Auto Diesel for Jet

[errant]

G'Day fellas, I'm currently in the planning stages of a boat build and need some advice on powerplants for the jet drive I plan to use (not finalised as yet).

I understand that the load differences between marine engines and auto engines are very different, many people do not recommend using one. This in mind, I have seen a lot of jet boats that use auto engines. I am wondering if the characteristics of the Jet make it a viable option. Also, the engine MUST be diesel as I cannot obtain ULP at a reasonable price where I live, I currently have to barge in 205L drums at great expense. Diesel is available and also does not go stale over time. Economy is also a major factor in choosing the Diesel as well as my being a Diesel Mechanic and loving the sound of a TD! I have been contemplating using a 2.4 4cyl TD Toyota Hilux/Surf plant. Will make 80hp no worries, my target figure. It will make it at approx 2400rpm, will this be sufficient RPM to drive a jet unit?

Cost ratio of a rebuilding my 2.4TD Hilux engine to a marine 80hp is about 1:4 so is a major factor in wanting to use one. If there were diesel outboards available, I'd just use one of them, but its a lost cause currently.

The boat is a 5m aluminium inflatable style and used for diving/fishing but shallow water operation is necessary due to the tides here which can vary 8m between Hi-Low.

So, my question is... Is the auto TD a viable option for a jet drive?

Cheers!

[Frosty]

Youve got the cart before the horse. Decide on a jetunit and impellor that will propell your boat at the speed you want, then ask the manufacturers for a suitable unit suggestion with Hp toque required to do that.

Its the only way to guarantee its success.

Auto trannies,--naaa. If it was a good idea we would all be doing it.

[errant]

Wasn't intending on utilising an Auto transmission... Just referring to the engine being a marinised automotive one rather than marine.

Am I just dreaming that I can use a 2.4L TD "CAR Engine"?

Thanks for the feedback though, I'll get in touch with some manufacturers for ideas.

Cheers!

[baeckmo]

There are two decicive factors in waterjet propulsion; first the Power/Weight ratio, and second the critical minimum speed. Due to limitations in inlet energy recovery, cavitation will limit the operation at full power-low speed.

Thus it must be understood that in general, WJ installations have to be designed for a minimum speed, fully laden and in adverse weather conditions, of ~25 knots! Anything less will result in reduced life-time and catastrophic efficiency. This limit then gives a minimum Power/Weight ratio that you must reach! Anything extra is a bonus that gives increased top speed.

Now, if you open the attached diagram, you will find that this minimum speed requirement will set a maximum weight limit with whatever engine output you have. For instance, if you limit your Hilux engine to some 80 hp, the maximum weight, fully laden with fuel, beer, ladies and whatever else there is to comfort you, of one metric tonne.

So: start with an honest weight estimate, make it pessimistic by adding 15 % to your final result. It may take a second turn in the design spiral if you come up with a weight that needs more hp to pass the limit, but that is what it takes to avoid expensive mistakes.

Then you come back and we will guide you through the engine/jet matching steps, and don´t listen to Frosty in this case.........

[errant]

Great reply baeckmo, thanks mate, just what I needed.

Hull - 600kg
Engine/jet - 300kg
Passengers - 200kg
Everything else - 300kg max
Total : 1400kg
+ 15% 1610kg

So about 600kg over for an 80hp! Would need to up that to 120+ to cover that weight I'm supposing. I'll have to look at a different engine, 120hp would not be reliable in a 2L-T.

120hp is a bit more than i wanted on a 5m, but it's not like i want to be at WOT 100% of the time. It could be put on 3/4 throttle to conserve the life of the engine and fuel consumption. But, would also have that extra oomph when needed.

I note that the Mercruiser 120hp engines are 1.7L, the engine I would use to make that power reliably would be 3L. Toyota 5L 3.0L diesels are very common here so that may be an option.

[Submarine Tom]

"Economy is also a major factor in choosing the Diesel"

If so, are you sure an inefficient jet is the way you want to go?

