The prop looks a lot like the one on the old British Seagull, which was designed for sailboats. So It may be that that type of propeller is best for moving heavy displacement hulls.
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outboard for a Mac Classic D/S
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Prop is 4 blade 8.7 x 5
I ran the starter - no gas so outboard would not start. In under 3 second bursts, I saw 55 to 58 amps peak currents (its an averaged reading from the linklite battery monitor). This is with a brand new un-broken in outboard in temps just above freezing so might be higher than what will be seen for warmer temps and when the outboard is broken in. My 80 amp fuse up near the battery did not blow.
I didn't measure what voltage I got at the starter.. but sure "sounded" like the batteries all the way forward in the Vberth with 4 gauge back to the outboard is going to work great.
The prop on the XL shaft Nissan is about 3 inches lower now than I had with the long shaft Honda BF8.
When I tightened the bolts, I ended up moving the outboard up by 2 3/8 inch and back by 2 1/8 inch. The clearance tolerance is fairly tight (like maybe 1/2 inch) but its full rotation.
Top bolts are 3/8 inch (currently grade 8, could not find SS), bottom two sets are 5/16. The top bolts have an aluminum bar "backing plate" on the other side of the transom. I plan to leave the motor on during trailering.
I ran the starter - no gas so outboard would not start. In under 3 second bursts, I saw 55 to 58 amps peak currents (its an averaged reading from the linklite battery monitor). This is with a brand new un-broken in outboard in temps just above freezing so might be higher than what will be seen for warmer temps and when the outboard is broken in. My 80 amp fuse up near the battery did not blow.
I didn't measure what voltage I got at the starter.. but sure "sounded" like the batteries all the way forward in the Vberth with 4 gauge back to the outboard is going to work great.
The prop on the XL shaft Nissan is about 3 inches lower now than I had with the long shaft Honda BF8.
When I tightened the bolts, I ended up moving the outboard up by 2 3/8 inch and back by 2 1/8 inch. The clearance tolerance is fairly tight (like maybe 1/2 inch) but its full rotation.
Top bolts are 3/8 inch (currently grade 8, could not find SS), bottom two sets are 5/16. The top bolts have an aluminum bar "backing plate" on the other side of the transom. I plan to leave the motor on during trailering.
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The Internet suppliers show the 3 blade prop, but in their parts they show 2 different 4 blade props as optional........
http://www.tohatsuoutboardparts.com/Props-and-Hardware.html#Aluminum
..... 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.
I can't believe there isn't snow on the ground in your pictures. Those guys in California thought they got all the rain, but some made it past them and ended up here in the form.......
....or snow. 31 inches of it over about 12 inches left from the 35 inches we got in December. If anyone is interested there are more pictures here....
http://purplesagetradingpost.com/sumner/macgregor2/misc-pics-11.html
c ya and thanks for taking the time to check the current thing,
Sum
Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID
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That is the 5 pitch. I wonder if that is maybe a little too low pitched, but it would just mean the outboard will be turning more rpm for a given speed, but since it also won't be loaded as much the fuel efficiency might stay about the same. I'd suggest that if you wanted a backup prop why not get one that is a 6, 6.5 or 7 as a backup and try both.
I'll get a second prop to have with us as a spare. Before I decide on it I would be interested in why yours came with the 5 though. I could see it on some other 26 foot boat that weighed about 5000#, but ours doesn't.
Good luck and I'll bet you are going to love that outboard. I know we are looking forward to getting one also.
On the weather I was use to 65 then -20 then 65 again within a week or two in Wyoming, but then we got Chinooks like you do. None here :cry:. I've never lived somewhere like here were it gets slowly colder in the fall and slowly warmer in the spring. No big changes here, well except for the wind, that is another story
.c ya,
Sum
Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID
Our Mac Pages
Mac Links
Ive never really seen the need to mess with prop pitch. It seems to be important for the X and M planning hull boats but that is only because they are trying to hit some top speed - ie, get maximum HP out of a engine/prop combo by allowing the outboard RPM to get into a max HP range.
