Warped plane vs constant deadrise

mmd: This has been a comment I've seen any number of times - and on several of the hullforms that I've posted myself over time.
So what's the solution? If we want sharp sections fwd to slice through chop and flat aft sections to provide an easily driven vessel that doesn't poke it's bows skyward at the 1st hint of speed, then surely we are stuck - at least to a certain extent - with a shape somewhat like Dims. The addition of a small keel would help somewhat no doubt......

 

I will agree that bow steering will not be an issue at moderate semi-displacement speeds and in calm to moderate seas. I was basing my comments on first hand experience gained through supervising the construction of FRP hulls with a fine, narrow bow such as that shown by Dim and having to deal with the quality assurance problems created by trying to laminate into such a deep, narrow recess. I subsequently operated these same hulls in seas of 1 to 2 metres and found that speed needed to be reduced to avoid bow steering well before the ride became uncomfortable due to pounding.

Also, I have designed several high L/B hulls that operate in the range of 20 - 24 knots in coastal waters of the north Atlantic with a similar underbody form but with a bow profile considerably more cut away below the waterline. These hulls are able to maintain a rapid cruising speed in a seaway with minimal pounding (fine entry sections forward) and no tendency to bow steer or broach when entering a large wave (minimal lateral area in the underwater bow profile).

I'm not trying to say that I have all the answers, just trying to make a suggestion based on my experience with these types of hulls. As with most boats, the form develops to accomodate the conditions and useage of the boat as well as the tastes of the client and designer. C'est la vie.

Will, my solution in past designs has been to (using Dim's model as the example) maintain the upper bow profile and hullform from station 2 aft, but to substitute an elliptical profile form for Dim's circular shape in the forefoot area of the bow below the waterline. This allows reserve buoyancy to be realized smoothly without pounding as the narrow bow submerges in a wave, but doesn't present the lateral area of the minimally-buoyant sharp circular forefoot that can be the cause of bow steering.

 

Ok, I take it you mean change the generally circular shape to generally eliptical one, when looking at the profile of the forefoot? But in doing so, you must reduce the deadrise here - or do you reduce the chine beam correspondingly to maintain the higher angles?
You make a good point about the difficulties of construction: not something that we (...well me) student designers would always consider.

ps. I think you and Dim are coming from the same view point. Deciphering his message, I think he intends the hull only be used in relatively calm waters....

thanks for the input

 

Hi, Will and mmd,

If I have correctly understood mmd and if we speak about more correct reconstruction of "Latitude 46" boat's hull - I completely agree with opinion mmd. I as completely agree with opinion mmd that narrow form of the bow parts of the hull create more problems at manufacturing the hull and accommodation of the various equipment. It is indisputable. But I as want to tell the following. If I had opportunity to maintain such boat I would try to do it in the conditions, similar that are represented in photos "Latitude 46".
I have tried still to work with the hull's model and for me that is represented in a picture has turned out.

water density = 1.025 t / cub.m.

Displacement 10.16 t
Length OA 50.000 ft
WL Length 49.187 ft
WL Beam 11.501 ft
Draft at LCF 1.774 ft
Prismatic coeff. 0.757
Block coeff. 0.355
Midship area coeff. 0.662
Waterplane area coeff. 0.713
LCB to amidship 4.047 aft ft
tpi 0.976 t / in

Dim.

 

I've run into this issue at work recently. We had an existing hull form which had a constantly decreasing deadrise angle all the way to the transom. We updated our molds with CNC tooling, and following our waterjet manufacturers advice, left the last 15% of hull length at a constant deadrise angle.

The resulting dynamic center of effort is considerably further forward as a result of the change. Though I knew the theory going into this excercise, I did not expect such a dramatic effect from such a seemingly minor change.

 

Hi All, again.

I am sorry for irrepressible enthusiasm. It's just very interesting for me. I have some free time and continued work above the model and have carried out as a first approximation the express - analysis of resistance. Calculation of resistance was made with the help of one of methods for planing hulls.

water density = 1.025 t / cub.m.

Displacement 10.42 t
Length OA 50.000 ft
WL Length 49.180 ft
WL Beam 11.499 ft
Draft at LCF 2.004 ft
Prismatic coeff. 0.725
Block coeff. 0.316
Midship area coeff. 0.602
Waterplane area coeff. 0.722
LCB to amidship 2.471 aft ft
tpi 0.988 t / in

Dim.

 

Resistance compare.

Dim.

 

No need to apologise for your enthusiasm Dim!
It's difficult to make out the differences between the 3 hullshapes represented in your resisitance graph. It appears that No.3 has flatter aft sections and bow sections which are more cut away - as mmd suggested. Is that all? There's quite a difference in the resistance for No. 2 as opposed to the other 2....

Incidentally, what did you use for the resistance calcs?

 

You are completely right, Will.

In this small work I tried to simulate the hull similar in the sizes "Andreyale 15m". Therefore these models should be considered not as three different models, but as development of one hull.
Basic difference exists between the first and second hull's forms (in the bow frames form).

( Many thanks mmd for his posts.)

But at me it has turned out small a draught of the hull on the second model. Therefore the curve of resistance for this model has passed little bit below. I managed to return to "normal a draught" on the third model. I have gone little bit further in development of the exterrier on this ( third ) model. For calculations of resistance I usually use methods to which me learned at university. This method is very close to method Lahtiharju.

Dim.

 

It's the last model with deck.

Dim

 

Well - now that's an impressive little presentation Dim!
How will the amount of 'rocker' that you have in the fairbody shape affect this boats performance? I was always under the impression that it is best to keep the fairbody and buttock lines as straight as possible. This will also make your waterlines more parallel - another desirable feature I believe.....

 

If I correctly have understood you, Will,
we speak about a bend of a keel's line downwards in a aft part of the hull. My opinion on for what it's made on "Andreyale 15m" and it's make on other boats. When the boat goes, there is a running trim. And the aft part of the boat's bottom starts to carry out a role anti-trim surface. Thus, the boat can receive an optimum running trim on the move, that will affect very favorably his going performance.
An other business - that up to elegance "Latitude 46" to me "as on foot up to the Moon" - as at us speak.

Dim.

 

I guess that at the likely speed at which the boat travels, the 'hook' in the bottom will have more positive benefit than negative. Though I would expect that it would make the potential bow steering problems that mmd referred to even worse....

Sorry, I can't understand what you are trying to say here Dim..... maybe that you think the Latitude 46 is very elegant? If so I most certainly agree. Though for me, I would need something a bit more practical - can't see two dalmatians, a 6 month old baby, diving and fishing gear all rattling around in something so elegant...

 

About humour.

Yes, Will,

I wanted to tell, that I think "Latitude 46" is more elegant, than my hull.

Dim.

 

Hi Willallison,

Andreyale 15 m is a semiplaning hull, her Froude number is only 0.64 at cruising speed. At semiplaning speeds boat will almost never jump out of the waves and therefore you need only make the hull "sharp" at the bow. Aft amidships just few degrees deadrise is necessary.

There is no point to use deep V hulls under Froude number 1, while deep V hulls need much more power and are akward to steer at lower speeds compared to semidisplacement hulls. Deep V surely makes jumping in the waves more tolerate at planing speeds (Froude number <1).

Dim: What a beaty...

I have designed something similar few years ago, she is only LOA 9 m

http://kotisivu.mtv3.fi/terho.halme/salmon.htm


Terho