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Hull speed

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geokeg

Hull speed

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Posts: 25

Nov 23 09 7:28 AM

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Does anyone know the hull speed for a Sea Ranger 19? I am interested in knowing the most efficient speed for long range cruising. I am considering a 21 or 22 also so those numbers would be good too if they are known by someone on the forum.

Thanks

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todge t

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Posts: 14

Nov 23 09 3:52 PM

I run my 19 h/t 2008 140 suzuki-8h/yamaha@4600 rpm,with the leg trimmed up it gives me 26 mph.At 6200 full rpm 37mph.4600 is the best for me easy on the motor ,and the best fuel economy. TT

thewoodman

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Posts: 685

Nov 23 09 5:19 PM

Hull speed for normal displacement boats (traditional displacement hulls) is:

V=1.34 (this can be as high as 1.51) x \/LWL (length at water line).

In theory, exceeding this speed will "drag" a boat under.

You have to ask Don, I'm sure he has played with the figures for the Arima. It is best used for racing sailboats and naval ships.

Don, what's the answer? For 17's, 19's and 22's?

The Woodman

corky12

hull speed

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Posts: 57

Nov 23 09 8:41 PM

The thing is your speed is limited to the sea conditions. In my 17 sr if i can cruise at 20-22 mph at 42-4400 rpm it is a very calm day. An average day I may be at 16- 18 mph at closer to 37-3900 rpm. Any rougher then it is difficult and a rough ride and I prefer to stay home. I have a 115 suzuki motor with a 17" prop. It came with a 19" and I traded some higher end speed for lower end control. I cruise from Bodega bay to fort Ross in n.Cal on a regular basis. With an Arima it is defineatly slow and steady.- John

LL

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Posts: 20

Nov 24 09 1:18 AM

Are you sure that 'hull speed' is really what you are interested in? As the Woodman says, this value (in knots) is equal to 1.34 times the square root of the waterline length (in feet--mathematicians out there will have to forgive the units in this expression which imply that knots = ft^1/2... it's just an empirical relationship). Basically, this is the speed at which the hull is riding in the trough between the bow wave and the stern wave. But typically people think about hull speed only in the context of displacement hulls, where it really is a hard limit on boat speed. Planing hulls, when adequate horsepower is applied, are designed to essentially 'ride up' the bow wave in order to escape from this trough, and once on plane the concept of hull speed no longer applies. Certainly a planing hull can be operated in a 'displacement mode', but I expect it would require only about 10 horsepower to get an SR19 to its hull speed, making that big heavy thing in the middle of your transom seem pretty extravagant. And going to a 22' would make negligible difference in hull speed, since this increases proportionally to the square root of waterline length (i.e., to double the hull speed one would require a boat 4 times longer...). Anyway, if you really want to cruise at the hull speed, it will be 5 or 6 knots, and require minimal power from your outboard. This might be the most fuel-efficient way to go, but I don't know how much less efficient it would be to operate at a 'minimal planing speed', which will depend on your configuration of boat balance, trim tabs, and engine specifics (prop specs, mounting height, trim angle), but will probably be in the range of 15 to 20 knots. So a 3 to 4-fold increase in speed by taking advantage of the planing hull. I would be curious to see what the difference in fuel consumption is between cruising in a 'displacement' mode at 5-ish knots and operating at 'minimal planing' speed--I've never done that experiment myself.

finfinn

Most efficient speed

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Posts: 176

Nov 24 09 3:02 AM

Hi, Geokeg. I tend to agree with the previous. You would get totally bored trying to go anywhere with Arima hull speed. The most economical cruising speed depends on your motor, prop and loading. You can impact it by trimming the outboard in or out. And if you have trim tabs they help too. The best thing to do is to get one of them fuel flow meters in addition to the speed. I have the Honda Smart gauges on my boat and can set it up to show me mpg. The number is of course relative and does not indicate mileage on ground, but it helps trimming the boat. With my skip tower SR19, 135 hp Honda, two batteries, 8 hp kicker and two people on board with full gas tank I get about 4 mpg at 3800 rpm and am running 20 - 22 mph depending on the water smoothness. Adding weight and speeding up burns more fuel. Weight is worse than speed - within reason.

