Moderate Speed... Any Weather

I want to build a boat under 23 feet that will be able to progress at 15 to 20 mph through conditions that most people would call "unsafe". My goal is to travel without concern for weather.

At present, I'm considering a 23' x 5' (vsv?) hull with constant 26 degrees deadrise. Being this narrow, it might have water ballast or lead ballast or both.

My questions:

- Is a monohedron shape inappropriate for speeds below 20 mph?

- Will a potentially heavy boat with 26 deg. deadrise plane at 15 mph?

- Should I give up and be content with a hull speed boat?



Thanks for your advice,

~ Lowell (A7)

 

Having designed/built vessels that:
" ....travel {with much less} concern for weather."

A few thoughts:

The short answer(s):
( A.) Maybe......
( B.) Only with difficulty....
( C.) Probably.

As:
- Semi - planing occurs right in the speed zone you are thinking of.
- Where fuel costs and the power needed, can easily be double
- of what is required at displ.....or planing speed.
This is especially true of:
Narrow, heavy, deep " Veed " boats.
Whereas, a lighter vessel - with a flatter " V " - will hop onto plane,
with little fuss and bother. But might have difficuty, in heavy seas.

What you are looking for; has been studiously debated for the last
100 yrs or so. - And it continues......

That's why they pay us designers, the big bucks.
Yeah... If only.......

You sound like a " belt and braces " kind of a guy.
So I doubt that the "radical" designs I favour - will interest you.

Planing vessels have only been around since light weight, powerfull
engines became available. So the engineering of said, planing vessels,
is still undergoing change and developement.

Whereas, displ. boats go back thousands of years....With many well
proven designs out there.

It's your choice.

'Regards.

 

Actually I'm quite open to exotic designs. I have researched extensively foils, surface effect, and catamaran designs, but other than computer controlled foils, none of them seem to be capable of rough weather at the size I'm looking for. Also, ease of building is paramount because I intend to build this myself. This means all but the most basic hull shapes (not to mention foils) are ruled out.

Of course speed vs. seakeeping has alway been the biggest compromise in most designs, but I was hoping that moderate speed would allow me to avoid the usual "I want to go 100 knots and have a marble jacuzzi" problem. Perhaps lowering my speed requirement and flattening the hull is the answer?

Has anyone heard of Port Erie Catamarans? The owner makes some improbable claims regarding ride that, if true, would be quite amazing.

http://www.porterieboats.com/

~ ALowell

 

Depends on two things your idea on moderate speed and what do you consider poor weather! Get it wrong with a 23 foot boat in 'real' weather and you'll be airborne rather than planing! Shortly after yo ustand a good chance of being in that comotose state that is sometimes known as 'dead' - don't get me wrong I wouldn't want to put you off doing anything; but the comment "travel whaterever the weather" and the mention of 20 knots and 23 feet together fill me with dread! Nor so much for you - if you wish to kill yourself thats your concern, I'm more concerned for the poor devils in your local rescue service who will have to come out to rescue you!! Once it's all gone to rats***

 

Always optimistic

 

I guess when I say no regard for weather what I mean is traveling in wind up to 15-20 mph. Obviously I'm not going out in a gale or hurricane. Just like when you drive a car -- you don't drive in a blizzard, but you do drive in rain, snow, etc.

Basically I don't want to have to wake up at 5 am to travel before the wind comes up -- I'm not some sort of daredevil.

~ ALowell

 

Tri - star:

Can you expand? Why not use constant vee for a 20 mph boat? Is it only used for boats that are at risk of leaving and re-entering the water?

Also, is there a method for predicting hp requirement based on weight and vee angle?

Thanks,

~ ALowell

 

Don't worry unduly about those sorts of comments. If they do come out to help you they will probably come out in a deep V open boat designed to run in any weather at about 20kts. You could then compare notes!

However, although the hull is slim, the tubes give them the beam to resist transfers upset even though they are hardly immersed at rest. They also have inflatable righting air bags on the arch (available from Henshaws) and immersion proof engines (available second hand from Barrus) to make them truly 'all weather'. Oh, they also cost 150000 USD.

One of these shows what it must look like to have one heave into view at the right moment, although in this case was taken from the stern of a larger AWLB.

 

To ALowell:
You have never been on one of my boats.

If you had, you would have less doubts - about certain, relatively
simple hull forms. That do indeed perform well:
- In rough conditions.

Armchair theorists are always bringing up foils.
A technology invented and proven to work in Canada - almost a
hundred years ago now. Obviously, there are also, some thing(s)
working against using them. In the every day world.
- Or else, why are they not every where you look ?
- After all this time?
If you have a few hours to spend, I can tell you why. Even with
computer assist - there are issues that remain.
That will not go away. You will pay for dinner.

People who have been on a " surface effect ship " know how
effective they can be. Although, with so little of the vessel,
displacing any water at all....it may be questioned if an SES,
can really be defined as a boat anymore !
Also, at present they dont come cheap - or simple.

Cats and Tri - hulls are more commonplace.
However; ever since 20 knot, " native " boats welcomed Capt. Cook's
5 knot ship to the Islands - vested interests have been in denial as
to the viability of these " primitive " vessels.
Particularly, in places like England and where you live.
In the North Eastern US.

