BACKGROUND
After Booth successfull rebuilt of the M18 I'm thinking about adding a second trapeze, a bigger mainsail and spi to my Marstrom A-cat. I know that Booth didn't win but my interest was focused on the fact that the boat could handle the extra loads without any failures!

Booth used the M18 with 150kg+ of on the wire and used a main beam that had a weight of 50 kg instead of 5kg. The reason for the heavy mainbeam was to reach the minimum weight of 130 kg. Booth used a cut down M20 mast, I will try to use the A-cat mast as it is.

Should I do it?

THE PROJECT

Crew
It will not be designed for "Team beef" but for teams around 130-140 kg, same as Hobie 16.

Sail
The area can be increased from 13.9 to maybe 16.5 m2 by adding more roach area to the main sail. The spi should be around 17 m2.

PROBLEMS
Mast
The top of the mast may be to bendy for the bigger roach and increased loads. The performance to windward in medium winds could suffer from this.
Increased compression of the mast by 30%, reason higher sheet load and another crew on the trapeze.

Main beam
Increased load on the mainbeam by 40%, reason higher sheet load and double trapeze. Booth had 50-60% more load on a WIDER main beam.

Hull
The hull needs to support 220 kg instead of 165 kg which means that the hull will be 4 cm lower in the water. The hull on Booth F18ht supported at least 280 kg and that hull was probably 10 cm lower in the water.

Performance
The boat is narrower than the F16 and 25 kg lighter but the sail area will be lower, 16.5 m2 instead of the F16 18 m2. The lower sail area will be compensated a bit by a 1 meter higher mast and uni rigg instead of main+jib.

/hakan

BACKGROUND
After Booth successfull rebuilt of the M18 I'm thinking about adding a second trapeze, a bigger mainsail and spi to my Marstrom A-cat. I know that Booth didn't win but my interest was focused on the fact that the boat could handle the extra loads without any failures!

Booth used the M18 with 330+ lbs of on the wire and used a main beam that had a weight of 110 lbs instead of 11 lbs. The reason for the heavy mainbeam was to reach the minimum weight of 286 lbs. Booth used a cut down M20 mast, I will try to use the A-cat mast as it is.

Should I do it?

THE PROJECT

Crew
It will not be designed for "Team beef" but for teams around 286 - 308 lbs, same as Hobie 16.

Sail
The area can be increased from 148 sq.ft to maybe 176 sq.ft by adding more roach area to the main sail. The spi should be around 180 sq.ft.

PROBLEMS
Mast
The top of the mast may be to bendy for the bigger roach and increased loads. The performance to windward in medium winds could suffer from this.
Increased compression of the mast by 30%, reason higher sheet load and another crew on the trapeze.

Main beam
Increased load on the mainbeam by 40%, reason higher sheet load and double trapeze. Booth had 50-60% more load on a WIDER main beam.

Hull
The hull needs to support 480 lbs instead of 363 lbs which means that the hull will be 1.5 inches lower in the water. The hull on Booth F18ht supported at least 616 lbs and that hull was probably 4 inches lower in the water than normal.

Performance
The boat is narrower than the F16 and 55 lbs lighter but the sail area will be lower, 176 sq.ft instead of the F16 192 sq.ft. The lower sail area will be compensated a bit by a 3 feet higher mast compared to the F16 and uni rigg instead of main+jib.

/hakan

Hello Hakan,
The max loads due to increased righting moment go up in small parts also. The max bending loads in daggerboards and daggerboard trunks go up directly directly with righting moment. Max rudderblade, rudderhead, pintles, gudgens, gudgen bolts and transom increase with righting moment. Max chainplate loads increase as well as chainplate bolts and the hull area immediately around the chainplate.
On a boat like an A class cat that has been through a major weight reduction program, all the fat has been taken out, there is little to no margin for increased loads anywhere. Taking a new/modern A class boat and adding a bigger rig and double trapeze is risky business. The devil is in the details.
Bill
PS If the mast bends too much, raise the hounds, ie 30% if the mainsheet loads are up by 30%. Also add double spreaders below to reduce the max unsupported mast span.

