Thanks
Author  Comment  

geokeg 
Hull speed 
Lead  
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 



todge t  
I run my 19 h/t 2008 140 [email protected] 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  
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  
The thing is your speed is limited to the sea conditions. In my 17 sr if i can cruise at 2022 mph at 424400 rpm it is a very calm day. An average day I may
be at 16 18 mph at closer to 373900 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  
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 feetmathematicians 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 fuelefficient 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 4fold 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 5ish knots and operating at 'minimal planing' speedI've never done that
experiment myself.


finfinn  Most efficient speed  
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  
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  
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  
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.
I am hopeing there is a sweet spot between 3500 and 4000 where I can hit 5 MPG. Alan 

LL  
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 8fold increase in aerodynamic drag; I expect that hydrodynamics behave similarly but don't know for
certain). Anyway, it looks like a 2fold drop in fuel efficiency is the cost for a 4fold increase in speed; we must each ask where our priorities lie!




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

thewoodman  
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  
EQ Marine, the Arima Dealer up in Anacortes has this great discussion on displacement, semidisplacement, 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 semidispacement range is 6.2 to 10.2 knots. Alan's chart pretty much indicates that the Arima has decent performance thru the semidisplacment 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 minivan of the sea, verses the SUV or sports car. I've come to love my minivan. 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  
Jay: At 7 1/2 mpg. I like to call my 16 SC the "Prius of the Sea". (ha) BTW; great post......thanks.


Threeweight  
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.
