Beam and efficiency

[Hippocampus]

Displacement means the weight of WATER a hull pushes aside when loaded to it designed plimsoll line. To achieve that displacement you can make it long and thin or short and wide. Prismatic coefficient is a crude measure of the frontal plane pushed aside and back in per unit of length of the hull when moving.
So it’s simple physics. At given displacement a hull that is less beamy and has a better prismatic coefficient will take less energy to move. As a cruiser for comfort and range displacement is key. More allowable displacement means more fuel, water and infill (fittings, engine weight, spares, tools etc.). Every time you add weight you displace more water but the increase in wetted surface effects and parasitic drag will be much less for the thinner hull as the frontal plane immersed at any station will still be much less than the beamy boat
Ideally you want the structure of the vessel to be as light as possible as to increase the useable payload at a given displacement but only if it’s not at the expense of durability and safety.
Issues of hull speed aren’t relevant to this discussion beyond saying it’s a function of lwl while underway. All vessels operating in displacement mode will need to climb their bow wave and require extra energy to do so. At a given displacement the thinner hull takes less energy to do so. Less frontal plane. The square root of lwl x 1.32 still holds but the S v gph plot shifts down.
End of day thinner hulls are more easily driven. The wrinkle is when talking about thinner you need to be only talking about the immersed portion of the hull. Occasionally you see boats with much flare on their sides but narrower at the water plane. That works fine on lakes and inland waters.

 

[Simi 60]

It may be similar but I would guess its heavier.

.

4 ft additional beam
But one level less

Even at modest speeds 3hp per Ton adds to the fuel burn rapidly

Doesn't seem to make a jot of difference to us

1/3rd full diesel and near empty water we get 7.5 knots @ 1150rpm
Add 4 tonne of diesel and 2 of water and we get 7.5 knots @ 1150rpm

 

[Nomad Willy]

FF wrote “Even at modest speeds 3hp per Ton adds to the fuel burn rapidly.”

3hp per ton is for FD boats.
My FD boat has 5 hp per ton and it’s a bit excessive.
Just a bit though.
There’s another Mitsu that’s a truncated S4L2 (what I have) .. a 4cyl.
The smaller one is 33hp. Very very close to perfect.
But a sailboat would need less yet.

But FF your friends 4’ wider boat has much more skin friction and wave making resistance.

 

[fgarriso]

https://vicprop.com/displacement_size_new.php

 

[trawlercap]

I'm checking with Sound Propellor in Seattle for my "new" boat. They designed and built the prop shown. It replaced a 25 year old design. We suffered with excessive vibration and cavitation. This new generation wheel is 1" thick at the tips, and foiled full length. It eliminated the previous problems, had zero...as in zero bubbles or cavitation. 50 feet behind the boat the water breaks the surface and turns white. We went from a 10 kt. boat to 12. At same RPMs we burned 20% less fuel at a faster speed. I started running at 9 knots and we burned 60% some times less. At $25K for this big wheel, it was the best $ we ever spent. Paid for itself pretty quick.

 

[O C Diver]

I'm checking with Sound Propellor in Seattle for my "new" boat. They designed and built the prop shown. It replaced a 25 year old design. We suffered with excessive vibration and cavitation. This new generation wheel is 1" thick at the tips, and foiled full length. It eliminated the previous problems, had zero...as in zero bubbles or cavitation. 50 feet behind the boat the water would boils. We went from a 10 kt. boat to 12. At same RPMs we burned 20% less fuel at a faster speed. I started running at 9 knots and we burned 60% some times less. At $25K for this big wheel, it was the best $ we ever spent. Paid for itself pretty quick.

Sounds like the old prop was totally wrong for the boat. Excessive vibration and cavitation are clear signs of a major problem or a prop with one or more blades way out of pitch.

Ted

 

[trawlercap]

YEs, the boat switched from King Crab (speed needed) with no nozzle. To a trawler (pulling power needed) Added a nozzle and re-pitched (even added to the flukes) the prop, but we suffered until I finally got my wish for a new wheel. Most of the fleet followed (Except the VP guys)

 

[O C Diver]

So the prop was way over pitched by adding the nozzle and dramatically changing the load (towing versus cruising to the fishing grounds).

Do you believe this boat has a substantial miss match in propeller to intended use?