But on a displacement hull boat like mine and with a 8 to 10 hp outboard, I typically have probably a factor of two reserve of power I don't even need. Trying to size a prop to get the last 20% of power doesn't really do anything for me, maybe I would go .1 mph faster?
What seems more important for a displacement hull where we have lots of reserve HP is a more efficient prop - ie, better "Lift to Drag". For a given drive need, the prop would require less HP out of the outboard and therefore, would get better miles per hour. This is important to me for example on the lake Powell trip where I motor a lot.
In regards to just efficiency, maybe a three blade prop with a higher pitch (ie, higher than 6 or 7) may be both a higher efficiency "foil" and also allow the motor to run a lower RPM - and could be a better prop for what I really need.
But, I really have no way to tell if the four blade low pitch or the three blade high pitch would be best. I doubt were getting some sort of "stall" and therefore inefficiency with any of the props we use on under 10 hp outboards.
The four blade prop sure does score some "cool looking" bonus points however..
Looks like this one from the link earlier: 8.7d 5p 4 blade (high thrust)3B2W645110
Regarding the spare prop, my extra immediate cash is going to be spent on a prop guard ring. This not only protects the prop so your not so likely to damage it but on the S and D boats, when the rudder is up about 45 degrees, it can come in contact with the prop - not good. If the rudder is all the way down, no problem. but when its partially up, problem. I have munched my rudder twice. Once in a rudder grounding which popped it up to about 45 degrees and once when the hold down rope broke and let the rudder fold up.
But on a displacement hull boat like mine and with a 8 to 10 hp outboard, I typically have probably a factor of two reserve of power I don't even need. Trying to size a prop to get the last 20% of power doesn't really do anything for me, maybe I would go .1 mph faster?
What seems more important for a displacement hull where we have lots of reserve HP is a more efficient prop - ie, better "Lift to Drag". For a given drive need, the prop would require less HP out of the outboard and therefore, would get better miles per hour. This is important to me for example on the lake Powell trip where I motor a lot.
In regards to just efficiency, maybe a three blade prop with a higher pitch (ie, higher than 6 or 7) may be both a higher efficiency "foil" and also allow the motor to run a lower RPM - and could be a better prop for what I really need.
But, I really have no way to tell if the four blade low pitch or the three blade high pitch would be best. I doubt were getting some sort of "stall" and therefore inefficiency with any of the props we use on under 10 hp outboards.
The four blade prop sure does score some "cool looking" bonus points however..
Looks like this one from the link earlier: 8.7d 5p 4 blade (high thrust)3B2W645110
Regarding the spare prop, my extra immediate cash is going to be spent on a prop guard ring. This not only protects the prop so your not so likely to damage it but on the S and D boats, when the rudder is up about 45 degrees, it can come in contact with the prop - not good. If the rudder is all the way down, no problem. but when its partially up, problem. I have munched my rudder twice. Once in a rudder grounding which popped it up to about 45 degrees and once when the hold down rope broke and let the rudder fold up.
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One comment regarding the 55 to 58 amp starter current I measured.
This web site http://www.bcae1.com/wire.htm shows a MAX fuse sizes per wire gauge. This fuse is sized to keep the wire from getting too hot and in our case, would be the fuse placed very close to the battery (specifically in the V berth in my case). This fuse should blow if there is a short to keep the wires from starting on fire.
From the web site, it lists 50 amp max fuse for 8 gauge and 125 amp for 4 gauge. But.. if I had a 50 amp fuse, I could have blown it starting the outboard (I saw peak of 58 amps).
While 8 gauge wire may start the outboard fine even with the long wire run (assuming the batteries are forward in the boat), the issue would seem to be the fuse to protect the wire from fire. I think four gauge wire and at least an 80 amp is better. I may go higher than 80 amp on the fuse.. blowing the fuse at a bad time - like drifting towards some rocks in high winds - would not be good.
Although.. you do have the backup option of starting the outboard the old fashion way - using the pull start..
This web site http://www.bcae1.com/wire.htm shows a MAX fuse sizes per wire gauge. This fuse is sized to keep the wire from getting too hot and in our case, would be the fuse placed very close to the battery (specifically in the V berth in my case). This fuse should blow if there is a short to keep the wires from starting on fire.
