FLOAT Design

[IanHB]

I have just been watching the video posted on rcsailtalk of the Spitfire full size catamarans.
It occurred to me as I watched them run out of buoyancy and the leeward hull go down the mine, inspite of the double trapeze crew as far aft as they could get, that in our model format we are pushing it uphill to say the least.
Without adding movable weight or foils we somehow need to gain extra righting moment when the tri floats are hard pressed.
Is it feasable to have flared sections and perhaps a raised foredeck to bring in that buoyancy as the floats are driven under or is there some down side to my thoughts?

[peter]

Hi Ian
I think I can answer this with some degree of experience. I have sailed r/c cat's and tri's. The design of the floats is not that important, if you look at the boats on our website, or the posted pictures on r/c sailing discussion you will see several different configurations in floats.

Reducing the capsizing problem can be helped by reducing sail area, adding T-foil rudders and seeing gusts coming.

I have found personally that also the correct balance of sails and just concentration are the main ingredients in preventing capsizes.

The power being generated through the rig as the boat heels drives the leeward hull down, this force is in direct responds to the action of the high aspect rig usually used by r/c multihull's.

It comes down to a centre of effort thing(explained much better by Andy McCullough in an article called
Heavy Weather sailing).

With a monohull, when it becomes over powered it wants to drive up to windward. A multihull, when it becomes overpowered it want's to go faster, the consequence of this extra power, generate through the centre of effort drives the leeward further down into the water.

As I said earlier reducing sail area, etc. is the best way to assist with this.

This is a long explaination of a simple question. I thought the explaination needed to be said first.

The short answer is no, movable ballast etc. is not (IMHO) the way to combat this. We have been racing our boats for 4-5 years now as a club, and no-one here has gone that way.

Extra bouyancy in the floats means more displacement, which means you are pushing more water, which means a slower boat.

Apparently the (perfect) design for a float is an upside down egg shape. Sharp entry and somewhere between 60 and 90mm maximum beam.

Another important part with the design of floats ( the entire boat in fact) is to ensure that when the boat heels, the centre of the float contacts the surface of the water first. If you go bow down you are asking for trouble. On the other side of this, if the stern contacts first there is probably to much rocker in the main hull which will cause the bows to drive down rather than through the water as the boat increases speed.

Hope this helps and explains things for you.

Peter

[MultiFast Is MultiFun]

Ian -

I must concur with what Peter has posted. If you watch the video clip, you will notice that the top of the mast (lever) is the force driving the boat over and down.

I made a small sketch indicating issues of stability. One way is to reduce the height of center of effort. Going from a high aspect raio light air sail/rig, to a low aspect raio for heavy air will provide benefits.



Small anti diving planes could be incoorporated into bow design, but would have to be pretty big to counteract the leverage the mast/sail will be giving you.

Both options Peter suggests are still the best.

[IanHB]

Thanks for your replies guys, I understand all that you are saying.
My concern is not that my tri is simply overpowered as I am dealing with that, with the fitting of smaller rigs with the subsequent lowering of the centre of effort and the added feature that I can now move the mast position fore and aft about 100mm to correct the balance.
I know that a multi takes more concentration and skill to sail than a mono, that the correct rig is important and the theory of weather helm etc. I know enough to be dangerous.
The scenario I am trying to cover is where you have on the correct rig for the conditions, can see the gust coming, but do not want to just dump the sail to get through the gust but want to accelerate away from the fleet who are left wallowing in your wake.
Adding buoyancy to the float can surely be done so that under normal conditions nothing is changed but as the float is hard pressed a large amount of extra buoyancy comes into play to keep that float up out of the mine.
It should not add any excessive extra drag, certainly not on a long term anyway.
Like you Peter I hate to add weight, see my post on sailtalk "adding lightness" so we are left with the only alternative, Add Flotation
[glow=red,2,300]What say you my friends[/glow]

[peter]

Ian

Like you I have thought long and hard about how to improve the stability of the multi's. Really you can't have a float that's shaped like a T.
I.E standard size to half way up then widens dramatically. The problem with this is that it will work more like a brake when it hits the water than a benefit.

So what are the other choices:-

Overly bulbious floats- that's fine but how far do you go.

Spray chines on the bows- angle of the chines is the problem here

Raise the height of the bow- have it on my boat now helps a little

When sailing to windward

Really the best way to handle the gust is to point slightly higher towards it then ease sheets just a little as the gust hits, by doing this you lose the top half of the sail. This allows you to stay on a "level platform"(often lifting the main hull) and then just drive the boat. In most cases this will prevent the capsize.

When reaching

Ease sheets and sail more downhill with the gust.

When running

Same as reaching

Hope this helps

Peter

[Matt L]

Lowering the CofE and getting as much volume infront of it are the most important things to do. For a cat putting most of the rocker in the aft third of the hull seems to help, kinda sucks the back down and the nose up. If you want to add volume to the bow build it high and keep it narrow, and make sure there is nothing on the bow that coud trip the boat, shroud fittings and forward beams can cause alot of drag where you dont need it. A good T foil rudder always helps, although they are of little use if the boat is heavy and is slow to accelerate in gusts, as if the boat is going slow they dont work.
This is what I've found with cat's, it may not all apply to tri's....

