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Old 03-28-2018, 03:23 PM   #41
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Nobody has mentioned the formula for hull speed for displacement boats = 1.34X the square root of your waterline.
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.
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Old 03-28-2018, 04:49 PM   #42
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..... 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.....
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.
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Old 03-28-2018, 06:29 PM   #43
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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.
AHA! that seems to be THE answer!
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Old 03-28-2018, 06:45 PM   #44
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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.
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Old 03-29-2018, 11:41 AM   #45
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"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.
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Old 03-29-2018, 12:03 PM   #46
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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...
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Old 03-29-2018, 02:31 PM   #47
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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.
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Old 03-29-2018, 04:37 PM   #48
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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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Old 03-29-2018, 05:33 PM   #49
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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.
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Old 03-29-2018, 05:41 PM   #50
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You speak to non boat people?
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Old 03-29-2018, 07:54 PM   #51
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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.
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.
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Old 03-29-2018, 08:21 PM   #52
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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!
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Old 03-29-2018, 08:40 PM   #53
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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.
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Old 03-29-2018, 10:49 PM   #54
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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.
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Old 03-30-2018, 06:43 AM   #55
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"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 .
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Old 03-30-2018, 07:34 AM   #56
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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'
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Old 04-01-2018, 03:28 PM   #57
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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)
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Old 04-01-2018, 03:52 PM   #58
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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.

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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
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Old 04-01-2018, 09:20 PM   #59
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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.
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Old 04-01-2018, 09:37 PM   #60
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Hull speed is no-way near efficiency. Slowing down a notch can easily reduced fuel consumption by 50% and more.
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