Rapido 5.7 Break In Prop Question

[wil7483]

Hey. I have a post in the Mercruiser Sterndrive forum about my new Rapido 5.7 SBC I just put in my Wellcraft Elite 222 XL. I took her out today to the ocean and put her thru the paces and wanted to get some opinions. Previously I had a 15p prop on it but I never did a wot run so I don't have any numbers. Today however I put my 17p on it and these are the numbers I were able to get. Mild chop at WOT I was able to get to 4300 rpm's and 43.6mph on the gps once I found the sweet spot on the trim. The boat weighs about 3900 lbs empty. Factor in the weight of the 62 gallons of fuel, grandson (24 lbs), daughter (120 lbs), and myself (270 lbs), plus gear (100 lbs), I'd say we were pushing about 4700 lbs give or take a few. Are these numbers respectable or should I expect more. I have a 19p prop on order and should be here soon. Doe's anyone think it will make any difference as far as speed, performance, etc. Since I am within my WOT range for this engine I am thinking that I am good but just want to hear what you experts think. I had a blast today personally speaking. Kind of think I wasted the money on the 19p but I can always have a spare.

Sorry for the video. Daughter waas the camerawomen and she couldn't figure it out.

 

[Tail_Gunner]

Re: Rapido 5.7 Break In Prop Question

Your probably going to need to down in pitch, While i do not know the rpm range for your motor i would assume that it would be 46-4800 rpm. If that is the case run the 15, you do not want to lug a new motor. 4300@ wot is to low...And along with that once she gets 50 hours on it it will or might lossen up a bit. But a 19 is way to much

Do you know the gearing on the leg..that would be helpful to run some slip numbers.

 

[wil7483]

Re: Rapido 5.7 Break In Prop Question

Your probably going to need to down in pitch, While i do not know the rpm range for your motor i would assume that it would be 46-4800 rpm. If that is the case run the 15, you do not want to lug a new motor. 4300@ wot is to low...And along with that once she gets 50 hours on it it will or might lossen up a bit. But a 19 is way to much

Do you know the gearing on the leg..that would be helpful to run some slip numbers.

1:50 ALPHA

 

[Tail_Gunner]

Re: Rapido 5.7 Break In Prop Question

Thats 7% remarkalble,Is the GPS speed and is your tach on within 100 rpm.. I know you have put a ton of nice work into that boat...but 7 is maybe a bit to good for any i/o...Your are not running a 4 blade by chance...and is the prop ss or alum??

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

Here go have some fun..somethings a miss dropping to a 15 give's you the exact same speed @ 4800

 

[wil7483]

Re: Rapido 5.7 Break In Prop Question

Ok, I took the boat to the lake today and tried all three of my props again. 1st I checked the timing and found that it was at 12 degrees BTDC so I adjusted it until it read 8 degrees BTDC. Then I took off across the lake a few times at WOT playing with the trim settings all the while. The 17 was on the boat and I was able to make the boat porpoise with it and the rpms actually got up to 4700. The speed was consistent with previous outings however. Top speed on the 17 with the trim set for the rpms to spin at 4500 was 39.3 mph. The lake was rough today with high winds. I turned to the 19 and went back out. The boat really do not want to turn that 19. RPM's only got to 4300 and that was with it fully trimmed out and the prop not biting the water. After adjusting the trim the rpms dropped to 4100 and the boats top speed was still only slightly better than the 17 at 41.6 mph on the gps. Lastly, I put the 15 on and was able to actually spin it up to 5000 rpms with her trimmed all the way out but the speed on the 15 was lousy. The best top speed I was able to obtain was 35.6 mph with the trim in the spot where it sounded and felt the best. So I am still dead set on the 17. It is within the rpm range, decent hole shot, and speed is acceptable for my purposes. Too bad I bent it up backing out after beaching the boat. Hit something underwater and when I took the boat out of the water the prop was really beat up. The price to repair is only slightly better than a new one. Any suggestions?

Yes of course... Get your self a stilletto bay pro 2 16 pitch. I can see from that first pic you have a lot of thrust, but you prop is a 3 blade alum..id bet a few dollar's on that.

Yes it was a 3 blade aluminum up until I destroyed it yesterday. Probably not much benefit in repairing it as opposed to just buying a new one cost wise.

A good 4 blade will lift that stern heavy boat you have up a few inches making the ride far more "Wateralbe"..smooother and probably faster. You have a lot of azz... err boat draggin there.

So since in my opinion the 17 is spinning better than the 19 you suggest a 16
as opposed to an 18p 4 blade? Why go down and not up?

That ignition is it electronic... dist that is.

It is a Thunderbolt IV HEI system, whatever that means. I am still trying to figure all of that out.

 

[sschefer]

Re: Rapido 5.7 Break In Prop Question

Wil7483 - When you go from a 3 to 4 blade prop you drop down an inch in pitch. The extra blade makes up the difference. 4 blade props are just now begining to rise in popularity. They've always been known as good for eliminating vibration problems and for their fuel efficency. The problem in the past has been folks not dropping down that inch and then complaining about poor 4 blade performance. You should and can get equivelant top end, decent hole shots and better fuel economy from a 4 blade prop.

It is a Thunderbolt IV HEI system, whatever that means. I am still trying to figure all of that out.

HEI -- High Energy (40,000v) transitor triggered pointless Ignition system. Timing 8 deg BTDC with stock weights springs and cam. May have vacumm advance or centrifigal or both.

This is still the preferred ignition system for single coil (coil is on top of the the distributor cap) non computer controlled engines. The only thing that they don't do well is handle heat. The transistor packs can breakdown when exposed to long periods of high temperatures. The solution to that is the aftermarket trans pack. Rotors and distributor caps should be changed frequently for top performance but will easily go 40,000 miles in a standard performance vehicle. Plug wires need to be 8mm or better. Plug wires 5 and 7 should be crossed on the 350 chevy (firing order 18436572 (note 5 and 7 are side by side and fire in succession) ) to prevent cross fire.

 

[Tail_Gunner]

Re: Rapido 5.7 Break In Prop Question

As above...Here's a fairly common rule

15@5000 3blade alum
drop 200 for ss 4800

15p ss 3blade should net 4800

Up a pitch..200 loss

16p ss 4600

drop 100 for the 4th blade

16p ss 4 blade 4500

Before you go any futher and i do not know if you have done so already. Make sure you have done the timinng correctly..Aka i do not know to much about that ignition so go back and state the exact type of ignition....clearly post your prodedure and ask Don if it has been done correctly. He is a great guy..he just does not guess and has a uncanny ability to diagnose a motor...pretty good sense of humor to if he ever let's it show...


http://www.boatfix.com/merc/Techbk/95/95HGB4.PDF

I might be pounding on this timing thing a little or the state of tune your boat is in... Mkos has a boat very similar to yours and it used to do 50..does 60 now....And you have a fresh 5.7 with a 4 barrel carb..no slouch. Good luck...Solas make a fine 4 blade alum much cheaper that a stilletto bay pro 2.

Stickly a personal opinion here, i would prop the motor to 4600- 4800...your not going to spend to much time there anyway but you would have sharp and quick response at normal operating speed's...And that would be a solas product alum prop...aka 16p ss up two hundred for alum thats 4700...a good place to be.

O