Planing speed/rpm

[ferari]

Just a curious question; I have a '73 36' DCMY with 440s. What are
other owners with similar power cruising at-when on plane? I've read
about the 80% of WOT vs. hull speed...What is your RPM on plane and
fuel usage?

[BW]

I'm looking at buying a 36 uniflite sedan with 440's. Can you shred some light on fuel burn rate at 12+ knots. I'm really Curious how much fuel I'd burn cruising slow.

Thanks,
Brent

[Harris Bob]

Hi

At about 1300 RPM per engine, you're doing 8 knots and burning 3.5 USGPM per engine

At 12 knots your consumption would be 8 USGPM (guess) per engine (.75 USGPM/knot) vs 1.14 USGPK @ 8 knots (35% improvement)

Sent from my iPhone

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[Hhct...@aol.com]

I burn about 8 gallons a hour at less than 10 miles a hour and when I pop it up to over 15 miles a hour goes up to abut 15  gallons a hour, tedc

[ferari]

Brent,

My numbers are exactly what Bob Harris draws out. Granted, I have a
DCMY vs. the sedan you are looking for, but the numbers are the same.
Interesting....

On Aug 15, 7:11 pm, Hhctuc...@aol.com wrote:
> I burn about 8 gallons a hour at less than 10 miles a hour and when I pop
> it up to over 15 miles a hour goes up to abut 15  gallons a hour,  tedc
>
> In a message dated 8/15/2012 3:32:32 P.M. US Mountain Standard Time,
>

[ferari]

I forgot to add the additional question, if you are going 8-9 kts and
your destination is X miles or hours away, is going 12 kts and getting
there a little quicker and burning a little more fuel about the same
(time vs. speed)? I think it's pretty close in numbers, give or
take.

I was just wondering what a 'like' power package i have is at in terms
of rpm and speed.

[waterguy]

I have a 1971 36 SS with 440's and an older Floscan fuel consumption meter.  I've found that at hull speed (around 7.5 kts; 1,200 rpm), I'm burning about 2 gph per side.  According to the Chrysler engine manuals, max RPM for these engines is 4,000, and they recommend not exceeding 80% of max rpm for sustained operation (3,200 RPM).  At 3,000 RPM, I'm cruising about 20 to 22 kts and burning about 8 gph per side.

To answer your "additional" question about the difference in speed vs fuel consumption, what you want to figure out is miles per gallon, not time vs. speed, unless your time has a value (e.g., could you be at work earning money vs. boating).  So at hull speed, burning a total of 4 gph, you're getting 1.875 nautical miles per gallon.  At 20 knots, burning 16 gph, you're getting 1.25 kmpg - 33% worse.  So over a trip of 100 miles, you burn 53.3333 gallons at hull speed over 13.3333 hours vs. 80 gallons at 20 knots over 5 hours.  You use 26 gallons more fuel.  At $4 per gallon, that's a difference of $106.66, but you save 8.3 hours on the trip time. 

 

Fine calculation of the miles per gallon figure requires a fuel usage meter and some method of measuring speed.  You can use a GPS that has a readout of Speed Over Ground (SOG), but only if you know there's no current or tidal flow to increase or decrease your SOG.  If there is, you need a speedometer that reads out your speed through water (STW).  My boat has a depth sounder that has a little paddlewheel in the transducer that provides a STW reading on the sounder.  Newer sounders and speedometers have a NMEA 0183 output that will interface with the newer flow meters and give you an instant readout of MPG. 

 

Personally, I like to cruise at hull speed, unless I've got to be somewhere.  Saves gas (read $$$$), and it's less stressful - at high speeds, it's hard to see a floating log in your path (a real hazard in Puget Sound and Northwestern waters) before you're on top of it.  Plus, if you do hit a deadhead, it'll do less damage to hull, props, shafts, rudders, etc. at low speed.  And it's nice to have a big speed reserve on hand in case the weather blows up and you have to run for a sheltered harbor.

[waterguy]

One other thing I forgot to mention - sorry to be so long-winded - is that if you're going to run at higher speeds, you've got to get the boat completely on-plane.  Otherwise, you're just digging a big hole in the water, and that's the least fuel-efficient way to run.  On my boat, I use the trim tabs to get the stern up, get the speed to 18-20 knots, then retract the tabs.  I've found that running at 12-15 knots just hogs the gas - my mpg drops to about .8 at 12 to 15 knots.

