16 Degree Deadrise

Hi,
I was looking for some input regarding a boat building project. I am purchasing plans to build a 27 foot rigid inflatable which will be powered by twin 150hp outboards. The construction is stitch and glue and the total weight of the boat will be approximately 1,950 lbs.

I advised them that I wanted to go open ocean outside the Golden Gate Bridge and my first view of the design shows a deadrise of 16 degrees.

My question is, how does this deadrise affect boat handling in open ocean conditions, I'm not looking for top speed but, control of the boat.

Thanks in advance for your help.

Mikey483

 

Sweet Sixteen

To mikey483:

- How much D. RISE.....? - Or as I prefer: DIHEDRAL.
{ BOTH sharks and pigeons vary, dihedral, to improve CONTROL.}

This is one of those eternal debates.
People go from 24 degrees of dihedral all the way down to 12 !
" Sweet 16 " - is often the choice of designers - between being
able to smash through waves and reasonable economy.

A 80 mph + " Go Fast " monohull will move towards 24.
Whereas, a quite cruiser - will lean towards 12.

Not to confuse:
However.....
The degree of dihedral, is not the only number that effects
control, seaworthyness and fuel expenditures.
- When you COMBINE other important factors: - WITH the
dihedral angle. All sorts of other varibles come into play.

With Sq. Root & Cube Root Multipliers showing up in several
areas. With interesting results.
i.e.
A. POUNDING - gets worse as the BEAM gets wider.
B. POUNDING - gets worse as the DIHEDRAL gets flatter.
C. PLANING - gets better as the BEAM gets wider.
D. PLANING - gets better as the DIHEDRAL gets flatter.

Lets say you have a 8 ft wide cabin cruiser. Very economical
at 12 degrees.
- But going offshore, especialy in the San Francisco area.....

We have built a seaworthy, 25ft offshore cat that used only
14 degrees dihedral....?!
- With 150 hp per side - we were clocked at over 40 mph.
- With a full load.
I've also driven a 25ft light weight cat, with a SINGLE stock,
300 hp Chevy, past 70 mph.
Both were 8 ft wide. The same beam as the cruiser.
How did we get away with it ?!

Because; the EFFECTIVE beam width - of each one of the hulls,
of the cats - is not more than 2 ft in width.
- One forth of 8 ft!
However, the impact force is significantly LESS than even
one forth. Due to all those wonderfull Sqs. and Cubes !

This allows us to sneak in a flatter dihedral - with out paying
the high price - that our fat little cruiser does.
That WILL slam much more, than our cats will.

We also get the added bonus of good fuel economy and/or
higher speed potential.

Your typ. RIB falls some where between these two camps.
As hydrodynamicaly, it is ( dare I say it ? ) ALMOST a tri hull.
Or cathedral hull.

{What saves the RIB from slamming as much as some solid bottom
" W " hulls - is that the outer " wings " of the " W " deflect readily
- when impacting head seas.}

The solid, centre hull of a RIB will be somewhere around
- only one third, to one half - the beam of said, cabin cruiser.
Resulting in less shock being experienced than with the cruiser.
- But, possibly, not as forgiving as our cats.
- Resulting in a compromise of 16 degrees.

Is logical....no..?

So now you can see, possibly, how the reasoning of the designer
of your intended boat went.
- And why; designer's oil lamps stay lite, long into the night.....
- Reconciling, all the multiple, conflicting varibles involved.

Cheers !

 

Wow - TriStar - that's quite some reply!
Personally, I would have thought that given the narrow beam of the rigid hull, you could comfortably incorporate more deadrise to improve rough water handling. This will be at some expense to lower speed handling, but given that RIB's tend to be driven quite quickly regardless of conditions, I would have thought that 20+ degrees of deadrise would be a good starting point.
Then again, as Tri Star suggests, I wasn't up all night reconciling, calculating and burning the midnight oil!

 

Thanks Tri-Star and Willalison,

I appreciate your input and to provide a little bit more info, the inside beam is 1600mm, the radius of the tube is 235mm.
Thanks,
Mikey483

 

AMOUNT of DEGREES ?

