Mileage vs Consumption

Gasoline isn't allowed on my boat. Thank you.

Insequent said:

I'll be interested in Fred's views as well. His rules of thumb are pretty good.

For what its worth, I have found the Hawaii Marine Company spreadsheet for Displacement and SemiDisplacement hulls pretty good also.

Displacement and Semi-Displacement (or Semi-Planing) Hull Powering Calculations, Description

It uses material from Gerr's Propeller Handbook amongst other sources. It has a small fee and user licence so I cant post the xls file I played around with using some of Art's info. I had to guess a few things, but for Art's Tolly I got a figure for SLR of 1.37 from it, giving 'hull speed' of 7.7 kn.

The xls file seems to manage the FD versus SD hull type issue reasonably well. For SD hulls, like my own, I find that SLR of 1.2 is still pretty economical. For Art, 1.2 is a speed of 6.8 kn so it seems to fit for his Tolly as well.

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TY, Brian

markpierce said:

Gasoline isn't allowed on my boat. Thank you.

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Mark - Don't be Scared! ... Ya Simply Gotta be Careful!!

Have you no wish/need for a dink w/ gasoline fired ob motor?

Insequent,
Good post and link. I was especially interested in what he calls "aft quarter butt exit angle". I've refereed to this several times as the QBBL or "quarter beam buttock line". He made mention of explanatory graphics but I found none. I was hoping to see a definitive angle that would define SD and FD hulls.

But there is more to it than just the QBBL angle. There are lots of boats w straight QBB lines and many (or even most) have curved buttock lines. There are also hooked aft buttock lines that are obviously concave usually to keep a SD hull from squatting too much. If they achieve that they usually make a big bow wave and are wet. But some just squat anyway and some even turn out right. But most FD hulls have curved (convex) buttock lines. I offer my own Willard as an example. Very nonlinear as well.

Art said:

...

Have you no wish/need dink w/ gasoline fired ob motor?

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What outboard?

markpierce said:

What outboard?

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Mark - I am impressed at how much you look like Popeye! Congrats!!

BTW: I was a competing rower in one of my early lives... Wanna race??

I really enjoy our 50 hp 39 knot WOT tow behind gasser "dink"!

Sounds like fun, Art, but your "gasser dink" doesn't appear to be able to mount on the Coot. (Rowing a boat was my first "real" boating experience, which I recall was on Clear Lake, CA some fifty-plus years ago.)

Racing? This looks like the way to go!



(Why are they all looking at me and not rowing?)

markpierce said:

Sounds like fun, Art, but your "gasser dink" doesn't appear to be able to mount on the Coot. (Rowing a boat was my first "real" boating experience, which I recall was on Clear Lake, CA some fifty-plus years ago.)

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Mark

On Coot, no need to mount a comfortable "4-seater" cruiser dink that is economical to run, powerful and fast. Short tow line or side tie for close quarters / slow speeds (canals, docking, fuel stops and the like). Longer tow line for cruises at speed. A tow behind "dink" (gunk holen run about) with good hull design will act well in even fairly rough seas. Fender pleasingly bolted to tow behind dink’s bow allows slow speed transom bumps during close-maneuvers leaving no marks. Often towed a 13' 3” Boston Whaler w/ 40 hp Johnson throughout New England coastal and inland waters for many years during 20th Century's 3rd quarter.

We love to tow our 14’ 8” 50 hp Johnson Crestliner “Stinger” behind our Tolly!

Try It - You'll Like It!

Art ... mark has a TRINKA

You can't beat a trink.


But on gasoline Mark's got his head in the sand.

manyboats said:

But on gasoline Mark's got his head in the sand.

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I wouldn't say that Eric. Mark broadened his horizons recently and moored 4' away from a gasser. Bliss felt privileged to accompany the Coot for an evening.

manyboats said:

Art ... mark has a TRINKA

You can't beat a trink.


But on gasoline Mark's got his head in the sand.

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Even ample gasoline fumes emanating from sand can explode to flame... Mark you've no place to hide! Fear can kill!! - LOL!

