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Multihull speed vs length


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I am trying to understand how multihulls progress on the speed scale when they increase in size. I understand that they go faster when they get bigger, but If you build them to the same "race level" but just scale up the boat. how much speed do you gain when the water line increases? This is only for boats that have an ama aspect ratio of greater than 10/1

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How long is a piece of string?

Straight scaling up is pretty simplistic. If LWL goes up 10%, then SA goes up 21% and Displ goes up 33%. You don’t need a 33% increase in displacement to get a 10% increase in length.

Fag Packet, using SeaCart 30 data:

A 10% straight scale up would give a theoretical ave speed increase of ~ 4%.

if you increased LWL, SA and Displ all by 10%, you would get a theoretical ave speed increase of ~ 6%.

If you increased LWL and Displ by 10% and straight scaled up SA (ie 21%) you get a theoretical ave speed increase of ~ 9%.

 

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I think an interesting metric is "Base Speed":

BSpd - Base Speed: An empirically derived indicator of the speed a given boat could average over a 24 hour period (Best Days Run) under a variety of conditions. It can be used to compare speed potential of one or more boats. It has been used for handicapping boat races involving a variety of boat types.

"Base Speed: A Simple Measure for Estimating Multihull Performance",

Richard Boehmer, Multihull International, No. 225, (APR'89) pp. 108-110.

BSpd. = 1.7*(Lwl^0.5)*(SA^0.352)/((Disp.*2240)^.253)

Also other interesting metrics on Multihull Dynamics Definitions Page.

There's also the issue of whether you're operating below or above displacement speeds - a multihull is just a displacement boat in light air so longer will go faster.

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That is exactly the metric I used...... although I have seen varying formulas for it and the version that I used may not be the latest. So for ave speed above, read Base Speed.

It is an interesting exercise to spread sheet out boats that you are interested in, and play around with LWL, SA and Displ parameters.

And if you are going to do it, get the base data off the MOCRA, OMR or Texel websites, not designer/marketing data.....

It gets even more interesting when you spreadsheet out changes to (say) OMR ratings and compare them with the Base Speed data changes.....

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I've noted that the differences in top end speed for a non foiling trimaran is a length dependent function.  That is, you can push a trimaran of 24ft to say 22 kts.  The longer the trimaran, the faster the top end is, but in no instance have I ever seen a non foiling trimaran go faster in kts than its length in feet.  Might have happened, just never have seen it documented.  OTOH, non foiling catamarans seem to be able to go faster than that at their top ends...for example, an 18 ft catamaran beach cat can exceed 18 kts of boatspeed.  This is just based on observations over the years in videos and personal experiences.  

Note that the top end speeds have little to do with boat ratings in general.  Which is OK since in practice, most races are done in sensible conditions and courses.   Boat ratings are an exercise in politics and observations plus some glittering mathematical explanations after the fact.    

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16 hours ago, MultiThom said:

... in no instance have I ever seen a non foiling trimaran go faster in kts than its length in feet. 

I agree.

I've sailed my 15' foiling trimaran a bunch of times without the foils, and the top speed for the day has almost always been ~14 1/2 knots. Never over 15.

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20 hours ago, MultiThom said:

I've noted that the differences in top end speed for a non foiling trimaran is a length dependent function.  That is, you can push a trimaran of 24ft to say 22 kts.  The longer the trimaran, the faster the top end is, but in no instance have I ever seen a non foiling trimaran go faster in kts than its length in feet.  Might have happened, just never have seen it documented.  OTOH, non foiling catamarans seem to be able to go faster than that at their top ends...for example, an 18 ft catamaran beach cat can exceed 18 kts of boatspeed.  This is just based on observations over the years in videos and personal experiences.  

Note that the top end speeds have little to do with boat ratings in general.  Which is OK since in practice, most races are done in sensible conditions and courses.   Boat ratings are an exercise in politics and observations plus some glittering mathematical explanations after the fact.    

As can many monohull “skiff” dinghies....

3 hours ago, Doug Halsey said:

I agree.

I've sailed my 15' foiling trimaran a bunch of times without the foils, and the top speed for the day has almost always been ~14 1/2 knots. Never over 15.

FWIW, a SeaCart 30 (referred to upthread) with 3 crew has a Base Speed of ~14.8. So the theoretical maximum speed would be double that, ie ~ 29.6 knots.

