Hull Speed

[Sadey]

I had an interesting conversation yesterday with a fellow Sam’s member. We were talking about hull speed. We both understand the equation specifics to get at a boat’s specific hull speed. The discussion was about the efficiency or benefit between running at hull speed or a slower speed. My understanding about hull speed is you get maximum efficiency at that speed, most forward progress for the least amount of fuel burned. Obviously, if you throttle back, you will burn less fuel, but you don’t go as fast and of course, inverse to that is throttling up and burning more fuel and going faster. Also, what impact does a displacement or a semi displacement hull design have on hull speed/efficiency?
So the question is if you run at anything less than a boat’s specific hull speed, are you wasting both time and money or just time?
Randy, you stay out of this. We’re not going to talk about beam.

 

[carolinacoast]

I am trying to learn how to use AI efficiently. Here is what I learned about your awesome question. I had some misconceptions.


AI answer: To answer your main question directly: If you run at anything less than hull speed, you are saving money, not wasting it. You are "spending" time to save fuel.

Here is the breakdown of why that happens:

1. Hull Speed is a Barrier, Not a TargetHull speed (1.34 x √LWL) isn't the point of peak efficiency; it is actually the point of diminishing returns. Think of it less like a target cruising speed and more like a "speed limit" for displacement. As you get close to that speed (say, moving from 8 knots to 9 knots), your fuel burn goes up exponentially, not linearly. You might burn 50% more fuel just to gain that last 10% of speed because the boat is fighting so hard to climb its own bow wave.

2. The "Sweet Spot"Maximum efficiency (Nautical Miles Per Gallon) is almost always found significantly below hull speed—usually about 20% to 30% slower. If your hull speed is 9 knots, your absolute best range and fuel economy will likely be around 6.5 to 7 knots. You will get there later, but you’ll arrive with a lot more fuel left in the tank.

3. Displacement vs. Semi-DisplacementYou asked about the impact of hull design:

• Displacement Hulls: These are the efficiency kings at low speeds. They slide through the water easily until they hit that "hull speed wall," at which point they just dig a hole in the water and burn fuel for zero gain.
• Semi-Displacement (SD): These have flatter sections aft that create a bit more drag at low speeds (turbulence), so they are slightly less efficient at 7 knots than a pure displacement hull. However, the rule still applies: An SD hull running at 7 knots is still vastly more efficient than the same hull running at hull speed (or planing speed).

 

[Sadey]

I never thought to ask AI. That’s an excellent description. Surely learned something, mom’s right, you do learn something new every day. I was combining aircraft and nautical fuel and distance traveled efficiencies. I thought hull speed and L/D max, where lift and drag are equal, were same principle. If you’re reading this Put Put, you were on the right track.

 

[carolinacoast]

And for a 53 the hull speed calc is 9.21 knots. Also from AI. LWL for that model is supposedly 47.25 feet. LMK if that is incorrect. That isnt a number I recall, but should tell my diver

 

[Captain Ralph]

We have the old FlowScans on out 12V71TI, 58x18 Bertram.
Deep V hull all the way back to the transom.
Fat asp keel to the first shaft struts.
Lower helm is open to the open stern. I rarely operate on the fly-bridge.

The FlowScan numbers are not accurate but repeatable.
They show a good increase in fuel consumption from idle to hull speed.
Our better slow (SLOW) speed MPG is around 5-6 kts thru the water. Just touch the throttles to make this, maybe 750 RPM.
Engine temps never come up, snail botes out run us. We can feel the wake from canoes. Not any wake of our own.
The exhaust noise rings/howls/resonates in my ears.

Just another 200 RPM, were at 6-7 kts, loosing approx 0.2 MPG. The engine temps are finally moving up but still below 140°.
We are competitive with some snail botes. We can still feel the wake from the snail botes.
A little bow line (not really a wave yet) is just coming off our stem. It may rock some canoes and small boats in the narrow parts of the river..
The exhaust tone is now tolerable.

Just before hull speed, the bow has yet to start rising. 1050 RPM. That line off of the stem can almost be called a wave now. Lost another 0.2MPG.
The exhaust noise starts to come back in my ears.

At 1150 to 1200 RPM, The bow has just risen a couple of degrees. We are beyond hull speed.
We have a bow wave. We have stern waves. We don't notice the wake from the little boats.
We don't feel the water ripple from the wind.
Engine temps are now over 160°
Our whole boat just changed personalities. She feels and rides much nicer.
The exhaust tone is now greatly reduced. I can hear people talk now.

Per my FlowScans, (not accurate but repeatable), What I just typed started around 0.8MPG to 0.45MPG thru the water.

Yes, slower - Better MPG but there are things to consider;
Engine temps is a big concern of mine.
Comfort of the boat.

When it is just us on the St Johns River for a few days, I try to watch the river currents for help and still push up the bow just a bit.

We still have to blow the slobber out of the Detroits every now and then. I don't watch the MPG here, just GPH during the quick run. That number is not pretty either but helps me ensure things are good down below.

I also have to mention, We make offshore passages. Hate the ditch.
We may operate outside day and night.
When it gets sloppy out there, The bow has to come up a lil more.
There is no MPG in my thoughts at these times.

MPG may not be the big concern for all of us. ☺☺

 

[SKYCHENEY]

And for a 53 the hull speed calc is 9.21 knots. Also from AI. LWL for that model is supposedly 47.25 feet. LMK if that is incorrect. That isnt a number I recall, but should tell my diver

That's too short. It's gotta be closer to 49.5
I'll measure it when I get over to the storage barn and report back.

