# Long - Skinny Power Boats

Discussion in 'Boat Design' started by SAQuestor, Sep 24, 2004.

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### Ilan VoyagerSenior Member

I'm too busy today for to write a long answer. I'll make a synthetic excel sheet (imperial and metric units) which, I hope, will help everybody for the calculations, and as always I shall bother all you with my ratiocinations.

Regards,

Ilan

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### PortagerSenior Member

My Calculations

I promised to check of George Buehler’s Troller 50 calculations, but first I wanted to give George an opportunity to check his calculations. George must be too busy, so here are my calculations.

First here is George Buehler’s data:

LOD: 50'
LWL: 44' 7-1/2"
Beam: 15'
WL Beam: 13' 9-1/2"
Draft at DWL: 6' 6"

The displacement is interesting because it changes rapidly depending on the loading. Here's de figures....

Displacement at DWL = 69,668 lb
floating 3” high = 61,42 (yes there is a digit missing here, use your imagination)
at 4” high = 59,497
at 3” LOW = 77,572
at 4” low = 80,074
at 5” low = 82,829

V/L .. Knots .... HP
1 ...... 6.96...... 12.9
1.1.... 7.65...... 19.8
1.15.. .8.0........ 24.3
1.20... 8.35...... 31.5
1.25... 8.70...... 42.4
1.30... 9.04....... 57.3
1.35... 9.39....... 77.9

First I noted that the V/L does not equate. The square root of 44.625 is not 6.96 so I determined that George must be using a LWL of 48.44’ instead of 44.625?

Next I started with Dave Gerr’s equations as follows:

Eq #1: D/L=(weight/2240)/(0.01 * LWL)^3

Where D/L is displacement/length ratio
Weight is weight in pounds
LWL = Length at Water Line in feet

Eq #2: S/L = 8.26/(D/L)^0.311

Where: S/L is the maximum displacement speed/(length)^1/2

Eq #3: Hullspeed = S/L * LWL^0.5

Eq #4: SHP = D/(10.665 / (S/L))^3

Where D = displacement in pounds

Plugging these equations into Excel and using a LWL of 48.44 I get the following:

V/L Gerr Speed Buehler
HP Knots HP
D=69,668
1 57.43 6.96 12.9
1.1 76.44 7.65 19.8
1.15 87.35 8.0 24.3
1.2 99.24 8.35 31.5
1.25 112.17 8.70 42.4
1.3 126.18 9.04 57.3
1.35 141.3 9.39 77.9

Note these calculations do not provide for waves, wind, current, skeg losses, shape corrections, … i.e. to are what George claimed. I consider these numbers optimistic predictions that you will probably never achieve in real life. Another significant point is the calculations assume good propeller efficiency, so if you are using a fixed propeller you will only approach these values at your design point and at any other speed propeller efficiency will drop off so power requirements will be higher and fuel efficiency will be lower. With all that said I would put more faith in Ilan Voyagers numbers than mine.

Now that we have an equation to estimate power requirements, it is a simple mater to estimate fuel consumption, provided we know the brake specific fuel consumption (BSFC) for the target engine. Engine manufactures will provide a curve for BSFC versus engine speed and it is usually a shallow bath tube shaped curve with a minimum point between 50% and 80% power. This curve assumes that the engine load or torque is optimized for that speed (rpm). If you compare the engines optimum power curve and a fixed propeller power demand curve, the propeller power demand curve is much steeper than the engines optimum power curve. Since you never want the propeller power demand curve to exceed the engines optimum power curve by more than a few percent (this would equate to overloading the engine, which is not a good thing) the propeller can only be optimized for the maximum speed you wish to achieve plus a few percent. At any speed less than the propeller design speed, the engine will be under loaded and BSFC will be less than ideal. To determine actual fuel consumption, you need to get a BSFC chart from the engine manufacturer that shows a family of curves of BSFC versus speed for different torque levels (they all have this data because they use the optimum point on each curve to determine the BSFC curve versus speed), but manufacturers are reluctant to give this data out and (unless you can get it in an electronic format) it is cumbersome to work with. In addition if you are in the process of selecting an engine from multiple candidates, evaluating each engine can be exhausting. Since the BSFC curve is a very shallow curve, the variation in BSFC is small over a typical operating range, therefore designers typically use an approximation of 0.055 (David Gerr’s recommendation) to 0.06 gallons/HP/hr (the marketing departments recommendation) for diesel fuel and 0.1 gallons/HP/hr for gasoline. Another factor that can impact range is BSPF ratings are usually in mass/power/time and we measure fuel in gallons or liters. There is significant variability in the density of fuel which will directly impact your range. As they say, your mileage may vary.

