outboard for a Mac Classic D/S

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walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Trade?? What was the wager on this bet
I would bet it is the 7 pitch.
Depending on the wager, I may be willing to trade.

Ok.. Im KEEPING the prop. But its partly because I like the name "High Thrust"!!

I hope to find out this weekend how efficient that prop is at sloshing water all over the place when used in a trash can.
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
.......... one is a 5 pitch and the other a 7 pitch. I was going to get the 6 pitch 3 blade and take the stock one off for a spare. I wasn't aware of the 4 blade ones. I would bet it is the 7 pitch. I think the 5 would be reserved for really heavy boats, like a pontoon boat.

Talk to the dealer if you have a chance. I'd be interested in what he recommends, or maybe he was the one that requested that prop. If it is the 7 pitch it probably will be really good for the boat............
After doing the further research this thread cause me to do and creating the spread sheets and so forth I'm willing to admit that at the time of that post I thought that the 5 pitch was maybe too low, but now feel it might be the best choice. So therefore I'm willing to trade an 8 or even a 7 pitch if it comes on our outboard for your 5 pitch. If your convictions are still where they were at the beginning of this then I feel you should follow suit and give up the inefficient 5 pitch for the more efficient 7 or 8 pitch :). I'll be doing you a great favor.

One last shot ;), the 5 pitch is "higher thrust" because it 'grabs' the water better at the lower speed and is not churning it up trying to go forward through the water faster than the boat will let it.

c ya,

Sum
 
Oct 8, 2008
362
MacGregor/Venture 25 Winthrop Harbor, IL Drummond Island,MI
Heated?......I didn't think so.......just info exchange. Beauty is we can change props if we need to:)

Airplanes have a wide speed range, so it is a compromise between reaching rated hp rpm for takeoff (trying to clear those obstacles)and not overspeeding when at cruise speed with a fixed pitch prop. Some smart person came up with the idea of a variable pitch prop. (way too expensive for our application) Now we can do both. My observation comes with reducing rpm at cruise with a high manifold(read cylinder)pressure and large prop(more efficient) equals better efficiency and range. Just figured it works that way with a boat prop as well. Since we are talking 4 stroke normally aspirated engines, I would think loading up that prop at our cruise speed of 6 knots would equal best efficiency. We certainly don't need high rpm (full power to do that) If I was pushing a barge....yes... low pitch, high rpm would be my choice. We can have some fun with this down the road and compare fuel burns.....granted you guys operate at higher elevations and are losing 3% per thousand feet, so you are already at a disadvantage. Good reason for lower pitch though.....hmmm a turbo for an outboard......stop thinking John......

J
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Speaking of not having turbo charging or fuel injection..

I want to be able to swap the jets on this engine. Ive heard that in order to keep people from messing with the engine, they make the jets somewhat difficult to get to (maybe an EPA requirement??). "Difficult" as in having to pull the whole carb off to get at the jets.

Also, seems like messing the the jets every once in a while might be a good way to keep them clean. I wonder if the jets are under the plate on top of the carb.
 

Attachments

Oct 8, 2008
362
MacGregor/Venture 25 Winthrop Harbor, IL Drummond Island,MI
jets

Not sure about your Nissan, but removing the carb seems to be standard procedure for juggling jets. I have to say except for the Mikunis on my old race sleds where they built them into the bowl from outside in case of temp changes during rounds. Two strokes are a little more critical of mixture. But don't let that slow you down. It does make cleaning easier for sure. A small piece of crud in a main jet can cost you money! The shelves of sled racers across the midwest are littered with pistons with nice little holes in the tops! You learn to "read" the plugs pretty quick or invest in Wiseco stock;)

J
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
Speaking of not having turbo charging or fuel injection..

I want to be able to swap the jets on this engine. Ive heard that in order to keep people from messing with the engine, they make the jets somewhat difficult to get to (maybe an EPA requirement??). "Difficult" as in having to pull the whole carb off to get at the jets.

Also, seems like messing the the jets every once in a while might be a good way to keep them clean. I wonder if the jets are under the plate on top of the carb.
They are in the bottom of the body. Looks like the bowl has to come off. Also they have a main jet and what they call a slow jet. Both are on the bottom of the body in the float bowl.

I think if you were having starting and/or idling problems I would start with the slow first. First though I wouldn't do anything. Just run it and check the plugs. I'd also see the effect on any warranty you might have if you do this yourself.

c ya,

Sum
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
No problems.. my yearly usage of the boat is a couple months at about 5000 foot el, a week at 3400, then the rest of the summer at 8600. No trips planned to sea level.. but hopefully in the future.

