G21 H5 gph?

[Teige Hunt]

I was wondering if anyone had a rough estimate for gph on a g21 with an H5 in it. Or even just the H5 in general if you don't have a g21. Whether you just cruise, wakesurf, or wakeboard all the time any input would be appreciated!

[greggmck]

I was wondering if anyone had a rough estimate for gph on a g21 with an H5 in it. Or even just the H5 in general if you don't have a g21. Whether you just cruise, wakesurf, or wakeboard all the time any input would be appreciated!

I have not owned an H5 but I have owned several H6 engines in the G23. I have measured fuel burn to be about 6.7 GPH while surfing. I also have about 800lbs of added lead weight and typically surf with a crew of 3-5. I don't have measurements while wake boarding but in general fuel burn will be about 2 times the surf burn rate. Since the H5 is a smaller engine in a smaller boat you could likely see about 5-6 GPH while surfing. About 2X that when wakeboarding.

[SilentSeven]

That's interesting. I would have thought that wakeboarding would be the same or less than surfing. Basis - boat is on a plane vs. plowing a big hole when surfing. RPMs would be up but load would be less. Ski guy...no reference...just surprised to hear the big difference.

[scttp]

Just sharing: My 2015 g23 450: fully loaded surf speed is basically 3k ish rpms. Fully loaded wakeboarding 4Kish rpms. So that’s all roughly to say we burn 33% more gas per wakeboarding time than surfing time. (There are of course a few other physics at play.)


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[toomuchuv]

I remember many many years ago... I saw this beautiful boat and was talking to owner at length about it... I asked the same question... His reply, which has stayed with me for years, was... If you have to ask how much gas, you shouldn't buy a boat..

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[greggmck]

The physics are such that the drag increases by the square of the speed. All things equal increasing speed requires significantly more fuel to overcome the increased drag. You can see this in fuel performance results for the many boattest.com results. Here are results from the MasterCraft X24. See: https://boattest.com/boat/mastercraft/x24-2020

[SilentSeven]

Interesting chart...thanks for posting.

The data supports my thinking that the boat is more efficient as it exits being a displacement hull (think surf) and moves to a planing hull (think wakeboard). You can see it in the MPG numbers....MPG decreases significantly in the 7 to 13 knot area and then rises again in the 19 to 23 knot zone.

Buuuuuut...it's pretty marginal...I might have expected a higher increase % than what's in those numbers.

[greggmck]

Plowing thought water might feel LESS efficient and planning on the surface might feel MORE efficient but in fact the exact opposite is true. This is why most transport ships are displacement hulls. The MPH data likely have more to do with the prop used for this application than the efficiency of the hull. A google search on the subject will show this. For example: https://www.soundingsonline.com/boat...l-form-for-you

A planing boat goes faster as more power is applied, with no clear upper speed limit because it flies, its bottom supported by water pressure much as an airplane’s wings are supported by air pressure. The only practical limitation to a planing boat’s speed, in fact, is determined by the size of the waves; the boat’s size, bottom shape, strength and integrity of construction; and the tolerance of its passengers.

The problem with a planing hull is its great inefficiency compared to a displacement vessel. It takes a great deal of power to get that boat up on the surface of the water, and the result is very low “transport efficiency,” as it’s called. Transport efficiency can be applied to any form of vehicle, and it simply refers to the amount of energy it takes to move a given weight of cargo a certain distance.

[SilentSeven]

Clearly you're well versed in this and I don't want to take us down a rabbit hole. But it's also fun to learn something....

My observation about is not related to MPH but to MPG. At a low speed, the volume of water displaced is low...but as you increase the speed of said displacement hull, it must displace water at a faster rate...this is expensive energy-wise as the volume of water to displace is now high and the water itself is heavy. The outcome of this is a larger wake. Now, as the hull starts to transition from displacement to planing, this is where there is a brief period where the friction of the planing hull requires less energy than high speed displacement hull. You can see this point in the MPG figures. Now...as the boat continues to accelerate, this benefit is lost and the MPG again drops but this time the energy is primarily used to overcome hull friction (and air resistance) and less water displacement. You can see this as the wake gets smaller as the speed increases.

In practical terms, I experience this as the boat rises to a plane and I have to throttle back else the speed rapidly increases.

[greggmck]

OK. I see your point. Yes, there is that period where the boat is climbing its own bow wave, is not yet on plane and is operating in an inefficient regime as a displacement hull. Accelerated past that point drag is reduced as the boat reaches plane but quickly increases as hull speed builds. In my prior post I did mean MPG, sorry for the confusion I miss-typed it at MPH. I believe surfing does not really operate in that no-mans land where the boat is riding up its own bow wave immediately before planing. If I recall that happens at about 14-16 MPH while surfing is done at 10.5 - 11.5 MPH and explains why all things equal surfing can be significantly more fuel efficient compared to wake boarding. All the best.

[scottb7]

You all about the paralysis by analysis...

The rpm's are so low when surfing that the gph is less then boarding. Yeah. Ok fine. Obviously rpm less gph will be less. See your own chart.

But me thinks you have to do the math. If you wakeboard for 10 minutes a set vs some dude surfing for 20-30 minutes a set...Probably not much difference in gas used.

[SilentSeven]

You all about the paralysis by analysis...

Nah...just a bit of extra time on our hands and some love for an intellectual discussion.

[greggmck]

You all about the paralysis by analysis...

The rpm's are so low when surfing that the gph is less then boarding. Yeah. Ok fine. Obviously rpm less gph will be less. See your own chart.

But me thinks you have to do the math. If you wakeboard for 10 minutes a set vs some dude surfing for 20-30 minutes a set...Probably not much difference in gas used.

That's by far the dumbest thing I have read in this forum. Go crawl back under your bridge.

[scottb7]

Wow. Thanks for the compliment. You the best. I knew I could get you going...You so easy.

Look at your chart. as rpms go up gph goes up...wow. big surprise...obvi wakeboarding use more gph then surfing...wow. You state the obvious on so many posts and i am dumb...

here your obvi b.s. "surfing is done at 10.5 - 11.5 MPH and explains why all things equal surfing can be significantly more fuel efficient compared to wake boarding"....yeah. you brilliant.

[toomuchuv]

That's by far the dumbest thing I have read in this forum. Go crawl back under your bridge.

Ouch. I hope you guys are buds....

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