To:
Willallison & mikey483.

I am very careful to "weigh" (pun intended) my words VERY
carefully in these tech. forums. As there are many people reading
these posts with multiple " degrees " {pun intended} who only
contribute - it often seems - to point out mistakes.
Although, this can be irksome, at times. In actuality - we must
welcome their vigilance. As it keeps things honest.
As, particularly with small boats, being inherently less stable than
larger vessels - there is considerably more risk, to life and limb.

The point I was making, is that:
- As the BEAM is reduced, of a hull, or hulls in combination
- one can reasonably expect, to be able to FLATTEN the planing
surface. While still MAINTAINING the same COMFORT and CONTROL.
(Because of all those Sq. and Cube root multlpliers.....)

i.e.
If you reduce a given BEAM by half, the actual impact force, is
reduced by at least - four times ! Or more.
According, to this - I gently suggest, Willallison, you might be
traveling in the wrong direction...
This is an important consideration. Because the deeper the V is:
- The more hp is required...to sustain a given speed.
Also, because of said, buddies - Cube and Sq. - even a couple
of degrees can have a huge effect.
{Which is why, as I stated at the begining, this is such an
ETERNAL debate.}

We havn't EVEN gotten into the WEIGHT factor much.
Where brother Cube starts to mess thiings up. A lot...

Consider this conumdrum:
- If the ability of a vessel to plane; goes up by the Sq. of the
wetted surface:
- And the SKIN surface - AND the WEIGHT of said vessel;
goes up, by the Cube of the length:
- At what point do these two factors meet and diminishing
returns set in ?

i.e.
A jet ski skips onto plane...whereas, a 100' yacht struggles
to achieve this. - And usually doesn't bother to try!

Please NOTE:
Even with the measurements, mikey483, you have submitted
- I will be exteemly reluctant to give you any firm numbers.

For some clear reasons:
(1.) You are PAYING a designer for plans and it is neither wise,
or ethical - to second guess - why they arrived at the design
decisions they did.
All I intended - was to indicate - just a FEW of the reasons
- WHY they have burnt a lot of mid - night oil, arriving at
said decistions.
(2.) Even with the lines in front of me....and having a hard time,
remembering ANY vessel launched - that came in, UNDER her
guesstimated weight. {'Cept one's, I have built.....Of course!}
- And given all, that's already been stated. How deep a " V " ?
- Is a quandry I'd be very cautous to decide on.

This might sound like a bit of a cop - out....
However, I will make one VERY FIRM declaration.

There is often - weigh, weigh - too much time spent at the
computer, " weighing" data..... I includ myself in this.
I'm a great beliver in an IMPERICAL approach to design.
Getting out into the field and finding out first hand.
The answers to one's concerns.

So, as follows:
( 1.) Go for a ride in an identical boat, to what you are
considering. In the intended conditions.
( 2.) Test out similer vessels, with dihedrals of 16,18 and on
- up to 24 degrees.

I GUARANTEE. After you come back from these test rides....
Wet and cold - but cheerfully charged up.....
You will have far,far - fewer questions.
Also, you will now be THE expert.... and we will be the one's
buying YOU drinks!

Cheers !

 

Tri Star - I wasn't trying to be irksome, nor pointing out any mistake's on your part. I don't claim to have a degree (I have graduated from the Westlawn Inst. of Marine Technology, and as such DO hold a diploma of yacht design however). If you consider yourself better qualified... jolly for you.

Having said that, the points you make are all valid. A deep-v will require more power ( a point that I already eluded to). The narrower beam will lessen the shock loads (though with RIB's this can sometimes be offset by the fact that the tubes come into regular contact with the water, even though their flexibility reduces the impact somewhat).
The designer no doubt took all of these things - and many others - into account in designing the boat. It very much depends on the intended service. I wasn't there when he was given his design brief, any more than you were. BUT - in my opinion, given the light weight that is typically inherent in rib's, greater deadrise can be carried without major detriment to performance. If it were my boat and I was intending to charge around at 40 knots in sloppy conditions I would incorporate greater deadrise. End of story.