So with the wide elevation range I use the boat at, I may be swapping jets several times each season and hopefully its not difficult.. doesn't sound too bad just having to remove the float bowl. I never messed with the jets on the Honda and had this same elevation changing routine. But I'm at least going to try it and see what difference it makes. I think the risk is using the high altitude jets at low altitude and overheating the engine.

Warranty - ???
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
......... I think the risk is using the high altitude jets at low altitude and overheating the engine...........
Not the engine as a whole, but the tops of those pistons as was mentioned above. Probably not covered under warranty....

Sum
 
Oct 8, 2008
362
MacGregor/Venture 25 Winthrop Harbor, IL Drummond Island,MI
jets

Wow....whats the record for thread length?....:D Can you get a tech manual for that motor? Good investment. Piston airplanes have adjustable mixtures for altitude variations. Full rich mixture also being used to cool cylinders and pistons. High altitude airports sometimes require leaning to obtain max power for takeoff. Having said that, I restored a 1940 Taylorcraft that had no mixture control on the carb. Not a problem when you rarely fly higher than 5000 msl( I have had it higher, but took a long time to climb...lack of mixture control didn't help, the real culprit was only 145 cubic inches). 4 stroke engines are not as critical of mixture as 2 stroke engines and the reason we would change jets (sleds)so often was to chase every last fraction of power. When running in equal classes, it made the difference. I guess it boils down to how well the motor runs. As Sum pointed out, the idle (slow speed)mixture might be adjustable to give you better throttle response to mid throttle, and then jet change for wide open throttle(read plugs). How often do we do that on these boats?

J
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Sumner and Flynfol, thanks for the discussion (and the spread sheet), I learned some things here.

I have to correct something I said "
The thrust must be traveling backwards faster than the boat is going so no matter what the prop pitch"
As the boat moves through the water, it must accelerate backwards some mass of water in order to move forwards. But the water accelerated backwards from the prop just needs to exit faster than it came in in order to get forward thrust. For example, if the boat is going 20mph, the water exiting the prop needs to be moving at 24 mph going out of the prop relative to the boat (ie, the water was accelerated by 4 mph by the prop). The water coming out has been accelerated backwards as the boat moves forward (F=MA). The slippage referred to earlier is a measure of difference in boat speed to "prop actual travel" so slippage is necessary in order to get thrust.

From the quote below "prop slip is NOT the same as propeller efficiency" and I think this is correct. Interesting to note about the term prop slip is that it to some extent tells you how efficient a boat is at traveling through the water. For a very efficient hull (like light planning powerboat), it takes relatively little mass*acceleration to propel the boat so I think you would get a low slippage percent number - ie, the speed of the water coming out of the prop is not much faster than the speed going in. A larger diameter prop would accelerate a larger volume of water so might also reduce the slippage number.

As you get to a more inefficient planing hull (like Mac M simply because of its size/weight and relatively small motor), you need to accelerate more water to make the boat move so the speed of the water exiting the prop increases compared to the speed going in. Ie, you have a higher slippage factor. Again, "prop slip is NOT the same as propeller efficiency" so no problem.

In the displacement hull where you are pushing up against hull speed and with a small outboard (and small diameter prop), you may need to accelerate water out of the prop a lot faster than what is going in to get the necessary M*A to move the boat. This pretty much matches what I see with my outboard where the water coming out of the prop looks like its moving a lot faster than the boat is. In this case, the displacement hull is operating at a relatively high slippage percent. Yet again, "prop slip is NOT the same as propeller efficiency" so this is OK - just the way a displacement hull with a small outboard works - and works fine.

If we were to just look at prop efficiency, Fylnfol suggested that a highly loaded foil moving at a slower RPM (ie, high pitch number) might be more efficient than a lightly loaded foil (lower pitch number) moving at a faster RPM. Plus, the outboard would be operating at a lower RPM. I cant back this up with a reference but also believe this is likely true.

The problem is that is that the high pitch number tends to operate at lower RPM for the same amount of thrust so in some cases wont allow the outboard RPM to get up to levels where the outboard is putting out its rated HP. If you want the outboard to get to its peak RPM power band for a given boat speed, you need to go to a lower pitch prop. The prop calculators give you a ballpark for this.

However, in the case above, we went to a lower pitch prop operating at a faster RPM so we could get more HP delivered because of the higher outboard RPM. But we also (according to what Flynfol and I think) went to a less efficient set of foil conditions. We noted that with a boat like a 26S and a 9.8 hp outboard, you "normally" don't need the peak HP so the trade-off was questionable.

