Top Speed?

[ris]

honda BFP60A gear ratio is 2.33:1

hope this helps
Richard

[Inquisitor]

honda BFP60A gear ratio is 2.33:1

hope this helps
Richard

Now I can see why Honda owners are justifiably proud of their motors. Your Honda is the first-ever example of a "better than perfect", perpetual motion machine.

[Be Free]

First run prop is 14 inch X 7 pitch (extra 850 lbs).
RPM. MPH
2000 6.8.
3000 7.8.
3500 8.6.
4000 9.5.
4500 10.7.
5000 12.6.
5500 14.6 WOT.
14 X7 without extra weight and fuel per hour
RPM - MPH - - GPH (fuel) - MPG
2000 - 6.1 --- .5 ---- 12.2
2500 - 7.3 --- .8 ---- 9.3
3000 - 8.5 --- 1.2 - --- 7.08
3500 - 10.2 - - 1.6 - ---- 6.38
4000 - 12.8 --- 2.0 ---- 6.4
4500 - 15.2 --- 2.6 - --- 5.85
5000 - 17.2 --- 3.7 - --- 4.64
5650 - 19.2 --- 5.1 --- 3.76

Did I figure the MPG correctly with knowing MPH and GPH?
Richard

Your MPG calculation looks right. MPH/GPH=MPG.

Why does your data show better performance below "planing" speed when the boat has more weight? The difference in top speed removing 850 lbs also seems a little high. I'm pretty sure 850 lbs should only make the boat sit a little over 1" deeper in the water.

[Jimmyt]

Lower in the water = longer waterline length/higher theoretical hull speed.

Or, tolerance in test data/recording... wind, tide, etc...

[Jimmyt]

Now I can see why Honda owners are justifiably proud of their motors. Your Honda is the first-ever example of a "better than perfect", perpetual motion machine.

Yep, my napkin says 16 mph at 100%...

Looks like the ratio is correct. So, either the tach or the speed indication (or both) have a little issue. Fiddling with the numbers makes me think the speed indication is the prime suspect. Unless he has a standard ratio foot on the motor. Even with that the numbers come out with a slightly negative slip.

RIS; how were you reading your speed?

[ris]

I can only obtain speed from my garmin GPS/chart plotter. I have no speed wheel. The fuel and rpm are from the Honda V3 gauge which is most likely more accurate than the gps speed reading. I appreciate the help in understanding the data. The data WITHOUT weight of 850 lbs was on a flat calm lake there was not even a ripple while we did the test. It was also about 89 F, very hot for February. This test was also after the boat came from the shop with new timing belt, plugs, thermostat ect., $1170 worth of maintenance so the engine was in best shape possible for the age.

Richard

[Jimmyt]

Thanks Richard,

I think we're having as much fun trying your figure out your data as you are.

How sure are you that your pitch is only 7? A higher pitch prop would explain the disparity easily. A 9-10" pitch would be the trick.

You should have noticed the difference between 13 mph and your 19.2 mph reading - if the error was actually speed. I've checked gps readings before and they are generally pretty good.

Also, the engine readings, coming straight from the engine electronics, should be spot on.

That leaves us with prop and gear ratio... motor had any lower unit work?

[Inquisitor]

... How sure are you that your pitch is only 7?

I was kind of leaning this way. Considering, your Honda 60 hp, with the 2.33 foot should be similar to my Mercury 60 hp, BigFoot with a 2.33 foot. With blurred vision, we basically have the same motors. I have a 14x11 prop. kurz has the same motor as I do, but has a 14x10 prop. He gets a little higher rpm at WOT, so I tend to think we have our numbers right. I just find it hard to believe that two motors of the same HP and gearing would use such different props. Especially... that a mere Mercury would be able to pull 60% more pitch than a Honda seems like a fairytale.

My rpm was from the VesselView dongle talking to the engine computer, so I feel pretty good about my 5220 rpm. Also, GPS is very accurate as long as current and winds aren't cheating. I'd bet easily as accurate at the engine rpm. BTW, in my test, my mast and rigging were at home and it was just me and the Admiral and a picnic. Just like your boat. I got 20.0 mph in that situation. Also when I plug my numbers into the app, I get a positive number slip percentage. (Higher than I think is optimum.)



As an aside... I believe my Mercury was designed and built in a partnership with Yamaha. They're kind of known for their acoustics... thus plenum and fuel-injection runners and exhaust are tuned pretty well. I look forward to doing a full battery of tests of speed versus fuel rate as you have.

[Jimmyt]

Yes. The gear ratio has to be 1.75 or lower to make the 7" pitch feasible. A 10-11" pitch brings it in with the 2.33 ratio. So, really starting to suspect the prop pitch number. Just checked your 2000 rpm at 6.8 mph. Same result.

3500 rpm at 8.6 mph shows a positive slip, but it's not high (about 14%). Since this is over hull speed, but not planing, I'd expect a pretty big number for slip here. A pitch of 9 shows about 33%.

My guess is, you had at least a 9" pitch prop on the motor when you took the readings. At least that's the easiest explanation. Unless you bought a used motor that had a foot swap, don't know how you'd wind up with a 1.75 or lower ratio...

You definitely get the fun mystery of the week award!

And, Inquisitor gets the Eagle Eye award for spotting the fact that the numbers were off!

[Inquisitor]

Yes. The gear ratio has to be 1.75 or lower to make the 7" pitch feasible. A 10-11" pitch brings it in with the 2.33 ratio. So, really starting to suspect the prop pitch number. Just checked your 2000 rpm at 6.8 mph. Same result.

3500 rpm at 8.6 mph shows a positive slip, but it's not high (about 14%). Since this is over hull speed, but not planing, I'd expect a pretty big number for slip here. A pitch of 9 shows about 33%.