From the web site, it lists 50 amp max fuse for 8 gauge and 125 amp for 4 gauge. But.. if I had a 50 amp fuse, I could have blown it starting the outboard (I saw peak of 58 amps).
While 8 gauge wire may start the outboard fine even with the long wire run (assuming the batteries are forward in the boat), the issue would seem to be the fuse to protect the wire from fire. I think four gauge wire and at least an 80 amp is better. I may go higher than 80 amp on the fuse.. blowing the fuse at a bad time - like drifting towards some rocks in high winds - would not be good.
Although.. you do have the backup option of starting the outboard the old fashion way - using the pull start..
Walt on the prop it isn't necessarily about getting the most speed. I agree that you aren't going to get any more speed. The pitch is all about how far the boat goes forward on each revolution of the prop. The 5 pitch wants to try and go 5 inches per revolution forward each revolution. The 7 pitch want to try and go 7 inches, about 30% farther.
If you take our dinghy with me in it alone it will get up on plane with the stock prop a 7 pitch for that motor. With the two of us it won't, but I might run the outboard at the same throttle position or higher. So now either the outboard won't pull up to the same rpm or it will and the prop will be slipping in the water and the boat won't go any faster. By changing the prop down to a 6 like we did the outboard is able to run easier as it can get to the correct rpm to match the slower speed.
Think about pulling a hill with your truck and no trailer. You might do it in 4th with 1/2 throttle and the motor isn't working hard. Now go back to the same hill with the trailer and you still might be able to pull it in 4th, but now you need full throttle and the motor is struggling. Shift to 3rd and the motor revs higher, but now you can back the throttle back off and the motor is running free. In all three cases you are going the same speed. In the first and last case the motor is not working as hard and will live longer. In the second case above the motor will probably heat up more and there is a chance of detonation (pinging) that can hurt the motor rather quickly. Now I'm not saying you are going to blow up your outboard with the wrong size prop, but you can make it more efficient.
The prop pitch along with the diameter is just like changing gears on a vehicle. I agree that put any prop on that you want with our boats and it is going to go forward and probably at hull speed. If you motor much why not try and put a prop on that was designed for the slower speeds our boat goes. Most of these small outboards come stock with props that are designed to push john boats and dinghies much faster than we can go, so they have a higher pitch prop stock on them. Yours came with just the opposite. The lowest pitch prop available. It will probably be about perfect.
On the prop guard I agree. I have one half built, but ran out of time on the last trip to finish it. We have got the prop into the rudder twice now also. I'll finish it this spring.
Sorry about the long prop info above. Mechanical stuff to me is like the electrical is to you. It is the area I get a little more anal on :cry:.
c ya,
Sum
Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID
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If you take our dinghy with me in it alone it will get up on plane with the stock prop a 7 pitch for that motor. With the two of us it won't, but I might run the outboard at the same throttle position or higher. So now either the outboard won't pull up to the same rpm or it will and the prop will be slipping in the water and the boat won't go any faster. By changing the prop down to a 6 like we did the outboard is able to run easier as it can get to the correct rpm to match the slower speed.
Think about pulling a hill with your truck and no trailer. You might do it in 4th with 1/2 throttle and the motor isn't working hard. Now go back to the same hill with the trailer and you still might be able to pull it in 4th, but now you need full throttle and the motor is struggling. Shift to 3rd and the motor revs higher, but now you can back the throttle back off and the motor is running free. In all three cases you are going the same speed. In the first and last case the motor is not working as hard and will live longer. In the second case above the motor will probably heat up more and there is a chance of detonation (pinging) that can hurt the motor rather quickly. Now I'm not saying you are going to blow up your outboard with the wrong size prop, but you can make it more efficient.
The prop pitch along with the diameter is just like changing gears on a vehicle. I agree that put any prop on that you want with our boats and it is going to go forward and probably at hull speed. If you motor much why not try and put a prop on that was designed for the slower speeds our boat goes. Most of these small outboards come stock with props that are designed to push john boats and dinghies much faster than we can go, so they have a higher pitch prop stock on them. Yours came with just the opposite. The lowest pitch prop available. It will probably be about perfect.
