Shoal Draft v. Full Depth Keel

[Dockhead]

Originally Posted by Polux

A twin keel system means not necessarily a bad upwind performance (small draft does). On RM (the blue boat that I posted above) they purpose the boats with twin keels and single keels and with their NA, Marc Lombard, they performed a lot of studies, on computer an on the water, to know what were the real differences between a really well designed twin keel and a slightly bigger draft torpedo keel.

Off course, the torpedo keel performed better but the difference would be negligible for most cruisers (I posted about that with the real numbers elsewhere). That's why the vast majority of RM clients (and they are many) prefer the twin keels, even on a sportive performance cruiser. Lots of advantages for some small loss.

Regarding a true blue-water boat for high latitude cruising, Pierre Delion /Pierre Rolland designed one, the Iroise 46. It is a fast boat in all points of sail, including upwind. not properly a motor sailor but the 50hp of the engine should be more than enough to the 9000kg of displacement and if not a 75hp would not be a problem.

Iroise 46 - Pierre Delion/ Architecture Navale

As a Channel sailor, I know the RM boats, which are pretty popular with the French sailors. You can see a lot of them walking the docks in Cherbourg.

Two keels gives double the wetted surface of one keel, so naturally there is a performance hit. You can claw some of it back by making the keels high aspect and deep, like what RM does, but the tradeoff is the same with high aspect and deep single keels -- draft and fragility. There's no free lunch here, and different sailors have different priorities.


And most cruisers are not willing to take on board these drawbacks for the sake of more upwind performance, which is why racing type underbodies with slender, deep keels, and very high aspect spade rudders are not found on most cruising boats. Tastes have become somewhat less conservative, but this balance still exists, and every design is a compromise.

The kind of person who would want a bilge-keel motor sailer would not want a light, relatively fragile, RM-type boat (as much as I like them, myself). It's a different direction and idea.

And I don't think I would want one myself. If I were going to go in that direction, I would not want to make the compromise of a twin keel. I'd rather just have an ultralight boat with a very deep high aspect keel and a very high aspect spade rudder and without a gram of extra weight. Then this purpose is fulfilled with the minimum of compromises to sailing performance.

If you're already going to have a bilge keel, then a sensible compromise is to give up altogether on windward performance and just use the motor to go to windward. Then you can have a ketch rig which has so many advantages, with just windward performance (and cost) as a downside. Then you don't mind the weight of the large engine, and you don't mind having less efficient but super strong keels, so you really don't worry much about grounding. And you don't mind about the windage of a pilot house, so you can have a great, warm, dry place to steer from in bad weather. You wouldn't even mind the weight of twin engines, with the huge advantages of redundancy and maneuverability.

Windward performance under sail is so hard to achieve -- to do it really well, we have to make a lot of compromises. Right now I'm chasing that -- waiting for a new set of carbon fiber laminate sails for my boat, and adding a blade jib for going to weather in strong conditions. Thinking about getting weight out of the boat, especially the ends. Thinking about sacrificing part of my 100 meters of 12mm chain to get weight out of the bow. Working on sheeting positions and barber haulers.

But that's expensive and a lot of trouble and by far not for everyone. Which is why many of these compromises to sailing performance make a lot of sense for a lot of cruisers. Not everyone sails the same way, in the same conditions, or has the same priorities.


Of course, the ultimate bilge keel boat is a catamaran . This is another set of compromises, also not for everyone, but fulfills a lot of these criteria, and with less compromise to sailing performance. Catamarans are in general much better motorboats than monos are -- one of the big advantages few people talk about. Why more catamarans are not designed for beaching, I have no idea. A crime, I think, to have that possibility inherently, and not fulfill it to the end.

[Neeltje]

Then, of course, there's the old Dutch solution. Upwind performance? Zip.

[Steadman Uhlich]

Dockhead quoted: "How about a pilothouse ketch with bilge keels, a super-reinforced rudder, a really big engine, and vast fuel tankage? Say, a ketch-rig Nauticat with bilge keels -- something like that. Or maybe even make it with twin propulsion engines, with one shaft behind each keel -- how about that?"

I like pilothouses, Nauticats, ketch rigs, two forms of propulsion, and find bilge keels interesting. So you hit several points that caught my interest.

