8mph 8mph 8mph

geoleo said:

That's funny, The ICW and the Okeechobee waterways have mile markers. Guess all those people in boats on them are not "real boaters" (maybe robots?) lol And the amateur Previous owner I bought my trawler from had all his instruments set to miles ---- he had only traveled 3600 miles every year for the past 15 years round trip from Maine to Stuart Florida using miles-he evidently was not a "real boater" I wish I was a real boater . I have only traveled in my own sailboat s over 26000 nautical miles offshore in the past 47 years.

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Do you call the pointy end the front, the other end the rear, one side the left, and the other side the right? I have no issue with dirt dwellers not learning the nomenclature of boating. It just seems to me that if you have enough of an interest to buy a boat and cruise, you should strive to use the nautical terms.

Btw, in case you weren't aware, the ICW was laid out and marked by government employees (not mariners) for Inland (inside the line of demarcation ) navigation. That's why I differentiated myself from a land locked boater in my previous post.

Ted

6.15 knots for Willy.

Sweet spot is a willy nilly expression that can mean numerous things.

1. Least vibration or noise is probably the most common.
2. Least fuel consumption. But that’s actually a speed almost
knows and it may be only about 4 knots.
3. Hull speed. Many think most things magical happen at hull speed.

And there are others like what the wife likes or what da book says. I think lots of us may just notice that even a bit above a certain rpm the noise seems to suddenly get a bit objectionable. Mark has said for years one knot below hull speed but that’s basically for his WLL and a full disp hull. Few fit into that slot on this fourm. 1 1/2 knot would be closer to cover all. And w many or even most SD hulls here hull speed would be hard to beat. After all SD hulls are designed to go faster.
But re fuel burn one knot below hull speed is a very good target to shoot for. Bigger boats burn more fuel and saving more would make even more sense. I thought the first time Mark said that, it was a good rule of thumb.

But sweet spot may just mean what speed we like the most. My point is that the “sweet spot” is not well defined at all but being inspecific on this forum has it’s advantages. How I got to 6.15 knots? Over time I noticed I very often picked 2300rpm and one day I did some back and forth runs and for all practical purposes the GPS said the average was 6.15.

'Most trawlers increase the beam along with the length, keeping the same basic ratio intact, meaning the same basic max speed, give or take a knot.'

Changes in the L/B ratio do not matter at displacement speeds till about 6-1 is reached.

More wetted surface at 6-1 means slower speed for same fuel burn, but less wave making at a higher speed is why skinny boats are built.

For most displacement boats waterline length is key to speed , weight is key to power required to get to that speed.

We run our 36' (40' OA) semi-displacement trawler at 8.5 knots, with the speed accurately ascertained by runnin' the measured mile monthly, before every Predicted Log contest.

ghost said:

We were dirt hotelers recently in Astoria. I pulled up ship finder and saw some boat named Alaskan Sea Duction on AIS just up river. You should reach out to them having a similar boat name and all.

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That is us at our winter dock! We are upriver about 40 miles or so...

ranger42c said:

But then sometimes we run at 20 knots.

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When I first bought Sandpiper, we topped off the fuel & water tanks, had 5 adults on board & went for a sea trial. The photo below shows the speed results. It does not, however, show the mpg! Suffice is to say I've never seen this speed again but I do run at 15-17 knots frequently. At 8 knots, she's about 1 mpg.

The waterline length to speed ratio only works for pure displacement vessels, many on this forum aren't displacement hulls. They just operate them as displacement vessels at displacement speeds.

My 30' hull has a less than 30' waterline length, since it is a double ender. I can run 7 knots (1 gph) or 5.5 (.42 gph) but the true "happy speed" is somewhere in between where the harmonics of the engine are disagreeable. At 7 knots it's a wet ride in any kind of chop, at 5.5 I am floating in a happy little bubble on the surface.

Nobody has mentioned the formula for hull speed for displacement boats = 1.34X the square root of your waterline.

AKDoug said:

I can run 7 knots (1 gph).

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That is awesome!

This is almost double nmpg my 17 foot key west, all of maybe 1200 lbs.

