Your on vacation..crank up the tunes, pop open an IPA, pedal to da metal...Looks like you need some rare sunscreen in the PNW this week.

 

Nope, I needed smoke screen on this trip.

 

.5 gallon/hr in a 40 feet boat at 7 knots....
I don't think so...
5.5 more likely but still quite optimistic I would think.

 

I did not think so either but I feel it should cruise 7 through the water under power. I guess I aim for 8.5 to hit 7.

What's a good RPM setting to hit the sweet spot?

Aventyre I'm a huge IPA fan I'll drink one in your honor.

Sent from my SM-G935V using Tapatalk

 

Trust your GPS. Far less likely to me mis-calibrated. Currents up here can be surprisingly big & rapidly changing. The first time I chartered here I kept seeing the course arrow on the GPS reversing 180 degrees. Turned out that local currents were in fact setting us back.

 

GPS speed is speed over ground, knotmeter measures speed over water. There will be always a difference unless you are moving in a bay without any current or wave. Knotmeter is good for sail handling. If you are cruising use your GPS, which will indicate your speed over ground

 

Knot meter is hull speed THROUGH the water, which will not compensate vs current for speed relative to fixed points on land.

Its kind of like the navigation problems given in basic aviation ground school.

You have a 12 knot wind from SSE
You want to go from point A to point B which is 500 miles directly N. Cruising speed of your aircraft is 135 knots at 7 gallons per hour.

Find the compass heading and fuel required.

There is enough information there for the solution.

 

The direction of laminar flow past the paddle wheel may vary with changes in the hull's speed. Air entrainment may also be affected so even speed thu water may be suspect for instantaneous readings The Gps speed over ground is less effected and reads what you've done , not what you're doing, Same with course / heading of course. This factor can get more interesting as you box the compass on a dark engineless night in the 6kt current of a rock studded channel

 

My 3gm30 yanmar 600 hours runs about .5 gal/hour running 3100 rpm to push my Alberg35 about 6 knots. If any of that is helpful. Speed measured via GPS, but I sail on the Patapsco River/Chesapeake mostly, not much in the way of current.

 

your gps SOG is accurate for speed over ground... current and all.

You speed log is likely off... fouled or needs calibration.

SOG is basically useless except to tell the wife when you will arrive somewhere.

Your fuel consumption has little to do with your SOG... more to do with boat speed... and consumption will be less efficient at you get to hull speed... 1.34 x sq rt of LWL.

Calibrate your boat speed on multiple reciprocal runs over a measured distance... better at slack tide.

I have a 3 cyl 36 HP Volvo and a 29'6" LWL and get about .6 gal / hr at 2,000 rpm WITH A CLEAN HULL and no towing of dinghy.

 

When you paid the big buck for the boat and the fee for the slip and the wine and the beer and the barbi steaks, you worry about the fuel thru an engine that you didn't even change the oil in. ???

 

More just interested in the sweet spot for the engine as far as RPM.

As someone here said there is a dramatic loss in fuel efficiency as you approach hull speed.

If it was a rental car if l would push it's capabilities but if it floats or flys, I like to baby the engine. I'll put 300+ nautical miles on this engine in two weeks. I want to make sure it gets me there and back.

That being said I now disbelieve the bill of goods sold me by the guy who checked me out in the boat. Today I bumped the throttle to 1800 RPMs gave me 8+ knots on the speedo, which showed about 7 on the GPS. I did see 9.7 knots on the GPS ridding the tail end of the flood tide through Dodd's Narrows this evening with some wild swirling currents on the tail end. It reminded me of my early days as a whitewater kayaker staying in the calm part trying to avoid the swirling eddys on the sides.

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experiment with your boat and don't expect others to answer your questions. Stop being lazy. Remember... people solved boat problems on their own before internet sailing forums.

 

People generally ask questions on here because they have real questions. Ridiculing them doesn't make sense.

If you don't have the patience to answer his / her questions without the snarky quips, just say nothing.

Comments like that aren't necessary.

 

Hey the Dodd Narrows is some big fun. Requires an IPA or after you get. A bit of the fun here:

 

Hey the Dodd Narrows is some big fun. Requires an IPA or after you get. A bit of the fun here:

little tug and big log boom dodd narrows bc - YouTube

Nicely Done! Somebody had their boat driving Wheaties that morning.

Totally agree with BP about finding the sweet spot with feel and sound. There are so many factors at play that you can observe on the boat that we can't observe here. Lots of resonances, prop pitch, bottom condition, current, sea and wind state are just a few.

You could start looking for a sweet spot around 75% throttle, work the throttle slowly up and down until everything feels right. When things stop rattling and shaking and the boat moves nicely through the water, you've likely found the sweet spot.

