Why Not Build a Motor Boat Like a Sailboat ?

[Wavewacker]

Beau, that's a first for me, are those amas rather flat? Was this something you added? Nice picture, BTW, your glasses appear to be dirty or was that from spray? (LOL)

And, can you have any weight in those, like extra fuel?

[u4ea32]

The only three powerboats I can think of that were directly derived from sailboats are the Gulfstars and Endevours (both previously mentioned), and the Willard 30s mostly built in the early 70s.

One needs to consider the way YOU will actually use the boat, before you can decide if any given boat is reasonable for YOUR uses.

If you want to drive around at high speeds over short distances while consuming painful quantities of fuel, there are any number of vessels available.

If you want to go cruising long distances while consuming little money but unlimited amounts of time, then (as far as I can find) there are zero suitable powerboats being manufactured in series production.

And that's the kind of boat I want for my next (and probably last) boat.

Like some others on this forum, I'm expecting to build a one off boat to meet my needs.

I'm actually looking more to row boats for inspiration, rather than sail boats, because sailboats are so distorted due to racing rule fashion, and need to support high levels of stability, while row boats need to be very low drag at slow speeds with nice levels of initial stability.

One key issue that most people misunderstand is that high initial static stability, such as from wide beam and hard chines, is the OPPOSITE of what one wants. High initial static stability simply forces the boat to roll to match the surface of the water, where what you really want is minimum roll (angle and rate). So wide beam, hard chines gives you both high rate and high roll: that's bad.

For low roll rate and angle, you want narrow beam with soft turn of the bilges, just like a row boat. You also then get far lower drag at low speeds.

To keep roll rates low, one wants a high moment of inertia around the roll axis. This means put the mass as far from the hull volume centroid as possible. Port and starboard, high or low, same effect. Some think its easier to put weight up high to slow the roll rate, and that does work, but obviously it also makes the boat capsize easier. Nonetheless, there are many commercial vessel capsizes that are due to this widespread practice.

For seakeeping in nasty conditions: When the boat is rolled to 90 degrees, recognize that the topsides and perhaps cabin sides can result in the center of buoyancy being far "above" where the waterline is painted. If the CG is even at WL level, there will be substantial righting moment. When the boat is inverted, the shape of the pilothouse and trunk cabin can make the boat very unstable upside down (just what you want) so the boat will roll right side up easily. Assuming you don't also fill up with tons of seawater during the capsize.

[dacust]

Originally Posted by u4ea32

The only three powerboats I can think of that were directly derived from sailboats are the Gulfstars and Endevours (both previously mentioned), and the Willard 30s mostly built in the early 70s.

....

There were also a few Albins. The main one I was familiar with was the Albin 25 which came in power or motor-sailer. From most reports I have heard, it made a rather poor motor-sailer. But as a motor boat, I had a guy that had one claim he did 10 knots at 1 GPH. It had a Volvo diesel, 3cyl if I remember correctly. Of course, way too small and way too short a range for what is being discussed in this thread, but an interesting design based on a sailboat.

The rest of your post was very interesting reading. Thanks.

-dan

[Wavewacker]

U4, your assessment of initial stability is bugging me. Initial stability is what I want in a power boat and that comes from a wide flat beam. Sailboats (mono) are designed to heal or roll initially and the flair of the hull provides secondary stability before we get really wet. So, I didn't really follow what you described above. Also, in my mind, if you place weight high on a boat with narrow rounded hull, like a canoe, you're going to get wet. If I'm wrong here, please tell me why I got wet when broadsided by a wake in my canoe (while looking the other way...LOL)

Converting a sailboat to a motorboat seems to me to be like changing a cat to a dog. All but the smallest sailboats use engines and I think they are "motoring" but they aren't powerboats designed for speed.

I read several articles about the modification of a small sailboat to motor, stripping the sails and rigging. The guy called it "Terminal Trawler" and used it primarily on rivers including the Mississippi. I think it was about an 18/20 foot O'Day or Catalina, but anyway he was advocating using a small cabin sailboat and stripping it down and motoring slowly with minimal power saving money.

I like that idea, saving money, but I could save even more if I sailed, to a point. There is nothing inexpensive about boating unless you use a canoe and don't lose your paddle.

