8000 lb cruiser twin merc 260's

[rbh]

Re: 8000 lb cruiser twin merc 260's

I'm pretty sure my prop diameter question has been long forgotten....

hahahahaha, should we go for a coffee and wait this one out.

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

Diameter
Defined as the maximum radius of one blade multiplied by 2.
The diameter of the circle scribed by the blade tips as the propeller rotates.
General rules:
Diameter usually increases as engine power increases and vice versa. (all other variables remaining constant)
Diameter increases for slower boats and decreases for faster boats.

http://www.olds.com.au/marine/terminology.html

 

[Bondo]

Re: 8000 lb cruiser twin merc 260's

I'm pretty sure my prop diameter question has been long forgotten....

Nope,... Not forgoten,... It's just taken 42 posts to come back to what I said back in post #8.....

 

[45Auto]

Re: 8000 lb cruiser twin merc 260's

Ayuh,.... I think you're putting Way too much weight into your quest for Diameter....
Especially, considering that you really have little or No choice in the matter...
Basically,...
Diameter is a function of Blade Design....
The Engineer who designs the prop, determines the diameter....
Generally speaking, the larger the pitch, the smaller the diameter,...
Conversely, the smaller the pitch, the larger the diameter....

You're exactly right Bondo. And the way the engineer determines the prop diameter and pitch is using the drag curve of the boat and the power curve of the engine with curves for different props, which I've been trying to explain for about 40 of these posts. Without the drag curve of a particular hull or prop performance data using multiple props on the same hull at the same weight and performance conditions, all you can do is a linear extrapolation from the single RPM/speed point.

The accuracy of that extrapolation will be determined by how linear the engine power curve and hull drag curve are at the point you are calculating from, and how closely your calculations conform to the slope of the curves through sheer luck. The problem with trying to specify props for our small (as opposed to a warship or oil tanker) recreational boats is that the optimum performance region is a very small area. If the boat is already operating close to the optimum point and you don't understand power vs drag relationships, you're going to waste a lot of someone's money having them buy props that have just a good a chance of moving AWAY from the optimum point as towards it. The engine power curves are relatively easy to get, the hull drag curves are more difficult. The Savistky method is typically used for drag calculations at different speeds on planing hulls, there's a link here to the Orca3D (hydrodynamic analysis program, used to predict performance of boats - Navy uses it a lot) site which uses the Savitsky method in their marine performance predictions for planing hulls:

http://www.orca3d.com/support/manual/index.html?resistance_savitsky_introduction.htm

and a link here to Savitsky's latest paper here. His method was originally described in 1964, and this 2007 paper added "whisper spray drag", which is the drag caused by the boat having to force it's way through the spray thrown up in front of it:

http://www.stevens.edu/ses/cms/file...f_Whisker_Spray_in_Performance_Prediction.pdf

This is the drag graph for a planing hull, from page 48 of the Savitsky paper. Notice the characteristic shape of it.

Here's another drag curve on a Power Available vs Power Required graph from Mercruiser on the development of the supercharged Verado outboards on page 3 at:

http://s3.amazonaws.com/resumetroy/Dresden+supercharger+paper-final2005.pdf

Again, notice the characteristic shape of the drag curve. What this graph shows is that with the engine propped as shown by the orange curve, it will max out at 120 km/hr. At that point the orange power available curve drops below the black power required curve.

I'll try to write something up and start a new thread with the explanation, since it's not directly related to diameter. It'll probably be a few days, the 2010 FIRST Robotics competition started yesterday and I'll be helping them design and build their robot for this years competition for the next 6 weeks. You can see last years robot at:

http://team2992.com/index.html

The competition is great, and it gives a lot of kids their first insight as to how engineering is used in the real world.

Hwsii, if you get a chance re-do your charts using the 23" pitch at 4800 RPM instead of the 19" pitch at 4800 RPM. That will allow the explanation of an under-propped case for the 19" and an over-propped case for the 27".

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

Well the problem I am having with that the engineer designing the prop has no idea who I am or what boat I might put his prop on.... I see that a certain model of prop generally has a basic design with a range of boats it is suggested for but that prop is avail in a variety of pitches and diameters....
Everywhere else online that I find any technical data suggests that diameter is a very important and often overlooked aspect of prop selection... I have read several bits stating that on sterndrives Diameter is generally limited by the physical dimensions of the outdrive thus causing a less than ideal application. Maybe I am missing your intended point but when a prop is offered in multiple diameters with the same pitch then stating that diameter is a function of pitch or prop design makes about as much sense as saying inseam is a function of wasteband and trouser design..... I am open to further explanation but at this point it just doesn't add up.