Ted

 

[Hippocampus]

Pretty amazing screw. Newbie question. How often is it usual to need to take it off for rebalancing assuming it’s never fouled by line or debris?

 

[trawlercap]

So the prop was way over pitched by adding the nozzle and dramatically changing the load (towing versus cruising to the fishing grounds).

Do you believe this boat has a substantial miss match in propeller to intended use?

Ted

O C Diver The boat in the previous post has that killer. That is the one I described as having so many good improvements.

I'm curious about my "new" boat. 56' waterline averages 8 kts. Main Engines: (2) John Deere Lugger LP66ST/M5661, 6-Cylinder, 168 HP, 3949 Hours (New 2002)

The props are pitched allow the engine to reach just under full RPM. We'll see, I have seen too much $ spent on engines when the props make a huge difference.

Using https://www.easycalculation.com/physics/classical-physics/hull-speed-calculator.php I should be at 10 kts.

 

[Peter Pan]

Scott, my boat has a 15'7" beam. When I'm running at idle (750rpm) I'm doing about 8.5kts and that is my most efficient running speed.

A way to find out for a specific boat is to use the formula for hull speed, which is:
Velocity in knots = 1.35 x square root of the waterline length.

So let's say you are looking at a boat that has a waterline length (LWL) of 30'

Velocity = 1.35 x square root of 30
square root of 30 = 5.477
Velocity = 1.35 x 5.477 = 7.4kts

The formula given above is correct for most boats having and approximate ratio of 3:1 (length to beam), however, very narrow boats, like the hulls of a catamaran, do behave different and are more efficient than conventional hull design.

 

[Hippocampus]

Add in wave piercing bows, lift from foils, Magnus effect and other stuff it all breaks convention even with mono hulls.

 

[Bryant]

Beam

How does the beam of a boat with a full displacement hull affect efficiency when cruising at a speed/length ratio of 1? Do narrower boats have significantly greater efficiency or does it not matter much?

My GB 36 has a beam of 12.5 ft. One FL 120 engine. On a good day I get 7kts. I burn 2 gals/hr with the gen running. I think most trawlers do about that. There are Trawlers, such as the Camano 31 that will do up to 15 kts but at a much higher fuel consumption rate. At the cruising speed of around 8 kts, most trawler designs aren't going to give you a much different fuel efficiency rate. Buy the boat you want.

 

[wallaceyachts]

I'm sure there are sound, calculated answers out there. I'm not that smart...I can relay some direct comparisons, though. I drive a couple pretty chubby girls.
It depends on so many things. Wetted surface, shape, displacement, prop efficiency, the list goes on and on. Home boat is 65' 20' beam 65 tons. Work boat is 120 tons 77' on deck 20' beam. Consumption on home boat is 5.3 gph, and work boat is 7 gph.

I've seen numbers like those on Grand Banks 42's.

I've had a few Taiwanese trawlers (Universal, Monk, etc) Those boats had Lehman 120's. My experience with the Lehman 120's has been they burn 2.5 gph whatever boat I've had them in. Some might be a tick under that but that's a good number to compare. Another Lehman recipient I had was one of those bridge deck cruisers from the 20's. Those are the really slender woodies that would be like the Elco commuters on the east coast, and the Lake Union "dream boats" out west. I had one that was repowered with a Lehman 120, and it consumed about 2.25. Light narrow plumb bow boats. She had a very sleek bottom and was so light, you could stand on the dock and move her with one hand.

I guess my point is, as it would apply to non-planing boats within the lengths that seem to be predominant here, that an extra foot or two of beam is not a big factor, or at least purchase criteria for me. I like the boats of abundant girth because an extra couple feet of beam adds a whole lot more space than a couple feet of length.

 

[Nomad Willy]

Wallace, I’ve had thoughts of how much I’d loose if I could build another Willard 30 w a 12’ beam. She’s 10.5 now. That would be a huge increase in volume and thus displacement if loaded properly.

There would definitely be more wave making action and more hull area under water. So some amount of added power would be needed unless I was to accept a small reduction in speed w same power applied through same prop. But I suspect it would be very small. But the additional ability to carry more weight and the added stability she’d be a whole new boat.

But Willard had a 36’ boat that would probably has a better hull for a boat w much more weight. If it was scaled down to 30 - 32’ it may be comparable to my W30 above. The W36 has a very full hull far bigger than most all boats.

But the only reasons these boats wouldn’t suffer from aspect ratio is the fact that they are FD boats.

