In defense of SD trawlers

[Tad Roberts]

Tad:
What is meant by operates? Can some boats exceed those numbers? Certainly all can go slower.

A lot of things might be meant, but I meant design speed as "operating". Every hull I design, power or sail, planing or displacement, is intended to operate at a certain (usually a narrow range) speed. The hull form is optimized to be most efficient at this speed, it may exceed or may not achieve the design speed.

If more power than I foresee is installed the boat may run faster than design speed. This usually results in the "snow plow" you see running bow up and throwing a large wake. If the boat comes out overweight (this happens often) she may not achieve design speed (underpowered).

Occasionally someone will state that their product operates efficiently at a range of speeds. That is partly possible, but the hull will still have a sweet spot where it runs best. Sometimes a boat will be required for two specific operating speeds, say a tug. For a usual tug the towing speed is the one that takes precedent, as that's were the money is made. But I've had commissions for dual speed cruisers, and in that case used a boost engine for the high end.

 

[Tad Roberts]

Tad: What shape hull rolls least?

Multihull......

 

[Tad Roberts]

Tad: What shape hull rolls least?

Seriously.....

Between displacement and semi-displacement boats.

Again we come back to speaking in wild generalities. There is no definitive (AFAIAC) choice here.

If you had two identical boats, except one was a vertical sided, shallow vee-bottomed, semi-displacement hull, and the other a slack-bilged round sided full displacement boat. If their beam was identical they would both follow (roll) a wave the same, if they were stopped in a sea. But the FD boat will roll further, slower, and some might say more comfortably. It will also take longer to stop rolling, which is why the owner installed bat-wings. The semi-displacement hull will roll more quickly but not as far, and stop rolling (damping) quickly. At speed the SD will roll a lot less due to what's called dynamic stability. But for some people this quick motion can be uncomfortable, as stated above by more than one person.

Of course the problem with the above scenario is that there's no such thing as identical except for bottom shape. It can't really happen. The semi-displacement boat will be of lighter construction, with no ballast, smaller tankage, more and bigger HP, shallower draft, etc. It would not make any sense to build the two hull types identical otherwise, so no one does it.......

 

[Donsan]

Just one feature(buttock angle) won't do it. Yes, as a very wide generalization buttock angle (rise) might be flatter on a semi-displacement hull than on a full displacement hull. But, I am a Naval Architect and I rarely look at buttock angle. I first look at prismatic coefficient, I look at displacement/length ratio, I look at intended operating speed(more precisely speed/length ratio), longitudinal weight distribution, HP to weight, and finally volume distribution (sort of back to CP).

I'm sure I've posted this before but for the record. Naval Architects define hull type; planing, displacement, or semi-displacement, by operating speed/length. We use something called a Froude number.

Fn = V/ sqrt Lg

V is the vessel speed (metres/second)
L is submerged length (metres)
g is acceleration due to gravity (9.8 m/s^2)

Displacement hulls operate at Fn less than 0.4
Planing hulls operate at Fn > 1.0-1.2
Semi-displacement vessels operate at Fn between about 0.4-0.5 and 1.0-1.2

To convert knots to m/s multiply by 1.9425

So a 30' waterline boat traveling at 7 knots is operating at Fn = 0.45

A 35' waterline at 14 knots is Fn = 27.195 / (sqrt 10.66 * 96.04)
= 27.195 / 31.99
= 0.84

Well, it appears from these calculations my Fu Hwa (35' waterline and 7 knots) is actually a borderline SD hull with a Fn of 0.42. and I miss the FD cutoff by slightly more than 0.02. Is that statistically significant?

Ouch, that hurts!

 

[eyschulman]

Tad thanks for your technical input love to find the gems among the chafe and learn something.

 

[tpbrady]

Donsan,

With the same boat as you, I don't think being on the cusp matters. It still only goes 6-7 knots is calm water and no current. At WOT (2350 RPM) I can push 9 knots along with using a lot of fuel and throwing out some smoke. I call that the emergency power setting only used to go through places like Deception Pass against the tide.