Getting back to the equations, multiplying SHP by the BSFC provides fuel consumption rate.

Fuel consumption rate = SHP * BSFC

And dividing speed by fuel consumption rate provides mileage

Mileage (NMi/gal) = Speed / Fuel consumption rate = speed / SHP * BSFC

Using 0.06 for the BSFC and substituting Eq #4 produces:

Eq #5 Mileage (NMi/gal) = Speed / 0.06* D/(10.665 / (S/L))^3
= Speed * (10.665 / (S/L))^3 / 0.06* D

At first glance it appears that the way to increase mileage is to increase speed, however since power requirements increase faster than speed so we know the to increase mileage we must reduce speed. See that attachment I provided for a graphical representation of this. Additionally, we see that mileage is inversely proportional to displacement, so the most direct way to increase mileage is to reduce weight. This is illustrated in the attachment by the Idlewild curve. Idlewild is another George Buehler design as documented at http://dieselducks.com/Idelwild.html . At less than half the displacement of the steel hull Troller 50’ and only 8% longer LWL than the Troller 50’, Idlewild provides a significant increase in mileage at a given speed. As you can see, reducing displacement is the best way to improve fuel efficiency.

On the subject of Idlewild, note that she is built in Aluminum. George says, “Personally, I think plywood is better because it’s considerably cheaper unless you’re hiring the labor of course; aluminum is fast to work so paying for labor is a much better deal than paying for a wood builder.” To me this means if you don’t put any value on your time plywood is cheaper, however if you value your time Aluminum may be a better option. In addition Aluminum will certainly be lighter, which improves fuel economy. Finally, as Ilan Voyager points out, wood/epoxy can be made durable provided you use enough epoxy and glass matt. However if you skimp a little too much, even in a small area, it creates a potential leak and a place for rot to start, conversely if you go overboard it adds weight. Finally, how well you build is a big unknown for potential buyers if you ever decide to sell, which is why home built wood/epoxy boats has much lower resale value.

Now you are probably saying that this a supposed to be the long/skinny boat thread, but beam isn’t part of the mileage equation. Well, actually beam is a key part of the equation but it is hidden in the displacement. Since displacement (in the nautical usage) is equal to the weight of water that the boat displaces,

D = LWL * BWL * draft * 3DPC * density of sea water
Where D is displacement
LWL is the length at the water line
BWL is the beam at the water line
3DPC is the 3 dimensional prismatic coefficient

Since we know we want to increase LWL to reduce the speed/LWL^.5 term, then we must reduce the beam, draft and/or prismatic coefficient to reduce the displacement. Lets leave changing the prismatic coefficient for now and just discuss reducing the beam and draft. The problem is if we reduce beam and/or draft we will loose stability. To accommodate reducing the beam, we need to lower the center of gravity. We could lower the center of gravity by adding ballast, but that would increase displacement and reduce mileage, therefore we need to reduce the center of gravity without adding weight. We can do that by reducing height and lowering height of high density items such as the engine, batteries, tanks, … As you can see, the draft of the hull actually helps reduce the center of gravity because it allows us to locate high density items lower than we could with a flatter bottom. If the hull is deep enough, then the equipment can be located below the floor of the living spaces allowing maximum living space without increasing the overall height too much.

Reducing the prismatic coefficient is basically going to a multihull configuration. By increasing the beam and pushing the displacement out to the sides we can increase stability and minimize displacement. However multihulls are difficult to make trailerable, although not impossible.

Well that is enough for now. I’m donning the asbestos underwear and wait for the fireworks.

