Not fast enough??

[fourwinnsdook]

I have a 17ft 86 four winns fish and ski with a 4.3 OMC, and the prop is a SS 14.5x21pitch. I am at 43MPH at about 4,400 rpm's should i be getting more out of it? It has to be trimmed good to get over 40mph. if it's not trimmed right it will knock 5MPH off in a hurry. A friend of mine has a 2006 18.6 chapparall with the same motor but runs at 60MPH (so he sais) Why is he so much faster? I know the newer 4.3's have more HP, but that's a big diff.

 

[steelespike]

Re: Not fast enough??

Not an expert but my guess is he isn't doing a true 60 if it isn't on a gps it could be off + or - 10% or more.Also without hp. rpm and prop demensions
it difficult to do a comparison hull design and weight can make a big difference in speed.It is normal to trim out for best performance.
I don't know the 4.3 reccomended max rpm but I don't think 4,400 is excessively low.Someone should be along with more info.
In the mean time you might do a compression check and check to see that the throttle plates are opening all the way.

 

[fourwinnsdook]

Re: Not fast enough??

from what i've read the max RPM for the 4.3 is around 4,800. Maybe a little less for one of my year. The rpms i think are pretty good. i'd rather not go much higher than that.Maybe just get a little more top end out of it. Maybe a slight prop adjust. I've also checked the throttle, and it is wide open. i'm not unhappy with the speed of mine, just in comparison with his (which is the same style) boat and probably heavier mine's pretty slow.

 

[SuperNova]

Re: Not fast enough??

I would venture that your friend is probably exaggerating a little bit, or he has an inaccurate speedo. He may have you by 6 or 7 mph if he's lucky, but there is NO way he's running 60mph. You would need to turn a 23" pitch prop about 4600 engine rpm to even come close to that kind of top end with 1.5/1 gears, which he doesn't have. He has 1.62/1 and he's probably running a 19" pitch prop. With the 1.62's you'd have to spin 5600 engine rpm with a 23 to hit 60mph. So don't sweat it--challenge him to a race.

 

[Zeusdafreak1]

Re: Not fast enough??

Supernova,
Could you explain a bit of your math there on how you come up with some of that. I am just looking for a bit of "cowboy logic" as to how to calculate it? It is interesting to us newbies to prop management.

 

[Tail_Gunner]

Re: Not fast enough??

http://www.go-fast.com/Prop_Slip_Calculator.htm

Now that is just a fairly good guide for your current performance it will not predict future performance

Fourwinns top speed would probably come from a Enteria in a 21 which would probably boost your rpm up to 4600 .......Crosse's finger's...... about 500 big ones

 

[Zeusdafreak1]

Re: Not fast enough??

Thanks.

 

[SuperNova]

Re: Not fast enough??

I don't really know, Zeus. I guess from experience. I've been around boats since I was born. I rebuilt a lot of them in the mid 70's to late 80's with my grandfather who'd been doing it since he got off the P.T. Boats after W.W.II and then I did a lot of high performance buildups in the 90's and experimented with props extensively because I found that most of the common theories and common knowledge weren't very accurate or reliable and I am a firsthand knowledge kind of guy. I wanted to find out for myself what worked and what didn't. Sometimes I am right and sometimes I am wrong, but I always learn something new.
--
Stan

 

[Zeusdafreak1]

Re: Not fast enough??

The link that Tail gunner supplied answered all my questions anyway. I am trying to build my own first hand knowledge but it sure helps to have a resource like this one.

 

[fourwinnsdook]

Re: Not fast enough??

Enteria?

 

[fourwinnsdook]

Re: Not fast enough??

What do you guys think a 14-1/4X19p 4 blade SS would do for me? better take off, and about the same top speed? I do alot of pulling skiers, so i guess the top speed isn't toooooo important, but right now i'm having a hard time staying on a plane under 20mph which is where i like to wake board at.

 

[Tail_Gunner]

Re: Not fast enough??

1. 18 pitch 4 blade stilletto baypro 11 smooth and lots of grunt quicker and easier plaining.
2. 19 stilletto advantge speed and good hole shot
3. 20 pitch merc enteria better than advantage in all catagories.

 

[SuperNova]

Re: Not fast enough??

The minimum speed a hull will plane at is more a function of hull design and weight distribution than prop selection, although sometimes trim tabs can help. And if you run some basic tests I think you'll find that prop calculator, while an interesting toy, is not super accurate. There are a lot more variables that influence speed and slip than that model fairly represents.
--
Stan

 

[Tail_Gunner]

Re: Not fast enough??

The minimum speed a hull will plane at is more a function of hull design and weight distribution than prop selection, although sometimes trim tabs can help. And if you run some basic tests I think you'll find that prop calculator, while an interesting toy, is not super accurate. There are a lot more variables that influence speed and slip than that model fairly represents.
--
Stan

So if one put's a 4 blade on a high hp boat with lot's of stern lift and cup , are you saying that wont influnce how the boat react's to the lift................ hmmm now if you put the good old smart tab's on a 20' foot boat are you saying it wont launch like a 12' skiff............

 

[SuperNova]

Re: Not fast enough??

You're right, T.G. I didn't say the prop had nothing to do with it, just not as much as people sometimes would like to believe, I think.