Prop Gurus

Golfman25

Well-Known Member
Sep 12, 2009
7,584
IL
Boat Info
1998 370 Sundancer
Engines
7.4 MPI
(moving from the 370 thread) So looking for some thoughts. New to me 1998 370 7.4 MPIs. 310 HP. WOT spec is 4000-4400. Current WOT is 3800 rmp. Assumed it was prop related as they are not the stock 3 blades but 4 blades. Per the SR parts manual, stock pros are 18 x 21 3 blades. So I assumed a previous owner just swapped props -- 18x21 4 blade. Just pulled the nuts off so I could get a look at the markings, I was half right. Current prop is 17 x 21 4 blade (Austral 1721L4Q). So they kept the same pitch but reduced dia 1 inch, assuming to make up for the RPM loss? Reading the interweb, there is conflicting information regarding effect of dia on RPMs -- some say little effect, some say 400-500 rpms per inch.

Looks like most owners here replaced their 3 blades with 18 x 19 4 blades (2 inch pitch reduction). My understanding is the 2 inch pitch reduction will offset the RPM reduction caused by the 4th blade.

So is my best bet just to add some 18 x 19s and enjoy? Thanks in advance.
 
@Golfman25 Thats the right math, 2" in pitch equals 1" in diameter and also 3 -> 4 blades. That math has been proven time and time again. So you should be good in doing htat.
 
I’ve always ignored the diameter. That’s deeper into prop science than I’m smart enough to understand and I believe mostly driven by blade surface, design, and desired pitch.

I think your math is right but you could consider an alternative to new props - take your 18x21 to a good shop with the info you have and let them repitch. That would probably save a fair amount and you’d end up with balanced and polished props that’s are now matched to your boat and desired WOT.
 
I’ve always ignored the diameter. That’s deeper into prop science than I’m smart enough to understand and I believe mostly driven by blade surface, design, and desired pitch.

I think your math is right but you could consider an alternative to new props - take your 18x21 to a good shop with the info you have and let them repitch. That would probably save a fair amount and you’d end up with balanced and polished props that’s are now matched to your boat and desired WOT.
The current prop is 17 x 21. Thinking of moving to the 18 x 19.
 
@Golfman25 Thats the right math, 2" in pitch equals 1" in diameter and also 3 -> 4 blades. That math has been proven time and time again. So you should be good in doing htat.
It seems like the 17 inch prop didn't pick up the RPMs as it's still short.
 
It seems like the 17 inch prop didn't pick up the RPMs as it's still short.

Not sure what your saying? You changed the prop(s) and re-tested today?

It seems the original prop is 18x22 3 blade cupped.

http://www.clubsearay.com/index.php?threads/1998-370-sundancer-transmission.36602/#post-403269

Now if over propping is your issue and not a hidden engine issue, then dropping from 18d to 17d will do something to your top end. I would expect to see ~200 rpm increase. To expect more then that is unrealistic. You can be at a point where there are engine/weight issues and just over propped for your situation.

A rule of thumb is leave the diameter alone and change the pitch. Dropping an inch in pitch will often wake the engines up on older equipment. in some cases cruise the speed at lower RPM or faster at the same RPM. Dropping 2" in pitch is a lot and would expect to see the fore mentioned 400 Rpm if everything is running correctly. if not it could only be 100-200 increase or none.

Taking too big of a swing and seeing little to no change would indicate something else is going on. Boat weight has changed (possible water intrusion) and/or engine(s) getting tired etc. If factory 3 blade is 18x22 then 18x19 4 blade should be spot on.
 
Not sure what your saying? You changed the prop(s) and re-tested today?

It seems the original prop is 18x22 3 blade cupped.

http://www.clubsearay.com/index.php?threads/1998-370-sundancer-transmission.36602/#post-403269

Now if over propping is your issue and not a hidden engine issue, then dropping from 18d to 17d will do something to your top end. I would expect to see ~200 rpm increase. To expect more then that is unrealistic. You can be at a point where there are engine/weight issues and just over propped for your situation.

A rule of thumb is leave the diameter alone and change the pitch. Dropping an inch in pitch will often wake the engines up on older equipment. in some cases cruise the speed at lower RPM or faster at the same RPM. Dropping 2" in pitch is a lot and would expect to see the fore mentioned 400 Rpm if everything is running correctly. if not it could only be 100-200 increase or none.

Taking too big of a swing and seeing little to no change would indicate something else is going on. Boat weight has changed (possible water intrusion) and/or engine(s) getting tired etc. If factory 3 blade is 18x22 then 18x19 4 blade should be spot on.

So boat is new to me. Current prop is 4 blade 17 inch dia, 21 pitch. That was a surprise to me as I assumed a previous owner simply changed to 4 blade at the same size 18x21 as the stock prop -- and thus the slight loss of RPMs. But as you said, the proper 4 blade prop should be 18 x 19, as is the case with most owners on this site. I have no idea why someone went to a 17 inch prop and have no idea if the prop is still to the original specs. So I am thinking, start fresh with the props many owners have reported work 18 x19 4 blade.
 
