Propeller Pitch

[TyBoo]

Dan -

You ever considered swapping the lower unit for a jet drive? No, wait, then you would be worrying about the size of your orifice.

[dogon dory]

Sorry, Folks - Post Deleted By Author

[AK Angler]

[TyBoo]

If only you had been so obsessed with semantics on another thread................

[dogon dory]

Sorry, Folks - Post Deleted By Author

[tpbrady]

I think it's time to reopen this subject and add some new ingredients to the mix.

I am still thrashing with thrashing trying to come up with the right prop combination for my CD22 with 90 Honda VTEC. What's confounding the situation is way the ECM manages fuel flow. I went through this whole thread and there really weren't any comments about fuel flow, versus slip at various rpm settings. I recall reading a book about the Wright brothers and when they had figured out lift and control they then tackled propellers and then I think it was Orville who wanted to quit as it seemed unsolvable.

So last night I plotted graphs of fuel flow on various boats using the 90 VTEC on the Honda web site against my own experience. When I plotted the points, 3 of the boats almost formed a single curve, and the other two boats were almost on a single curve. The first 3 were planing hulls and other two were essentially catamarans (a Glacier Bay and a pontoon boat). My fuel flow was like the twin hulls up to 3500 rpm (just on step at 10 mph) and then went rapidly above the planing hulls and stayed there.

My fuel flow was essentially the same with either a 15 pitch or 17 pitch Solas 4 blade. The reason for the spike in fuel flow is exiting the lean burn mode (like opening the other two barrels on a Holley 4 barrel). In the past with carbureted engines and many fuel injected, fuel flow was a nice almost straight line. With computers making the decision it went non linear. My engine with the load and prop combination came out of lean burn at 3500 RPM, the planing hulls at just above 4000 and the twin hulls at 4500. I then started looking at the prop pitch and diameter on the twin hulls. The pontoon boat was using a 14" 11P prop and the Glacier Bay was was twin engine boat so not a good subject to look any further.

Based on what I saw, the lean burn feature kicks out based on engine load. I don't know how it measures load but on the old 4 barrels it was vacuum. I looked at prop slip and found the pontoon boat had a large amount of prop slip over the entire rpm range, anywhere from 46% down to 24% at the high rpm range. My prop show about 55% slip at which point passing through 3500 rpm it started to bite the water better and slip dropped to 12%at WOT. At about the same time it started to bite the water, the engine exited lean burn and fuel flow went from 2.1 gph at 3500 rpm to 6.2 gph at 4500 rpm and speed went from 10 mph to 16.5 mph.

Based on this I am seriously considering changing the techique I use to determine WOT. Rather than trim the motor up until the prop ventilates and then drop it a little, I will stop WOT throttle testing, when trimming the motor stops increasing the speed. The next prop I intend to test is a 3 blade 14 inch in diameter prop of 13 pitch. I'm taking Dan's lead on that on bigger diameter and try to ease the engine load at the midrange rpm where I typically cruise so I can postpone the opening of the other "two barrels".

Things I have learned:

a. Prop slip generally decreases as rpm increases. As you reduce prop slip engine load can increase.
b. Fuel consumption is not linear as rpm increases.
c. I'm glad Orville didn't quit, but I wish he had studied marine propellers.

Tom

[tsturm]

[tpbrady]

That's probably what it is. I found reference to it in the shop manual. The shop manual is very detailed, but doesn't say anything about how it works.

The science experiment continues. It gives me a reason to take the boat out when I don't need any more fish and its raining.

Tom

[thataway]

You have to look at the hull form --the C Dory planes at a much lower speed than the majority of boats tested--and thus other boats are not good comparisons. The Glacier bay and Pontoon boats are both non planing--displacement cats--no relevance to the C Dory.

The prop slip is less at higher speeds because the boat is planing more effeciently.

What is your WOT? Your boat should be proped so that it will reach WOT--if not you you may lug the engine at lower speeds--in the curve where the boat is just starting to plane.

If you have a flow meter, it will pay for itself and this is the best way to determine what the effeciency of the boat is--miles per gallon. Gallons an hour are rarely relevant.

[C-Gypsy]

Tyboo? I can resist no longer...

What is the best prop for my CD-22 with a Honda 90?

It has a blue stripe (the boat, not the Honda).

It has power tilt (the Honda, not the boat).

If you can answer that I will contact you about my yellow car. It don't run anymore.

You coming to NC next weekend? If so I'll buy you a beer. Perhaps two if they have a keg.

I am old & single and confess I spend an hour or so every evening on this site.

It almost feels like "home, West Virginia". Of course I am from PA so I get lost a lot.

Don't think John Denver ever owned a C-Dory anyhow. If he did it probably had the wrong prop.

My hat is off to you gentlemen; this is quite a website!

Al

[dogon dory]

Sorry, Folks - Post Deleted By Author

[tpbrady]

Al,

Why don't you consult the oracle out at Kitty Hawk and see if Orville can help? I'll let you know what I find.

Bob, I was using a fuel flow meter to determine my fuel efficiency. The reason I was looking at the Glacier Bay and Pontoon boat was the weight and not the hull form. I am at planing speed when the fuel flow spikes as the ECM takes the motor out of the lean burn mode. It also happens to coincide on that particular prop will slip taking a dramatic drop from 46% to 29% or there abouts. Both the Glacier Bay and pontoon boat stayed in the lean burn mode longer due to lighter engine loading. The thought on going to a 13 inch pitch 14 inch diameter prop was to reduce engine loading in the typical cruise rpm range, say up to 4500 rpm. My current WOT RPM with a 15 pitch 4 blade Solas 13.25 inches in diameter is 6300 but that includes another 200 rpm that produced no more speed just more fuel flow. I would consider the WOT in this condition to be 6100 rpm. The specs on the new 90 VTEC place the WOT range as 5300-6300 with 90 HP produced at 5800 rpm.

The VTEC is a strange beast to figure out with lean burn shifting fuel flow around. With a light boat and the 15P prop it stayed in lean burn to nearly 4000 rpm. Add 200 lbs and it drops out at 3600 RPM. The difference between 3900 in lean burn at 12.5 mph and 4100 not in lean burn at 14 mph is 4.3 mph versus 3.1 mpg. It never recovers that back with a 15p prop. With a 17p three blade it reaches 5800 WOT, doesn't stay in lean burn mode once you try to go on step. It does give pretty decent economy at 22-24 mph of 4.2 mpg. What I am trying to determine is the best compromise between higher speed economy 18-22 mph, and low speed economy 10-12 mph. Right now I can have one or the other but not both.

This weekend the test will be a 3 blade 13P 14 inches in diameter and a 4 blade 13.25 inch 16 pitch prop. The last test will be a 4 blade 14 pitch prop. I forget the diameter. After that I quit. I'll just randomly select a prop out of my box and adjust accordingly.

Tom
Now I know why airplanes have variable pitch propellers and mixture controls.

[C-Gypsy]

I guess I could spend this weekend reading "Stick & Rudder" again.

Dang. Was hoping to go fishing a while. Reckon I'll have to read fast!

Al