Want some prop guidance

I have around 500 HP bbc on a 1.5 Bravo 1

Boat is a 1994 VIP Vindicator mini cigarette boat really. Looks like a cig just in a smaller package, its only about 23.5 feet. 4000lbs probably closer to 5000 with gas and people.

The drive sits pretty low in the water, I'd say the cavitation plate is slightly lower than the bottop of the hull.

I do have trim tabs.

Thank!

Re: Want some prop guidance

Ayuh,.... Have ya Run it,..?? What sorta numbers ya gettin',..??

Rpms at Wot,..??

What prop, 'n pitch,..??

Re: Want some prop guidance

As Bondo....Need some current information and statistics to begin with.

Re: Want some prop guidance

dude at the end of my docks runs a similar rig- big block chevy, but nowhere near that kind of HP.. more likely around 400 instead..

the engine should tap out at 6kRPM, and it's comfort zone should be 5200~5500... that will be dependent on valve train, more specifically whether you're running flat tappet or roller cam, and either stamped or roller rockers.. w/o either, your not going to want to go over 5200RPM.. On a build that strong, I'd wager a roller cam (hydraulic lifters) was used, and I can't imagine cheaping out on the rockers- but I'd certainly ask, or even just look for yourself.

BBC's produce hella torque down low.. I'd HOPE a marine camshaft or something similar was introduced to limit inversion- not that I buy completely into that, but it is still a concern- which would lead me to believe it is in fact a marine cam or similar profile, with little or no overlap w/ intake and exhaust valves... which means is 'likely' produces power off idle all the way to redline instead of being designed for a very specific range (trucks like range low, cars a wider range, strip cars want it high)....

so- range (if you don't know it, maybe you know what to ask the builder now) is likely in the 5.2k~5.5k comfort range.. power over full range (likely).. heavy boat (4k~5k#), cigarette type hull... 23.5 LOA, likely a 22' waterline..

what is your target? top speed? hole shot for drag? cruising/economy?

I'll guess you'd be served well with around 25p five progressive and vented blades of 'high five' goodness as a widely available off the shelf prop...

Re: Want some prop guidance

You'll want to look at the Mercury Mirage and the Bravo 1. A starting point for pitch is going to be hard as you don't have a point of reference but I would guess 24-25 pitch.

Run through Mercury's prop selector and see what it comes up with:

http://www.mercurymarine.com/propellers/prop-selector/#

Re: Want some prop guidance

well, I had a hi-tech 4 24p prop on there this past weekend, and it is soooo not optimal. I'm thinking it doesn't give enough lift to the boat. I was only spinning it to 4400 RPM at about 52 in some pretty rough chop. Now at first, I was concerned it was the engine not producing the power it was supposed to but I'd have to be like 100 hp down for it to be spinning that poorly. I know that's not the case, everything has been quadruple checked; and some even more than that. So, I know its the prop, and I've been doing some research it seems thats and outboard prop anyway that most guys use with a jack plate (I don't know though, it came with the boat)

I have the stock 3 blade 21p prop that I had on it that I'm going to try this weekend. I was able to spin that with some mild upgrrades on the previous engine before, to about 5000.

I hear alot of mention about the Mirage and the B1, so I think those are good choices.


It does have a marine type came (the Crane 731 cam) Hydraulic Roller with Roller rockers. I also have some IMCO Powerflow manifolds with as long as risers as I can run with silent chioce. So far, Reversion hasn't been an issue at all, and i've done alot of idling too.


Thanks for all your help so far guys!

Re: Want some prop guidance

the fifth blade on that high five will lift the stern more.. the vents will allow a better hole shot by providing a controlled ventilation- and allowing the prop to spin up like a prop one if not two pitches shorter.. the progressive blade geometry will bite in during the hole shot forcing water off the leading edge/lower pitched portion of the blade (somewhere nearing 24ish pitch), but as it gains velocity and bite it will shift further back on the blade where the pitch is actually higher than 25p, or likely nearing if not just over 26p... a good cup, effectively lowering pitch in corners and biting stiffer in the water will assist in preventing cavitation..

what is your drives ratio?

