Bravo III testing results

[jumpjets]

I recently replaced my tired 320HP merc 6.2L MPI with an ATK 383 small block. It's rated at 345HP and 445TQ. My boat weighs about 4500ish lbs.

With the OEM 24P merc props, I was reaching 5200RPM rev limiter at about 80% throttle. That would net me 47mph, regardless of how many people or fuel quantity on board. The boat seemed to be underpropped. I figured I could get more out it. My prop slip calc iphone app says 12.5% slip

I bought a 26P and 28P bravo III propsets on ebay to see if I could get more performance.

I started with the 28P, hoping for more max speed. The boat still planed quickly, and the cruise speed was fantastic. I could cruise at 39mph at 4000rpm. At WOT, however, I could only get to 4900rpm, and GPS speed was 45mph. With 2 people on board, it dropped to 44mph and 4800rpm. My prop slip calc says I'm at 23% slip.

I'll try the 26P prop set soon, and let you know how it goes. I was hoping with the 28P set, I'd get a 57mph top speed.... but it appears prop physics are much more complex than I'd imagined.

 

[tpenfield]

I think you'll find that the 26P set is the best match. From what you described, the 28P is too much. In terms of not going any faster with the greater pitch, what you are running into is the power limits of the engine. Power physics actually . . . not prop physics . . . You will probably see speeds of around 50 mph.

If the engine would let you go to a higher RPM with the 24P set, my guesstimate is that it would go to about 5500-5600 RPM and a speed of about 50-51 mph. . . then it will reach the power limits of the engine.

 

[alldodge]

Agree the 26 will be better and the 345 crankshaft HP motor is like 320 at the prop

 

[jumpjets]

Thanks guys. Additional question: Will prop condition have a major effect? The 24 and 28p prop sets have very minor scuffs and scrapes on the blades. I had to 26p props reconditioned, balanced, and buffed and re-worked at a prop shop.

If I got the 28p props re-worked, would I get anymore RPM or speed from them? I don't want to spend $150 on prop shop labor if it's likely to net zero gains.

 

[alldodge]

If I got the 28p props re-worked, would I get anymore RPM or speed from them?

Don't see that happening. If the props are scuffed and dinged up bad, they will cause the slip to increase. If slip increases then the rpms go up

Polishing a prop is only good for looks, its not going to help much else hen comparing polished to dull finish

 

[jumpjets]

Tested the 26p set today.
5200rpm, 49mph, 16% slip
4000rpm, 35mph, 22% slip
3000rpm, 27mph, 20% slip

These props had a rough shimmy at 1400-1800 rpm. I’m nervous they still aren’t balanced. They also cavitated with the drive trimmed full up/aft on plane. The 24 or 28s didn’t do that.

I was able to weigh the boat today too. It’s 5520 lbs with a full 85 gal tank of gas, anchors, and dock lines. More than I thought.

Tomorrow I’m going to put the 28P back on and remove my teeny tiny flame arrestor. I have a much bigger K&N arrestor that ought to flow considerably more air.

 

[Scott Danforth]

you dont have enough motor to spin the 28's on your heavy boat as indicated by your run with the 26's

what was the ATK motor balanced at? was it neutral balanced? under balanced?

What RPM can you spin it to?

as stated, your 345hp crankshaft is about 315-320 prop. so you did not gain any motor.

 

[jumpjets]

you dont have enough motor to spin the 28's on your heavy boat as indicated by your run with the 26's

what was the ATK motor balanced at? was it neutral balanced? under balanced?

What RPM can you spin it to?

as stated, your 345hp crankshaft is about 315-320 prop. so you did not gain any motor.

Scott, I checked my ATK motor build sheet, and it doesn't say anything about being neutral or under-balanced. I can call them and find out if you think it's critical data.

I put the 28p back on today and gave it one more shake just for fun. I fixed my pitot-static speedometer on the boat, and I used an iphone GPS speed app. I also don't fully trust my tach, so I hooked up a timing light with digital RPM indication as a backup to the tach. The tach is reading is dreadfully inaccurate. It is generally 200 RPM high, but sometimes it'll be low. It's all over place.

Todays data with 28P props
3000 rpm, 30 mph, 17% slip
4000 rpm, 41 mph, 15% slip
WOT 4670 rpm, 49 mph, 13% slip

On the last couple WOT runs, my Faria-beede tach was at 5200 rpm, and my pitot speedometer was at 54mph.

My ultimate goal was to increase cruising speed with a light load of 2-3 people, a dog, and a cooler. I think this engine will be OK cruising between 3000-4000 rpm with the described load.

Right now I'm inclined to leave the 28P set on the boat. Unless, of course, you guys tell me it's a horrible idea and I'm sure to frag my new engine. I rarely run around at WOT, and when I do it's for 1 minute or less. I'm keeping a set of 24p props in the garage. If I'm taking 6 people on the boat or carrying a heavy load, I'll swap the 28's out for 24s. I can swap those props in 15 minutes.

 

[alldodge]

Tested the 26p set today.
5200rpm, 49mph, 16% slip
4000rpm, 35mph, 22% slip
3000rpm, 27mph, 20% slip

Todays data with 28P props
3000 rpm, 30 mph, 17% slip
4000 rpm, 41 mph, 15% slip
WOT 4670 rpm, 49 mph, 13% slip

I would use the 26P

 

[tpenfield]

Looks like 49 mph is your top speed . . . the 26 P is a better match.