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On 10/17/2020 at 10:59 AM, MultiThom said:

I've noted that the differences in top end speed for a non foiling trimaran is a length dependent function.  That is, you can push a trimaran of 24ft to say 22 kts.  The longer the trimaran, the faster the top end is, but in no instance have I ever seen a non foiling trimaran go faster in kts than its length in feet.  Might have happened, just never have seen it documented.  OTOH, non foiling catamarans seem to be able to go faster than that at their top ends...for example, an 18 ft catamaran beach cat can exceed 18 kts of boatspeed.  This is just based on observations over the years in videos and personal experiences.  

Note that the top end speeds have little to do with boat ratings in general.  Which is OK since in practice, most races are done in sensible conditions and courses.   Boat ratings are an exercise in politics and observations plus some glittering mathematical explanations after the fact.    

Interesting comment which I am sure is right. F18 type cats also don,t go much faster than 20kn. Not sure about formula 40 type cats going much more than 25kn either. 

Reality is though it is all about the averages. A formula 40 type  cat will beat most boats around because their average is so high. Our 50' cat will get higher peak speeds than the formula 40s but in anything under 20kn they beat us by many miles.

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On 10/16/2020 at 5:59 PM, MultiThom said:

in no instance have I ever seen a non foiling trimaran go faster in kts than its length in feet.

On 10/17/2020 at 10:54 AM, Doug Halsey said:

I agree.

I've sailed my 15' foiling trimaran a bunch of times without the foils, and the top speed for the day has almost always been ~14 1/2 knots. Never over 15.

 

Our very own @Tom Kirkman has taken his 14' Weta up to 20.68 kts according to the internets. 

Reality IS certainly all about averages. With my old classic plastic I never felt slow when the wind was blowing, but rather when it wasn't. 

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10 hours ago, Floating Duck said:
 

Our very own @Tom Kirkman has taken his 14' Weta up to 20.68 kts according to the internets. 

Yah, not sure I believe him.  He's exaggerated before and hasn't shown any gps tracks.  

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On 10/20/2020 at 2:31 AM, Paranda said:

Roughly 3,8 times the square root of the waterline length in feet and do not believe exagerated figures of boatowners about their own boats...

Using your formula:

A Seacart 30 can only do ~ 21 knots, and  an MOD 70 can only do ~ 32 knots?

MOD 70’s have been recorded offshore going over 10 knots faster.

Adjust your multiplier to match MOD70 speed of (say) 45 knots you get~  5.3.

Use 5.3 multiplier for the Seacart 30 and you get ~ 29.6 knots......

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1 hour ago, Sidecar said:

Using your formula:

A Seacart 30 can only do ~ 21 knots, and  an MOD 70 can only do ~ 32 knots?

MOD 70’s have been recorded offshore going over 10 knots faster.

Adjust your multiplier to match MOD70 speed of (say) 45 knots you get~  5.3.

Use 5.3 multiplier for the Seacart 30 and you get ~ 29.6 knots......

Mod 70 is foil assisted though, makes a difference

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6 hours ago, bushsailor said:

Mod 70 is foil assisted though, makes a difference

Just did the numbers on Beau Geste, using the data off the current OMR ratings spreadsheet, and with 500 kg extra for crew, the Base Speed is 22.5 knots, doubled for max speed gives 45 knots! ......Base Speed doesn’t allow for foiling.....

It is all fag packet stuff, but there you are. With foiling, still more speed to come.

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On 10/19/2020 at 5:31 PM, Paranda said:

Roughly 3,8 times the square root of the waterline length in feet and do not believe exagerated figures of boatowners about their own boats...

3584FC52-6AA5-4644-A9B3-7CAA18B465D8.jpeg

Agree for the owners claims !

They trust instantaneous GPS readings which are very delusive !

Three days ago I recorded 14.7 kt max speed reabeam reaching on my 14' Paper Tiger, strong wind on a lake, flat water. I was glad to be near my record. Careful analysis of GPS record showed it was more 13.7 kt. However it looks like you can exceed your hull length max speed by surfing on a wave, which is not really representative of  boat performance

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On 10/19/2020 at 8:31 AM, Paranda said:

Roughly 3,8 times the square root of the waterline length in feet and do not believe exagerated figures of boatowners about their own boats...

 

19 hours ago, Sidecar said:

Using your formula:

A Seacart 30 can only do ~ 21 knots...

And an F-25C/82R can only do ~ 18.8 knots?  

Sure I'll take a rating based on that!!!

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12 hours ago, Sidecar said:

It is all fag packet stuff, but there you are. With foiling, still more speed to come.