Put another way what if you kept the weight and volume of Soverneign and gave her a typical rec trawler hullform? When that thought struck me I saw an absolutely horrid boat that nobody would want short of perhaps one needing the mother of all wake board boats. And she’d make a wake that would limit her speed to 4-5 knots most of the time in civilization.

So IMO FD boats need not be held to the confinements of SD design re aspect ratios and displacement.

 

[wallaceyachts]

Wallace, I’ve had thoughts of how much I’d loose if I could build another Willard 30 w a 12’ beam. She’s 10.5 now. That would be a huge increase in volume and thus displacement if loaded properly.

There would definitely be more wave making action and more hull area under water. So some amount of added power would be needed unless I was to accept a small reduction in speed w same power applied through same prop. But I suspect it would be very small. But the additional ability to carry more weight and the added stability she’d be a whole new boat.

Totally non-scientific on hp required, or prop selected, but the hull speed remains 7.12 ish. If it’s a new build, I would tuck a sweet smooth Yanmar in it. They have a nice selection with incrimental bumps in hp of 10 from model to model. I can’t imagine anything much greater than a 50hp would be needed...just from the hip with my opinion.

 

[Nomad Willy]

Yup,
About 42. And I have 37 installed. Just a guess though.

Hull speed is meaningless. I cruise at one knot below hull speed. Any more speed eats too much power. I cruise at 6.15 knots at 2300rpm. 100 more rpm would (I think) more than make up for the wide beam. No re power required.

But this is just mental exercise.

I like your high aspect ratio prop on the sailboat.

 

[Caballero II]

How does the beam of a boat with a full displacement hull affect efficiency when cruising at a speed/length ratio of 1? Do narrower boats have significantly greater efficiency or does it not matter much?

See L. Francis Herreshoff's "Marco Polo". Built to maximize hull speed/efficiency. Reported to roll like a m/f...er. So, just saying. It's the boat I would want, but for the adverse roll/comfort characteristics.

 

[Nomad Willy]

Re beam & efficiency has anybody been passed by a large barge going 15-18 knots?

No swoopy lines, hard corners and flat bottom.
Easily goes right on past your trawler.

 

[thataway]

One boat (and there are at least one similar) is Idlewild which cannot be ignored. 55'LOA, 52 1/2' LWL, 11' Beam and 10' beam waterline. Power 55 hp cruising speed 6.5 knots and fuel burn 1.3 gallons an hour. Circumnavigated.

Was hard chine, aluminum construction. with moderate amount of rocker.

 

[Scott Harris]

So, I read a bit about the Idlewild Expedition (thanks for that reference). The boat was 57' long and 11' wide for a l/w ratio of 5.18. It traveled 6.5 kts on 1.3 gallons for a kpg ratio of 5.

Now let's compare that to more typical FD trawler. Eric's Willie is 30' long with a beam of 10' resulting in a much smaller l/w ratio of 3.0. Yet it travels 6.15 kts with 1 gallon of fuel for a kpg ratio of 6.15.

So the Willie is more efficient than the Idlewild while having a much smaller l/w ratio.

So what is the advantage of having a narrow boat?

 

[Nomad Willy]

Hello Scott,
It’s got less wave making resistance.

BTW Willy (with a y) has 10’ 6” beam.

And she has a rather raked bow (like an N46) so her WLL is only 27’ 6”. The lower WLL gives her more efficiency below about 5.25 knots ( may be higher) and less speed than a typical 30’ boat.

 

[Scott Harris]

Hi Eric,

I will do the calculations again. In order to compare apples to apples, how many gallons per hour does Willy use at 6.5 knots?

 

[Nomad Willy]

Scott,
I’m not much for calculations but I did make an effort on a run up to Ketchikan and it was close to but not 1gph. I have a better prop now so Call it 7/8ths of a gallon.

Edit:
I see you asked for burn at 6.5 knots. Almost never go that fast. A guess would be 1 1/3 of a gallon but ........ ?

 

[Art]

Scott,
I’m not much for calculations but I did make an effort on a run up to Ketchikan and it was close to but not 1gph. I have a better prop now so Call it 7/8ths of a gallon.

Edit:
I see you asked for burn at 6.5 knots. Almost never go that fast. A guess would be 1 1/3 of a gallon but ........ ?

