Righting moment question- expertise required
Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=11523
Printed Date: 12 Jul 25 at 1:02pm
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com
Topic: Righting moment question- expertise required
Posted By: flano
Subject: Righting moment question- expertise required
Date Posted: 21 Jun 14 at 4:15pm
|
Please help I'm currently sailing a 16ft skiff, which a lot of you guys would probably not be that familiar with. Class rules specify 3 crew but only 2on trapeze. My current crew consists of myself, skipper, hiking at about 75kg at 6ft6". On traps my sheethand is a big bopper at about 115kg and 6ft 3" with my f'hand around 5ft 10" and 70kg. My question is, if I decided to start steering from the wire and swapped my f'hand with a 40kg kid who would now take up the hiking role on the boat, potentially how much righting moment would I be likely to lose? The goal would be to obviously drop 30kg out of the boat without compromising our righting moment too greatly. To work it out accurately I imagine you would need to know the width of the boat which is close enough to 1.8mt. The ultimate goal would be to give the 16ft skiff national titles a real shake. Anyone that can give me some solid feedback on this will get a significant thanks in my acceptance speech after taking the crown. Also be welcome for a beer at my place whenever visiting our tops sailing Mecca in Belmont, Australia.
|
Replies:
Posted By: flano
Date Posted: 21 Jun 14 at 4:27pm
|
Thought I'd add a photo just to give you a bit more to work with 
|
Posted By: Daniel Holman
Date Posted: 21 Jun 14 at 4:35pm
|
I get 5094Nm at current config, 4922Nm if you get the kid hiking (assuming a 40kg kid is 1.5m and hikes pretty hard) so 3.5% RM loss which is fairly minimal tbh. Don't know lightship for a 16, lets assume 130kg giving 400kg sailing displ in current trim - you can extremely crudely assume resistance is proportional to displ, so you will lose 8% of displ. I reckon that would help esp in a lighter series, assuming the kid can hike / trim / take abuse / hand out the beers / manage all the fishing tackle :-) Looking fwd to redeeming by beer. |
Posted By: flano
Date Posted: 21 Jun 14 at 4:40pm
| Daniel, your a legend. Exactly the type of info I was after. Probably should have mentioned that crew work is likely to take a significant hit as well. So any change would need to see a substantial improvement in straight line speed to overcome a slight loss in going round the corners. |
Posted By: flano
Date Posted: 21 Jun 14 at 4:44pm
| Not sure on overall weight of the boat when it hits the water. Hull weight is about 74kg with all fittings. Mast around 15kg, boom 3 or 4kg + the rest I'd assume we'd be close to 100kgs all up. |
Posted By: Daniel Holman
Date Posted: 21 Jun 14 at 4:48pm
|
Cool. Well a good I14 is 100kg when it hits the water (measurement scope is ex mast, boom, foils, sails etc) and I assumed there would be a bit more boat with the 16 - built in wings, more surface area etc. All the best with it! |
Posted By: PeterG
Date Posted: 21 Jun 14 at 4:49pm
|
The answer wouldn't be that hard to work out - though having the kid's height would help (makes less difference hiking that trapezing, but it's still relevant. But all anyone could come up with as a number would be something like the % loss of righting moment. To convert from that to would it improve your chances of winning would depend on a lot of other factors - how windy will the races be not being the least. Are you generally overpowered, or underpowered? My advice would be give it a go, and if you could do it with another boat to sail against so much the better. For what it's worth, making some simple assumptions, I reckon your total crew righting moment is going to be be reduced from something like 406 kgm to 391kgm, while total boat mass goes down by 30kg. So not a large drop. A definite winner in light wind (if the kid is as good a foredeck hand!), might cost a bit in strong winds though? Whatever you do don't sack the sheet hand - he makes up more than 50% of the crew righting moment! ------------- Peter Ex Cont 707 Ex Laser 189635 DY 59 |
Posted By: flano
Date Posted: 21 Jun 14 at 4:59pm
Don't worry, I won't be sacking the sheethand, look at the size of him. Not the sort of bloke you want to get offside .
|
Posted By: rb_stretch
Date Posted: 21 Jun 14 at 7:10pm
| I'm amazed you can get down to 75kg at 6'6". I would consider myself a slim 6'6" and come at 97kgs. If I could get near your weight I might even be able to try a moth....10 years of trying suggest this will remain a dream.... |
Posted By: Daniel Holman
Date Posted: 21 Jun 14 at 7:29pm
|
Pretty sure Giles Scott is 6'6" and must be about 97 kg, and looks pretty slender. Edit - he was low 80s at that height in his laser days and was a beanpole! |
Posted By: flano
Date Posted: 22 Jun 14 at 4:11am
| Yeh I've always been super thin. Just one of those blokes who can't put on weight. The Moth might be the next item on the bucket list. |
Posted By: Chris 249
Date Posted: 23 Jun 14 at 8:07am
|
Flano, I think Scotty Ellis skippered from the wire to 3rd place in the nationals a few years back, didn't he? Someone around the club should be able to tell you how it worked out. |
Posted By: flano
Date Posted: 23 Jun 14 at 1:21pm
|
Yeh sailed against Scott that year, actually my brother in law sailed sheethand with him. Scott was definitely one of the guys I thought of when going through this process. He did a good job of it but his forward hand, although small was still a great hand and I think took the wire off the breeze all the time. Another guy who did it was Bruce Savage and he was runner up in Geelong back in 2007 but same formula as Scott. If I were to go down this path i would have to committ to steering from the wire right round the track. Also my sheethand would have to take on most of the forward hand responsibilities.
|
Posted By: Chris 249
Date Posted: 24 Jun 14 at 12:01pm
|
Ah, OK, sounds like you've got it sorted. I may have sailed with your brother in law once; my brother (one of Scotty's arch rivals from Laser days) did the first year in the 16 with him, and I was going to be their fill-in crew if I was ever needed. If I'm thinking of the right guy, he was good - and I'm also feeling old if he's married these days! |