Up a pitch for better GPH/MPH?

[Mordekai]

Re: Up a pitch for better GPH/MPH?

I'll probably will swap to a hustler prop with the same pitch.

and holy hell, flo scan are expensive, I know boating is expensive but 400 bucks for a floscan!

 

[180shabah]

Re: Up a pitch for better GPH/MPH?

flo scan are expensive...

Agreed, but if the goal is REAL fuel savings - that little gauge will "force" you to be more restrained.

Do I think it is worth it for the average boater, using less then 100 gals a year. NO! Takes too long to pay for itself.

 

[45Auto]

Re: Up a pitch for better GPH/MPH?

I think a better analogy would be a running person. Running uses more calories per hour than walking. So if you run a specific distance you use a lot of calories for a short time. If you walk, you use fewer calories for a longer time. --Net expenditure is the same amount of calories to go the same distance.

Very bad analogy there. The difference in MPG between best cruise and WOT on most planing hulls is usually more than 50%. In other words it'll take you longer to get there, but you'll only use about 2/3 as much fuel.

 

[Mordekai]

Re: Up a pitch for better GPH/MPH?

Agreed, but if the goal is REAL fuel savings - that little gauge will "force" you to be more restrained.

Do I think it is worth it for the average boater, using less then 100 gals a year. NO! Takes too long to pay for itself.

So far this year I've used up about 300 gallons ... 5 weeks into owning this boat and still got another 2 months atleast of this season

 

[overkill]

Re: Up a pitch for better GPH/MPH?

this is sounding like a job for mythbusters!

 

[itsaboattime]

Re: Up a pitch for better GPH/MPH?

this is sounding like a job for mythbusters!

uh huh...............

 

[balphin23]

Re: Up a pitch for better GPH/MPH?

I feel that trying to compare MPGs in a boat is rediculous as you are not traveling on a flat surface at anytime. Try and think of a boat as an airplane. It would be better in my opinion for him to use a flow meter on his gas. They did this on Myth Busters to prove a myth on tailgates on trucks. If they guy installed a flow meter, and his only concern is fuel economy he then could go to a shop and try multiple props to see which one actually is giving him the best fuel economy.

IMHO he won't like the prop that gives him the "best" fuel economy.

I would go with a 4 blade with a 1" step down in pitch to start the testing. This should give him a starting point for best all around cruising.

 

[Tail_Gunner]

Re: Up a pitch for better GPH/MPH?

3 blades will give the best top end
4 blades will give you the best midrange and lower planing speeds
You cant have both....Not yet anyway

If you want to try one solution would be to pick up a 19 pitch 3 blade stilletto and compliment the prop with nauticus trim tabs thats about 400 and they should lower plaining speeds (increasing fuel econ) along with a lower pitch..increasing hole shot....as far a speed goes stilletto props are amoung the best...(good overall lifters..reducing wetted surface lessons drag that increase's effeciency)

Or just pick up a 4 blade amita very cost effective in a 20p...

Props can effect the character a boat has..such as lower midrange punch and hole shot..4blades...Or over all top end...3 blades...Thats a opinion take it for what its worth..

 

[45Auto]

Re: Up a pitch for better GPH/MPH?

I feel that trying to compare MPGs in a boat is rediculous as you are not traveling on a flat surface at anytime. Try and think of a boat as an airplane. It would be better in my opinion for him to use a flow meter on his gas. They did this on Myth Busters to prove a myth on tailgates on trucks. If they guy installed a flow meter, and his only concern is fuel economy he then could go to a shop and try multiple props to see which one actually is giving him the best fuel economy.

I have 2 different props for my boat, a 19" pitch and a 23" pitch. I also have a flowmeter. The 19" pitch uses 2.5 GPH at idle, and 40 GPH at top speed. It obviously uses anywhere between 2.5 and 40 GPH at part throttle. The 23" pitch also uses 2.5 GPH at idle and 40 GPH at top speed, and anywhere between those numbers at part throttle.

Maybe you can help me and tell me which one is giving me the best fuel economy? Here's a couple of pictures of the fuel flow at 3100 RPM with each prop if that will help you.

19" Pitch:

23" Pitch:

 

[drivehard]

Re: Up a pitch for better GPH/MPH?

I have 2 different props for my boat, a 19" pitch and a 23" pitch. I also have a flowmeter. The 19" pitch uses 2.5 GPH at idle, and 40 GPH at top speed. It obviously uses anywhere between 2.5 and 40 GPH at part throttle. The 23" pitch also uses 2.5 GPH at idle and 40 GPH at top speed, and anywhere between those numbers at part throttle.

Maybe you can help me and tell me which one is giving me the best fuel economy? Here's a couple of pictures of the fuel flow at 3100 RPM with each prop if that will help you.

19" Pitch:

21" Pitch:

What is the GPS speed at these two conditions???

 

[Tail_Gunner]

Re: Up a pitch for better GPH/MPH?

When arguing or debating prop effeciency take the number one factor into account...Hull type ...Are you guys debateing a 18-26 boat with a high hp to weight ratio.....Or are you pushing a 26' or above crusier....Two totally different worlds..

 

[45Auto]

Re: Up a pitch for better GPH/MPH?

What is the GPS speed at these two conditions???

I cut these pictures out of the instrument panel pics that include the GPS speedometer. Why would you need it? I am trying to think of my boat as an airplane, as balphin23 suggested. I am interested in how airplanes calculate best fuel economy also.