10′ Trinka Yacht Tender


10′ Trinka Yacht Tender | ellisboatbrokerage.com

[FONT=&quot]

manyboats said:

Insequent,
.....I was especially interested in what he calls "aft quarter butt exit angle". I've refereed to this several times as the QBBL or "quarter beam buttock line". He made mention of explanatory graphics but I found none. I was hoping to see a definitive angle that would define SD and FD hulls.

Click to expand...

Eric, if you bought the Hawaii Marine xls template the graphic is on the 'References' tab. And that is taken direct from Dave Gerr's Propeller Handbook, p13. My copy of the book is on the boat so I cant check Gerr's discussion of it at present. You now have me curious about the QBBL for D to SD transition. I’ll have to do more research. Its more than 2 deg, but how much more I don’t know. [/FONT]

[FONT=&quot]The xls template refers to 2 deg or less where SLR >2.5. That’s getting toward the SD to planing transition. At 17 kn Art's Tolly gets to SLR 3, maximum possible for SD mode (Gerr). So his installed power is a good balance with his hull shape.[/FONT]

[FONT=&quot]My QBBL is 2.5 deg. I don’t believe I get near proper planning. I think this is in part due to QBBL and in part too much weight. When delivered new, boats like mine all had trim tabs fitted to try and counteract stern squat and keep the bow down. With trim tabs, the boat could get to 17kn at WOT. This is an SLR of about 2.48. It seemed to me that using trim tabs for this purpose was just forcing a hull shape to perform above where it should, at a cost of high HP and fuel requirements. [/FONT]

[FONT=&quot]Above about 10.5 kn I really noticed the stern squat and got a feeling of plouging through the water rather than planing. It was still noticeable at 16 kn. So I normally would not try for more than 10 kn SOG unless I had current assistance. Considering this, when I repowered I went down to 402 HP from 540 HP. I now get 11 kn at WOT, without trim tabs (I removed them) for an SLR of 1.6. But 10 kn SOG without current assist still has a reasonable bow attitude. This is my high speed cruise mode. Using the Gerr formulae and my boat’s info, SLRmax (maximum possible speed for displacement mode, which calc incorporates DLR) is 1.428 and thus Vmax is 9.8 kn. So I’m into the SD mode, but not into it very far before my QBBL and weight give serious stern squat and really hurt performance. If I go back to 7.5 kn, SLR is about 1.1. That is clearly displacement mode, and my low speed cruise or economy setting. [/FONT]

[FONT=&quot]Now to the point of all the above discussion – I don’t see any sharp breaks from SLR 1.1 to 1.5, it’s a smooth change in HP or fuel used and speed change. My preliminary conclusion is that for me to get good SD performance, QBBL should be less or the boat should be returned to its as-delivered light ship weight. Neither is feasible. But what I can do is add the 5’ extension that many folks have done with their Mk 1’s to get a longer LWL. Unsubstantiated reports of an extra 1 kn. I’d like that. I just didn’t like the quotes to do it. I’m also starting to think that the definitive QBBL number Manyboats (and I) would like to see is going to be quite low.[/FONT]

The most efficient boat, don't burn any fuel

Insequent said:

[FONT=&quot]Eric, if you bought the Hawaii Marine xls template the graphic is on the 'References' tab. And that is taken direct from Dave Gerr's Propeller Handbook, p13. My copy of the book is on the boat so I cant check Gerr's discussion of it at present. You now have me curious about the QBBL for D to SD transition. I’ll have to do more research. Its more than 2 deg, but how much more I don’t know. [/FONT]

[FONT=&quot]The xls template refers to 2 deg or less where SLR >2.5. That’s getting toward the SD to planing transition. At 17 kn Art's Tolly gets to SLR 3, maximum possible for SD mode (Gerr). So his installed power is a good balance with his hull shape.[/FONT]