What I am looking to discover is what is the limiting factor at play here.
Is there an interferance between the spinning prop and the av plate of the drive causing an undesirable condition when less than 1" clearance or is 14.5 actually calculated as the best diameter for cruising speed on the hull.....
I completely understand that without exact measurements lots of math anf lots of real world testing prop selection is a bit of a crapshoot, but I just can't see how this question could be unanswerable.... even if the answer is less than certain..... I will keep looking for the answer if for no other reason than I am curious

In any case THANK YOU TO EVERYONE INOLVED for any and all efforts here.

 

[DaNinja]

Re: 8000 lb cruiser twin merc 260's

Nothing to contribute really.
I just wanted to say that this exchange of information exemplifies why this is such a great forum!

Great exchange! I feel like I've been through a physics class.

 

[hwsiii]

Re: 8000 lb cruiser twin merc 260's

Auto45,

If you could shrink the width of those chart pics you posted in #29 down some, this thread would be a lot easier to read. They're forcing the edges of the forum off the side of my screen.

I have resized all my pictures, so I hope the new size fits your screen better.

I am very glad to see we are able to have an intelligent discussion without anyone resorting to bashing of any kind, that makes it very pleasant and enjoyable for everyone involved.

For this discussion we need all of the props to be the same make and model so that we are comparing apples to apples.

Auto45 Post 27

To finally get to your prop diameter question, you'll find that if you propped your boat for maximum efficiency as opposed to maximum top speed, you'd be using a bigger diameter, high pitch prop that would be incapable of reaching the recommended maximum RPM of your engine (just like overdrive in a car), because the additional drag due to the bigger diameter increases exponentially at higher speeds and RPMs.

Auto45 Boat Performance Secrets

FUEL ECONOMY

You can see that if you want to cruise at 30 MPH any of the props could do it. It only requires 80 HP. So you close the throttle until the engine is only making 80 HP. But which one would give you the best gas mileage? The biggest pitch prop would be your friend here. At 30 MPH the engine with the 19 is turning 3000 RPM, the 23 is turning 2500, and the 27 is turning 2000. Look up these RPM on the engine Dyno Chart (Chart #1) to find the max power available at that RPM. With the 19, you need 80 HP out of the 250 available at 3000 RPM. So the throttle is open about 80/250 or 32%. Same 30 MPH for the 27 pitch turning 2000 RPM, it needs 80 HP out of the 150 available to it at 2000 RPM. So the throttle is open 80/150 or 53%. Since the 27 pitch is turning less RPM, it needs more throttle to deliver the same power (80 HP) as the 19 pitch at 30 MPH. The same amount of fuel and air is required to be pumped through the engine to deliver 80 HP to run the boat no matter what speed the engine is turning. Since the 27 pitch engine is turning slower, it must take in bigger bites of fuel and air to pump 80 HP worth through it. The engine must make 80 HP to drive the boat, but it must also produce the power to suck in the air it needs.

This is why the bigger pitch prop with the bigger throttle opening will give you better fuel economy. The wider throttle opening required for the 27 pitch to make the 80 HP makes it easier for the engine to suck in the air that it needs. So the engine has to develop less power (therefore use less fuel) ABOVE the 80 HP required to drive the boat to suck in its air. This is also the reason higher gears (overdrives) in your car give you better gas mileage.

With my very limited knowledge of motors I am trying to figure this out. You say there is a 21% difference in the throttle opening between the 19? prop and the 27? prop, and yet, also using your numbers there is a difference of 33% less RPM being turned. So how is the engine able to get the extra air to keep the extra fuel mixture from running rich, because it is turning 33% less RPM and the throttle is only open an extra 21%. So I don?t really understand the efficiency gain here.
I always understood that when an engine cannot reach the manufacturers minimum recommended RPM it is termed LUGGING, and the reason you don?t do this is because the cylinders build up a deposit of carbon from the unburned fuel, which can and usually does create a major engine problem at some point. But I believe that QC has the real answer here.

QC Boat Performance Secrets Post 26

Originally Posted by PhatboyC View Post
How does anyone know if the engine is lugging to much from an oversize prop?