Eric - Willy 4.89 miles per gallon? Heck... that's coming close to 1/2 the mileage of your and my big ol' Buicks.

 

[Scott Harris]

Idlewild...L/W = 5.18_____knots/gal = 5 at 6.5 kts_____displacement 30,000
Willy.......L/W = 2.62_____knots/gal = 5 at 6.5 kts_____displacement 16,000

These two boats have the same fuel efficiency at 6.5 kts despite Idlewild having a 2x L/W ratio. Idlewild has almost twice the displacement, but I thought the effects due to added displacement were minimal in FD boats.

Can I infer from this that beam is not that important when it comes to efficiency, or am I missing something?

 

[thataway]

Now let's compare that to more typical FD trawler. Eric's Willie is 30' long with a beam of 10' resulting in a much smaller l/w ratio of 3.0. Yet it travels 6.15 kts with 1 gallon of fuel for a kpg ratio of 6.15.

So the Willie is more efficient than the Idlewild while having a much smaller l/w ratio.

So what is the advantage of having a narrow boat?

The Willard has much softer buttock lines, with rounding to the stern. This in its self is a more efficient full displacement design. There are several reasons that Idlewild was designed and built with flat surfaces and the hard chine aft. I have owned 2 full powered pilot house sailboats LWL 38' and 40', respective beams 12.5' and 14.5', Displacements 30,000# and 60,000# which would go 6 knots burning 1 gallon of diesel an hour. I was using about 20 hp to do that in calm conditions. (Engines: 4 cylinder Ford Lehman or Perkins 4-236) Both boats could do 8 knots under sail or power.

Another other point is that Idlewild is a proven circumnavigator with her standard tankage of 1000 gallons. The longest leg was 4,495 miles, with a few gallons left over. How many 55' trawlers have a range of over 4500 miles and how many get this fuel economy?

Would any of the Willard 30 or 36 footers be capable of doing a circumnavigation, or going thru the NW passage? The larger vessels are able to carry stores and fuel for longer passages. I suppose if you outfitted a Willard 36 with a Sabb 10 hp single cylinder diesel or even the 2 cylinder 20 hp, and controllable pitch prop--that you might get the range. The problem would come when you had to beat into adverse conditions and not having the HP to do that.

The design of hull shapes is way beyond this thread, but sometimes the most efficient is not the most sea kindly design.

 

[Nomad Willy]

Aspect ratio effects boats in different ways.

If a design was very dependent on narrowness and you changed it ...
a. Whereas the boat got narrower it likely wouldn’t change much. But ..
b. Whereas the boat got wider it may suffer tremendously.

Also in your example (I don’t know “Idlewild”) the most important (usually) element of design is aspect ratio .. Length/Beam. Willy burns 1gph but there are numerous 36 to 40’ boats that burn a bit over 2gph that are way bigger than twice as big as Willy.

And fuel burn numbers expressed on the forum are frequently not even close to reality. Many (perhaps even most) just use the gallons they put in the fill hole and the hours shown on the hour meter. Usually that will show considerably fewer gph burned.

My engine is (for a newish engine) a bit less efficient for several reason the most significant being it’s a “pre-chamber” design combustion chamber. It’s quieter and smoother. I was aware but felt fuel consumption on Willy was still going to very affordable. Also I liked the glow plugs or heaters. Moving to Alaska dependable starting was considered important.

Anyway there’s lots of variables.

 

[Nomad Willy]

thataway,
Yes a Willard 36 would probably be more fuel efficient at 6 knots. Just like my W30 would burn less fuel per hour at 5 knots. BUT mpg would probably only get slightly better.

But for Scott’s mission the W36 would be better if he could afford twice as many batts. His range would be better and he could have more domestic things aboard. But finding an old (cheap) W30 to refit or convert will be next to impossible.

Speak’in of batts mine here is down to 10%. Later ....

 

[mvweebles]

I don't know Idlewild and their journey. But there is credible documentation that a Willard 36 powered by a Perkins 4.226 went from San Diego to Honolulu in October 1987, a distance of about 2300 nms. She did a tad under 6 kts on average and burned around 350 gals of diesel, around 0.9 gph over 18 days. At some point, she returned as she is currently based in the PNW area.

I am a huge fan of the Wm Garden designed W36, unless to answer a dare of a low cost trawler, I would not recommend it for a journey such as this. Windows are large and the deck house is decent but not overly robust.