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 |
AlexE
May 23
2006
|
| But Aluminum will still crack evnentually with any flex. Some of the Alloyw
are much stronger, but they will still crack with flex. 5086 is probably
the stongest of the aluminum alloys for boat building, stronger and more
saltwater resistant than 5082, but can not be formed like 5082 without more
cases of cracking. |
Calif
May 24
|
| 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.. |
Jack
May 24
|
| 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
May 24
|
| And if you do hit things ..... how about trashing out a cheap aluminum prop
and not a shaft? |
Forrest
May 24
|
| 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 |
Forrest
May 23
|
| The aluminum isn't flexing. Really. Get a 12 SS with a generous cup and you'll be golden. Rob |
trainfan1
May 23
|
| 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 |
RCE
May 23
|
| I don't see many die cast or sand cast airplane wings up here in the
north country... Rob |
trainfan1
May 23
|
| 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. |
Calif
May 24
|
| 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 saltwater marine use. http://elvis.engr.wisc.edu/UER/uer99/author1/content.html RCE |
RCE
May 24
|
| 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 |
RCE
May 24
|
