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Old 07-04-2020, 02:44   #91
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Re: I'm under propped. What should I move up to?

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Originally Posted by mcarthur View Post
Very interesting - where did the 2.84 come from?

The number is called the SL ratio. Dave Gerr has some interesting notes about it, and believes the maximum SL ratio for a vessel is
Code:
8.26/power(DL ratio,0.311).
The DL ration (displacement-length) is
Code:
Displacement in long tons (2240lb) / power(0.01 * LWL in ft,3)
which would be an SL of about 2.4 for you (assuming weight is 7 long tons and 51' LWL?) rather than 2.84. Still extremely high, giving 17 knots! And of course it says nothing about how much power is required to get to that speed
It’s the K factor mentioned by BigCat69 here that makes the difference. I’m between a 10:1 or 12:1 L/B ratio off the top of my head.

https://www.sailnet.com/forums/sailb...ull-speed.html

Driving 1200 miles to the other boat right now and moving a summer’s worth of provisions aboard to go quarantine.

I’ll check back in on this thread after that. Thanks so much for all the input. Everyone. It’s been a very enlightening thread so far.
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Old 07-04-2020, 04:15   #92
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Re: I'm under propped. What should I move up to?

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2) The high thrust propeller does produce more static thrust, and, perhaps because of the exhaust being pulled through the propeller in reverse, it produces significantly less reverse thrust than forward thrust.

Bill
Any fixed blade propellor will perform poorly in reverse, the reason is of course the leading and trailing edges are swapped, a correctly designed blade if you look at it there is very much a difference between the two plus wash out of the airfoils etc.
Quite a few of the feathering props do well in reverse because quite frankly they are very poorly designed blades from an aerodynamic view, they are simply just flat plates, so that reversing a flat plate doesn’t change its efficiency that much.
The only prop that I am aware of that operates very efficiently in reverse is the Autoprop, and only because it’s blades flip around in reverse so that the leading edge is still the leading edge.
It apparently was designed by the British in WWII as a propellor to get a landing craft off of a beach, if you ram a boat onto a beach and are being shot at, you want the most possible reverse thrust to get off the grounding
Surprised you hit on the exhaust in reverse, most don’t realize that, I didn’t even think about it until I read about Kiekhaefer’s original design patent from the 50’s on the thru hub exhaust
It’s really a brilliant design cause in forward at speed it forms a vacuum, which will exhaust scavenge, which one a two stroke especially can greatly increase performance
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Old 07-04-2020, 05:40   #93
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Re: I'm under propped. What should I move up to?

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Surprised you hit on the exhaust in reverse, most don’t realize that, I didn’t even think about it until I read about Kiekhaefer’s original design patent from the 50’s on the thru hub exhaust
It’s really a brilliant design cause in forward at speed it forms a vacuum, which will exhaust scavenge, which one a two stroke especially can greatly increase performance
We worry so much about the effects on propeller performance of small bits of air that might suck around the "anti-cavitation plate" or the bubbles formed in the areas of the propeller that are under vacuum, but we don't think about the huge amounts exhaust gas that flood over the propeller in reverse. It has a significant effect on reverse thrust. I see it's effect on thrust when I try to stop my Tanzer 22 when entering a slip, and I see it when I try to tow with my dinghy operating in reverse.

My 7.5 Johnson outboard has a depression on its port side above the anti-cavitation plate that is about 1/2" in diameter and 1/2" deep. My dealer told me that the bottom of the depression could be drilled out allowing the exhaust to exit above the anti-cavitation plate. Because that increases the exhaust noise, the depression can be tapped and a pipe plug inserted to restore normal operation. I never bothered.
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Old 07-04-2020, 08:20   #94
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Re: I'm under propped. What should I move up to?

If the engine is ticking off the rev limiter you aren't using the hp you already have by a large margin. IF you are hitting the drag wall it should be at lower rpms anyway. Mfgs spec for that engine is around 5500 and a good prop target. A $30 handheld tach on the flywheel is a cheap way to find rpms without breaking the bank. For starters I would try a couple inches more pitch and see what direction that takes you. Forget hacking up the engine.