[FONT=&quot]My QBBL is 2.5 deg. I don’t believe I get near proper planning. I think this is in part due to QBBL and in part too much weight. When delivered new, boats like mine all had trim tabs fitted to try and counteract stern squat and keep the bow down. With trim tabs, the boat could get to 17kn at WOT. This is an SLR of about 2.48. It seemed to me that using trim tabs for this purpose was just forcing a hull shape to perform above where it should, at a cost of high HP and fuel requirements. [/FONT]

[FONT=&quot]Above about 10.5 kn I really noticed the stern squat and got a feeling of plouging through the water rather than planing. It was still noticeable at 16 kn. So I normally would not try for more than 10 kn SOG unless I had current assistance. Considering this, when I repowered I went down to 402 HP from 540 HP. I now get 11 kn at WOT, without trim tabs (I removed them) for an SLR of 1.6. But 10 kn SOG without current assist still has a reasonable bow attitude. This is my high speed cruise mode. Using the Gerr formulae and my boat’s info, SLRmax (maximum possible speed for displacement mode, which calc incorporates DLR) is 1.428 and thus Vmax is 9.8 kn. So I’m into the SD mode, but not into it very far before my QBBL and weight give serious stern squat and really hurt performance. If I go back to 7.5 kn, SLR is about 1.1. That is clearly displacement mode, and my low speed cruise or economy setting. [/FONT]

[FONT=&quot]Now to the point of all the above discussion – I don’t see any sharp breaks from SLR 1.1 to 1.5, it’s a smooth change in HP or fuel used and speed change. My preliminary conclusion is that for me to get good SD performance, QBBL should be less or the boat should be returned to its as-delivered light ship weight. Neither is feasible. But what I can do is add the 5’ extension that many folks have done with their Mk 1’s to get a longer LWL. Unsubstantiated reports of an extra 1 kn. I’d like that. I just didn’t like the quotes to do it. I’m also starting to think that the definitive QBBL number Manyboats (and I) would like to see is going to be quite low.[/FONT]

Click to expand...

Brian... yours, Fred’s, Eric’s and some other TF members inputs are priceless.

Add to my boat’s stats - post 48: Tolly’s top condition 255 hp gassers push her 16 to 17 knots (at 1 +/- nmpg); just before quadrajet secondary opens (3,300 +/- rpm with trim tabs in play). WOT’s 4,400 rpm reaches 21/22 knots (.05 nmpg – if lucky, Ouch!). We often cruise with twins pushing us at 6.5 to 7 knots for about 2 nmpg (3 + gph). For lackadaisical loafing-along we sometimes run on either one screw at 5 + knots and around 2.75 nmpg. If we’ve need to cover distance rather quickly the 1 nmpg 16/17 knot full plane speed is enjoyable, and rather quiet too (especially on fly bridge) w/ synchronized gassers in well insulated engine compartment under salon sole.

Believing in 20% safety-fuel reserve (40 gals in our case): Tolly’s 2 – 100 gal tanks give us approx 160 mile range using twin screws at 16 + knot full plane / 320 miles at 6.5 knots / 440 miles at 5 knots on one screw.

Not too bad for a beefy, comfortable, sea worthy, and fully self contained pleasure cruiser (sometimes referred to by others as a “Trawler” – lol)... We do like our easy to live with Tollycraft. She’s affordable and reliable as all get out!

Nmpg, gph and mile range calcs are considering our Tolly fully loaded... but, they do not take into account tow behind runabout or gen set usage. I deduct 10% off these calcs regarding on-board fuel-stock as compared to range availability when both those added factors are included for a cruise.

Here’s the trip summary for both Colombia to Aruba. We had some positive current and 15 kts on the nose (average).

279 miles 1.46 gph 3.82 mpg

And here’s the trip summary from Bonaire to Trinidad: 425 miles. We had contrary current of at least 1 knot and as much as 2.5 knots for the entire trip.

First 224 miles: 1.5 gph 3.2 mpg
Last 201 miles: 2.0 gph 2.0 mpg

The first half we had lighter head winds and less current. We typically cruise at 6-6.5 knots, burn 1.8 gph and get 3.8 mpg with light winds and no current. The paravanes were in the water the entire trip which costs us about 3/4 of a knot.