[Dockhead]

Originally Posted by Steady Hand

I like pilothouses, Nauticats, ketch rigs, two forms of propulsion, and find bilge keels interesting. So you hit several points that caught my interest.

The ketch-rig, pilot-house motorsailer is pretty common here. They often have quite small rigs, though.

To continue the fantasy: I would give such a boat a good bit of ballast and a decent size rig -- SA/D of at least 15, maybe more. I would provide it with a variety of sails for reaching and running -- a fisherman, a mizzen staysail, etc. A yankee jib and a furling staysail on the inner forestay. Try to make it a really good sailing boat despite the lack of keen upwind performance.

Then with two main engines, you won't need a separate genset, so you almost claw back the weight and cost of the second power plant. You put large heavy-duty alternators on both power plants and charge a substantial LiFEPo bank. The alternators should be sized so that they will provide a reasonable load to one of the main engines so you can run one of them just for charging when necessary.

Then you would operate the machinery as you would on a catamaran -- use both engines for port maneuvers (what a luxury). You have some kind of variable pitch feathering props on both shafts for excellent motorsailing, and to get the thrust right for different situations of using both engines or just one at a time. You use both engines when you need a lot of power, like powering into stronger conditions; just one at other times, switching between them like on a cat. This is good for the machinery since you keep a better load on in lighter conditions, by switching off one engine and feathering the prop.

You have completely separate fuel tanks and systems for the two engines for redundancy. With day tanks, in a nice walk-in engine room with workbench.

You'll want twin rudders so that the props wash on them for maneuverability. You don't want the keels to be too long -- unlike usual motorsailer practice. Try to maintain some sailing performance -- you're giving up just on the last part of performance upwind. But twin rudders bring other advantages -- redundancy, and easy mounting of fully redundant autopilots.

One design challenge will be to maintain a decent sailing cockpit despite the pilothouse -- these functions fight with each other for deck space and visibility, but both are essential.

The pilothouse should have a nice steering position with unobstructed view forward, windshield wipers, all controls and instruments. And a nice stand-up chart table with big drawers for charts. Should not be too tall or blocky -- you don't want to completely give up on windage. Should be narrow enough that you have decent side deck access.

[Polux]

Originally Posted by Dockhead

...
Of course, the ultimate bilge keel boat is a catamaran . This is another set of compromises, also not for everyone, but fulfills a lot of these criteria, and with less compromise to sailing performance. Catamarans are in general much better motorboats than monos are -- one of the big advantages few people talk about. Why more catamarans are not designed for beaching, I have no idea. A crime, I think, to have that possibility inherently, and not fulfill it to the end.

I agree generically and I believe bilge keels or twin keel as I prefer to call them are outdated by the new swing keels with all ballast on them. Even the new RM has one available on his new boat as an option. Regarding RM keels to be fragile I don't agree. They are a single steel construction that is bolted to the bottom of the hull. Never heard of any problem with them and there are hundreds of RM around.
Regarding motoring cats I agree fully. They make more sense regarding the displacement motorboats and their number is increasing sharply. Anyway many use condo cats mostly as motorboats.
In what concerns beaching a cat does not offer the advantages of the big legs of a RM namely in what concerns to clean the hull or even to apply new anti-fouling, besides most would not be on their Iron keels but on their hulls and they could be superficially damaged by the contact with sand and small pebbles.

[Polux]

Originally Posted by Neeltje

Then, of course, there's the old Dutch solution. Upwind performance? Zip.

You have one of those? Beautiful boat and great for living aboard. Congratulations. Do you enter those crazy races with them? Awesome spectacle

https://www.youtube.com/watch?v=DFmCaNM_0bI

https://www.youtube.com/watch?v=NNybmi3kI9M

[bmz]

A lot depends on where you sail and the size of the boat. I sail in the Chesapeake--with both shoal and deep keels--and I will never go back to a shoal draft again. I sail a 34 foot boat with a 5'8" fine lead keel. It not only points higher, but it's sails faster and better on all points than my previous shoal draft. Shoal drafts usually have far more water resistance and wetted surface. Inasmuch as almost everyone obeys 6 foot markers and depth lines, there is nowhere that a shoal draft can sail that I can't. This is an important point; if you are going to obey 6 foot markers, you gain nothing by having a shallower draft. 99% of the Chesapeake has a sand or mud bottom; before I had a GPS, I would run aground at least once per year. But all I had to do was crank up the diesel and back off. Run aground with a shoal draft, And you are stuck.