Tom

 

[eyschulman]

Donsan,

With the same boat as you, I don't think being on the cusp matters. It still only goes 6-7 knots is calm water and no current. At WOT (2350 RPM) I can push 9 knots along with using a lot of fuel and throwing out some smoke. I call that the emergency power setting only used to go through places like Deception Pass against the tide.

Tom

See you in Deception pass. I have also found that 9 or more knots is a big help in our PNW passes and rapids. What I like to have is at least 4-5 K more than the current in order to keep good rudder control. When I did sail boats this got dicey at times had to really time things well. With SD hull and some juice it is much easier to deal with rapids especially up north where there are chains of rapids with fixed deadlines for slack or near slack.

 

[FF]

>Tad: What shape hull rolls least?


Multihull......<

But the violence of the snap as it follows the water surface may not be to every ones liking.

Especially if the boat is properly built , light as can be for its length.

 

[Nomad Willy]

eyschulman wrote;
"See you in Deception pass. I have also found that 9 or more knots is a big help in our PNW passes and rapids."
It can be an advantage but most of the time we just go slower. We get where wer'e going but slowly. Waiting for slack water has almost never come to pass w our 6 knot boat. We have waited for slack water at Dent rapids and Seymour Narrows but all the SD boats did too.
Speed bucking current is over rated IMO.

 

[eyschulman]

Back on track to the post #1. The new issue of Wooden Boat magazine has an article featuring the motor boat designs of Nigel Irons. The concept of Fast very economical SD boats is featured in his design concepts with his explanation of the mechanics of such boats. Pack rats and boaters seeking the cottage on the water would not be pleased. Nevertheless as posted with many examples here and elsewhere a SD can deliver good fuel burn #s and the option of speed beyond the 1.33xSR of WL.

 

[Nomad Willy]

Actually you mean "option of speed beyond a knot below the 1.34x sr of WL."
FD boats do not cruise at hull speed. If they do they are very overpowered and over throttled or not FD. Actually a half a knot below HS will work for many.

 

[Tad Roberts]

Well, it appears from these calculations my Fu Hwa (35' waterline and 7 knots) is actually a borderline SD hull with a Fn of 0.42. and I miss the FD cutoff by slightly more than 0.02. Is that statistically significant?

Ouch, that hurts!

No worries, I did write that FD is up to around 0.4-0.5, and SD is between 0.4-0.5 and 1.0-1.2. These are soft fuzzy limits that are generally agreed to by many naval architects. Then we have Eric stating that full displacement is 0.5 knots below 1.34 x sqrt LWL.

A 30' waterline at 7.4 knots = Fn = 0.48, and is equal to the old speed/length ratio of 1.34 x sqrt LWL. That's the traditional top of FD, but today most agree that absolute limits don't work and aren't useful.

To add confusion there are other defining factors. One is the amount of dynamic support. As a hull accelerates from standstill, it first sinks a little bit (measured at the center of gravity) and then starts to rise as speed increases. The sinking part is generally considered FD. Once she starts to rise it's semi-displacement up to some percentage of her weight (popularly 50%) being supported dynamically, from there to 100% dynamic support is planing. There is no way to establish this in real life, but it's easily observed in a towing tank.

 

[Pgitug]

Be careful when you start talking FD sailboats. You have to consider air draft. With heavy rains in the Carolina's and easterly winds from the Atlantic a sailboat can be "Stranded" due to high water I on the ICW. Not being able to clear the fixed bridges. Speaking from many personal experiences.

 

[Nomad Willy]

TAD,
MOST ANYBODY ON TF knows what hull speed is. What I said is that the speed to run them is .5 to one knot below hull speed. I think most (even on TF) think FD trawlers operate at hull speed. Perhaps my wording was bad. Sorry.

 

[FF]

The ONLY folks that attempt to operate at hull speed seem to be the sailors , as wind energy is free and broad reaching in 17K + with whole sails is exciting.

Most marine motorists will LRC at Sl of about 1
Thats the Sq RT of the actual underway LWL in K.

The only forced confusion is the ICW motorists that quote statute miles (that what their charts are calibrated for) .

So 6K is about 7stat , which sounds faster