wanna convert jet boat to outboard

Hi currently i have a 94 dynasty jet storm 15' 6'' with a 90hp force/mercury sport jet , the boat is kinda slow on take offs and does not go very fast about 30 -35mph. im looking at converting the jet drive to a outboard motor sense the 90hp jet drives are not very reliable ignition wise and are kinda slow for this size boat. what i am wondering though is what size outboard i should be looking for to put on the boat. i know i need to install a jack plate i have one that will hold up to a 150hp outboard but i know i dont need a motor that big for a boat of this size. all i want is a motor big enough to make the boat go about 40 and pull tubers i was thinking something in the ranges of a 75hp to a 125 hp but i don't wanna go to big or to small. any ideas. thanks

Re: wanna convert jet boat to outboard

Does that boat have a capacity plate? If so, it should have a max HP rating. Aim for 75% of that rating.

Have you thought of simply swapping your jet lower unit for a standard prop lower unit?

Re: wanna convert jet boat to outboard

Is this an outboard or inboard jet? If it's an inboard jet, I doubt if you'd be asking the question because converting to a prop would be a major reconstruction of the entire boat.

Assuming outboard, is it an "aftermarket" conversion, or did it come from the factory with the jet l/u installed? This factor may impact the feasibility of converting back to prop because factory installed jet l/u's usually don't have the shift linkages needed for a prop l/u gearcase. Aftermarket conversions have all the parts you need, you just need to find the appropriate prop l/u and install and cable it.

This factor ("factory" jet vs. "aftermarket" jet) also impacts what powerhead is actually in that engine. If "factory", what is labeled "90" might actually be a 115 or 125 powerhead because the labeling might reflect the actual HP at the jet, which is about 20-30% less than if it was a prop. If "aftermarket", it's probably an actual 90 hp powerhead, which means it is actually producing 60-70 hp and if you convert to prop, or buy a similar propped engine, you will be seeing a big jump in power.

More info? I've been running jet outboards for quite a while and am familiar with conversions since I have both jet and prop l/u for my outboard and it can be swapped back and forth pretty easily.

Re: wanna convert jet boat to outboard

its a inboard jet drive mercury sport jet 90hp. and its not ll that much haul reconstruction just fiber glassing the hole where the jet drive was and installing a jack plate and a new outboard with controls. the boat weigh just over 1000 pounds and will hold 6 people or 900 + people and gear. it does not have a max horsepower rate but im thinking a mercury 90 horse outboard would work might be kinda big sense the jet drive only puts out around 65 hp.

Re: wanna convert jet boat to outboard

313b0ss429 said:

its a inboard jet drive mercury sport jet 90hp. and its not ll that much haul reconstruction just fiber glassing the hole where the jet drive was and installing a jack plate and a new outboard with controls. the boat weigh just over 1000 pounds and will hold 6 people or 900 + people and gear. it does not have a max horsepower rate but im thinking a mercury 90 horse outboard would work might be kinda big sense the jet drive only puts out around 65 hp.

Click to expand...

Aha, I completely missed that you'd written "sport jet"! Sorry, disregard my lengthy post.

In my opinion, you're not only dealing with a HP issue, you're also dealing with needing to figure out if the hull will actually handle the major change in the powerplant. For the actual work on the hull to do what you want to do, you might post to the Boat Topics forum; I think I've seen discussions on I/O-Outboard conversions there. You might get some good practical advice.

Just off hand, the factors I might see coming into play are that the weight support of the engine and angle/torque of the thrust forces are quite a bit different with a floor mounted sport jet shooting straight out the back vs. a transom/jack plate mounted outboard whose thrust torques the engine and the bracket.

There might also be different support needed back there for trailering as w/ an i/o type of setup, all the weight just rests there, while an outboard as you might imagine, again kind of hangs off the back and can torque the transom as it bounces along. It might actually require some reinforcement of the rear of the boat.

It will probably require some change in your trailer set up because the outboard will definitely change the balance and therefore the tongue weight, and might affect handling if it's too light on the tongue.

But I hope it works out as easy as you hope...Good luck with your project. I'd be curious to see how it works out.

Re: wanna convert jet boat to outboard

thanks for the info on the trailer as it is the boat doesn't have much tongue weight so i will have 2 look into changing that some but that not a huge problem. as for the hull of the boat i was planning on reinforce the back where the jack plate would be mounted. was thinking i would fiberglass a 3/4" of plywood to the hual and then make up a steel frame that would mount the back of the boat 2 the bottom . so it would be jack plate bolted thru the hull and plywood and the steel frame which would take the stress off the back and spread it out along the mounting area that the sport jet motor sits. the i would turn part of my motor cavity into a live well for fishing. but still not sure on what size hp engine 2 get . also what brand outboards are know for reliability. i hate this force 90hp because the thing always has ignition problems ive had it die on me 3 times when i was out on the water lucky for me i have a trolling motor.

Re: wanna convert jet boat to outboard

Hey, just wondering if you ever went through with this conversion. I have a Four Winns fling (almost the same boat) and wanted to do the same set up.

Thanks,

I am getting ready to do this did you ever get yours done

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