I love general formulas. Not being sarcastic, I really actually do . It helps to quickly get an approximation of speed potential. Can't get lost with these being absolute numbers however...

Seems there are a couple of ways to go about approximating speed. The most useful ones (I think) all revolve around ratios of multipliers of some sort (in order to have fair comparisons across a wide range of boats);

- Multiple of sqrt(LWL) (the ol' classic 1.34 x LWL for leaded boats)
- Base Speed
- Multiples of wind speed (My personal favorite) 

So for example, using sqrt(LWL), If you have a traditional 4ktsb displacement boat;

- You will be able to normally go 1.34*LWL
- As @patzefran correctly points out, if you are surfing down a wave you might be momentarily higher, lets say 1.75*LWL
- Reduce weight to a ULDB, and you are now at 2*LWL
- Add partial assist foils (IMOCA), and you are at 3*LWL
- Full foiling? 4*LWL

What is interesting about all this, is that it really gives you an idea of how performant a boat is or can be! @gspot an F-25C might be able to reach higher multiples than say, an F24. Figuring out how and why that might happen, is then fun part. Maybe you are full carbon. Maybe you don't have bottom paint. Maybe you only sail in perfectly flat lakes. Maybe you use a bucket instead of an electric head. etc etc.

 

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22 minutes ago, Floating Duck said:

I love general formulas. Not being sarcastic, I really actually do . It helps to quickly get an approximation of speed potential. Can't get lost with these being absolute numbers however...

Seems there are a couple of ways to go about approximating speed. The most useful ones (I think) all revolve around ratios of multipliers of some sort (in order to have fair comparisons across a wide range of boats);

- Multiple of sqrt(LWL) (the ol' classic 1.34 x LWL for leaded boats)
- Base Speed
- Multiples of wind speed (My personal favorite) 

So for example, using sqrt(LWL), If you have a traditional 4ktsb displacement boat;

- You will be able to normally go 1.34*LWL
- As @patzefran correctly points out, if you are surfing down a wave you might be momentarily higher, lets say 1.75*LWL
- Reduce weight to a ULDB, and you are now at 2*LWL
- Add partial assist foils (IMOCA), and you are at 3*LWL
- Full foiling? 4*LWL

What is interesting about all this, is that it really gives you an idea of how performant a boat is or can be! @gspot an F-25C might be able to reach higher multiples than say, an F24. Figuring out how and why that might happen, is then fun part. Maybe you are full carbon. Maybe you don't have bottom paint. Maybe you only sail in perfectly flat lakes. Maybe you use a bucket instead of an electric head. etc etc.

 

I favor the aussie texel version for base speed to compare boats across platforms.  I don't bother with all the add ons and political crap that got added in for racers (where you have to measure all the sails at different spots along the luff, and how much the rotating mast length is, and ....).  You can use published numbers although you should realize that the published weights are most often wrong by up to 25%.  OMR and Multi2000 prefer to use a TCF, a time correction factor, so their basic formula is even simpler: TCF = rl ^0.3 * rsa ^ 0.4 / rw ^ 0.325.

Basically  a dimensionless number length to the 0.3 time sail area to the 0.4 all divided by the weight to the 0.325.  Gives a good comparison.  But it doesn't account for lots of things like chop if you sail in it routinely or course selection if you are always on a reach--also doesn't account for pointing ability, some boats don't point worth a damn while others point like champs.  You should just use the upwind sail area since you spend more time going to weather than downwind.  If you want to add in the downwind kite, I think the rule of thumb is to use 15% of the kite area.  You should also add crew weight for your normal crew size--can even use it to estimate how much slower you will be with an additional body on board.

http://www.texelrating.org/site/pub/Pagina.php?paginaid=81

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2 hours ago, gspot said:

And an F-25C/82R can only do ~ 18.8 knots?  

Sure I'll take a rating based on that!!!

With a multiplier of 5.3 that makes it ~ 26.2 knots..... How does that sound? Or better still, what is you multiplier?

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31 minutes ago, Sidecar said:

With a multiplier of 5.3 that makes it ~ 26.2 knots..... How does that sound? Or better still, what is you multiplier?

That seems plausible based on the reports of others with more experience on the boat.

We're still learning the boat but 15 knots is "routine" under all sail plans and we've done just over 20 under kite without really pushing the boat too hard, lots of flotation left etc. 

We had the opportunity last weekend to really light her up with good breeze and flat water, but unfortunately our rudder cassette broke just after rounding the windward mark. so we put a triple reef in the main and sailed the last 10 mile leg to the finish by steering with a canoe paddle and sail trim, averaging 6-8 knots with blips over 10 with no rudder! 