Hakan,

I might suggest (if you haven't already done so) you contact Goran on this project with your concerns. Since he built your boat, he is the one who can tell you the reality of your project. Good luck!!

Bob

Hello Hakan-

Both Bill and Bobcat have good points. One factor I would take into account is the second crew/s height and weight. I saw two crew that were at least 120lbs apart [150 + 270] and both on the trap. Of course the heavy guy was forward and the bow was buring alot. What is the bouancy factor of your boat hull? I have heard there are two 22s in the Pcola area with a full load out sailing.

Send me a pic of what you're doing,

thom

I haven't done any real modifications to the boat yet, I'm still trying to get more facts.

The main reason for this idea is that the A-cat platform from Marstrom seems to be so strong that it could handle double trap and spi without breaking.

If we use the Booth configuration of his F18ht as an upper limit and remove 50 kg (110 lbs) of boat weight and 20 kg (44 lbs) of crew weight and reduce the beam from 2.5 to 2.3 meters it might work. All the racing Booth did was in flat water or? Bigger waves might lead to failure....

/håkan

The Marstrom A Class hulls are famous for their strength.

According to the Marstrom website:
The M18 uses the same hulls from the A Class,
Is 2.5m wide, instead of 2.3m,
The mainsail is 15sm instead of 13.94sm,
The boat weighs 78kg instead of 73 kg,
And of course it has a 15sm gennaker.

I doubt if Booth would even have tried to build his boat without checking every single modification with Goran Marstrom.
Supposedly the ICCT boat was made fron the same hulls and hardware with an M20 mast.
The hulls didn't break.
The mast didn't break.
The daggers didn't break.
The rudders didn't break.
The hardware didn't break.

I think the issue of strength has been resolved.

To all the above.........Yet......

The boat was used for a couple of weeks max and it has not broken yet.

I would urge caution and contact Goran Marstrom and discuss it with him.

And while you are talking to him, ask why they have not responed to my enquiries about the M18

>>The Marstrom A Class hulls are famous for their strength.

This is not a correct conclusion from this test. As a matter of fact Booth made skilled use of how the design parameters behave when loading up a platform.

Let me run through the points


>>Is 2.5m wide, instead of 2.3m, : Does not seriously increase loads on the platform. the increases are accompanied by equal magnitude reductions.

>>The mainsail is 15sm instead of 13.94sm, ; Doesn't matter ; righting moment determines the laod on a platform not the sail area. Amount of sailarea only determines how much more you need to depower a rig at the same high wind conditions

>>The boat weighs 78kg instead of 73 kg, ; This is neglectable


>>And of course it has a 15sm gennaker. Spinnaker does not introduce higher loads on the sidestay than when sailing upwind. It does not load the hulls up more than a jib does on the same upwind course. I haven't done the calcs on mast compression yet. Mast foot and beam however are unimpressed by a spinnaker. The increase in load is simply to small when compared how much a beam is overdimensioned to get the optimal beam stiffness. Beam is never in risk of breaking only flexing to much.


>>The hulls didn't break. : The hulls are build to resist shock and point loads this make the hull alot stronger than necessary for the sailing loads. Only serious point of concern are the sidestay chainplates. These do experience increasd loads to 170% to 200 %


>>The mast didn't break. : It was a M20 mast ; it was designed for double trapeze spi use.


>>The daggers didn't break. : This is interesting, These see increased loads to again 170% - 200 %


>>The rudders didn't break. : Rudders are less critical than the daggerboards. so if the dagger hold ..

>>The hardware didn't break. : Yes well, that is because for small open boats there is pretty much one line of breakstrength used on all boats from 14 foot to 22 ft.


>>I think the issue of strength has been resolved.

Yet but the issue of flexing hasn't. For example It is not difficult to design a mast thatwon't break but it is difficult to design a mast that won't break and flexes in the right way and to the right amount to make a boat well behaved.


>>The boat was used for a couple of weeks max and it has not broken yet.