The "high Thrust" 4 blade prop I have now I think will allow me to punch out a lot of water when the boat is going slow because it will allow the outboard RPM's to get up into the power band. This prop will also likely get max hull speed out of the 26S at full throttle as it should allow the outboard to get into a better power band. But I think it will not get as good of gas mileage when I run it on long trips because its lightly loaded with lots of blade area (high parasitic drag) and generally higher RPM's. Most of the summer, I use maybe 2 gallons of gas total so the efficiency wont be an issue and the high thrust will be nice to have (and fun to have) for most of the things that I do such as motor around the lake a little and for docking. Sumner said I will like this prop, I think he is correct.

But.. maybe if I get a spare prop, it might be a high pitch one and if was trying to squeeze the gas mileage, I could swap to this prop for my trip where I motor a lot and have limited gas.

One final thing.. for a given prop diameter, it must accelerate some mass out the back end in order to propel a boat at some speed. The speed of the mass determines the slippage number. Note that no matter what pitch of prop you use, you have to accelerate the same mass out the back so the slippage number is independent of prop pitch. No problem once again because "prop slip is not the same as propeller efficiency".

The quote from the web site Sumner referenced is below:

Prop slip is a measure of how efficiently the boat is going through the water. Prop slip is not the same thing as propeller efficiency.

If the prop did not slip at all as it screws through the water, each rev theoretically propels the boat the forward a distance equal to the prop pitch. (a 15 inch pitch prop theoretically propels the boat 15 inches in one revolution). The propeller revolution rate is the engine rpm divided by the gear ratio. This propeller rpm times the prop pitch determines the theoretical distance that the boat should have moved in one minute, which can be converted to a theoretical boat speed in miles per hour. Due to prop slip the boat is actually going slower than the theoretical speed. Normally on the nature of 10 to 20% slower at top speed. This ratio is found by dividing the actual boat speed by the theoretical boat speed to get a ratio. Then, this ratio is subtracted from 1. The result is the propeller slip. In real life, you also have to do a number of unit conversions between hours, minutes, miles, feet, inches, etc. All of these calculations and conversions are done for you internally in the RBBI Prop Calculator.
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
Sumner and Flynfol, thanks for the discussion (and the spread sheet), I learned some things here.

I have to correct something I said "

As the boat moves through the water, it must accelerate backwards some mass of water in order to move forwards. But the water accelerated backwards from the prop just needs to exit faster than it came in in order to get forward thrust. ............
Walt is this your conclusion as to how a prop works or can you send me to a site where I can read more on it?

I've always read that a prop works similar to a nut on a screw. The screw has a pitch, so many threads per inch. Turn the nut and each revolution it moves up the screw the distance of the pitch. The prop is trying to move forward through the water the distance of its pitch each revolution and take the boat with it. Of course it can't due to working in a fluid, so there is some slippage. Which leads me to this....

The quote from the web site Sumner referenced is below:


Quote:
Prop slip is a measure of how efficiently the boat is going through the water. Prop slip is not the same thing as propeller efficiency.

If the prop did not slip at all as it screws through the water, each rev theoretically propels the boat the forward a distance equal to the prop pitch. (a 15 inch pitch prop theoretically propels the boat 15 inches in one revolution).
I'm interpreting this different than you are I believe. I read that to say if you have the proper prop on your boat (most efficient) that has the best pitch, diameter, and blade shape for your application you will have less prop slip and the boat will move through the water more efficiently and will have prop slippage in the 10%-20% range. To do that you need an efficient prop, one that slips the least at the cruise speed you want to operate the boat. Above or below this speed it might not be the most efficient. If you want to increase acceleration or stopping you might go to a lower pitch and give up some cruise efficiency.



Let's say the chart above is close to correct. If you wanted to run close to hull speed like you quoted at Lake Powell with the 5 pitch you would probably be turning the outboard at about 3500 rpm. That would be almost 8 mph if the prop had no slippage, but it does so let's say with slippage you would be running close to 7 mph.

At 3500 with the 8 pitch the prop would be trying to propel the boat up around 11 1/2 to 12 mph, which it can't, so it would just be slipping a lot.

For the 8 pitch to be efficient with about 10%-20% efficiency the outboard would be turning less than 2500 rpm. Looking at the chart and interpolating it looks like it would be running around 2000 rpm. Now like you have said at 2000 HP it 'might' be putting out enough HP to push the boat, but I wonder about that. I also wonder what rpm the engine is most efficient at. Also as you have pointed out the 8 pitch will kill throttle response and acceleration in forward and reverse.

