440 kawasaki jet ski for a build but what size boat

[capt_dugout]

Hey guys I am new here and have been looking but can not find any info on sizing a motor with a boat for the jet john conversion. I have a 400 Kawasaki jet ski to use but have no idea what size boat to use. my father inlaw (150lbs) and myself (180lbs) will be the usual fishers/passengers. any help would be appreciated.
I have a 1965 alumacraft semi v hull with a early sixties model 35HP mercury outboard with electric start and choke but it has no charging system for the lights and we like to night fish. the jet john build would also allow us to get up in shallow spots w/o a $70-$100 prop rebuild each time igot into the rocks on bottom.

 

[catmansteve]

First off, I'll give my disclaimer of having limited knowledge of pwc's and inboard jets, but I'm kicking around the idea of a similar build, so I'll give my thoughts.

That's a pretty small motor, I'm gonna guess 25-40 hp? I would say you'd be safe with a 1448 boat or possibly a 1648 if you kept it very light. I have a 1648 with a 40/30 outboard jet, and it's the bare minimum for my boat to be able to get on plane quickly and run efficiently with two people and optimal weight balance, so I would err on the small side and stick with a 14' boat to be safe. You should aim to keep the gross weight, including hull, motor, and passengers somewhere around 800-1000 lbs max for decent performance. I'm defining "decent" as plane in 3-5 boat lengths and top speed of 20-25 mph, but again, these are just guesstimates.

 

[mercury jet]

I have to agree with cat 14/48 sounds good 16/ 48 at max. My old jet was a mercury 25-20 and i found out the hard way that it was not going to do the job. jets are a diferent breed than props lighter the better!!!!

 

[PSG-1]

That is definitely a small engine. The Kawasaki 440's were stand-up skis, intended for one passenger. I've been riding stand-ups since 1986, my first ski was a Kawasaki 550.

With a 440, you'd have to stay with a 12 foot boat to have any kind of power to plane off with passengers and gear. And then, it's liable to put so much load on the engine that it will sieze. Might not do it the first time, but it will happen.

Building one of these boats, when it comes to the type of engine needed, you have to consider all the factors. The first being hull weight of the ski vs. the boat. And when I say hull weight, I mean total weight, once you have installed fuel tanks, decking, etc. Then, you have to consider surface area of the hull of the boat vs. the hull of the ski. A ski is more narrow, and shorter, a lot less surface area and resistance to overcome to plane off.

Another factor.... passenger rating....what type of ski did the engine come from? If it came from a 2 or 3 seater type jet ski, then, it will probably be adequate for the job. But if it came from a single passenger stand-up ski, then, trying to use it to push a lot more weight is going to lead to engine failure. Adding a bigger pump just makes it worse. Sure it will probably plane it off, but again, it's overloading the engine, which will lead to failure.

Back in my teenage days, I'd sometimes take a girlfriend with me on my Kawasaki 550. Yeah, I know, it's a single passenger ski, this in itself would probably amount to a ticket for exceeding passenger capacity of the vessel. But the point I want to make is that even though we probably both weighed only 120 pounds or so, it was all the ski could do to plane off with that extra person. And this was only a few times, taking relatively short trips. I think the longest trip I made like that was from my house in Cherry Grove Beach to Bird Island at Little River inlet, cutting through the sound, probably a distance of 4 or 5 miles. I'm sure if I had run like this for long enough, I would have killed an engine from over-loading.

Most of us who have built jet johns have used 700+ cc engines (around 85 HP). Mine's an 1100 cc 4 stroke (160 HP), pushing a 1648 duracraft.

 

[lovedr79]

i had a 440, it would only move about 30mph with me on it when i weighed 150lbs. i wouldnt recomend using it.