On the prop guard I agree. I have one half built, but ran out of time on the last trip to finish it. We have got the prop into the rudder twice now also. I'll finish it this spring.
Sorry about the long prop info above. Mechanical stuff to me is like the electrical is to you. It is the area I get a little more anal on :cry:.
c ya,
Sum
Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID
Our Mac Pages
Mac Links
I'm just going to wait and see what happens in our case. I don't have a fuse, but a breaker. It is easy to reset. I'll have to see if it trips or not and how the outboard sounds starting. I'll run a battery closer as a test. I might just move up to the #4 as now the generator along with the 120 watts of solar might start to be a problem also.
Thanks for checking the amps during starting. I appreciate that,
Sum
The dingy example is like the X, M example. You are trying to get max HP to get the boat to plane. My point is I just don't need the max HP with the displacement hull. I pretty much never run at wide open throttle (which you are likely doing to get the dingy to plane).
I really have not seen any need to mess with pitch on either of the two displacement hull / outboard combo's Ive had. Pretty much whatever was on there has worked fine.. Going to a higher pitch might have allowed the motor to run slower - better MPG. Going to a lower pitch might have allowed the outboard to run at a faster RPM - better peak HP delivered. Ive got more HP than I need with the displacement hull. Ive never felt like the engine RPM needed to be different "so it wasnt working as hard".
If I had a 3 hp on the 26 foot displacement hull where I possibly needed to get everything possible out of the engine/prop, prop pitch might then become important.
edit - another example, if you had a 20 hp outboard on your dingy rather than a 6, you would not have cared much about prop pitch (unless maybe you were trying to get it to go 40 mph).
I really have not seen any need to mess with pitch on either of the two displacement hull / outboard combo's Ive had. Pretty much whatever was on there has worked fine.. Going to a higher pitch might have allowed the motor to run slower - better MPG. Going to a lower pitch might have allowed the outboard to run at a faster RPM - better peak HP delivered. Ive got more HP than I need with the displacement hull. Ive never felt like the engine RPM needed to be different "so it wasnt working as hard".
If I had a 3 hp on the 26 foot displacement hull where I possibly needed to get everything possible out of the engine/prop, prop pitch might then become important.
edit - another example, if you had a 20 hp outboard on your dingy rather than a 6, you would not have cared much about prop pitch (unless maybe you were trying to get it to go 40 mph).
A couple of years back there was a discussion on the (pig ears) prop and one of the big inprovements was backing or engine braking.
If you make a prop protector make it go compleatly around the prop. It seems that some thrust is lost to water slipping off the edges of the blades and this corrects this and gives better thrust. The tugs that move the big container ships around have this. If you go to Florida it will protect the Mantees too.:dance:
If you make a prop protector make it go compleatly around the prop. It seems that some thrust is lost to water slipping off the edges of the blades and this corrects this and gives better thrust. The tugs that move the big container ships around have this. If you go to Florida it will protect the Mantees too.:dance:
Since youse guys aint goin nosewhere soon here is some reading.
http://www.trailerboats.com/output.cfm?id=1210105
http://www.trailerboats.com/output.cfm?id=1210105
The article posted has the same mentality of picking a prop based on maximum thrust at some boat speed. Ie, they took a 9.9 outboard and got 5.7 mph with the standard prop but got 6 mph with a high thrust prop (I think the boat was a pontoon boat or something like that). In both cases, the outboard was probably run at max throttle to get these speeds and probably had higher RPM with the high thrust prop.
There was also a mention of improved control - apparently they needed higher thrust for control (compensates for not having some sort of keel under the boat I guess).
What I never see is someone picking a prop based on efficiency. For example, when motoring long distances, my speed is about 6.5 mph (about..). I tend to run it a "little faster" rather than "more efficient" and the outboard is throttled way back.
For me, the criteria for picking the prop would be to try the different props at this speed and then see which one got the best miles per gallon. I'm fairly sure this would pick a different prop than using the conventional criteria listed in the article - or what I almost always see discussed on the Internet.
