Tunnel boat design and optimizing etc...

[racetuner]

Hi all, new to the forums but only as a member, been surfing for some time now.

Im undertaking to design a new hull for a certain form of racing which for now I would assume leave unnamed. I do have some specific questions tho for those in the know or those who have experimented with tunnel boats of various designs.

I have an idea to increase the amount of bow lift of a certain style hull in what I feel would be a fairly dramatic fashion. This particular hull as it sits now tends to rely on what seems to me to be a signifigant amount of drive" trim" to obtain the bow lift needed to acheive the desired angle of attack for the hull (I suppose)

In doing this, other than increased drag(which I am not even sure I would induce with this method) is there any reason why this would be a bad thing? My thinking so far is this: If one can get that same bow lift thru other means than using the propulsions systems "trim" would that force not make for a faster boat by vectoring more of that thrust perpendicular to the waters surface rather than at some angle up from it? I.e. the fastest boat would be one which could throw no signifigant "roostertail"?

I'm actually thinking this design would increase lift to the stage where weight would even need to be moved forward in the hull, which from my basic understanding can also make for a more "stable" tunnelboat.

Thoughts?

Also just how much lift, as a percentage lets say, do some tunnel boats achieve? How much effect does the top deck have even if its not directly "above" the tunnel portion of the boat in terms of lift, not drag if that makes sense as a question.

Is there signifigant lift generated by the shape of the bottom of the sponsons in relation to the lift generated by the shape of the top deck?

I suppose those last few questions are based on the premise that I have read a bit about which states that sometimes upwards of 80% of the lift a wing can produce is due to its shape rather than just simply the angle of attack of a flat plane (i.e. a sheet of plywood inclined to the flow of air etc)

I have a question about frontal area as well but Im not yet sure how to word it.

Im sure these are far more complex q's that I think but any intelligent thougths would still be apreciated.

[HakimKlunker]

Hi there!
Certain new hulls with certain new ideas can only be commented when it is clear what we are talking about. You may try and get information about comparative certain other boats if only you would specify

[Ad Hoc]

I think HakimKlunker summed it up nicely. All too vague.

And also you need to be clear about the objective. Are looking for a reduction of say XX in one aspect, solely, and ignoring the rest of the boat, ie increasing the performance/efficiency of just one aspect?
or
Looking at the boat/design as a whole unit, ie a total improvement all round?

The two are very different propositions.

[Alik]

Buy a book 'Secrets of tunnel boat design' by James Russel. Basics are well explained there.

[racetuner]

If this design change works it will likely be seen as a radical departure from current thinking and as such I am definitely not about to go and announce the specifics of it on a public forum.
I was merely looking for dicussions of a general nature.

I am aware of Jim and his book and have read many of his articles etc already

[Ad Hoc]

"...as such I am definitely not about to go and announce the specifics of it on a public forum. ..

So why ask for comments if you're not prepared to explain in detail what "it" is? No details....no debate!

[Dan Ellison]

Detailed answers require details. Getting people to valdate your design without details will be tough. Playboat or raceboat? How fast? how big or small? inboard, or outboard? True tunnel or mod vp.Weight, Hp rating, speed target. ALL these factors play a role in design

[Jimboat]

racetuner - sounds like you're giving alot of thought to your new hull design ideas. Advancing ideas for more efficient hull designs always generates lots of opinion and often some excitement.

It's a bit hard to visualize the design features that you're thinking of, but anyway...

most tunnel hulls don't have a problem generating lift forward (in the bow), but rather usually have a design challenge to get the CofP sufficiently far enough aftward to achieve dynamic balance at higher velocities. So increasing bow lift may not be an advantage to most tunnel hull applications, particularly racing. There are many, many forces acting on an operating tunnel hull. As the hull increases in velocity, all of these acting forces change, and their relative significance to the balance of the hull change too. Since most of the weight of a performance boat is aftward, it's usually an advantage to find a way to move CofP aftward and move static weight forward.

A well balanced tunnel hull will run most efficiently with "neutral trim" - and in fact as velocity and aerodynamic lift increases, some tunnel boat setups require minor negative trim. Boats that require significant positive trim to "fly" are usually found to be underdesigned in the aerodynamic department, for their intended application.

Certainly you're right that the the fastest boat would be one which has little "roostertail". (see my article "Why do boats create Roostertails"?)

as for weight distribution, the trick in tunnel boat design is to achieve a dynamic stability throughout the hulls operating velocity range. the Dynamic CofG often "moves" quite significantly from fore to aft to fore as the boat accelerates through the velocity range. Matching the Dynamic CofG to CofP and static CG is always a tricky business with tunnel hull design, and a compromise to stability at some range of the velocity spectrum.

Some tunnels will achieve over 70% aerodynamic lift (I assume that is what your question related to?) The deck surfaces and the tunnel roof surface forms an aerofoil that generates a huge amount of aero lift, particularly at higher velocities. Racing tunnel hulls will generate even more than this amount of aerodynamic lift at maximum velocity.

The hydrodynamic lift & drag of the sponsons is also important to both efficiency and dynamic stability. there are many factors that influence the efficiency of the sponson lift, however it's safe to say that a flat, true smooth sponson pad will always be the most efficient. Different deadrise, width (aspect ratio) and trim angles are used to optimize the performance of each particular hull and it's operating expectations.

For more info on aerodynamics with tunnel hull design, check out the articles:
"How Aerodynamics Can Increase Your Hulls speed"
and
"Where Does Lift Come From?"

[HakimKlunker]

Originally Posted by racetuner

If this design change works it will likely be seen as a radical departure from current thinking and as such I am definitely not about to go and announce the specifics of it on a public forum.
I was merely looking for dicussions of a general nature.

I am aware of Jim and his book and have read many of his articles etc already

My friend: No idea is so new as if it was not thought about before. You want advise for free, but return nothing. Maybe you get to your workshop and produce your own results, then and surprise us all to be boat-building-Einstein. Or: Keep on talking about nothing. Good luck

[jtd]

First off you cant reinvent the wheel, or in this case the boat!

Second, A Idea is only worth money to someone who has the money, and the means to develop, fund and produce it.

and last with all of the development in tunnel boats thew the years, all of the current designs have gone trew such vigorous wind tunnel testing that I HIGHLY doubt your idea will change tunnel boat design as we know it now!!!

that said break open your piggy bank, pour your hart & soul into it, and prove me wrong.
PS, buy the book THE SECRETES OF TUNNEL BOAT DESIGN you need to read it.

[racetuner]

Its amazing on a board that is all about designand exchange of idea's that so many of the replies to this post have been so negative!

Thanks Jim for both your post and time on the phone, you got me even more excited about my idea's and I hardly thought that was possible!

Cheers