Bilge v Fin

ghostlymoron

ghostlymoron

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Reading the current PBO article on drying out the question of reduced performance of twin keels is mentioned. Is this fact or myth? It would be an interesting feature for the mag to have a head to head of fin v bilge to see what the truth is. It seems to me that a heeled bilge keeler has one keel vertical at say 10 degrees so might perform better. Obviously, twin vertical keels wouldn't have this and performance would be worse due to the extra wetted area.
 
Having sailed a David Thomas twin keel for a 2 years I'm quite impressed with it's performance. I sailed up wind in about a force 3, a Moody 36 was a few boats lengths ahead to start with I managed to catch it up but not quite over take.

We both anchored in a bay, but i left 30 min before him. The wind had now picked up to a force 5 (still sailing upwind). The same Moody (who I had a 30 min head start on) caught me up and passed well ahead. I was just going sideways there was nothing I can do, he punched his way upwind where as I just went sideways.

Once the wind picks up bilge keels seem to be at disadvantage, the slamming is teeth gritting.

On an upwind sail into a force 5 I sit wishing I had a fin keeler. Being in a quite anchorage in stepping distances of the shore I'm glad I made the right choice.
 
Racing around the cans in the Solent yes you would slowly fall behind the pack with twin keels. Back in the real world the husband and wife out for a gentle weekend cruising are unlikely to notice the difference. Newish sails, a clean hull or lots of tweaking the sails is likely to produce a bigger advantage if you want.

This is an interesting review by one Moodysabre:

http://www.ybw.com/forums/showthread.php?t=203360&highlight=bilge

Pete
 
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Depends on which twin keelers are being compared with which fins. Generally, if you have a hull which is available in either version, the fin will have a better performance.

In a race, our Moody 346 twin keeler beat the pants off 2 Jeanneau 352 fin keelers, both of which eventually dropped out. On the long outward leg, we were very hard on the wind gusting F9 and they just couldn't get a grip. 2nd place went to a GK26, which finished 30 mins behind us so, it doesn't always follow that fin is better, particularly when looking at high aspect fins with spade rudders such as the Jeanneau.
 
ghostlymoron said:
Reading the current PBO article on drying out the question of reduced performance of twin keels is mentioned. Is this fact or myth? It would be an interesting feature for the mag to have a head to head of fin v bilge to see what the truth is. It seems to me that a heeled bilge keeler has one keel vertical at say 10 degrees so might perform better. Obviously, twin vertical keels wouldn't have this and performance would be worse due to the extra wetted area.
Click to expand...

ask yourself, how many racing boats have 2 keels ;)
 
I wonder if any boat designer has really designed a twin keel boat properly.
Do they always design the fin keel version and then add the bilge keeler as an option, knowing that they are offering a flawed concept.
Have you read my link in post #2?

Perhaps designing a boat as a bilge keeler as the only option and looking for performance would deliver a different proposition.
 
Lakesailor said:
I wonder if any boat designer has really designed a twin keel boat properly.
Do they always design the fin keel version and then add the bilge keeler as an option, knowing that they are offering a flawed concept.
Have you read my link in post #2?

Perhaps designing a boat as a bilge keeler as the only option and looking for performance would deliver a different proposition.
Click to expand...

RM are the only people I know of that make the perfect twin keeler. Although If I'm right they also make a fin keel version?
 
IMO There are conditions when certain boats will excel, going to windward it is rare a bilge keel will... As Tom illustrates it can happen but not that often.

Off the wind its a different story again.

Keep a bilge keel clean and they will surprise you and others but at the end of the day Keels on most boats do 2 things.

Stability: Its much better to have All your ballast at the end of the keel to counteract the wind, than half hanging down in near neutral position.

Stop boat moving side ways: Here I think deeper is still better.
 
onesea said:
Stop boat moving side ways: Here I think deeper is still better.
Click to expand...

keels-3.jpg
 
3 or 4 times?

Don't forget most bilge keels you see are stubby little legs that are attached to the bottom of a floating caravan.
Most bilge keelers were not designed to do other than draw less than a fin and allow the boat to sit down without falling over.

The comparisons being drawn are not starting from an equal footing.
 
For racing performance on a given hull, all other things being equal, single keel wins (illustrated by yardsticks). The biggest advantage of a single keel is when going to windward; the boat points better, and has less leeway. It's a basic fact of hydrodynamics - long and narrow creates much less induced drag for a given side force.

However, for cruising in waters with significant tidal ranges, such as Brittany and the Channel Islands, far more anchorages are available to boats which can take the ground. So bilge keelers and lifting keels have more destinations.

And the associated tidal streams affect your journey time far more than boat performance in these waters.

So a cruising boat will time his tides, have two keels, and go to more places more quickly than the late rising single keeler who missed the tide, hoping his improved performance would help him catch up!

Different in the Mediterranean and Baltic though . . .
 
jimbaerselman said:
For racing performance on a given hull, all other things being equal, single keel wins (illustrated by yardsticks).
Click to expand...
I think I'll go off and top myself.

Yardsticks only measure the performance of existing boats (and their crews).
Existing bilge keels are generally a compromise for shallow draught, drying out purposes.

It is not a level playing field.
 
Lakesailor said:
I think I'll go off and top myself.

Yardsticks only measure the performance of existing boats (and their crews).
Existing bilge keels are generally a compromise for shallow draught, drying out purposes.

It is not a level playing field.
Click to expand...

A twin keel boat deigned for performance can often outsail a similar fin keel boat that is not performance orientated. However, in most cases where the same boat is available with either, the fin will outperform the twin. Examples fom the rating figures, some back to back tests when such boats were popular and even the RM, which is arguably the best performing twin keep, the single keel version outperforms it according to the designers figures.

As Jim suggests these differences become less important if one considers all round capability as cruising boats, particularly in tidal waters where passage times are influenced by tides and draft limits access. Similar with shallow draft versions of fin keel boats - Loss in outright performance can be compensated by extra versatilty, but this advantage disappears if you sail exclusively in non tidal deep water.
 
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