Engine / trans. / shaft tutorial?

[Seth Wood]

It's getting to be time to think about engine options for the 30' sailboat I'm building (11,000 lbs. displacement).

I know VERY little about this, so I'd like recommendations on how to learn as much as possible. What are the best books? Web sites?

The most immediate question is: what size hole will I need for the prop shaft? Are most shafts for this size boat/hp about the same size? I'd like to be armed with at least a little knowledge before calling engine manufacturers.

I'm ready to learn, I'm just not sure where to start. I had a look at Nigel Calder's Maintenance book; that was a good start. What else? Let the enlightenment begin....

[John E Hardiman]

Seth;
Usually, the speed desired and the prop are the first things selected in propulsion design. For a sail auxiliary of ~30' this means a hull speed of ~5 knts. Now given a hull drag calculation (way too complicated to explain here, e-mail me) for 5 knts, we come up with a necessary thrust. Given the thrust and the speed of advance (in this example 5 knts) we come up with an effective horsepower (ehp). Using the ehp and a couple of "judicious" constants we come with shaft horsepower (shp). Now shp is a function of torque and rpm. The prop selected determines the rpm and torque and must match the shp found the other way reasonably. This torque sets the shaft diameter, this rpm sets the gear ratio (i.e. transmission) given the selected engine.

As the selection of a propeller sets everything else, I suggest that you should shoukld think long and hard about what you expect out of propulsion then consult someone experienced with this before selecting a engine/trans/shaft/wheel. And yes; manufacturers usually have suggested "sets" based upon speed/ehp.

[John B]

The modern trend is to err on the over powered side. My fathers 30 footer came out with a 10hp but was repowered with an 18. The new motor is lighter and he has some ability to punch a sea now.I could have fitted a 15 to my boat ( 31ft static w/l)but went for 27. I know people who have fitted 30 hp to 30 footers for hardly any weight gain and get the " get home " factor and the smoothness of 3 cylinders.That's where I would start .

[Seth Wood]

John H. and John B., thanks for the guidance. What I'm leaning towards is Westerbeke's 11A One. Although it delivers a little less HP than I'd prefer, I like that it's hand-cranked.

It sounds like my engine needs will determine the prop size, which will determine the shaft size and hp/torque required, along with gear ratios. And then, working backwards, I need to ensure the engine delivers the appropriate torque/hp down the shaft to the propeller.

See what I mean when I say I know VERY little about this?

I'm glad to hear manufacturers often provide these as "sets," if that's what you mean, John H.

Still, I'm woefully undereducated on this. Any other pointers, basic or otherwise?

[John E Hardiman]

By "sets" I mean engine/transmission. Shaft and props, because there are so many variables, are still up to the builder. See if you can beg/borrow/steal a copy of Skene's Elements of Yacht Design or the update by Kinney. My '81 updated edition has a whole chapter on this for the SANTA MARIA which has a 29'0" DWL.

[Syd MacDonald]

Hand cranked? Is this a back-up procedure or due you have to crank it every time you start it? I don't think I'd like that in the confines of a sail boat.