Top speed 252 with 502

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thunder550
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Top speed 252 with 502

Post by thunder550 »

Watching Fozzy's videos inspired me to post this.... :lol:

During the purchase process for my boat I had a few people make comments hinting that I should be able to get low-to-mid 60's out of it. 1995 252, 502 Mag, Bravo 1 with the factory 3 blade 14-5/8 23 pitch prop. At wide open I run about 4800 RPM, both the in-dash speedo and GPS on my phone read about 55 mph. Our elevation here is about 1200ft and it's been warm when I've opened it up, above 90 degrees ambient temp. I haven't been in perfectly smooth water to where I can trim it all the way up and make it levitate like Fozzy's boat does, but I can't see that making a 5+ mph difference. Would elevation, temp, and trim make up that much? In my dad's 1998 252 502 with the B3 and factory props I think I remember getting about 62 out of it. I feel like I'm missing a few MPH but not sure. What's yall's opinions on this? Normal? Time to strap on a couple of JATOs or just eat more broccoli?
1995 Cobalt 252 - 502 Mag EFI - Bravo 1 - 21" Revolution 4
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cmattj
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Re: Top speed 252 with 502

Post by cmattj »

Density altitude does play a role as you are down on power there..

Your true elevation with a low barometer and low water grains has you at a 3500 feet true elevation when calculated most likely.

your prop set up is good but seems to have a bit of slippage with the numbers you give.

Check your outdrive to see if it's a 1.5 ratio for 1. Maybe it's a old school elevation ratio 1.65?

You may want to check props for nicks aswell..
my 502 in my Donzi with a 3 blade mirage " plus" 25 pitch would tap the limiter at 5050 rpm. @ 71mph

You should be 62-65 at sea level

Still flying good though!!
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Re: Top speed 252 with 502

Post by thunder550 »

I forgot about DA calculations. Just looked up actual elevation and some weather history for the last day I really opened it up:
DA.jpg
I'll have to check drive ratio next time I'm in the boat. I've wondered about the prop as well, it's not beat up by any means but looks like it may have run through some mud or sand at some point, there's some light scuffing on the blade surface and I just filed a small nick off one of the edges (can't remember for sure but I think it was a trailing edge) last week. I need to get a spare anyway...might be time to buy a new one and keep the current one as a spare. Last week while we were at the lake I had a couple moments pulling a tube, making tight turns where I got what I believe to be prop ventilation. I chalked it up to the way I was driving, maybe another sign of something?

Hard to tell in these pics but some of the light scuffing is visible if you look close. I'll try to get some better pics.
IMG_20170903_102537.jpg
IMG_20170903_102531.jpg
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Re: Top speed 252 with 502

Post by thunder550 »

Another prop pic...
prop.jpg
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Re: Top speed 252 with 502

Post by thunder550 »

Hmmm well after looking at prices for new SS props...maybe I'll take this one in and have it worked over, then order an aluminum one as an emergency spare :lol:
1995 Cobalt 252 - 502 Mag EFI - Bravo 1 - 21" Revolution 4
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Re: Top speed 252 with 502

Post by AsLan7 »

.
Thunder550.
Under ideal conditions, that's a good 60-62mph hull you got there with your 415 horse Merc & 23" pitch prop.

Your prop condition will cost you 1-2 mph.
Your Density altitude calc. Will cost you 3-4 mph.
Your described water conditions will cost you 1-2 mph.

That's 5 mph on the low end and 8 mph on the top end.

If you are currently hitting 55, then that makes sense based on your current weather and set up.

Our 24' CS Fountain with your 502 Mag was a good 76-78 mph boat on gps with 27" four-blade chopper prop at 1,000 D.A. Granted it was a 13 degree dead-rise with a ventilated running pad but your hull is still capable of speeds in the 60's. 8-)
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Re: Top speed 252 with 502

Post by thunder550 »

Thanks for that analysis!

Will my 4800 RPM number come up as well as those other conditions improve? Meaning...do the conditions you listed above drive the max RPM number, or is that an unrelated thing? Seems that others are seeing 5000-5200 RPM out of their 502's, I've been wondering why I seem to max out at 4800.
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Re: Top speed 252 with 502

Post by cmattj »

Prop looks good in the pics.. sure every nick can cause slight lack of bite but that doesn't look bad at all. And 3 blades are slippy anyway but tend to be faster in WOT so maybe just have it checked. It does calculate to about 20% percent slippage if it a 1.5 ratio.

And no worries on the turning.. the prop was probably just lifting higher in turns and airing out . My twins will do that as she leans over trimmed for straight driving.

Yes.. at 4700 feet she wont find that 5000 -5100 so I don't thinks shes tired or slow. That is actually strong numbers for that type of air you were running in. Nice set up!
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Re: Top speed 252 with 502

Post by thunder550 »

I'm going to put prop refinishing on my winter list along with stripping and painting the drive (and new stickers...gotta get that last 10-15 hp out of her :lol: ).

You guys are awesome, thanks for sharing some of your knowledge :D
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Re: Top speed 252 with 502

Post by thunder550 »

Did some searching on prop slip numbers. I'm assuming 12% isn't achievable with a bigger boat like this but hey it's good to have goals right?
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Re: Top speed 252 with 502

Post by Big Block Power »

Ok my 2 cents. I would think that engine should turn 5000-5200 rpm at What ever altitude your at if your propped correctly. Does it say anywhere on the engine covers what your specs are? Otherwise I'll look later. How do you use your boat? Water sports or cruising and top end.
If you don't mind losing some top end and want a killer hole shot and midrange switch to a revolution 4 prop. Prob around a 19 pitch. It's a great prop but you will lose a few mph with a 4 blade prop but it won't cavitate on turns it will hold. Great for water sports. Ok another great prop is a Enertia. Very efficient prop. You may want to go down a pitch to get your max rpm. Don't buy a aluminum. Even if you get your prop reworked it will change how it performs unless they are really good. They can tweak yours if they are good. I think your leaving a bit on the table. The mirage plus is a good prop. The cooler it is the faster she will run get some chop under the boat she will run. Last weekend At 60 degrees and a good two foot chop my boat ran the fastest it ever did. My hull isn't known for speed. I was loaded down and got a easy 60
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Re: Top speed 252 with 502

Post by Ytmsn »

Thunder I have a 502 as well. Comes up on plane in two seconds and runs 55+. Different props would probably make it run 60+. But the last time we went out I never got over 25. I'm probably more like Tuscany in that we just cruise kinda slow and laid back listening to tunes and enjoying the ride!

Y'all know these aren't race boats, right? :)
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Re: Top speed 252 with 502

Post by Big Block Power »

Y'all know these aren't race boats, right? :)
[/quote]

What you talking about :-)
Go get your grey poopon :-)
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Re: Top speed 252 with 502

Post by thunder550 »

I just want it all! Tow a tube, get on plane fast, do 105 mph, fly to Texas, BBQ a steak on the manifold, use the prop to dice up some tomatos.
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Re: Top speed 252 with 502

Post by thunder550 »

Ytmsn wrote: Tue Oct 17, 2017 8:53 pm Y'all know these aren't race boats, right? :)
I'ma put some racing stripes on this beeeeeeeach



Beer's doing a little talking tonight wheeeee.
1995 Cobalt 252 - 502 Mag EFI - Bravo 1 - 21" Revolution 4
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