A Photo & Discussion Forum for Wharram Design Enthusiasts
We are building a Tiki 38 and are thinking of usin g 2 small inboard diesel engines (12HP each) with a slanted long shaft that (not Z drive).
Does anybody have any experience with this solution and can also recommend which make to choose?
Personally I'd much rather use diesel than gas as well but appreciate the simplicity of the original design and am going to stick with that. Shame that Yamaha hasn't updated the 9.9high thrust in 20 years though. Would be nice to be able to buy a fuel-injected outboard. The only diesel outboards are really too big and heavy for the T38.
I do wonder about one issue doing it with diesels and long shafts. I was on a Pahi 42 recently with a small diesel running a long shaft. According to the owner the engine was useless in a swell since the prop was out of the water so much. Something to bear in mind when you're designing it. A motor is not much use if you can't use it to go to windward against any kind of sea...
As for brands, I have no personal experience except with the small, bulletproof Yanmars but there are a few brands of small stationary diesels available, often used in generators. Kubota has a good reputation, in agriculture and industry at least.
I know some people have successfully installed diesels on the bridge-deck area, using a long shaft like the Thai boats have. I can't see why this would come out of the water if it is installed right? Surely it is adjustable? Personally we have twin Yamaha 9.9s on our Pahi 42. Certainly are able to push us against strong headwinds if needed (but prefer not to do that if possible!!). Very good point with outboards is that you can take them off for winter storage and service.
What type of engine set up you would choose depends on where you use the boat . If you sail in area with ( normally) no strong currents , steep waves, etc then
two outboards of about 10 - HP will be just fine . Advantages and disadvantages of outboards have already been pointed out previously .
If you do need more grunt then one or diesels would be nice.Installing a diesel is a lot more complicated then an outboard .In addition to the engine you will also need a gearbox ,this assuming you would like to be able to go reverse as well.
If you buy a ready made marine engine it comes with a gearbox . If you buy an industrial engine you will need to find a gear box as well and make a bell housing to marry the two together .This is a specialist's job.
Comes the next and biggest problem which is how to drive the prop(s). I have yet to see a long shaft arrangement which works well , does not take lots of usable space and is reliable .
Installing the diesels inboard a T 38 is not practical as you will lose a lot of living space ,so this is not practical..
Our Tiki 38's have a single 24 hp diesel mounted just aft nr 2 beam . The engine does not drive a gearbox but a hydraulic
pump . The pump in turn drives two hydraulic motors, one in each hull . Hydraulic motors are small and take up no usable space . Props are driven in the conventional manner by a straight shaft. Manoeverability is outstanding. She turns on a sixpence. In flat water she does 7- 8 kts flat out cruising speed is 6kts.
Advantages of hydraulic drives are that you can put the engine(s) pretty much wherever you want and you lose no interior space. Disadvantages are weight, rather complicated installation and in case of trouble ( say, a broken hydraulic hose) you can not improvise repairs . Hydraulic drives are not exactly cheap either . If I was to do it again I'd have two smaller diesels ,each with it's own hydraulic drive,rather then the single diesel . This for reasons of redundancy . As a whole I am very happy with the hydraulic drives . Our boats lead a hard life but I have not yet experienced any serious trouble with the hydraulics.
As a whole : if you can get away with using outboards I'd use those . If you nevertheless decide for diesel then I'd use hydraulic drives. If you choose outboards but think you might at a later stage install diesels and hydraulic drives it makes a lot of sense to already make the prop shaft tunnels and block them off until required . They are a lot easier to make when the boat is being built then retro fitting them