Deadrise, How important?

[Flex]

Hi all. Im feverishly researching all sorts of things in anticipation of buying myself a larger plate boat.

I've got a few options on the list new and second hand, but a decision thats killing me is hull design.

Im looking at a riptide with a 17 degree deadrise and a new fisher/bluewater with 21/23 degree.

How much of a difference in performance and fuel economy would these 2 hulls be? (all boats being around 6m). Would the difference be significant? or hardly noticeable? (disregard stability at rest).

[PinHead]

performane..the 23 should be better...deadrise is measured at the transom..the entry lines also have some effect on the handling of the boat.
fuel economy..I doubt you can compare boats in regards to fuel economy as weights, laods carried etc etc ..so many variable.
does actual fuel economy really enter into the choice of boat or is it more for the range required? If the latter, just get a bigger fuel tank fitted.

[BM]

Deeper the deadrise the better the ride. Although 2 boats of equal weight but differing deadrise will not necessarily mean the deeper vee hull will ride that much better.

Deep vee and extra weight will be a better riding hull.

23 degree deadrise would really want a hull weight around 1000kg. What do the boats you are considering weigh?

Cheers

[Flex]

Its more range. I am eventually planning to be able to the boat up past cooktown or 3-4 day extended trips on my days off as I do shift work. So having the ability to travel 200-300km round trip is needed+a bit extra. but performance in rough conditions would be paramount, as carrying more fuel wouldn't be that difficult and Im assuming boat weights are identical. I wouldn't imagine fuel consumption would be that significant between a 17 or 23 degree..but Im no expert.

The boats im considering are a 6m riptide loaded would weigh around 1 tonne.

Or a fisher/bluewater 6metery which would weight roughly the same.

[BM]

whats the bare hull weight though (without engine)

[nautilus]

General rules of thumb:

Deeper the deadrise (DTD), softer the ride
DTD, the less stable the boat will be at rest
DTD, the worse the fuel economy

If the deep v has wide chines, it should be ok at rest.

If you are considering travelling large distnaces and it is a plate boat, I suggest having an extra fuel tank built in, or carry the fuel. The differnence in fuel economy is unlikely to be huge, but a softer ride over long distances would win hands down for me every time.

I had a look at both websites and I think I would be leaning towards the Fisher.

Just my 2 cents

[Catlover]

More deadrise doesn't mean better ride in all conditions when it gets really rough some boats with too much deadrise flounder between planing and falling off the plane and also tend to broche in a following sea if you want the best of both worlds buy a cat

[Flex]

whats the bare hull weight though (without engine)

Bare Hull weight dry is 900kg,

[BM]

Should go well then. 200kg for the engine, another 200 or more for fuel and gear. So 1300kg+ and a 23 degree vee would be a good riding hull.

[Stuart]

Flex

Sites like this are great to get information from, but when it comes to this sort of question you rely should be consulting with a naval architect. You have asked a question that will get you a different answer for every post as can be seen here. When I was looking at plate alloy hulls I didn’t even bother asking questions here or on any site for that matter because everyone has different opinions and ideas on what works and what doesn’t. I consulted with a naval architect and got all the answers I was seeking from the trained experts, hence I ended up with exactly what I needed in my boat. Dead rise is just as important as the forefoot of the boat “entry point”. I think way too much emphasis is placed on the dead rise of a hull these days and the forefoot is never even mentioned, yet it’s what pierces the waves and contributes to the overall ride and feel of the boat. By the way, chines do not add to stability when at rest. Beam equals stability how ever chines do add to some of the boats directional stability when underway. Please contact a naval architect flex, you wont regret it.

Stu

[chop duster]

Flex,
First off - what size boat are we looking at here? 6.5m> or <
You say you are looking to venture up past cooktown. Where are you leaving from?
Going North it's more than likely gonna be a following sea. All is good with little deadrise. You spend a couple of days fishing and piss-farting about in partially protected waters. All is good with little deadrise (probably better than a large deadrise).
Then...
Wind starts to blow up. Me starts to think it might be time to go home. Point the bow to a Southern heading (head sea). This is when you are going to wish you bought that boat with that little bit of extra deadrise!
Might not be that much of a help although may get you thinking of your priorities.

