Skegs and/or keel on twins

Had someone interested in our boat and he wanted to know if there were skegs or keels to protect the props and rudders. Not on our boat, but I am carious, with the boats owned by TF members, are skegs and/or keels found on a few, some, many, most twin engined boats?

Thanks,

Bob

Keeps are common. Some like that on my old Hatteras are deeper than the running gear, which helps protect it some.

Was that on a twin?

I have only seen good protection on a British motor sailor that had twin keels for beaching .

With 20 ft tides some sort of method for taking the ground is common.

But on a cookie motorboat ,I have not seen it yet.

I've seen some custom builds with twins and twin skegs, but the only production boat I can think of with twin skegs is the Great Harbour line. I'm sure there are more, but few and far between examples.

FF said:

I have only seen good protection on a British motor sailor that had twin keels for beaching .

With 20 ft tides some sort of method for taking the ground is common.

But on a cookie motorboat ,I have not seen it yet.

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Look at Marlows.

All the twin engine Nordhavn's have dual skegs, one for each prop, and a centerline keel.

Some thoughts on the dynamics of twin keels;

In the fwd part of the hull water is pushed aside by the fwd motion of the boat and the shape of the bow. Amidships the outward moving water meets stationary water and the outward movement stops. You can see that the water line underway amidships is lower by up to several inches. As the boat moves fwd the water that the bow pushed out is stopped for a brief moment and then it returns coming back (inbd) at the boat aft.

So the water surrounding the hull moves outboard, pauses (roughly amidships) and then returns aft. Water at the bow moves outboard and water aft moves inboard. So if one knew the angle of the returning water the outbd twin keels could be installed so as to be parallel to the water flow. Otherwise extra drag would result from the water not flowing parallel to the keels.

However, directional stability may be enhanced by the fact that the keels being on either side and the inflow of water aft being inbd on both sides in opposite directions. It would be much like a twin w rudders adjusted outbd at the trailing edge. More drag but increased directional stability. Of course unlike the rudders the keels would not be adjustable but since cruising boats travel at about the same speed most all the time most of the extra drag would be eliminated by aligning the keels to the assumed water flow. That is w the trailing edge of the keels slightly inbd. Toed out to put it in automotive front end alignment terms.

Assuming this is true (I'm quite sure it is) to get the same amount of drag as one keel twice as large at normal speeds adjusting the keels to the water flow would eliminate (mostly) the extra drag of the keels not being in line w the water flow. But if one was to build the twin keels parallel to the center line the keels could be made slightly smaller because the inbd flowing water would aid directional stability.

And one aspect of twin keel drag that cannot be reduced is interplane drag. The moving water (fluid) is forced between two appendages. This is one of the reasons biplanes are not as efficient as monoplanes.

So some amount of increased drag or decreased directional stability should result from the twin keel arrangement. I think most twin keels are smaller than half the size of a single keel. Krogen's twin keels are clearly smaller by being much shorter than most keels but the center of area and thus side pressure is further aft and thus more usable. But most twin keels don't offer as much prop protection as a single keel.

And there's more to it like structural considerations, not only underway but when aground.

BobH said:

Was that on a twin?

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Yes. All Hatts are twins, even the one sailboat they made 3 of.

In addition to Marlow and Nordhavn I seem to recall the Krogen 58 having shoes as well.

And Selene´s with twins.

Eric,
How about having the twin keels converging toward the stern?
That way they may force an increased water flow, and pressure, aft.
This may tend to lift the stern, decreasing the "squat" and stern drag.

Ted

Northern Lights has a keel and prop pockets to have a shallower draft (3'1") and some protection for the running gear.


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Ted,
Good think'in but I think it would increase drag, slow the flow between the keels. I assume you mean w props at the ctr of the trailing edge (end) of the keel. But I'm sure you're right it would give lift aft. But trim tabs would probably be more efficient. Even more efficient would be a hooked bottom aft. The later Albin 25's had such a bottom aft.

A thought though is what if one was to install two keels closser together. Close enough so to be about 1.5 X the prop dia .. (Or a tad closser .. the gap between the keels trailing edges or ends. At cruising speed perhaps the prop would pump enough water out of the space between the keels to neutralize the pressure difference or even reduce it. That is .. the difference between on each side of the keeks. Perhaps you could call it a "forced inflow" for the prop that may have benefits. One of them may be as you implied that drag may be reduced. Increased thrust from increased feed to the prop and decreased drag from the hull. But does it do that? Big question. Probably not.

A new thought usually leads to another.

It's called "stirring the pot"

Ted

If protecting propellers and rudders have a high priority, some additional drag probably isn't so important. Believe it's important there is a connection from the keel to the bottom of the ruder.

Twins can have that too Mark. Just as easily as singles.

An advantage to a steel vessel using twin keels is that since steel is so strong a keel can be fabricated from one thickness of plate. There will have to be the prop shaft tube and that will be wider but that prop tube will be not very far from being horizontal and thus not far from being parallel to the flow of sea water. So not much extra drag from the very thin keel. One could fab a steel plate keel to fit the bottom of a FG boat and bolted on. Two steel keels and rudders typical as per Mark's pic (thank you Mark) would make a good rudder/keel design for FG or steel boats w twin keels. Steel rules?

Mark,
What are the scalloped shaped things ahead of the prop and behind the keel proper? i would'nt think the stern tube would need extra support. Maybe someone thinks it reduces turbulence.

I seem to remember Gulfstars with twins had twin skegs and shoes. Seemed a good idea. Be comforting if the IG had them.

Nomad Willy said:

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Mark,
What are the scalloped shaped things ahead of the prop and behind the keel proper? i would'nt think the stern tube would need extra support. Maybe someone thinks it reduces turbulence.

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I'm clueless. Nevertheless, no harm. Strong is good.

Another example of a single keel and twin prop relationship

Re # 20 above it looks like the list resulting from grounding would not push the props hard against the bottom as the hull is deep and the props are quite close together.

Mark,
I actually thought somebody thought it was an addition that made the keel look better and was trying to draw you out.

Great Harbour all have twin engines with skegs that the running gear runs through. There is a sand shoe at the bottom. I know of one Great Harbour that was set on the bottom by a falling tide with no problems to the running gear.

Our 88 Defever has twin rudder posts and a center keel a bit lower than the props.

twin Cummins

To confirm what was mentioned about-the Krogens, like the Nords, have a centerline keel that tapers off at abut 3/4 of the length, with twin keels w/skegs. As Nomad(IIRC)noted, there are water flow issues with twin keels. On the Krogen, the keels are designed to direct and enhance the waterflow through the props. The 58 can sit on its own bottom. Pretty sure that is true for all the KK twins.

Mirage Great Harbour N-37 bottom with keels. These are all (Ns, and GHs 37-47' x16') designed like this. Lou Codega a WEBB Institute graduate MIT Naval Engineering masters degree holder is the Naval Architect/ Naval Engineer PE who designed these purpose built boats. He posted once to answer many "mistaken" opinions regarding hull performance vice design. He is presenting in Newport at Trawlerfest this Sept. on hull design performance applications. It will be a fascinating seminar to attend.

Nice tip on Codega's presentation at Newport, Joe. Sure would love to be there.

My Mainship 34 with twin Yanmars has dual skegs.