Any Ideas

[fin-addict]

have only used the boat three times in last six months due to wife's illness/passing. son has used it about the same amount of times. of course she has been maintained and run at the dock. son went out last week and got 4800rpm's/37mph @ wot. normal is 5400-5500 rpm's and 40-42.2 mph wot. realized bottom has not been cleaned for awhile, diver not doing cleaning anymore. found new diver, said bottom had growth and some oysters. now super clean. went out yesterday w/son. normal load and fuel, 1/2 tank gas. jumped up on plane as usual. normal rpm to mph throughout throttle range. at wot, max rpm was back to 5400. here lies the question, mph @ wot was 37.7 just a tad better than when the bottom was dirty. tried different trims (no jackplate) and could not get it better. min. loss of 3 mph. any ideas/suggestions.  note, boat has not been out of water for some time. may be due to wet hull. in past with other boats, after being hauled and sat on the hard for a week or two have always picked up a few mph. think this may be the cause. thanks, fin

[polliwog]

I would say it's not due to the hull as long as it is clean.  These boats don't soak up or lose water weight in that short a period of time.  My Hewes sits in the water all summer and doesn't vary at all between being launched in the spring  and being hauled in the Fall for winter storage .

The puzzle is different speed vs WOT 5400rpms.  Are you sure the sump pump has cleared any standing water?  That would certainly slow you down.  My sump pump switch went on the blink and I had a bunch of water in the bilge.  My only thought so far.

[hawg]

Old Fuel? Bad Fuel? Low Octane Fuel?Ethanol Fuel?

[mulligan]

Could your performace have been falling off from something like a worn prop and you have noticed it until you put it up for a bit?

Also keep in mind the hot water/air temperature will slow your top end some.

[lurem]

I am with Mulligan....Warmer weather and my boats slows down. Could you have bent or damaged a blade on your prop?

[Nauti]

Running up current?  That would be better than my other guess; prop damage.  Good luck!

[whichwaysup]

last possibility - given that the boat is in the water and has been sitting in the water - is it possible that you have water in the bilge that isn't visible when at rest due to the angle of the boat?  Have you tried putting her into a bow-high position and checking the bilge (in the back obviously)?

Doesn't seem like temp is going to cost you 3 mph.  Sounds to me like something happened to the prop - you are getting RPM, but not bite . . . 

[JEM]

One other thought, has the prop hub come loose so that its starting to slip a little at WOT.  Just a thought......I like riddles.

[fin-addict]

to answer all questions.

1. bottom just cleaned 3 days ago.

2. pumped out rain water from three days of heavy rain before leaving dock. once in river, rode bow high till rest off water was out. this is normal practice every time I take the boat out. check the auto bilge switch, safety check always. 

3. prop has not been hit or damaged.

4. boat has not been out of water for the last 1 1/2 years since last bottom job. not put up at all.

5. heat and water temp never seemed to effect wot last three summers.

6. just added fuel a month ago. same marina for last 27 years, have had no problems.

7. on way out was running against a incoming tide, thought that might be it. running back, running with the incoming tide, still the same, not the problem. hoped it was the cause.

8. when throttling up, constant bite to wot. thought about prop slip but reaching my max rpm as always.

* again, throughout the entire throttle range running the way it has always run for the last three years except for wot, -3 mph. confused to say the least. thank you all for your feedback. fin

[mulligan]

The prop does not have to be damaged to lose your top end, might just be normal wear.  Not long ago I sent my prop in to get "refreshed", no damage was done and it still shined and looked good.  I picked up close to 5mph at the same rpm.  Just a thought.

[whichwaysup]

Okay - let's break this down with the broad list of potential causes by category - maybe we can work together to build a complete list of potential issues::

Increased Drag:  This would include things like your hull not being as clean as it was last time you ran the higher speeds, a keel guard coming off and creating drag, a trim tab partially deployed or falling off, a new piece of equipment dragging, or some other cause.   Don't forget about drag above the water line too - any new t-tops, poling platform, windshields?  Bent skeg on the engine?  Does the boat weigh the same?  Do YOU weigh the same (Personal question, but something to consider)

Decreased HP Power from the engine:  Seems odd to get your RPM if your engine isn't generating the same power.  Tempted to eliminate this category, but will leave it here for brainstorming.  We mentioned fuel quality, but we didn't talk fuel filter, fuel lines, etc.  Does seem strange that you are getting the RPM though . . . 

Mechanical issues:  Assuming the engine is generating the same power, something between the power head and the prop that is generating the issue.  A prop slipping would fit in this category (which you eliminated), but are there other potential causes that would too?

Prop Efficiency:  This one seems to me to be the most likely culprit.  If you are getting the same RPM, there is no change in the amount of drag above or below the water line, and there are no mechanical issues . . . This guy would be the first place to start.