Re: Want some prop guidance

As far as I know, the ratio is 1.5...I've never checked to verify though.


and I never asnwered your original question. I'd say a good all around prop. I need a holeshot becuase I do pull skiiers, tubers, wakeboarders. Obviously I want to go fast, but that extra 2 mph that guys fight for doesn't bother me. All around good performance. I really don't care too much about fuel economy, not that I hate the environment or anything - but, it is boating after all. whats 1.9 mpg compared to 2.3 mpg?

Re: Want some prop guidance

aye... in light of that:

wakeboarder: needs lower planing speed, decent hole shot (-1 pitch)
skiier: needs good hole shot, decent speed if slalom (-1p)
tubers: lower speeds on plane, hole shot not important.. (-1p)
engine speed: you need to be able, for the safety of your engine, to reach WOT as rated by the builder comfortably, not struggling.. (0p)
top speed: you need to keep the arse of that thing planted, be capable of reaching WOT, and keep a slip rate between 10% and 15%... (+1 or maybe even 2p)

you've ran a 21p to 5k on previous build to 5kRPM.. the 24p only rev'd you to 4.2kRPM.. I'm going to suggest that the blade geometry is responsible for the limited rev on that 24p, or something else is amiss..

the 1.5:1 gearbox will necessitate a drop in pitch (-1~1.5p)...

I think a boat that size, at that weight, with that power, for that use, translated through that drive would make a good candidate for either the high five (though I'm going to amend my suggestion of 25p and drop to 24p with vents open, or 23p with vents closed), or a s/s four blade 23p w/ aggressive progressive pitch, vented as well, or a turbo style three blade s/s, vented in 24p..

if you are to experiment using a prop shop's inventory, I'd rec those or something akin to them for defining your parameters and narrowing your search..

Re: Want some prop guidance

PowerAddict said:

I was only spinning it to 4400 RPM at about 52 in some pretty rough chop.

Click to expand...

If the 1.5 ratio is correct, that calculates to 22% slip. That's awful bad when running speeds that high. Leads me to wonder what you are doing with trim tabs, and trim, while running top speed. Are you optimizing both after hammering down to find your absolute maximum top speed? aka...limiting your hull and drive drag to the absolute minimum possible?

No disrespect, but don't want to take anything for granted. Just thought I should bring this up in the event that you're a "set 'em and leave em" type of guy, as I know several boaters who are. You might find you simply need some practice or training on how to use the trim and tabs properly. Buddy of mine has a 29 footer with twin 454's. He was complaining that it could only get to about 60 mph and that was it, everyone in his club kept telling him it should be much faster than that, and dealer kept telling him there was nothing wrong with the boat. I later asked if I could drive it once and he obliged. Took me about 30 seconds of working the switches to get it up into the low 70's and backed out of it while still accelerating due to increasing traffic...he was stunned as it had never gone that fast before. I asked him what he was doing with all the trim stuff when he drove, and he just said he left them all the way down all the time...cuz he didn't even know what they did...I gave him a quick basic lesson.

I'm certainly no racing pro on that stuff either, but be aware that it makes a HUGE difference in potential top speed, holeshot, cornering, and max RPM if you're not actively working both trim and tabs to optimize for your hull, load distribution, and water conditions, every single time you drive...

Re: Want some prop guidance

No offense taken. Part of this could be due to lack of action with the tabs. Do you go full down when taking off and then full up on plane (or adjusted up accordingly)? Or what is your usual procedure?

However, I found out more. The prop I'm using is made for like a 19' bass boat. It probably truly is slipping something fierce.

Re: Want some prop guidance

No real procedure. Just feel. But generally speaking yes, full down for better hole-shot, and up outta the way for top speed...or at least level. They do impact top speed because they effectively lengthen the hull, and if your boat needs a little more stern lift to go faster they can help get it up a little higher out of the water...but this is a double-edged sword because that sometimes drops the bow down and causes it to plow. You just have to experiment.

Even more critical is the motor trim. This needs to come up too after planing out. For top speed usually you trim it out as far as possible until the prop just starts to blow-out and your speed drops a little, then trim back down so the prop bites again. This will usually get you top speed.

It's all feel and experimentation. And it varies with chop, glass, and load. So you find a different setting for both every single time you drive if you're an enthusiast trying to get the most out of it.

I have no idea what a "Hi-tech 4" prop is. Alum or stainless? A motor and boat of that size is begging for stainless. If you're running aluminum it's flexing like mad and you have no idea what the pitch really is while its running in the water...

Re: Want some prop guidance

its stainless. The three blade is stainless as well.