Islander's hull speed calcs. to 7.76 kts. I find I like 1400 - 1600 rpm cruise which is usually between 6-7 knots. My boat is semi displacement and I have seen 12 kts @ 2200 rpm but the engines are working hard. Over time, running my usual cruise speeds I find I'm burning between .8 - 1.4 gpm fuel per engine which seems right in line with most Perkins 4-236s I've heard of in similar boats.

Kevin

1Sailor said:

Nobody has mentioned the formula for hull speed for displacement boats = 1.34X the square root of your waterline.

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So with Klee Wyck that is 9.18 and for Libra that is 9.66. Can't say I ever run much at those theoretical hull speeds. They both seem most comfortable and efficient at around 1600 RPM and (you got it).....8kn.

ghost said:

..... I push the throttles forward until the engines make a sound that matches my mood.....

........Its really funny after awhile how little the exact speed seems to matter.....

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Me too Ghost.

Holding my usual 1400 - 1450 rpm, I've surfed to 14 knots in a perfect 4m swell, and also bashed into a nasty 2m chop at 3 knots, but I still average around 7.

That rev range may not be the most economical, but zoom back just a bit and the exact numbers become less relevant to the joy of cruising.

schrater said:

To the original question, it’s not just length, but also beam. Most trawlers increase the beam along with the length, keeping the same basic ratio intact, meaning the same basic max speed, give or take a knot.

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AHA! that seems to be THE answer!

The 1.34 constant is not an absolute and prismatic coefficient (sorta hull shape from frontal area....rough idea) has a lot to do with the speeds we are talking.

"Nobody has mentioned the formula for hull speed for displacement boats = 1.34X the square root of your waterline."

Most cruisers have no interest in "Hull Speed (1.34) as that is the point where the vessel is stuck attempting to climb her bow wave , with her transom dragging a foot or more underwater in the trough.

Uncomfortable on board and perhaps 3x the fuel burn of SL .9 to 1.15 speed range.

Full displacement hulls don't have a point at which they attempt to climb over their bow wave and plane. I think you misunderstand "hull speed"...

It's a maximum EFFICIENT speed for a displacement hull, if you exceed it your hull will begin to dig down, pull your exhaust underwater at the stern, and consume huge quantities of fuel for minimal gain in speed.

I agree that most full displacement vessels cruise at slightly less than "hull speed" as theoretically calculated. If your vessel is trying to climb over it's bow wave you are not operating a displacement vessel...

I prefer to think of the theoretical Hull Speed of 1.34 x WL as the speed at which you are making a wave in which the boat just fits. You are not climbing, nor are you exceeding the length of your own wave. If you like going faster than that, your bow remains at the front of the wave. After all, it is your bow that is initiating the wave making, so by definition it must remain at the point of origin. Since the wave dips to a low point 1/2 way back to the stern end of the wave, if you travel in a wave that is moving faster than your hull speed, you will always be travelling in the front portion of that wave, so your stern gets further from the stern end of the wave as you go faster. As your stern moves towards the low, middle of the wave, it "squats".
We are all familiar with the concept of needing more power, hence more fuel, to go up hill. That is exactly how the boat is acting, with the bow always at the top of that hill and the faster you push it, the stern getting further down slope.

Does anyone have a Nav Man, I had one on a gas boat once. It identified the sweet spot and helped with range. They are not cheap and once I find the sweet spot, range is not so important with 400 gal of diesel and coastal cruising. The boat feels right at 7 K so although I like toys, I may skip it.

again, 1.34 is just an average number/ constant.

hull shape especially transom shape/ flow changes that number a bit.

unless I am crazy, all the boats on boat design I studied explain this fairly well.

Prismatic coefficient also affects hull speed to a degree.

Simi 60 said:

You speak to non boat people?

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Greatlaker221 said:

Does anyone have a Nav Man, I had one on a gas boat once. It identified the sweet spot and helped with range. They are not cheap and once I find the sweet spot, range is not so important with 400 gal of diesel and coastal cruising. The boat feels right at 7 K so although I like toys, I may skip it.