Well the only real answer to your question above is when your boat will not reach the "window" of your recommended WOT operating range. I believe yours is a 320 bhp, and I also believe that the WOT range is 4800 - 5200 (Please Verify). If you cannot reach 4800 RPM at WOT with a typical load, then you are technically over propped. Most here would say that if so, you are damaging your engine at all operating conditions. I tend to disagree as once you are below WOT you are operating at less than max torque at all points except when you are pushing the throttle all of the way down for any reason. I believe this is more a question of degree than an absolute yes or no. For example, if your engine made 4750 RPM at WOT I don't think you would have any issues when running it at 3800 RPM forever . . . However, if you could only make 4000 RPM at WOT then I would suggest even 3000 RPM could be detrimental as extended high load operation at low RPM is not real good for a Gasoline engine of any type. It's not gonna blow up in 30 seconds in either case, but for the best combination of flexibility, performance, fuel economy, speed, acceleration and engine life, a prop set that gets you near the middle of the range will typically be best.

Originally Posted by PhatboyC View Post
Generally they say add lots of weight (people, gas) and see if your performance decreases allot. This only tells me if the rpm-prop configuration is satisfactory to my needs. What about the engine? When is it harder on the engine to ride with a higher pitch prop?

This doesn't make sense. If you load up and cannot get in the WOT range then all of the above would apply.

45Auto Boat Performance Secrets

You can see that max speed in this case would be right at 60 MPH with the 23 pitch prop. The 27 pitch runs out of power at about 58 MPH, and if you want to go over 54 MPH with the 19 pitch you have to over-rev it.

To sum up, it takes 300 HP for this boat to go 60 MPH, 290 HP for it to go 59, and 280 HP for it to go 58 MPH. Note how this follows the "10 HP per additional MPH" general rule (which is controlled by the slope of the "Power Required" curve). You can also see how the 23" prop is fastest because it puts the engine's HP peak directly on top of the "Power Required" curve. The 19" pitch puts the engine's HP peak to the left of the "PR" curve, so the engine is overrevving before it reaches max speed. The 27" pitch puts the engine's HP peak to the right of the "PR" curve, where it is unable to develop enough power to accelerate to it's potential 300 HP peak before the drag overcomes it.

45Auto Post 38

hwsiii Post 36
So if what your chart shows is different, I have to believe my mathematical calculations, as they are simple and straight forward.

Can't control what you believe.

If you want to ignore horsepower and drag that's up to you.

Your chart shows 59 MPH with a 19" pitch at 5450 RPM (270 HP if we extrapolate the dyno chart).
Your chart shows 59 MPH with a 23" pitch at 4502 RPM (290 HP from dyno chart).
Your chart shows 59 MPH with a 27" pitch at 3835 RPM (250 HP from dyno chart).

So you believe that the same boat takes 3 different amounts of power to go the same speed? Or do you believe that the engine makes the same amount of power at 5450, 4502, and 3835 RPM?

I believe MY MATH, I also KNOW that in order for the boat to do 59 MPH with 10% prop slip the motor HAS to turn the above posted props at the RPM I stipulated. Whether the motor can attain those RPM or not, I do NOT know. And since the motor HAS to attain 3,835 RPM, and 290 HP per your calculations, at those RPM with the 27" prop then it CANNOT reach even 59 MPH if it does not. So you tell me.

If you want to ignore horsepower and drag that's up to you.

The Math tells the truth, or we wouldn't even have a way to know what prop slip is, much less be able to Quantify it. So it DOES take HP and Drag into account when you do a Prop Slip Analysis. How can you question the math when it is used worldwide in aviation as well as boats.​

H

 

[hwsiii]

Re: 8000 lb cruiser twin merc 260's

Kevin, commercially manufactured props are designed for MOTORS and NOT boats. They are specifically designed for certain SIZE motors, and that is why the barrel is an exact size, and the diameter ONLY reaches a certain size as well.

You don't believe what I said about changing the blade surface area, and don't worry about the diameter so much. But if you will just Google, Mean Width Ratio and Developed Area Ratio and read that information you will be able to better understand what I am talking about.


H

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

Will do...... I most certainly do believe you about the blade area..... I am just still looking for answers on the diameter issue also.... One prop can be used on engines from a very wide range.... for example an alpha 4 is designed for v-6 outboards and all alpha sterndrives up to 250 hp..... That makes no designation for which engine for which prop but rather the consumer is left to choose the diameter and pitch for his/her application....
As I've said before I am fully aware that it may just turn out that the recomended sizes really are best for the planing hull.
I really believe that the hull design is the major player in economy.....

 

[hwsiii]

Re: 8000 lb cruiser twin merc 260's

45Auto, I am sorry I didn't run the new analysis you asked me to run with the 23" prop at 4,800 RPM, I just forgot as we all do. And would you also do the analysis with RPM we talked about as well.