Anyway, the 45 gallons gives me about 110 mile range, or 35+ miles there, 35+ back and 1/3 tank in reserve. Which is why I am looking at additional fuel capacity solutions. Going into larger Arimas will marginally increase the hull speed, but your most economical cruising speed will again depend on the fundamentals. With added weight and horsepower your mpg will go down. But your tank may hold more, so the range can actually grow from your current. And your comfort both cruising and at he dock will improve - at a price tag though. But what the heck, it is only money

Tight Lines!

finfinn

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Posts: 176

Nov 24 09 3:12 AM

LL
Reread your post. As I told above, my "most economical" planing speed is 20 - 22 mph at which I get about 4 mpg. I have not run a speed/mpg curve, but seem to recall that "no wake" areas (4 - 5 mph) I get about 5.5 mpg. Will have to check it out next time out. The worst mpg is just before going on plane, when it drops to around 2 mpg. Will try to remember to run a speed/mpg curve and report that. Anybody else with fuel flow and speedo willing to give it a try?

Tight Lines!

geokeg

Hull speed

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Posts: 25

Nov 24 09 8:13 AM

Thanks guys, good discussion so far. I don't have a fuel flow meter but it seems that 4.5 to 5 knots has been where I can run without requiring much power. So, I guess I didn't ask the question very well relative to planing. My sweet spot planing speed seems to be 20 knots. I asked the question because some days I don't really have any where to go but just like a slow cruise along the shore line looking at the scenery.

Any more info or opinions would be appreciated.

George

asautt

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Posts: 322

Nov 24 09 12:08 PM

Here is a chart I did last week. There may be some anomalies. I only did one run at each RPM setting, some with the wind some against. I was by myself so running the boat and maintaining a proper watch were primary priority, recording data secondary. Speed is GPS SOG.

RPM	SPEED MPH	GPH	MPG

650	2.2	0.18	12

1000	3.5	0.29	12

1500	5.5	0.66	8.8

2000	6.9	1.43	4.8

2500	8.2	2.64	3.13

3000	13.9	3.83	3.53

3500	17.8	4.23	3.97

4000	23.6	5.05	4.64

4500	26	7.56	3.53

5000	30.3	8.8	3.2

5500	33.8	11.54	2.92

I am hopeing there is a sweet spot between 3500 and 4000 where I can hit 5 MPG.

Alan

LL

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Posts: 20

Nov 24 09 3:42 PM

That is a nice dataset, I don't have the instrumentation to easily generate such numbers, so I am green with envy

. I would guess that about 5.5 mph is near the hull speed, where you are getting about 9 mpg. As you start to exceed that slightly, you are effectively powering up the backside of your own bow wave, sorta like driving uphill, and your mpg starts to plummet. Once you are applying enough power to reach the crest, efficiency starts to come back up to reach your 4.6 mpg sweet spot where most of the hull is out of the water, minimizing the effective displacement and wetted surface area, and thus minimizing that component of the hydrodynamic drag. Any further power actually increases drag, since drag increases with speed (in aerodynamics, drag is proportional to the cube of speed, i.e. increasing speed by a factor of 2 results in an 8-fold increase in aerodynamic drag; I expect that hydrodynamics behave similarly but don't know for certain). Anyway, it looks like a 2-fold drop in fuel efficiency is the cost for a 4-fold increase in speed; we must each ask where our priorities lie!

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thewoodman

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Posts: 685

Nov 24 09 4:49 PM

Wow

Poor old Don must be shaking his head over this series.

Bottom line, I have a hole in the back I put gas in, sometimes 4 gallons, sometimes 10. I go fishing, push this level forward to go fast, pull back to go slow.