We like to motor about 4-5 knots usually when on the motor, so with the 5 pitch that would be about 2500 rpm. If you like to motor around 6 knots then maybe the 6 pitch would be ideal. See if you can find the rpm that the outboard is most efficient at and prop for that at Lake Powell allowing for 10-20% prop slip.

(ADDED LATER: We put an 8 pitch, lowest I could find on the 8 HP Honda we have had and it has worked just fine. I'm thinking that we will probably start off with a 6 pitch on the Tohatsu after thinking this over.)

c ya,

Sum

Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID

Our Mac Pages

Mac Links
 
Jun 3, 2004
1,863
Macgregor 25 So. Cal.
I am willing to admit that the lower pitch on the high torque prop is less to enable the motor on a displacement hull to be smaller and rev into it's power band.

Let's look at the size and shape of the blades because this is where I think the big difference is.

First the prop is about as big in diameter as will fit under the cavitation plate.

OMG, just look at the size of those blades! I believe this difference causes the prop to bite and not slip allowing the motor to put more torque to the water.

Kinna like putting taller and wider tires on a car, more contact with the road.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Sumner, your conclusions seem to depend on this statement:

I'm interpreting this different than you are I believe. I read that to say if you have the proper prop on your boat (most efficient) that has the best pitch, diameter, and blade shape for your application you will have less prop slip and the boat will move through the water more efficiently and will have prop slippage in the 10%-20% range
I don't think this is correct (but of course could be wrong). What I read is that prop slip is more about the hull efficiency - NOT prop efficiency. If you are moving a hull at some speed, it takes some force to move it at this speed. This force is generated by the prop accelerating a mass of water in the opposite direction the boat is moving. It does not matter how efficient the prop is doing this or how much pitch it takes to do it, it is the mass and acceleration that makes the boat go forward. A less efficient hull and a given blade diameter will have a higher slippage number because acceleration needs to be higher. A more efficient hull for the same blade diameter will have a lower slippage number because acceleration can be lower.

You also make the conclusion that slippage has something to do with efficiency. It might have some influence but I don't think it is key here and the reference web site says
Prop slip is not the same thing as propeller efficiency
I don't have a reference web site for you on any of what I said - it could be wrong. I'm pretty sure we always operate at fairly high slippage numbers already with our displacement hulls - have been for a while with my Honda and I get around 12 mpg pushing the hull speed a bit. Likely I'm operating at fairly high slippage here and I don't think it has a significant influence on the efficiency I'm getting.

I'm going by the assumption that conventional things like lift to drag are what influences the prop efficiency. I don't think we have any control on slippage, it is more of a number which regards a hull's efficiency at a given speed. As I said before, I don't think that pitch even has an influence on slippage - no matter what, for a given mass (ie, prop diameter), you have to accelerate the water at the same speed regardless of the prop pitch configuration - and that is what defines slippage. The high pitch prop just accelerates the water at a lower RMP than a low pitch prop does.. Both props need to provide the exact same acceleration of the water mass for the hull speed.

I think Pitch is mostly about getting the outboard RPM into the best power band for the top speed you expect to achieve..
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
If the prop did not slip at all as it screws through the water, each rev theoretically propels the boat the forward a distance equal to the prop pitch. (a 15 inch pitch prop theoretically propels the boat 15 inches in one revolution). The propeller revolution rate is the engine rpm divided by the gear ratio. This propeller rpm times the prop pitch determines the theoretical distance that the boat should have moved in one minute, which can be converted to a theoretical boat speed in miles per hour. Due to prop slip the boat is actually going slower than the theoretical speed. Normally on the nature of 10 to 20% slower at top speed. This ratio is found by dividing the actual boat speed by the theoretical boat speed to get a ratio. Then, this ratio is subtracted from 1. The result is the propeller slip.
I wanted to clarify one other thing. The quote above uses prop pitch to describe "how far the boat would travel in the time of one revolution" and compares this to how fast the boat is actually traveling to get the slippage ratio. I noted that the distance the prop "travels" must ALWAYS be greater than the boat travels as this is how the prop generates thrust. The greater the thrust required, the "longer" the prop must travel compared to boat speed. Ie, a less efficient hull for a given speed will have a higher prop slip. For a given thrust required to push a boat at a given speed, I think you will find that you get the same prop slippage number regardless of the pitch you use in the equations. Key here is the "same thrust" and this would assume things like a 5 pitch prop going two revolutions produces the same thrust as a 10 pitch prop going one revolution. Is this exactly correct - don't know but suspect is close enough.

