Monohull Rules Questions

[Ralf Moser]

Hi,

fiddling about my own designed monohull i would like to clarify some design aspects:

1) Strakes are defined as a strip, max. 5/16inch deep.

Strakes are not specifically defined as having vertical sidewalls and horizontal running surfaces:

Using a rectangular L-Profile, with equal length of the legs, with a height of the triangle <5/16.

Attached to the hull this will lead to a warped strake surface (depending of the basic hull-surface, let`s look at a hull with a variable V from bow to stern).

The surface could be tilted inwards the flater the V will get.

Tilting outwards i would expect no issue.

The rules are fullfilled (not restricted is allowed), but i created a local tunnel with the hull and the strake. It is small, and maybe not heavy effective, but it is mathematical there.

Legal?

Or put it in a extreme view: using a rectangular strip, 5/16 height, and simply put it on the hull surface. Highly visible triangle forming a local tunnel

2) The strakes ends have to be faired, not being squared (to the keel from bottom view i think):

What is defined as being faired, 89,9degree? 45degree?

3) Strakes in between midpoint and transom of the hull have to be parallel with the keel:

Is this spec related to a certain view?

Thanks in advance.

 

[Kris Flynn]

I am thinking you might want to do some sketches/drawings Ralf...should help get the juices flowing

 

[RaceMechaniX]

Ralf, I will ping Bill about this again.

Tyler

 

[Ralf Moser]

Thanks Tyler.

I will make some sketches.

 

[John Finch]

Ralf,

If you look at the IMPBA rule book technical standards appendix 1- mono hulls you will see the top picture has strakes on the chine of the hull that tuck in and make the boat illegal. Same goes for strakes closer to the keel. The flat riding surface so to speak cannot form a tunnel when the boat is standing straight up. When the boat banks for a turn there will be a small tunnel that cannot be helped. As for fairing.....I have seen it done many ways at many angles. So long as it is not an obvious straight line step it will be ok.

As for strakes being parrallel at the rear half of the hull.......parrallel to the keel is what we want. As the strakes move forward to the bow they get out of parrallel because of the warped plane bottom at the bow.

personally....I have experimented with strakes for years and my contention is we don't need them at all on our models unless we run totally wet with heavy loads, and who races with a boat like that! I used them on my Twin Craft hulls simply to strengthen the hull and give a good plant for the engine rails.

John

 

[Ralf Moser]

Thx for the reply.

Ok, strakes theoretical running surface have to be horizontal or angled outwards.

Is this also relevant for the strake shape in between bow and mid point?

The fairing would be enough if the strakes edge is angled like 95 or 100 degree to keel.

As you desribed the strake gets warped in front of the hull.

Wenn the hull surface itself is warped between mid point and transom you have the same issue.

If i keep the strake parallel to the keel from bottom view, it is good.

From transom view also.

But from side the strake will angled upwards relativ to the keel, if the transom v is flatter than midpoint of the hulls vee.

This is the pov-issue iam struggeling with.

I planned without strakes, but the shape got way to radical so i need the support of strakes currently to balance lift of the hull:/

 

[Darin Jordan]

For those who don't know, Ralf definitely has a "clue" about Mono design and operation... Yeah... He's THIS guy!

https://www.youtube.com/watch?v=EhD7K9ettBk

Whatever he's working on is bound to be AMAZING!!

Go Get-Em, Ralf!!

 

[John Finch]

Yes, the strakes will rise as they go forward from the side view and that is ok because you probably have a warped plane bottom. So long as you are not trying to make steps out of strakes I think it will be ok. Hard to tell without drawings or photos. Sounds like you have some ideas for a mono and just want to make sure it fits the rules. Hard to beat what you already did at 104 mph. That was awesome. So far I have only been up to 92 mph on the IMPBA timing lights with a mono. Just a standard deep vee like yours. I made a delta design 20 mono from scratch that Kently Porter and I took the nitro record with years ago that worked really well with a 10 degree vee at the keel and then went to a deep vee moving up to the chine. Always fun to build a scratch build and see how it works.

As for the trailing edge of the strakes that end before the transom...........So long as they are rounded as they fair into the hull they are ok. A straight line from the outer edge of the strake to the hull is a step even if at an angle. I think you have an idea what I am saying. You can always send me a drawing or photo and I can make an official call for you as Mono Technical Director for the IMPBA organization. I am not sure on the NAMBA rules.

John

 

[Ralf Moser]

Hello John,

as mentioned i took a brake

I did even 108mph, and i recognized that i am near the physical limit with the boat. The monos have to (instead of catamarans) go through different transistions of balancing water and air during accelerations to this high speeds.

If you are at peak >110mph and want to adjust the setup for that speed the result is often contrary. Sometimes the boat was not able to reach even 90mph after an "optimization".

A catamaran is simple as you have a standard vee shape for water forces (liftig the cat at slow speeds) and a non wetted tunnel for air. With the monos we have to handle both media on one surface.

My personal experience and view: 93mph is something like a wall for standard vees. You get there with experience and adjustments very fast but then some weird things arise. The other wall i found at 110mph. This was the trigger to built my own design. Still in the optimization phase the compromise of power, speed and stability looks good for my Corvus. Moment of truth will happen when i put in the hot setup.

As i do see already some water-air balancing issues at high speeds i have to do some decisions of how to proceed (adding stuff, modify hull shape etc.). This is only possible by knowing the allowed degrees of freedom.

Regarding the starting point of these thread: the picture from the other mono thread describes the point i had.

I coloured the critical lines. Blue is nominal water, yellow is the theoretical parallel strake running surface. This boat has angled strake and chine surfaces (red) which introduce locally a small tunnel w.r.t. the water surface line.

This is a natural thing with warped hulls when you add simple rectangular profiles to add a strake on a warped hull.

And a lot of work to make the strake surface to fit the yellow angle.

I do not want to argue about established monos being illegal, i only want to clarify if it is worth looking into that stuff as "tuning"

Regards

Ralf