Selective Memory:  okay, we've all done it.  I remember fish bigger than they actually were, my boat newer than it actually is, my physique (significantly) better looking than it is . . . Are you SURE you saw that higher speed?   

Inaccurate electronics:  Could the device you are measuring with be faulty?  Are you using the same machine to measure?  Anything else different?

Of course, most airplane wrecks are a function of multiple small causes coming together in the wrong way at the wrong time . . . may be a combination of factors above.  The biggest question is - what has changed?

 

 

 

 

[JEM]

Next, I assume the tach is reading correct rpms.  Might be the next thing to check.  The only thing I can think of, if turning a true 5400, just like it always has...... its either trying to push more weight, or there is an issue with the prop.  It seems if it was trying to push more weight (as in water in the hull, or more gear), you would loose rpms.  In that case,  I would probably pull the prop and have it checked/reconditioned at a reputable prop shop. 

[fin-addict]

nothing has changed at all including my wt. gear aboard as always. no drag anywhere. tabs full up, cannot trim with tabs, stb. tab not working right now. do not recall if when running 40-42 mph if tabs were used or full up as now. MIGHT BE THE CAUSE but would not think so as there is no tab drag and she runs flat w/o tabs and bow is just right with no plowing and water breaking on hull area as it always had. using oem tach for rpm and gps for speed. matching same as always throughout rpm/speed range. including idle and running no wake speed. PROMISE!!!!, have written down on paper since owning boat rpm vs speed  at different rpm settings so I know if something is off. have always been able to hit 39-40 mph. a few times 42 mph @ 5600 rpm. nothing has been added or removed from the boat. this is really no big deal but would like it the way it has always been. will look into a prop refurb.

[DaveC]

All great suggestions and answers, I would look at the fuel filter first and of course make sure the fuel is good with no water...

[hawg]

Any chance the GPS got switched from MPH to Knots? 

[fin-addict]

46 minutes ago, hawg said:

Any chance the GPS got switched from MPH to Knots? 

was thinking of that. will take a look but think not.

filters were changed a few months ago but will look at that too.

checked log book, stb. tab was not working last time she ran 39-40 mph so tabs were full up.

ME SO CONFUSED, LOL

cannot stand when I cannot troubleshoot a problem

JUST CHECKED, GPS SPEED SET AT MPH, rule that out!!!

[mulligan]

What did you have for breakfast? Maybe an extra biscuit?

If you normally run 39 and now you are at 37.7 that is only 1.3 MPH diffrence.  Losing 1.3 I would think is environmental.  Heck +/- 1mph could be something as simple as air temperature, salinity, extra 5gal of gas, ect.

[SCFD rtrd.]

You say the bottom is clean, but how clean is it? After all, you used a new diver to clean the hull this time.  If I wax my hull, it runs about 2 mph faster. If it sits in the water for four days, I loose 2 mph.

 Do you have bottom paint? If not, I don't understand why your gel-coat is not blistered from sitting in the water for an extended time!!!!!!!!

[justfish]

Fin, I'm no mechanic but have had my fair share of lil issues over years and I usually try and take a systematic approach.  It's easy enough to do a compression test, so do that first. While plugs are out, might as well replace em. Empty water separator and filter under cowl, I usually add some sea foam to water sep filter before replacing.  

See if you notice anything out of ordinary while doing that and see where it gets you.  

 

I have never really kept mine in water so not sure how quick growth can happen other than at end of week in keys every year I have barnacles, so maybe if your water is warmer than normal it could be growing quicker ?

[fin-addict]

as stated in second post, bottom is painted, with a barrier coat and then top of the line bottom paint. done 1 1/2 years ago. usually get 2 1/2 years between jobs. no blisters. bottom was cleaned 2 days before going out. bottom gets cleaned every 2 months, sits in brakish water. fresh gas a month ago, marina uses a stabilizer in their gas. plugs and large filter replaced less than a year ago.

to all of you, right now feeling like a bit of a fool and my have wasted all of your time and effort in helping me figure this out. I stated the load was the same as usual, not. additional gear was a cooler placed in the front storage locker instead of under the leaning post, tackle bag and 8' cast net also in that locker. son's gear. this may be the problem but he has brought all that before without a problem as I MIGHT recall or not really CHECKED the numbers. fin

[JEM]

Not a problem.  We have all done the same.  I knew if you were getting the same rpms, the only thing that could cause a loss of speed would be weight, drag, or prop.  Hope you have it figured out!

[fin-addict]

I will keep all posted if this was it or not, thanks much guy's. fin

[Capt. Troy]

If the bottom got really dirty with Barnacles and growth an overly aggressive cleaning can really rough up the bottom. Which equals more drag and less speed. I would look here if you turning the normal rpm's. If it is an ablative paint it will wear and begin to smooth out over time if you use the boat a good bit.