Regards;
Mike Schooley

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### Ilan VoyagerSenior Member

Portager, take out the abestos underwear (abestos is bad for the lungs): I have just read your post: everything looks fine and I agree with the method of calculations (how I wouldn't? Dave Gerr knows his job...).

The difference of HP comes from I've used an different equation I had under hand (It's why I said; rough calculations). Probably the true ciphers are somewhere between our ciphers, plus the corrections. To know the exact ciphers has finally no importance; the important is to know that Mr Buehler ciphers are impossible.

I'm bothered by such big discrepancies, so I'm reading some old books as I want to find the other methods used on old displacement hulls.

I wouldn't said better about the corrections to make and about the importance of the propeller, gear ratio. I agree totally that the principal penalty is weight; a boat must be as light as possible without sacrifying reliability and on small boats you have to make choices about accommodations and amenities.

The method of consumption evaluations is perfectly exposed: there is a also a method by taking the best torque RPM of the engine (it's almost always where the engines have the best specific consumption and are the most efficient) and calculate the gear and propeller for the best cruising speed at this RPM. That sacrifies generally top speed but gives the best mileage at the best cruising speed. The perfect propeller is the variable pitch one as you can adjust perfectly the load of the engine whatever the sea conditions.

The lone assertion I do not agree it's about low prismatic coefficients of multihulls.

It's easy to give a high prismatic coefficient to a power slim hull: entry curves are smooth and are only a small fraction of the hull, the central part (where prismatic coefficient is close to one) is the longuest fraction of the hull , and the stern when wide (about 75 % of the max beam) with a good keel curve has also a fairly high prismatic coefficient. A PM of 0.65 is not difficult to obtain with a slim NPL hull or an elliptic sections hull.

That which may change a lot is the block coefficient: The block coefficient represents the cargo capacity if a hull in a given length, and for example a NPL hull, with its close to V sections, has not a very high block coefficient.

I think that some illustrations will be needed, so these 2 notions will be better understood. I'll work on that. Now I have to get some sleeping hours.

Best regards

Ilan

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### PortagerSenior Member

I wanted to see how accurate Dave Gerr’s 0.055 gal/HPhr rule of thumb was and if that approximation was causing me to over or under estimate mileage and range. Since I plan to use either a Steyr Model 164 or a Yanmar 4LHA-DTP, I downloaded the Steyr Model 164 performance curves from http://www.steyr-motors.com/products/products0006.htm and measured power and brake specific fuel consumption (BSFC) versus engine speed. I then plotted BSFC versus power, as shown is the first attachment. Dave Gerr’s rule of thumb appears a little conservative over the middle power range (i.e. from 25 HP to 150 HP, but it is overly optimistic at the ends of the power curve. Next I added a trend line, shown by the dashed line, and used trial and error to find the best fit. As you can see a 6th order polynomial provides a good curve fit. The error between the curve fit equation and the measured data ranges from -1.42% to +1.39%, but by leaving out the last two points that are outside the range of the curve fit, the error ranges from -0.82% to +0.55% which I think is good enough. Note: if you attempt to utilize this curve fitting technique, make sure that you increase the number of significant digits on the text box of the equation or the round off error will cause erroneous results.

Now that I have an equation for the Steyr Model 164 BSFC versus power, I can calculate the BSFC based on the power requirement and then multiply by the power to determine the fuel consumption rate. The second attachment shows the mileage prediction versus speed for Portager using the BSFC curve fit and using Dave Gerr’s 0.055 gal/HPhr rule of thumb. To Dave Gerr’s credit, I notice that the curves are very close, however his rule of thumb is slightly optimistic below 6 to 7 knots and a little pessimistic above. It should be noted that these predictions are accurate for controllable pitch propellers, however if you are using a fixed pitch propeller it would only be accurate at the weight and speed that the propeller is designed for.

The third attachment, range versus speed, shows a similar characteristic although the difference is slightly more pronounced. Range improvement at speeds above 6 to 7 knots if a welcome benefit. The reduction in range is above 2,600 NMi (light) and 2,300 NMi (loaded), which is greater than my maximum design range of 2,200 NMi (Los Angeles to Hawaii) so it is doubtful I’ll ever operate in this speed regime. To better illustrate the difference between the BSFC curve fit and Dave Gerr’s rule of thumb, I plotted the difference in range versus speed in the fourth attachment which shows a 200 to 240 NMi improvement in range between 9 and 10 knots. The fifth attachment shows the percentile difference which shows up to a17% improvement is range between 9 and 10 knots.