45Auto Post 38

hwsiii post

And if you will run your chart with RPM instead of using HP you will see that I am right.

I'll re-do the chart with RPM for you and you'll hopefully be able to understand where you are wrong. Please re-run your calculations based on the 19" prop at 4800 RPM (292 HP). EDIT - Use 23" at 4800 RPM, it will allow the demonstration of an under-propped and over-propped case.

I do have an ego, just like everyone else does, but I try very hard to not let it get in the way of the facts, I just want to know the TRUTH.

What I have been thinking all along is that we should be talking about props that are not so extremely different in pitch, I believe that if we actually used props that are just 2" different in pitch instead of 4" in pitch, the motor would be less strained at both ends of the spectrum, unless you believe you don't really gain much more fuel efficiency with only 2" changes in pitch. I will run an analysis on 2" pitch changes just so you can see what I am talking about.

Prop Slip

I also have crouch's formulas and Savitsky's calculations in my spreadsheets as well, but NOBODY would ever give me all the numbers and information I need to do an analysis with Savitsky, I do not have the "whisper spray drag" incorporated into though and I am sure it never will, although it is another drag consideration. I think it is very funny that he doesn't really use the length of the boat as a parameter in his formulas.


H

 

[45Auto]

Re: 8000 lb cruiser twin merc 260's

So how is the engine able to get the extra air to keep the extra fuel mixture from running rich, because it is turning 33% less RPM and the throttle is only open an extra 21%.

This is an excellent example of how trying to force the non-linear real world into linear equations can cause errors. Throttle setting is NOT proportional to RPM, it's proportional to load. That's why my numbers came out that way, 80/250 of the available power with the 19" pitch is 32%. 80/150 of the available power with the 27" pitch is 53%. RPM has NOTHING to do with it, so there's no relationship like you're trying to force between RPM and throttle setting.

Let's say you have a truck with a manual transmission (just so I don't have to argue about torque converter slip). Get in your truck and ride down the road at 50 MPH in 4th gear. You can easily calculate engine RPM if you know tire diameter, rear end ratio, and transmission ratio (same thing you're doing with boat props using prop pitch, drive ratio, and prop slip). For example purposes say your truck engine is turning 1500 RPM at 50 MPH. Probably cruising along easily at about 1/4 throttle.

Now go hitch a 15,000 pound boat to your truck. Ride down the road at 50 MPH in 4th gear. Since tire diameter, rear end ratio and transmission ratio haven't changed, you're still turning the same 1500 RPM at 50 MPH. Problem is you need a LOT more power, so your throttle is going to be open a LOT further - probably floored. Engine is turning the same RPM in both examples, but throttle setting is totally different. This is because throttle setting is proportional to LOAD, NOT RPM.

As another example, let's say your boat does 4800 RPM at WOT. Put your boat in neutral and see how much throttle it takes to reach 4800 RPM. Not much!!!. Imagine that, exact same RPM, totally different throttle settings due entirely to the amount of load on the motor. In the first case, your throttle is obviously at 100% (pretty much the definition of WOT). In the second case (neutral, very low load) it's probably not even 5%.

On top of that, throttle area is NOT linear with throttle plate angle. Moving your THROTTLE LEVER to the 50% position does NOT throttle your engine 50%, although it does open the butterfly 50%. You can see that from the sketch below. I got the areas at 25%, 50%, 75%, and 100% butterfly angles, which is what you get with your throttle lever set at 25%, 50%, 75%, and 100% (now THAT is a linear relationship, cause they're tied together with a cable and don't have to worry about hydrodynamics, fluid flows, Reynold's numbers, Froude numbers, etc, like you do when you're trying to predict prop performance).

The areas shown are the area in square inches that's open to flow on a 2.5" diameter butterfly.

And in addition to that, flow is NOT proportional to throttle area opening. Just because you have 50% of the throttle bore area clear doesn't mean you get 50% of the flow.

In other words, pretty much NOTHING to do with fluid flows (water or air, props or carburetors) is linear. All I see in your spreadsheets is straightforward linear RPM relationships.

Look at the oil tanker p/d diagram in my previous post #30. They kept everything the same except the prop pitch. They cut the pitch in half (from 24 feet to 12 feet) while maintaining the same vessel speed of 14 knots. If you ran these numbers in your spreadsheet it would show that the prop RPM would double. Just like if you run it with a 19" pitch and a 38" pitch at 59 MPH it will show the 19" pitch prop RPM will be double that of the 38" pitch RPM.

