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#1
posted to rec.boats
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Prop choice: 12 or 13
Hi
I have a Tohatsu 40c (2 cyl) that now has a Tohatsu 10½ x 12 aluminum prop. The motor turns about 6000-6100 at full throttle (max allowed 5800). If i buy a stainless steel prop (I was thinking of a Michigan Wheel Rapture), should I get a 12 or 13 pitch? Someone told me that in a case like this, if the revs are slightly high, get a stainless prop with the same pitch as the alu prop, that'll take some rpm off, presumably because the alu prop flexes ever so slightly and the steel does not. The boat is 14 ft and weighs about 350+ kg including me. Comments? Suggestions? Rgds Alex |
#2
posted to rec.boats
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Prop choice: 12 or 13
"AlexE" wrote in message ... Hi I have a Tohatsu 40c (2 cyl) that now has a Tohatsu 10½ x 12 aluminum prop. The motor turns about 6000-6100 at full throttle (max allowed 5800). If i buy a stainless steel prop (I was thinking of a Michigan Wheel Rapture), should I get a 12 or 13 pitch? Someone told me that in a case like this, if the revs are slightly high, get a stainless prop with the same pitch as the alu prop, that'll take some rpm off, presumably because the alu prop flexes ever so slightly and the steel does not. The boat is 14 ft and weighs about 350+ kg including me. Comments? Suggestions? Rgds Alex I can't picture an aluminum prop continuously flexing without breaking. A good one for "Mithbusters". What are you using the boat for? Do you want to zoom from one side of the lake to the other as fast as you can ... or pull the kids around on a sled or a skier ... maybe tow a dingy? I would just stick with the cheaper aluminum and increase the pitch. How much depends on the above. Good luck |
#3
posted to rec.boats
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Prop choice: 12 or 13
"Forrest" wrote in message ... I can't picture an aluminum prop continuously flexing without breaking. A good one for "Mithbusters". What are you using the boat for? Do you want to zoom from one side of the lake to the other as fast as you can ... or pull the kids around on a sled or a skier ... maybe tow a dingy? I would just stick with the cheaper aluminum and increase the pitch. How much depends on the above. Good luck Better stay out of airplanes. Those aluminum wings flex big time. If they didn't, the wings would break off the first time the plane hit some turbulence. Metal will flex without breaking as long as it does not exceed it's yield strength. The yield strength of aluminum alloys is similar to that of carbon and stainless steel. RCE |
#4
posted to rec.boats
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Prop choice: 12 or 13
RCE wrote:
"Forrest" wrote in message ... I can't picture an aluminum prop continuously flexing without breaking. A good one for "Mithbusters". What are you using the boat for? Do you want to zoom from one side of the lake to the other as fast as you can ... or pull the kids around on a sled or a skier ... maybe tow a dingy? I would just stick with the cheaper aluminum and increase the pitch. How much depends on the above. Good luck Better stay out of airplanes. Those aluminum wings flex big time. If they didn't, the wings would break off the first time the plane hit some turbulence. Metal will flex without breaking as long as it does not exceed it's yield strength. The yield strength of aluminum alloys is similar to that of carbon and stainless steel. RCE I don't see many die cast or sand cast airplane wings up here in the north country... Rob |
#5
posted to rec.boats
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Prop choice: 12 or 13
"RCE" wrote in message ... "Forrest" wrote in message ... I can't picture an aluminum prop continuously flexing without breaking. A good one for "Mithbusters". What are you using the boat for? Do you want to zoom from one side of the lake to the other as fast as you can ... or pull the kids around on a sled or a skier ... maybe tow a dingy? I would just stick with the cheaper aluminum and increase the pitch. How much depends on the above. Good luck Better stay out of airplanes. Those aluminum wings flex big time. If they didn't, the wings would break off the first time the plane hit some turbulence. Metal will flex without breaking as long as it does not exceed it's yield strength. The yield strength of aluminum alloys is similar to that of carbon and stainless steel. RCE Not true. There is no B-50 bearing type rating for aluminum. As long as you stay below the yield strength in the flex in steel, that is true, but aluminum wants to crack anyway. Airplanes just keep track of any cracks and when they exceed a defined amount the part is replaced. I would assume the airframe is designed so a single part failure is not catastrophic. |
