new boat owner prop?

[kScrown]

Great website! But ive read alot of info about props and im confused? So i just purchased the boat in my sig a month ago (first boat)also forgot to add its 190hp. I was at the lake to day, third time out and heres the story.
Boat has a 3 blade 14.25x21p prop, aluminum, stock im guessing.
At WOT boat does 52mph @4150rpms trimmed up and handles well. Thats with 3 people and full tank of gas.
Long story short i chipped a blade today and am wondering what to buy for my next prop. I will probably have a boat load of people and sking and tubing most of the summer. When i bought the boat the dealer recommended a 3 blade 19p for a better hole shot but ive never pulled a skier so i dont know how the 21p will respond. Also i here everyone talking about their 4.3v6s turning 4800rpms at WOT.
Is my tach wrong? 4150rpms @WOT
Is my speedo off? 52mph
With my rpms where they are if i go to a 19p that should put me at 4500rpms and may not lose any speed right or will i lose a bunch of top speed? Any advice is very much appreciated!!!

 

[NHGuy]

Re: new boat owner prop?

Sounds like something's incorrect. My bet is the tach is wrong. Reason I say that is using a prop slip calculator I found that a 21 pitch prop couldn't do that speed.
Look on the back of your tach, there will be a dial with positions for 4, 6, & 8 cylinder engines. Set it to 6. Then tell us your results.
Someone who knows the math could tell you without another water test, would just need to know what setting the tach was on during the 52 mph test.
You can also confirm your speed more precisely if you test it with a GPS.
Lastly, since the boat normally comes with a 19 pitch prop you may have a correct tach reading and an ambitious speedometer. In this case the engine is lugging. That is meaning it can't make recommended RPM at wide open throttle. Using a prop that's too tall will yield max speed with a light load, so use it when you want top speed and will have a light load.

AND, if the engine speed is 4150 at WOT go down to a 19 pitch, it will improve your holeshot, might actually improve top speed, but most importantly it will allow the engine to go to it's rated RPM.

 

[kScrown]

Re: new boat owner prop?

So tachometer is set to 3pulse 6cyclinder. I believe this is correct. The engine doesnt sound like its lugging but after a cruising around the lake and then shutting it off, the engine "desiels down". So maybe it is only doing 4100rpms and lugging if the engine is dieseling down on shut off. I cant check with GPS as my current prop is ruined.

 

[steelespike]

Re: new boat owner prop?

A lightly loaded 4.3 isn't likely to deisel from running at 4100 rpm.More likely poor timing ,to high an idle, wrong plugs etc.It can't make 52 at 4100 but 52 or more is a likely speed of a 4.3 in an 18ft.
So I guess you have to prove one of the gauges wrong.Sometimes if you rotate the tach switch a few times it will work better.
Your prop may be dinged up but you can likely get it repaired at a prop shop.

 

[kScrown]

Re: new boat owner prop?

Yeah ur right about the dieseling down, i COMPLETELY forgot that i need to adjust the idle screw, it idles too fast. Good call spike! So anyway lets just say that it is running 4100 at WOT, should i go down to a 19p to get the rpms up? The only thing that bothers me is that if the tach is wrong and i go to a 19p i could potentially be overrevving or if i stay with the 21p i will be lugging and totatlly suck with a boat load of people. I guess im just confused on which direction to go. Thanks for the input so far! One other question can aluminum props be repaired?

 

[Silvertip]

Re: new boat owner prop?

Step 1 is to check the timing and adjust idle speed where it is supposed to be. You also need to let the engine idle for a couple minutes before shutting down to prevent dieseling. Then you need to verify the tach. Your numbers don't compute so there is no point changing props until you determine whether speed is wrong, tach is wrong, or the gear ratio in the lower unit has been altered from stock.

 

[kScrown]

Re: new boat owner prop?

Ok sorry for continuing to bump my post but i was looking around this evening at the boat and i seen that the gear ratio of my outdrive is a 1.43, seems high geared doesnt it?

 

[steelespike]

Re: new boat owner prop?

Yes according to my info 1.43 is for a 5.7 280 and 320.Yours woud be 1.66.
While you should confirm your tach; with that gear ratio you will likely need to go to the 19 it could add 3 to 400 rpm and get you closer to the 4600 max rating.Also will improve hole shot which it probably needs with a 1.43 ratio.

 

[kScrown]

Re: new boat owner prop?

Ok sorry to bring up my old thread but i bought a new ss 14.25 x 19p 3 blade stiletto and gave it a run today and this is what i come up with: 47 mph (gps)and speedo @ 3800 rpms.. So by going from a 14.25 x 21 aluminum to a 14.25 x 19p ss ive lost 5 mph and 300 rpms. Im lost and dont know what to try next?????

 

[Mac525]

Re: new boat owner prop?

it seems like you original prop has been repitched and or mismarked also you need to verify the tach (some timing guns have rpm display)