Conference vicki::boats

    
      Hmmm, a good set of circumstances for speculation. Lets see....
    
      I also doubt that your adding a battery will affect your fuel
    mileage so much that you notice it. One thing that rings in my mind
    about your fuel mileage is that the carburetors were just messed with.
    It is easily possible that the jets were changed as per some factory
    recall or that some other change (like float height) was changed which
    has this affect on your mileage. Do you *know* what they did to them?
    
      Of course, no matter what is done to the engine, it has no bearing on
    how fast the prop needs to turn to push the boat a given speed. Is your
    prop still in perfect condition? No nicks, dings, etc.? Are you *sure*
    that the bottomn of the boat is just as clean as it was at this time
    last year upon which you are basing your tests? It does not take too
    much buildup at ALL to slow down the boat and that would do exactly
    what you are noticing (higher RPM needed for a given speed and an
    increase in fuel usage).
    
      As far as being over or under propped goes... it is a common misnomer
    that lowering your RPMs will reduce fuel usage. It does work if you are
    also reducing the WORK the engine is being required to do, like when
    you back off a bit from WOT - that saves fuel. But increasing pitch to
    reduce RPMs actually *increases* the amount of work your motor must do
    and usually will not save you any fuel at all. (Unless of course you
    are vastly underpropped). It is also harder on the engine in the worst
    way - it increases the load on the engine at ALL speeds. If you cannot
    exceed redline with the lightest possible load in the boat, then you
    are not underpropped.
    
      My votes goes for gunk on the bottom. It can and does vary from year
    to year as to how much and how fast it grows.
    
    					Kenny

    
     MOst powerboat speedometers are the pitot tube type - terribly
    inaccurate and lack repeatability. The gauge mechanism itself can
    bind/be tight as a result of corrosion, a small water leak (due to 
    a puncture or leaking connection) can throw them off. The only kind of
    marine speedo I trust is the paddle whee type- but those are big $$$$
    
     IF seem to have less pitch- I would say that the engine is now simply
    running better. Getting more rpm for tyhe same throttle position says
    to me that you have fixed some kind of problem. I'd get the prop
    changed. IF your prop is not a cupped prop, get it cupped, that will
    add the equivalent of 1-2" of pitch, and you will also get a gain in
    effiency.
    
    Dick
     

    I am getting speed info three different ways - the paddlewheel on my
    fishfinder (which is my main speed reference), the boat speedometer
    (pitot tube - yes its not terribly accurate), and my loran (which gives
    me ground speed, but I take the average running both with and against
    the current/wind).  All three of these are in general agreement.
    
    I think the bottom is pretty clean.   
    

Jim,
	I too would suspect fouling. If the prop is in the same condition
as last year and it takes more RPM to reach a given speed, there must be 
more resistance. Either a fouled or damaged hull would be the most likely
source of the increase. I think its unlikely that 50# would make that much
of a difference. 
	Regarding the prop pitch; I didn't understand what you said about WOT.
4,500 to 5,500 is a very large range. I think my target is 5,200 to 5,800
(on a 200 hp Mariner) and I consistantly get 5,600 (plus or minus depending 
on load). What is your WOT RPM? 

Paul

    I believe the spec sheet on my engine says full throttle rpm = 4500
    to 5500.   It does seem like a wide range.
    
    At WOT, I'm looking at something like 4800 rpm.
    
    

    Re: 1017.2
    Dick, can any propeller be "cupped" or just outboard or I/O props?
    Can I take the prop off of my straight inboard and have it cupped?
    Where?  At WOT I am running right at 4400 RPM which my manual says
    is good if I'm properly propped.  My current prop is in excellent
    condition and the boat seems to move right along.  Top speed is 
    between 35 and 40 but the pitot tube speedometer has an accuracy of
    + or - about 10 MPH so I can only guess what I am really doing.  
    Does a cupped prop give a better hole shot, top end, or both?  Is
    "cupping" a prop expensive?  
    Thanks, Wayne

    
    Jim - Moving up 2" in pitch will decrease your RPMs by about 300 at
    WOT. Judging from your current WOT RPM you sure do not want to do that.
    You are already at the bottom end of what you should have for rpms.
    
    Wayne - cupping is most beneficial in outboards and I/O's because it
    allows them to run 'closer' to the surface of the water (actually in
    the air slightly) without excess ventilating (What most people call
    'cavitating') It will end up giving the boat a bit more speed 1-3 mph
    IF all the other things are done right. Ie. you have to trim the drive
    up enough to get the prop into the correct position and you have to
    remember that cupping a prop adds the equivalent of 2" to it's pitch.
    So if you cupped your prop, it would lower your rpm's just as if you
    added a non-cupped prop with 2 more inches of pitch. I would think that
    for an inboard the only benefit would be if you were experiencing
    excess slippage or if it tended to ventilate too easily in sharp turns.
    