This is also interesting as indeed damage to beams and other stuff is nearly always the result of metal fatigue and low tension ruptures. These two do require above anything else "time". Still I trust Goran to design his products in such a way that both failing mechanism are unlikely to occure on his boats. Mostly these tow failing mechanisms are the result of lapses in design.


Wouter

Hakan,

I was a competitor at the LAC so had a close up look of the "M-18HT". I believe the reality was a world class talent sailing the boat resulted in it doing as well as it did.

The "M-18HT" simply did not float on her lines. Mitch felt that the boat could be fast in light air and flat water. As expected, the boat was difficult to sail downwind in breeze. The beams were slapping the waves and Herbie was having a difficult time staying on the wire. From our observation, the boat looked like it was pitching a lot and the waves and chop were not that big (offshore wind).

The boat was no faster than the Bimare boats. In the light air racing, it appeared to be no faster upwind. Downwind, Enrique and Jorge always seemed faster (Mitch lost the challenger berth on a downwind leg).

You do have to remember that nearly 50 kg of weight was added to the boat to make it measure in to the HT class rules. That did not help. Mitch did not know what he was going to do with the boat (I don't think he and Herbie plan to race it). He felt if he sold it, he would remove the 50 kg that was added to it. You could then no longer race it as an 18HT.

Bottom line - you can do it, the rig and platform may be able to handle the loads but you better keep that crew weight very light. I second others in you should consult Goran.

Bob Hodges

I talked to Goran today and he has the same opinion as Bob, the boat will not float on her lines and it will loose the A-cat feeling.

My calculations gave me a floating line that is 1.5 inches above the correct one and that might be enough to loose that A-cat feeling, but will the boat be slower?

To sum things up, the boat can handle it but it lacks volume in the hulls and the mast might be to flexible to handle more roach on the main.

I'll try to do some tests later on with a total crew weight of 130 kg (286 lbs) and see how it compares to a singel handed A-cat.

Thank you all for your input!

/hakan

You estimate 40 mm extra draft when putting some 140 kg (double) - 75 kg (single)= 65 KG extra on it.

My estimate would be at least 50 mm, probably more. (When sailing on one hull, the other just lifted clear of the water)

Typically I use as "an rough extra draft" estimate the following formula :

(Length hull * width at widest point deck * 2/3 * 1/1000) * extra draft in mtr = kg's extra

or

5.49 * 0.30 * 2/3 * 1/1000 * X = 65 This leads to an increase in draft of 59 mm. 50 % more than your 40 mm estimate.

Even when using a shape factor of 3/4 (instead of 2/3) I arrive at 53 mm Increase in draft. Theoretically the shape factor of 3/4 is linked to a hull that is rectangular in crossection and that from the top looks like it is made up of 1 rectangle of halve the hulls length and one triangle of the halve the hulls width. It can easily be argued that this is the most extreme case with the lowest increase in draft. Real draft figures MUST be higher than any estimate this 3/4 approach produces.

P.S. The 2/3 approach compensates for the roundings of the hull from the 3/4 basic shape but still can be regarded as a underestimating approach.

Wouter

Surely Booth and Marstrom knew before the A class hulls were put together as a two person boat with spin that the narrower hulls would sail lower in the water to find the displacement to support the total sailing weight. That is a given. What it comes down to is a trade off between friction drag, wetted area drag, and wave making drag. The finer A class hull will sit lower in the water and have more wetted area and have more friction drag. At speed where wave drag becomes important the finer A class hull will have a lower wave drag characteristic. A wider hull of the same length will float higher in the water and have less wetted area and therefore have less friction drag. At speed this wider hull will be pushing the water further sideways to move ahead each boat length and this makes more wave drag. So what it comes down to is that in light winds you want fatter hulls with less wetted area and in strong winds you want finer hulls with less wave making drag. Everything is a compromise.
Bill

Maybe that is why Shark catamarans do so well in light air?

billrob

Your arguement leaves out the rear beam slap factor. When a lightweight, tall rig boat, like the HT (or a modified 2 man A cat with a spin) is sailing in chop downwind the rear beam will hit the chop and cause the boat to pitch forward. Read Bob's discussion above, he witnessed this in action. The PUR boat was pitching in the chop and the crew had a hard time staying on board. If the boat is slapping and pitching in the waves, you are slowing the boat dramatically and if the crew doesnt have his wits about him, he will get thrown forward and then you are swimming. So your arguement is flawed unless you have flat water, which usually doesnt happen with higher winds.