However, as you can tell from the tanker diagram, in the real world the prop RPM did NOT double. It was 80 RPM with the 24' pitch at 14 knots, your prediction would have been 160 RPM with the 12' pitch at 14 knots. In the real world the 12' pitch only turned about 125 RPM, only about a 50% increase instead of the 100% increase you would have predicted.

I'm really supposed to be designing a robot instead of doing this! Best thing anyone that really wants to learn this stuff could do is take a couple of semesters of fluids and aerodynamic courses.

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

while we're at it lets consider that the carb issue is often even more complicated with a progressive 4 barrel rochester with small primaries and large secondaries

 

[Bondo]

Re: 8000 lb cruiser twin merc 260's

Ayuh,... I'm going to throw yet another wrench into your quest for the Perfect Prop...

I run the Torque-Shift prop from Land & Sea....
It's no longer in production, but used copies are around,+ a fella bought up all the replacement parts for 'em, so parts Are available...

I have mine set up to run 11" pitch at idle, which is Perfect for trolling... I can bump the throttle abit, keeping it at or slightly below 1000rpms, yet trolling reasonably Slowly...
I have the upper limit set to about 20/ 21",... it's adjustable out to 26"...
When I drop the hammer, the motor jumps to 4800rpms,+ Stays there, while the prop catches up as the boat accelerates to catch the rpms...
It'll almost dislocate the shoulders of a waterskier,+ Topend is still a respectable 42/43mph@4800rpms....
Not bad for a 74 Starcraft 221 Islander, 4.3LX, Alpha 1, Fully charter dressed fishin' barge...

While I've never bothered with the Efficiencies of it,...
It's still the Perfect Prop for My application....

 

[hwsiii]

Re: 8000 lb cruiser twin merc 260's

45Auto, I know that my analysis works very well and DOES come up with answers that are very very close to REALITY.

45Auto Post 51

In other words, pretty much NOTHING to do with fluid flows (water or air, props or carburetors) is linear. All I see in your spreadsheets is straightforward linear RPM relationships.

That is because they ARE linear relationships, I used 10% prop slip for all of my calculations, which reduces the fluid dynamics and boat drag coefficient errors involved in the analysis .

45Auto Post 51

Look at the oil tanker p/d diagram in my previous post #30. They kept everything the same except the prop pitch. They cut the pitch in half (from 24 feet to 12 feet) while maintaining the same vessel speed of 14 knots. If you ran these numbers in your spreadsheet it would show that the prop RPM would double. Just like if you run it with a 19" pitch and a 38" pitch at 59 MPH it will show the 19" pitch prop RPM will be double that of the 38" pitch RPM.

However, as you can tell from the tanker diagram, in the real world the prop RPM did NOT double. It was 80 RPM with the 24' pitch at 14 knots, your prediction would have been 160 RPM with the 12' pitch at 14 knots. In the real world the 12' pitch only turned about 125 RPM, only about a 50% increase instead of the 100% increase you would have predicted.

But, if you will notice on EVERY prop pitch change I made in these posts I use the SAME EXACT 10% prop slip for EVERY prop, that is why I can use Linear math. The closer the pitch is between any two props that are being exchanged, the closer my analysis is to the exact speed and RPM that can be expected by changing pitches, in the same make and model of prop. 95% of all prop changes needed in this forum are 4" or less and it is normally only 2".

I went to Mercury's website and ran the numbers you gave above on the tanker and these are there results, and they are the same as mine, if you use the same 26% prop slip for all props, the way I use 10% for my prop analysis.

If anyone would care to verify this data just go to Merury's Prop Slip page and input the same data and solve for % of prop slip.

Here is the browser address:
http://www.mercuryracing.com/propellers/propslipcalculator.php


24' Pith and 80 RPM 26% Prop Slip 16 MPH

12' Pitch and 160 RPM 26% Prop Slip 16 MPH

12' Pitch and 125 RPM 26% Prop Slip 13 MPH

12' Pitch and 125 RPM 6% Prop Slip 16 MPH

In order for this prop to attain the 16 MPH at 125 RPM, the prop slip has to drop to 6% at those rpm. And since the diameter is constant and NOT VARIABLE like our props (as Pitch goes up Diameter goes down), in any particular make and model of our props. It is NOT comparable to the props we use on our boats, as it is a constant Diameter prop that is rotated about its axis to mechanically change pitches. And it is NOT comparing apples to apples.