    					Kenny

    
     I 've run bioth cupped and regular props- (curently running regular,
    because I got 4 props when I purchased the boat- and I usually "ding at
    least one a year. The marina keeps moving my slip and I have found lots
    of propellor eating rocks at moon low tides.
    
     My eperience with a cupped prop is that at low rpm's You get the
    effect of the basic low pitch (lots of torque and acelleration, while
    getting the effect the the higher apparent pitch due to cupping at the
    higher rpms. Any decent propellor service can cup a prop- but beware
    that it is like adding 2"" more pitch and may bog down the engine-
    unless you decrease the prop diameter.
    
     Dick
    

re .6

a)	Yes

b)	H & H  in Salem Mass., or probably any other reputable prop 
	repair shop.

You didn't ask but,,,,

c)	Gee, I thought most all ski boat props were (are) cupped.
	(pretty sure mine is anyway)

d)	I think I've seen it on  H & H  repair price schedule that 
	they'll cup a prop thats in for repair...  for a reasonably
	small fee, though I don't remember how much - maybe zero if
	they're repairing it. 

e)	H & H    stands for   Haskell and Hawes   and I think thats 
	the way they're listed in trhe phone book, so if you call
	information for their number it helps to know this (-:

	Reg

Jim,
	I agree with Kenny in Re .7, if you are at 4800
RPM @ WOT I don't think you would want to increase the 
pitch. Of course the final decision is a trade off 
between top end and acceleration (among other things).
	On the other hand, a 15" pitch seems small
unless the boat is underpowered.   
	Is your current prop stainless? If not you
could get a little increase in both performance and
economy by switching over.
	How's your overall performance? Does the boat 
get up on plane quickly? Is the top end speed 
acceptable?

Paul

    Paul,
    
    That's what's so weird about this.  The boat feels about the same
    as last year.  The boat will plane in acceptable time (might be a
    while with full load most of whom seem to want to sit in the transom
    seat!).  The top end speed is fine (35 mph on a 20:1 weight/hp ratio
    is perfectly decent).  The cruise speed is fine.  The boat feels great
    particularly since the low end (under 2000 rpm) problem was fixed.
    
    I'm guessing my gas mileage is around 2.5 mpg at cruise.  Unfortunately
    I don't have exact numbers from last year but it was over 3 mpg
    somewhere.
    
    I just thought of another thing, though.  I've had problems with my
    trim gauge this year.  Would mistrim account for this much difference?
    

    
     Mistrim *could* do it. How do you use your trim? A good starting point
    is to trim all (or almost all) the way down when going slowly and
    for accelerating to plane quickly. After planing, you should trim up
    until you get a few more RPM's. I usually see 200-300 extra, either on
    my father's boat (4K lbs) with a 170hp SeaDrive or mine which has a 
    XP100 on about a 1200 lb boat. If your mileage really has gone from 
    3 to 2.5 I would still find it hard to believe it is the trim.
    
      That 115 is the same motor as mine but with bigger carbs, more
    compression and a more highly tuned exhaust. It may not even be making
    it's full rated power at the lower end of the approved RPM band which
    is also something to consider. I don't see why you NEED to get a prop
    that has 2" less pitch, but the next time you buy one you may want to
    do it. Then again, needing 3800 RPMs to maintain plane is very high - 
    you will likely save a lot of gas if you go to a Doel-Fin or the like
    because it will bring the minimum planing RPM down. Under your current
    conditions, your best fuel mileage will probably be at about 4200 rpms.
    Most OB boats get their best fuel mileage around 3500, but you have to
    be well on plane before you start getting good fuel mileage.
    
      If your WOT RPM is 4800 with no extra people, your next prop really 
    should have 2" less pitch. The same thing happened with my boat exactly. 
    My WOT rpm was 4800. I went down 2" in pitch when I went to stainless. 
    I now get 5400 with the lightest weight in the boat which is perfect.
    (Though I can trim it out to 5600) :^) My top end speed increased about 
    3 MPH and my mileage increased some, too. The all around performance is 
    MUCH better. Oh yeah, I get a solid 4mpg and on an easier planing boat, 
    I'm sure it would even be a bit higher. With a 27 degree deadrise and
    no planing strakes, my Larson takes a bit more speed than many boats
    before it is planing comfortably.
    
    				Kenny
    
    And Rick - No comments about my 6k rpm runs. :^)

To sum up the previous replies, the relationship between boat speed and
rpm should remain constant, no matter how the motor is actually running.
If you're really turning 400 rpm more at a given speed than last year, 
it has to be that the boat is either harder to push through the water, 
or the prop is not pushing it as efficiently as it once was.

