TMI - 4 or 5 Blade - 575 SCi

VP · 05-23-2005, 02:28 PM

Ok - read a bunch of threads and now I am really confused.

I have a 2001 single step, 29 Ft Fountain with a 575 SCi ----- 1.5 XR Drive -- stock 30" Bravo 1 prop. --- I just freshened the engine.

It runs over 80 mph (on GPS) any time I want to, runs 82 most any time I want, and 84 in just the right conditions after long WOT time getting there.

(I increased the boost from a stock 4-1/2 psi to ~ 6 psi to do this, with No intercooler)

Last year I was pushing the rev limiter (tach reads 5400 rpm), but now after freshen job, with a full tank of fuel (100 gals), on fairly calm, cold (48 degrees) fresh water, I an right into the rev limit, and only running 80 - 81 mph on GPS.

I am assuming I increased power with the freshening --- I also assume the full load of fuel, plus the cold water is creating some extra drag and slip (i think) therefore the lower speeds.

I believe the peak hp (where i want to run at) is at 5100 to 5200 rpm ------- so I think I need to re-prop to get the rpm's down, the prop slip down, and my speed back up.

I was thinking a lab'd 32" B1 would be the ticket but the Hydro P-5 is being talked about a lot -- NEED HELP !!!

throttleup · 05-23-2005, 03:28 PM

We have found that the P5X works very well on the 29 Fountain. We worked with the Xtreme F1 Fountain and found the stock P5X ran the same top end as the lab Hydro Quad IV which was used for racing. The Quad IV was used to set 2 kilo records in Factory 1 on the same boat.

Your slip is higher than it should be. The 5 blade may bring it a little closer to where it should be. I think you are still going to need a spacer to lower the prop shaft a little deeper. You should be able to run the speeds you are running now with a 28 Bravo 1.

Matt

VP · 05-23-2005, 03:34 PM

But shouldn't I be expecting higher speeds out of a prop change if I am over-rev'ing now, and slipping too much ?

throttleup · 05-23-2005, 05:43 PM

You will get your best top end speed when the prop slip is at it's most efficient. Too much or too little prop slip is either over or under loading the blades on the prop resulting in lost efficiency.

In your case, with excessive prop slip, the boat will go slower with a higher pitch prop with a lot of slip verses a lower pitch prop with less slip. By lowering the drive, in your case, you may find additional speed, both top end and cruise.

Matt

VP · 05-23-2005, 09:05 PM

I understand what you are saying now -- that kinda sucks that my drive may be too high -- it is in it's stock location right from the Fountain factory -- i can't believe they would be that aggressive with the x-dimension on a production boat -

I just loaded my boat to take testing at another lake tomorrow - friend of mine has a pair of stock 32" B1's on his 42' -- I am going to try them for the hell of it to just see what happens --

When i went to load it, it ran right up to 5400 rpm at 82 mph without any effort at all -- on flat water too --

i could hear the engine running against the rev limiter -- wasn't even WOT yet - i noticed i was at 7 psi boost too - that is where i am getting the extra power verses last year a 6+

If the weather holds off and i get some testing in, i will let you know the results.

throttleup · 05-24-2005, 08:08 AM

It's not unusual for a boat to have the drive mounted too high. Better too high than too deep. Putting a spacer in is no big deal and doesn't take long. If you run into the rev limiter that easily at 80-81 then a spacer is almost needed for sure.

Matt

VP · 05-24-2005, 07:26 PM

Ok Matt -- here is a little bit of data from testing today ---

55 degree ambient temp ---- 12" + chop --- full (110 gals) fuel

1. Baseline run with my stock 30" B1 RH prop.= 5450 rpm 82.0 mph -- rev limiter kicking in (all GPS readings - all trim the same on all runs)

2. stock 32" RH B1 prop. = 5150 rpm -- 80.2 mph

3. stock 32" LH B1 prop. = 5150 rpm -- 79.8 mph

4. back to stock 30" B1 RH = 5450 81.9 mph -- rev limiter kicking in

throttleup · 05-25-2005, 07:51 AM

Your slip numbers are high, too high. I would work on reducing the slip before working with props. When you can run the 30 Bravo 1 at 82@5100 rpm, or so, then the drive height is right. Then it would be time to work props.

I would expect an increase of top end speed when the drive is lowered and the slip reduced. I would recommend a spacer of 1" to start with and see what your numbers are.

How does the boat get on plane? Is there a rooster tail at top speed? How much trim do you have to put into the drive to get top speed?

Matt

bbladesprops · 05-25-2005, 11:35 AM

Fountain has continued to raise their drive heights over the past half dozen years. Always looking for more top end.

I agree with everything Matt has been telling you. Your slip is too high. You're better off lowering the drive first to decrease the slip%, then work with the props.

Going to 5 blade props right now would be a band aid for the actual set up issue.

VP · 05-25-2005, 03:09 PM

Ok -- to answer the questions ---

How does the boat get on plane? ------ it requires 3/4 throttle - full "in" on the trim - No tabs needed - Zero blowout 95% of the time, or cavitation or whatever the real term is - not real fast coming up but I don't use full throttle either unless 6 people in boat.

Is there a rooster tail at top speed? ------- No rooster tail -- i can make one but when i get it fine tuned to get max speed, there is only about a 12" high max rooster tail, which shoots off toward the starboard direction a little.

How much trim do you have to put into the drive to get top speed? --------- well my indicator is reading about 3-3/4 when i seem to get max speed - my spacer in the slot in the drive, where the through-shaft goes for the rams, in located on the inside (towards the transom) so the drive is pre-positioned in verses out - and i can run the drive out to 5 without loosing a lot of speed.

so --- what do the two of you think now ?? does it sound like the drive is still to high ???