Big HP vs Small HP

[Rufus]

If fuel flow data are handy (dedicated sensor or from an electronically controlled engine) it's easy to create a speed/fuel flow chart. Just go out on a dead calm day and write down the numbers at evenly spaced engine RPM. Do a run against any current, and then do another in the exact opposite direction. Average the two.

If an engine curve and a prop curve are available for the hull, do the same thing but record engine rpm and speed. Then use the two curves to extract ballpark fuel burn at each rpm/speed. The prop curve for our boat is surprisingly accurate....at least when compared to cumulative real world fuel burn at our normal cruise speed.

The same methodology can be used to establish comparative data for single versus twin engine efficiency gains. It's about 5% for our 44 when operating at the same slow cruise speed of 8.0 knots. (The fuel burn improvement would increase at slower speeds where rudder angle would be much less).

Bottom line...get the engine and prop charts for your hull, or install a device that measures fuel flow on at least one engine. If an engine and prop chart are not available for your hull, it says something about the boat builder....

BTW, a Grand Banks is not a displacement or a "by design" semi-displacement hull, nor are the typical Taiwan Trawlers. They're planing hulls...typically poor performers at higher speeds with too small engines, and awful performers if they have engines big enough to get over the hump.

 

[Don L]

If my theory (chart above) is close, running between 1.2 and 1.7 is tricky because you're not reducing time by much but you are reducing fuel economy quickly.

Peter

Op here and still reading as sometimes we got back to the topic.

I will say on my 41' sailboat I have years of fuel use and speed data for like 2000 hours.

I have learned from that that motoring at 5 kn gives pretty much the same mpg as motoring at 6.5 kn. If fact on paper the 6.5 kn seems to give better mpg (not enough to really consider).

While this just seems wrong I believe it is because you have to consider the whole boat not just the engine. There is engine efficiently, prop efficiency and hull efficiently.

 

[rslifkin]

While this just seems wrong I believe it is because you have to consider the whole boat not just the engine. There is engine efficiently, prop efficiency and hull efficiently.

Agreed. Sometimes the increase in hull drag as speed increases is offset by other factors that lead to better efficiency.

 

[twistedtree]

Yep - I think that's right. Us Trawler owners have a "Prius Mindset" were it's all about reducing consumption. But that ignores the time-side of the equation. And let's face it, vast majority of us have boats capable of near-planing speeds.

If I get a bit bored I may convert the raw data on TTs GB plot to single data plot similar to what I posted above (speeds converted to S/L vs knots).

I'll go a couple steps more granular:

First, I'll challenge the notion of S/L 1.34 as the "right cruise speed." I suspect it's quite a ways past optimum efficiency. Throttling back to 1.1-1.2 delivers a decent fuel benefit without much time penalty.

I totally agree. There is nothing sacred about S/L 1.34 It's just a point on a continuous curve. It's like the mythical 50% depth of discharge for lead batteries.



I'll also say there is no "optimum efficiency" point, at any speed. Fuel efficiency is measured in NMPG. In a boat, that ALWAYS drops as you go faster. There is no inflection point in the curve that represents or even suggests optimum efficiency. It's all downhill, all the time. There are places where the curve climbs faster and where it climbs slower, but it's never flat, let alone drops.


When people talk about a "sweet spot" for cruising speed, it's just a balance point that you are comfortable with in terms of NMPG and time to get where you are going.

Second, inline with TT's conclusion, if you're going to go past 1.34, get up and go. I suspect there's a dead-zone between S/Ls of approx 1.3 and 1.8 where small changes in speed (and time) have outsized impact on efficiency. If time is your limiting factor (i.e. wanna get there sooner), crank 'er up. Fewer engine/generator hours and not much of a fuel penalty.

Peter

 

[mvweebles]

So I plotted the GB data from TT's PDF. I think it shows the difficulty in simply measuring as described by Rufus (and is a common method). It's one thing to write-down numbers, but when you plot them in X-Y scatter vs two series, gaps emerge. At the slow-end, it becomes more difficult to measure accurately and the data in tension (frictional losses) become more prominant and skew the numbers. There is no way to measure near-zero consumption, but there is a point where the boat will not move but at least a little fuel is being consumed - efficiency is zero. What that looks like and where the curve peaks is difficult - there has to be a 'hump' in the curve somewhere. I know, easy to say "so what?" but I think it matters to explain the curve in general.

Bottom line is the GB 42 plot shows the efficiency 'hump' at below 0.4 SL which is 2.5 kts. I cannot reconcile that the frictional losses have not imparied efficiency by then. I would also expect more of a penalty from displacement speed to planing speed - from 0.6 SL to 4.0, it's fairly linear with just a minor bend at ~1.3-ish. I don't buy it - taking a 40k lb boat from in-the-water to on-the-water is a big transition. My point being - hard to accurately measure the data, even harder to interpret.

Peter

 

[Don L]

what is this S/L people keep posting about?

 

[rslifkin]

what is this S/L people keep posting about?