I hope this is helpful. If you would like a copy of my spreadsheet, send me an email and I’ll send it to you, but keep in mind it if a roung engineering spreadsheet with no comments.

Regards;
Mike Schooley

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### FAST FREDSenior Member

For an early look at Long efficent boats , Yachts In a Hurry , C. Philip Moore ,W.W.Norton & Co. is great!

Of course the 20's era was using 300 to 500 hp gasolene engines that weighed 3000lor more each!

Although every attempt was made for lightweight construction & outfitting , these boats were HEAVY!(at least by todays standards.)

There were in the ball parek on LB ratios , from the many examples given:

45x10...56x11...75x12'10...68x12'6...60x16...46x9...72x14..

From the list begining with the A's

Seems today we should be able to construct a modern foam GRP or aluminum vessel that would be seaworthy and fast , while being cheap to run.

Does anyone have a concept for the fuel milage at cruise speed , say 12K+ that could be obtained?

From my playing with the simple graphs avilable to me , I feel 5 NMPG would be barely atainable with an optimised design.

Any way to better that? or?

FAST FRED

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### PortagerSenior Member

If you want 10 NMi/gal at 12 knots, then the fuel consumption rate will be 1.2 gallons per hour. Using a brake specific Fuel Consumption (BSFC) of 0.050 gal/HPhr, then the power level must be 21.82 HP. Solving the power equation for displacement allows you to compute the maximum displacement to achieve the desired 12 knots speed. The attachment shows the combinations of length and displacement that provide 10 NMi/gal at 12 knots.

Some data points are at;
Length Maximum
Waterline Weight
Ft lbs
70 8,970
100 15,316
125 21,405
150 28,138
175 35,454

These weights seam quite low to me, but maybe it is possible. If it were accomplished it would be so light that I would worry about it bouncing around like a cork if it encounter heavy seas.

One idea that comes to mind is a smaller vessel with inflatable sections between the bow, mid and stern sections. When the inflatable sections were inflated the total length would increase to the required cruising length. Once the vessel arrived at its destination or conditions became too rough, the vessel could be deflated and the rigid sections reconnected.

Regards;
Mike Schooley
Designing "Portager" a 48' transportable trawler

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### FAST FREDSenior Member

A 70 ft boat that was only 9000lbs would be quite an achievement , considering the minimum engine weight , fuel & water & supplies , necessary weights like ground tackel .

Doubble that , say 18,000lbs would be far easier , and would not need exotic construction or titanium anchors.

AS a past multihuller (45 Nicol Voyager) there is little hassle with being ON the water , rather than being IN the water, although the motion is different , its hardly bad.

What does your computer say about a 18,000 lb boat optimized for long range passages at speed?

Thanks

FAST FRED

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### PortagerSenior Member

You set an aggressive performance goal; therefore it stands to reason it would require an aggressive weight reduction effort.

On the plus side, you only need 22 HP, so the engine weight will be the least of your concerns. Unless you also want to be able to go faster than 12 knots, which would require additional power and thus additional weight.

If you double the weight, then you double the required power and fuel consumption rate, which cuts your mileage in half.

Regards;
Mike Schooley

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### 8knotsA little on the slow side

Portager, Your in the wrong biz there my man, I envy tour skill at crunching numbers.
Well Im not sure what we are considering "long and skinny" but one of my many concept doodles I will attach, she has a wl length 77' with a beam of 18' draft 2'9" my rough numbers give me 112,000 disp DL of 110 and a BL of 4.27. I cant remember all the particulars but seems to me I had figured on a pair of 170hp will drive her to SL 1.5 or about 13kts. I think she is a little beamy for the long skinney concept but I think she needs it considering the top hamper factor. In my opinion it is still a good compromise in length and usable space. with good speed performance with but modest HP. I'm still fidling with the idea of the "Seabright" box keel Gerr is so fond of. I think you could drive her to SL 2.0 before fuel consumption gets out of control. A pair of 700hp Luggers will drive her to SL 2.5 (apx 21kts) drinking about 28gph @WOT 2100rpm ease her back to 1700-1800 and yor cruising @ a reasonable clip without breaking the bank. Keep in mind she is an "able coastal cruiser"
Some time ago there was an article in PMM titled "Passagmaker lite" By a Tad Roberts. I'm guessing it is our own Tad that posts here from time to time.
It is a good read and a great overview of what you folks are trying to do.