Thanks for the info guys, all very usefull numbers. However, to be able to compare apples with apples the base criteria must be the same for all. Speed being the most significant factor here. I picked 7 knots because most 40 foot lwl boats can do that economically. 6 knots is even more economical and is also about the average speed my trawler makes on longer trips. We should not add in generator usage or towing a dink, Stabilization is a factor that is more difficult to account for, but on a round bottom trawler I would say we just have to eat that fuel usage because it is absolutely neccessary. A flatter bottom has more initial stability and doesnt need as much or any stabilization. So, we can call stabilizers just part of the ship for these purposes. If we calculate into the equation the lower cost of gasoline and the lower/lesser maintenance cost of a gas engined boat we may be surprised at the difference. My trawler is 52 foot oa, 42 foot lwl, 16 foot beam, about 50,000 lbs, Krogen designed full displacement hull, with twin 4-53 DDs at 120 hp each. At 7 knots (IIRC) we got about 2.75 mpg. This boat has been in the hard for several years now for a refit. My sportfisher is 50 foot oa, 43 foot lwl, 16 foot beam, about 48,000 lbs. pure planing designed hull, with DD 8v92s at 550 hp each. Top speed is 30 knots, making OPEC smile. At 7 knots on 1 engine I can get 2.5 mpg. At 10 knots it gets about 1.5 mpg. Keep the numbers coming. Thanks

Insequent, Kulas, Larry and Art,
Best discussion on speed, hull shape and power we've ever had.
I assume that your (Insequent's) boat is similar to Daddyo's. I've seen his pics and other boats out of the water. I'm still reading and absorbing your post #79. Especially your last paragraph. "[FONT=&quot] I’m also starting to think that the definitive QBBL number Manyboats (and I) would like to see is going to be quite low." To make a SD hull closer to a planing hull a lower. Increasing the angle (in numerical terms as in 6 degrees rather than 4 degrees) would increase efficiency and require the boat to go slower to realize the increased efficiency.

Oh I just figured it out. You're referring to the number of the QBBL angle that would divide FD hulls from SD hulls. I read about this on BoatDesign.net and someone gave a definitive number to this end. I don;t recall the number, it's source or if the number just came out of somebody's head. Could be the latter but I suspect not. I searched on BD.net several times but came up empty handed. However if such a number was found or established it still would only be a basic discussion point that would give some scope to this issue as there are so many other variables. Variables like if the displacement was carried in close to the keel or out toward the chines. An extreme example of the latter would be a barge.

AS I pointed out earlier a relationship between the cross sectional area of the deepest portion of a hull compared to the area of the submerged transom could also be a rule of thumb to the same end and of course have the same limitations if not more. I carried the opinion for some time that any boat w it's transom above the WL identified itl as FD and any "significant" amount of submerged transom identified a SD hull.

To study, evaluate and learn about this is one thing .. of interest to only a limited number here of TF but to have a rule of thumb to quickly identify a SD boat from a FD boat would have very significant value to many. I say this as there seems to be an extremely wide range of opinions as to what a FD and SD hull is and how to identify the each.

Two avenues toward definitive success would be to establish numerical documentation that would put every hull neatly in it's appropriate box or to narrowly define the capabilities of each, measure the performance of each and put in the appropriate box. That could be useable or beneficial to the NA but of little use here on TF.

It's fun talking about this and probably leads to bits of new knowledge for many. That in itself makes it worth discussing even if we fail to establish a definitive rule of thumb.

Since I opened the door to all for coffee at Barnes & Noble in Bellingham WA this morning (10:30) I better get going so as to not be a no-show.
[/FONT]

My sportfisher, with a fairly sharp bow entry angle, if ran bow down and stern clear of the water would come close to FD. It was not designed to run that way but it is doable and to some extent at low speeds this is exactly how it runs. Add some power and the big props will really drop the stern, like almost 12 inches. a lot more at higher settings. At 25 knots the wake is substantial. My trawler has this design built in, and no flat planing area aft. The wake it leaves is very small.At 7.5 knots the wake behind my sportfisher is about the same. Mpg is not that much different and with modern electronic diesels even at 600 hp the mpg would be almost indecernable between the two. Possibly, the trawler could benifit from the new tech, but that has not been proven. It still takes about the same amount of diesel to push the same size, weight and approximate hull configuration thru the water at 7 knots.

