Twin keel research

Chandler's thoughts on 'balancing' a single keel at low tide has been pretty standard practice aroud the West Coast of Scotland and the Hebridean Islands where few 'marinas' or jetties exist - but extreme tidal variations do.
'Sea legs' as they are called really do work. However, when in place heavy physical activities should be performed with caution (no plump ladies or ceilidhs aboard - unless accommodated on the centre line).
The legs can either be removed (but not those of plump ladies) and stored on the deckhouse roof - or swung up and lashed against the rubbing strake.

 

Help for Beginner with Twin Keeled Boat

To Whom It May Concern,

My husband and I are in the process of purchasing a sailboat for the purpose of offshore cruising. We are both 24 with limited sailing experience and we are throwing everything we have into this endeavor.

We have found a sailboat that we are in love with. It is a Marimba 35, a one-off, designed and refit by Michael McGrath from Victoria, British Columbia. Michael has succesfully sailed it for five years offshore and it is completely outfit for our intended destination.

The boat seems to be completly ahead of its time, all the boats we have found in our area subscribe to traditional design features. Our reservations about the boat are primarily to do with its design features that we do not understand. Namely the hull (Monocoque construction, strip planked Western Red Cedar with fibreglass overlay) and the keel (twin-keel "medium-aspect foils with an extended 'torpedo' foot"). We have done some reasearch into the construction of each and it seems that "if it's done right" it is perfect for our purposes. However, we are not in a position of experience to know if it has been "done right". The only person we know who understands the design of the boat seems to be the owner himself and we obviously cannot rely entirely on his opinion.

I have an extensive list of some of the design particulars. If you, or somebody you know has a better understanding of effective twin keel design can you refer us? We will soon have the plans for the boat as well as a photographic record of its construction. We are looking for
somebody who can independantly evaluate this material as to the soundness of the boats construction and educate us as to its design principals, and theoretical handling. We are trying to get a sense of what the potential drawbacks are for this boat so that we can evaluate it better when we take it out for sea trial and have it surveyed.

Thankyou for your help.

Drea Pinnegar
604.845.6666
todreamalife@yahoo.com

 

I've read and re-read this thread a number of times with much interest and have looked at all the links, papers etc that have been referred to.
I'm currently working on a 50-odd ft aluminum sloop with twin keels, but I can't find much in the way of hard data as far as CLR, lead, keel location etc goes. There's quite a bit that suggests a more aft location of the keels, but litle else.
I've attached an image of the preliminary model I'm working on. The boat isn't required to be a rocketship, simply provide comfortable handling.
Any suggestions, questions etc welcome......

 

Marimba Twin Keel Evaluation

I might suggest you post some of that info and photos on this forum and we'll all have a go at it. You might put a few of the photos on the posting itself, and the others in the photo gallery under your name.

Regrettably I've not revisted this subject tread in quite some time, and the timing now is poor for me as I'm getting ready for a 3 week trip to Panama

Add your data as well. You may not get a timely response right now as it seems a lot of forums are slow at the moment...or maybe its just my preoccupation with other matters.

 

One easy approach to compare performance between a twin bilge keel and a fixed keel, is analyzing the performance of both systems in the same hull.

There are several boats that offer the two options, one of them is the Legend (made by Hunter), another the RM, a French fast line of boats, designed by Marc Lombard and made by Fora Marine (there are also several semi-custom French boats that also offer both options).

RM's are interesting and unusual boats (they offer water ballast, are made of plywood and offer spacious interiors) . The boats have always deserved enthusiastic reviews by French the specialized press. The RM 1050 was elected boat of the year in 1999, and the RM 1200 has come in 2nd place in this year’s election. Knowing the quality and quantity of the French boat production, it is quite an accomplishment for a very small company.

At Fora Marine you are going to find very enthusiastic and nice people and I am sure they will make their knowledge on twin keels available to you (once a friend of mine has been interested in one of his boats, he has been at the shipyard and they have given him a first class welcome, explaining him everything about the boat…and he was only interested in a used boat that was not even sold by them).

http://www.opalmarine.com/
http://www.rm-yacht.com/detail_gamme.cfm?id_gamme=24
http://www.rm-yacht.com/phototheque/phototheque.cfm#

 

The twinkeels have advantages over single keels: lower draft and the possibility to dry out.

The disadvantage is a lower performance, but these facts are known - so if you can live with that the choice is not so bad.

 

look at the Engish made Snapdragon sailboat! they have a large following in the cheasapeake bay in virginia

 

I'm familiar with the advantages and disadvantages of twin keels - I'm just having trouble finding data regarding the finer points (I suspect because there isn't a great deal of such info out there...)

Brian (and anyone else...) preliminary specs are:
LOA 15.25m
LWL 14.2 m
BWL 4.3
Displ. 16000 kg
LCF 57.2% wl
SA approx 120m^2 (need to refine this still)
Lead 14.6%

Any input is much appreciated
Thanks

 

One cruising disadvantage is when you run aground ,
you become a fixed object till the tide or a barge crane gets you off.

Single keeled boats can sometimes be healed enough to get unstuck , with out help.

The advantage of a beachable boat can bite!

FAST FRED

 

Why have you chosen for 2 rudders? 1 rudder seems better to me for handling under power.

I think the lead for twin keels are the same as 1 keel, shallow draft keels are usually a bit more aft than deep keels.

I have raced a 21' twinkeel Hunter against a 1 keel version, upwind in a breeze it was faster and pointing higher. Downwind a bit slower, also got a bit stuck when surfing down waves. Keels toeed in, angled outwards and had asymetric profiles. The section at the bottom was a bit wider than at the top lowering CG and enlarging footprint.

Arthur

 

To keep the boat steady when beached?

 

Brien, I know that. What I am saying is that the RM guys have a lot of knowledge in the fine tuning of the concept. I have heard that differences in performance between their boats equipped with twin keels and with fin keels are very small.

 

Windvang - 2 rudders as I have 2 propellers - so for slow speed manouvering to have the thrust over the rudder.
Keels (and rudders?) are toed in about 2 degrees and are asymetric

 

Here's another stranger...

Anyone any idea what this has to do with keel science? I'm at the point of buying her to find out...
regards,

Sander

 

Will,

The published papers on twin keels do not mention fore & aft location in any meaningful way. Remember that the keels must carry the ballast in the correct fore & aft location as well. Though the twin keel profile will be smaller than that of the single centerline keel, the quarter cord should be in approximately the same position fore & aft.

There is little data on balance available because every hull-keel-rig combination will balance differently. Balance may change dramatically at different heel angles. We have some few rules of thumb that sort-of work, but each designer must do his own research, that is find out how your hull design balances with a given keel and CE position. Then make adjustments in the next boat. This is painful experience, but that's why they pay us the big bucks!

All the best, Tad