Planing Hull at Disp Speeds

TollyWally, I have often synced speeds with other boats to find best paths - trouble is, many guys can't get the racing blood out of their veins and I have found, sometimes decades later that I was wrong on a given path because somebody nudged their throttle forward a bit, way back when. I like watching drifting flotsam from an anchored vantage (I have run up to a 50 mile radius 100 to 200 times a year since 1980 and every tidal micro-current becomes important with expensive diesel. The trick is in realizing that the shortest route is often not the shortest distance over ground. Also, sometimes eddies form on big tides but not small ones, and tidal current timing varies fron tide relative to size.). Tug captains that frequent Washington tell me that your Juan De Fuca will have up to twelve distinct currents that they look for on a given transit. Nevermind the big red arrow on the current charts!
I'm not following the error - why is a flow-meter unreliable at low speeds?
One gallon an hour at seven knots - Now that's a thing of beauty!

 

Mark,
I'm not convinced of an error in my flowmeter at low speeds but Easy brought up the spectre of low speed unreliability so perhaps I'll do a little experimenting. For some perverse reason I enjoy that sort of thing.

I learned to sail playing in the Tacoma Narrows. A wonderful schoolroom for learning to ride currents and back eddies. There is significant tug traffic through there for additional instruction for those who care to observe.


Back in the day we'd head for a tavern on the water in Gig Harbor on our schedule not nature's. Running up the Narrows against a flood tide you need to stay in a back eddy very close to the shore. We'd be sailing but would have the motor running as we were skirting the shore extremely closely to stay in the back eddy and wanted to be able to power out if something went amiss. We had a 10 horse honda for power. The first of the 4 strokes, it was very quiet compared to the old evinrude. With the motor idling in neutral after a bit I noticed a curious thing. Even though we were in neutral the prop would put a minute load on the motor. I found I could actually hear the motor varying as the load would rise and fall depending on the push from the back eddy. We had a vary sensitive (for the time) speedometer for racing that would track minute differences in speed for tuning the sails. It was cool watching the needle swing to the changing pitch of the motor idling in neutral. It was pretty hairy sometimes sailing within 10 feet of the beach but we were young, fearless, and thirsty.

Driftnetting in the Inlet naturally gives one a whole new appreciation for the subtleties of current interaction. In the late 80's when gps units became availbale for mere mortals it was a vast improvement over loran for tracking speed over ground etc. when drifting. If you kept in close contact over the radio you could sometimes hit the change of tide for more than one set if your radio group buddies were positioned correctly and paying attention to the gps.

I always use my flowmeter for sensing current. Coupled with a gps it really lets you tune your route. There is nothing I like better than to glide with the tide.

 

I better get one on my new power. I watch the northeast side when going to Gig Harbor - once in awhile, one can see a boat get out of shape in the whirlpools off that point on a flood. If a one minute drive once or twice a year can show me several boats getting current-whipped or even spinning, it must be a common occurance. Sorry for the tangent.

 

The swirlies in the Narrows are nothing like what you have between Anchor Point and the Forelands. LOL, don't worry about the tangent, it's easy for a planing hull to be moving at displacement speeds bucking those currents!

 

With a waterline length of just 27ft, I manage 6.5 nmpg (imperial) with my (planing hull) Graphite.... what say ye now ..?

...sorry, only pic I could find on my work 'puter...

 

That's pretty damn good. Tell us more, I don't recall you mentioning this before. ( could be an oversight on my part) How do you do it?

 

Aha! That explains why it is can be so difficult to plot a straight course in a busy waterway that will miss everyone! This is a particular challenge in a big lake with cottages all around when each boat is heading in a different direction. Half the boats that I am planning to pass ahead of seem to get to the intersect point sooner than expected. I wasn't factoring human behavior.

 

If I told you, I'd have to consider you to be a tourist.... and shoot you..!

It's nothing magical... a relatively slippery shape, narrow WL beam and light weight....

 

Hi Will,
I'd guess the efficiency comes mostly from light disp. Look's to me like the beam is the same at the gunnel as it is at the WL. I like the forefoot a lot but the roof seems visually heavy. I always like three face/plane cabin fronts. Oh Will, sorry I misled you about the accuracy of meters. I have zip zero experience w flow meters but other meters seem to be less than perfect. I can't think of a meter that would be more meaningless on my Willard but to put the record straight it burns .85gph (US) at 6.15 knots turning 2300/3000 rpm. My last boat (Albin 25) made 7.35 knots consuming .75 gph. I like to cruise all day and not worry about fuel (the Willy carries 100 gal). Lots of space up here and ancient, you can plot and run all the straight courses you want and only rarely will anyone get in the way.

Easy

 

Yes - the light weight is definitely a major contributor. Beam at WL is 2.36m, at gunwale it's 2.8m - there's a quite significant knuckle in the topsides.

Found a couple more pics...

 

Will,
If I remember right we have to knock 20% off to translate to yankee gallons. That's still nothing to sneeze at. I'll duck when you shoot for more particulars.

What powers her? Cruising weight? Cruising speed?

Easy,
What do you heat her with?

 

Here's a fuel consumption chart... that way you can convert to anything you want....

In answer to the q's -
Yanmar 6BY 260Z (260hp through a Merc Bravo 3 drive)
Around 3000kg
12 - 25 knots (top 32)

 

Thanks Will,
I'll study this a bit. LOL your boat makes my boat suck!

 

No such thing as a bad boat I say! You can enjoy being on the water just as much in a 10ft tinnie as you can in a 150ft superyacht...

 

Indeed, you can. I always feel best out on the water. I have a heavier cabin cruiser 28' which I love dearly but...there is always a but is there not? Your boat whips it on speed and milage

The old magazine report weight was about 6500# I need to weigh it with a strain guage on a travel lift but I'm guessing maybe 7500# now. With the motor in it's current state of tune probably 200-220 hp.
1.5:1 reduction 17/15 wheel. Very modest v aft. I get maybe 1.5 npg on a real good day. 1.25 is a realistic day to day number.