My guess is, you had at least a 9" pitch prop on the motor when you took the readings. At least that's the easiest explanation. Unless you bought a used motor that had a foot swap, don't know how you'd wind up with a 1.75 or lower ratio...

You definitely get the fun mystery of the week award!

And, Inquisitor gets the Eagle Eye award for spotting the fact that the numbers were off!

It sounds like you have a real feel for this slip thing. I read the Mercury guy's article https://www.mercuryracing.com/prop-school-part-6-slip/ and even have a question out to him. He states that slip must be there to actually give thrust and defines ranges of (1) No, (2) too little, (3) good, and (4) too much, but never gives numbers. He uses one example at WOT that comes out to 15% but doesn't even say which category that belongs. Here you've brought up hull speed and my head about imploded. I can rationalize (not with logic, mind you ) that a slip goal might be different at different speeds and/or different expectations.



It sounds like most people are interested in Top Speed (ah... dah... its the topic title ) but I... and I would bet (ris) would be more interested in maximizing range. I'm sure that would be in the pre hull-speed range, but what kind of slip percentage would help achieve that maximizing efficiency at some given speed? My initial and gut feeling was I should want zero slip, just because slip signifies energy loss. But as the article says, that implies the diameter and/or area of the blades is too high and I'd be wasting HP on prop rotational drag...

It makes the brain hurt... I need a number or at least a theory. Can you put me out of my misery?

[Inquisitor]

And, Inquisitor gets the Eagle Eye award for spotting the fact that the numbers were off!

Type some numbers in the app... hmmm... see negative number

[Be Free]

The RPM and fuel flow supplied by the engine a probably the most accurate part of the test. The GPS speed can be wildly off over a very short period of time. I was just going over logs from an old trip last night and noticed that I hit 68 knots at one point! Obviously a bad reading. If I had not been recording raw data I would never have seen it.

The GPS is not actually measuring speed. It is calculating speed from the distance and time between two consecutive position fixes. As long as the speed values were recorded over a few seconds they should be accurate as well. As long as it has a few data points to work from your MFD or phone or whatever is displaying your speed will smooth out the occasional anomaly.

I feel good about his numbers from the second run pending verification of his prop and final gear ratio.

[Jimmyt]

And, Inquisitor gets the Eagle Eye award for spotting the fact that the numbers were off!

Type some numbers in the app... hmmm... see negative number

Giving a hoot about prop slip/efficiency.

Being inquisitive enough to find an app to calculate prop slip on someone else's boat.

Being smart enough to know that negative slip smells like a trash can full of last week's shrimp boil.

Throwing out a super snarky, but not mean spirited, quip about your findings including the phrase "perpetual motion machine".

Pretty good in my judgement.

This is a bit out of my wheel house, but I do find it interesting. I'm more of a pump and fan guy than a boat prop guy. Just browsing around, I see numbers in the 30-40% range for displacement trawlers. High speed planing boats might get down in the 5-10% range at top speed. And, I'm seeing numbers everywhere in between.

Wish I could tell you how to pick one on the first try, but right off, it's not coming to me. It seems like quite a bit of data would be needed just to find the target engine RPM, and target boat speed. Might be easier to try a few props and see if a trend emerged... in the speed range you're considering.

[ris]

My motor was new 60 hp Honda with a 70 hp foot. called the 60 hp BFP. The prop that came with the boat is a stainless 13 3/4 by13 prop. WOT was 4500 rpm at 19.5 mph. Since I was not getting over 5000 rpm we bought a aluminum 14 X 9 which produced 5250 rpm at 21.5. Wanting a higher rpm just to get midway between 5000 rpm and 6000rpm the stated WOT rpm for the motor I had a prop shop knock the pitch down from 9 to 7. So the prop is supposed to be 14 X 7. And the numbers are different from the 14 X 9 because I bought another 14 x 9 after I had the first changed to a 14 X 7 and used it in the test. Next time we do a trip I will load the boat with stuff and fill the ballast then run the test again. Back on page 2 of this discussion I posted all the info I had for all three props.
When we first did this test in 2016 Sumner who is fairly sharp said he would just stick with the 13 3/4 X 13 prop as we were going to go slow most of the time anyway and not worry about getting above 5000 rpm. Probably should have listened to him. But was trying to do as Honda suggested. I know nothing about motors. Just glad I can change the oil. Thanks for your time and help.

Richard

[Jimmyt]

Since you bought the motor new, we can rule out the gear ratio as suspect.

That leaves us with the prop that started out as a 14x9 - but was modified down to a 14x7 pitch by a prop shop.

Let's look at some of your numbers from your page 2 info comparing your 14x9 prop to your modified 14x7

14x9: 2,000 rpm; 6.5mph; 11.1% slip
14x7?: 2,000 rpm; 6.1mph; -7.2% slip

So, while it clearly isn't a 14x9 any longer, it also isn't a 14x7. Negative slip means the boat is moving faster than a perfect prop of that pitch could drive it. Numbers look like around a 14x8.45. So, they did work on your prop. It just didn't make it to a 14x7 - which is understandable. Seems there would be no way to re-pitch one perfectly due to the hub configuration.

Just for chuckles, your 13.75x13; 2,000rpm; 8mph; shows 24.3% slip. Moving faster at lower engine speed, but also slipping a bit more. To really zero in on it, you'll need the fuel numbers to go with your speed and rpm info. Your 13.75x13 would seem better given your desire to cruise around 8mph, but there is a lot more slip, so fuel number comparison at 8mph would be interesting.

Changing the oil certainly puts you ahead of a lot of boaters.