There is no way a prop chart is going to dial it in. Prop charts are a compromise and normally where you start, not end, in dialing a prop in. Be aware that more pitch will give more speed in smooth water but less power when punching into high winds or chop. Been there a few times myself and ended up having two props for the boats with undersized engines. One small dia high pitch (rpms matched to max torque curve which gave reserve max rpms) for best speed in the ICW and the other larger dia with flatter pitch for max rpms and power to punch through high winds and chop. The difference being the speed prop would do 6-7 kts in flat low wind conditions and 3 kts against 20kt winds. The power prop would do 6 kts in high winds but run the engine flat out all the time whether in calm or high winds/seas. After changing more than a handful of props on production sailboats I've learned most people have no clue how inefficient their props really are.
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Old 08-04-2020, 00:03   #95
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Re: I'm under propped. What should I move up to?

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Originally Posted by BBill View Post
If the engine is ticking off the rev limiter you aren't using the hp you already have by a large margin. IF you are hitting the drag wall it should be at lower rpms anyway. Mfgs spec for that engine is around 5500 and a good prop target. A $30 handheld tach on the flywheel is a cheap way to find rpms without breaking the bank. For starters I would try a couple inches more pitch and see what direction that takes you. Forget hacking up the engine.

There is no way a prop chart is going to dial it in. Prop charts are a compromise and normally where you start, not end, in dialing a prop in. Be aware that more pitch will give more speed in smooth water but less power when punching into high winds or chop. Been there a few times myself and ended up having two props for the boats with undersized engines. One small dia high pitch (rpms matched to max torque curve which gave reserve max rpms) for best speed in the ICW and the other larger dia with flatter pitch for max rpms and power to punch through high winds and chop. The difference being the speed prop would do 6-7 kts in flat low wind conditions and 3 kts against 20kt winds. The power prop would do 6 kts in high winds but run the engine flat out all the time whether in calm or high winds/seas. After changing more than a handful of props on production sailboats I've learned most people have no clue how inefficient their props really are.
Yay!

My fast sailing cat flys under power (1200kg 2 x 9.8 Tohatsus with the standard 3 blade props, just under hull speed, 10 knots clean.

I also have a few planing power hulls, to push big loads I have used low pitch four bladed props but I hate them. Sure they have lots of thrust out of the hole but at cruising speeds of say 25 knots they are inefficient running at least 300 rpm higher than two inches more pitch on a 3 eared prop and alot noisier (because of the higher rpm. The four bladed prop also cavitated badly in sharp turns.

So if there are higher pitched props try at least two more inches
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Old 09-04-2020, 12:04   #96
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Re: I'm under propped. What should I move up to?

It involves bending the tips to increase thrust.
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Old 16-05-2020, 04:48   #97
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Re: I'm under propped. What should I move up to?

Ok, was thinking about this issue this morning.

The gear ratio for the Yamaha High thrust 25 is 2.42:1. It spins various high thrust props from 8-14” pitch.

The gear ratio for my Evinrude etec 30’s is 2:15:1. They are spinning a 7” pitch 4 blade propeller.

Are they really that different??

What if I modified the etecs?

Would more cup on the 7” prop help a little? Would it be better to do a custom prop that’s bigger and cut off the anti cavitation plate to fit it?

Can’t justify the money to buy new engines really.
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Old 16-05-2020, 05:48   #98
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Re: I'm under propped. What should I move up to?

Some cup or a different design of prop very well might help.

I just ran the numbers through the Michigan wheel calculator (set for a 4 blade prop) and for a boat speed of 10 kts and 6000 RPM target with 30 hp and a 2.15:1 ratio, they suggest an 11x7 prop, as you already have. Moving the boat speed up to 12 kts suggests 11x8.

Changing the RPM target to 5500 (low end of the range for that engine) brings the suggestion to 12x8 at 10 kts, 11x9 at 12 kts.

So you're in the ballpark size-wise for getting it as good as you can with those engines, but some changes to prop design, cup, etc. can likely give some improvement.

Unfortunately, you're likely to get stuck with something better than you've got now, but still less than ideal, just due to the limitations of the available off the shelf outboards.

Now, if you could figure out a way to change the gearing in your outboards, you might see some bigger gains. The efficiency numbers and thrust output go up a bit according to the calculator if I change the ratio to 3:1 (which would put you in the 13 - 14" prop diameter range). 3.5:1 ratio (14 - 15" prop diameter) improves things even a bit further. I wonder if you could cut the cavitation plate for clearance and get a lower ratio gearset made for the lower units?
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Old 16-05-2020, 05:48   #99
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Re: I'm under propped. What should I move up to?

I have been thinking about this too.

FIRST thing I would try is a pair of new props. Compared to other options this is low cost and will provide valuable info.