Wouldn't a well protected prop be better?

-Tom

[errant]

I had thought about that too Tom and its still on the cards. Just exploring the possibilities. It may be the way to go too, being that I could use a smaller engine to do the same job, making it even more efficient. I have considered an Outdrive as well as a shaft drive too.

Main question is still whether or not an automotive Turbo Diesel is suitable and will be reliable when used in a marine application, assuming its built correctly and of the proper HP.

[Frosty]

There is nothing beats the efficiency, price, performance, weight, compact, reliability , ease of repair or replacement of a suitable outboard for a 5 meter RIB

Jets are not maintainence free, diesels are noisy heavy, I would think seriously of your direction.

[errant]

Did you even read my 1st Post? You can't get a new diesel outboard and Gas ULP is DOUBLE the price of Diesel here, transport etc, many factors. For a 44 gallon drum, "Gas" is costing me $500 USD. Also, the boat will be moored and I'll be unable to flush an outboard regularly. Direction thought about seriously, many many thousands of times. Whatever the new engine is, it MUST be diesel and NOT use raw sea water cooling.

[Frosty]

I read it and answered it, but you still want to talk engines before you know what jet you need.

My opinion of outboards still stands. There are diesel outboards but I havnt seen one.

[errant]

At this point Frosty, I do not care what jet/propulsion system I need. I'm not that close to doing anything about it yet. I just want to know whether or not I can use an Automotive turbo diesel to drive whatever I get in the end.

Once I know whether or not I can do that, I'll find out what jet I need, then match an engine. If I can't do that, then I'll have to look at other options as mentioned earlier, such as getting a proper marine diesel.

Believe me, I'd much, MUCH, rather grab a diesel Outboard from the shop and bung it on, it would be that simple. If it were that simple, I wouldn't be here asking for advice.

However, I have been looking for one for 3 years and they just DO NOT EXIST in any form that is suitable, obtained easily or have parts available for. The ones that do exist are under 40hp or over 175, either discontinued models, "In development" or produced by a company that only exists overseas.

This is the reason I want to use an automotive TD, they are everywhere, parts are easily available and cheap. Not to mention I can rebuild it myself and do any repairs to it. The latter applies to a Marine diesel also, except that the initial purchase cost is about 400% of that of an Automotive TD. Then it would cost me double in parts to rebuild it.

[Frosty]

Yes you can use an automotive deisel, the Yanmar 6LP 250HP is a Toyota Landcruiser engine 1HD,--I have 2 of them Toyota is written all over them.

I believe they now use the BMW range of diesels in that HP range.

Diesels for a 4 wheel drive application are generally set up for torque and torque is what you need for boats.

Jets however are not quite the same they dont screw like props, hence you really need to talk to the jet manufacturer.

[Mr Efficiency]

Five metres is mighty small for a diesel powered planing boat. The hull design would be crucial, and I suspect the list of suitable candidates very restricted. Is this something like the stabi-craft idea ? Is there gonna be much room for people after fitting the drive combo in the guts of it ? Some boats don't adapt to jet drives, most of the good ones have cutaway forefoots which helps make up for the lack of lateral plane at the rear, with no drive leg there, and the higher line of thrust you get with a jet.

[errant]

Yeah, its very similar to a stabicraft. The engine I was originally planning on using would be a fine fit, but it is not powerful enough. Like you said, 5m is small. I mentioned above that i though the 120hp needed for a jet drive was too big, and it is. I'm pretty well buggered when it comes to anything other than an outboard it looks.

[Mr Efficiency]

In your case being a diesel mechanic and the ready availability of engines, plus the fuel price issue, you are pushed in a certain direction, but those engines need marinising which is not a minor or cheap job. And jets sap power like nothing else, your boat would likely disappoint performance-wise. For open water use, jet boats really don't have much going for them, but of course they shine in their natural habitat of river rapids or the like, or as surf rescue boats where you want to avoid chopping limbs off people with propellors, or bottoming out on sandbars.