 

[carolinacoast]

Here us an answer from a different AI platform:
For the 53 Hatteras Motor Yacht hull, the published length at the waterline is about 47 ft 3 in (47.25 ft), and this hull is the same basic 53 MY hull used through the 1970s into the 1981–1985 production run. ��Most brokerage and spec listings that actually quote an LWL for the 53 MY show 47.25 ft, and they do not distinguish a different waterline length for early‑1980s boats versus the earlier 1970s examples, because the underwater hull remained essentially the same 53 ft hull mold.

 

[Freeebird]

I never thought to ask AI. That’s an excellent description. Surely learned something, mom’s right, you do learn something new every day. I was combining aircraft and nautical fuel and distance traveled efficiencies. I thought hull speed and L/D max, where lift and drag are equal, were same principle. If you’re reading this Put Put, you were on the right track.

But I wanna talk about beam width!

Actually, it was somebody else who wanted to do that. Yes, the lower the RPM, the better the MPG, but when it comes to total cost of operation, it gets interesting again.

You can’t run a turbo diesel, at slow RPM for long periods of time. Then it becomes a matter of spending more money on secondary things because of the longer time to get from A to B. More run time on the generator, for example. Paying a delivery Captain is another along with food, marinas, toilet paper…

Nobody runs slow because they enjoy going slow. As someone else said, hull speed is the fastest you can run without significantly impacting your billfold.

Trying to determine an exact LWL is a waste of time as the formula for hull speed will change very little for a few feet. The biggest thing with hull speed is RPM to achieve it on slack water.

If you don’t maintain that RPM into a current vs pushing up or back on the throttles to maintain your determined hull speed, it’s all for naught as speed through the water is what matters.

 

[madhatter1]

I wanna go fast

 

[Sadey]

I do miss the speed sometimes.

 

[rsmith]

I’ve been running a 62 Neptunes for a neighbor C18’s having real time fuel flow speed and mpg or more gpm in front of you is pretty amazing. Solid 30 kts @ 100gph makes my smile I’m not paying the fuel. But we’ve done some river cocktail cruises and I was showing him at 7.62 kts 8 gph total is almost 1gpm. Bump it up to 8.5 kts is 14gph or almost double. I don’t know the LWL to calculate theoretical hull speed, but given the huge drop in range with a very slight increase in speed makes me wonder how accurate these theoretical numbers are.

 

[SKYCHENEY]

I notice the same with my new setup. Watching the gpm in real time, you can really dial it in. 8kts is 6GPH, 9kts is 8.5gph, and 10kts is 12gph, and its goes up fast from there. At 19kts, its 46gph and 20kts is 56gph.

 

[Sadey]

I miss the FF info for that very reason but at over $5000 for the parts last time I looked it wasn't worth it. Although 1500 hours later maybe it would have paid for itself.

 

[dar636]

I miss the FF info for that very reason but at over $5000 for the parts last time I looked it wasn't worth it. Although 1500 hours later maybe it would have paid for itself.

That formula for upgrades ($$ vs. hrs.) my Admiral uses with great effect.

 

[Chasemmc]

A related question: if lower RPM's don't allow engine to fully come to temp, why not change the thermostat to higher temp?

 

[rsmith]

Thermostat has nothing to do with it it’s the combustion temperature in the cylinders and exhaust valves that burn off the carbon. The thermostats are shut at low speeds higher thermostats won’t do anything. Ever see a trucker put cardboard over the grill? It’s because the radiator or heat exchanger is provided too much cooling and the restriction is to get the engine temp up into control range of the thermostat.

 

[Captain Ralph]

A related question: if lower RPM's don't allow engine to fully come to temp, why not change the thermostat to higher temp?

The thermostat is at a higher temp. It will not open until 180°.
It's the extra cooling capacity that keeps everything cool at higher HP that keeps things real cold at lower HP (thermal productivity).
I remember some engine MFGs experimented with two state cooling to help keep temps up at low HP demands. It turned into some issues and was mostly discontinued.

If we had small engines that never made big HP, they would run a lil harder, keep temps up and run more efficiently, like a trawler.
But we have big engines, designed to make big HP and the second we start them up, the checkbook gets smaller.

 

[Chasemmc]

I have questions, so I'll start a new thread.
Thx
Mark

 

[Pascal]

Except for my 53, everything I ve run in last 18-20 years has had flow gauges being full electronic engines. It tells you a lot.

Here are some real world numbers about fuel economy approaching hull speed. Yes the boat is bigger but the principles are the same

Here is my original flow charts I did when I got on the boat 5 years ago and just doubled checked them while cruising in the Exumas in 20’ of water and calm conditions. The 110 LOA has a 91’ LWL which results in a theoretical hull speed of about 12.4 knots. The numbers are very telling

Rpm	Gph	Speed	NMPG	250nm
1150	23	11.0	0.48	521usg
1200	28	11.6	0.41
1250	32	11.9	0.37
1300	37	12.1	0.33	757usg
HS	12.4

Just below hull speed we get 0.33 nmpg which for a 250 nm run from Miami to Staniel requires 757 USG of fuel. If we slow just little over a knot down to 11, we get 0.48 nmpg burning 520 USG.

So, saving two hours at the higher speed would cost us 230 gallons. Not worth it. Just like if we were to do the trip on plane at 20kts, we d be burning about 1600 USG!

 

[Buccaneer]

What are the numbers for 8-92TAs on a 70 CPMY ?