Outboard vs Jet Drive propulsion capability

I know that there's some power loss in a jet drive vs. a traditional prop driven outboard. I also know that the jet drive size/setup has a lot to do with how that power is delivered.

Anyhow, what I'm trying to figure out is how large of a boat I can use for my potential jet boat project. I think the general accepted ratio for boats is 1 HP for every 25 (at most) pounds of weight at full load.

However, the 2 stroke, 700cc engine I'm planning to use to motivate the project was apparently rated at 80hp using 1997 standards. That means that it could run a boat up to about 2000 pounds. The engine/drive setup seems pretty small for that. 

so.... Sea-doo/rotax 718?  

for nearly the same money you can usually find a 787/781 which makes about 110hp if you haven't already bought the motor.  its a little more complicated with its power valves but that lets you set them for a broad flat power curve, or to reserve max power for the last 10% or so of throttle.  

edit:  if you are using a PWC powertrain, keep in mind they would do about 50-55mph but only weighed around 5-600lbs.  the larger models seadoo made using the same motors had more "boat-like" top speeds, for example the explorer RIB would do about 30-35mph while a 2 or 3 seat ski would make around 48-52mph.  

This is relevant to my interests.

What size boat are you hoping to run?

My thinking was to use this 15' 59 Dura Craft aluminum, but looks like the splashwell would need major surgery. I imagine 80hp in a boat that light would he a hoot though.

Dura Craft Link

captainawesome said:

This is relevant to my interests.

What size boat are you hoping to run?

My thinking was to use this 15' 59 Dura Craft aluminum, but looks like the splashwell would need major surgery. I imagine 80hp in a boat that light would he a hoot though.

Dura Craft Link

There are a few videos on YouTube where people basically cut the motor mount-pump shoe-jet drive out of an old ski and graft it into a boat - usually a Jon boat. The challenge you'll run across is probably steering as the pump nozzle is short travel so like 1/2 turn lock to lock. 
 

if you start with a fiberglass boat to begin with, it makes it even easier to glass the ski parts in, but the driveline would be quite a bit longer than even an I/O setup, so probably some floor modification would be required too. 

a 15-16ft runabout would be a great start, as seadoo put single 718s in their 16ft boats, as well as twin 718s and single 787s. The challenger 1800 even had twin 787s. 

Jets don't become efficient until you get to rather large HP numbers.  I'll give you a little story to demonstrate.

Back in the day, outboards were rated at the powerhead.  A 40 hp outboard actually put about 35 hp to the prop.  So if you bought a 40hp outboard with a jet lower drive, you might only be putting 25 hp to the water.

Newer outboards are rated at the prop.  A 40hp outboard puts 40hp to the water and the powerhead might make 47 hp.  That changed the way a jet outboard had to be rated.  Now they're usually marketed as a 60/40, meaning they took the 60hp outboard (which might make 68 at the crank) and have to advertise it as a 40 hp motor because that's all that makes it to the water.

Jet drives have fewer parasitic losses, but the liquid-liquid thrust is not efficient.  With a prop, the power loses are proportional to the power.  If you lose 15% power getting to the prop, you lose a lot more power with a 200hp outboard than you do with a 20hp.  Jets on the other hand have a more fixed loss of (let's say) 20hp.  At low hp levels, they lose up to 50% of their power.  At large hp numbers, that 20hp loss is only maybe 4%.

Jet Skis don't have jet drives because they're fast, they have jets because the liability of people's legs being in close proximity to a swirling, 150 hp food processor is a non-starter.

All of those fun jet boats from the 60s and 70s often had a featherweight hull and a big block making 400 hp, and they still only made it to about 70-80 mph at best.  If you were somehow able to hook up an outdrive to that same 400 hp, you'd be looking at 120 mph or more.  Actually, you'd be looking at the ceiling of a hospital room, but I'm speaking hypothetically.  Or, to look at it another way, you put 150 hp in a relatively light jet ski with a jet drive and it does 60 mph.  But if you put a 150hp outboard with a prop on a 16' bass boat that is 8' wide and weighs three times as much.... you still get 60 mph.  Jets don't make a jet boat fast.  Making enough power to overcome the jet's inefficiencies makes them fast.

