Vee hull features.

[Simon.O.]

I have a 30" DV (22deg deadrise) on the build.
The sides are on and bottom is skinned. Foredeck is framed and will be skinned tomorrow.

Bottom hook
There is slight hook in the bottom in that when placed on a flat surface the transom touches and about 10" froward touches. There is a slight hollow between. I believe this is a common feature on many of the produced hulls such as the DF.
Is it good bad or otherwise ?
I have enough skin thickness to be able to sand most of it out if it has to go.

Spray rails
Attached is a sketch of two different spray rails I have seen.
I will initially build and finish the hull with out any rails.

If I fit rails then is there a preference for either of the two I have drawn ?
Do rails have to go all the way to the bow?

That will do for now. I am sure I will have more questions later.

Thanks.

[sean450]

Simon.O.
From what I have seen you don't need spray rails on the boat. Some boats like my Muck mono doesn't have any on the bottom of the hull like in your illustration.
The spray rails for this hull is actually the overlap of the deck and freeboard. You will have to run the boat to see if water wants to climb the free board and wet down the deck.If it does, add some rails.

[paulwilliams]

There is slight hook in the bottom in that when placed on a flat surface the transom touches and about 10" froward touches. There is a slight hollow between. I believe this is a common feature on many of the produced hulls such as the DF.
Is it good bad or otherwise ?

I'd say it's good if the designer intended it to be there, and bad if not, and especially bad if it buggers the handling.

Paul

[Simon.O.]

I'd say it's good if the designer intended it to be there, and bad if not, and especially bad if it buggers the handling.

Paul

Given that I am the builder and drawer of my plans take from photos of DF hulls I would say that the hook is not a design inclusion.
However if there are a lot DV hull sthat have a "hook" and go well then it will stay.

Do I need a really flat bottom surface (each side of the keel) or is a mild depression / hook allowable.
I can assume that the keel must be dead straight/flat , no hook or depression allowed here ?

How may it bugger the handling ?
I am new to fast monos, riggers are more my game, cats too.

This should be a very good discussion....I hope.

[EPower]

Given that I am the builder and drawer of my plans take from photos of DF hulls I would say that the hook is not a design inclusion.
However if there are a lot DV hull sthat have a "hook" and go well then it will stay.

Do I need a really flat bottom surface (each side of the keel) or is a mild depression / hook allowable.
I can assume that the keel must be dead straight/flat , no hook or depression allowed here ?

How may it bugger the handling ?
I am new to fast monos, riggers are more my game, cats too.

This should be a very good discussion....I hope.

From my understanding Seaducer monos have a slight hook in the keel, i just looked at both my Mhz hull and they have no hook whatsoever

[paulwilliams]

Given that I am the builder and drawer of my plans take from photos of DF hulls I would say that the hook is not a design inclusion.
However if there are a lot DV hull sthat have a "hook" and go well then it will stay.

Do I need a really flat bottom surface (each side of the keel) or is a mild depression / hook allowable.
I can assume that the keel must be dead straight/flat , no hook or depression allowed here ?

How may it bugger the handling ?
I am new to fast monos, riggers are more my game, cats too.

This should be a very good discussion....I hope.

Simon,

Let me make it clear I'm no expert on monohulls either - outriggers are more my thing too.

Generally speaking, the starting point for setting up a monohull is a straight, flat bottom, with no hook or rocker. However, in order to correct torque roll, it is possible to modify the shape of the hull, for example to add wedges to the right hand side, or to introduce a very, very subtle hook with a thin layer of filler.

The monos I've played around with recently have all been stepped, and these behave quite differently to a boat without steps - usually they're much more nervous and don't turn as well.

Hook/hollows is usually considered a bad thing because it acts like a big trim tab, forcing the nose down and making the boat run very wet, and therefore killing speed - unless the designer of the boat has purposely hooked the bottom to avoid using trim tabs.

I would suggest that the logical course of action is to start with a flat, straight "neutral" hull and proceed from there. If you have very bad torque roll/prop walking, try correcting these with adjustments to the drive angle and by biasing the weight to the left. If this doesn't work, then think about re-profiling the hull bottom, or perhaps running spray rails on the right only, or reducing the spray rails on the left to alter the lift on each side.

When evaluating another design that has hook, you would need to know whether the hook is a consequence of poor quality manufacture, or a deliberate design feature. A badly made hull that works well is pure luck.

Paul

[785boats]

Hi Simon.
I run one of those 25"Delta Force copies & that hull has a hook but is only about 4inches long. Very stable without trim tabs.
The way I look at it is this.
The hook as Paulwilliams suggested tends to push the nose down.
By angling the strut up slightly the driveline thrust raises the nose pivoting on the transom. This balances out the force of the hook.
This then allows the hull to ride up on the trailing edge of the hook (transom) & run a lot drier. When the sweet spot for the COG is found by moving the batteries the hull should run with a slight nose up attitude & just on the transom & prop.
If the nose tends to keep lifting & wants to flip drop the strut angle a tad and/or move the packs forward a bit.
If it looks too wet do the opposite.
This is purely subjective on my part, but I consider my hull well trimmed if it leaves the water after hitting a bit of chop or another boats wake & flies through the air at exactly the same attitude as on the water & lands the same way. In my opinion, the thrust line, the COG, the attitude & the aeronautics are all in perfect harmony. Like I said, purely subjective & based on nothing except that it looks right.
Also being able to do 5 laps at full noise around our course without lifting off & under control most of the time means that it's about right too.
All the best in your new designs.
Paul.