I picked this outboard in order to get electric start, two cylinder (smoother), 4 cycle, XL shaft. I did not pick it for the 9.9hp as I really probably need only about 1/2 of this. When you have more power than you need and with a low drag displacement hull, Id rather pick the prop based on efficiency. But.. I don't see this discussed and I probably will not test different props myself.
I think the high thrust prop will be fun to have..
Things like braking and reverse will of course be important. Wonder how the four blade high thrust prop does on these.
There was also a mention of improved control - apparently they needed higher thrust for control (compensates for not having some sort of keel under the boat I guess).
What I never see is someone picking a prop based on efficiency. For example, when motoring long distances, my speed is about 6.5 mph (about..). I tend to run it a "little faster" rather than "more efficient" and the outboard is throttled way back.
For me, the criteria for picking the prop would be to try the different props at this speed and then see which one got the best miles per gallon. I'm fairly sure this would pick a different prop than using the conventional criteria listed in the article - or what I almost always see discussed on the Internet.
I picked this outboard in order to get electric start, two cylinder (smoother), 4 cycle, XL shaft. I did not pick it for the 9.9hp as I really probably need only about 1/2 of this. When you have more power than you need and with a low drag displacement hull, Id rather pick the prop based on efficiency. But.. I don't see this discussed and I probably will not test different props myself.
I think the high thrust prop will be fun to have..
Things like braking and reverse will of course be important. Wonder how the four blade high thrust prop does on these.
The following is for anyone that is interested. I am not trying to tell anyone what prop to put on their outboard. Do what you want, but a year ago I didn't understand what the 'pitch' really did.
As I said in an earlier post think of the pitch as threads per inch on a bolt. More thread per inch means you turn the nut more times to move it the same distance on the bolt. With the prop a 5 pitch wants to screw itself forward 5 inches in the water each revolution. An 8 pitch prop wants to go forward 8 inches. The problem or good thing is that the water does not provide the same 'bite' as the bolt shank. This makes it a lot more forgiving in being sure you have selected the right prop pitch for the job.
I did a quick spreadsheet (sorry if there might be errors) using the gear ration in the 9.8 HP Tohatsu/Nissan. From the data you can see that at 1500 rpm a 5 pitch prop is trying to move the boat 3.41 mph if there was no slippage between the prop and water, which there is. The 8 pitch which is suppose to be the stock prop on a 9.8 HP Tohatsu would be trying to move the boat at 5.46 mph. If the boat is moving at an actual speed of say 3 mph with both props and the same rpm the outboard with the 8 pitch has to be slipping in the water a lot more than the 5 pitch. Not too efficient.
Now go to 3500 rpm and the 5 pitch is trying to move the boat at almost 8 mph. Of course it can't with our boats, so there is some slippage. Look at the 8 pitch prop. It is trying to move the boat at almost 13 mph. There is going to be a lot of slippage.
Now lets say we are in a heavy current/wind condition and we need to use the HP to overcome the conditions above those required to push our Mac at say 6 mph. We give the outboard almost full throttle and are running it at 5500 rpm. The 5 pitch is now trying to move the boat at 12 1/2 mph. There of course is slippage between the prop and water which decreases the efficiency (thrust) of it, but look at the outboard with the 8 pitch. It is trying to propel the boat at almost 20 mph. There is going to be a lot of slippage, loss of bite, loss of thrust and inefficient.
Will the 8 pitch propel our Mac? Sure. Is it the most efficient prop? No. There is a reason they make different pitch props.
This isn't about HP and getting the boat up on plane in our case it is about putting on the most efficient prop that we can. My dingy is better now with the lower pitch prop on it. I didn't do it to increase speed, but to improve efficiency. I think regardless of the HP of your outboard putting on a lower pitch prop will have its benefits if you have a sailboat. If you do have a small HP motor the prop will become more critical since you need to have a prop that will allow it to get up in the HP rpm range.
Most of us buy a second prop to have as a spare, especially if you are going any distance from your trailer or slip. This is a basic safety issue. Why not try and size the second prop to be more efficient for your particular use and put it on the outboard and save the one that came with it as the spare. All the better if the outboard comes with the correct prop in the first place, like I think is the case with the one Walt's dealer put on. A lot of people's dealers probably aren't going to do that for them.
