23ft Mercruiser 260 5.7 prop question

[duststang]

I have a 1986 23ft Celebrity cuddy with a 260hp 5.7 Mercruiser, Alpha 1, alum prop 15.5x15. I am new to boating and trying to learn so have a few questions.
I notice after 3800-3900 RPM doesnt seem to go much faster. the motor is running 4200 RPM but doesnt seem to be much faster, is this normal? At WOT the max RPM with one person is 4200 RPM . I thought ideal RPM for this motor is 4600?
I notice it takes a little to plain even using trim tabs (not good at) especially when I have a few people. they always sit in the back. the front of the boat comes up really high out of the water, not sure if this is normal? once plained I can cruise around 3000 RPM on plain no problems. When trying to tube it seems harder to get plain. Any suggestions on prop changes or am I doing something wrong? any help/advice is appreciated.

 

[Philster]

Re: 23ft Mercruiser 260 5.7 prop question

Need to know the gear ratio of the drive. Could be switched from stock and not have enough reduction, because a 15p is small on a 5.7 V8.

Consider the health of the engine and have it checked.

Consider the integrity of a 26-year old runabout and whether it is waterlogged (heavy).

It's barely getting to the bottom of the max RPM range when lightly loaded, so there is concern. Can you report the top speed via GPS please?

(Yes, you can turn the engine a little faster and not go faster, because props slip)

 

[duststang]

Re: 23ft Mercruiser 260 5.7 prop question

I am not sure how to tell the ratio and don’t own a GPS. I had it out yesterday (light load & full tank) and if I goose it with trim tabs down it would plain decently. I would goose it to 3200-3500 RPM it would plain and then I would reduce to 2800-3000RPM and cruise. Maybe I wasn’t throttling enough to get it on plain? I figured probably not good to WOT until moving (like a burnout in car)? I did not max it out yesterday but the speedo (probably not accurate) read was 32ish at 3500RMP. When cruising at 3500RPM and goose it you notice it pulling until above 4000RPM seem not to be pulling much. Maybe I have slipping or am I out of the power/torque range? It seems that very few boats run a 15 dia prop does the dia matter? I forgot to mention that the alum prop recently had fixed because had big chips missing . I had someone tell me that the alum prop flex a lot and will hurt performance? The motor runs great starts first crank never any smoking or missing, has good compression, recent tune up. Maybe I will weigh the boat to see if it is water log? I didn’t think it would be because never stored on a lake always out of water.

 

[Philster]

Re: 23ft Mercruiser 260 5.7 prop question

MAJOR Variable: your prop sounds like it might be junk or of unknown pitch/size/config after a repair. I can't see how your engine should be turning that little thing and not spinning the RPM faster.

Forget diameter for now. I'd think that combo should spin a 19p... a 17p at worst. It can't spin a busted 15p (maybe it's fixed right now, but I don't trust it).

Can you borrow a prop from someone so we can get a baseline somewhere? I'd try to borrow a 17p prop for starters.

 

[duststang]

Re: 23ft Mercruiser 260 5.7 prop question

I will see if I can find good 17p to borrow. Should I just open it up till max RPM? (of course not blow up)

 

[Philster]

Re: 23ft Mercruiser 260 5.7 prop question

Yeah, with a good prop. If you have to, try not to exceed 5000 RPM. Hopefully, you can actually get close to that.

Fresh prop; make a WOT run in two directions and note the speed and RPMs. Pretty important stuff.

 

[duststang]

Re: 23ft Mercruiser 260 5.7 prop question

no luck on borrowing a prop, I think I will purchase a new one any suggestions on where to start or even what orignally was on the boat 1986 Celebrity 225 cuddy single 5.7 260hp mercruiser boat weight is around 4400. should I get a 3 or 4 blade? a friend told me that a 4 blade will help out of hole and sometimes top end both. anyone experienced this?