The boat can be made harder to push by changes in weight or weight 
distribution, hull shape or damage, or excess drag due to bottom growth.  
It can be pushed less efficiently by a damaged prop or skeg, changes in 
trim angle (as you already suspect) or prop height in water.

If you're really going to get to the source of the problem, you can't
discount any possibility, as you seem to when you said "I think the 
bottom is pretty clean".  You have to *verify* it.  Just because it was 
clean at this point last year isn't a guarantee that's it's clean this 
year.  Less effective bottom paint or this year's colder water temps may 
be causing a change in the amount or type of bottom growth.  If the hull 
and prop are not damaged, and the bottom really *is* clean, that pretty 
much leaves either the trim angle or the extra weight from the added 
battery.  Have the trim gauge fixed, and try removing the extra battery 
and see if the situation improves.

One other thing...  As in any mechanical troubleshooting, if you want 
to *know* what caused the problem you should only change or fix one thing 
at a time. Otherwise, if you change two things and the problem goes away
you may never know what the source was.

re: >>  And Rick - No comments about my 6k rpm runs. :^)

No problem Kenny, I won't say a thing about how you revved it to 6K.  
Mum's the word.   8^)

Rick

    Folks,
    
    One other possibility is the hull water logging. Fiberglass hulls will
    tend to absorb water over time if left in the water. If the boat is
    being trailered this would not be a problem.
    
    I had an older outboard with a '70 115 several years ago. If the boat
    was out of the water for a while it would go 35-37 mph by my speedo.
    After 2-3 months in the water, and the hull clean ( scrubbing with a 
    mask and snorkel ), the boat would be 3-4 mph slower from the hull
    being heavier. Let it dry out on the trailer for a week or 2, it would
    be back up to speed.
    
     Chuck
    

    If the hull were heavier I would notice it riding lower and I know
    exactly where it sits in calm water, empty, because that's where
    the bottom paint line is.
    
    Well I screwed around with a bit this past weekend, because of the
    broken trim gauge I found was was tending to overtrim.  But that
    only seemed to account for 100-200 rpm.  Maybe I'm losing my mind
    but I'm almost positive I could hold plane at 3200 rpm with a couple
    of passengers last year.  The bottom is clean as a whistle.
    
    Could anyone who reads this file with an outboard powered boat in the
    18-22' range with displacement around 2000 lb and an engine in the
    90-150 hp range post (or send me mail) your performance stats?
    (Minimum cruise speed to stay on plane/rpm/fuel consumption,
     and the same stats at WOT?, oh yeah, and also prop pitch?)
    
    Thanks a lot.   
    
    

RE: Note 1017.15 
>> If the hull were heavier I would notice it riding lower and I know
>> exactly where it sits in calm water, empty, because that's where
>> the bottom paint line is.
   
Well, not necessarily.  An extra 100 pounds of water would make a *barely*
detectable difference in how the boat sits at rest, but could make a 
big difference in performance when it all flows to the stern with the
boat underway.

>> Maybe I'm losing my mind but I'm almost positive I could hold plane 
>> at 3200 rpm with a couple of passengers last year.  The bottom is 
>> clean as a whistle.
  
Since you're somewhat unsure that the boat is doing anything different 
from last year, you should start by mapping the performance stats of your 
own boat.  Post them here, and ask questions based on what your boat is 
actually doing. It could be that you're chasing a problem that doesn't exist.

>> Could anyone who reads this file with an outboard powered boat in the
>> 18-22' range with displacement around 2000 lb and an engine in the
>> 90-150 hp range post (or send me mail) your performance stats?
>> (Minimum cruise speed to stay on plane/rpm/fuel consumption,
>> and the same stats at WOT?, oh yeah, and also prop pitch?)
   
While the performance stats of another boat may be interesting reading
(for some of us!), comparing your boat to any other boat is pointless.
Even 2 identical boats will rarely perform exactly the same.  The stats
of boats in the size and hp range you listed can vary *SO* widely that
they are totally irrelevant to anything your boat does.  As I mentioned
above, concentrate on the stats of your boat and try to determine whether
you actually have a problem.  There are only so many things that can cause
an increase in rpm at a given hull speed, and most of them should be fairly
easy to detect.  As mentioned in earlier notes, they are trim angle, hull
weight, hull drag or resistance, prop slip, and now possibly just your
imagination!  8^)

Rick

    There's no water in the bilge.  I looked.
    
    OK, here's my (approx) performance data for comment.
    
    Boat is a 1990 Sunbird 20' Cuddy displacement 2200 lb deep-V hull
    deadrise approx 18 deg with a Johnson 115 Outboard.  Both boat and
    motor are in excellent condition and have never been damaged.  Prop
    13x17 Aluminum (and there have been several different props of that
    size on it this year).
    
