different kind of prop question

[Peels]

OMG there is too much kaka on the web about this topic. so lets add to it LOL

anyways, we had two nice days to get out and use it after buying in the fall finally! LOVE IT! spent all day polishing the gelcoat. looks brand new!

to the question...when we bought our boat. 2004 glastron sx175, 3liter merc.
it came with a "spare" prop, which is actually the stock one. it is 14.5" 19p.

On it in its place, is a Michigan wheel company "rapture" SS prop. it is 14.25 19p.

My question is, what did the previous owner "gain" by doing this?

To me... (new to boats) it feels faster outta the hole than Id expect. but it maxes out at 36mph. And thats at a sh*tload of RPM..... would I add speed and kill acceleration by slappin the stocker on there? In my noob head, a larger diameter prop would mean more drag but more thrust.

it feels "lurchy" outta the gate too...and when docking.

these arent really complaints, as I am new to this...I really don't care mostly...

just information gathering here. As I pick up experience, i'm sure it wont matter much....

 

[nathan]

if you are not at max rpms then you can go smaller but you will reach the max rpms quicker. if you go bigger it may hurt the hole shot but it should give you more MPH but you do not want to go too high and not turn enough rpms

 

[Peels]

thanks for the explanation.

makes sense. feel like the analogy of putting smaller tires on your car is somewhat similar.

works a little less hard at low rpm, with less mass to spin, so less resistance.

If Im bored one day ill slap on the stocker and see what happens.

just that the SS looks cooler-which is also important. LOL

 

[Peels]

also....

my apologies for slapping this in the wrong subforum

I didnt even see the correct one in the technical section...


<N00B>

 

[Tablerocker]

Prop selection can indeed be a frustrating and near 'black magic' type of thing sometimes. SO many different factors. I've introduced more people than I can easily remember to boating over the past 30+ years, and...sigh...usually got to help them with their problems.
Diameter, pitch, cup, number of blades, material, type of hull, outdrive or outboard 'X' dimension...it can be crazy. I remember one boat in particular, a 20' Mariah that a friend of mine had bought, it had this god-awful 15" diameter, 19" pitch aluminum prop that I'd never seen the make or type before, it was truly hideous. And it made that boat frickin' DANGEROUS to drive, as any sort of moderate left turn would cause the boat to literally heel over onto its side! Like, up on about a 50 degree angle, causing a blowout and total loss of control. We IMMEDIATELY headed back to the trailer and went to the local marina and they were gracious enough to give us a Quicksilver aluminum prop in 21" pitch to try. It was 500% better right out of the box, but the engine would only pull about 4000 rpm (260 hp Mercruiser 350). Eventually he went to a 19" pitch Quicksilver and had much improved overall results.

The point of my rambling is, you need to find a friendly marina or boat dealer that has a good prop selection and try several of them, do this on the same day, same fuel and boat load, etc. Note things like you have already, you're looking for things like cavitation, where the prop just won't 'bite', looking for transom lift of each prop, and watching the engine rpm to where you can hit WOT limits. Talk to people with similar rigs as yours, see what they are using. A lot of props these days are 'tunable', with vent ports you can customize to your best result.
Once you've found a good prop, consider whether the testing conditions will be what you normally boat at, keeping in mind that in reality you'll have several hundred more pounds of gear, fuel, people, etc on board during normal use, and that this use may be fishing one day and skiing the next, calling for a couple of option props to have.
Finally, if you do your testing when it is nice and cool and the air is dense and full of oxygen (and thus horsepower), you'll need to remember that you'll experience a far 'doggier' response in the middle of the summer.

 

[Peels]

well, to finish this thread, I swapped them, and took the boat out...

the results of stock prop vs the aftermarktet on it:

aftermarket was probably 15 lbs heavier to start with it is stainless steel...

FULL boat i gained 6 miles per hour, and revs out to 4100 rpm nicely before it would, but it felt like being in too low a gear..... a smidge laggier out the hole(not really noticeable), but its a 3.0... ANNNNND we pulled a skiier out with 7 people in the boat.


so yay.

on a more disturbing note(not related) this was the first time I have pulled a skiier, having only done it myself 15 yrs ago failing miserably... He does great...i go for my turn, realize we are in UNMARKED shallow water.

I get up on the skiis, then fall.....the prop was in the mud! my wife thought she broke it...

SCARY! all is well, lesson learned. Coralville Lake we go to often, is WAAAAAAAAY down.
I saw boats down there....but when we pulled up to them....they were both seadoos. LOLOLOL