# boat design questions

Discussion in 'Boat Design' started by HowardH, Jul 22, 2013.

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### HowardHJunior Member

I have questions regarding the stated specs associated with boat designs.

Displacement figures. Is that boat weight or total weight capacity to displacement waterline? What determines displacement waterline? If the numbers are capacity how does one determine boat weight so as to figure carrying capacity of people, gear, etc?

Hull speed. How is that determined? How accurate is the published number? Many, perhaps most, of these designs appear to have never been built. I draw that conclusion from the lack of associated photos of actual boats to go with the design drawings. With that in mind how are the numbers arrived at? I would be disappointed to build a boat with a published speed of ten knots only to discover I'm getting eight. What would be considered a reasonable margin of error?

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### quequenSenior Member

Howard, what designs are you talking about? Link to them or name them, then you may receive some opinions.

"Hull speed" is often used to point a velocity equal to V (m/s) = 0.4 x ((9.81 x Lenght of Waterline in meters) ^ 0.5)
It is not necessarily related to the top speed of the boat.

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### HowardHJunior Member

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### lewisboatsObsessed Member

Displacement is the weight of the amount of water the boat will replace at the design waterline. When the boat is designed all the weight of the components is calculated to find the lightship weight and trim. Some designers will start with a target total displacement and design the hull to fit the bill... allowing for people, supplies and stuff... basically the "cargo". Others will design a hull close to what they envision and then calculate the scantlings and "stuff" and perhaps adjust the hull to tweak it then recalculate. There is a thing called the "design spiral" that a lot of designers work to. You start with the basics then adjust for this and that, re-calculate, reshape readjust until everything fits as neatly as possible or you have to chuck it out and start again. Predictions can be made as to performance and they will vary in accuracy according to the experience of the designer. A designer who has done a number of hulls that are similar can give a more accurate estimate than someone who is trying an entirely new design (to him/her). However... until the hull is built (Exactly) as designed and loaded (Exactly) to specs you will not know how accurate the estimate of hull speed will be. If the designer used the basic rule of thumb and didn't account for large deviances in hull shape ie: really short and fat with piggy lines or long and lean with smooth flowing lines then the estimate will be rather off.

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### quequenSenior Member

Beautiful boats, all of them!. TAD is a member, he can explain what you're requesting in detail.
Anyway, usually on displacement motor boats the published velocity often refers to the "design" velocity, that is the velocity where the boat will reach his best performance (fuel/distance).
That number is not necessarily related to the top speed.

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### PARYacht Designer/Builder

Displacement figures published can vary widely, from delivery weight to full up month long cruise ready and any place in between. It's unfortunate there aren't better instilled standards for this, but most do list displacement with tanks partly filled and a partial compliment of crew, stores and equipment.

Determining displacement is part of the volumetric requirements for any design. Yes, everything has it's weight and location aboard plotted, all adding up to a total, longitudinally, vertically and laterally. This is the only way to determine how the boat will float, compared to how you want her to float, which is an important distinction.

Hull speed (theoretical) is determined by taking the square root of the waterline's length and multiplying with a speed/length qualifier. This qualifier can be arbitrary, but for many sailboat or displacement powerboat hulls, it's about 1.3 - 1.35, but this speed is dependent on many factors. Some hulls just can't go this fast with the available power, while others can easily exceed this speed, so some level of understanding is required when playing with these types of calculations. The same is true of higher speed vessels (different formulas), where shapes and power available can dramatically affect the predictions.

Most designers are pretty careful about published speeds. If a design is intended to travel at 10 knots and the best you can do is 8, you'd justifiably be pissed, but a reputable design, wouldn't have this issue. You might run 10.2 or 9.8 knots, but you'd be in an acceptable range. With each build, there's always some discrepancies, in regard to material selections, slight difference in the build, modifications not on the plans, etc. These are the things that cause major differences in speed and performance, not the formulas to make these predictions.

If truly interested in these and other formulas, plus all the seemingly esoteric stuff about designs, get Dave Geer's book "The Nature of Boats". It's available at the book store here or any book seller.

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Don't trust those guys and their numbers, they probably don't know what they're talking about......

The displacement waterline is wherever the designer has drawn it. The immersed volume below that line equals the design displacement. Beyond that it is up to the builder to follow the plans, not make any wacky changes unless he fully understands the consequences, and the boat will float somewhere close to her design waterline.

No boat floats precisely on her design waterline, few float higher, most end up floating lower. This is due to the designer making one set of assumptions and the builder another. "Surely an electric windlass, extra battery, and chain rode won't make much difference?" "Surely a hard-bottom inflatable and 15HP outboard, and gas, and fishing gear, won't make any difference?" Bill Garden called it, "going by the building shed to see what they've done to me today."

The largest factors in determining hull speed are length and displacement (weight). Then there are a bunch of lesser factors like bottom roughness, form factors (wavemaking), propulsive coefficients, wetted surface, etc. Most estimates will be accompanied by caveats such as, "at design weight", "flat water, no current, no wind, clean bottom, correct propeller, etc." I can't speak for others but I certainly try to publish conservative speed projections. Speed and power projections for planing and displacement hulls are well understood by naval architects, the gray area of semi-displacement is less well understood. After 30 years you get a bit pessimistic about such things. I can't recall ever being in trouble because a boat didn't achieve some speed, but I've never guaranteed such things (due to numerous factors beyond my control) either.

There's only one well known designer I can think of who publishes completely unrealistic speed and power projections on his website. There are threads on the subject on this forum.

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### nzboySenior Member

All nice boats, Tad also my favourite designer .The thing is they are different boats with different uses .Notice some are quite "heavy displacement "They will need ballast for them to sit on their lines. Some trawlers simply have large fuel tanks for ballast This can affect seaworthiness if they aren't kept full! Boats with ballast will be more seaworthy but will need more power. A coastal cruiser with shallow draft may not need ballast and will be easily powered . As for Tad comment on designer power ratings It is true but needs to be qualified Eg. I have 4.236 perkins at 1200rpm and no load burns 3lph so generating 15kw If I run at 2000 under load 50kw and 10 lph So I think these designers are quoting power needed at propeller in calm conditions A well known designer is quoting powering to 40% loading in calmish conditions at displacement speed A trawler I have been on had twin detroits Under calm conditions he only ran one engine But if things got rough 2 Another 50 ft trawler had a115hp Gardner engine But has since replaced with Derated V10 MTU 250hp continuous .Because at times his old engine was overloaded for the conditions New motor can idle all day But has big capacity when needed, fuel consumption more economical with modern motor

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### HowardHJunior Member

Thanks for he replies.

Tad would the Wedgepoint be suitable for aluminum construction?

Which gives a lighter boat? Wood or aluminum?

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The Wedgepoint is designed to be built from sheet material (plywood) and could be adapted for aluminum construction. 3/16" aluminum plate (the lightest one would use in the hull) is 2.64 pounds per sq ft, 1/2" Occume ply is about 1.25 pounds per sq ft. The metal boat will be heavier, but how much heavier depends on many details.

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### HowardHJunior Member

Oh boy why didn't I think of that? Five minutes of research and I could have got the answer for myself. At least part of it.

Aluminum is roughly twice as heavy. Is there a rule of thumb for finished weight? For instance the wood hull might require more and heavier frames etc. I would think that things such as epoxy, fasteners, fillets, fiberglass, etc would add substantial weight to the wood hull but wouldn't be needed with the aluminum hull.

12. ### El_GueroPrevious Member

But, wouldn't a comparison with Anzel be better than Frank Lloyd?

Wayne

13. ### dskiraPrevious Member

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