As you can tell via my general calcs as well as last paragraph on post # 75 - I believe in and recommend caution regarding calculating fuel usage and therefore the reserve supply. Ya just never know when or why those precious last drops of fuel might be required!

If we are collecting data:

Grand Banks 47'
56k lbs (it's a heavy boat)
Twin Cummins 500s
7.1kts, 2.25 NMPG (7.1 is the closest data point that I have)

Ok, some more data:

Ocean Alexander 50 Mk1
60,000 # half load
Twin John Deere 6068, 201 HP each
7 kn, 2.7 NMPG

But it seems I have a lead foot. Looking at daily logs for 2000 nm since my repower I typically am getting 1.3 NMPG at 9.3 kn.

I did a bit more research, and here are a couple of very good pdf's. If you used boat design.net then you might remember Eric Sponberg's series on Design Ratios. On his website he compiled the posts into an updated pdf, and I've attached it. Eric's "primer" is very readable, and whilst it is more sail oriented there are great sections for powerboats.


That file lead me to Dave Gerr's article in Masthead. Also available to download, here it is. The key article is p12-17. In it he discusses things raised earlier in the thread. It is also a very readable discussion for folks like us. He has a great graph for how the 1.34 number varies, discussion of the Quarter Beam Buttock line (and the graphic Eric was looking for) and importantly his updated formula for SLR. It is revised from the one in his Propeller Handbook. It is truly worth printing and keeping pages 12-17.....

THANKN YOU Insequent.

I remember Spoonberg and that he's into sailboats. He's put out very good stuff for a long time and I now have it (thanks to you downloaded for good) and I'm off to read. All interested should look at pg 14 of the "Westlawn Masthead" DL and see the good illustration of the QBBL (quarter beam buttock line) and it should be big help understanding the difference between full disp and semi disp hull design.

"lead foot"? You are probably a sucker for a nice big wake fullmof action (like me) and just like to see the seawater foam. Pleasure boating isn't it?

Insequent said:

Ok, some more data:

Ocean Alexander 50 Mk1
60,000 # half load
Twin John Deere 6068, 201 HP each
7 kn, 2.7 NMPG

But it seems I have a lead foot. Looking at daily logs for 2000 nm since my repower I typically am getting 1.3 NMPG at 9.3 kn.

I did a bit more research, and here are a couple of very good pdf's. If you used boat design.net then you might remember Eric Sponberg's series on Design Ratios. On his website he compiled the posts into an updated pdf, and I've attached it. Eric's "primer" is very readable, and whilst it is more sail oriented there are great sections for powerboats.


That file lead me to Dave Gerr's article in Masthead. Also available to download, here it is. The key article is p12-17. In it he discusses things raised earlier in the thread. It is also a very readable discussion for folks like us. He has a great graph for how the 1.34 number varies, discussion of the Quarter Beam Buttock line (and the graphic Eric was looking for) and importantly his updated formula for SLR. It is revised from the one in his Propeller Handbook. It is truly worth printing and keeping pages 12-17.....

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Thank's for the links...

I posted parts of the Sponberg article (pretty sure it was that one) in one of the last threads here on hull design/efficiency. It was in defense of my trying to post similar info but was rebuffed when someone was adamant (as always) that 1.34 was a hard and real number for all displacement boats as well as the QBBL as the "determining factor" with little regard to prismatic coefficient or other key design features.... Also they suggest that a quick glance at a bad photo and magically telling you your boat is displacement over semi-displacement is not really practical.