 

[Art Burtis]

Dear Mark, i.e. waterguy.  Excellent description of GPH and GPM in accordance with STW and SOG time-travel rates as they may be calced by combining or separating the use of GPS / Floscan meter / Velocity-over-water speedometer... etc.  Understandable to the ength degree!  I owned a Uni, currently own a Tolly, and have looked at several Uni’s this year for possible purchase.  During my decades of boating I’ve explained similar to your explanation in email below over and over again to boat owners.  It really is simple math and simple reasoning.  Your depiction should help some boaters better understand the actual, simple reasoning incorporated into GPH, GPM, STW, and SOG in relation to personal time preferences, Money $pent, safety from deadheads, and reserve fuels (i.e. speed) to get out of harm’s way if required.  May I add... at hull speed there is also a pleasing calm that occurs in one’s mind as the scenery (be it land views or the joy of watching sun glistening waves) has time to “sink-in”; not to mention the peacefulness of greatly reduced engine noise and the reduced wear on entire driveline!  Happy Boating!! - Art

 

Art

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[bushwood]

Thanks to all for the great responses.  I half thought this site was fading out, but not true.  In the 4+ years we have owned our boat, we have always been a 8+ knot boat for the exact reasons others point out.  However, I like to educate myself along the way and ask the questions. 

 

It is time for me to save for a flo-scan or like product.  I do the math on hours and gallons every time I fuel up and pleasantly surprised each time, but I am also curious about my usage at 12, or 18 or 7 knots.

 

Thanks again!

[George]

From what I like to think I know of this subject I must say thank you the numbers quoted reflect my 29' ulrichsen with twin 318s.. My uniflite is still waiting to be launched and will have more electronics. I came to close the same by reading gas gauge and traveling distance on my charts (definitely not the most accurate way) my uni is a 32 1978 but what I don't get is how does one come up with hull speed??? I am familiar with the calculation for a displacement hull, can the same formula be used on a semi displacement or planning hull??. Things that make you go hhmmm. 

Sent from my iPad

[BW]

Thank you!

[bushwood]

Waterguy,

 

I have same power and hull but I'm not close to your rpm/speed ratio.  Can you tell me what size props you are swinging and their pitch, etc?  I'm obtaining 8.5 kts at 1800 rpm and 13 kts at 2800.  I can get my max rpm but my speed is much slower.

 

My prop shop put my 4 blades props at this pitch. 

 

Thank you! 

On Tuesday, August 14, 2012 4:25:53 PM UTC-7, ferari wrote:

[Georgeviking]

Curious??? How are you guys determining hull speed??? We all have planning hulls so the calculation used on a displacment hull does not apply.
Is there a formula for planning hulls???

[Georgeviking]

[ferari]

George,

I'm not aware of any other formula for hull speed than the one that is
used. In trying to look deeper into your question, I looked at a few
places and the most logic one I kept coming to was the formula 'we'
use for hull speed. I don't know that it matters whether it's
'planing' or displacement.' It is based on your LOA.

http://en.wikipedia.org/wiki/Hull_speed

[Art Burtis]

Not LOA, but rather WLL upon which the formula works. "Hull Speed"
calculation is same for nearly any style hull design. In that, Hull Speed
represents pre planning attitude of a hull staying parallel to water surface
while it "slips" through the water. Soon as the power becomes sufficient to
begin to change its attitude by raising the bow and droping the stern of any
hull it has then exceeded its Hull Speed. Most efficient speed for best GPM
is normally 5% less than calced Hull Speed.

-----Original Message-----
From: unifli...@googlegroups.com [mailto:unifli...@googlegroups.com]
On Behalf Of ferari
Sent: Sunday, September 23, 2012 8:12 PM
To: UnifliteWorld
Subject: [UnifliteWorld] Re: Planing speed/rpm

[Georgeviking]

Ok so on a typical 30 boat theoretical hull speed is app 7.5 knots on a displacement hull the power needed to push the vessel faster than that is extremely
Inefficient which would cost heavily in fuel cost, we operate planning hulls which are design to have the ability to be able to climb onto it's bow wave, now the correlation between the bow and stern wave is still the same that hasn't changed enabling the vessel to greatly exceed hull speed " which is not a limiting speed"
However gasoline engines are inherently more efficient at higher rpm then lower, unlike a diesel engine so by trimming the boat to level it and reduce friction along the water, it may at times be more economical to operate at higher speeds. I'm sure there are many factors that come into play here also. On new boat tests many are more economical in the area of 3000 - 3500. Granted travel at hull speed is more efficient than adding a few knots and plowing trough the water but without accurately calculating fuel consumption to distance traveled it is a tough call .......installing flo scans on my 32 this winter so I will be back next spring with results.