LONG Bilge Keels

[FAST FRED]

On my converted Navy 50 Utility Launch the past owners were lobster men and modified the boat to suit their needs.

The boat has 2 bilge keels about 30 ft long and 7 inches deep.

What is interesting is the keels have holes about 3 inches in diameter , about 4 in apart for the full run of the keeks.

Any Ideas on the purpose and efficency of this style?

AM willing to seal the holes if it would result in faster transit speeds.

FAST FRED

[dionysis]

My first though is they can only slow you down. I have not read of any use for such anywhere. I say fill them in.

[water addict]

The holes probably have almost no increase in straight line drag because they will be within the boudary layer. They were probably put in there for added roll damping. The holes will allow some water to pass through as the boat rolls, hence turbulent eddies will be formed- more than if the bilge keel was solid. More energy will be imparted to the surrounding water in the form of eddies, and thus more damping of roll motion.

[dionysis]

What a good idea water addict. Most probably the right one too. I still think thay give added resistance though.

[FAST FRED]

Sounds good ,
as they were probably MORE interested in not rolling while stopped , pulling a pot , as stability under way.

Thanks!

FAST FRED

[DaveB]

Wow... I'm impressed... I am just a student and have very little experience in the industry, but for them to come up with that's pretty amazing/ingenious... even to think of the idea wheather it works or not!

Where are the holes located (near hull, middle, or towards the outboard end)?

Often times there are holes or gaps on the inboard side to avoid hardpoints... If you attach a bilge keel to hull plating it will stiffen it a great deal more than the surrounding plating which can cause large stress concentrations... The holes might have also made it easier to bend and fit the keels...

Seems like they're a good thing any way you look at it... Interesting...

Cheers,

Dave

[FAST FRED]

The bilge keels are 30 ft long and about 6 or 7 inches deep.

The units are GRP just glassed in place (decades after ) with FG tape.

The holes are 3 to 4 inches in dia and there almost the full run of the bilge keel 3 or 4 in apart..

The boat is kept on anchor (no dock) at all times and is fairly smooth while anchored in a moderate blow.

FAST FRED

[DaveB]

Originally Posted by FAST FRED

The holes are 3 to 4 inches in dia and there almost the full run of the bilge keel 3 or 4 in apart..
FAST FRED

Thanks for the reply...

Where are the holes located vertically? (near the boat, outboard, or in the middle)

Cheers,

Dave

[Sean Herron]

Hello...

I love these strange questions - I know you said your thingees are glass - but on the steel boat I am working on the same thing appears - actually they are getting cut back for new AB stabilizers - the holes are torch cut from one by 5 inch stock which bends longitudinally along the belly for say 25 feet - they are fully cut out to the edge of the stock (U-shaped) which is then welded to the boat...

Now I am not going to try and bend one by five steel bar to a hull without 'KERFING' it and heating the BeeGees out of it too - sounds like your situation is also one of those shop floor solutions...

My vote is for 'bendability' from an 'edge panel' situation...

What is even more fun is to pull panels and find all the felt pen scribbles left by the builders and want to be comedians and poets - the above refit boat has bits like DAS HERE ISDA MOTABELLIE on the cork as you go into the engine room - I personally can relate to these builders...

SH.

[mackid068]

Bilge keels are actually ok for some purposes; fishing being one of them; but speedwise, because the wetted surface is increased so greatly, they aren't great. By the way, how are those old Navy launches? Do they work nicely, by which I mean, can you get them up to at least 10 kts.?

[FAST FRED]

"can you get them up to at least 10 kts.?"

Sure, they go 12K with the minor load of a small deck house, but the fuel consumption is normal, about 10GPH for the 160 or so HP it takes.

We crawl at 8K on 3.5 gph which is only 1400rpm.(finance$ dictated) , but the speed IS there if one has the diesel to burn. Located in Middletown CT , if you wish a ride.

FAST FRED

[normbaker]

Hi Fred,

I'm back on the West coast after living in a small fishing village in South West Nova Scotia, off and on, for the last ten years. The lobster boats there retrofitted the same strakes that you are talking about and the NA who first described their function was right on the button.

Lobster season is winter in the Bay of Fundy when it gets real mean, especially wind against current. I would watch these boats coming and going from the warm safety of my home office overlooking the harbor entrance and didn't envy those crazy Blue Nose's one bit.

When they work their lobster traps they're side on to the wind and sea and even though the boats are usually 45' x ~18' they would still roll hard enough to almost throw you out of the boat, according to one fisherman I spoke to. The newer boats are still 45' long but they have taken the beam out to 22' to gain more stability.

They pull a lot of the boats out of the water at the end of the season and I have seen the longitudinals on the hulls and they are usually glassed on solid wood. The NA's idea of turbulent causing holes to increase the dampening effect is a good one, but I guess that technology hasn't reached out East yet.

[FAST FRED]

The bilge keels are on the bottom about 5 ft from the keel , which is before the bilge begins to turn up.

From others with similar hulls my speed / fuel burn seems to be the same , although the surface area under my boat is probably 1 or 2 % more .

Probably the state of repair/tune of the engine and the bottom preperation has more to do with speed or fuel burn.

For someone wanting to build a cruiser , or fishing boat , Boats and Harbors frequently lists almost NEW ex Navy hulls for $35,000 to $50,000.

These have UNDER 5 or 6 total hours and are of course Fire retardant resin and full foam flotation.

If you just need an open launch to hold 150 of your closest friends , it will work great !

Unfortunatly they come with the Cummins pick up truck engine , rather than a heavy duty monster like my DD 6-71 , but for the price the 5000 hr engine should be fine.

For serious offshore work I would repower to a John Deere , but this years cruise were "Looping" the Hudson , Erie Canal , a lake or two and the CN canals to Ontario , then back thru Lake Champlain & down the Hudson .

Can walk ashore most places , so the old 1944 Grey Marine will keep thumping another 500 to 800 hours for this summer.

FAST FRED

[zcg0085]

FAST FRED
my 50' steel defever has a bilge keel running about 30' by 12" no holes but in a conversation with a navy marine engineer...this was the cheapest form of damping ( up to 50%) works quite well low cost no maintenance.

[Raggi_Thor]

Ventilated bilge keels, that is with a lot of holes, is quite normal on small fishing vessels here in Norway :-)