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They don't make Navman any more. They are hard to come by. I was set up to buy a used one once, but the seller ended up deciding to keep it.

Life is good at 5-6 knots. At WOT I may achieve 7 knots, but I can't see over my wheel house as I plow through the water!

I can run at 8 kts but I burn a lot more fuel at that speed than I do at 7-7.5 kts. I typically will just set the throttle at the gph that I feel like burning. Whatever speed comes out is fine with me. It only matters when I am trying to do some planning as to arrival times.

My boat/engine:

Max rpm: 2400, no boat speed increase over 2200 rpm.
Max speed reached at 2200 rpm; boat speed 7.3 knots
Cruise speed at 1800 rpm; boat speed 6.3 knots, fuel consumption reduced 55% from 2200 rpm.

"I prefer to think of the theoretical Hull Speed of 1.34 x WL as the speed at which you are making a wave in which the boat just fits. You are not climbing, nor are you exceeding the length of your own wave."

You are describing the hull reaction at the Sq Rt of the LWL, what a NA calls "SL".

Some boats make big waves , big drag at SL x .9 others can get to SL x 1.15 .

When at SL x 1,34 or so most boats are bow up, stern dragging underwater .

With enough power Hull speed can be beaten , surfing a wave from astern can add enough push to add a knot or two.

For cruisers to find the "sweet spot" a graph of RPM vs boat speed will show where adding RPM does little for speed .

With no tach or GPS simply observing the boats hole in the water move aft to see when the bow rises or stern sinks works too.

Once the nicest speed is decided its simply a matter of propping to be at that speed with the engine happy.

A spot of paint on the tach face will allow easy returning to cruise speed in busy waters .

The actually hull speed changes based upon 'load out', weight distribution, waves, current and of course wind speed and direction against the superstructure.

So you have two identical boats, running together, it boils down to load out and weight and distribution. Plus how clean the hull is too.

The square root of the WWL X 1.34 is a 'starting point'

Here’s a formula I recall from some uscg exam years ago regarding fuel / speed usage. It’s not just water line length:
Lesser speed is to lesser fuel used as greater speed is to greater fuel used CUBED. This probably explains why my sweet spot is more related to quiet grandkids on boat mean 6.3 knots and less than a gallon an hour. Noisy grandkids (or trying to get home quicker) equal 5 gph. (And 8 knots)

tallswede said:

Islander's hull speed calcs. to 7.76 kts. I find I like 1400 - 1600 rpm cruise which is usually between 6-7 knots. My boat is semi displacement and I have seen 12 kts @ 2200 rpm but the engines are working hard. Over time, running my usual cruise speeds I find I'm burning between .8 - 1.4 gpm fuel per engine which seems right in line with most Perkins 4-236s I've heard of in similar boats.

Kevin

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When we moved from sail to power with a 40’ Silverton we were well used to slow motion. I travel with one engine getting there and the other engine coming back. Each ran at about 1750-1850 RPMs. We average around 1.4-1.6 mpg. At these RPMs the boat’s speed is around 7K

Hull speed hasn’t got anything to do w efficiency ..... directly. It’s just a number having to do w a wave, it’s speed and length.

Efficiency and hull speed probably shouldn’t be used in the same sentence. It leads prople on TF to think hull speed is the most efficient speed to go. It’s not. And AKDoug “most” boats on TF are SD .. not “many”. And our WLL is 27.5’. The hull speed is 7 knots. I may not even be able to reach hull speed w my 37hp but you almost certainly can. And I could do (for all PP) everything I have done w Willy in the last 10 (or so) years w 33hp.

But SD boats are different and probably over 95% of the hulls not planing hulls here are SD. Much more complicated than FD. And many here think they have FD hulls that have SD hulls.

But the “sweet spot” probably has almost everything to do w vibration and noise. And engines don’t care if they are vibrating or making noise. So your engine isn’t “happy” at a given rpm at all. It’s you that are experiencing less vibration and/or noise.

Hull speed is no-way near efficiency. Slowing down a notch can easily reduced fuel consumption by 50% and more.