Power to speed ratio for displacement boats

Discussion in 'Boat Design' started by misanthropicexplore, Apr 23, 2018.

1. Joined: Apr 2018
Posts: 79
Likes: 14, Points: 8
Location: Upper middle Missouri River

misanthropicexploreJunior Member

Near as I can tell these are the main factors of boat design, not counting wallet thickness, from a power point of view. The important thing to me here is that I get the order right. This is from most important factor to least:

(1.) Displacement: All other things being equal it takes less energy to move less mass.
(2.) Slenderness: When displacement is equal, the slender boat takes the least energy to accelerate.
(3.) Entry: When the slenderness is equal, fineness of entry comes into play.
(4.) Wetted area: etc.
(5.) Sectional profile: etc.
(6.) Skin surface quality: etc.

If I'm looking at this right, then at the same displacement, a box end barge made with a l/b ratio of 10:1 will take less power to push up a certain speed than round sectioned catboat with a l/b of 2:1. Does that seem right?

Is there something I am missing?

2. Joined: Aug 2002
Posts: 15,604
Likes: 1,097, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

gonzoSenior Member

This should help:

Attached Files:

• boat speed chart05272015.jpg
File size:
194.8 KB
Views:
445
3. Joined: Apr 2018
Posts: 79
Likes: 14, Points: 8
Location: Upper middle Missouri River

misanthropicexploreJunior Member

4. Joined: Oct 2008
Posts: 7,184
Likes: 1,105, Points: 113, Legacy Rep: 2488
Location: Japan

Yes, the SOR.

Without knowing the SOR, all you're doing is taking one element, one chain or link, if you like, and assume this one chain is the absolute, in terms of the design. It is not.
There are many ways to get the best out of the hull, hydrodynamically speaking, but it is all indicated by the SOR. Since the SOR may imply or infer a hull shape that is very draggy and not particularly useful at all. Expect...it satisfies the SOR.

In layman's terms, it is like thinking about the size and shape and quality of an egg, when making a Victoria sponge cake, yet ignoring all the other ingredients. Since you're assuming that THE egg is all that matters. There is more to a Victoria sponge cake, than just the eggs!

5. Joined: Apr 2018
Posts: 79
Likes: 14, Points: 8
Location: Upper middle Missouri River

misanthropicexploreJunior Member

That makes sense. The reason for no Statement of Requirements is I'm still trying to understand things in the most general, first principles sort of way first.

6. Joined: Oct 2008
Posts: 7,184
Likes: 1,105, Points: 113, Legacy Rep: 2488
Location: Japan

Then I would suggest you start reading some very good books on the basics of hydrodynamics. That will make things clearer and more complex at the same time as the more your learn leads to yet more questions!!

7. Joined: Nov 2012
Posts: 811
Likes: 62, Points: 28, Legacy Rep: 41
Location: Delta BC

JSLSenior Member

Check out various coefficients like 'block', 'prismatic', etc.

8. Joined: Nov 2003
Posts: 19,133
Likes: 489, Points: 93, Legacy Rep: 3967
Location: Eustis, FL

PARYacht Designer/Builder

Read "The Nature of Boats" by Dave Gerr. In developing a refined SOR, you don't need to have any clue about the beam/length ratios or anything else in regard to hydrodynamics. The SOR typically forces your (the designer's) hand in this regard. You may desire one aspect, but other portions of the SOR dictate you (the designer) take a somewhat different approuch. It's this very thing that has nearly every designer, NA or engineer mostly bald or if like me, still wallowing in baldness denial. The only time this doesn't apply is when developing a very narrow set of variables, like that of a racer. On these, the compromises are still quite confounding though in general, you stay within the rules and class requirements, cheat where you can get away with it (if you don't you'll be mid pack) and cut it as close as you dare, just to remain competitive. I've seen sailboat racers that were cut so thin to the line, the first time they really torqued up the back stay, it literally folded the yacht in half. This tends to makes SOR decisions easier, though in this case maybe a tad too close to the line.

9. Joined: Apr 2018
Posts: 79
Likes: 14, Points: 8
Location: Upper middle Missouri River

misanthropicexploreJunior Member

I confess, that from what I know about hydrodynamics, aerodynamics seems easier. With aero if you know the shape and the reynolds number, you can very easily ballpark everything you need to know. With boats that doesn't work. Graph some random coefficient between different boats and instead of nice flat lines, or smooth exponential curves, you get lines that wave all over the damn place with weird low and high points at certain fractions of froude numbers...which makes sense from a wave making resistance point of view...but it makes it really hard to guess a ball park figure.

The reason I don't want to make an SOR yet is because I have don't want to be the kind of "customer" that I have learned to hate over the years. I supplement my income with a little construction contracting. Light, strong, cheap material, fast labor. I can give you any combination of those three that you want, but I can't give you all four...but that's what people ask for. I don't want to be the guy asking for a hurricane proof, 40' ULDB papercrete boat for \$5k.

I'll see if I can get that book.

10. Joined: Oct 2008
Posts: 7,184
Likes: 1,105, Points: 113, Legacy Rep: 2488
Location: Japan

Which is the reason why you need to understand the SOR rather than focusing on the absolutes!!!

11. Joined: Nov 2012
Posts: 811
Likes: 62, Points: 28, Legacy Rep: 41
Location: Delta BC

JSLSenior Member

Aerodynamics and hydrodynamics both deal with solids moving through a fluid. The difference is s.g.(density), one fluid is compressible (air) and the other (water) is not.

Last edited: Apr 25, 2018
12. Joined: Jan 2005
Posts: 418
Likes: 68, Points: 38, Legacy Rep: 56

fredrosseUSACE Steam

Adding to JSL's comment, the phenomenon of "Free Surface Flow", where the atmosphere meets the water, complicates things by an order of magnitude or more. Designing a submarine hull is relatively easy when submerged, compared to surface craft.

13. Joined: Nov 2003
Posts: 19,133
Likes: 489, Points: 93, Legacy Rep: 3967
Location: Eustis, FL

PARYacht Designer/Builder

Aero and hydrodynamics are quite similar, using many of the same formulas, though water being about 50 times more viscous. If you can handle basic 12th grade math, neither of these subjects are very difficult, though truly understanding them, will take considerably longer, for many of us a life time.

14. Joined: Apr 2018
Posts: 79
Likes: 14, Points: 8
Location: Upper middle Missouri River

misanthropicexploreJunior Member

Yeah, that seems to be the case to me. Just rotate a NACA 0015 and you have a pretty OK sub hull, mostly regardless of displacement or desired speed. Whereas displacement designs don't scale well because of waves, and the fact that resistance to speed depends on froude number, which changes at the boat heels. D

15. Joined: Oct 2009
Posts: 5,055
Likes: 531, Points: 113, Legacy Rep: 1485
Location: Midcoast Maine

DCockeySenior Member

In low speed aerodynamics, below 200 knots or so, the effects of compressibility of the air are ignored.

Fred is correct. Free surface effects are a major complication for surface vessel hydrodynamics.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.