So boat is new to me. Current prop is 4 blade 17 inch dia, 21 pitch. That was a surprise to me as I assumed a previous owner simply changed to 4 blade at the same size 18x21 as the stock prop -- and thus the slight loss of RPMs. But as you said, the proper 4 blade prop should be 18 x 19, as is the case with most owners on this site. I have no idea why someone went to a 17 inch prop and have no idea if the prop is still to the original specs. So I am thinking, start fresh with the props many owners have reported work 18 x19 4 blade.
Reading back through the thread, it looks like we know what they are stamped but we don't know if they have been changed/modified, so I would be having them checked/tuned and then make decision's from there.
If nothing else, you get a good prop baseline moving forward.
 
Reading back through the thread, it looks like we know what they are stamped but we don't know if they have been changed/modified, so I would be having them checked/tuned and then make decision's from there.
If nothing else, you get a good prop baseline moving forward.

@Golfman25 This is the only way to proceed. You need to find out where you are at before you can make a change. Especially since you are having issues that are unexplained.
 
Ok so reached out to a couple of prop shops for recomendations. One came back with 17 x 20 med cup 4 blade (which is one click down from what I have now 17x21). The other came back with 18 x 20 #5 cup 4 blade, similar to what has been recommended by those here on the forum. Online calculator came back with 18 x 19.5 (similar to what some users are reporting). I have found a pair of used 18 x19 4 blades for under $1000.

So thoughts.
1) Get current props checked, and change the pitch to 20 if need be.
2) Go with the 18 x 20 recomendations.
3) Grab the 18 x 19s, cause how can you go wrong.
4) Stop worrying, grab the sunscreen, some beer and enjoy your boat. Spend the money on a new stereo :)
 
Ok so reached out to a couple of prop shops for recomendations. One came back with 17 x 20 med cup 4 blade (which is one click down from what I have now 17x21). The other came back with 18 x 20 #5 cup 4 blade, similar to what has been recommended by those here on the forum. Online calculator came back with 18 x 19.5 (similar to what some users are reporting). I have found a pair of used 18 x19 4 blades for under $1000.

So thoughts.
1) Get current props checked, and change the pitch to 20 if need be.
2) Go with the 18 x 20 recomendations.
3) Grab the 18 x 19s, cause how can you go wrong.
4) Stop worrying, grab the sunscreen, some beer and enjoy your boat. Spend the money on a new stereo :)

I’d go with #1 which should leave some
Money for #4’s beer

I just like the idea of a pro taking your current state measurements and setting the prop to match.

I can understand buying another set as a spare but I’d still want a shop to check them before running
 
Ok so reached out to a couple of prop shops for recomendations. One came back with 17 x 20 med cup 4 blade (which is one click down from what I have now 17x21). The other came back with 18 x 20 #5 cup 4 blade, similar to what has been recommended by those here on the forum. Online calculator came back with 18 x 19.5 (similar to what some users are reporting). I have found a pair of used 18 x19 4 blades for under $1000.

So thoughts.
1) Get current props checked, and change the pitch to 20 if need be.
2) Go with the 18 x 20 recomendations.
3) Grab the 18 x 19s, cause how can you go wrong.
4) Stop worrying, grab the sunscreen, some beer and enjoy your boat. Spend the money on a new stereo :)
I would go with 1. 2. & 4.
 
So to tie up some loose ends here. Had the props removed and took them to get checked. They checked pretty good and are in fact 17 x 21 4 blades. They are from 2001 so 22 years old, Austrial props. In decent enough shape but do have some pitting. So talking with the shop they aren’t sure there is a lot of value in a pitch change as you’ll still have pitted 20 year old props. Did their calculations and came up with an 18 x 19 4 blade. So probably going with new 18 inch props keeping the old as an emergency pair.
 
I would go with 1. 2. & 4.
So to tie up some loose ends here. Had the props removed and took them to get checked. They checked pretty good and are in fact 17 x 21 4 blades. They are from 2001 so 22 years old, Austrial props. In decent enough shape but do have some pitting. So talking with the shop they aren’t sure there is a lot of value in a pitch change as you’ll still have pitted 20 year old props. Did their calculations and came up with an 18 x 19 4 blade. So probably going with new 18 inch props keeping the old as an emergency pair.

So much for my advise.
 
So the numbers are in. Took the new props for a short shakedown today. Acme 18 x 19
WOT was about 4100 RPM, Gps @31.5 mph. (vs. 3800 RPM, 28-29 mph)
Good cruise at 3000 RPM, Gps @ 22.8 mph (vs 3200 RPM, 22-23 mph)
Did 2600 RPM while still on plane, 15.1 Gps. (vs. 2900 RPM, 16.5 mph)

Didn't really push it, trim it a lot, or do several runs. All in all, I'm pretty happy. :)
 

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