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### OldfartNew Member

Examples

I just stumbled on your discussion but have been interested in long skinny boats for some time. I haven't read thru everything so forgive me if I am redundant. On of the best examples I have seen is Bill garden's Tlingit, a 62 foot "ferry launch" with a 7'6" beam built from lumberyard 2x4's and plywood. It was built mainly as a carrier for an old 20 HP (463 cubic inch!) Easthope engine with which it could run 10.2 knots. It is detailed in Bill Garden's "Yacht Designs". Another boat you might be interested in is Phil Bolger's Breakdown Schooner from "Boats with and Open Mind". Also there was the Energy 48, basically and Alden ocean shell scaled up to 48'. If fuel prices keep going up these boats will come back.

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### FAST FREDSenior Member

"If you double the weight, then you double the required power and fuel consumption rate, which cuts your mileage in half."

Of course different programs will have different output , but when I use

doubling just the weight does NOT double the thrust required to go the same speed.

Also the very tiny diesels seem to be mostly rated for pleasure boat , not 24/7 work as the more industrial engines are.

The Yanmar would probably last at about 600lbs or so for 100 Hp max. and 20-50hp for the long pulls.

The newest common rail tech would be great as its more efficent ,and vastly quieter , BUT the lack of get home operation would be really scary in a lightning storm.

HAS anyone come across a mechanical injection timed diesel , that gives modern fuel consumption, and 24/7 realistic service?

FAST FRED

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### PortagerSenior Member

Coincidentally number crunching is also a prerequisite for engineering.

I like your Seaka 75. I crunched the numbers on her and to do a S/L of 1.5, 13.16 Knots, she needs 311.6 HP. I calculate her “Hull Speed” using Dave Gerr’s equations of S/L = 1.92 or 16.82 Knots, which requires 650.76 ponies. At S/L = 2 power requirements are 738.63 HP. S/L of 2.5 requires 1442.63 HP or 721 per engine. Fuel consumption would be 79.345 gph (see attached graph) using a BSFC of 0.055 gal/HPhr. To get 700 HP at 28 gph would require a BSFC of 0.04 gal/HP hr. All in all I’d say your memory is pretty good, but I’ll have to look up the BSFC of the Luggers, 0.04 is quite impressive.

Fast Fred try looking at Kubota Super 03 series http://www.frontierequip.com/kubota/kubota.htm#Super3 you can also get Kubota engines under the Beta lable http://www.rhby.com/betaengines.html and from Nanni(SP?) amoung many others. These are high efficiency long life water cooled diesel engines.

Regards;
Mike Schooley

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### 8knotsA little on the slow side

Yep, I blew it on the fuel consumption of 28gph. But im glad you agree with the lower SL HP requirements. Gives me a little faith that I am learning.
here is the link for the Luggers
http://www.lugger.com/northern-lights/PDF/L6140AL2.pdf
She would be a head turner at speed I think, So long as you were in flat water.
Thanks for your time mathing her out!
8knots

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### PortagerSenior Member

Fast Fred;

I forgot to mention I could not use the calculator at http://www.users.globalnet.co.uk/~fsinc/yachts/spreads/fred.htm . Perhaps I have stale Java?

Anyway, I thought we were discussing monohulls. The hull resistance calculator is based on the equations and formulae in chapter 3 of "21st Century Multihulls" by Joseph Norwood Jr. I question it applicability to monohulls and I do not know if its predictions have been validated in practice. Just out of curiosity, when you double the displacement (not just the weight) how much does the power requirement increase?

Regards;
Mike Schooley

15. ### Dr.khaniGuest

application of surface driving system