small inboard diesel or outboard

I,m starting to look at power requirements for my build , a tristar 31 trimaran, disp 5200. Any pros and cons for an outboard or small diesel other than price. thanks rick

 

Go the four stroke outboard

I am great fan of modern 4 strokes like the Yamaha big foot series. They are quiet, sip juice and leave no turbulence in the water when retracted. A built in and composite pivoting sled is a great option to get them into the water when needed and out when not.

The cost of a shaft, stern gland, fuel tank, sound proofing, filters and folding prop will be about the same as the outboard, before you buy the diesel motor (was for my situation 10 years ago)

A friend of mine has a cat with twin 15hp diesels - folding props. They struggle to get the boat up to 7 knots in a calm and the thing vibrates away and is noisy as hell. On my cat I have a single 25hp four stroke and can leave him for dead. Under sail he has to drag folded props, shafts and struts around.

You will have more space inside if you go the outboard option. Look at how Jim Brown put the outboard on his 31ft tri.

http://www.outrig.org/outrig.org/Videos.html

cheers

Phil

 

I agree with Phil. A 10HP Yamaha outboard with the big prop and workboat gearing will handle your needs. One can push my 37' past 7 knots and will power into a sizeable wind. A 15 HP for a 31 would probably get you above 8 knots if you are thinking about the Panama Canal.

 

There are a lot of advantages to outboards particularly if you are cruising extensively I agree with Phil that a pod is the best application as it gets your motor out of the water when not in use and provides good access for repairs and maintenance and clearing the prop. You would want a reasonably longshaft outboard if it's transom mounted as pitching can cause your prop to pop out of the water.

The downsides of outboards are charging for your batteries is low from an outboard and fuel is more expensive a lot of people who run outboard equipped boats carry an additional small petrol generator to top up their batteries. You also have to be careful about carrying your petrol to prevent any risk of fire or explosion.

Diesels will crank away all day and all night with low fuel usage they provide good charging ability and great reliability. I'm not familiar with the layout of the tristar 31 in terms of engine bay access you really want to be able to get the engine out without having to walk it through the inside of the boat or remove sections of deck if access is poor I would not go the diesel option. Another consideration is weight diesels are not as heavy as they once were but still weigh a fair bit so positioning of motor and fuel tanks for good weight distribution is important. I also dislike having shafts through the hull or saildrives they are additional maintenance and subject to corrosion particularly if you are berthed in a marina where the water is full of stray currents this means more maintenance for you and less sailing, I've seen a retractable shaft for an inboard diesel on Boatspeed's website but its all a bit complex and it looks like trouble to my eyes.

 

There is a moter well designed for the outdoard on the tristar but I dont like the idea of a hole that big in my hull. I plan on a dagger board rudder, so the outboard would have to be off set or even on one of the wings. thanks for the replys, rick

 

Diesels are all not bad

I should have also put in that my mum's cat could do 8 knots with twin Yanmar 15s. They are a much better setup than my mates. The props were very pricey.

I also agree with Corley about charging. That said it is nice not to have to run the diesels every day to charge the battery. We could go into a long winded discussion about how best to use a diesel but they really should be run with load to avoid glazing the bore (so I am told). So running them to charge batteries or power the fridge could shorten their life.

Where I don't find the theory fits the facts is with the fuel safety issue. I see a lot of people with big diesels in their cats and they talk about petrol being more volatile which I agree with. BUT most have big 10 -15 Hp engines powering their RIBs. So they still lug around lots of petrol to power the dinghy. They may carry less pertol than me but only slightly. On our 3 years up and down the coast we used less than 300 litres of fuel. Carried about 60L most of the time in the 20 litre tank and two jerry cans. Jerry cans stored in non sparking (no electrics) overwing voids - easy to do in a multi. Fuel tank stored in draining motor pod.

One reason you can use less fuel is because the boat will sail better in the light. Use the saved money to buy a big reacher on a furler and when your peers are motoring you will be sailing. Less drag and weight and bigger sails.

I will be making a small generator for Kankama before we go away in three years to help the solar panels. It will be based on a typical 1000w generator motor and coupled straight onto an alternator. A small one could crank out 30 AH when needed for about 2 litres of fuel. Not bad. My friend with twin diesels has a generator - it is a diesel one.

One of the best things about outboards is that every 6 years or so you sell it and buy another one - for about the same cost as a reco on the diesel. I would really look at Scrimshaw's pod. My pod is great. It uses composite materials, even composite hinges and is bulletproof. It will probably do a better job for you to do similar rather than a well.

You can tell I like to sail can't you - turn the motor off!

cheers

Phil

 

Where are pictures of Scrimshaw,s pod. thanks rick

 

Check out the videos of Scrimshaw on outrig.org They are entertaining. I think Scrimshaw video #6 has the outboard pod.

 

I certainly endorse the outboard option. I had an inboard Yanmar on my 30' cat and seemed to spend most of my time head down, *** up in the engine bay fixing one thing or another. Quite apart from the initial cost and hassle of fitting the engine, shaft, skeg, control lines etc, the ongoing maintenance costs were horrific.

Conversely, the 15 HP 2-stroke long-shaft outboard I eventually added pushed the boat along at much the same speed as the Diesel and with far fewer problems. It actually never failed me but if it had, it would merely have been a matter of taking it ashore for service. No contest!

Alan

 

More about the friendlier nature of outboards. When one becomes recalcitrant you simply invite it into the salon or the cockpit for a polite discussion of the alternatives, in good light, and in a situation where you move it rather than yourself to study the nasty bits.

Due to the much higher number of engines sold, parts are slightly cheaper and more readily available, and there are no components requiring a clean room and specialized test instruments to make right.

Lastly, repropping to maximize efficiency is a much less harrowing experience; for example, you won't need a diver or a haul out to do it!

Still, there is a warm a fuzzy feeling about motoring into the night with the comforting thumpitythump of a small diesel that's charging the batteries, making water, cooling the reefer and keeping all the electronics happy....

In your case, a 20hp Yanmar on a saildrive will do the trick, giving you 6 knots on a half gallon an hour, and its a lot easier to install than a shaft drive.

I have both. I can't decide which I prefer.