As I mentioned above I'm not trying to get anyone to change their prop just giving them some info that is pretty parallel to what is in the link that Rick posted above.
c ya,
Sum
Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID
Our Mac Pages
Mac Links
As I said in an earlier post think of the pitch as threads per inch on a bolt. More thread per inch means you turn the nut more times to move it the same distance on the bolt. With the prop a 5 pitch wants to screw itself forward 5 inches in the water each revolution. An 8 pitch prop wants to go forward 8 inches. The problem or good thing is that the water does not provide the same 'bite' as the bolt shank. This makes it a lot more forgiving in being sure you have selected the right prop pitch for the job.
I did a quick spreadsheet (sorry if there might be errors) using the gear ration in the 9.8 HP Tohatsu/Nissan. From the data you can see that at 1500 rpm a 5 pitch prop is trying to move the boat 3.41 mph if there was no slippage between the prop and water, which there is. The 8 pitch which is suppose to be the stock prop on a 9.8 HP Tohatsu would be trying to move the boat at 5.46 mph. If the boat is moving at an actual speed of say 3 mph with both props and the same rpm the outboard with the 8 pitch has to be slipping in the water a lot more than the 5 pitch. Not too efficient.
Now go to 3500 rpm and the 5 pitch is trying to move the boat at almost 8 mph. Of course it can't with our boats, so there is some slippage. Look at the 8 pitch prop. It is trying to move the boat at almost 13 mph. There is going to be a lot of slippage.
Now lets say we are in a heavy current/wind condition and we need to use the HP to overcome the conditions above those required to push our Mac at say 6 mph. We give the outboard almost full throttle and are running it at 5500 rpm. The 5 pitch is now trying to move the boat at 12 1/2 mph. There of course is slippage between the prop and water which decreases the efficiency (thrust) of it, but look at the outboard with the 8 pitch. It is trying to propel the boat at almost 20 mph. There is going to be a lot of slippage, loss of bite, loss of thrust and inefficient.
Will the 8 pitch propel our Mac? Sure. Is it the most efficient prop? No. There is a reason they make different pitch props.
This isn't about HP and getting the boat up on plane in our case it is about putting on the most efficient prop that we can. My dingy is better now with the lower pitch prop on it. I didn't do it to increase speed, but to improve efficiency. I think regardless of the HP of your outboard putting on a lower pitch prop will have its benefits if you have a sailboat. If you do have a small HP motor the prop will become more critical since you need to have a prop that will allow it to get up in the HP rpm range.
Most of us buy a second prop to have as a spare, especially if you are going any distance from your trailer or slip. This is a basic safety issue. Why not try and size the second prop to be more efficient for your particular use and put it on the outboard and save the one that came with it as the spare. All the better if the outboard comes with the correct prop in the first place, like I think is the case with the one Walt's dealer put on. A lot of people's dealers probably aren't going to do that for them.
As I mentioned above I'm not trying to get anyone to change their prop just giving them some info that is pretty parallel to what is in the link that Rick posted above.
c ya,
Sum
Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID
Our Mac Pages
Mac Links
Sumner, correct me if I'm wrong but your making the assumption that more "slipping" equals less efficient and basing the argument on that.
I cant say this is wrong.. but I don't know how you could back that up. Can you?
Regardless of the prop pitch, you need to accelerate a mass of water backwards in order to get thrust. The thrust must be traveling backwards faster than the boat is going so no matter what the prop pitch is, they are always "slipping".
What I would understand more is lift to drag. For example, for a given thrust (a mass of water accelerated), the "foils" on a lower pitch prop would be operating at a lower angle of attack but have a higher velocity. In the lift to drag equations, the lower angle of attack has less induced drag. But the higher velocity gives higher surface friction drag. So which "dominates", I really don't know.
Walt first off I think you have the ideal prop for the situation. I think if the outboard would have come with what I see is the 'normal' 8 pitch prop you might not of been so happy. The dealer did you good and I don't think that was an accident.