45Auto Post 51

I?ll be busy the next couple of days, I?ll try to put something together this weekend. But if you want to add RPM to the ?Power Required vs Power Available? graph, it?ll look something like the one below, NOT what you did. Each pitch would require it's own RPM scale on the same axis as speed. I added them below at the max HP point and the point where each prop runs out of power against the power available curve. You can see that the 19" pitch would do about 57 MPH, the 23" pitch about 59 MPH and the 21" pitch about 61 MPH. It would have been easier to explain with a separate graph for each pitch before I combined them, but we're limited by the forum constraints.

hwsiii Post 36

Prop Speeds with 10% Prop Slip

I notice NOWHERE in your Motor and Prop curve analysis is Fluid Dynamics or Aerodynamics used, does that mean your results are less acceptable, I don't think so.

I also see that two of my numbers using my Linear Math are also within about 3% of your numbers while the middle one is dead on, and your numbers are not exact.
And that is while we are using an 8" difference in pitch, which is an actual increase in pitch of 42%. These props jump from 19" to 27" in pitch, so

I think anyone would have to say my method works very well when comparing it to yours.

Whether either one of us uses Fluid Dynamics or Aerodynamics does not matter.

I DO agree that my numbers are not EXACT, but if you are willing to spend the time and effort, and also have the people coming to this forum collect ALL the information required to run a FULL analysis using fluid dynamics and the correct drag coefficients for their boat and motor, more power to you. I have all of Savitsky's equation's, except Whisker spray drag, in an Excel spread sheet, but most people that come in here are certainty NOT willing or knowledgeable enough to give me ALL the information required to fill this out to see what speed they should be able to attain with their boat and motor, and even if they did it would only be good at ONE angle of thrust only, because Savitsky's formulas all assume that the boat has an INBOARD motor that NEVER changes the angle of thrust but instead uses trim tabs to change the running angle of the boat, and it can only bring the bow down and not up, while our outboard and inboard/outboard motors ARE able to bring the bow up or down, which in turn means we are able to maximize our speed and RPM.

Do YOU have ALL the power curves required to do an analysis of all the different motors and boats that people ask about on here, NO of course you don't, so you are ONLY able to to do it on motors that you have the CORRECT power curves for. Doing it without the true power curves is just like what I do here, the best approximation you can come up with. I do my best to educate people on here and give them more knowledge about props than they had before they came here so they can make more intelligent decisions and have more knowledge than they had before they entered this forum, and I am very happy with the results that I have seen.

I have posted my required information questions on here MANY MANY times and NEVER gotten a response from the person inquiring about what prop they need, because they don't know the information I am asking for and are not willing to spend the time required to find it. Some have even asked me "Are You For REAL". But, I don't mind, because if they are willing to try, and expend the effort required to find a better prop for their boat, then I am willing to do my best to help them.


H

 

[beny]

Re: 8000 lb cruiser twin merc 260's

Hi all
My hats off to you two! I am learning a lot.
But I need to throw you one more curve.
What happens when you go to a single engine.?
dos it change?
Thanks
Beny
my boat a 1980 formula 26 9.5 twin 260 merc

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

Unfortunately Beny there are alot of errors in this thread.... There have been alot of apples being compared to oranges..... I started this thread looking for an answer to a question..... here we see what happens when you put an engineer and a layman in the same room..... both have good points but are from different schools of thought.

No, single engine doesn't change the math but rather just the parameters.....


btw... Bond-o those props look great..... gonna be watchin ebay for some...

 

[hwsiii]

Re: 8000 lb cruiser twin merc 260's

Smoke, I am very sorry to have taken over your thread, please forgive me, BUT I did find the information you were looking for. So I hope that helps.

This information is straight from Dave Gerr, and he is a world renowned Naval Architect.

From his writing it appears that HP, RPM and SPEED, all make the decision for the diameter of a prop.


Propeller Diameter

H

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

no worries...
Thanks for the new info..... An equation that backs what I had been finding bits of.... That math is beyond me though
One of those fancy tables with the above equation would be fun to play with

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

hey H....
Any chance of putting the above into a calc or at least a simplified table of some sort?

 

[hwsiii]

Re: 8000 lb cruiser twin merc 260's

Kevin, that formula relies on knowing blade area ratios and manyyy other aspects of props that we do not have the information available for to do the comparative analysis of. That is only part of the equations needed to do the analysis. And it requires doing many multiples of some of them in order to complete the analysis. This is really designed for finding the correct propeller for large commercial vessels and custom made props.


H