Hull Speed

Good morning Fish, and Merry Christmas Eve.
Being just a normal captain, and no Brainiac I've always determined my most efficient running speed with my gut ("feel") based on my experience. Today I'll also let a Flowscan do my thinking for me. I also check the speed curves supplied by manufacturers from time to time although mostly just out of curiosity. The term "hull speed" was something I never ran across until I joined YF. I still consider my gut a more accurate indicator, 2nd to a FLowscan, because it takes into account LWL, beam, draft, hull shape, weight, current and water depth, but I always like to expand my knowledge. Today, as I followed your link I found that hull speed really isn't used as an accurate indicator, and instead the "Froude number" is used.
So for the Brainiacs on the Forum, I'm curious if and how many boaters do this calculation to determine their sweet spot.

I tend to use my gut or experience for determining hull speed. Generally diesels are most efficient at 1000 rpms or less (ones that top out at 2350rpms). But hull speed can also be drastically effected by the balance of the boat and where weight is concentrated, such as whether your fuel is in a foward tank or aft tank. But generally I start at idle and take a notice of speed, then up it each 100 rpms, usually you'll see a .5-1 knot gain at slow speeds. When you up it another 100rpms and only see a 0.2 gain, you've passed the hull speed generally......and I back it down 100 or 50 rpms.....You'll also see a change in running attitude or your wake or both.

The 75' Hatteras I used to run had a 10' extension put on, it had a foward, midship and cockpit fuel tank. When trimmed right at 1000 rpms you'd see 10.2-10.5 knots.....if you got off 30 gallons too much of fuel drained out of the foward or cockpit tank it would slow down to 9 knots at 1000 rpms.......As soon as you noticed it slowing down, I'd valve both engines to whichever tank was too heavy and 30 mins later it'd start picking back up.

Maybe you meant that the hull's most efficent running point (I am assumming based on fuel burn or MPG) usually can be reached by the horsepower developed at 1000 rpm?

I know of no diesel engine manufacturer who would claim peak engine efficiency is achieved at 1000 rpm for a 2350 rpm engine

Yes the hull's most efficient running speed.

Diesels burn a miniscule amount of fuel at 1000 rpms or less compared to say 1950 rpms. For example: a 12v71 TI, burns 6GPH at 1000 rpms, at 1200rpms you're looking at double or 12.5 gph, at 1950 rpms 40-45 gph.

Most yacht hulls are most efficient at hull speeds. It takes a proportionately lot more hp to keep a boat on plane.

So hull speed has nothing to do with engine EFFICIENCY, right?

Not necessarily. How much fuel does a diesel use idling without a load, next to nothing. If you look at a diesel that is turbocharged, the amount of fuel (gph) used per hp made increases exponentially the more rpm's you turn the engine after the turbo comes online.

I would typically make my own speed/fuel burn chart. Since you have a mechanically injected engine with no electronic fuel consumption data output, you will either have to rely on CAT fuel burn data from their manual, using the Cubic Propeller Demand Curve for estimated fuel consumption or install a Fuel Flow Meter System like a Floscan.

Load up your boat as you would normally, bring it up to full rpms, and take speed readings at max rpm and every 100 rpms below that down to idle. Now you will have to match the fuel data from the CAT manual, and this will give you an estimate of fuel burn for 1 engine at that rpm setting. Multiply by 2 and that gives you your estimated total fuel burn (not including a generator) for that rpm setting. Divide the speed by the total fuel burn and you will get an estimated mpg, and look at your data for the best fuel efficiency point. My guess is that it will line up to or below the 1500 rpm 9 knot range, before the turbos are spooled up.

At 2700 rpm. you seem to be about 100+rpm low, meaning if all engine parameters are good and the hull is clean, you are a bit over-pitched at the moment. Typically, you would like to see about 2825 rpm at wide open throttle, but that is another discussion.

You are missing the point. Fuel burn at idle with little to no power means nothing to this discussion. By your analogy, an efficent diesel would not be turbocharged, and this is contrary to what is going on in Industry.

This thread is about finding the most efficient operating setting of the engine in relationship to the resistance developed by it's hull form, hence the opening question on theoretical hull speed.

Merry Christmas

To add one more thing, best you know your true waterline length. For sailors in the crowd who already know , the importance of WL is immense.

I already answered the hull speed and efficiency in an earlier post related to the OP's question.


A turbocharged diesel has a more efficient hp made versus fuel used than a non-turbo'd diesel generally at cruise rpms or the rpm it's designed to run at or 80% load, and makes the engine more efficient with fuel used versus horsepower made or put out in the upper rpm's where you have boost. Under the turbo boost at lower rpm's they're actually less efficient because the turbo is causing a minor restriction, and you have larger injectors which don't meter the fuel as efficiently at lower rpm's. Turbo diesel efficiency might be vital information if your looking at a fully loaded generator that you run at a constant rpm, or a boat that is constantly running at cruise and your looking for maximum speed/performance/fuel burn at cruise rpm. The OP is not.

Sorry, your are not providing accurate information again, I would leave the Engine Analysis to guys like Marmot, etc.

Marine diesel engines are typically designed for peak output at peak load. I don't know of any manufacturer who would currently design the engine to 80% load or whatever cruise rpm's and accept the aditional 20 - 30% leftover as just bonus. It just doesn't happen that way. They do design today's engines to meet certain emission criteria at defined rpm's, as dictated by the EPA.

What restriction at low (idle, 1000, 1500???) rpm's does a waste gated Turbo contribute in the combustion process? news to me...

Today's diesels have their fuel metered electronically, independent of Injector size.

This has nothing to do with a generator. You stated "Generally diesels are most efficient at 1000 rpms or less (ones that top out at 2350rpms)." and that is not an accurate statement, it just relates to two things: 1. A 2350 rpm diesel will burn less fuel at 1000 rpm than WOT. 2. Hull Speed will be achieved at lower rpms, maybe even 1000 rpms, depending on how the specific vessel was powered.

Is this your way of stating that a 2350 max rpm diesel is most efficiently operated at 2350 rpm?

No it is not.

Isn't that sort of like saying you are going as fast as you can when you can't go any faster?

There hasn't been an engine built that didn't produce the same output as the load consumed.

In a nominal situation, then yes.

Not including the point when the engine is overloaded and lugged down, but that is a different topic, not related to the thread.

There are plenty of Commercial engines built and tested at 110% overload, but sold and warrantied at the 100% rating, but this is not typically seen in yachts under 100'.