[Flex]

Hit the nail on the head there Chop Duster I'd be leaving from cooktown heading north, buggerising around in protected waters,Islands etc and coming home into the famous north QLD trade winds. So doing the trip safely is the biggest concern(I'll be doing it with another boat more often than not)

Plus i'd more often than not be heading offshore from Yeppoon when i cant get up north, whitsundays etc.

[chop duster]

mate if a head sea is going to be part of the norm. I would be looking for some deadrise for sure. 20 and up.
My pick would not be plate ali, but you have prob got your reasons.
best of luck! and may the fish be a bitting

[Wahoo]

HI Flex

talk to your boat builder mate, just tell him what you want/need and they will get it right, its there job, ( boat building) , i know the builders you have posted get feedback on there hulls

best of luck

Daz

[Greg P]

There is a lot more to the hull design than the 23 deadrise on the Fisher. I looked at all those brands and more before I pulled the trigger - in the end it was a no-brainer for what I wanted to do in the boat and where I wanted to do it.

[johnny roger]

Excuse my ignorance guys, but...what exactly is a 'deadrise?' and what does it do?:-/

[Blackened]

G'day

Deadrise is the angle of the hull in relation of the keel to the horizontal.... how much V is at the back of the boat pretty much.

More V, better it cuts but mnore horsepower required, less V, less horsepower but more it pounds.... but there's ALOT more to it than that

Dave

[johnny roger]

thanks blackened, i think i get the jist of it.

[cbs]

As a qualified practising naval architect, I have designed boats with both shallow and deep V configurations. As with many things, it is really all about compromise and achieving your targets and goals. Deadrise at transom is not necessarily the be all and end all in determining whether a boat will perform well. Sure the general rules of thumb above hold true but they aren't the whole story.

A little while ago, I have designed a ~7m boat with a 12 deg deadrise aft in order to promote easy planing and low power consumption and at the moment am working on a slightly larger design that starts with 23 degrees at the transom. This boat will be destined to go blasting across the bay at more than 40 knots while the first will cruise at around the 20 knot mark. Both boats are similar in size, and although very different in configuration and purpose I made effort to achieve as deep a forefoot entry as practical. In both cases, the deadrise at the 25% wl mark (ie the forefoot) was within 2 degrees of each other which in my opinion is important in reducing slamming in short chop.

You might also ask, well if a shallow deadrise hull require a little less power to achieve the same speed, then surely if your goal was to have an extremely fast boat in flat water then this would be the way to go? Sound logical right? Unfortunately this is not necessarily so, as there are some dynamic stability issues with flat bottom boats at very high speed that a deep vee (~26-28 deg) will overcome.

As I said, everything is a compromise to achieve a good result, and there is much more to it than just deadrise angle. I feel that deadrise angle is something that marketing types have taken ahold of and run with as the basic concept is easy to understand and indeed physically measure on any given hull in order to make some comparison.

Once again, just touching the surface...


cbs

[BM]

I agree cbs,

Although the original question related only to deadrise.

As you have mentioned there is a lot more to it. As you without doubt know, length to beam ratio is also quite important in terms of stability.

International Marine found this out when they stretched a Bertram 28 and made a 31 footer many years ago. A good friend of mine was involved and he said the additional length without the beam to support it made a pig of a boat. Only a very small number were made and they were then dropped.

Johnny Roger, just in case you are still confused consider this. If you were looking at the stern of your boat and with a straight edge you drew a line from one chine across to the other (chine is where the side of the hull meets the bottom) and then at the centre of this line you drew another line at right angles so it intersects the very dead centre of the bottom of the hull. This angle you have formed is the deadrise angle.

Cheers

[BM]

Here you go Johnny Roger,

Knocked up a quick pic for you to hopefully make it easy to understand.

Cheers

[Catlover]

Some good info there cbs. In your opinion which hull shape would you choose, the moderate 17-18 degree or the 21-23 degree remembering he is looking at a 6.0m trailable boat and the area he is using the boat. If it was my choice I would go for a test run in a couple of different boats and choose the boat that can do most things well. A more all-round boat.

[Flex]

Good post CBS. So forefoot and beam play just as an important role, aswell as weight distribution.

I see alot of boats with very pointy sharp forefoot, others with a very blunt nose, reverse chines etc.

How does each aspect of a boat affect performance? In terms of running into,following a sea, side on,chop etc.