Airwick was in the same race and hit 17 knots on his F-24 mk II on that leg - if only our rudder hadn't broke!!!

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2 hours ago, Floating Duck said:

Seems there are a couple of ways to go about approximating speed. The most useful ones (I think) all revolve around ratios of multipliers of some sort (in order to have fair comparisons across a wide range of boats);

- Multiple of sqrt(LWL) (the ol' classic 1.34 x LWL for leaded boats)
- Base Speed
- Multiples of wind speed (My personal favorite) 

Don’t get too carried away with it ..... 

Most “contemporary” monohulls can do a lot better than 1.34*LWL^0.5, even before you get into ULDB’s and canting keels, without surfing.

Multiples of wind speed? You mean Bruce Number? Which gives an indication of power, not speed. And a boat with a high BN can’t necessarily carry it in winds needed for max speed, but it can help to get the averages up.

This from Richard Woods:

"People try to simplify hull design and performance predictions, formulae like the Bruce Number and KSP spring to mind. These coefficients rely only on basic sail area, displacement and length dimensions yet purport to give an accurate indicator of performance. It's easy to show that these formulae cannot be relied on if you consider that a Tornado would have the same rating whether it was sailing forwards or backwards! I suspect the latter is slower! Its probably as accurate as predicting car speed from the kerbside weight and engine horsepower."

And in fact since the BrNo has no length factor the answers are especially meaningless. For example, try a boat with length 1ft, sailarea 1sqft and weight 1lb. How fast will it really be?

A better simple multihull performance indicator is the Texel rating

http://www.texelrating.org/pub/Pagina.php

 

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My experience on the F-24 is that there is a definite "wall" between 19-20kt: I have done in the 18s quite a few times but only exceeded 19 a couple of times.
Things feel under control at 18 but very different at 19...
This was all in flat-ish water (i.e. chop only) so with a waterline length of 23.5ft that's a multiplier of around 4...

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34 minutes ago, Airwick said:

My experience on the F-24 is that there is a definite "wall" between 19-20kt: I have done in the 18s quite a few times but only exceeded 19 a couple of times.
Things feel under control at 18 but very different at 19...
This was all in flat-ish water (i.e. chop only) so with a waterline length of 23.5ft that's a multiplier of around 4...

Do you think your "wall" is a function of flotation, drag from underwater appendages, or something else?

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31 minutes ago, gspot said:

Do you think your "wall" is a function of flotation, drag from underwater appendages, or something else?

It is mostly drag based on my F242 experience.  The drag is from beam and folding system appendages getting into the water.  Back in 2012 I posted a video of my boat going through the finish line with 4 people on board at 19 kts (actual sail was in 2004).  Gust hit at the finish line and you could get a good view of what was the drag.  Unfortunately, that video is blocked by YouTube because of the music I used in it.  If you aren't in the USA you might still be able to view it, title is "Best of Puppeteer".  

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Just found the polar below for a Seacart 30.....Max Speed by that data set is 24 knots, which gives a multiplier of 4.4.

I suspect that  multiplier possibly increases with boat length due to relatively less wave interference at least until you get to “wall” speeds of ~40 knot displacement/planing due to non lifting foil drag and ~ 55 knots on lifting foils due to cavitation.

Interestingly, MOD 70’s and AC75’s have fairly similar numbers all round, including Bruce Numbers and Base Speeds in displacement mode. So even without fully lifting foils, an AC75 (so designed) should be able to do ~ 43 knots top speed.

But getting back slightly more to topic, if you play around with the 3 main performance variables, increasing LWL by, say, 10% has significantly more  impact than 10% increase in SA and even more if you just reduce Displ by 10%... You are better of by reducing weight and then putting it back again as extra length.

Seacart 30 data suggests: 15.6, 15.3 and 15.1 knots respectively up from 14.8 knots Base/Ave Speed.

 

 

766F3A81-FB8B-49A6-BC18-1BDA458A9E43.jpeg

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3 hours ago, Airwick said:

My experience on the F-24 is that there is a definite "wall" between 19-20kt: I have done in the 18s quite a few times but only exceeded 19 a couple of times.
Things feel under control at 18 but very different at 19...
This was all in flat-ish water (i.e. chop only) so with a waterline length of 23.5ft that's a multiplier of around 4...

