Variable Deadrise

[Fisherman]

I am looking at 25-28' alloy boats.

I've noticed some alloy boats have variable deadrise from bow to stern, while others, after the bow portion of the boat, maintain constant deadrise to the stern. The Wooldridge pilot house, for example, is one that appears to have constant deadrise most of the length of the boat. The Seawolfs and Fishrite Sea Storm both have variable deadrise much farther back from the bow it appears.

How do these differences in deadrise affect performance?

[buck3]

Not completely sure. But more dead rise equals more rocking at rest. Then dead rise affects down swell ride as well.

I am just guessing here but a variable dead rise will rock less at rest and have a more stable down hill ride. It gains this by sacrificing some upswell performance.

Again that just a guess so someone that knows what they're talking about will be here shortly.

[JETTYWOLF]

I'll take a "stab" at it...

Variable dead rise = mo' $$.
plain ole deadrise = mo' $$ you can put into other cool things.

C'C'mon

[kmorin]

One bottom is called a monohedron where the V is constant from the transom forward, and the up sweep of the chine (viewed from the side) and the taper of chine (viewed from the top) is all that brings the V to the bowstem.

The other bottom is called a 'warped' and attempts to give a mix of performance. The warped or variable sheet is twisted as it comes forward increasing the V as it lifts and tapers and is a bit more work at tack-up.

Performance wise- the monohedron will not slam if the boat is run at higher speed in a head sea as its has more V or deadrise father aft, so if the boat comes off a swell and gets launched its entry is 'sharper' or more gradual than the warped bottom where the entry is less of an angle in the after 2/3 the hull. One element is to add enough V at the transom of a warped boat so that it is not completely flat at the stern. This also helps minimize pounding because there is always more V than the stern as the bottom goes forward. But, unless the warped boat is kept flat or at a low pitch by the bow when its running, generally these hull forms will have more impact in a head sea at higher speeds. If the forefoot or sharpest part of the warped hull is kept as the main point of entry then they run as smoothly as the monohedron boats until you get at high angles of pitch.

The gain is fuel use, the warped bottom will use less fuel in the lower 2/3 of the speed ranges of both boats since there is less V surface, there is less water to 'move aside' and because of these facets of those hull they will plane 'sooner' in the power curve and overall speed compared to the deeper V or monohedron boat. Monohedrons are mainly for higher speeds where regularly the bow would pitch up or high seas where you will regularly want to maintain high(er) speeds in heavier weather than you'd expect to do in a shallower deadrise hull.

Generally monohedrons aren't the most effective bottom for boats that weigh less than three tons since they result in a very 'rolly' skiff. But a shallow V monohedron would offset that tendency to some degree.

A 28' footer with a warped bottom that begins at the transom as a 14-16 degree V and runs forward increasing in deadrise, compared to a monohedron bottom that is 22-25 degree all the way forward; is sort of a designer's preference as the hulls will perform very similarly. But a hull with 10 degrees at the transom warped forward may only end up with an entry of 35-40 degrees while the 22-25 degree monohedron ends up with 55 degrees. Then the sharper hull can be felt as 'ride' and the flatter hull will be seen as more fuel 'efficient'.

In a following sea the balance is between the forward 1/3's lift compared to the after 1/3's lift where the stern almost always 'beats' the bow. So that sea state the longer, deeper V's bottom shape tends to keep the hull on course more easily compared to the warped bottom which has to rely more on a keel or the outboard's skeg area or perhaps bottom rails to provide lateral area that resists the sideways forces at the stern.

cheers,
Kevin Morin

[Chaps]

And if you are shopping the builders in British Columbia tend more towards deeper V hull forms than the Washington builders

[speedboats]

Your decision to go either way may also depend on your operating conditions / locale. As we've tended to specialise in variable deadrise / warped hulls, the ability to generate more lift at the transom for load carrying while maintaining a reasonable amount of ride comfort plays big into the equation. Also for performance of higher speed vessels the ability to have the chine out of the water at 2/3 length when performing turns means it is less likely that you will 'over-generate' lift along the inside edge causing the boat to lean over (very uncomfortably) to the outboard side of the turn.

Another plus side of the warped hull is the ability to draw less water (less draft) once up on plane. This is particularly helpful when operating in areas along coastline, in rivers and over reefs. Coupled to a waterjet pump you can vastly decrease the amount water needed.

The lack of lift under the front 1/3 can be compensated for with lifting strakes, although there is a balance of being helpful and over-doing it. Over done and you might as well have a monohedron at lesser deadrise as the ride becomes hard when the strakes slam into the water in much the same way a very wide chine does.

