etc) gives you heartburn, then by all means run at that sweet spot. For boats burning anything above 20 - 30 gph that can be a significant number but I suspect most folks with boats in that category probably don't lose much sleep over fuel cost. To me, time is money both in recreation and business. I generally want to get where I'm going so I hammer down. 50% increase in speed vs the little difference in fuel economy is just not an issue for me.

 

[QC]

Re: Cruising Speed vs WOT RPMs

Either way you still need to know speed. I do have GPS, the pitot speedo, and the locator speedo so I do have a fair idea how fast I'm going.

That is the only point I have been making, in this thread and the last time we debated this . . .

Edit: BTW, in an airplane, when you're gonna run out is slightly more important than where . . .

 

[kenmyfam]

Re: Cruising Speed vs WOT RPMs

If I have to calculate that close to make the next fuel stop, I will take a couple of spare cans with me to top up if necessary.

 

[72SideWinderSS]

Re: Cruising Speed vs WOT RPMs

There has been alot of advice givin on here about safe boating.
It has been recomended many times: 1/3 out 1/3 back and 1/3 left for reserve.

In flight training they teach, for VFR flight, enough fuel to arrive at your destination PLUS a 30 min. reserve.

That is done by measuring your route on a chart, calculating your corrected air speed (for a given rpm and burn rate, given alt., reported wind speed and dir) and estimated ground speed for time en-route

Record your take off time, use your chart to get the distance traveled, calculate your speed on that leg using time en-route, verify your eta and fuel remaining.

It has not failed me yet.

Regards,
jimmy

 

[jmbfin]

Re: Cruising Speed vs WOT RPMs

Wow! There is a lot of good information in this thread. I certainly have learned plenty and I thank everyone for their participation.

My original question was basically, would it be beneficial to try to prop my engine so that it would reach the top of the recommended operating range which is 6000 RPM.

If yes, then should I try a 3-blade prop. Would a 17" pitch or 15" be preferable? How much RPM would I gain going from a 4-blade to a 3-blade? Would it be possible to gain 2 or 3 mph on the top end with minimal loss of hole shot. If no, then the 15" pitch 4-blade would be the correct prop for my application only because it allows the engine to operate at higher WOT RPM.

I was asked how I determined cruising speed. I determined 4000 RPM as the most economical operating RPM, either by MPG or GPH, as posted by Yamaha Performance Bulletins for the F115 on several boats with similar weight as mine. That is the reason I chose to include my speed at 4000 RPM. It appears speed at 4000 RPM is only important if comparing props that also reach the top to the WOT RPM recommended range.

My truck operates more efficiently on the highway at 60 MPH than it does at 70MPH. I seldom drive on the highway at 60 MPH. However, if I needed to stretch out my range for any reason, that information would be valuable to me. When I am on the boat, I try to be a little more relaxed and not pressed for time. I enjoy cruising and the feeling of freedom while on the water almost as much as fishing. I treasure the escape from masses of people and traffic.

I am new to boating. I assumed when I purchased my boat that it would have the best prop available to match the powerband of the engine to the weight and design of my boat. After reading on this site and another boating forum, I have learned that may or not be true. Do not assume. Therefore, I am experimenting with props. I have only tried 4-blade props.

I am grateful to the more experienced boaters offering their opinions and ideas. Thanks to the internet and the contribution of information from members here, years of experience can be gathered in a few days.

Would you invest the time and money in a 3-blade prop to try to get the F115 at 6000 RPM at WOT or would you be satisfied with the 15" pitch 4-blade that turns 5800 RPM at WOT?

 

[Ron G]

Re: Cruising Speed vs WOT RPMs

A rule of thumb is to prop for wot.you said your range was 5800-6000 the 15 is right there.how does the 4 blade run???

 

[jmbfin]

Re: Cruising Speed vs WOT RPMs

A rule of thumb is to prop for wot.you said your range was 5800-6000 the 15 is right there.how does the 4 blade run???

The 4-blade runs strong. I figure I get on plane in less than 4 seconds, have good midrange acceleration, and top speed about 40mph using the digital Yamaha gauge. GPS speed may be less as I am not sure of the accuracy of the Yamaha gauges though I have read they are fairly accurate.