I may get a higher pitch prop for my spare - and then do a test at lake powell where I go through 3 gallons of gas with the high thrust prop and then 3 gallons with the high pitch prop - keeping the boat speed constant. As I mentioned, I dont know if anything I said is correct so the experiment might be interesting. I thing the high thrust is going to be fun as long as it doesnt bust my transom riser in half..
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
.......... This force is generated by the prop accelerating a mass of water in the opposite direction the boat is moving. It does not matter how efficient the prop is doing this or how much pitch it takes to do it, it is the mass and acceleration that makes the boat go forward...........
Please show me one reference to this somewhere pertaining to a prop driven boat. I might see it on a jet boat?

Does a tug propel the mass of its tow with an equivalent mass of water with the prop to move forward? Does a plane move an amount of air equivalent to its mass with its propeller to move forward? When I swim do I move an equivalent amount of water to equal my mass with my hands to move forward in the water?

I guess we will just have to agree to disagree on how a prop moves a boat forward through the water:) and I'm sure we will both get to where we are going.

Fuel usage and RPM really don't really equate.
Rick why then do we use overdrive transmissions in our cars to achieve better gas mileage? Now saying that there still is an rpm that an engine is going to be most efficient at gas mileage wise. Most cars now seem to be in that range somewhere between 1800 and 2100 rpm even though their peak torque might be between 3000 and 3500.

Ok I got to get out in the shop and get something done :),

Sum
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Does a tug propel the mass of its tow with an equivalent mass of water with the prop to move forward? Does a plane move an amount of air equivalent to its mass with its propeller to move forward?
A tug moves forward because it accelerates a mass of water backwards creating a force equal to the drag of the hull (and its load) as it moves through the water. Mass of water * acceleration of the water = force to move tug through water at some speed. As the tug travels through the water, it leaves some water displaced from where it originally was before the tug came through. I don't think we need to consider gravity and buoyancy here.

If we consider the blade of a prop like an airplane wing, it will have both lift dependent on the angle of attack, area and V**2 and parasitic drag dependent on the area and V**2 (V is velocity). So a low pitch foil needs to move at a higher V in order to generate the same lift as a high pitch foil. But we also have the drag go up with the square of velocity. So my high thrust prop with low pitch is going to need to operate at a higher rotational velocity to generate a certain amount of lift. And because of both the large surface area and the higher velocity, it could have fairly high drag. In more normal use, I'm not going to care much however as maybe its an extra 1/2 gallon of gas over the entire summer.

How can you be thinking about swimming with all that snow you've got?



Ok... back to work.
 
Oct 8, 2008
362
MacGregor/Venture 25 Winthrop Harbor, IL Drummond Island,MI
thrust vs. drag

With an airplane, at equilibrium(cruise) thrust will equal total drag (parasitic and induced). If it is accelerating, then thrust is greater than drag. Efficiency comes in at the point where your propulsion is the most efficient and fuel consumption is the least. That is where I figured like an airplane you would want the slowest rpm with the highest cylinder pressure. Large diameter, high aspect ratio props on planes are the most efficient. Dynamics in a fluid I'm sure are different. Some good head scratching going on here............too fun.

J

Sum, were you a teacher at some time? If not you certainly would have been a good one;)
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
......Sum, were you a teacher at some time? If not you certainly would have been a good one;)
Thanks, I did teach shop for a couple years on the Cheyenne Indian Reservation and then taught computer classes (MS-DOS and programming in Basic) at the Junior College where I lived in Wyoming. I was self taught in the computer stuff, but a college doesn't care and can get by the requirements for prior education that public schools have to follow to some degree.

My computer business (retail) grew too big and I had to give up the teaching. I liked the teaching, but it is a lot of work. Hardest most tiring work I ever did and for a couple years I delivered milk on a retail route to over 600 homes in a night and I've had other physically demanding jobs. Good teachers don't get the credit they deserve and there are a number of teachers out there that should be doing something else.

I'm going to go out and re-finish my canoe paddles just in case I end up with the wrong prop :).

c ya,

Sum

Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID

Our Mac Pages

Mac Links
 
Oct 8, 2008
362
MacGregor/Venture 25 Winthrop Harbor, IL Drummond Island,MI
Walt,

I think you are a scientist;)

J
 
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