Deadrise at transom

[Timicrinn]

I need some schooling on this term...deadrise at transom. I am confused with which is the better ride in rough seas versus which is not. Example...a boat I am looking at has a deadrise at transom of 6 degrees...does that mean I am looking at a flat bottom boat? What would be a good solid deadrise for cruising to Alaska and other inside passage explorations and provide the smoothest ride? I need to pick your collective brains...thank you very much.

 

[Nomad Willy]

That boat’s bottom angles up a bit as in it’s not flat. The bottom forms a V and the angle of the bottom to the horizontal is the deadrise. A flat bottomed boat has no deadrise or you could say it’s 0 degrees.

V bottom boats all have deadrise. And the characteristics of the boat running in/on the water is commonly known such that one knows a boat w lots of deadrise will be smooth riding and require more power than boats having less deadrise. How they turn or how stable (directionally or roll-wise) will be known by those that are knowledgeable also. So if you look at the deadrise in degrees much can be generally known about that boat.

 

[Comodave]

A boat with 6 degrees of deadrise isn’t going to be the best boat for rough water. A deep V boat generally is in the range of 20+ degrees of deadrise. They will handle better in rough water, but they won’t be as fast or as efficient as a lower deadrise. A flatter bottom will be more efficient and faster with a given power. It is all a trade off, you don’t get the best riding without giving up something. A higher deadrise will be more tender, rolly, at rest. One is not better than the other, just different. You need to decide which fits your needs/desires better. Good luck.

 

[DNT99611]

It depends some on what you are talking about. A 20 foot boat going 40 knots and a 50 foot boat going 8 knots are two different things. A long, heavy, slow boat is going to ride much better than a short fast one does.

 

[Comodave]

It depends some on what you are talking about. A 20 foot boat going 40 knots and a 50 foot boat going 8 knots are two different things. A long, heavy, slow boat is going to ride much better than a short fast one does.

Those are very different boats. Given similar size boats a low deadrise will ride rougher, faster and be more stable at rest. A high deadrise boat will ride better in waves, be somewhat slower and more tender at rest. You do sorta need to compare apples to apples not oranges.

 

[FF]

Deadrise is only one tiny portion of a boat design . The entire boat package must be created to work together towards a specific goal.


The best insurance is purchasing a boat designed to do what you hope to do from a NA that has designed that type of boat before with good results , and from a yard that frequently has built that style of boat multiple times.

 

[OldDan1943]

Boat out of the water. Draw a straight line at the transom, port and starboard and a straight line up and down at the chine.
I think the dead rise angle is that angle created between the hull and the lower straight line.
I could be wrong and if so, I am sorry and feel free to jump on me with an angry mob with burning torches.
If I weren't so lazy, I'd look it up and confirm it.

Okay, I looked it up. I think I am close, if not spot on.
https://southportboats.com/blog-post/deadrise-what-does-it-mean-and-what-does-it-do/

 

[psneeld]

Deadrise is only one tiny portion of a boat design . The entire boat package must be created to work together towards a specific goal.


The best insurance is purchasing a boat designed to do what you hope to do from a NA that has designed that type of boat before with good results , and from a yard that frequently has built that style of boat multiple times.

Deadrise carried aft on higher speed boats is to create softer landings between waves.


Deadrise on slow boats probably mimics a more rounded hull for rolling characteristics and more buoyancy without increasing wetted surface.

 

[rslifkin]

With planing hulls, a lower deadrise hull will have to slow down sooner / more in steep chop to avoid pounding. If it's got a sharp enough entry, you just slow down, trim down and let the pointy end do the work. A high deadrise hull can run faster, as the waves hitting further back won't cause pounding as easily.

When running in a beam sea or downwind, deadrise isn't a huge concern. And deadrise is far from the only design factor that matters in terms of ride quality. Look at an old Hatteras. Fairly low deadrise, but heavy and pointy up forward and known for riding well (just not fast).

 

[PPandE]

Deadrise is only one tiny portion of a boat design . The entire boat package must be created to work together towards a specific goal.

True dat. Deadrise has been studied at every possible angle, including negative numbers. The extreme of which would act something like a catamaran or tunnel hull at speed. Everything between the bow pulpit and the transom will have some effect on the ride. Talented designers make extensive use of the interaction between features such as chines, pad and deadrise to achieve the ride they are looking for. They must balance the trade offs between such things as draft, weight, speed, efficiency and cost in order to build a product that is marketable.

I would suggest that a useful approach might be to mention specific boats considered for sale and then tap the collective wisdom here to learn the handling characteristics of each final product. More than likely someone here has first hand knowledge of almost every popular cruising hull on the market. And the better you can define your cruising style the better you can choose those trade offs.

 

[Comodave]

Boat out of the water. Draw a straight line at the transom, port and starboard and a straight line up and down at the chine.
I think the dead rise angle is that angle created between the hull and the lower straight line.
I could be wrong and if so, I am sorry and feel free to jump on me with an angry mob with burning torches.
If I weren't so lazy, I'd look it up and confirm it.

Okay, I looked it up. I think I am close, if not spot on.
https://southportboats.com/blog-post/deadrise-what-does-it-mean-and-what-does-it-do/

Ok, not sure if we are trying to say the same thing or not. If you were to set the boat level side to side on the ground. If you would measure the angle between the ground and the bottom of the boat at the transom, that is deadrise. Does that make sense?

 

[psneeld]

......

 

[Comodave]

......

Exactly, much easier to say with a diagram than words.

 

[OldDan1943]

Exactly, much easier to say with a diagram than words.

+1

 

[Willin]

Trawlers don’t go fast so are designed with a fine entry forward ( some more than others) and very little deadrise aft. My GB has a wet ride due the fine entry with little flair. Flatish deadrise aft also makes for a more stable boat at low speeds

 

[Comodave]

Grand Banks tend to be a wet boat since they don’t have much if any flare forward.

 

[AndreHuizing]

A purist would say that deadrise is not a relevant topic on a Trawler forum as it is only relevant on (semi)planing hulls. In the 60s the introduction of Deep-V hulls was kind of a revolution in offhore (rough water) racing. When introducing this hull form in pleasure craft the design was modified (hence: Modified deep V). Main modifications were to decrease the angle of deadrise going aft and introduction of chines and pads in several form. A lot of this had to do with gaining stability (a pure deep V is quite unstable).
Displacement hulls are a totally different beast with several forms including round and S-bilged. Also here designers introduced modifications to gain stability and efficiency. E.g. look at a Nordhavn or Selene and you'll see that there is hardly any deadrise (nearly flat bottomed) on the stern while fully rounded more up front. However the goal of all this is very different from deadrise on hulls that are supposed to take lots of hp and ride on their bow wave to get past their theoratical hull speed (i.e. planing).