# V-bottom or flat bottom?

Discussion in 'Boat Design' started by HJS, Oct 14, 2020.

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### HJSMember

V-bottom or flat bottom?
Calculate, do not guess.

#### Attached Files:

• ###### Viberö 6,0 x 1,3 Sammanställning diagram text eng.pdf
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### RightboatNew Member

The v- bottom hull cuts through the waves better than a flat bottom boat and when the boat is stationary, the v-bottom boat is more stable in waves. Flat bottom boats will slap the waves harder when you are relocating on a wavy lake, causing more water to enter the boat.

Looks like a V-bottom boat ...

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### baeckmoHydrodynamics

Statements based on calculations, which in turn are based on what method and how are they validated?

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### cracked_ribsSenior Member

Essentially, then, you found it was possible to keep the flat submerged on account of the lower trim necessary, thereby presenting the stem to the waves, but the shallow V trimmed up more and presented a section of the bottom to the waves, which hit harder?

It does seem reasonable that a submerged flat would be softer than a trimmed up V.

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### srimesSenior Member

Interesting!

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### BlueBell"Whatever..."

Both, a compromise.
Calculation: 1 + 1 = 2

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### HJSMember

My clients demand optimization, never compromises.
This approach is always crucial for the end result.
The whole is more than the sum of its parts. 1+1=3

JS

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### Mr EfficiencySenior Member

What is the cruise speed with this 6 hp engine ? Can't be fast enough to expose the underbody aft, and with the only apparent difference being the underbody aft, naturally the trim angle will be less with the flat afterbody, allowing the forefoot to be more immersed, all that could change as powering was increased.

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### gonzoSenior Member

If this is an engineering analysis required for a client, free opinions are not going to cut it. You will have to pay for it. There are many members that can provide that service if you explain in detail the requirements.

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### DCockeySenior Member

What is there to calculate?
Why does it matter?

Or is it a rhetorical question that you provide the answer to in the attachment?

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### BlueBell"Whatever..."

HJS,

Perhaps this is an English issue but if you are involved with boat design and you think compromise is bad, then you may need to go back to school.

Had I called it a "combination" instead, would you have found that more palatable?

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### cracked_ribsSenior Member

That is my interpretation - I believe I have seen the same comment attached to a number of Sass Jurgen designs.

I find them quite interesting; I wonder however if the narrow waterline beam, even though it seems to be very efficient, results in very tender boats. I fish a lot and a tender boat is a little annoying as a fishing platform. Otherwise I think the Victoria design is quite appealing. I have the plans for the winning entry Marissa, though, because a broad beam is an advantage to a couple of people walking around in the boat at rest.

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### philSweetSenior Member

Following on from baeckmo, you show two hulls and think one is better than the other one. Okay, it would be really strange if they turned out to be identical. Why should we think either is good?

In general, the more variables, the better you can shape a hull to a given condition. But understand if you have optimized the flat bottom hull wrt 2 chine locations, then if you go with a file bottom, and identical stability numbers, and identical displacement curve, then the optimum chine locations will be slightly different, the wetted area will minisculely less, and the ideal sections will have some warp to them, flattening aft. The warp is very sensitive to speed, but at your speeds and power, there will be some warp.

I agree that the running trim of the file bottom will be a tiny bit greater based on the change from static trim. But the cg should be adjusted such that the running angle of the hull is virtually identical to the flat bottom version; and the boat would just settle bow-down a touch more at rest. The wave impact should be less for head-on sea state in the file bottom design if both are trimmed for best economy. Slamming is a terrible drag on economy. And we are talking about tenths of a degree here at 6hp. It shouldn't even show up on the graphs.

So when running the comparisons, at a minimum, match the following-

1. Curve of displacement (shape), and actual displacement unless there is some built-in weight bias to the feature under investigation. In that case, account for the weigh difference.

2. Some practical stability metric. Metacentric height might be okay, but it is a very weak basis of comparison when looking at optimum hull shapes. I prefer something like the change in VCG in a 6 DOF model through a heel of 15 degrees or more. This will optimize to a different set of sections vs just using the metacenter on a two chine design, and it will produce a warped bottom fore and aft on boats such as yours with a high transom beam ratio.

3. Match the chines at the bow. The bow sections can be identical for this comparison.

Then attempt to optimize wetted surface area and lcg for the running conditions of interest.

This style of boat doesn't want to lift very much when semiplaning. It doesn't have a lot of excess stability to start with, so it won't benefit from shedding excess stability when transitioning to planing the way a fatter boat would. That's one reason flattish bottoms over the center half of the section work really well on this type. It gives the sections a steeper angle at the waterline than a typical vee hull would.

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### Mr EfficiencySenior Member

A 100kg, 20 foot boat, I would be concerned about being blown over like a tumbleweed, as much as anything else. In either axis. In any event with a featherweight boat where the load is typically humans weighing in total more than the boat, all calculations of stability and trim depend on how they behave, and where they place themselves. You certainly would not have two people right aft, without the need to counterbalance that weight.

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### HJSMember

Statement from the boat builder:
"......... I expected the Keyhaven to have a soft ride because of the slower speed, but it takes a lot of water disturbance to even notice. I have put a lot of hours on it already, and the good way that it handles still does not feel natural to me (that is a good thing). I have had multiple people approach me that were surprised by how it moves in the water. ......
......... The boat rides very soft in the water, it handles great, and it is very capable in rough water I have been in 3 foot waves with white caps. The boat can be seen in action here. ....."

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