RPM, Planeing out and Prop Pitch Question.

[r0caldwe]

What should wide open throttle be while using a 17 pitch prop on my 1992 cobia Monte Carlo. I am running a 235 5.7 Mercruiser Alpha with an Alpha 1 outdrive. Surprisingly I'm not getting nearly the excelleration that I thought I would out of this boat.

It is also taking my boat a very long tim to plane out. Does anyone have any suggestions?

 

[r0caldwe]

Re: RPM, Planeing out and Prop Pitch Question.

What should be the approximate RPM at WOT and how long should it take me to achieve this?

 

[tmh]

Re: RPM, Planeing out and Prop Pitch Question.

You'll need to provide some more info. What is current WOT rpm and speed? Size and weight of boat? What kind of prop?

I think 4200-4600 is proper WOT pm for that engine but someone will be along soon who knows for sure.

Do you know if your boat is a bit waterlogged The flotation foam tends to soak up water over that many years, especially if it was left outside much.

 

[r0caldwe]

Re: RPM, Planeing out and Prop Pitch Question.

The boat is a 23 foot Cobia Monte carlo which weighs in at about 3500LBS. My estimate on speed is approx 35 due to a non working speedometer. The WOT the RPM meter reads approx 4800 RPMS.

I would guess the the boat is a bit water logged. Is there a way to fix this? The prop is a 17 Pitch 3 blade, I'm unsure about any other specs on the Prop.

 

[180shabah]

Re: RPM, Planeing out and Prop Pitch Question.

If 3500 is accurate, then you are not pushing an excessive amount of weight. However, a boat that size would be the perfect candidate for a set of trim tabs, adjustable if you can afford them, or smart tabs if you want to save money.

A question, I should have asked to start with:
- When you take off rom idle, are you triming the drive all the way down/in?

 

[r0caldwe]

Re: RPM, Planeing out and Prop Pitch Question.

Yes, When I take off from Idle I do trim the boat all the way down/ in. What should this boat's Idle RPM be? I currently have it idleing at about 900 RPM's.

Do you recommend dumping the throttle from the start or gradually excellerating. I am currently gradually accelerating.

 

[MikDee]

Re: RPM, Planeing out and Prop Pitch Question.

At 4800rpm, with a 17" prop, you should be doing approx. 43.5mph not 35? That is of course if your hull bottom is fairly clean, & Tach is right?
My formula:
RPM divided by gear ratio, times pitch -1" = speed - slip = Top speed

4800rpm divided by 1.5 reduction gearing = 3200 prop rpm X 16 (17" pitch prop minus 1) = 51.20mph minus 15% slip = 43.5 mph

 

[steelespike]

Re: RPM, Planeing out and Prop Pitch Question.

I/Os are known to sometimes struggle out of the hole.Some folks vent the prop and as suggested tabs would help.
Be sure it is well tuned. If prop is aluminum a change to stainless in the same pitch might help all aspects.As far as soaked up water put it on a trailer and weigh it.

 

[wca_tim]

Re: RPM, Planeing out and Prop Pitch Question.

The boat is a 23 foot Cobia Monte carlo which weighs in at about 3500LBS. My estimate on speed is approx 35 due to a non working speedometer. The WOT the RPM meter reads approx 4800 RPMS.

I would guess the the boat is a bit water logged. Is there a way to fix this? The prop is a 17 Pitch 3 blade, I'm unsure about any other specs on the Prop.

Just how waterlogged??? might try weighing it... also might try checking the tach... if it were off it could change the situation a good bit.

otherwise I like the other suggestions...

 

[r0caldwe]

Re: RPM, Planeing out and Prop Pitch Question.

I am going to look into tabs, what price range should I be looking for? Are there any specific ones that you suggest?

I am going to put new plugs and plug wires in as soon as I get the time, but I honestly don't think that the boat needs it.

Can you elaborate a bit more about venting the prop, I am unfamiliar with the terminology.

 

[tmh]

Re: RPM, Planeing out and Prop Pitch Question.

It sounds like you are fairly inexperienced as a boater (not a rip on you, so am I) so I'd suggest "Smart Tabs" which require no operational adjusting as you are boating. Maybe $200 or so (I bought mine a few years ago) - fairly easy to install but some transom drilling so if uncomfortable doing "critical" boat work you can hire it done for not too much. They'll likely help your planing quite a bit, keep the bow down on take off which is safer also.

That's not a ton overpowered for a fairly big boat, deep V hull(?) and maybe some extra water weight. Find a local truck scale, and weigh it on the trailer then either find out how much your trailer weighs or weight it when the boat is in the water. My guess is, MAYBE just 3500# empty, then gas and people and gear - you may have 4500# there which is the main reason it's a bit sluggish. However, aside from tearing the deck out, etc. not much you can do if it is overweight!

Hopefully, in tuning it up, etc. you'll ge a bit more out of the engine. However, you may just wind up with an underpowered boat. Smart tabs and a GOOD SS prop (maybe a 4-blade) may put you back into the performance range you're happy with. At 235 hp is that a 2bbl carb? Converting to a 4bbl will get you a few more hp, not a ton but maybe enough to make a difference in a borderline powered boat.

For me, i did those things (except the 4bbl conversion) , it ran much better.....then I put in a bigger engine anyhow and now I'm happy (until I think of something else to do). Still have the Smart Tabs and just plunked down a lot of $$$ for a Stiletto bay Pro II 4-blade SS prop. This may be one you'd look at also as it is supposed to have excellent lift for stern-heavy boats (according to Prop Guru "Waleyehead" in the prop forum here).