If the slippage of the prop becomes excessive it will cause cavitation, less than that it is just less effecient. You have the right prop, so you are going to be just fine. I'm trying to put this out there for someone like myself that might order an outboard online like we are going to do and that might come with the 8 pitch prop.
I've tried my best to explain it in my words. As I said before why do they have different pitch props if there is no reason for them? Here is a quote from a different site that is maybe explaining it better than what I've done.
http://www.rbbi.com/folders/prop/propcalc.htm
I was happy to see that their speed/rpm is about the same as mine. I didn't put in a slippage factor, which is of course needed as the prop is trying to bite into a fluid.
There are a number of sites that are basically saying the same thing, just google the subject.
c ya,
Sum
Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID
Our Mac Pages
Mac Links
In terms of efficiency, I would think that the lower rpm and higher cylinder pressure would be more efficient(higher pitch). Cavitation (blade stall) would be more prevalent at launch or slow speeds and high power settings. Since speed isn't really a concern with sailboats, thrust would is. Granted having a ducted prop, (jetpump) would increase efficiency, and safety, but blade design and tip clearance is a huge factor in the way that works(if you are trying to squeeze every last bit). The safety factor is obvious and worthwhile by itself. Since a 10 hp outboard has probably twice the power required to attain hull speed, I would go with the largest diameter(more efficient) highest pitch prop I could find. Only two cents worth......granted my experience is with airplane propellors/jet engines but still a fluid dynamic.
Fairwinds and warmer weather......J
Fairwinds and warmer weather......J
I can see this with a car engine/transmission/rear-end where there is no slippage (torque converter locked up) in the drive train. The engine rpm will follow the road speed.
If you have the boat outboard at 3500 rpm in the situation above with an 8 pitch prop the prop is trying to go forward at a theoretical speed of almost 13 mph. The boat can only do a maximum of about 6 1/2. So what is the prop doing to the water? In my mind I see it churning the water up and not biting as efficiency as it could and moving through it.
Find me one web site or other documentation from an outboard manufacture or prop manufacture that says in our situation we should go with a high pitch prop. Not trying to put you on the spot, but everything I've read and my reasoning goes against the high pitch prop. Do they put high pitch props on slow planes?
Thanks for making me think more on it though,
Sum
Flynfol, I would agree with this. One thing we haven't worried about here is blade stall - and a higher pitch blade would be more likely to stall - but then its operated at a lower speed - less likely to stall. Stall may be more dependent on blade area. For a given thrust, the forces would be spread over more area for larger blades so maybe localized pressure gradients (which cause stall) could be less. But larger blades would have more surface friction drag - and this force goes up with the square of velocity - very significant.
The calculators you find on the Internet are all based around power boats where you want to go as fast as possible given some HP and the RPM it occurs at. These are good if you have a power boat with a 50 hp and its not going as fast as you would like. Or, your dingy cant get up on a plane and you know it could if you changed the prop a little and got just a little extra thrust to get over the planning threshold.
I'm repeating myself now (and I guess also Flynfol), but a displacement highly efficient hull with excess HP capability is possibly not optimized by the power boat calculators as they do not optimize for efficiency - only max HP delivered out of a particular motor and prop. I would optimize for efficiency and think Flynfol is probably correct about highest efficiency.
Sumner, the dealer had nothing to do with the prop selection. I don't know if they are shipping that prop standard now. When you go to order, maybe you can find out (Nissan 9.8, electric start, XLshaft, manual tilt). I think I am really going to like it. The one thing it should do is push a lot of water when I need it. Like if I need to stop. Maybe it will make getting in and out of docks easier.
Good (maybe a little heated??) discussion..
We will probably buys ours here....
http://www.onlineoutboards.com/Tohatsu-9-8-hp-MFS9-8A3EFUL.html
and in the specs they say............
Propeller:8.5" Dia. X 8.0" Pitch 3-Blade Aluminum
If ours comes that way would you like to trade? I'll be glad to
and you will get a more efficient prop out of the deal .I'm finished on the subject, but still stand behind what I've posted and what I've seen on site after site.
c ya,
Sum
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