The LCG then becomes a concern due to the way the hull generates lift as previously mentioned. Heavy items need to be moved aft (important during design) and the skipper needs to be aware of how he loads the vessel. While the vessel needn't be 'mollycoddled', awareness of the difference between the two design types is always good, afterall, you wouldn't drive your truck the same as your sports car.

[pjay9]

I sure am glad that I am in between AK and NZ...what a great explanation of the two hulls designs. Now let's throw a bit of a question here...If it was catamaran built for speed rather than displacement...what would be the choice of hull? Now if the Cat was built for semi-displacement, what would be done. And what about those cat with the hulls that are not equilateral?

Here in Seattle we have a very successful GRP hull builder called DELTA that produced for years a 43 and 50 version of their charter boat with a semi displc hull...it was so good at being a charter boat there are many still plying the waters of Lower 48 , AK and CND.

What made it so good at crossing the bars and being a stable platform was hull shape the 50 was faster than the 43 with the same power...all to do with waterline length (I recon!)..it would be fun to see some discussion on this too. I don't see the hull on the Delta site...but I saw a fairly new one last summer made into a grand coastal cruiser. Capt PJ

[Katoh]

Gentleman
Very interesting thread and one thats very close to me, I have tried to search for the differences and found certain opinions but nothing to definite.
The boat I am rebuilding has constant deadrise 17.5deg at transom 18deg at midship an only changes very close to the bow, now were are talking about a 20' planning hull. with an I/O if that makes any difference. The most that I read on the subject is on the Boston Whaler site forum http://continuouswave.com/ubb/Forum4/HTML/004764.html interesting reading if you have some time to spare.

Kevin, your explanation is as always very thorough, but confused me a little.

Now Sorry I can not give any an practical experience into this post as my vessel is still shore bound. From my understanding Variable deadrise came about to reduce the impact on the boat at speed, while trying to achieve two things, one stability, the other fuel economy.
You can get a very fast smooth riding boat by introducing a lot of deadrise, but it will rock and roll, very unstable and will need more hp to plane, then the opposite side of the coin go flat bottom very stable at rest, needs lower HP to plane but kidney belts are essential!
So the concept of sharp entry at speed combined with low deadrise at stern give stability and fuel economy. So they Say.

Again I am no expert but would love to learn, I would like to find out out my constant (say18deg) deadrise hull competes with these new hulls that start at transom 18deg and at midship might be 25-30 deg and rising from there. My thoughts I might not keep up to variable dead rise hull when its rough, but I will have a much more stable fishing platform than one in any condition. If I'm wrong Ill love to know!

[speedboats]

katoh

That's basically it in a nutshell. With the transom deadrise being the same, the warped hull will be a little more tender at rest but will be more comfortable once the weather is less than perfect. The finer entry at the bow will cut through the waves without 'slaming' atop them.

The Surtees and BarCrusher 6.1m boats have a similar length and deadrise to the warped hull you are describing. They both overcome the tenderness at rest with a floodable ballast chamber along the length of the hull. More volume of water is held forrard, but this helps to give the boat more mass along the keel line at rest helping to reduce tenderness.

As you pointed out, the vessel will still be less tender than a vessel of comparable length and weight than a vessel with a monohedron deadrise at the 25 degrees these have at amidships.

[Katoh]

Speedboats
Thanks for the confirmation, Its going to be really interesting to see how this rebuild is going to perform, with its marginal deadrise of 18 and constant at that might end up being a real banger. The funny thing is when compared to say a BC and you mentioned the 6.1, its nearly 1/2 meter longer and over 200mm wider in the beam even though its only a 6m, but that would make a good topic for another thread, Th e Rubbish Written on boats!

[speedboats]

The funny thing is when compared to say a BC and you mentioned the 6.1, its nearly 1/2 meter longer and over 200mm wider in the beam even though its only a 6m, but that would make a good topic for another thread, Th e Rubbish Written on boats!

I've bought that up with the guys at Surtees Boats (whom BC copied). They measure their boats from the fore-most point to the aft-most point, INCLUDING the bow sprit and motor bracket. This means that their 4.85m boat (formally called a 4.7m) grow to be 5.1m once a longer bow sprit is welded on and sold to the Pacific Islands as such. I think it is mis-leading, but it is an easy way to get potential customers to think they are getting a bigger boat for the same money as the smaller boat of their competitors...