Presumably, a lot of earlier the tri amas were significantly shorter and less “speedy” shapes than their main hulls.....That would bump up the  multiplier a bit? Most recent tri amas go for same (immersed) LWL (or on some Graingers tris, longer) than the main hull. Seacart 30 and Mama Tried being some?

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3 hours ago, Raz'r said:

Multi23 hit a wall at 18. It was the centerboard messiness.

I have been passed my Multi 23's when reaching, they are useless on most points of sail but they reach very fast.

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If you look at beach cat foils for up to say 20kts, the tri's have low speed foils yet can sail at higher speeds. Hence imho they are being held back
Corsair 28 has a rudder profile suitable for a J24 for example
I think its the modern ( beach cats) versus old traditional tri's with no real development class to copy.
The ama's look modern on some yes but foils??

Rudders that swing up, yuk

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7 hours ago, gspot said:

Do you think your "wall" is a function of flotation, drag from underwater appendages, or something else?

I'm guessing it's just a combination of things. The foils definitely are definitely fairly "fat" so that could be a good part for it, but I'm thinking it also has to do with the ama drag.
At that point the main hull is is mostly riding on the flatter aft section so should be able to skim at higher speed but the ama is plowing through the water and its drag might be increasing exponentially at that point.
It's also shorter overall so if you used the ama length the multiplier would be higher.

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11 hours ago, Sidecar said:

Presumably, a lot of earlier the tri amas were significantly shorter and less “speedy” shapes than their main hulls.....That would bump up the  multiplier a bit? Most recent tri amas go for same (immersed) LWL (or on some Graingers tris, longer) than the main hull. Seacart 30 and Mama Tried being some?

Speak of the devil:

F3C97D95-A425-47E6-A58B-F1733C3F4A9B.jpeg

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19 hours ago, MultiThom said:

It is mostly drag based on my F242 experience.  The drag is from beam and folding system appendages getting into the water.  

 

16 hours ago, Sidecar said:

Presumably, a lot of earlier the tri amas were significantly shorter and less “speedy” shapes than their main hulls.....That would bump up the  multiplier a bit? Most recent tri amas go for same (immersed) LWL (or on some Graingers tris, longer) than the main hull. Seacart 30 and Mama Tried being some?

 

11 hours ago, Airwick said:

 I'm thinking it also has to do with the ama drag.

At that point the main hull is is mostly riding on the flatter aft section so should be able to skim at higher speed but the ama is plowing through the water and its drag might be increasing exponentially at that point.

It's also shorter overall so if you used the ama length the multiplier would be higher.

Just to focus on the ama drag theory for a second, I have definitely noticed that the amas and struts of the F24/Sprint750s do plow through the water when the boat is loaded up.

The F-82R on the other hand seems to have much more flotation and/or dynamic lift in the amas than the F24/Sprint750. 

For example, here are some photos of our F-82R fully powered up and close reaching in about 20 knots of breeze. if it weren't for the choppy sea state, the bow knuckle on the leeward ama would be out of the water, whereby the leeward ama on the F24/Sprint750 would be substantially submerged in the same conditions.

Sitting side-by-side at the dock, the beams and struts of the F-82R are also about a foot higher out of the water than the F24/Sprint750, also resulting in substantially less drag when pushed hard.

Needless to say, length is only part of the equation!

image.thumb.png.956e46937cb684e2f36b0ebc6a06eaa0.png

image.thumb.png.78a28b3e690030f28b8711fde64d584a.png

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What happened on my F242 is when we're powered up with kite, I'm all the way back in the stern holding onto the mainsheet since the main controls the heel mostly and can keep you safe until the wind is so big that the main has no impact on heel, then you have to rely on the spin sheet crew to release that sheet to keep you from a pitchpole in a gust.  That crew is also aft and outboard and the sheet is wrapped around both winches to give grip but not in the self tailer.  The boat, fore-aft, is then bow up...which also means the aft leeward ama is submerged (like you can see in the photo above).  The outboard beam typically hits before the strut.  Trouble is, the gusts usually drive the bows down as well so   you    just     stop and the stern pops up.  If everything is released, you plop back down, clean your shorts and start sailing again.  One of the reasons I got rid of the cam cleat on the mainsheet in favor of a spinlock was the fact that if you are really loaded up (wimpy mainsheet cascade on stock F242 was only 5:1--8:1 was much better), you can't unload the mainsheet quickly.  Had water at the mast base on the main hull that time.

About fat foils...I don't think that was much of a factor since most of the time on my boat I had the daggerboard lifted when going downwind with kite.  

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