Thread: Anyone built a mini jetboat? 3m?

Hey guys, starting to build a 3m jetboat with a Seadoo 787 rfi and 787 jet unit. Was just wondering on what tips you could give on the build and what steering, throttle, intake and grill etc was used, cheers guys

@Spanners had a go at a couple I think.

Steering use a 40series Morse cable. Lighter more simple and a lot cheaper then wire rope/pulley set up.
Throttle 30series Morse cable with a triple SSS foot pedal works well, don’t recommend a hand throttle (maybe that’s a personal thing)
Intake ASP cast one, they are set up for jet ski engine heights and well tested etc also can get grills to suit, or easy enough to make just set it up correctly..
Intake can be made easy enough but sometimes it ain’t worth it for the price of a well designed cast one it can save a lot of time/hassle.

Other advise would be don’t use a 2 stroke... (probably a personal thing as well) although quite suitable for light little skinny water basher

Tagging along regarding the hours of a JetSki motor. The internet says around 300 but I would like to hear real experiences

Couple of guys in Rotorua have built them with good results. Videos on YouTube of them zipping around in creeks.

A lot of those two stroke ski motors are getting on now with unknown history, a lot of small boats with 2 strokes come up for sale often and often for a reason..
My opinion your far better off using a small car engine for not a lot more weight, Get a shit load better fuel economy, more reliability, last 100x longer, less noise, if you were to cook it scrap it and grab another short block for cheap and go again, similar power or more power if want to spend the $ on the engine..

A set of rings and a gasget set on a two or 3 cylinder 2 stroke doesnt cost much compared to rebuilding a foursroke tho.

I never managed to wear one out but had two secondhand two cyl strokes of unknown hours and would have put at least 100 hours on one of em and one new 3cyl two stoke.
The new one had its first service at 10 hours (new plugs and check over) and 100hr services after that the same as most 2smoke outboards....I only put about 80 hrs on that one but reportadly they would do 400hrs hard use before a freshen up (rings, clean powervalve etc) if not being thrashed I imagine they would do a lot more as 2stroke outboards will do thousands of hours.

You are better off getting the cast housings made for the mini boats. They have a bigger intake. My brother had one. The engine was unreliable. I sunk it in willows when it crapped out in the river. Replaced it with a suzuki swift motor. Went good for a bit till he got the grill clogged with stones and then over heated it. New engine and then sold it. I have a mate has a couple he built and have been really good.

stick a rotary in it

Originally Posted by Dev

A lot of those two stroke ski motors are getting on now with unknown history, a lot of small boats with 2 strokes come up for sale often and often for a reason..
My opinion your far better off using a small car engine for not a lot more weight, Get a shit load better fuel economy, more reliability, last 100x longer, less noise, if you were to cook it scrap it and grab another short block for cheap and go again, similar power or more power if want to spend the $ on the engine..

I know fuck all about jet boats other than they look like great fun, but have walked past and had a nosey at a couple in gas stations and was surprised to see K series Honda engines lurking. Those things would be bloody cheap ($500) to get ahold of, they're in about 10 different models of Honda, and just swap over the ancillary bits and bobs. You could literally stockpile a couple at that money/availability. I know of several that are super and/or turbo charged in sports cars and go like the hammers of hell.

Originally Posted by Tommy

I know fuck all about jet boats other than they look like great fun, but have walked past and had a nosey at a couple in gas stations and was surprised to see K series Honda engines lurking. Those things would be bloody cheap ($500) to get ahold of, they're in about 10 different models of Honda, and just swap over the ancillary bits and bobs. You could literally stockpile a couple at that money/availability. I know of several that are super and/or turbo charged in sports cars and go like the hammers of hell.

Vtec just kicked in bro!

Originally Posted by Cam7mm08

Hey guys, starting to build a 3m jetboat with a Seadoo 787 rfi and 787 jet unit. Was just wondering on what tips you could give on the build and what steering, throttle, intake and grill etc was used, cheers guys

What hull are you using? As others have said, the cast ASP intake with a matching grill is a good option. I’ve finally finished a 3m Wattscraft, just have a couple of things to tidy up. Has been in the water already. I reckon a foot throttle is good over the hand throttles. My opinion only though. Think ahead when fitting it out. I’ve put in a bench seat but won’t suit long legged buggers so consider bucket seats for extra leg room. Keep it light and simple.

Originally Posted by Tommy

I know fuck all about jet boats other than they look like great fun, but have walked past and had a nosey at a couple in gas stations and was surprised to see K series Honda engines lurking. Those things would be bloody cheap ($500) to get ahold of, they're in about 10 different models of Honda, and just swap over the ancillary bits and bobs. You could literally stockpile a couple at that money/availability. I know of several that are super and/or turbo charged in sports cars and go like the hammers of hell.

I have a k24 turbo on ethanol in a 3.8m.It is a little more complicated than you make out but k24 is a good choice but depends on what you want in a boat.A $500 dollar motor is a drop in the bucket in the overall build price.

Certainly sink the hours into them when building esp when you have teething problems, id do it again but wouldn’t use the cunty Rotax 787

From a kit set hull 400-600 man hours depending on build quality and how well your workshop is set up is not uncommon. A lot of knowledgeable chaps on the jbnz forum.