I would use a prop similar to what you have now just to avoid introducing additional variables. I think I remember you now have Solas 4 blade props in 7" pitch, so stay with Solas 4 blade. I would go to 9" pitch, 2 inch jump, just to see what happens. Run the boat collecting data on speed, fuel economy, rpm, evidence of ventilation, etc. Give the results a hard think. Then if you are not fully.satisfied you will have good quality information to make your next move.

Going through a few prop changes you show you what the engines you already have are capable of. I am thinking they will be fine.
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Old 16-05-2020, 05:49   #100
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Re: I'm under propped. What should I move up to?

I’ll try this a different way, you have a car that in its highest gear will top out at 100 MPH at redline, but will only go 30 MPH when it hits redline, the clutch isn’t slipping, so something must be, in this case it must be the tires spinning. Increasing the gear ratio won’t help, you need bigger tires that won’t slip.

You have a boat instead, that the propeller pitch means that at redline it should be going over 30 mph, except it’s not, so the propeller is slipping, increasing the gear ratio by increasing pitch won’t help, you need bigger props that won’t slip.
Or just slow down where they props aren’t slipping and accept its not as fast as you want it to be or as fast as others claim.

In my opinion the only thing I can think of that might work is a Kort Nozzle, shroud the propeller, that’s often done in big boats that pull very heavy things like Shrimp boats that are restricted in the size of prop that they can swing.
Same thing in aircraft is called a ducted fan, it takes the wing tip vortices and directs them into thrust, it’s actually more efficient than an un shrouded Prop, although GE was enamored for some reason with an unshrouded turbine. I kept teasing them that they just reinvented the turboprop.
A Kort Nozzle needs to have very little clearance between it and the prop, I have no idea if you can buy one and bolt it on, I know that there are Manatee and lifeboat guards that look like a Kort nozzle, but doubt they would work as the clearance is probably excessive
https://www.kortpropulsion.com/products/kort-nozzles
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Old 16-05-2020, 07:54   #101
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Re: I'm under propped. What should I move up to?

I definitely understand that my “tires” are basically bicycle tires on a Ferrari here and the wind resistance isn’t even being overcome before the bicycle tires spin. I get that. You explained it beautifully earlier in the thread.

I’m thinking how people have suggested buying Yamaha F25 high thrust outboards, but digging deeper, I don’t see that much of a difference between those and what I currently have. That’s what my last post was about.

Or potentially about fitting bigger tires. Cutting off the anti cavitation plate would allow me to fit wider tires. So I mentioned that as a maybe above.




Quote:
Originally Posted by a64pilot View Post
I’ll try this a different way, you have a car that in its highest gear will top out at 100 MPH at redline, but will only go 30 MPH when it hits redline, the clutch isn’t slipping, so something must be, in this case it must be the tires spinning. Increasing the gear ratio won’t help, you need bigger tires that won’t slip.

You have a boat instead, that the propeller pitch means that at redline it should be going over 30 mph, except it’s not, so the propeller is slipping, increasing the gear ratio by increasing pitch won’t help, you need bigger props that won’t slip.
Or just slow down where they props aren’t slipping and accept its not as fast as you want it to be or as fast as others claim.

In my opinion the only thing I can think of that might work is a Kort Nozzle, shroud the propeller, that’s often done in big boats that pull very heavy things like Shrimp boats that are restricted in the size of prop that they can swing.
Same thing in aircraft is called a ducted fan, it takes the wing tip vortices and directs them into thrust, it’s actually more efficient than an un shrouded Prop, although GE was enamored for some reason with an unshrouded turbine. I kept teasing them that they just reinvented the turboprop.
A Kort Nozzle needs to have very little clearance between it and the prop, I have no idea if you can buy one and bolt it on, I know that there are Manatee and lifeboat guards that look like a Kort nozzle, but doubt they would work as the clearance is probably excessive
https://www.kortpropulsion.com/products/kort-nozzles
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Old 16-05-2020, 10:43   #102
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Re: I'm under propped. What should I move up to?

What you guys should do is read David Gerr's book..."the propellor handbook"

The art of choosing the right propellor is dependent on many, many things. Too many to relate here, but the book explains all. It tends to be a bit technical with numerous graphs, etc, but when you are done, you'll understand this science a lot better.
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Old 16-05-2020, 11:07   #103
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Re: I'm under propped. What should I move up to?

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I have Evinrude etec 30hp engines on my 7 ton cat. I'm considering brining her up north to work on. It's a 1400nm trip. To maximize fuel economy I'd like to get the most speed possible out of the boat at the 3 gallon per hour burn rate. I have no income currently and every dollar counts.