 

No Worries - No offence

To Willallison,

Please don't take offence - I was not referring to you personaly.
I have been taken to task, on other threads by various people, on
some minor points in the past.
Also, I was just trying to inject a little humour... with my bad puns.
Forgive me.

In terms of the question asked:
I personaly, know very well - how deep a " V ", I would favour.
However, I see these forums, as eductional first and foremost.
{And have learned a lot myself, from them.}
So I tend, to avoid giving out, pat and simple answers.

Also, as a designer as well, I am loath to comment on
other's work. Especialy, from second hand information.....
I agree, also, neither of us, know what is going on with
whoever, drew the lines for the boat and his client.

I'm also serious - about my suggestion - that real life testing
can be very usefull. That's how I proceed. That's why I am
very clear, myself, as to what dihedral I prefer.

By the way guys, it needs to be said:
- The dihedral at ENTRY, is also a concern - and is no small matter.
Even if every brocure for RIBs, around the world - all agree on the
same, identical: - XX degrees of " V "
- at the back of the boat....
I will be suprised if you get the same consensus for the " V "
at the BOW.........Or..how WET a boat, it will be.

It's hard to shop, entirely, by brocure....

Now it's time to take my own advice - turn off the computer
- and get my gear ready for tomorrows sea trials.

Cheers !

 

Now we're singing from the same song sheet!

Mikey - I suggest you go back to the designer and ask him/her why they chose 16 degrees deadrise. As both TriStar and I have said - there's more to it than simply the deadrise to consider....
BTW, are these stock plans, or ones that you've had commissioned especially for yourself?

 

For a RIB, the deadrise is very important. The vertical position of collar too.

Zodiac Cherokee 22 degres deadrise but low collar=hard pounding
New Zodiac Open 550 about same deadrise but collar are higher=soft ride.

Somes builders offer more than one deadrise moderate V for little ripple or deep-V for choppy water.

Does your 16 degres is a constant deadrise or variable ? What is the midship deadrise ?

Do you have the transom design to know collar position ?

At the first look, 16 degres is a very low deadrise for choppy water...

 

Hi Mickey

For the first look, your design is in the midrange for capacity in choppy water. Take a look on this picture. This is a boat for soft ride in rough water. The collar are very high. Deadrise is in 24 degres range at transom.

http://www.polarisboats.com/p/fiberglass_hull/neptune_pics/neptune7.jpg

Some builder offer two different hull shape for their RIB

 

The OB engine seams to be not very powerfull for a deadrise of 24 deg....

Huge loss on speed

 

To jfblouin:

No worries, about English spelling/grammer.

Most of the time, people will only make a fuss - if they don't agree
with your numbers !

I suspect the Polaris boat shown is often sold to the Coast Guard
and/or the Military.
Who can afford the fuel costs that is inherent in a 24 degree " V ".

Also, they will not have the option of returning to safe harbour or
slowing down - when on a "mission".

Cheers !

 

The Polaris "Neptune 27'-6"" has the option as OB up to 2 x 300 hp.....

Better for speed with such deadrise....

 

300 hp !

To RANCHI OTTO:

Look closer....Mr. Otto - at the photo.
Perhaps it is an inboard/outboard ? Not an o/b.
Rated anywhere, from 300 to 400 odd hp - hiding INSIDE the boat.

To add fuel to this " V " fire:

Saw Skater Cat adds recently - that proclaimed....guess what?
A Sweet Sixteen angle - on it's hull(s).

Gee Wilikers !!
How the Hull ! - Do those Cheeky Blighters get away with it !
Considering they WIN race after race. In smooth OR rough water.
In the neighbourhood of 100 mph. Pretty tough 'hood !

By the way lads - Mr. OTTO has worked on some fairly quick
boats..... Delivered to some very demanding clients.

You may perhap...request from him - some definitive,
mathmatical rational - in respect to the TRUISIMS being stated
here. With such absolute conviction.

Remember to say please.

Cheers All !
,

 

The 2 x 300 are indicated as max. power in the POLARIS WEB.

Deadrise=22°.....Hull shape=ASD(Arrow System Design)....Speed=100 m.p.h.....Nice?