# Higher Cp at higher speeds equates to less resistance?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Dec 1, 2020.

1. Joined: Jun 2019
Posts: 109
Likes: 11, Points: 18
Location: Tucson, AZ

### dustmanSenior Member

Thanks Ad Hoc. Just to make sure I understand you:

By prismatic hump you mean the speed at which I'll experience maximum bow up trim? This will be a particular speed range in which I'll experience added resistance due to the trim? Is the trim at this speed due to a pressure differential or due to trying to climb the bow wave?

I was under the impression that you want your actual entry angle at the bow as low as practical to initially disturb the water as little as possible. Then you want the transition from that angle to max beam to be as gradual as possible. Entry angle and transition need to be more and more gradual as your design speed increases. Am I on the right track with my thinking here?

The design draft of the hull itself should wind up being 12 to 14", with minimal rocker; the bow and stern will terminate about 5" above the point of maximum draft.

I should note that I am planning for about a third of the length to be straight(parallel midbody). Is there any real advantage at these speeds to make the hull a continuous curve? I have it in my head that having some parallel midbody might help with keeping the boat from rocking front to back by moving more of the buoyancy towards the ends, in effect lowering the cp.

I was planning to make the exit angle slightly less than entry angle, pushing the LCB slightly aft. A lot of people say this decreases rocking significantly.

I'm not sure I understand what you are saying here: "Then do a similar, in profile view, from midships, at the deepest draft, to the transom, but leaving around 60% of the draft at the transom."

Why do you recommend an LCB of 8%? To counteract bow up trim?

Magic carpet, I like the sound of that. Am hoping to decrease resistance as much as possible so I can minimize powering requirements and all the associated structural requirements.

At what point does increasing LD ratio give you diminishing returns? I'd like to sail well in light winds but still be able reach 15 knots in good conditions.

2. Joined: Jun 2019
Posts: 109
Likes: 11, Points: 18
Location: Tucson, AZ

### dustmanSenior Member

What do you all think of a hull like this in moderately rough sea states?

3. Joined: Oct 2008
Posts: 7,047
Likes: 975, Points: 113, Legacy Rep: 2488
Location: Japan

### Ad HocNaval Architect

Dustman,

Ok..in picture form:

Ignore the PMB - parallel middle body.
Red Herring!

Too many minor questions in your post, that require a lot of explaining... happy to do so, but it wont be a 5min fire side chat!
You may need to do plenty of background reading too..

Hence the condensed version in picture format.

DogCavalry and bajansailor like this.
4. Joined: Jun 2019
Posts: 109
Likes: 11, Points: 18
Location: Tucson, AZ

### dustmanSenior Member

Oops, correction, draft will be 6-7".

5. Joined: Oct 2007
Posts: 2,211
Likes: 746, Points: 113, Legacy Rep: 37

### bajansailorMarine Surveyor

Dustman, have you got any sketches of your boat please?
Including a side profile showing the sail plan if possible?

I was thinking earlier that you were designing a power cat, and then I later saw mention of how you would like to be able to sail at 15 knots in good conditions.
You also mentioned earlier a displacement of 1,200 lbs - does this include crew weight?

6. Joined: Jun 2019
Posts: 109
Likes: 11, Points: 18
Location: Tucson, AZ

### dustmanSenior Member

I never understood why you would taper the hull so much vertically instead of horizontally, doesn't this contribute to bow up trim?

7. Joined: Oct 2007
Posts: 2,211
Likes: 746, Points: 113, Legacy Rep: 37

### bajansailorMarine Surveyor

Is this in reference to Ad Hoc's sketch?

The taper that you refer to (also called rocker) is good - ideally you want it so that when you are floating at your load waterline, the transom is just kissing the waterline, or very slightly submerged.
If you had no taper / rocker at all, you would probably have extra drag from transom immersion, not to mention an unsuitable prismatic coefficient, and maybe too much buoyancy aft.

Ad Hoc likes this.
8. Joined: Oct 2008
Posts: 7,047
Likes: 975, Points: 113, Legacy Rep: 2488
Location: Japan

### Ad HocNaval Architect

You need to design the boat first rather than discussing just abstract numbers!!
Then once you have your weights and matching hydrostatics can you consider Qs like this.
It also depends upon your speed range...which as you have given, should be relevantly unaffected as your LD ratio is high....assuming you achieve this of course!

And... this assumes you'll be spending most of your time at the speeds you noted.

bajansailor likes this.
9. Joined: Oct 2008
Posts: 7,047
Likes: 975, Points: 113, Legacy Rep: 2488
Location: Japan

### Ad HocNaval Architect

Me too!!

But motor/sailing, shouldn't really influence this too much as the hull has a high L/B and LD ratio... so it is of to a good start

bajansailor likes this.
10. Joined: Oct 2009
Posts: 4,997
Likes: 488, Points: 83, Legacy Rep: 1485
Location: Midcoast Maine

### DCockeySenior Member

Moving volume and therefore buoyancy towards the ends means the maximum cross-sectional area will be small for the same displacement, and therefore Cp will increase.
Cp = Volume displaced / (LWL x Area max)

fastsailing, Ad Hoc and bajansailor like this.
11. Joined: Sep 2011
Posts: 6,737
Likes: 491, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Sorry but, if we talk about "moving", not "adding", volume, as I understand it, you can move submerged body volume forward or aft, without changing the total displacement and without changing the area of the maximum section. The maximum area section, as normal, does not have to be in Lpp / 2. The shape of the frame area curve changes but not the total displacement. Is that called an "affine transformation" (literal translation from Spanish)?
This is precisely what is done to achieve an adequate LCB for the current weight distribution without varying the other magnitudes of the boat.
Where am I wrong?.

12. Joined: Jun 2019
Posts: 109
Likes: 11, Points: 18
Location: Tucson, AZ

### dustmanSenior Member

Yes, I made a mistake typing that.

13. Joined: Oct 2009
Posts: 4,997
Likes: 488, Points: 83, Legacy Rep: 1485
Location: Midcoast Maine

### DCockeySenior Member

Prismatic coefficient can be defined using either the maximum cross-sectional area, or the using the cross sectional-area at mid-ships which may or may be the maximum cross-sectional area. For many vessels the two areas are the same, but for some vessels, particularly those with considerable "drag" (keel sloping down towards the stern) the maximum cross-sectional area is located away from mid-ships. I prefer using the maximum cross-sectional area.

It is correct that it is possible to move volume towards the ends of the vessel without altering the maximum cross-sectional area. However for many vessel shapes this will introduce unwanted hollows or other peculiarities into the shape of the vessel and curve of areas. Frequently a reduction in mid-ships area / maximum cross-sectional area is needed to maintain the desired shape of the vessel and curve of areas when moving volume towards the ends of the vessel.

14. Joined: Oct 2009
Posts: 4,997
Likes: 488, Points: 83, Legacy Rep: 1485
Location: Midcoast Maine

### DCockeySenior Member

To be clear, Dustman and I are not the same person. I don't know what he made a mistake typing.

15. Joined: Jun 2019
Posts: 109
Likes: 11, Points: 18
Location: Tucson, AZ

### dustmanSenior Member

I hope it's clear from my posts that it's a double ender. The majority of the stern taper will be in the horizontal plane. When I say the hull will be similar to a double scull, I mean quite similar, except for having a plumb bow and the keel line will be a little sharper for more of the length. Part of the reason for this is that the method of construction will be simplified greatly by doing it the way I plan, and at least in my head will result in an exquisitely hydrodynamic form.

BlueBell likes this.
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.