Hull Design Questions

[Mark Mills]

I happen to find myself in the middle of a powerboat design for the first time ever. I have been trying to learn as much as I can as fast as I can, with the aid of Doug F, some good stuff from Levi and DuCane, as many papers I can find from offices like Don Blount, and some very good material from sites such as Tad Roberts I am kind of getting a feel for it.

So the first question, of many I bet, is how relevant is prismatic for planing hulls? Over a volume froude number of 3 does prismatic have any relevance? With a number of hull designs aimed at gentler accelerations in rough seas (like the "VSR" Venom Ribs http://www.venomribs.com, or even the very fine bowed Sonny Levi designs) the Cp must be lower than for a fuller bowed design. Does it make any difference what the front half looks like as long as the back half is conventional and working properly? Does a higher prismatic help getting over the hump quicker, or does it make the hump larger than it needs to be?

[Alik]

I do not use prismatic for planning craft hull shape design. Prismatic uses terms of volume, but volume is generally no meaning for planning mode...

Better guideline is bottom area/deadrise on several stations. If use 10 stations: Stn.1 - responds for spray making and seakeeping performance, Stn.3 - for slamming loads; Stn. 5 - planning performance; Stn.10 (Tr.) - directional stability.

[Mark Mills]

Thanks Alik, that makes sense. Any thoughts on whether narrower deeper sections forwards cause difficulties with steering in following seas? In sailing we would call it the 'bow rudder' effect... Or whether with less reserve bouyancy higher up there is a problem with stuffing the nose in?

Mark

[Alik]

Yes this is right, but on planning power boats the forefoot is never so deep as on sailboats with destroyer bow. For powerboat the waterline in the bow should not be concave.

[Mark Mills]

You have gotten directly to the type of hull shape I have mind - why not have hollow waterlines forward? They wont be immersed much when planing, and it seems to work fine on boats like the Venom Rib I mentioned above, and the Wally Tender design... I suppose you could imagine fairing in a pointy bit onto the front of conventional planing hull design, would it make much difference?

[tom28571]

Hollow forward waterlines offer an exponentially increasing buoyancy when running into waves. This creates slamming which can also give structural problems. A convex cross sectional shape forward offers a much easier entry into waves that is more comfortable and less strain on the hull structure.

[Alik]

We did sketches of torpedo boat style hull recently, very similar bow concept to what You show. Yes, should consider if concave area is immersed. I meant submerged bow/waterline shape in my previous post.

[Mark Mills]

Tom,

Thanks for further input, I really am trying to see what I am missing here.

I would have thought that was more an issue with sectional (vertical) volume distribution rather than longitudinal. Based on a curve of areas I bet the longer finer bow actually has a gentler rate of change of bouyancy than a more conventional shape, since it can be viewed as a regular bow with a fine cone stuck on the nose: it retains all the bouyancy the original hull had, and adds a bit more in a narrow, and therefore gentle rate of increase of volume, shape.

I am not sure I think this type of hull is any better, I am sure it is just a styling thing, looks a lot better for my .02c.

[Alik]

This is our 'torpedo boat' body plan.

[Alik]

And 3D sketch...

[Mark Mills]

Alik - thats exactly the sort of thing I am playing with, I see you have put effort into making sure those forward waterlines havent gotten hollow. I really appreciate your input, thanks for sharing, it looks great.

Mark

[Tad]

Mark,

As you know a great deal of the design decisions you make for any boat are tied to styling, and the bow shape of a powerboat is no different.

But...the more important factors are speed, loading, and operational sea state. The Wally and VSR (Adrian Thompson?) type of piercing shape works (I guess) for them with very light loading. But I wouldn't want to be caught in one crossing the Gulf of Georgia into a summer westerly with the tide against the wind. In those conditions all the (moderate sized) planning boats are down to high idle speed, rolling and pitching horribly. But the deep, fine, high-bowed older style boats can knife right into it and remain quite dry on deck. The Wally would (I believe) be trying to stick her nose through every second wave.

I don't think you can divorce bow from stern form in any boat, they play different roles but they still have to work together. I would like to see the transition area between bow and stern in Alik's hull be smoother and more gradual. The boat appears to get too blunt, too fast, and I would like the chine higher and narrower further aft. I believe the more gradual transition will make the boat more comfortable to ride in, but it will cut into interior volume.

There's always a trade off.

No, I don't worry about the CP of a planning hull, but it is very handy to set a target (usually .72) when you're at the preliminary sketch stage. In minutes you come up with how deep and wide the midsection is and from that the size of the transom. A simple reality check is always useful.

IMO Getting the LCB in the right place for the speed is the most important factor in creating a sucessful hull, but always within the styling boundaries you have set.

Tad

[Mark Mills]

Tad,

Thanks for pitching in, I really appreciated some of the info on your website when trying to get familiar with the tradeoffs at hand - in particular the form and function article, I guess nothing has changed in your target LCB's since then? By the way on a different note the Yellow Cedar example design hit a home run with me - I have long enjoyed looking at Atkins 'Motor Boats' and that seemed to be a beautiful revival, it reminded me of your PBB Passagemaker article.

I certainly imagine the limiting sea state for effective Wally tender use is less than some other designs, I guess the design brief for my project is as a 28' rib/tender replacement, not as a deep water fishing platform, so interior volume concerns and suitability for extreme conditions are a bit further down the list, I am just hoping to produce a 'euromodern' styled, decent riding Deep V hull that has lower accelerations in moderate chop (Solent, San Francisco Bay, Puget Sound) than some of the kidney pulping Ribs that are popular. These days if a slightly narrower than average beam helps reduce the powering requirements all to the good.

I know if someone came from left field with an 'odd' racing sailboat design I would view it with some scepticism, so I am trying to work out the obvious flaws in my plan early, with all of your help!

Mark

[hmattos]

We build RIBs in three different hull forms here at Explorer Marine, in sizes from 5 to 10 metres. The key issues are the bow forefoot shape and the bow deadrise angle, for rough weather bow entry. The planing conditions - speed, porpoising, chine walking, stability and power requirements are determined by the stern deadrise angle and the spray rail designs. The high speed turning and / or side slip is controlled by the sharpness and shape of the keel and the spray rails and strakes. We build at 28 degree 22 degree and 15 degree deadrises for a good choice to serve each customer requirements - see www.explorermarine.co.uk
Good luck
Hugh Mattos C.Eng.M.I.Mech.E.
www.explorermarine.co.uk

[janero13]

hi
some one have rib hull plan?
also if molds for sale
seknas