As in the topic is a narrower boom any better? There's such a variation in hand sizes out there. Is going from 27mm down to 25mm really going to make any difference, it's only 2mm ok about 6mm in circumference. What are the supposed benefits? Any scientific studies been done on this? My bike handle bars are 22mm plus grips 3mm thick = 28mm.
Anyone know what size is a tennis racket handle or golf club handle is, you'd think those high earning sports would have researched the best ergonomic diameter.

Tennis racquet grips are different sizes. I played squash at state level and I used a very narrow grip, I didn't like the fat grip on grip feel.
When you are hanging your weight off the boom the diameter can make a difference to how well/long/tight you can maintain your grip. Easily enough to exaggerate a test , hang on a monkey bar and hold onto the different thickness poles to see which is easiest.

It's not till you try your old thicker boom that you realise the difference .
Golf grips come in a few different sizes and can feel different depending on the tape under them if any , a major improvement I found is with the putter - a huge super stroke grip on the handle made finding cups alot easier for some reason

for me it's more about the grip comfort than the diameter.

some booms have thin/harder grip than others and some have nice plush grip.

apparently thinner is more comfortable and you can sail longer etc.
I did notice some comfort benefit of the 25mm booms.
It was however a very minor thing, going back to a 27-28 now I can still sail the same but they are far stiffer, so I don't think the thin boom is all its made out to be for larger folks.

Im a largish bloke. Definately feels more comfy n easier to grip my NP RSX boom than the larger diameter Severne AluRace boom. Horses for courses though id think.

I'm only 61kgs & coming down to Severne Engima was a godsend for my little hands. Feels way more comfortable in the waves

Size of hands plays a role. It seems that a smaller grip may pinch the skin more but are better with gloves.
I never had issues with wider grips even with gloves though. I find harness lines length, harness adjustment and boom height/mast track position more critical.

Depends what you are used to. What diameter is the first thing you grab hold of in the morning?

Well normally mine is a mug of tea first but I am an OAP...But I was recently talked into thin booms and regretted it, mainly because I have big hands and a 28mm is no problem at all, whereas with 26mm its as if I am holding a pencil. My wife a petit 5 foot just loves the 3 new booms... so now I am selling one... and keeping 28mm. Obviously I should have fully tried one first but thats difficult from Exmouth.

I'm a windsurfer and a flatwater kayak coach. For paddle shafts, the thinner ones result in a lower chance of developing 'tennis elbow ', the pain in the outer part of the elbow. Simply easier to gang on to with less 'grip' needing to be applied by the hand.
Guessing the same for booms.

When I started windsurfing in the late 90's, booms were 30-32mm and was the norm. I consider 28 skinny.

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TD55 said..
I'm only 61kgs & coming down to Severne Engima was a godsend for my little hands. Feels way more comfortable in the waves

Yes being small with little hands I love the smaller diameter ones.