Speed / Length ratio. It's speed in kts divided by the square root of the waterline length of the boat. So for a 40 foot waterline, an S/L of 1.0 would be 6.33 kts. Theoretical hull speed is at S/L 1.34, which would be 8.48 kts for a 40 foot waterline.

 

[mvweebles]

Theoretically, S/L of 1.34 is the speed at which the boat becomes trapped by its bow and stern wakes and has to climb up on plane, which takes a lot of power to levitate the boat. Long skinny hulls are different.

Somewhere I have a picture of a 45 foot pilot boat alongside a 500 foot ship both moving at around 15 kts. Ship is throwing zero wake, pilot boat is damn near planing.

Peter

 

[Don L]

So motor at the square root of the hull length would be considered a good displacement speed to use for economical motoring????

And anything past 1.34 times that comes greatly decreasing mpg?

 

[DDW]

Here are the speed/economy curves for my AT34, actual measured values at rpm from 1300 to 3050 redline. There is a flat spot between about 2100 and 2400 where it is climbing over the (considerable!) bow wave and maintaining efficiency while speeding up, then it turns down again. S/L of the lowest speed in the chart is 1.1. That said, we cruise typically at 1200 or 1250, 6.9 - 7.3 knots depending on fuel load, and 1.8 - 1.9 G/H. By running at 1100 I can get the economy over 4, but I am not patient enough. Engine is a Cummins QSB 380. At 1250, putting out about 40 hp.

 

[Art]

Go Horsy - GO!

 

[mvweebles]

So motor at the square root of the hull length would be considered a good displacement speed to use for economical motoring????

And anything past 1.34 times that comes greatly decreasing mpg?

It applies to sailboats too, not just powerboats. Probably easier to see on a sailboat - when pressed hard, the bow wave and the stern wave are pretty visible. 1.34 is a number. I don't recall how it was derived but it's often repeated as the boundary layer speed for a standard hull. I like to say my full-displacement Willard 36 (33-ft waterline, S/L of 1.4 = 8 kts) would not do 8-1/2 kts if she had a pair of Pratt & Whitney's bolted to her deck. I am not certain, but I suspect that the fuel burn difference between 6-1/4 kts (1.1 SL) and 7-1/2 kts (1.3 SL) would go from 1.0 gph (6.25 nm/gal) to 1.5 gph (5.0 nm/gal). For a 20% increase in speed, a 50% increase in fuel consumption.

Peter

 

[ranger58sb]

I did a quick plot of what I would expect to find - peak would be in the 0.9-1.0 S/L range and drop off pretty quickly after 1.2. Other note is my guess would be that for Semi-Displacement boats with enough power to plane, efficiency flattens out pretty early, meaning might as well throttle-up and get on plane vs push a bow-wake. Overall fuel consumption won't change.

Thoughts? Is there better information vs a semi-educated guess (emphasis on guess)?

I totally agree. There is nothing sacred about S/L 1.34 It's just a point on a continuous curve. It's like the mythical 50% depth of discharge for lead batteries.

what is this S/L people keep posting about?

Speed / Length ratio. It's speed in kts divided by the square root of the waterline length of the boat. So for a 40 foot waterline, an S/L of 1.0 would be 6.33 kts. Theoretical hull speed is at S/L 1.34, which would be 8.48 kts for a 40 foot waterline.

So motor at the square root of the hull length would be considered a good displacement speed to use for economical motoring????

And anything past 1.34 times that comes greatly decreasing mpg?

Beebe speak to all that... IIRC I think he landed on 1.0, 1.1, or 1.2 depending on boat, hull, situation, weather, how the mule felt that day, etc.

-Chris

 

[Hippocampus]

Given all boat designs are different with different prismatic coefficients and parasitic drag is there a simple way to calculate “best” cruising speed. Have heard this expressed as a percentage of hull speed. Is there merit to that approach?

There’s a notable difference in sailboats. I don’t know to what extent is translates to power. Much discussion about boundary layer, wetted surface and parasitic drag. Although nothing is really new the current generation of slice of pizza boats have just about the same wetted surface regardless of heel angle. Prior boats tended to dig a hole when overpowered and often went faster when reefed. Flat is fast was the mantra. Also to some extend until recently sailboat hulls were balanced. By this they mean the plane at the water line was fairly symmetrical. Recreational trawlers tend to widen from the bow and then stay about the same. Recently there’s been a shift in powerboat design in part derived from the thinking behind ultralight sailboat sleds. Low displacement and long with very low prismatic coefficient. If done right the wall of hull speed is overcome to some degree . Then you have hydrofoils and all bets are off.
So I read this thread keeping in mind most of us have traditional designs. But I wonder what physics apply to LDL, wave piercing bows and other more novel design features?

 

[Don L]

Go Horsy - GO!

I just have wonder the need of this post

 

[Hippocampus]

Have several friends contemplating LDL boats v traditional . TT has been very helpful explaining the downsides of LDL. You hear the upsides from the manufacturers. It’s fun having friends and having clue about what they are saying. Interacting with knowledgeable people without skin in the game helps me get there.