I currently have the Evinrude 4 blade, 11 inch x 9 inch pitch prop on these motors. They hit the RPM limiter very easily at 8 knots and that's it. 3gph at 8 knots. Expensive trip.

I should be able to change the props and get a better top end out of this for the trip up.

How do I go about finding the right prop? I know aluminum should be fine. I want ones that don't ventilate easily. I know I need to pitch up.

Do I need 4 blade or 3 blade?

Looking at solas props. I just need to go 10+ knots under power at full tilt to get the expense of motoring up down to something more manageable for Covid-19 no income times. I'm looking at $2500 to move it otherwise. Trying to bring that down by getting the right props.

Looking at your first post what you want isn’t possible, you want to go faster and burn less fuel, that’s not how it works.
If saving money is your concern you need to run at 6 kts or less and maybe on one engine.
Spending money on props is not going to save you money, and certainly not buying other motors, you have what you have, you need to learn how to save money motoring on what you have, and I’m certain that’s go slower and most likely on one engine.

I m sure you have watched all these work barges etc that run around everywhere with big as in 100HP+ outboards on them, but run at 5 kts or less. They do that because they actually can’t go much faster, I’m sure they got that old motor at a very good used price is why it’s there, not that their barge can go 15 kts with a 150 pushing it.
I’m not calling your boat a barge, but if saving money is the primary concern, it’s best you operate it as if it were.

Your doing the ICW? Most traffic runs 6 kts or so plus or minus 1 kt, you’ll be in good company.

Cutting your anti cav plates is just going to ruin the resale value of your motors at least. You need a bigger prop. But also a lower gear and cutting the anti vac plate isn’t changing the gear ratio.
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Old 16-05-2020, 11:42   #104
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Re: I'm under propped. What should I move up to?

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What you guys should do is read David Gerr's book..."the propellor handbook"

The art of choosing the right propellor is dependent on many, many things. Too many to relate here, but the book explains all. It tends to be a bit technical with numerous graphs, etc, but when you are done, you'll understand this science a lot better.
Exactly the right word. Art.

I have that book (somewhere) and read it sometime in the last decade. I’m smart enough to know this art is best discussed and gone over with people who have a lot of experience in the art.

It’s barely a science.
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Old 16-05-2020, 11:54   #105
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Re: I'm under propped. What should I move up to?

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Looking at your first post what you want isn’t possible, you want to go faster and burn less fuel, that’s not how it works.
If saving money is your concern you need to run at 6 kts or less and maybe on one engine.
Spending money on props is not going to save you money, and certainly not buying other motors, you have what you have, you need to learn how to save money motoring on what you have, and I’m certain that’s go slower and most likely on one engine.

I m sure you have watched all these work barges etc that run around everywhere with big as in 100HP+ outboards on them, but run at 5 kts or less. They do that because they actually can’t go much faster, I’m sure they got that old motor at a very good used price is why it’s there, not that their barge can go 15 kts with a 150 pushing it.
I’m not calling your boat a barge, but if saving money is the primary concern, it’s best you operate it as if it were.

Your doing the ICW? Most traffic runs 6 kts or so plus or minus 1 kt, you’ll be in good company.

Cutting your anti cav plates is just going to ruin the resale value of your motors at least. You need a bigger prop. But also a lower gear and cutting the anti vac plate isn’t changing the gear ratio.
Except the already incredibly low pitch I’m running would change the ultimate gear ratio with the water, no? There are 2 different gear ratios involved here. One in the lower unit and one where the metal meets the water. Getting that larger diameter prop would involve cutting the cavitation plate, unfortunately. Otherwise I’m stuck with the current prop.

My original post was thinking I was at max rpm with no slip with too small a pitch. So that would definitely be wasting fuel as the whole thing would have been too geared down. Like driving to cross country in 1st gear.

Later, from a video I put up, the consensus was that the prop was shooting out a jet of water at like 30mph while I was going 8 knots. I accepted this theory which is your theory.

If that’s the case, then I’m wasting the majority of the fuel I’m using creating a narrow jet of water behind the boat because (again, as you said) the prop is too small in diameter, so the traction isn’t there where the rubber meets the road.

I do always run on 1 engine. I do 8 knots WOT on a single engine. The other engine is off.

When I see displacement power cats that are the same length and 4-5 times heavier than my boat zooming along with some big Outboard’s on the back, I can’t help but to wonder why things aren’t working in a similar way for me. Especially given my boat is currently somewhere around 12,000-14,000 lbs displacement and so easily driven it’ll do 4 knots on a very windy day with no motor on and no sails up.
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