As far as the error on the knot meter goes, I'd say just go with the GPS, there's a good chance the data from the mechanical log is just gibberish and I wouldn't be bothered with trying to figure it out on a rented boat.

 

The sweet spot is hit by feel and sound, not GPS or knot meter.

 

said the actress to the bishop.

 

Slightly OT...

This site and others like it tend to be a go to place for noobs. They will often ask questions assuming (correctly) that there are more experienced people who have the answers. The alternative is to get a few books and do some reading... search thru YouTubes which have all manner of instructive how to's or search google and follow the links and use this to inform you.

I think SOME or maybe MANY of the questions indicate laziness... and that is something that can be dangerous out there on the water.

Nothing wrong with intelligent questions from someone who is struggling with new material. But too many questions sort of lower the discourse level of the site.

Maybe

 

Back to "speed"

Boat speed is useless if the transducer is fouled, the hull around it is fouled and / or it's not calibrated. When the transducer is clean and calibrated it should be accurate enough to trim your sails... which you can't use GPS SOG for... simply not precise or it's "filtered/smoothed".

The B&G speed instrument I have is precise to 0.01. I suppose the accuracy is limited to the amount of friction... the concept of measuring the speed is basic and fool proof... speed is measured by rotations of the paddle wheel or impellor. Of course it is not perfectly accurate... but orders of magnitude more accurate that the SOG.

There is also a "trend" indicator on the Boat Speed read out.... an "apostrophe" means speed is trending up.... a "comma" means it is trending down... neither means the speed is constant.

You can see ever change in sail trim... as long as the conditions are constant... but you can also see every puff or change in the sea... waves and so on. None of this "shows" on SOG as "instantly" as it does with the transducers.

Boat speed can be damped as well... a sort of averaging

GPS will tell you when you will arrive at a set mark... if you program this...

Remember... you can be anchored in a strong current and your boat speed will tell you that you are moving but the GPS will tell you that you are not. Or drifting and the GPS will indicate speed and the boat speed may be reading 0.

Sailors need to understand THEIR instruments and THEIR boat...

Boat speed is useless when you are not calibrated and clean... and properly damped.

GPS is useless for fine sail trim...

Lookie lookie.... GPS reads 7.0... Boat Speed reads 6.47 with an UP speed trend

 

Back to "speed"

Sailors need to understand THEIR instruments and THEIR boat...

You do realize this is a charter boat right?

But back on topic FWIW, we lived on and sailed the Salish Sea for a year and I can remember the knot meter and GPS matching only a few times. There are a lot of currents in play! Every time I was convinced that one or the other was broken the would sync up again on a particular passage and make me doubt my sanity again :grin

My 38' averages about .56 gph although we cruise a bit slower than optimum (2200 rpm vs 2400 rpm) and get anywhere from 5.5 knots to 6.5 knots thought the water depending, again, on currents.

If you are truly interested learning about the local currents, pick up a current atlas (CHS - Atlas of Tidal Currents) and the 2017 Current Atlas Tables (2017 Current Atlas Tables - Waggoner Cruising Guide) and start making your self crazy trying to route plan. Personally I think its a bit of a waste of effort, but it will sure explain why your speed drops suddenly as you shoot through Discovery Passage on what you thought was an ebb tide - I thought I'd screwed up something...

 

This scenario sounds suspiciously like a charter company trying to fake the solution to a problem they haven't fixed. Did someone whack the prop and they installed the wrong replacement? Is the motor failing? Who knows.

Assuming all is proper, you should run the motor to wide one throttle on flat water and note the max rpm. Only takes a few seconds. Then run her in cruise somewhere between 60% and 80% of max rpm. As poker said, sound and feel matter, so vibration may tell you where in that range she wants to be. Lower, is obviously much better on fuel consumption. Who's buying the fuel, btw? Included in the charter, or do you return her topped up?

p.s. If you're not getting close to hull speed at 80% of max, she is either fouled, has the wrong prop or the wrong engine.

 

SO what if there are currents and you want to know what the boat is REALLY doing. Use your GPS to determine when it's slack water and motor around and see how the knotmeter and GPS compare. And because sometimes the current is so subtle you won't even get a 0.1 knot reading I set the GPS to display, get this, actual Lat/Long. If you check the 3rd digit to the right of the decimal (assuming it's deg, min decimal min) you can see ANY movement of the boat.

This isn't rocket science. And I can guarantee your 40'er isn't doing 7 kts at 0.5 GPH.