2 mpg, 10 mpg, 10 mph, 50 mph. When I get home the only thing that I think about is what is in my icebox, were my friends happy and did I have a good time.

Don, Mr Arima thanks for my boat that gets xxxx mpg, xxxx mph and is within my budget for a great time.

The Woodman

asautt

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Posts: 322

Nov 24 09 6:20 PM

I don't know why anybody who put the time and effort into designing and building a safe, stable, economical, efficient craft like the Arima would shake their heads about the owners of the boats trying to get maximum fuel effiency.

When I'm heading out to get my gear in the water I can make an informed decision on whether I wanna get there as fast as possible, knowing I'm wasting gas and decreasing range, or I can slow down to max range planeing speed and get still there pretty quick.

Alan

Threeweight

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Posts: 570

Nov 24 09 8:12 PM

Woodman, of course you aren't worried about fuel consumption. You have an old Johnson that burns fuel faster than Kieth Richards goes through scotch and cigarettes

finfinn

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Posts: 176

Nov 24 09 11:06 PM

Alan,
Great dataset. Can you please post data on your boat - size, power etc. I could probably find it on the intro, but getting lazy......

Tight Lines!

asautt

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Posts: 322

Nov 25 09 11:42 AM

Fin,
1990 SeaChaser 17, 115 E-Tec, 15" 4 blade prop, full canvas up, loaded out with "normal" load... kicker, full tanks, 2 batteries, down riggers, etc.
Alan

thewoodman

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Posts: 685

Nov 25 09 12:08 PM

Assault

I implied that Don was shaking his head because we were talking hull speed, which loses its meaning when you hit plane as the Arima is designed to do. It is apples to oranges.

I did love your chart, now if I could ever get my "old Johnson" to 4000 at 23.6 mph I would be in dreamland.

Threeweight, love your Keith comparison. Like Keith my old Johnson smokes too much and drinks a way too much, but like like Keith, when performing it does a heck of a job.

New motors with instal, etc. and other items the cost would be the same as it cost new in 88. One day, like Keith, it will go up in flames.

Meanwhile, couple of extra gallons, and a little less speed, it still performs.

The Woodman

Jay in Kitsap

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Posts: 314

Nov 25 09 3:14 PM

EQ Marine, the Arima Dealer up in Anacortes has this great discussion on displacement, semi-displacement, and planing boats.

http://www.eqmarine.com/R...emidisplacement_hull.htm

Each boat falls somewhere on the line for its best speeds, I am not fully sure but I believe that the Arima is a bit more of a planing boat
than the Rosborough but the deep Vee hulls are way out there. For a 17 foot Arima the semi-dispacement range is 6.2 to 10.2 knots. Alan's
chart pretty much indicates that the Arima has decent performance thru the semi-displacment range, pretty similar to its performance in the
planing speeds. Once on plane, I recall that a substantial proportion of the energy is keeping it up on plane, but as speed rises so does the drag

Eq Marine also has some other good information about boat selection. Yes, it probably is true that we all pilot the mini-van of the sea, verses the SUV
or sports car. I've come to love my mini-van.

Actually Mr Arima and Don did quite well refining the hull parameters to make a real performance boat, but their idea of performance was to handle
most sea conditions safely and comfortably. Just smile when you compare your fuel bill to the power boat with twin 300 HP engines, I believe they measure
their milage in gallons per mile no MPG. They get to hang out on their boat in the marina because they cannot afford to fill up.

Yachter Yat

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Posts: 340

Nov 25 09 4:55 PM

Jay: At 7 1/2 mpg. I like to call my 16 SC the "Prius of the Sea". (ha) BTW; great post......thanks.

Yat

Threeweight

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Posts: 570

Nov 25 09 9:45 PM

Hmm, that is actually an interesting marketing idea the folks at Arima might want to take advantage of. With gas at 3 bucks a gallon, and the economy where it is, it is certainly appealing to me.

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