Brand New 5.7 Vortec - Only 3000 RPM on water?

[HT32BSX115]

Re: Brand New 5.7 Vortec - Only 3000 RPM on water?

Question: Using the Prop Slip Calculator I see that I have like 9% - 12% slippage? does that sound right?

Prop slip for a single prop is usually around 12-15%. If it's lower, one of the parameters (speed or RPM) may be off a little, assuming your ratio and prop pitch is accurate.

I get about 7-8% with my Bravo III (dual props usually have a little more than half the slip of a single prop.

As the hydrodynamic drag increases the slip increases..... For example, If you tie your stern to the dock, and run the engine at static RPM (max RPM attainable when holding the boat), .....assuming you don't break the rope holding you or break the dock etc.....your slip will be exactly 100%.

If it were possible to to go the "theoretical" speed, the slip would be exactly zero. (which of course is not possible) http://www.go-fast.com/prop_slip.htm


Most large cruiser type boats have more slip than smaller sport type boats that are lighter with a lower "whetted" area on plane...

You can actually control the "whetted" area a little by trimming the drive and/or with tabs. If you trim the bow down, you push more hull area onto the water and increase drag.

Trim the bow up and you reduce drag. Tabs have a lesser effect and there's some interaction between the two. I can really slow my boat down by driving the bow down at top speed.....




Regards,


Rick

 

[QC]

Re: Brand New 5.7 Vortec - Only 3000 RPM on water?

Here's what I think . . .

- No you are not getting less than 12% slip and probably 15% is more reasonable.

- Don't do any more testing without
a) Fixing the throttle
b) Using your trim
c) Bringing your GPS

- Don't calc from past data . . .

- Don't stress about what pitch you end up with. If it's right, it is right. I don't care if you end up with a 9 inch or a 17 inch. What works, works.

 

[Maclin]

Re: Brand New 5.7 Vortec - Only 3000 RPM on water?

Stating the obvious and yet difficult-to-achieve here, but a single engine boat that big really needs a dual prop type drive...

Could be like QC says, it is what it is regarding prop pitch required.

 

[HT32BSX115]

Re: Brand New 5.7 Vortec - Only 3000 RPM on water?

Stating the obvious and yet difficult-to-achieve here, but a single engine boat that big really needs a dual prop type drive...

Could be like QC says, it is what it is regarding prop pitch required.

Before I installed the 454 Bravo III, I wouldn't have believed you.

I believe it now though.

My Liberator comes out of the hole more than twice as fast as it EVER did (even when new) with the Bravo III and basically the same HP.....(Actually a little less.... The 454 is rated at 330hp and the 460 was rated at 340hp)

 

[Mkos1980]

Re: Brand New 5.7 Vortec - Only 3000 RPM on water?

I'd add some more timing too. Isnt stock 8* Advanced? I'm running 10* with my Vortecs.

 

[waterone1@aol.com]

Re: Brand New 5.7 Vortec - Only 3000 RPM on water?

Wow, interesting thread ! I have to agree with many above posts : 1) put a shop tach on that motor and verify what your tach is saying. 2) That hull, with a 350/5.7L single, is NOT going to do 46 mph.....ever ! Before you drive yourself crazy and spend needless money, get back to the basics on why you purchased a 27 foot Sea Ray Sundancer.....it couldn't have been about top end speed ! Get your rpm's verified and post back if there is still a problem.