Timing and prop change

[kloobs2305]

I bought a 1988 Sea Ray 270DA (twin 4.3l 175hp) 3 seasons ago and found out last year that sometime in the past the prop was changed from spec'd 14x19 to 15.25x15's.

The boat ran well but was getting around 1.25mpg and it seemed that I had to run at higher RPM than normal (>75%) to get reasonable cruising speed. (ie 25mph,3700 rpm)and so I thought that going back to the spec prop 14x19 I could get much better speed/mpg. I also had the engine tuned and discovered that the timing was 0deg instead of 8deg BTDC as specified. Changed the timing to spec and changed the prop to 14x19's.

When I did this, my WOT RPM came down from spec 4500 to 4200. On initial start my WOT RPM would not get above 3700RPM until after planing which could take up to 15seconds If I had 4 people on board. I did get higher speed at equivalent RPM but couldn't cruise below 23mph/3600 RPM because the boat would come out of plane. Fuel efficiency was no better, and perhaps slightly worse.

I went back to the old props, kept the 8deg BTDC timing. Now planing is 5 seconds as before with a lot of people, I can hold plane at 3300RPM/20mph, cruise at 224-25mph at 3600-3700rpm. Assume fuel efficiency is same as before.

Does this make sense or should I look at the engine to determine if I have a problem there? Any reason why the timing was set to 0deg?

Any comments?

Thanks in advance.

 

[Haut Medoc]

Re: Timing and prop change

Well, 27 feet is alot of boat.....
Is the bottom clean?....
I would do a compression test on both engines.....
When was the last time it had a full tune up?....
IE, cap, plugs, wires, rotor?......

If you leave it outdoors in the weather, you may have a problem with water logged foam.....
I think the 15 x 15's are masking other problems.....
There should be no need to use anything other than oem to get to wot.....​

 

[180shabah]

Re: Timing and prop change

What is your current WOT RPM?
What is the WOT range for your your engine, per Mercruiser?

 

[kloobs2305]

Re: Timing and prop change

Spec WOT rpm is 4500 and currently with the 15 pitch is 4500. With the 19 pitch it was around 4200.

 

[kloobs2305]

Re: Timing and prop change

Full tuneup this summer and that is when we went to 8deg BTDC from 0deg which is spec. Compression check not done. Bottom seems to be clean and good bottom paint. Also with the 15pitch running like it did last summer.

 

[180shabah]

Re: Timing and prop change

Where are you getting the Spec for your WOT. It wil not be a single speed. It will be a range ie. 4200-4600 or 4400-4800.

Also, something just isn't adding up here. When you increased pitch by 4" from 15 - 19 your RPM only dropped by about 300. The difference should have been double.

 

[evilratgirl]

Re: Timing and prop change

Hey Klobbs, I'm not going to claim to be an expert but a few things you might want to keep in mind as you work thru your question.

You mention this is a 1988, 27' boat pushed by twin 4.3's. First thing that comes to mind is an older heavy boat pushed by what should be ~170hp carbed engines.

Not knowing the hrs on the engines, they may be alittle tired. Even if a compression check shows compression in spec, you can bet they aren't putting out the power they did when new.

I would highly suspect the hull isn't in new condition either. No matter how well you take care, there's nothing like new slipery gelcoat. The 200 rpm/inch pitch is a good guideline, not actually linear.

For an actual definative answer you may want to look for what the boat origionally weighed new. Then get it weighed to see if there's any water weight gain. Get the engines a compression and leak down check. Stick a vacuum gauge on it which will also give an idea of valve and carb condition.

Basically I really can't getting tripped about dropping the prop pitch alittle to meet wot on a 29 year old boat.

 

[kloobs2305]

Re: Timing and prop change

Thanks for the replies. BTW the spec for WOT is 4400-4800.