#6
posted to rec.boats
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Prop choice: 12 or 13
"Calif Bill" wrote in message nk.net... "RCE" wrote in message ... "Forrest" wrote in message ... I can't picture an aluminum prop continuously flexing without breaking. A good one for "Mithbusters". What are you using the boat for? Do you want to zoom from one side of the lake to the other as fast as you can ... or pull the kids around on a sled or a skier ... maybe tow a dingy? I would just stick with the cheaper aluminum and increase the pitch. How much depends on the above. Good luck Better stay out of airplanes. Those aluminum wings flex big time. If they didn't, the wings would break off the first time the plane hit some turbulence. Metal will flex without breaking as long as it does not exceed it's yield strength. The yield strength of aluminum alloys is similar to that of carbon and stainless steel. RCE Not true. There is no B-50 bearing type rating for aluminum. As long as you stay below the yield strength in the flex in steel, that is true, but aluminum wants to crack anyway. Airplanes just keep track of any cracks and when they exceed a defined amount the part is replaced. I would assume the airframe is designed so a single part failure is not catastrophic. I said aluminum alloys. Any of the structural aluminum alloys will flex to their yield strength number without cracking. The internal monocock frame structure of a modern jetliner flexes a foot or more over the length of the wing. Next time you fly, note the relative position of the wingtip to the fuselage while the plane is on the ground. Then, watch as you take off and note the position of the wingtip. Note table 1 at the following link. There are several aluminum alloys that exceed the carbon steel yield strength of 60 ksi. The 5000 series alloys are of particular interest for sal****er marine use. http://elvis.engr.wisc.edu/UER/uer99...1/content.html RCE |
#7
posted to rec.boats
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Prop choice: 12 or 13
"RCE" wrote in message ... "Calif Bill" wrote in message nk.net... "RCE" wrote in message ... "Forrest" wrote in message ... I can't picture an aluminum prop continuously flexing without breaking. A good one for "Mithbusters". What are you using the boat for? Do you want to zoom from one side of the lake to the other as fast as you can ... or pull the kids around on a sled or a skier ... maybe tow a dingy? I would just stick with the cheaper aluminum and increase the pitch. How much depends on the above. Good luck Better stay out of airplanes. Those aluminum wings flex big time. If they didn't, the wings would break off the first time the plane hit some turbulence. Metal will flex without breaking as long as it does not exceed it's yield strength. The yield strength of aluminum alloys is similar to that of carbon and stainless steel. RCE Not true. There is no B-50 bearing type rating for aluminum. As long as you stay below the yield strength in the flex in steel, that is true, but aluminum wants to crack anyway. Airplanes just keep track of any cracks and when they exceed a defined amount the part is replaced. I would assume the airframe is designed so a single part failure is not catastrophic. I said aluminum alloys. Any of the structural aluminum alloys will flex to their yield strength number without cracking. The internal monocock frame structure of a modern jetliner flexes a foot or more over the length of the wing. Next time you fly, note the relative position of the wingtip to the fuselage while the plane is on the ground. Then, watch as you take off and note the position of the wingtip. Note table 1 at the following link. There are several aluminum alloys that exceed the carbon steel yield strength of 60 ksi. The 5000 series alloys are of particular interest for sal****er marine use. http://elvis.engr.wisc.edu/UER/uer99...1/content.html RCE Another quick comment on aluminum. Although there are some high strength alloys, the yield strength and ultimate strength (where it permanently takes on a new shape or fails) are very close to each other, unlike steel. RCE |
