Need. More. Speed.

[Circles47]

Ok, I guess I need to explain the above statement. I have a 2004 18.5 Crestliner Sportfish, with a 125 HP Saltwater Merc on it. The engine is one of the 2+2's, and the hull is aluminum. It seems to me, that with 125 hp's, top end should be higher than 34 mph.
The prop is original, a Merc Quicksilver Black Max 13-1/4 x 17, right 3. Motor is rated for 5250 RPM.

I took it out today to check WOT Rpm's, and it's running about 5200-5250, and occasionally a spike higher (5400), and consistently 34 mph on speedometer and GPS.

We use this boat often for watersports, and the power is very sufficient to pull an adult skier out of the water quickly.

I'm planning on purchasing a S/S prop, and understand that it will deliver more power and speed than the current stock prop.

My question is this: Since my current power is sufficient, can I increase the pitch on a new S/S prop from 17 to 19, throwing all the "new" performance towards top end and still maintain my current power? If I understand correctly, each inch of pitch change equals 200 RPMs, so going from 17 to 19 would decrease my RPMS at WOT to 4800-4850. That seems like a bit too much WOT decrease if accurate.

Does anyone have any suggestions? Would be greatly appreciated!!

 

[steelespike]

Re: Need. More. Speed.

Any prop change can be unpredictable when you also change material it becomes even less predictable.
If you could find a dealer that will loan you props with the understanding you'll buy one it would make things bearable.
I deffinately wouldn't change pitch unless one of the gurus suggests it.
You presently have about 16% slip,a little high. A ss prop should improve the slip which could lower rpm but it should get the boat up better,increasing rpm.blades are thinner so should be easier to spin.blah,blah.
With my limited knowledge start with the same pitch.
There are guys here with real hands on experience so they may be able to suggest in more detail.

 

[pokman]

Re: Need. More. Speed.

In my opinion I would try a 19p ss and see where that puts you. I am no prop expert but I had a 94 125 2+2 on a 17ft rinker runabout. The weight of your boat and mine was probably similiar. I could run 41 with a 19p aluminum, and about 44 with a 21p aluminum. These are two different types of boats I know but I would think you should be able to see around 40.

 

[QC]

Re: Need. More. Speed.

Motor is rated for 5250 RPM.

Actually it's WOT range is 4750 - 5250. A 19 may in fact be a good prop for speed for you, but . . . most likely your hole shot will suffer.

 

[Circles47]

Re: Need. More. Speed.

Thanks alot for all the advice!

Actually it's WOT range is 4750 - 5250. A 19 may in fact be a good prop for speed for you, but . . . most likely your hole shot will suffer.

That's interesting that the range is actually 4750-5250......it has always seemed at WOT that the engine RPM's were plenty high. Now according to the tach, they're pushing the upper limit....would I be better off dropping those RPM's a tad at WOT?
Also, QT, in making the transition from aluminum to stainless, I should see an increase in hole shot and speed, right? Is there a chance with changing the pitch from 17 to 19, that the speed would maximize, and the hole shot wouldn't increase like it would have if I had went with a S/S 17 pitch prop? In other words, stay the same as it is now, which would actually be a decrease in hole shot with a S/S prop. Feels like I'm not making any sense!! :facepalm:

Also, I've seen some props somewhere with an 18 pitch. Any input on changing pitch by 1 instead of 2?

Since my first post I found my boat weight to be around 1500, with an estimated total weight of around 2700, gear, ppl, fuel, and everything else.

 

[QC]

Re: Need. More. Speed.

Common misconception. If you had two props, one stainless, and one aluminum, but otherwise identical. Your hole shot would actually get worse. What kinda carp is that guy spewin'?

There are two reasons that SS props are faster. One thinner blades, and two, less flex. The aluminum actually flexes and helps with hole shot. So going from a 17 AL to 19 SS, I am expecting some top speed increase, but a noticeable drop in hole shot (less quick).

 

[Circles47]

Re: Need. More. Speed.

Common misconception. If you had two props, one stainless, and one aluminum, but otherwise identical. Your hole shot would actually get worse. What kinda carp is that guy spewin'?

There are two reasons that SS props are faster. One thinner blades, and two, less flex. The aluminum actually flexes and helps with hole shot. So going from a 17 AL to 19 SS, I am expecting some to speed increase, but a noticeable drop in hole shot (less quick).

So would your recommendation be to stick with a 13-1/4 x 17 S/S?? Any guess on the projected hole shot and top end with one of these?

 

[fishndirk]

Re: Need. More. Speed.

Maybe I'm confused and you can set me straight QC but I love anything to do with physics and this convo is really sparking my interest. Wouldn't aluminum flexing during ecceleration be a slight decrease in pitch angle of the blades momentarily and therefore a slight decrease in the flow of water they are pushing meaning less hole shot. I'm sorry I'm not trying to ruffle any feathers its just my thinking was totally the oposite on this. Please tell me otherwise if I'm wrong on that

 

[QC]

Re: Need. More. Speed.

You are on the right track except for one thing. RPM goes up with the effective pitch change, so it's like a little mini downshift and you get a little more power to the screw.

 

[fishndirk]

Re: Need. More. Speed.

Thanks QC for pitching in, once again my appologies but with questions comes knowledge. So the bigger factor is the speed of the rotation vice the pitch angle? I would also suppose that piston displacement and gear ratio have a major play in this. I guess the overall bigger question is; what are the bigger factors that play a part in all this if there is even an answer to that?

 

[Circles47]

Re: Need. More. Speed.

With the information I've supplied, would anyone care to make a prop recommendation?