I was wondering if a 3-blade stainless steel 15" or 16" pitch would get the engine all the way to 6000 RPM, gain 2 or 3 mph top speed, and maintain the hole shot of the 17" pitch 4-blade. I want to get the maximum performance this F115 has to offer. I may be there now, just not sure.

 

[Tail_Gunner]

Re: Cruising Speed vs WOT RPMs

Lot's of info up there, personally i just a set of smart tab's and all your holeshot plane issue's would be handled as best they could be.

Since you have a O/B id stick to a 3 blade say............Mecr enertia prop for max speed and be done with it..........

http://www.mercurymarine.com/uploads/images/1817/PROP_Approval_Rev_3.mpeg

 

[Ron G]

Re: Cruising Speed vs WOT RPMs

see below woops

 

[Ron G]

Re: Cruising Speed vs WOT RPMs

If your up on plane in 4 seconds thats pretty darn good!!!The 16 should put ya at 6000. with a little more top end.see if ya can borrow a prop and run a 15 or 16 just to see.the enertia has some very good numbers that kenny tested.all of this said is asuming your set-up is close as far as engine height and so on.

 

[flybuddy]

Re: Cruising Speed vs WOT RPMs

I just re-propped and tested it today. Went from a stock 3 blade 14.25 diamter 17 pitch to a SS Stiletto with same numbers. (I also have a Yamaha F115). I went from 5800 WOT and 40 mph to 5225 WOT and 38 mph. (same 4 folks on board both runs). Not sure if 3 blades will get you much other than a slight increase in efficiency.
I'm a novice at boating but have very extensive aviation experience. There are lots of product claims in aviation that are greatly exagerated. I expected something similar with boating speed/efficiency claims. At least in aviation, the products don't slow you down. The hype I read on the Stiletto promised both a better hole shot and higher top end. Both numbers went south. Guess I learned a lesson. At least the mods are cheaper.

 

[Silvertip]

Re: Cruising Speed vs WOT RPMs

Losing 600 RPM in your scenario seems like something is drastically wrong. Either the original prop was not a 17P or perhaps was repitched at some time. Speed or RPM may have been mistakenly read in either the original or post-change test, water conditions, engine operation, trim setting, etc could also affect the test. It normally takes a couple steps up in pitch to lose that many RPM. Stainless or aluminum is more efficient so there will generally be less slippage.

 

[Tail_Gunner]

Re: Cruising Speed vs WOT RPMs

I just re-propped and tested it today. Went from a stock 3 blade 14.25 diamter 17 pitch to a SS Stiletto with same numbers. (I also have a Yamaha F115). I went from 5800 WOT and 40 mph to 5225 WOT and 38 mph. (same 4 folks on board both runs). Not sure if 3 blades will get you much other than a slight increase in efficiency.
I'm a novice at boating but have very extensive aviation experience. There are lots of product claims in aviation that are greatly exagerated. I expected something similar with boating speed/efficiency claims. At least in aviation, the products don't slow you down. The hype I read on the Stiletto promised both a better hole shot and higher top end. Both numbers went south. Guess I learned a lesson. At least the mods are cheaper.

No hype with the Stilletto line, iv'e had vovlo,mirage and merc prop's on a I/O, The Stilletto outran and outlifted them all, Sounds like........... Ohh i see you have a deck boat.......

You might want to start a new thread and see if Walleyhed could suggest a better fit for that boat. May i ask did you boat ride higher in the water or lower with the Stilletto .............lower im sure.

 

[Tail_Gunner]

Re: Cruising Speed vs WOT RPMs

Dunno how that happened sorry for the double post.......

 

[flybuddy]

Re: Cruising Speed vs WOT RPMs

Hey tailgunner--don't know where you got the picture but that is the exact model of my boat (even the color)--and yes--it did ride lower. Also on conditions--identical, same river, same folks on board and no mistakes on the readings. The pitch and diamter are the same (triple checked). From what I've read, a cupped prop will behave like one with about 2" more bite--but even that won't explain that much RPM loss.