I would hope reading these links will give TF members some insight to the reality of boat design and see that while complicated to a degree...that the complications of design are for fine-tuning small changes in performance...not gross ones....and that trying to "guess" performance from a few tidbits of internet data is ridiculous.

psneeld said:

... trying to "guess" performance from a few tidbits of internet data is ridiculous.

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That's not likely to stop anyone.

This thread brings to mind the Weather Channel gimmick of naming storms that meet some self-defined parameter. Why shouldn't a boat designer contrive a ratio of some sort and sell it?

I just developed a new one myself ... it is the ODS ratio and is a measure of how many angels get their feet stepped on while dancing on the head of a pin per angel per unit of pinhead area; it is the Overlap per Dance Step ratio.

Give me some time to fine tune the formula and I will include dance efficiency in there somehow. Speaking of which ... what happened to the "efficiency" idea that was tossed around so freely at the start of this thread?

psneeld said:

I personally like dollars/nautical mile.....last year it was about $1/NM ...this year should be the same....

Makes trip planning easy....3000 NM trip is gonna cost me $3000 in fuel..

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Sorry guys, just got back on line and am feeling feisty having just entered Saddle Cay from the east during a easterly blow with the current coming out. Never saw less than 9 feet, but that's another story.

Bring in the Bahamas, I have finally had the time to analyze fuel and electricity on Dauntless.

Between tt and the other rick, I actually think you are all saying the same thing, though in a bit convoluted way.

The gph is exactly where I expected it, and based on engine rpms. At 1600 rpm, I get 1.6 gal per hour. At 1800 it goes up to 2 and at 1400 about 1.4 gph.

So gph is a good lowest common denominator, to compare engines, but not necessarily boats!

Now, mpg is dependent on do many factors that it can really only be used on a case by case situation.

Once boats are thrown into the mix, then, just changing the prop will change nm per gal or mpg, let alone everything else.

Mpg is a tool I can use on a case by case basis. E.g. I discovered that the last 100 miles of the icw, my mpg went from 4 to less than 3, mainly from all the bridge wait times. Also, current plays a big role.
If I take the time to calculate avg speed, I can then use that to call mpg, but I pretty much don't bother anymore.

So, in sum I have been pleased that my initial expectation of $1/nm has held up well.

Later.
Richard on Dauntless in the Bahamas.

Dear Ann Landers

In May we will be driving from Arizona to Seward Alaska. Part of the driving trip will be on a ferry. How do I calculate MPG for the Suburban? Then during June through September we will be cruising on the West Coast in our boat. I'm really befuddled as to how to calculate efficiency, GPH and NMPG for the entire summer's journeys.

BTW, my credit card which I will use to cover the cost of the ferry, truck gas and boat fuel may have been hacked at Target. Do the bank fees to make good on this count towards my dollars per nautical mile?

Awaiting your reply, I remain,

Anxious in Arizona

sunchaser said:

Dear Ann Landers

In May we will be driving from Arizona to Seward Alaska. Part of the driving trip will be on a ferry. How do I calculate MPG for the Suburban? Then during June through September we will be cruising on the West Coast in our boat. I'm really befuddled as to how to calculate efficiency, GPH and NMPG for the entire summer's journeys.

BTW, my credit card which I will use to cover the cost of the ferry, truck gas and boat fuel may have been hacked at Target. Do the bank fees to make good on this count towards my dollars per nautical mile?

Awaiting your reply, I remain,

Anxious in Arizona

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PERFECT!! :lol:

MPG, would be the only way to compare this subject. GPH means nothing to the fellow on the other boat. I realize there are contributing factors and a decently accurate number is difficult to get. It is still the best comparison for these purposes. My opinion is that a house on a square bottom barge, 40 foot lwl and 14 feet wide weighing 50,000 lbs pushed by a pair of appropriately sized diesel engines will get close to the same fuel usage (in mpg) as a similar sized "trawler".

anyone that can't do ALL these different calculations in their head doesn't belong behind the wheel or earn the title captain anyway...my Ouija board is way more accurate than any floscan.....

so what's the question?

ps...real men use metric anyhow....