I think I have come full circle in looking for the boat I think I "need" just as the OP is thinking. I have been seriously considering building a boat as mentioned by others and asking dumb questions and what I have found is that there is no way I can build a boat cheaper than I can buy one, regardless of type or size being comprable. (But I'd still like to build one just for grins)

Sorry for the ranting, I'd like to hear what someone who knows has to say about turning a sailboat into a power tri for greater stability on say a 30/32 footer with a 25hp outboard, pushing to hull speed in real world conditions. If the OP mentioned his requirements I missed it. So as to range, since most sailboats are not equiped with much fuel capacity, adding fuel tanks might be a problem. I don't think anyone would really want lose fuel tanks in the cockpit to move around.

[Mick Edge]

Get a Cat it is the very best of both worlds twin fast engines,Great free Sailing in wind, lots of room CATS RULE

[u4ea32]

Originally Posted by Wavewacker

U4, your assessment of initial stability is bugging me. Initial stability is what I want in a power boat and that comes from a wide flat beam. Sailboats (mono) are designed to heal or roll initially and the flair of the hull provides secondary stability before we get really wet. So, I didn't really follow what you described above. Also, in my mind, if you place weight high on a boat with narrow rounded hull, like a canoe, you're going to get wet. If I'm wrong here, please tell me why I got wet when broadsided by a wake in my canoe (while looking the other way...LOL)

Hi Wave, I just noticed your response.

To make sense of this, you need to keep the issues separate. The problem with using your canoe to understand this phenomenon is that everything is coupled, not separate: you constantly move your high center of gravity to keep the boat balanced. If you don't, you get wet. Canoes, kayaks, and bicycles are tightly coupled, dynamically unstable systems. A yacht is dynamically stable. That is a very big difference.

Lets talk about each issue independently: 1) initial stability, 2) moment of inertia, and 3) center of gravity.

First, initial stability, also known as form stability: stability due to the shape of the hull, turn of the bilge, beam, water plane.

In protected water, where one only has little waves, then you are right: lots of initial stability is a good thing. Then you and your guests can stroll around and ignore wakes without worrying about wine glasses toppling over.

I was referring to the rather unusual and rare situation of covering serious miles at sea, where the waves are large in comparison to the size of the boat. Its probably far better to just avoid crossing oceans, or at least to ignore the unavoidable discomfort of such crossings. But lets say you really want a boat to be as comfortable as possible (still uncomfortable!) in waves that are much longer -- peak to peak -- than the boat is wide.

A boat with extremely high initial stability will try to stay level with the water -- not with the horizon. Therefore, the boat will rock'n'roll the same as the water under the boat. Bad.

Example: Consider a 36 foot power boat with 15 foot beam is traveling beam to large seas, waves coming from the starboard side. The boat has extremely high initial stability, either very flat bottom or is a catamaran or (worst of all, as I will explain later) a trimaran. Consider as a wave peak passes under the boat from abeam: first, the boat will be tilting one way, the angle of heel being the angle of the face of the wave, lets say 20 degree to port. As the peak passes under the boat at 15 knots -- 25 feet per second -- the boat will roll 40 degrees in less than a second! Wine all over the place. Bad.

Second, lets introduce moment of inertia. This is not the center of mass, or the average location of all the masses, but the separation of masses.

One way to reduce the RATE OF motion in waves is to increase the moment of inertia. Think of a spinning top or gyroscope. The spin causes the top to have a very high moment of inertia, and so it tries very hard (actually exerts force) to stay level. It is very hard to make a gyro tilt quickly. Slowly, you can roll it any way you want, but the gyroscopes moment of inertia makes it hard to tilt quickly.

Having masses widely separated, like having a tall mast and a deep ballasted keel of a sailboat, provides a high moment of inertia. That is why sailboats have much more gentle motions than powerboats. And that is why adding weight aloft on a powerboat, like adding a tuna tower, reduces the roll rate. Without the keel (a low center of gravity) you might flip over, but you will flip over nice and gently.

Third, is center of gravity.

You still need positive stability to very large angles of roll (heel): the center of gravity must be low enough that the boat does not tip over. But you need high moment of inertia (weight spread apart) to reduce roll rate.

A boat with low form stability, high moment of inertia, and low center of gravity is what you want to cross oceans.

Of course, the most comfortable way to cross oceans is in the front of a large jet!

Oh: the reason power trimarans are the worst configuration possible at sea is because they combine high initial stability with minimum moment of inertia and a high center of gravity.