odd problem

[Cptkid570]

Hey Everyone,
24' bowrider with a gen 1 alpha 1, 260hp Mercruiser, 4bbl.

I typically cruise around 25mph. Oddly, sometimes it takes the engine 3500 rpms to reach that speed and sometimes I get it at 3000 rpms. Same weight in the boat. Boat is trimmed the same in both situations... and same water conditions. Engine sounds good and runs very smooth.

Could it be the coil and sometimes the plugs aren't getting as much spark and I have a coil issue? This happens during the same outings, so it's burning the same fuel (not like it does it one week and not the other -- it happens during the same day)

If you think it's a coil issue, are there any coils that are more high performance than others?

I just don't understand why sometimes it takes more throttle to go the same speed..

Tach is good.. it's been tested.. and, that said, I can tell that I do have to give it more throttle.

Any suggestions?

 

[Philster]

Re: odd problem

Your post implies slip somewhere in the mechanicals after the engine.

 

[iop]

Re: odd problem

"same water conditions" could be tricky.... what kind of waters are you cruising in?? Wind and tide current play a major role in my rpm/mph ratio...

 

[Cptkid570]

Re: odd problem

Outdrive is new (SEI).. definitely no slip at the engine coupler.. prop is stainless and doesn't appear to be slipping either..

 

[Cptkid570]

Re: odd problem

Salt water.. but in open waters, so not a strong tide (gulf of mexico). Pretty much non-windy conditions..

 

[iop]

Re: odd problem

On the days that you have to rev up more there are ghosts pushing against your bow...

have you tried looking at your condition with the incoming and outgoing tides.
if your I/O is Ok its unlikely it will perform randomly.

 

[JustJason]

Re: odd problem

When your 5 miles offshore it's pretty tricky to see/feel the currents unless your a seasoned salt.

It's normal. Air density, air pressure, air temperature, humidity, all effect engine output power. Surface currents, subsurface currents, air currents ect will all effect RPM vs speed ratio.

If you don't believe me go boating on a pond...

 

[bruceb58]

Re: odd problem

How are you measuring the speed? GPS?

 

[Cptkid570]

Re: odd problem

Measuring by GPS.. I appreciate all of the replies.. i really don't think it's the tide or waves or wind.. I will be going the same direction going 25mph at 3000 rpm, slow down for a minute, start back up again and suddenly it takes 3500 to do the same speed.

 

[bruceb58]

Re: odd problem

Sounds like a tach problem. Get a shop tach to see if it says the same thing.

 

[wired1236]

Re: odd problem

Like has been said before, there are a lot of factors. I have noticed it's also how you get to the that speed. What I mean is if I slow down to a cruising speed of 25mph from like 35, the boat has been on a higher plane and will have to work less becuase it's gotten itself out of the water already and it's easier to sustain the plane. If I speed up to 25mph and don't go over that speed, often it will take a higher RPM to stay there due to the fact it's still pushing a little more water since it's not gotten a chance to fully plane yet. Yes, the boat is technically "on plane" at 25mph but not as much as it is at 35mph.

Make sense?

 

[Cptkid570]

Re: odd problem

Wired - I will check that out.. Good thought..

 

[NHGuy]

Re: odd problem

I found something similar but wrote it off to pitot speedometer inconsistency. So to read Wired's thought, take it up past 25 mph onto a good flat plane and then back it down to 25, maybe it will then be at your lower RPMs. I believe I have had that occur in my boat.

 

[JoLin]

Re: odd problem

Salt water.. but in open waters, so not a strong tide (gulf of mexico). Pretty much non-windy conditions..

Crazy thought... when's the last time you checked for growth (like barnacles) on the outdrive, props, tabs and bottom?

I've seen it many times and it's happened to me. Boat sits, growth forms... many times the simple act of running it through the water will clean enough of it off that you gain back some performance.

This is why many of us need to pull the boats a couple times a season just to scrape the junk off. Fairly easy to do in the water, too, at least on the metal parts. Put on your PFD, tie yourself to the boat, and get in the water with a scraper/putty knife...

My .02

 

[Cptkid570]

Re: odd problem

JoLin - thank you for the thoughts, but the boat is kept on a trailer...I guess I should've put that above..