    Cruise:  3700 RPM
             23 MPH
             10 GPH (2.3 MPG)      (This is very approximate and also
                                    includes starts)
    
    WOT:     5000 RPM
             32 MPH
             Consumption never measured
    
    
    

RE: Note 1017.17
>> Prop 13x17 Aluminum (and there have been several different props of that
>> size on it this year).
   
That's it!  The fact that you've been changing props is one bit of data 
you hadn't given us up til now.  Not all 13"x17" props are created equal, 
and two props of equal dimensions but different designs can produce *VERY* 
different performance characteristics.

Different brands of props (i.e. OMC and Michigan) may have the same pitch
and diameter, but be otherwise completely different.  Changes in blade 
surface area, thickness, curvature, cupping, etc. can EASILY account for 
the 300-400 rpm difference in cruise rpm you think you're seeing.

My Sunbird SPL174 ran 41mph with the stock 13x17 aluminum prop.  It did
42mph with a 13x19 Michigan stainless prop.  I tried another unknown brand
13x19 stainless prop, and top speed went down to 35mph, with prop slip 
like you can't imagine.

I believe that you've got a similar problem with the prop(s) you're using,
or a bad tach or speedometer.  The numbers you gave for top speed (5000 
RPM, 32 MPH) indicate you've either got a serious gauge problem or are 
seeing 20% prop slip, as shown below:  

The formula for calculating theortical speed is 
shaft RPM x pitch /1056  =  MPH   (thanks Reg)

Your motor's gear ratio should be 13:26, which would put shaft rpm at 2500.
Thus:
2500 x 17 / 1056 = 40.24mph theoretical top speed.  

Allowing a max of 10% prop slip gives a top speed of 36.2.  Your 32mph 
indicates 20% prop slip, which is terrible. No way should it be more than 
10% with your boat and motor combination.  Under 10% is normal, which means 
you should be doing at least 36-38 mph.  And the numbers bear that out, as 
the Johnson 88hp on my Sunbird SPL174 with the same hull design and power 
to weight ratio was capable of 41mph with the 13x17 prop.

Start checking your gauges and prop slip...

Rick

    I didn't give you that data because the props were all OMC.  I nicked
    up one prop this season when I hit mud and changed to another one.  I
    was trying to indicate that prop damage wasn't a factor although most
    of the Summer and most of these observations have been on the same
    prop.
    
    What's the chance OMC gave me a prop of the wrong pitch.  If I plug
    15" into your formula, I come up with 35 mph.   Also isn't a smaller
    pitch prop consistent with
          - higher cruise RPM  (min planing speed)
          - lower WOT RPM?
    
    I'm frustrated that I haven't kept stats like this throughout all
    the time I've had the boat, but it wasn't until lately that it
    felt "wrong".    I'll check the prop.
    
    
    

RE: .19
I realize that a "damaged" prop wasn't a factor so you didn't tell us
about the prop swap, but just the fact that it's a "different" prop could 
be a factor.  From what I understand, there is an allowable "tolerance"
on these things, and two props of even the same brand may not be identical.
Especially if one's been repaired by a prop shop.  I've seen props that
had been "shaved" down to way less than their stock diameter to repair
knicked blades. The right way is to weld them up and regrind them to the
stock tolerances, but some shops cut corners (no pun intended).

re: >> What's the chance OMC gave me a prop of the wrong pitch.  If I plug
    >> 15" into your formula, I come up with 35 mph.
    
It's doubtful they gave you a 15" prop. Your WOT rpm (5000) doesn't indicate
that you're under propped, as it most likely would if a 17" prop is the
correct one and you were running a 15".

>>   Also isn't a smaller
    >> pitch prop consistent with
    >>      - higher cruise RPM  (min planing speed)
    >>      - lower WOT RPM?

You're half right.  A smaller pitch would give both a higher cruise rpm *and*
a higher WOT rpm.  Chances are, with too small a prop you'd be able to over
rev the motor.  At minimum you'd be able to rev past 5000.

At this point it's hard to diagnose the problem because you've got conflicting
symptoms.  Top speed rpm's are pretty normal at 5000, although you should be
able to pull up near 5500.  The 20% prop slip that the formula says you have 
would normally be associated with over revving, yet it's not over revving. Top
speed is *way* off compared to what you should be seeing at either 5000 or 5500
rpm.  I know from what I've seen and read that your boat *should* be doing
somewhere up near 40mph, not 32.  Your symptoms are really weird, and don't 
seem to add up.

Do you have access to another 13x17 OMC factory prop, one that you *know* has 
never been repaired?  Try that out.  If the problem persists, I'd see about 
verifying your speedo and tach.  Get a "shop" tach and run it on the motor, 
and running alongside another boat with an accurate speedo should give an 
idea whether your's is off.

    Good luck!
    
Rick