 

[Mordekai]

Re: Up a pitch for better GPH/MPH?

Can you pm me the speed? Based on your meter my assumption is wrong

 

[Tail_Gunner]

Re: Up a pitch for better GPH/MPH?

.......im going to get into some i-net bs..............45 is using numbers from a barge..(30') and you have a little bitty 18 foot hull....there are no similarities...One is a planing hull another is a displacement hull

2.5 gph is the tip off...Wheres hiswii when you need him.....


Your boat







Boat

• Length Over All: 18 Feet and 1 inches.
• Draft: 1 Feet and 5 inches.
• Beam: 7 Feet and 7 inches.
• Fuel Tank Capacity: 20
• Max Load: 1050
• Dry Weight: 2270

Engine

• Model Name: Alpha 1
• Model Year: 2010
• Hours: 1
• Cylinder: 4
• Horse Power: 135

Or 45 are those numbers from your 22' bow rider????

Here is a example of props...lift drag and results...The first pick is a 3 blade ss 21 doing 55 the second is a 4 blade 20 doing the 54...Now which provides the most performance....Question 45 is it the hull or the HP or maybe the lift and blade area... which prop provides the most invigorating or overall performance



3BLADE

4blade

The 4blade prop is a significant improvement over the 3 blade...It will one have a better hole shot...smaller pitch and more blade area....It will hold the hull on plane at lower speeds...better econ aka 1800 rpm and your planeing not crawling on the back of the pressure wave....from 20 to 40 your running on less wetted surface again increasing both acceleration and econ...Now at Wot theres a diff the 3 blade is traveling 1 more mile in a hour...about 1 minute of gas...but most I/O's spend very little time is spent there and a great deal of time just on plane to 30-40 mph and thats where the 4 blade shines

 

[45Auto]

Re: Up a pitch for better GPH/MPH?

Those numbers are from a 27' high-performance express cruiser (planing hull) with twin 270 HP engines. Why does the type of boat matter in determining fuel economy? Would you figure it differently based on whether it was a planing hull, a displacement hull, an airplane, a truck, or a car?

The fuel flow shown is for BOTH engines combined. It doesn't get real good fuel economy, obviously the owner is interested in which prop set to use! I was just wondering if maybe Balphin23 or Silvertip could help me out with some advice on fuel economy.

Here's the test data from the exact same boat. I was going from memory on the 2.5 GPH at idle and 40 GPH at WOT. It was probably actually 1.5 GPH and 42 GPH as shown in the test data below. I've included the noise level data (dBA) with it in case that will help with the "aircraft" method:

 

[Tail_Gunner]

Re: Up a pitch for better GPH/MPH?

Hmm i was a bit sharp angled there...Simply said small lightweight hulls can achieve results that larger hulls cannot and vise versa..There common enemy is drag no matter how you look at it..3 blades have less drag than a 4.....4 blades have more lift reducing wetted area...until the drag of the 4th blade outweights the benefit of lift which generally begins to occur aruond 30 hence lowering planing speed and better econ in general...

http://webcache.googleusercontent.c...cy+planing+hull+size&cd=1&hl=en&ct=clnk&gl=us

Theres a good read on hull dynamics....any time the drag coefficent goes down econ goes up....Its simply a matter of getting the hull up and planing quickly at a speed or rpm the motor operates the most effeicently..The hull really is the secret and how it balances.. the prop type and trim effect that....the rest were stuck with...aka weight cams heads etc..umm most of the time,

So pick your posion its always the same 3 for speed 4 for lift aka midrange punch again less wetted surface area which will result in better econ.

 

[tommays]

Re: Up a pitch for better GPH/MPH?

I had a full collection of props on my 19' 3.0 boat with GPS/fuel-flow to tell what was going on

13 x 17 When you just gotta take a big load

14 x 19

14 X 21

14 x 20 4 blade was are favorite all around prop

I would not say there was a huge difference in MPG in the props compared to using the fuel-flow to fine tune trim and RPM

 

[SRC8778]

Re: Up a pitch for better GPH/MPH?

45, the reason they want the speed is that distance is a necessary part of the equation to figure fuel economy. If, however, we assume that prop slip is the same, which I am sure it isn't, then the 23's are providing considerably better fuel economy. 16% better fuel economy at a speed that is 21% higher. Even if we assume increased prop slip, I think it is a no brainer that on that boat and under those exact conditions when the pictures were taken that the 23's are providing better fuel economy.

 

[bob johnson]

Re: Up a pitch for better GPH/MPH?

I cut these pictures out of the instrument panel pics that include the GPS speedometer. Why would you need it? I am trying to think of my boat as an airplane, as balphin23 suggested. I am interested in how airplanes calculate best fuel economy also.

but for this thread it is ALL about fuel economy...they guy is spending money to save fuel...

is he? or isnt he

to determine fuel economy one has to know how far they got in that hour they burned 13.9 gallons of gas....


vs the hour that they burned 13.3 gallons

maybe he gets great fuel per hour but runs out before he gets home

40 miles
30 miles
35 miles



in the end it is miles per gallon....just like a car...if he is worried about economy... and too bad he was late for dinner


I take my boat out I go over to island X anchor and fish and then i come home and I used 10 gallons of gas...

its miles divided by gas used for the OP

for someone else its thrills per gallon....!! yeah!!

bob

 

[QC]

Re: Up a pitch for better GPH/MPH?

45 is well aware of the need for speed for GPH to be valuable info. That was his point . . .