 

SO what if there are currents and you want to know what the boat is REALLY doing. Use your GPS to determine when it's slack water and motor around and see how the knotmeter and GPS compare. And because sometimes the current is so subtle you won't even get a 0.1 knot reading I set the GPS to display, get this, actual Lat/Long. If you check the 3rd digit to the right of the decimal (assuming it's deg, min decimal min) you can see ANY movement of the boat.

This isn't rocket science. And I can guarantee your 40'er isn't doing 7 kts at 0.5 GPH.

This is a fine way to find out if there is current running... Of course you may have to wait until you find slack tide to then do you speed runs to calibrate the boat speed. And you need to know that the bottom around the transducer and the transducer itself are not fouled.

 

I'm pretty happy with the speed data I get from a modern Chartplotter GPS. I haven't had a functioning paddle log in a couple of boats and don't miss it.

The VMG vector on the Chartplotter gives me more data than I could get from a paddle log.

Unless I'm trying to decipher a current though, I just leave it set to read out (small screen). On my data readout I have speed, max speed, moving average and trip.

For sail trimming purposes I focus on speed trends more than absolute speed. If you make an adjustment and the speed is trending up, then your adjustment has been effective, if your speed is trending down, then your adjustment was ineffective.

Between VMG vector and those 4 data points, I feel I have a good enough picture for the kind of sailing I do.

I just use a fairly entry level handheld Garmin Chartplotter, nothing fancy, but it gives me all I need to know for boat speed.

 

Well Jordan, did you find a sweet spot somewhere in Desolaion ?

 

Sure did Capt Len, It was great meeting you and appreciate your hospitality. It was an epic trip. I plan on writing up a journal of the trip some day soon.

This scenario sounds suspiciously like a charter company trying to fake the solution to a problem they haven't fixed. Did someone whack the prop and they installed the wrong replacement? Is the motor failing? Who knows.

Assuming all is proper, you should run the motor to wide one throttle on flat water and note the max rpm. Only takes a few seconds. Then run her in cruise somewhere between 60% and 80% of max rpm. As poker said, sound and feel matter, so vibration may tell you where in that range she wants to be. Lower, is obviously much better on fuel consumption. Who's buying the fuel, btw? Included in the charter, or do you return her topped up?

p.s. If you're not getting close to hull speed at 80% of max, she is either fouled, has the wrong prop or the wrong engine.

Thanks for your suggestions.

You hit the nail on the head. I got the boat for a great deal. 14 days on a 40' Jeanneau deck salon for $7,000, about 5 grand below what I would have paid from a "good" company. As the saying goes you get what you pay for.

I rented this boat from a guy that owns 4 different sailboats he is a semi-retired contractor who hustles his charter business each summer to pay the rent. The price is right on the boat and most everything is working flawlessly. The boat does have some shortcomings though, the most important being no cup holders. This boat has 3 cabins, two heads, refrigeration, but no cupholders? It is almost a luxurious boat but misses the mark by one or two things.

Somewhere further down the line of minimum required equipment in my opinion behind a cup holder and a stereo with either bluetooth or an aux input is a working speedo. On the second day of mostly motoring, I realized the speedo was consistently 1-3 knots over the GPS speed. Too long a time period to be tide related. I check the Kalman filter on the GPS and it is not clogged so I know it is a speedo problem. I recently diagnosed and replace a broken speed impeller on this exact system Ray Marine ST60, on a boat I crewed on.

If the speedo reading was zero or under GPS speed consistently I would suspect a fouled sensor but since it was consistently reading way over GPS speed I suspect bad calibration or a glitched central computer. I didn't have the equipment or the vested interest in this boat as I did the other and am too busy on this trip to be troubleshooting broken items. Plus I have nowhere to put my beer down while working on the boat.

Since my posting of this I have had spotty internet coverage but read some of the suggestions. I have been setting the power so the speedo says 8.5 and have been getting 6.3-7.0 on the GPS speed. On day 5 while heading out of Pender Harbor, to test the RPM range and get a feel for the boat. I brought the power up and had my wife look at the RPM gauge. 1800 she read. i said "you mean 2800?", "Nope 1800". I had her steer while I ran the throttle up. The RPMs stop at 1800 while the engine continues an obvious increase in output. The engine is roaring, the stern is squatting, a large froth of prop wash comes off the back of the boat and the speedo goes up to 11. That's right the boat "goes to 11".

So the speedo and tachometer are not really working, so i will go with BoatPokers' suggestion of operating the motor by feel. Eventually I found around 9.0 on the speedo had me average around 6.8 knots on the GPS. After 50 hours of motoring I had to put in 37 gallons of fuel, for a burn rate of about 3/4 gal/hr.