Bow half angle of entrance

I don't know about the others, but this is the MFV listed in skaraborgcraft's image.

The Morgan Giles Archive - MFV https://www.morgangilesarchive.org.uk/index.php/the-vessels/ww2/mfv

 

Ok, thats fine. Still does not tell me where to measure from. What makes it pathetically inefficient if not its bow angle, something else?

 

And this is probably similar to the the "G L Watson" design, being almost exactly the same size.

1960 G L Watson Motor Yacht Former Mfv for sale. View price, photos and Buy 1960 G L Watson Motor Yacht Former Mfv #116007 https://dailyboats.com/boat/116007-buy-g-l-watson-motor-yacht-former-mfv-for-sale

 

I play with lots heavier slightly shorter trawlers. It doesn't seem possible to have that big of a difference, but anecdotally I've seen it.

Our big builder is Fred Wahl marine, he builds 58 footers 26-30 feet wide around 125 tons. They burn fuel like a Kuwaiti refinery fire. They are slow, like 6 knot loaded speed. But they are relatively cheap and he can have them out the door in under 8 months.

Hansen is the opposite, they have made one. Almost identical outside specs and tonnage, but it's much smoother looking below the water line. It took talented men with grey beards and big wheels to make the smother transitions compared to the Wahl. It's shockingly more efficient and faster than the equivalent Wahl. Fiends of my mine fish on it and they blow by me like I'm parked, where as my boat is so slow I'm only passing wahls... does the 400k and 4 months warrant the lifetime of objectively better performance... apparently not as Wahl sells 20 to 1.

 

When you have "cheap" fuel, economy is not the biggest issue, but over the working life of a boat, it all adds up.

Going by sail performance alone, a fully bluff bowed Dutch type barge is going to get passed by a fine bowed Thames Barge of the same capacity, so "easily driven" comes from having a Cp closer to the desired speed -length-ratio. Yet bow entry angle is bought up often enough that it has to play a part.

 

I knew a guy who had a very similar boat, converted from trawler to offshore guard duty to a dive boat, and running a Gardner 6LXB, 150hp. Im still trying to wrap my mind around going just as fast on half the hp with a 4LW.

 

One of the things a naval architect needs to become familiar with is the physical representation of dimensionless coefficients like the midships and prismatic. Think about: it a midships coefficient of 0.5 would imply a section shape of a straight line from keel to waterline beam. Less than 0.5 implies a concave hull section and more than 0.5 implies a convex section becoming perfectly rectangular at a midships coefficient of 1.0. A circular arc is the shape that maximizes volume for surface area, it has a midships coefficient of 0.785; so better performing hulls for a given tonnage have midships coefficient closer to 0.785 than 0.5. Similarly, the prismatic coefficient...a coefficient of 0.5 implies a straight line from the FP to Midships to AP.... a prismatic coefficient of 0.785 implies an elliptical shape from bow to stern.

So a midships coeff. of 0.594 means the section is slack bilged and therefore needs to carry the midships section further along the hull because it needs to make a specific tonnage. This is shown in the prismatic coeff. of 0.645 which makes the ends fuller in general. Conversely, the second easier driven hull has a midships coeff closer to 0.785 and is able to thin the ends because it can carry the tonnage more easily, as shown by its prismatic coeff of 0.537.

Of course these are generalizations based on the assumption of smooth transitions in the hull form. You would really need to look at the sectional area curve and the body plan or wetted area curve to know exactly how this is going to effect powering. By playing with weights and volumes you can make very efficient vessels with high midships coefficients, low prismatic coefficients, and very fine entrance angles.....such as the US third generation fast battleships.

 

I get all that.

It does not look like im going to get an answer regards to measuring point, without any drawings to compare.

Ii already have a hull that can achieve a Speed-Length of 1.9 , I dont know what the bow angle is and if it can be tweaked to gain better performance with less hp........ hence the original post.

 

For V/(sqrt L) = 1.9 then your prismatic coeff should be 0.62-0.64. Displacement length ratio (volumetric disp/(0.01 L)^3 )should be between 30 and 50. B/T should be ~2.25 and maximum section should be 60 to 70 % aft of the FP. This will all help decrease the entrance angle. Which, as I said earlier, is generally an artifact of other hull design considerations.

 

BTW, are you familiar with TLINGIT?

Bill Garden's Tlingit https://www.boatdesign.net/threads/bill-gardens-tlingit.54853/

 

Yes, I am a fan of easily driven hulls, but that would not fit in my shed or able to be towed down small country lanes for weekend launching. Even if i extended the fore-foot of my bow, i would gain WL length and a finer entry. I may try that full size on a mock-up.

Thanks for effort.

 

The DLR on this hull was 164, beyond the 133 suggested would be required. It is a "box keel" design, and I summise it was getting lift from its keel base as speed was gained, as the "theory" suggested the numbers would not work. It was certainly beyond my expectations and an interesting experiment.

 

A floating object is to be moved through the water. If it is moving in displacement mode then it has to push some water out of the way. The rate of movement of a particle of water is the determining factor for the force required to move that water. I have used the term rate which is the acceleration of the particle. The higher the acceleration the more force required. That takes us to the familiar Newtonian postulate..F=Ma. That little "a" is the kicker. It is apparent that a larger angle of the bow will cause the acceleration to increase for a given location on the forward surfaces of the boat.

We could measure the various instantaneous angles and calculate acceleration rates. Integrating all of them could give us some information. I suspect that it is sufficient to observe that long skinny boats have more gentle entrance angles. They require less input force to make them go at a given velocity than fat boats do.

 

Nigel Irens was musing about this very subject, and mentioned, IIRR, that during COVID lockdown that he spent a fair amount of his time pondering easily driven motorboat hulls that would go gently (willingly?) into forced mode, and found that squeezing a toilet paper tube at both ends 90 degrees from each end gave some inspiration. I found this irresistible, and being a fan of skinny boats and intuitive design, tried it with a paper towel tube ( I think the stern can be squeezed to taste, but it is big fun to mess with, and rocker can be moveable feast, as well as bow aoa…):

 

I would say that is already a given, and why i used the example of 70 ton trawlers of almost the same length, beam and displacement, but different entry angles. A picture of the waterplane areas would have been more usefull than a bad description of half angle bow entry.