outboard stainless prop question.
 

I currently have a 13.75 inch X 15 pitch aluminum prop on my 70 hp
Evinrude. The prop was really beat to death by it's previous owner, and
it is needing replaced. I use this boat for diving and skiing mostly, it
runs about 30 knots at present, and yanks me up out of the water quickly.
I was looking to go stainless, but they all seem to have a much higher
pitch, like 19 or more. What could I expect from say 13.25 X 21? I would
guess it would lug my motor down, and be very poor on pulling a skier up.

Rule of thumb with an I/O (so I've been told) ... if you use say a 21
aluminum, then use a 19 steel. In other words, decrease the pitch when
switching to steel.

"gudmundur" wrote in message
...
I currently have a 13.75 inch X 15 pitch aluminum prop on my 70 hp
Evinrude. The prop was really beat to death by it's previous owner, and
it is needing replaced. I use this boat for diving and skiing mostly, it
runs about 30 knots at present, and yanks me up out of the water quickly.
I was looking to go stainless, but they all seem to have a much higher
pitch, like 19 or more. What could I expect from say 13.25 X 21? I would
guess it would lug my motor down, and be very poor on pulling a skier up.

It can't be that bad if you are still using it (although you probably
shouldn't). You can get it repaired for a spare, or repaired and
re-pitched for everyday use. Find a good prop shop and talk to them.
There are a variety of factors that affect the performance of a prop.
You can also have them add a "cup" or vent it, for example. Whatever
you do you don't want to exceed your maximum RPMs at WOT and get a new -
or repaired- prop on there right away!

If you are happy with the hole shot for skiing, you might be able to
gain more top end with the right tuning with the prop you have. From
your post it seems that you are unhappy with the top end speed but boat
propellors rarely give you the best of both worlds.

Talk to the experts. It's well worth it.

Dan


gudmundur wrote:
I currently have a 13.75 inch X 15 pitch aluminum prop on my 70 hp
Evinrude. The prop was really beat to death by it's previous owner, and
it is needing replaced. I use this boat for diving and skiing mostly, it
runs about 30 knots at present, and yanks me up out of the water quickly.
I was looking to go stainless, but they all seem to have a much higher
pitch, like 19 or more. What could I expect from say 13.25 X 21? I would
guess it would lug my motor down, and be very poor on pulling a skier up.

Rule of thumb with an I/O (so I've been told) ... if you use say a 21
aluminum, then use a 19 steel. In other words, decrease the pitch when
switching to steel.

I'm not sure that's entirely accurate from my own experience. I had a
aluminum 19 on my 190hp Merc and it ran WOT at about 5400RPM (when I bought
the boat). The manual states 4400-4800rpm for WOT.

I read and heard that the adjustment was (rule of thumb) 1" pitch/200 rpms.
So I increased the pitch to 23 with a SS prop, and I'll be damned if WOT is
at about 4600rpm now...perfect. Maybe it would have been different if I
stayed with aluminum, I don't know. YMMV

--Mike

"bowgus" wrote in message
...
Rule of thumb with an I/O (so I've been told) ... if you use say a 21
aluminum, then use a 19 steel. In other words, decrease the pitch when
switching to steel.

"gudmundur" wrote in message
...
I currently have a 13.75 inch X 15 pitch aluminum prop on my 70 hp
Evinrude. The prop was really beat to death by it's previous owner, and
it is needing replaced. I use this boat for diving and skiing mostly, it
runs about 30 knots at present, and yanks me up out of the water quickly.
I was looking to go stainless, but they all seem to have a much higher
pitch, like 19 or more. What could I expect from say 13.25 X 21? I would
guess it would lug my motor down, and be very poor on pulling a skier up.

mgg wrote:


I'm not sure that's entirely accurate from my own experience. I had a
aluminum 19 on my 190hp Merc and it ran WOT at about 5400RPM (when I bought
the boat). The manual states 4400-4800rpm for WOT.

I read and heard that the adjustment was (rule of thumb) 1" pitch/200 rpms.
So I increased the pitch to 23 with a SS prop, and I'll be damned if WOT is
at about 4600rpm now...perfect. Maybe it would have been different if I
stayed with aluminum, I don't know. YMMV

--Mike

That's a pretty fast 190 hp. 55 mph?

What hull is that Merc in?

Rob

That's a pretty fast 190 hp. 55 mph?

What hull is that Merc in?

It's a 1995 Reinell 184 BRXL. Let me qualify that MPH statement. The stock
speedo has seen 55 in smooth water and no wind. However, we know how
accurate those things are. I'm going to use a GPS next time out and see what
I really get. Prolly closer to 50. Bottom line really is that WOT is now
within specs with the new prop, and she goes "pretty fast." g

--Mike

"trainfan1" wrote in message
...
mgg wrote:


I'm not sure that's entirely accurate from my own experience. I had a
aluminum 19 on my 190hp Merc and it ran WOT at about 5400RPM (when I
bought the boat). The manual states 4400-4800rpm for WOT.

I read and heard that the adjustment was (rule of thumb) 1" pitch/200
rpms. So I increased the pitch to 23 with a SS prop, and I'll be damned
if WOT is at about 4600rpm now...perfect. Maybe it would have been
different if I stayed with aluminum, I don't know. YMMV

--Mike

That's a pretty fast 190 hp. 55 mph?

What hull is that Merc in?

Rob

mgg wrote:

That's a pretty fast 190 hp. 55 mph?


What hull is that Merc in?

It's a 1995 Reinell 184 BRXL. Let me qualify that MPH statement. The stock
speedo has seen 55 in smooth water and no wind. However, we know how
accurate those things are. I'm going to use a GPS next time out and see what
I really get. Prolly closer to 50. Bottom line really is that WOT is now
within specs with the new prop, and she goes "pretty fast." g

--Mike

If you are running the 1.65:1 gearset, I think you'll find you're
pushing 54-55 if your tach is right.

Rob

On Thu, 31 Mar 2005 21:23:59 -0500, trainfan1
wrote:

If you are running the 1.65:1 gearset, I think you'll find you're
pushing 54-55 if your tach is right.

================================

Assuming 10% prop slip, that's what I calculate also.

51 mph at 15 % , and 48 mph at 20%.

Assuming 10% prop slip, that's what I calculate also.

51 mph at 15 % , and 48 mph at 20%.

Would a SS prop reduce slip?

--Mike

"Wayne.B" wrote in message
...
On Thu, 31 Mar 2005 21:23:59 -0500, trainfan1
wrote:

If you are running the 1.65:1 gearset, I think you'll find you're
pushing 54-55 if your tach is right.

================================

Assuming 10% prop slip, that's what I calculate also.

51 mph at 15 % , and 48 mph at 20%.

On Fri, 01 Apr 2005 06:12:07 GMT, "mgg" wrote:

Assuming 10% prop slip, that's what I calculate also.

51 mph at 15 % , and 48 mph at 20%.

Would a SS prop reduce slip?

=================================

It's hard to say, but it might. The advantages of stainless over
aluminum are structural rigidity (less flex) and surface smoothness
(less friction). Improving either quality could decrease slip which
would have the effect of increasing speed at comparable RPMs.