Dinghy development: Trim

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Trim

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=11122
Printed Date: 15 Jul 25 at 10:59am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com

Topic: Trim

Posted By: hobbiteater
Subject: Trim
Date Posted: 04 Oct 13 at 9:21am

as a noob sailing those grp smods with asymmetric kites it is now obvious that one of the main things we need to do is get the weight forwards...

The question is why? - not the reason for doing that which i get but the reason why the rearmost 2 feet of boat are on my boat - if i take an alligator saw to that part surely the boat will be lighter, i wont be able to sit in it and it would be cheaper to make...

Is this a really stooopid question, i guess they no what they are doing so i guess the quest is still why but why is the last 2 foot of my boat there?

Strikes me as odd that these (most?) boats arnt designed to allow both crew to easily be positioned where they need to be 99% of the time - it gets more cramped the further forwards you go and the thwart (yes ive seen the icon thread) is always too far back...

Maybe i need to reach in a hoolies 0.0001% of the time sailing as im supposed to race up and down arent i?

Replies:

Posted By: iitick
Date Posted: 04 Oct 13 at 9:26am

If you stay forward ALL the time whatever the wind then it accounts for why you are wet a lot of the time.

Posted By: yellowwelly
Date Posted: 04 Oct 13 at 10:04am

Originally posted by hobbiteater

as a noob sailing those grp smods with asymmetric kites it is now obvious that one of the main things we need to do is get the weight forwards...

sounds like you are sailing it at the wrong location....

Posted By: RS400atC
Date Posted: 04 Oct 13 at 10:42am

Not sure which SMOD you refer to, but in light weather, in most boats, you need to get the weight forwards to avoid the transom dragging. When it's lighter still, speeds are so low you don't need the waterline length, then it pays to get further forwards still, and as you say, get the last two feet of boat out of the water. This reduces wetted area, hence surface drag.

At planing speeds, or even around 'hull speed', the flat aft sections are providing lift, so the transom is not dragging, the wake will be smoother.

Posted By: Rupert
Date Posted: 04 Oct 13 at 11:20am

Effectively, the modern planing wedge type hull has already lost the back 1/3 of the boat. If you look at old style hull shapes, the exit of the waterline shape is far more tapered, so the water flows more smoothly. With the planing boat shape, the last 3rd is just dead weight getting in the way and stopping the boat going fast when planing. However, you lose the smooth waterlines, so have to do something about it in non-planing conditions. Heeling the boat a little and moving forwards will stop the transom drag.

However, not sure what boat you have, but I've not sailed an Assy boat where there haven't been times I'd wanted it longer in a blow!

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Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686

Posted By: transient
Date Posted: 04 Oct 13 at 11:59am

The flat rear end of a planing boat is not efficient drag wise in displacement mode, i.e. light winds. The flat arse is very useful when planing though.

Some non planing hulls work better if waterline length is maximised.

Dragging the transom in displacement mode is not good in either type of hull.

Good trim will not only vary with different hull types it will also vary with different crew weights. If the helm is heavier than the crew then getting forward will be more important...If the crew is a lardy half the work is already done.