 

[Art]

I just have wonder the need of this post

Got bored. Having read same general items over and over again... Just thought a bit o' levity might bring a smile. Obviously, not to all!

 

[Don L]

Have several friends contemplating LDL boats v traditional . TT has been very helpful explaining the downsides of LDL. You hear the upsides from the manufacturers. It’s fun having friends and having clue about what they are saying. Interacting with knowledgeable people without skin in the game helps me get there.

I am a sailor and don't understand the code letters. Do they impact the HP thread question or I move along?

 

[SteveK]

Got bored. Having read same general items over and over again... Just thought a bit o' levity might bring a smile. Obviously, not to all!

Put a smile on my face. As with most threads they derail after the question is answered many times.

 

[DDW]

The 1.34 isn't an arbitrary number, though it is a number along a smooth curve. It comes from the equation for velocity of a wave in water: v = (L * g / 2Pi)^0.5, pulling out the constants and rearranging units etc. you get 1.34 * L^0.5. It is the speed of a wave with length L. The idea is that the bow and stern wave peaks now coincide, any further speed requires climbing on the bow wave and leaving the stern wave behind.

The hull doesn't affect that number, but the shape affects the slope of the curve around it.

 

[Nomad Willy]

I don’t understand why the horse posts exist. If you don’t like a thread just look at another thread. A no brainer IMO.

And as long as some are talking it’s justifiable to be active. Guys have something to say … post.
Or go to FB or wherever…..

 

[OldDan1943]

Maybe he expect the Feds to ban the entire forum?
OR
You can channel the channel.

 

[Art]

I don’t understand why the horse posts exist. If you don’t like a thread just look at another thread. A no brainer IMO.

And as long as some are talking it’s justifiable to be active. Guys have something to say … post.
Or go to FB or wherever…..

Eric... Please don't be offended at my attempt to get smiles. Humor Eric... Humor... Just in humor! Smiles are good for your heart... wives usually like it too... if/when their/we Ol' men smile once in a while; as we consistently read TF posts!

= = :lol: = =

 

[Don L]

Put a smile on my face. As with most threads they derail after the question is answered many times.

I don"t feel the orginal question got "answered many times" plus no one makes people read or comment. So when they do on an "answered many times" thread 8-9 pages in it is really about them having a need.

But I know what to do.

 

[Hippocampus]

Enjoy the wisdom and experience of many here. Look forward to learning something new. Thought some of comment about sail c/w power were off based. Also some of the comments about power didn’t include the range of offerings.
Perhaps you have no interest but others wish to expand their knowledge or correct prior misconceptions. Amount and nature of HP directly relates to the hull it’s being put into and the mode of service. Free feel to ignore my posts but have no apologies for them.

 

[mvweebles]

I am a sailor and don't understand the code letters. Do they impact the HP thread question or I move along?

I forget the exact acronym, but LDL is essentially a long skinny powerboat. Steve Dashew's FPB is the poster child but there are others (a pic of an FPB is attached).

TT is shorthand for TF poster "TwistedTree," the name itself being a mangle of his boat's name "Tanglewood."

Peter

 

[Hippocampus]

Interesting to look at HP decisions on the long low displacement hulls produced by Gladding Hearns in Somerset MA. They are the largest producers of pilot and patrol boats in the US. Made of Al and propelled in the main by waterjets. Operate commonly at 30-45kts where waterjets become quite efficient.
Seaton has taken these Hunt designed deep V hulls and put conventional props and a cruising interior in to them. Made to operate at mostly displacement and semi displacement speeds. Goal was create a brick sh-thouse boat on a very well proven hull for a million (or more) less money than what currently available in the niche market of Al LDL boats like the FPB derivatives.
Find it hard to get my mind around taking a Hunt deep V and making it a cruiser. How do you calculate what power to put into it? Think for all these Low Displacement Long powerboats hull speed isn’t the primary concern. Would think as long as the penalty on range isn’t too severe pile on the HP.

 

[Don L]

Ok thanks for the answers. I have pretty much come to the conclusion that as in post #1 that the size of the engine in the same boat going the same hull speed or less is same mpg for all practical purpose.

Will mostly disregard when comes buying time on basis of just HP.

Going to unsubscribe so go at it with as much drift as makes everyone happy.

 

[Marco Flamingo]

I'm lucky. No need for graphs, charts, horsepower calculations, prismatic efficiency coordinates, effective waterline dynamic ratios, fuel burn estimations related to barimetric conditions, total gross weight effecting deadrise runout, etc.

If I go faster than 7 knots, I get there too soon.

 

[twistedtree]

Ok thanks for the answers. I have pretty much come to the conclusion that as in post #1 that the size of the engine in the same boat going the same hull speed or less is same mpg for all practical purpose.

Will mostly disregard when comes buying time on basis of just HP.

Going to unsubscribe so go at it with as much drift as makes everyone happy.

I think that’s the bottom line.

Hint…. When a debate never ends, it’s because the answer doesn’t matter…..