A two man spin boat needs more freeboard, not less. The rear beam needs to be up out of the chop. Most the HT's have raised the rear beam and this solved the problem.

So I would argue that a wider hull - like the Bimare HT Hull would work better than a finer hull in practice.

Bill

Hi Bill,

The speed at which wave making resistance dominates is actually at the lower end of the speeds beach cats operate. Once they exceed about Fn=.5 viscous resistance is the largest term, and it quickly increases with increasing speed.

Talk to you later
-colin

Bill,
I was only trying to point out the difference in boat performance due to hull "Fineness Ratio". I was not attempting to explain the race results.
If the Booth boat had 100 pounds added to the main beam near the center of pitch rotation and the other boats have that same weight spread out over the entire length of the boat, there is going to be a very large difference in pitching moment of inertia between these two configurations. The boat with the centralized weight sailing downwind is going to pitch more and to a greater degree and more quickly and more violently than a boat with the weight distributed over its entire length. The low pitching moment of inertia boat is going to be more difficult to sail downwind than the standard boat.
The boat with the low pitching moment of inertia very well could find a slight speed advantage sailing to windward in a chop where it could respond more quickly to the changing slope of the surface of the water. The normal boat will tend to crash through the chop/waves more where the low pitching inertia boat will tend to ride up and over the waves and not crash through the waves as much.
This is not the first time the F18ht rear beam dragging problem has been brought up on this forum. Sounds like a design goof to me or a boat designed to sail on small lakes, small chop.
Bill

AS USUAL WEEZY IS ARGUING ABOUT SOMETHING THAT WAS NOT SAID OR IMPLIED. HE JUST DOESN'T UNDERSTAND ENGLISH VERY WELL.

>>The Marstrom A Class hulls are famous for their strength.

This is not a correct conclusion from this test. As a matter of fact Booth made skilled use of how the design parameters behave when loading up a platform.
WHO SAID IT WAS FROM THIS TEST? THE MARSTROM HAS BEEN FAMOUS FOR MANY YEARS FOR STRENGTH. WHY DON'T YOU KNOW THIS?

Let me run through the points


>>Is 2.5m wide, instead of 2.3m,
: Does not seriously increase loads on the platform.
I DIDN'T USE THE TERM "SERIOUS", YOU DID. BUT YOU AGREE THAT LOADS ARE INCREASED.
the increases are accompanied by equal magnitude reductions.
ONLY IF NOTHING ELSE CHANGES.


>>The mainsail is 15sm instead of 13.94sm,
; Doesn't matter ; righting moment determines the laod on a platform not the sail area.
WRONG! A SAIL IS SUBJECT TO INSTANTANEOUS LOADS FROM GUSTS AND OTHER SHOCKS. YOU NEED TO LEARN ABOUT "ACCELERATION". YOUR SIMPLETON STATIC CALCULATIONS DON'T DETERMINE MAXIMUM LOADS. COMPARE THE INSTANTANEOUS LOADS OF A SLOW FLIP TO A VIOLENT PITCHPOLE.
Amount of sailarea only determines how much more you need to depower a rig at the same high wind conditions
WRONG!! SEE ABOVE


>>The boat weighs 78kg instead of 73 kg,
; This is neglectable
NO DUH!!! THAT IS MY POINT. THE ONLY WEIGHT GAIN IS CAUSED BY THE ADDITIONAL EQUIPMENT.