Thankfully; there is a sea change coming.
Both the US and British Navies have spent and will continue to
spend Billions of Euros / Dollars to answer your very question !
(Amazing how big a response one can get - by starting a Thread on
Boat Design Forums.)
Narrow, deep " Vees " - Semi-planing ships - Cats and Tri - hulls:
- Have all been tested and orders are in place. For all of the above.
( Except perhaps, note: For foil supported ships.)
I say thankfully, as soon, I will feel less alone in the wilderness,
calling into the wind.
As after sea trials, of all the ships above. This debate of several
hundred years: will finaly simmer down.
- Then I will be able to get some rest and will not be required to as evangelical in the future.

Cheers !

 

tri - star:

You don't have to tell me not to use foils. If you remember, I already mentioned that home building rules them out.

Crag Cay:

I've seen lots of pictures of RIBs where the tubes are well clear of the water at rest. What are they there for if not to stabilize the boat at rest? Do they prevent tips in quartering/beam seas?

It seems to me a RIB might actually ride better with the collar removed - this would have the same effect as removing chine flats and therefore lift. Obviously this would make them unsafe, but I guess I want to build a collarless RIB by counteracting the instability with greater freeboard and ballast.

But my main question is still unanswered: Does anyone have experience running deep vee hulls at moderate to low speeds? What is the minimum planing speed for a 26 degree hull?

 

To ALowell:

Re:
( A.) - "...why not...constant " V "....".
( B.) - "...hp...based on weight and V angle ?"

( A.)
I choose my words very carefully in these matters.
Note: I said " Maybe..."
As it depends.... On various factors.
We were involved in the design of a 65 ft vessel that was
constant in it's V, forward, to mid ships. However, not for all
the reasons that you might presume.
i.e.
If jet drives are a possibility. Then a constant V, well forward:
- Is a requirement. Not really an option.
As you do not want any confused flow ahead of the intake.

Also, if you are happy to stay under 20 knots - and prop(s)
are intended - there are a many ways to reduce the " hump "
getting on plane. That are now open to us.
They might not includ the simplisity of a constant V.

( B.)
FRANCI OTTO is very generous and precise with mathmatical
formulas.
In fact I think he posted something very close to your question,
just a few days ago.
If you check all his posts you will find answers - to most of your
concerns - and beyond.

Cheers !

 

tri - star:

I intend to use outboard power, so this would apply to me. What are you suggesting to reduce hump? Flatter aft sections?

Thanks for the info.

~ ALowell

 

"...experience running deep vee...."
Yes, I do.

It is just is not practical.
There is a lot of reasons - why not too.
It's absolutely the worst speed range for a deep V.
The vessel's bow will be hard to see over much of the time.
The steering will be sloppy.
It will hunt back and forth.
Want to lean over this way and that - both from waves
on the beam and cross winds.
In bad weather, it will be going on and off plane all the time.

A deep V shines at high speeds. But they do not like to trundle
along - hovering close to dipls. speed.
All the time devouring prodigous amounts of fuel.

".....min....planing speed for...26 degree hull."
A lot depends on how much beer you bring on board. I'm serious.
And where you stow it !
You are looking for a simplistic answer for a very complex problem.
Once again - I refer you to OTTO.

It is ABSOLUTELY a myth that the V angle is the only
determinant - on the onset of planing. The C. of G. can have
as much effect. Or more. So there is no easy answer to your
concern.

Time for you to build some decent sized models and find out
for your self.

That's all folk's.... turning off the computer. Have to see a
man about a boat.

Cheers !

 

One thing Crag they won't be sending the RNLI they don't operate further than some thirty miles from the UK! (especially in ILB's)

And taking the 'collar' off a Ridgid Inflatable Boat means it ain't Inflatable anymore!

Yep Raggi, that's me every optomistic, but he ain't going out in really rough weather - I wouldn't even call it rough weather! Bit of lop is all, do't get interesting until it's close to 30 Knots! but you got to establish what he's going into and his knowledge (there again I think that's been done by the fact that he's posting.......so my original warning rests!)

 

If you don't mind, you can address your statements to me, instead of referring to me in the third person (it's rude).

As to knowledge, let me say that I am a USCG licensed captain, 100 Tons. As to weather, I think most people tend to seriously overestimate wind speeds - 20 knots is a lot of wind, especially when you consider the shoals and currents I encounter on a daily basis. Anyone familiar with Woods Hole knows how treacherous it can be even with no wind at all.

Furthermore, let me say that I am frustrated with the widespread assumption that I know nothing about boats. A RIB without it's collar isn't inflatable? Oh gosh - I hadn't realized that! Explaining to me that vee angle is not the only determinate of speed is telling me something I already know. When I ask about it, I am assuming other variables (weight, CG, CB, etc.) are within average limits. I know a deep vee hull is crappy at hull speed, why else do you think I would be asking about minimum planing speed? And I know that variable deadrise will fix the problem, that's why I started the thread - to find out if people knew about alternatives.

Sorry if I'm sounding a bit huffy - I guess I should have made myself clearer from the start.

Thanks anyway,

~ ALowell