Went from 19 inch to 15 inch pitch, gained 200 rpm?

[chelmick]

I have a 1989 Regal 255 XL Ambassador with a Mercruiser 260hp V8. I was running a 19 inch pitch prop but couldn't get above 2600 rpm's. Very slow to plane. So I swapped on a 4 blade 15 inch pitch prop. I get on plane much faster now, but only picked up 200 more rpm. Full throttle is 2800 rpms and about 32 mph. Is there something I'm missing? I used the Mercruiser prop selection guide.

 

[Sea Rider]

Welcome to Iboats,

What was your I/B wot rpm performance with previous 3 blade prop assuming you had one. Have gained just 50 + revs per less pitch decrease. Check I/B probably with tech issues not delivering full power ? Seems the rule of thumb that states + 200 revs per less pitch on 3 blade props dials much better on heavy combos.

Happy Boating

 

[jimmbo]

How long have you had this boat? What shape is the engine in? Compression, ignition advance, are the secondaries on the carb opening, is the carb clean or all gummed up?

 

[chelmick]

The 3 blade prop was the 19 inch one that gave me 2600 rpms. I just bought it last day of summer, last year. The engine runs and idles very smooth, but I have not checked the ignition advance or cleaned the carb. Sounds like a good place to start! What does I/B mean?

 

[gddavid]

It sounds like your tach is off. That looks like a heavy boat and I doubt it is really going 32 mph at only 2800 rpm. Was your tachometer replaced? it needs to compatible with your ignition system and be set for the correct number of cylinders (although, I think it would read even lower if you had it set for the wrong engine).

 

[chelmick]

Looks like the original tach. In neutral, I can rev it to 4400+ rpms on the tach. I was thinking 32 mph was a pretty decent top speed for a 4000 lb, 25 foot boat. But the motor sounds like it only running at midrange rpms, at wot.

 

[Sea Rider]

Could your interior hull be waterlodged ? check, water adds overweight to a already heavy boat. So now at full throttle you're revving at 2800 wot revs, right ? What's the max wot rpm range for that Mercruiser 260 HP ?

Happy Boating

 

[Maclin]

Tach has to be off, way off if the Speed is accurate. An 8 cylinder hooked to a tach set for a 12 cylinder would show 2800 when the rpm is actually 4200. Not sayin' this is the problem, however When I run the prop calc-er with 1.6 ratio and 15p and 11% slip at 32mph it calcs to 4050 rpms.

Same tool gives an error with the 2800 rpm plugged in, says "You have selected a speed that is faster than the prop can go at these conditions with no slip."

 

[Sea Rider]

If he's having a bad tach reading how can I/B not be over revving already going 4 pitch down in size, that V8 surely has a rev limiter, right ? If everything concerning the engine is right, that's a healthy in top shape engine, is it possible that his current 4 blade prop is not doing its homewirk right on his application ? If we don't know min-max wot range for that V8 can't have the slight idea on how much more rpm is needed to go for full max wot revs.

Happy Boating

 

[gddavid]

Using a 1.47 ratio drive and the prop slip calculations for that speed and prop yield a prop slip of -15% so the numbers seem off. I don't see how the tach would be off but it is the only explanation I can think of. I had that engine and drive in a 25' heavy walkaround and the numbers were nowhere near that, not the same boat but the numbers still seem way to far off to make sense. Max rpm for that engine is 4800.

edit: I did not see Maclin's post and my post is pretty much redundant. I doubt that engine has a rev limiter unless built into the solid state ignition system but I don't think so. The 260 hp is a non-vortec 350 chevy with a 4 barrel carb. When those secondary barrels opened up, it could consume some fuel.

 

[Maclin]

If he's having a bad tach reading how can I/B not be over revving already going 4 pitch down in size, that V8 surely has a rev limiter, right ? If everything concerning the engine is right, that's a healthy in top shape engine, is it possible that his current 4 blade prop is not doing its homewirk right on his application ? If we don't know min-max wot range for that V8 can't have the slight idea on how much more rpm is needed to go for full max wot revs.

Happy Boating

Not sure what your "I/B" means, but, Merc 260 V8's are all about the same on MFG recommended top RPM range, 4200-4600. If the 32MPH is correct and drive ratio is known (almost certainly around the 1.5 range) then we can know it is not over-revving.

The poster has a Stern drive, also know as Inboard/Outboard or I/O. Your I/B term seems to mean InBoard and that is not what he has.

 

[redmen62]

Must be something with Regals for this age... I'm having the same problem with my 277 Commodore with twin 305's. I'm about to start my own post about this. I'm going to follow yours too!

 

[chelmick]

I'm going to work on the boat this weekend and check out all the suggestions. The hull looks and feels dry and solid. Going to look at the choke and carb secondaries. Also going to look at the tach and make sure it's set right. Keeping my fingers crossed! Thanks for all the help!

 

[spoilsofwar]

How are you measuring the speed? GPS?

32mph @ 2800rpm on that barge is impossible. There's no way.

 

[Sea Rider]

If having the right connections along pulse set right, could still have faulty reading issues. Borrow a shop tach and double check.

Happy Boating