Anybody powered a boat with jet ski works?

My mahogany runabouts have very little vertical room for a 4 cylinder inboard engine- the only ones I've used are a Subaru and Mercury's Vazar sterndrive- which they don't make anymore. My boats are light weight, deep V hulls that don't require a lot of torque to drive them. So I've been thinking about buting a new jet ski and salvaging the engine and drive train. I recollect a WB article a few years back about this very same idea- and it was used successfully in a modern planing hull. How about some analysis here guys? What are the other considerations? I'm much attracted to having all the systems worked out- and all I have to do is transfer them to my hull. And I'd have the added benefit of taking a sawsall to a brand new jet ski and eviscerating it.

 

You build beautiful boats ,but the all up weight of the smallest is nearly a ton according to your website. You really need a larger diameter jet to give you satisfactory performance ,shame to compromise. Have you considered resumping with an angled sump ,and laying the engine over ,or even drysumping.

 

I remember that WB story and it was a new build 26ft lobster boat and the guy bought a brand new very large PWC as a donor,they claimed better fuel economy and quite respectable performance.
Steve

 

I suspect your hulls are too deep in the belly for reasonable expectations from a jet drive.

 

As Par noted your hull is not set up for a jet drive, but that doesn't mean that you couldn't redesign it below the waterline to make it work. If you look at some of the larger jet skis they are getting close to 1,000 pounds of empty weight, and some of the larger ones have a capacity of 500 pounds, so if you consider how much you will yank out of the boat by going to the lighter and smaller engine, you could be getting into the range where it might make sense. You would need to be down close to 1,000 pounds empty for it to work, but that could be possible. You are just going to have to make sure you keep it light to make it work.

With as much power as the biggest PWC's drive systems can put out you will have plenty of power. Bigger question is the design of a hull that will work with as much weight forward as you will have with two people in it and all of that weight forward ahead of the engine.

Your double cockpit boat is getting heavy for this drive, assuming that you put 4 people in it, and all of that weight is forward, so both of those are an issue. If you had an aft cockpit with the engine in the middle that might work in terms of balancing out the boat.

Might be easier to design your boats with more height so that you can accomodate a more typical 4 cylinder engine standing up and put in a I/O or a v drive. The PWC route is going to be relatively expensive unless you find some new units that were damaged in shiping and could be salvaged for a reasonalble price. A classic I/O or a V drive with a straight 4 or a V6 auto engine is going to be a lot less expensive than buying a jet ski and just taking out what you want.

 

you're right yellowjacket

a 4 cylinder marine engine- like the Vazar is just perfect for my boats- but they don't make them anymore and I'm having trouble finding ones to buy

 

Nice boats, great idea, but man it's going to cost you , and not just monetarily. The odds are stacked against you but you could make it work... eventually. So, if that's where your heart (and wallet) are at, go for it, all the power to 'ya my friend.

Other considerations: a flat four, Saab, Porche. OR, how about a turbine with reduction gearing? Yellowjacket could light up your world on that one! He is very knowledgable in that area.

-Tom

 

At first i thought you meant putting a jetski jet drive on the stern of a sailboat...that seemed like a good idea...for a second...lol

 

Yellowjack come back please

I think you are right- I'd need a flat bottom for a jet ski and mine is nearly a 17 degree deep V. Stern drive is perfect and weight distribution is about right. Is there a 4 cylinder plus I/O that you think might have a low profile?

 

I am reluctant to incur the wrath of my peers particularly those with so many rep points,so I will phrase this as a question. That way it appears less confrontational. Why is this not a good jet hull ?True the forefoot is a little deeper than ideal ,but there appears from the photos to be plenty of lift in the bow, it has a constant deadrise ,no warping, and longditudinally straight over the planing area ,and no sign of a hook in the transom. A flat bottom as suggested is of course out of the question but surely 17 degrees will work well. As regards weight distribution if you look on the Naiad Dynamics site you will see the perfect jet for the larger of these two hulls ,this will put most of the jet outside the boat. So having played the devil's advocate, is this a good idea ?
No of course it bl**dy isn't.This compromise will ruin a beautiful concept.
, this is a Gentlemans Runabout circa 1930's there weren't any jets then ,so unless you can find an art deco model it won't look right and the performance envelope of a jetski power plant would be the final straw.

 

Here are the dimensions of the 3.0L GM marine engine that has been around for decades.
http://www.gm.com/experience/techno...s/specialized/marine/REV_2011_3000_Marine.pdf
4 cyclinder and measures 25"x25"x25" with manifolds

 

A jet certainly could be fitted, but you'll need to alter the bottom a fair bit to get good inlet flow conditions.

 

Could you elaborate a little please I'm still not with you

 

The bottom of the boat will need a clean entry into the jet inlet and a deep V makes this difficult without some hull bottom modifications. A simple flat, carved into the bottom will suffice, though you'll lose a little bearing area likely not enough to get all excited about, except at quite high speeds. So essentially a redesign of the framing system and affected stringers, keel, it's batten, planking, etc. will need to be employed from say about 1/2 way to 2/3's aft of the immersed entry, to the transom.

 

Thanks for reply PAR I don't know if you are familiar with the jet I referred too ,they have been around for 30 years or so ,a great testament to the designer. Much of the inlet ducting on this jet is aft of the transom ,the forward part comes as a premoulded insert for the hull. Although this is intended to be moulded into GRP they can be pre-reinforced, reflanged, and bolted into wood or aluminium. I have fitted several of these and have found that this ducting will merge seamlessly with a V of about 17 degrees no more than a couple of feet or so from the transom and provide excellent intake characteristics. This jet is not intended for very high speed ,but it works well up to about 50kts in my experience, perhaps more.