[Cheechako]

Originally Posted by Steady Hand

Dockhead quoted: "How about a pilothouse ketch with bilge keels, a super-reinforced rudder, a really big engine, and vast fuel tankage? Say, a ketch-rig Nauticat with bilge keels -- something like that. Or maybe even make it with twin propulsion engines, with one shaft behind each keel -- how about that?"

I like pilothouses, Nauticats, ketch rigs, two forms of propulsion, and find bilge keels interesting. So you hit several points that caught my interest.

Yeah... but what kind of anchor gear?

[Dockhead]

Originally Posted by Cheechako

Yeah... but what kind of anchor gear?

Ah, well, a boat like that might have a bit of a bowsprit, and could have a massive fishing-boat style horizontal windlass and a stout sampson post. You might have two anchors on either side of the sprit.

Mmm, I like this boat more and more . . .

[Polux]

Originally Posted by bmz

A lot depends on where you sail and the size of the boat. I sail in the Chesapeake--with both shoal and deep keels--and I will never go back to a shoal draft again. I sail a 34 foot boat with a 5'8" fine lead keel. It not only points higher, but it's sails faster and better on all points than my previous shoal draft. Shoal drafts usually have far more water resistance and wetted surface. Inasmuch as almost everyone obeys 6 foot markers and depth lines, there is nowhere that a shoal draft can sail that I can't. This is an important point; if you are going to obey 6 foot markers, you gain nothing by having a shallower draft. 99% of the Chesapeake has a sand or mud bottom; before I had a GPS, I would run aground at least once per year. But all I had to do was crank up the diesel and back off. Run aground with a shoal draft, And you are stuck.

Yes you are right regarding to be more difficult to take out of a grounding a bilge keel boat but regarding those studies I talked about made by RM regarding similar boats one with twin keels other with a monokeel, with not very different performance results, the twin one had 1.65m draft and the monokeel had 1.95m of draft so you have to take that in consideration. Twinkeels work better than monokeels in what regards a lower draft.

[Neeltje]

Originally Posted by Polux

You have one of those? Beautiful boat and great for living aboard. Congratulations. Do you enter those crazy races with them? Awesome spectacle

Yes, but since the only other 2 that I know of on the East Coast are in Baltimore and Rhode Island respectively, I don't see a race in the near future. Besides, I'll first have to learn how to sail before I even think about racing...

[Polux]

Originally Posted by Neeltje

Yes, but since the only other 2 that I know of on the East Coast are in Baltimore and Rhode Island respectively, I don't see a race in the near future. Besides, I'll first have to learn how to sail before I even think about racing...

How the well they went there, sailing I presume? Not bad for a boat designed for shoal waters

By the way do you know that the therm Yacht is linked to boats with that kind of lateral board? Boats like this one:



and also to performance boats since the yacht therm in Dutch "jacht" comes from "jagen" that means hunter? They were used by the "police" to chase contraband boats, a bit light the powerful police fast boats are used to chase drug fast boats.

[Neeltje]

Originally Posted by Polux

How the well they went there, sailing I presume? Not bad for a boat designed for shoal waters

By the way do you know that the therm Yacht is linked to boats with that kind of lateral board? Boats like this one:



and also to performance boats since the yacht therm in Dutch "jacht" comes from "jagen" that means hunter? They were used by the "police" to chase contraband boats, a bit light the powerful police fast boats are used to chase drug fast boats.

Fact is, Neeltje's a "Tjalk". She looks just like the Skutzes you'll see racing around Dutch waters in the video clips, but she's considerably heavier.
Mine was originally a manure barge with sails until Dan Rowan turned her into his pied-à-terre and canal cruiser in Paris.

[Neeltje]

PS - I think all of them were transported here. I'd rather cross the Atlantic on Kontiki than on this type of vessel.

[Pete7]

Originally Posted by avb3

That's funny. Those keels presume a tidal variance of more than 2 feet.

Indeed we do, tonight we had a tide of 4.7, that's meters. Surrounded by sandy shoals and muddy estuaries those keels were a natural choice.