#8
posted to rec.boats
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Prop choice: 12 or 13
Forrest wrote:
"AlexE" wrote in message ... Hi I have a Tohatsu 40c (2 cyl) that now has a Tohatsu 10½ x 12 aluminum prop. The motor turns about 6000-6100 at full throttle (max allowed 5800). If i buy a stainless steel prop (I was thinking of a Michigan Wheel Rapture), should I get a 12 or 13 pitch? Someone told me that in a case like this, if the revs are slightly high, get a stainless prop with the same pitch as the alu prop, that'll take some rpm off, presumably because the alu prop flexes ever so slightly and the steel does not. The boat is 14 ft and weighs about 350+ kg including me. Comments? Suggestions? Rgds Alex If you want high quality stainless "made in the USA" consider PropCo http://www.propcopropellers.com/ Found them at the boat show this year and purchased from them direct. Very nice indeed, also avaiable at Bass Pro Shops.. Cheers: Capt Jack R.. I can't picture an aluminum prop continuously flexing without breaking. A good one for "Mithbusters". What are you using the boat for? Do you want to zoom from one side of the lake to the other as fast as you can ... or pull the kids around on a sled or a skier ... maybe tow a dingy? I would just stick with the cheaper aluminum and increase the pitch. How much depends on the above. Good luck |
#9
posted to rec.boats
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Prop choice: 12 or 13
AlexE wrote:
Hi I have a Tohatsu 40c (2 cyl) that now has a Tohatsu 10½ x 12 aluminum prop. The motor turns about 6000-6100 at full throttle (max allowed 5800). If i buy a stainless steel prop (I was thinking of a Michigan Wheel Rapture), should I get a 12 or 13 pitch? Someone told me that in a case like this, if the revs are slightly high, get a stainless prop with the same pitch as the alu prop, that'll take some rpm off, presumably because the alu prop flexes ever so slightly and the steel does not. The boat is 14 ft and weighs about 350+ kg including me. Comments? Suggestions? Rgds Alex The aluminum isn't flexing. Really. Get a 12 SS with a generous cup and you'll be golden. Rob |
#10
posted to rec.boats
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Prop choice: 12 or 13
Around 5/23/2006 2:48 PM, AlexE wrote:
Hi I have a Tohatsu 40c (2 cyl) that now has a Tohatsu 10½ x 12 aluminum prop. The motor turns about 6000-6100 at full throttle (max allowed 5800). If i buy a stainless steel prop (I was thinking of a Michigan Wheel Rapture), should I get a 12 or 13 pitch? Someone told me that in a case like this, if the revs are slightly high, get a stainless prop with the same pitch as the alu prop, that'll take some rpm off, presumably because the alu prop flexes ever so slightly and the steel does not. The boat is 14 ft and weighs about 350+ kg including me. Comments? Suggestions? Rgds Alex My setup is almost identical, except a bit heavier (~430 kg with me, gas, and normal gear): I've got a Nissan 40d (3 cyl) that came with the same 12" pitch prop on a 14' fiberglass runabout, and I couldn't even go full throttle without hitting the rev limiter. I went with the highest pitch replacement aluminum prop Michigan Wheel makes (10.2" X 13.9"), and I have been extremely happy with it. Revs are perfect, 5400 at full throttle (manual says the max should be 4500-5500), and the top end picked up almost 7 knots. It'll still pull up a big skier and even uses less fuel to cruise at the same speed as with the old prop. Now, you don't need to lower your RPMs as much as I did (the two cylinder is a little more rev happy, so the 13" would probably suit your boat well. You might even be able to squeeze a little more speed out of the 14" and still be inside your ideal RPM range. I've got very little opinion one way or the other with stainless vs. aluminum. I don't tend to hit things, so I opted for the cheap aluminum over the durable SS. -- ~/Garth - 1966 Glastron V-142 Skiflite: "Blue-Boat" "There is nothing - absolutely nothing - half so much worth doing as simply messing about in boats." -- Kenneth Grahame ~~ Ventis secundis, tene cursum ~~ |
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