Balance of a trimaran: NEEL 51

Dear experts
I am the happy owner of a NEEL 51 trimaran. So far sailed 7000 nm. Over this period we have both learned a lot and optimised a lot. Love the layout, volume and performance.
But, she has one "issue" that I have not been able to fix. She has too much weather helm for my taste. Often the rudder is in 8-10 deg both on broard reach and lighter winds as well as upwind in stronger winds.
Mast had a rake of 4 deg when we took ownership and I have reduced that to 2 deg but without any totable change on weather helm.
We of cause do what we can to trim the sails and when we are really good we may achieve 5 deg weather helm.
I am considering to reduce rake further, but will need to shorten the forestay. So, before going there I thought I might ask some expert advice and thoughts.
Does multihulls have the same considerations regarding CE and CLR as a monohull or are there other variables to consider? Is it realistic to obtain less weather helm or is this just the nature of big trimarans?
Other considerations:
- boat is heavier than "light displacement racing configuration" because of all the equipment and reinforcements put in by NEEL. This might influence the balance compared to the architects dream. She rarely lifts a hull.
- performance version with carbon/dynema mast/rig.
- not possible to sail her with just the main.
- perfectly possible to sail her with just the genoa or code-0 even upwind.
- at some point we considered to add 4-5 foot to the stern or increase the length of the empty keel, but sure that would require some careful calculations first.

Any thoughts?
Thank you!

 

I'm far from an expert but you asked for advice and thoughts ;-).

Not much advice, your craft is almost infinitely more massive than what I'm used to where my weight is a significant portion of displacement. I'd check with the original designer for advice.
Thoughts; It may well be that the boat is really happy with 8 to 10 degrees of weather rudder even if the skipper isn't, beats the heck out of lee helm, if you can tack through 90 deg +/- with reasonable leeway let it be and enjoy the boat.

 

Thank you Skip Johnson. I have tried to contact the designer, but with no luck (which is also what I hear from others who have tried). When tacking upwind we can go 32 deg to apparent wind, but have 15 - 20 deg leeway making our track more like 100 - 120 deg tacks. Going less high say 40 deg does not reduce the leeway enough to compensate. I should have mentioned this in the original post as this is also a reason we would like to improve the balance.

 

Thanks for the info, that's a lot of leeway. I'm intimately familiar with the situation on a much smaller scale. It's unfortunate the design source is silent.
Probably more keel area forward would be technically the best solution, but a fair amount of work.
Best wishes on a solution

 

Adding to the hollow fiberglass keel should actually not be the hardest job (more difficult is to find a haul out for 9 meter beam). But I am confused by your suggestion to add keel area forward. I am thinking I need to move the center of resistance aft and to do this add keel area aft eg. make the keel a foot longer aft?

 

Mea culpa, you're right (proanauts(nuts)) sometimes get confused.
Easier way to move CLR aft would be to enlarge rudder, might not have to haul boat. If the keel extension is free flooding it might be possible to attach same while in the water. OTOH a solid lightweight extension would add a bit if displacement on the correct side of CB I think.

 

Let me start by saying that it's not really possible to diagnose the problem remotely, so this are theoretical considerations and things for you to try.

Unless you know the builder did something specific to your boat that's different from stock, you shouldn't assume the boat is heavy because of them. This boat was designed for cruising, it's intended to have all the equipment on it in lightship. If the boat is overweight first to be looked at is the user.
One thing you should try is removing all the loose stuff from the ama's and moving them to the central hull. Even better would be to empty the boat entirely but that's not always possible. Then do a test sail and see how she behaves.

The fact that you already decreased rake without much effect makes me suspect your sails. If they are dacron and blown out, they simply have to much shape, and that moves CE. Most multihulls have very little "lead", out of shape sails can affect them much more. Flattening or reefing the main should have a marked effect on weather helm.
My advice is to take a sailmaker that does multihulls on a test sail, see what he says about your sails.

 

"Hydro Drag Angle" (eh)

Low Aspect Ratio Keel

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"When tacking upwind we can go 32 deg to apparent wind, but have 15 - 20 deg leeway making our track more like 100 - 120 deg tacks."

 

Upwind must also be taken into account when the wind increases the "Aero Drag Angle" (ea) increases

For Example

25 Knots Upwind
tiny 19.64 ft Boat, Monohull



Look

Aero Drag ... Hull (!): A20 and 21

Hydro Drag induced by the Lift (lateral force) produce by the Keel (a good one): A24

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They are large quantities (quite a few Newtons) that add up to a lot.

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Article Dogstar 50 https://www.shuttleworthdesign.com/Dogstar50-article.html

 

1967

"Aero Drag" ... Obsession

The doomed crew below deck:

sweating and sweating, moving the big winches while two enjoy the fresh air

 

"Hydro Drag" ... Obsession

 

A small detail that can be important in some cases:

Down/Sidewash

Downwash - Wikipedia https://en.m.wikipedia.org/wiki/Downwash

If the keel has a 10-degree Angle of Attack (Leeway = 10°) ....

if the rudder is behind the Keel, it may have/see a 5-degree or 1-degree Angle of Attack, depending on the keel's Aspect Ratio

 

Balance

Above in the first diagram the Hydro Drag based Balance is by Andrew Claughton, and the Aero Drag based Balance is mine.

https://www.andyclaughton.com/