I know from my tinny in a small chop i put more weight at the front and it slaps less, when im in a big scary following sea if I load the boat up at the back the ride feels totally safe. but if I have people sit at the front it feels like its gunna broach. and running into a moderate swell...well i still dunno how to take it..slow,medium or yet to try fast to skip along the top..

[Alchemy]

Hi Flex,

I have none of the tech knowledge such as that displayed by CBS & BM, but do own a 6.6m Fisher with the 23 degree deadrise.

I spent last night anchored up behind Cape Bowling Green, thinking I'd get up early and beat the forecast change home. Well it beat me, and I awoke to a stiff 25-30kn SW at 430am.

At 7am I pulled the pick and headed for home, with a 1.5 - 2m beam on sea for most of run. This was ok as I was able to cruise in the troughs most of the run to Cape Cleveland. I had no problem planning at 15kn at around 3000rpm on my 200 Suzi 4stroke. When I rounded Cape Cleveland and headed straight into the sea I maintained this pace, but pulled the bow down a bit with tabs. Wasn't the fastest trip I've had, but we did it easily and without getting belted by every second wave.

Like I said, nothing technical there just a first hand recount of a run in an ordinary sea in a hull you're looking at.

Regards,
Dave.

[Stuart]

Flex

As I have already stated in one of my previous posts on this thread, get professional advice from a naval architect. I have been down this path in conjunction with a naval architect and the information he or she can give you is invaluable and some thing you will never forget. I spent 3 months with my naval architect and the amount of information I have learnt is phenomenal. What you learn can be taken into any boat purchase from that point on. You wouldn’t purchase a house based on what a mate has told you, you get professional help and advice from the start.;)

Stu

[disorderly]

Hi Flex,

I have none of the tech knowledge such as that displayed by CBS & BM, but do own a 6.6m Fisher with the 23 degree deadrise.

I spent last night anchored up behind Cape Bowling Green, thinking I'd get up early and beat the forecast change home. Well it beat me, and I awoke to a stiff 25-30kn SW at 430am.

At 7am I pulled the pick and headed for home, with a 1.5 - 2m beam on sea for most of run. This was ok as I was able to cruise in the troughs most of the run to Cape Cleveland. I had no problem planning at 15kn at around 3000rpm on my 200 Suzi 4stroke. When I rounded Cape Cleveland and headed straight into the sea I maintained this pace, but pulled the bow down a bit with tabs. Wasn't the fastest trip I've had, but we did it easily and without getting belted by every second wave.

Like I said, nothing technical there just a first hand recount of a run in an ordinary sea in a hull you're looking at.

Regards,
Dave.

On the flip side of this coin I also got hit by this same SW front only about an hour later....However the deadrise on my stacer is minimal and it took me 4hrs and 15 minutes to cover 60 km's directly into it...
The ride in Dave's Fisher was probably a bit more comfortablehttp://www.ausfish.com.au/vforum/../yabbfiles/Templates/Forum/default/wink.gifhttp://www.ausfish.com.au/vforum/../yabbfiles/Templates/Forum/default/grin.gif.
My flatter bottomed boat was getting airborn rather than slicing through waves at only 5-10 knots....the human body was not designed to cop such a beating.http://www.ausfish.com.au/vforum/../yabbfiles/Templates/Forum/default/sad.gif
It caught me by surprise as well Dave.http://www.ausfish.com.au/vforum/../yabbfiles/Templates/Forum/default/shocked.gif

Scott

[Alchemy]

On the flip side of this coin I also got hit by this same SW front only about an hour later....However the deadrise on my stacer is minimal and it took me 4hrs and 15 minutes to cover 60 km's directly into it...
The ride in Dave's Fisher was probably a bit more comfortablehttp://www.ausfish.com.au/vforum/../yabbfiles/Templates/Forum/default/wink.gifhttp://www.ausfish.com.au/vforum/../yabbfiles/Templates/Forum/default/grin.gif.
My flatter bottomed boat was getting airborn rather than slicing through waves at only 5-10 knots....the human body was not designed to cop such a beating.http://www.ausfish.com.au/vforum/../yabbfiles/Templates/Forum/default/sad.gif
It caught me by surprise as well Dave.http://www.ausfish.com.au/vforum/../yabbfiles/Templates/Forum/default/shocked.gif

Scott

Hi Scott,

I had a 6m Quinny and that is why I sold it! I would've hated to have been coming back today in my old boat. Though in saying that, if you take your time as you did, then its bearable.