[hoj]

I am back from vacation and the tri's will be a priority. I have looked at all my amas and the plans that I have. The nose heights differ from 3 1/4" to 4 3/4" with out a quick round up. The beams vary from 2 1/2" to 4". The x-sections vary from egg shape narrow up to wide up and the Pulse has 25 degree v shaped bottom with vertical sides and fairly flat top. There has to be a difference in performance but I have no other tri's to compare to. All main hulls of the Nightmare's are the same? I am going to finish the two that are similar MKI and MKIII. The amas are the same but MKIII has a 3 1/4" nose height by being sectioned vertically. MKI has 4 3/4". I am going to make all the amas mounts the same and be able to change to different types. Also all the amas the front touches first with 6 to 8" of the rear out of the water. I am setting MKI up so I can put different wedges to change it so the front and rear will be parallel to the water. MKIV's are very slim 2 1/2' beam but average height. I would appreciate all your comments. I am going to make plans of all the boats and try to somehow have at least the shadows available. I can't do it but I hope someone here can if I send them hard copies. I can make full size complete drawings as long as they are returned. Will continue on other subjects.

[ABC]

Have you tried raking the mast back?
I sail a full size Taipan Formula 16 catamaran very similar to the Spitfire and when the wind gets up we tend to rake the mast further aft which tends to produce more forward urge than pitchpoling urge.

I'm currently developing (very slowly I might add) a Mini 40/Formula 48 RC model of Team Philips though I haven't got it on the water yet and haven't sailed an RC multihull before so I'm not aware of the limitations.

cheers,
Andy.

[Matt L]

Unfortuately mini 40 rules dissalow twin rigs (You are not alowed two mainsails, its in the rules under either sails or rigs...) or I would have built somthin along those linees already!

[MultiFast Is MultiFun]

Matt said:

Unfortuately mini 40 rules dissalow twin rigs

Matt - just one of the reasons "why" the Formula 48 Class rules were developed over here.

If one stops and gives a really good consideration, you would see, that the class (Mini40) which is promoted as a development class, has a lot of "little" details which say otherwise.

While length, beam and sail area might be considered a restriction, those are clearly there and evident they control the general overall "SIZE" of the boat. Within the F-48 Rules, every effort was made to allow and promote creativity. The ONLY restriction for the F-48 Class on development is on the use of kite sails. And to detail a bit, the only reason for this restriction was the ISAF has not (as yet) brought forward any sailing rules that define issues such as starting, finishing, overlaps, equipment touching, etc. that would need to be addressed if free flying kite sails were allowed.

If (when) ISAF in their "traditionalist" views recognize that kites can power boats around the course and adjust their rules to accept this fact, I am sure the F-48 Class will quickly move to change the class rules to allow kites.

Come on over ! Set yourself free !

[Matt L]

Another reason I shoud move to the US!!

[peter]

In respect of the mini40 rules. The multiple mast point is being addressed as we speak, hopefully it will be passed.

I would like to know what other restrictions there are that would then restrict F48 and mini40 coming together as one class.

Peter

[hoj]

I have a topic started on RCSailing (design/build Nightmare) which Ernst Zemann has replied. I hope to get it going and keep it going till I have the Nightmare's complete. It will start with the design of the boat and going into construction from start to finish with anyone else joining in. I put it there because Ernst came on. I have the design of the main hull going now please come over as I don't want to post in two spots and there has been very little here till recently. How many will not come over. Then I will post here too. I just hate typing.

[MultiFast Is MultiFun]

Hoj -

I made my one (and probably only) post on RCSailing Net forum regarding Ernst and his designs. You are aware of my past issues with Ernst, and I simply prefer not to discuss anything with him any longer.

I did hint about it - and will make it very clear over here on this forum - until I see "ANY" of Ernst's designs show up in entry lists (better yet if they were in the upper part of a finishing list) I simply could care less what the guy says. Ernst right now is only one step ahead of Doug Lord, in that Ernst has photos of boats to point to - which Doug didn't have. Still, living in a country only a few hours from France or England where many multihull races take place, Ernst (like Doug) never has taken the time to enter, sail and prove how superior his designs really are. It is almost 2005 - and Ernst was self promoting his designs back in early 2000 - and as yet - no official race results. Wonder why?

I have had several French and British r/c multihull sailors indicate they want nothing to do with the guy. We have two builders over here that are done with him, and a guy in Canada. Add in yourself and a fellow from Finland, and you are getting up there in numbers that seem to indicate it might be best to steer clear of him.

Of course, in your case you have an investment in whatever parts and pieces he sent you, so if you feel you really need his input - that is your choice.

Please remember that the line drawings for PULSE "ARE" restricted in distribution to only those who have purchased the plans and paid for the royalties to the designer. Also, they are copyrighted. They are not for free distribution over the internet and I would request that you honor that agreement with the designer.

[Idealist]

I would like to participate in this topics, if I may. -
It's not my intention, to cause any troubles. -
So - could we give it a new chance? -
Ernst