[Simon.O.]

Paul and Paul. Thanks for the very detailed answers.
As I am building this I can do as I please, my boat used for fun and straight runs.

I will finish it flat in regards to the bottom and rocker/hook.
I have found that it will need 2 more lengthwise rails inside the hull skin from amidships to the transom. This will flatten the hook and prevent any flexing. Sealed and varnished 3mm balsa on cross grain doesn't flex a lot anyway. I will just be making it a little stiffer.

I hear you both on the trim tab, spray rail and torque wedge bit and will keep all of these featurse off unless they are needed. Any laterral ballast can be induced by moving the rx and pack to the left, any more an I can either add hull features or weight.
Moving on.

Strut height.
I am happy with what strut angle does on a mono, and have a rough idea on what strut height can do.
Raising the strut will unload the prop increasing rpm but at the same time lower the transom wetting the rear and inducing drag. These are two opposing forces.
Have I got this bit right or is my theory off here.

Given a 30" hull with 22deg deadrise roughly where should the shaft centre line be for kick-off. I use the shaft CL as a measure as there are so many different struts that the CL is a more accurate measure.
I will be running a 062 wire down a flooded shaft box so I will have the ability to adjust up to 5/16" or 8mm vertical movement on the strut.
440 prop for starters.

What say the clever people ??

[785boats]

Strut height.
I am happy with what strut angle does on a mono, and have a rough idea on what strut height can do.
Raising the strut will unload the prop increasing rpm but at the same time lower the transom wetting the rear and inducing drag. These are two opposing forces.
Have I got this bit right or is my theory off here.

Simon
Nearly.
By raising the angle of the prop, yes the transom is pushed down into the water slightly but this increases the angle of attack of the hull. This allows three things to happen.
Less hull in the water, just the last couple of inches or less, instead of 10 or 12 inches if the hull is running flat or "Wet" The amount of friction on the water that this reduces gives a marked reduction in load on the motor & therefore temperatures, while resulting in higher revs & speed.
The angle of attack of that part of the hull in contact with the water causes the hull to ride up higher on top of the water also.
More air also gets under the hull to help lift it because of the angle of attack. I know you are familiar with this concept with your work on riggers & hydros.

The cumulative effect of the above is less drag, increased speed & reduced load on the electrics. This far outweighs the slight downward pressure created by the increased angle of the thrust line. After all we are only talking of about 1 or 1-1/2 or 2 degrees at the most.

Maybe someone else with more experience than me can chime in & add more info regarding the finer points of strut angles.
Hope this helps a bit.
Paul.

[Simon.O.]

Paul, that too has been printed and filed in my reference folder.
Strut angle is now sorted. One down one to go.

Let's see who can give me the beans on strut height for monos.

[G Doggett]

Hi Simon

The strut height on my DV hulls has always been determined by how low I could mount the rear of the stuffing tube inside the rear of the hull. Simple as that !
In most cases the shaft then extends parallel with the keel, with the prop distanced behind the transom at 10% of the hull length.
For example a 30 inch hull has prop at 3 inches behind transom.
I think that if you have to angle a strut more than 1 - 2 degrees up or down , you are compensating for a problem somewhere else.
Works for me.
Graham.

[paulwilliams]

In my experience, if you raise the prop too far out of the water the boat becomes very unstable, and will be more likely to spin out or flip. Raising the strut allows you to use bigger props and unloads the motor, but at the (probable) expense of stability, thanks to increased torque effects.

Running the prop deeper loads the motor more, but does calm things down a lot.

However, these are generalisations - until you build the thing and run it, you simply won't know.

As Graham says, typically you run the shaft as low in the apex of the vee as possible, unless you have a European-style shallow vee mono with a solid shaft - these often exit the transom quite high up.

Paul

[EPower]

In my experience, if you raise the prop too far out of the water the boat becomes very unstable, and will be more likely to spin out or flip. Raising the strut allows you to use bigger props and unloads the motor, but at the (probable) expense of stability, thanks to increased torque effects.

Running the prop deeper loads the motor more, but does calm things down a lot.

However, these are generalisations - until you build the thing and run it, you simply won't know.

As Graham says, typically you run the shaft as low in the apex of the vee as possible, unless you have a European-style shallow vee mono with a solid shaft - these often exit the transom quite high up.

Paul

I have just been looking at your website Paul there is a whole lot of very good information
Adrian

[DISAR]

Very nice info here about monos. I just want to add that I was told in the past that for a Delta Force hull the centre of the shaft should be about 12mm - 0.47" above the keel, and when placing the strut too deep creates lift on the transom and not proper handling. Unfortunately I have not tried different strut heights to know the difference.

[paulwilliams]

I have just been looking at your website Paul there is a whole lot of very good information
Adrian

Thanks!