Regards,
Dave.

[arvor]

As a qualified practising naval architect, I have designed boats with both shallow and deep V configurations. As with many things, it is really all about compromise and achieving your targets and goals. Deadrise at transom is not necessarily the be all and end all in determining whether a boat will perform well. Sure the general rules of thumb above hold true but they aren't the whole story.

A little while ago, I have designed a ~7m boat with a 12 deg deadrise aft in order to promote easy planing and low power consumption and at the moment am working on a slightly larger design that starts with 23 degrees at the transom. This boat will be destined to go blasting across the bay at more than 40 knots while the first will cruise at around the 20 knot mark. Both boats are similar in size, and although very different in configuration and purpose I made effort to achieve as deep a forefoot entry as practical. In both cases, the deadrise at the 25% wl mark (ie the forefoot) was within 2 degrees of each other which in my opinion is important in reducing slamming in short chop.

You might also ask, well if a shallow deadrise hull require a little less power to achieve the same speed, then surely if your goal was to have an extremely fast boat in flat water then this would be the way to go? Sound logical right? Unfortunately this is not necessarily so, as there are some dynamic stability issues with flat bottom boats at very high speed that a deep vee (~26-28 deg) will overcome.

As I said, everything is a compromise to achieve a good result, and there is much more to it than just deadrise angle. I feel that deadrise angle is something that marketing types have taken ahold of and run with as the basic concept is easy to understand and indeed physically measure on any given hull in order to make some comparison.

Once again, just touching the surface...


cbsthis is why why we should look outside the circle.as for ours an arvor 20 the fine entry and gullwing aft works for us and as c b s said its more than just deadrise.its an art .

[cbs]

I agree cbs,

Although the original question related only to deadrise.

As you have mentioned there is a lot more to it. As you without doubt know, length to beam ratio is also quite important in terms of stability.

International Marine found this out when they stretched a Bertram 28 and made a 31 footer many years ago. A good friend of mine was involved and he said the additional length without the beam to support it made a pig of a boat. Only a very small number were made and they were then dropped.


Cheers

BM,

Length/Beam ratio not so much, but increasing either length or beam without increasing displacement too much will give an increase in initial stability. Beam giving more bang for buck obviously.

Whilst I do not know the exact particulars of the Bertram 28/31 extension, there are plenty of boats that have been extended/shortened to make a new model with good result. I am thinking that there is more to this than length /beam ratios alone to turn the newer boat into a "pig". There are plenty of long skinny boats out there that work quite well.

[cbs]

Good post CBS. So forefoot and beam play just as an important role, aswell as weight distribution.

I see alot of boats with very pointy sharp forefoot, others with a very blunt nose, reverse chines etc.

How does each aspect of a boat affect performance? In terms of running into,following a sea, side on,chop etc.

I know from my tinny in a small chop i put more weight at the front and it slaps less, when im in a big scary following sea if I load the boat up at the back the ride feels totally safe. but if I have people sit at the front it feels like its gunna broach. and running into a moderate swell...well i still dunno how to take it..slow,medium or yet to try fast to skip along the top..

I'm sorry mate but thats just too general a question to give a good answer in a short time. I can say that a sharp entry is desirable, and good chines are also a preference to me, since they not only provide lift, they also help keep me dry...

I actually wrote my thesis 15 or so years ago on the effect of hull form parameters on resistance would you believe. What I did, was take a truck load of towing tank data (smooth water) and try to correlate to various hull parameters in a non-dimensional manner (in order to compare different size boats to each other). I just dug out the section on deadrise angle and it shows a defininte increase in resistance in the 15-20 degree deadrise region, however there were data points for vessels with deadrise angles higher and lower to this mid range are that did not conform to the "pattern".... As I said before, its all a compromise.

FYI, the boats looked at were commercial vessel types of planing hull form. (Hence they were tested in the first place - expensive job to do)



Common sense would suggest you choose a hull that will do well in conditions that you are to spend most of your time on the water experiencing. If it blows up, and slowing down is the option, then so be it.