 

[QC]

Re: Need. More. Speed.

what are the bigger factors that play a part in all this if there is even an answer to that?

Voodoo

I'm kinda serious.

You're still over thinking the flex thing a little. It is simply a temporary lower pitch, so hole shot improves. Yes gear ratio, and displacement and ultimately power all matter, but we are comparing the same grouping of those things, but two different props. Usually the Al will have better hole shot. The SS will actually lower WOT RPM a little, but will deliver higher top speeds due to less flex (more efficient) and also thinner blades helping to compensate for the lower flex and maintain a little of the associated lost RPM.

 

[fishndirk]

Re: Need. More. Speed.

With the information I've supplied, would anyone care to make a prop recommendation?

I'm gonna say based off what you've provided, you would be ok to move to a slightly higher pitch prop if it is a good quality stainless steel one. Please understand though that the diference your talking about may only be a few mph. Any more than that would require a re power or boat hull change. The boat you have described is not like a high performance bass boat that can do 60mph easy. A ski boat that can do over 40mph is a hell of a good ski boat. You just gotta ask yourself what you want and what your willing to pay.

 

[fishndirk]

Re: Need. More. Speed.

Do this in case you really really wanna know. Take your WOT rpm (say 4700rpm) and divide it by your lower unit gear ratio (say 2.08). Multiply that by your propeller pitch in inches, that's inches per minute. Divide that number by 12, that's feet per minute. Multiply that number by 60, that's feet per hour. Divide that number by 5280, that's mph. Gear ratio is a big factor here there's no shortcuts. .02 could mean a few mph difference. The end number is the maximun speed for which you could ever expect your boat/motor combo to run at. It is impossible fr it to go any faster at those numbers. Your actual speed you can expect only goes down from there depending on boat weight, hull shape, weight distribushion and endless other factors. All of these factor into what is considered prop slip

 

[Circles47]

Re: Need. More. Speed.

Do this in case you really really wanna know. Take your WOT rpm (say 4700rpm) and divide it by your lower unit gear ratio (say 2.08). Multiply that by your propeller pitch in inches, that's inches per minute. Divide that number by 12, that's feet per minute. Multiply that number by 60, that's feet per hour. Divide that number by 5280, that's mph. Gear ratio is a big factor here there's no shortcuts. .02 could mean a few mph difference. The end number is the maximun speed for which you could ever expect your boat/motor combo to run at. It is impossible fr it to go any faster at those numbers. Your actual speed you can expect only goes down from there depending on boat weight, hull shape, weight distribushion and endless other factors. All of these factor into what is considered prop slip

Thanks for the input, fishndirk! I did the calculations you suggested, and ended up with 40.44 mph. I assume that 40.44 is as fast as the motor would go "unattached" to a boat?! I would love to get 40 mph out of it, and it seems like there should be a way to get closer to that. The boat itself doesn't seem all that heavy compared with the hp pushing it.

 

[Circles47]

Re: Need. More. Speed.

I see that Merc makes a Quicksilver Silvarado Vengeance S/S prop 13 x 18. My current prop is a aluminum 13-1/4 x 17. How would my performance be affected from 13-1/4 to 13 diameter? Would it be a better choice to go from 17 to 18, rather than 2 pitch sizes (19)?

Thanks again for all your consulting!

 

[Circles47]

Re: Need. More. Speed.

You presently have about 16% slip,a little high.

Sorry for all the posting...my brain keeps rolling this all around, trying to make an educated choice!

Your calculation of 16% slip was dead on. How would I go about reducing that slip? Does increasing prop diameter reduce slip??

 

[QC]

Re: Need. More. Speed.

I am seeing a 2.07 ratio and getting 41 MPH theoretical, so we are all close.

I assume that 40.44 is as fast as the motor would go "unattached" to a boat?!

To help with clarity, I would say "as fast as the propeller would go if it unattached to anything". This might help explain that load on the propeller, any load, will result in some level of "slip". And your case I think 12 - 15% may be as good as it will get because she is not fast enough to get most of the boat out of the water where sub 10% slip numbers come from.

So if 12% slip is in the cards (not a guarantee by any means), then a target speed potential of 36 may be achievable. It's just going to be about getting the right prop to match your speed and load and that's where my Voodoo comment comes in :facepalm: Could be a 4 blade 18 that could get it done, or a 3 blade 17 or 19. That's the challenge. And I would not venture to guess at this point. My problem with the 19 is that you are going to take two hits to hole shot: 1) SS and 2) 2". Both will negatively affect hole shot, while potentially helping with top speed. This is why I think a good 17 or 18 would be best. With the 18 being a 4 blade you could get a triple combo working against hole shot in that example: 1) SS, 2) 1" and 3) 4 blade. No way to really know without trying, and that's why steelespike suggested working with a good dealer to try a few.

 

[fishndirk]

Re: Need. More. Speed.

Good call QC. I would also bet that if the theoretical speed is right around 40mph than you couldn't expect the actual speed to be more than about 37mph with that style of hull. Deep V's always have more boat in the water. Instead of getting on top of it they have more an effect of pushing the water out of the way. This tends to slow things down. Smoother ride but more water resistance.

 

[Circles47]

Re: Need. More. Speed.

OK, based on the recommendations of those more knowledgeable......I know, I know.....you accept no responsibility... , I ordered a Turning Point S/S propeller, 13-1/4 x 17, same as my current. I'm guessing the hole shot won't change enough to be real noticeable, and the top end will only increase 2-3 mph. I'll let you know how she turns out. Thanks again!