>>And of course it has a 15sm gennaker.
;Spinnaker does not introduce higher loads on the sidestay than when sailing upwind.
I DIDN'T SAY "SIDESTAY". SUPPOSEDLY, WHEN SPINNAKERS HAVE BEEN TRIED ON SOME A CLASS BOATS THE SIDE LOADS RIPPED OFF THE WINDWARD BOW.
It does not load the hulls up more than a jib does on the same upwind course.
THE SIDE LOADS ON THE BOWS FROM A SELF-TACKING JIB ARE MICROSCOPIC COMPARED TO A SPINNAKER.
I haven't done the calcs on mast compression yet.
DON'T BOTHER, IT WILL BE WRONG.
Mast foot and beam however are unimpressed by a spinnaker. The increase in load is simply to small when compared how much a beam is overdimensioned to get the optimal beam stiffness. Beam is never in risk of breaking only flexing to much.
MARSTROM ALREADY CALCULATED THE LOADS AND BUILT THE BOATS AND THEY DIDN'T BREAK. WHO ARE YOU?


>>The hulls didn't break.
: The hulls are build to resist shock and point loads this make the hull alot stronger than necessary for the sailing loads.
NO DUH AGAIN!!! REMEMBER, I SAID "FAMOUS".
Only serious point of concern are the sidestay chainplates.
These do experience increasd loads to 170% to 200 %
WRONG, WRONG, WRONG!

>>The mast didn't break.
: It was a M20 mast ; it was designed for double trapeze spi use.
NO DUH THREE!!!

>>The daggers didn't break.
: This is interesting,
WHAT DOES THAT MEAN?
These see increased loads to again 170% - 200 %


>>The rudders didn't break.
: Rudders are less critical than the daggerboards. so if the dagger hold ..
WRONG!! YOU CAN'T RACE A BOAT VERY WELL WITH BROKEN RUDDERS.

>>The hardware didn't break.
: Yes well, that is because for small open boats there is pretty much one line of breakstrength used on all boats from 14 foot to 22 ft.
WRONG!! THAT IS YOUR MOST RIDICULOUS STATEMENT YET. IF YOU ATTENDED REGATTAS YOU WOULD SEE THE DIFFERENT SIZES OF HARDWARE ON THE DIFERENT BOATS.

>>I think the issue of strength has been resolved.

Yet but the issue of flexing hasn't.
WHO SAID THE MARSTROM HAD A FLEXING PROBLEM? EXCESSIVE FLEXING OF A COMPOSITE MATERIAL CAUSES EARLY STRUCTURAL FAILURES. WHEN HAVE MARSTROMS FAILED?
For example It is not difficult to design a mast thatwon't break but it is difficult to design a mast that won't break and flexes in the right way and to the right amount to make a boat well behaved.
DID YOU FIGURE THAT OUT YOURSELF OR DID YOU READ ONE OF THE HUNDRED DISCUSSIONS ABOUT THAT SUBJECT?


>>The boat was used for a couple of weeks max and it has not broken yet.

This is also interesting as indeed damage to beams and other stuff is nearly always the result of metal fatigue and low tension ruptures.
WHO SAID? WHAT ARE YOUR SOURCES FOR THIS STATEMENT?
These two do require above anything else "time". Still I trust Goran to design his products in such a way that both failing mechanism are unlikely to occure on his boats. Mostly these tow failing mechanisms are the result of lapses in design.

Wouter

PITIFUL, JUST PITIFUL!!!

>>MARSTROM ALREADY CALCULATED THE LOADS AND BUILT THE BOATS AND THEY DIDN'T BREAK. WHO ARE YOU?


Well, I'm actually the guy that took a lightweight design of smaller dimensions and modified in almost every way as described in these posts.

This "fantasy boat" as you like to call it, is sailing and still very much in one piece, is now being used by AHPC as the blue print for their new Taipan F16 and has improved on the older design in stiffness among other things.

I didn't just guess at what would happen, I worked it out before applying the mods to the boat.

Having gone through all the stages from conception to completion and validation I think I can claim a little knowlegde in this respect and with regards to the mods considered by Hakan.

However, the question that remains is :

SO WHO THE *%#$@ ARE YOU in this little pissing contest ?

Wouter

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