Jets (in most situations) should be a drive choice as opposed to a performance choice.  Shallow waters, proximity of props to humans, and other outside factors have a large role in choosing a jet drive, but (oversimplification here) jets are to performance boating as automatic transmissions are to motorsport.  It's rarely chosen because it is a better-performing means of getting power to the water.

Brett_Murphy (Forum Patrón) said:

However, the 2 stroke, 700cc engine I'm planning to use to motivate the project was apparently rated at 80hp using 1997 standards. That means that it could run a boat up to about 2000 pounds. The engine/drive setup seems pretty small for that. 

 

Depending on hull design, an 80hp jet wouldn't even get that thing on plane.  A prop would, but doubtful a jet would.

I shoot for about 15 lbs per hp as a minimum with a prop.  Jets would need less weight per hp.  Current boat, for instance is an 18' I/O that weighs 2130 empty (probably north of 2500 with me, gas, anchor, etc) and has a 140hp engine with a prop.  Top speed is about 35, and it takes a bit of time to get on plane.  That ratio is about 18 lb/hp.  Can't pull a skier out of the water unless they're very light.

Curtis73 (Forum Supporter) said:

Depending on hull design, an 80hp jet wouldn't even get that thing on plane.  A prop would, but doubtful a jet would.

I shoot for about 15 lbs per hp as a minimum with a prop.  Jets would need less weight per hp.  Current boat, for instance is an 18' I/O that weighs 2130 empty (probably north of 2500 with me, gas, anchor, etc) and has a 140hp engine with a prop.  Top speed is about 35, and it takes a bit of time to get on plane.  That ratio is about 18 lb/hp.  Can't pull a skier out of the water unless they're very light.

I've got a 20' Key West dual console for family boating, this would be a build just for me. I already own the jet ski, having picked it up for $150, and I don't really want a jet ski.  So this is more of an exercise in potentially getting use out of this vs. selling it.


The goal is a shallow water 2 person fishing skiff. I'm not looking for it to be fast. It'll probably spend most of it's life getting poled around really shallow water, but I want it to be able to get on a plane to and from the boat ramp. To this end, I'm looking at boats around the NC area. There are some tri-hull boats for cheap, but they're dogs. There are some fiberglass row boats available, but the hull design of those isn't optimal. A old bass boat might work. There is a 16' Key West Center console available, but that seems too big.

In reply to Brett_Murphy (Forum Patrón) :

14' or less with a Jon-boat hull is your best bet. The shallow-v or flat bottom will plane easier, and smaller boats are lighter. There are some tweaks with different pitch impellers that you can do, but those will be for keeping the motor in the powerband, not outright hotrodding. You may need to add trim tabs to the hull to help it plane. 

https://youtu.be/C2FZLV8_hOQ 

this guy grafted a Yamaha power unit into a 14ft job boat. The yami has some disadvantage due to the factory layout of the power train, the skis in those years had a driveshaft that ran back to the jet pump so as you'll see they don't leave much room in a Jon boat. 

I'll leave this here. JetBoat

I used a 50 hp Yamaha jet drive and a 15' boat that probably weighed in at 400 pounds. The finished product probably weighs in at 800+ pounds. With 2 people in it it will run 25-30. It will also run in 6" of water.

Curtis73 (Forum Supporter) said:

All of those fun jet boats from the 60s and 70s often had a featherweight hull and a big block making 400 hp, and they still only made it to about 70-80 mph at best.  If you were somehow able to hook up an outdrive to that same 400 hp, you'd be looking at 120 mph or more.  Actually, you'd be looking at the ceiling of a hospital room, but I'm speaking hypothetically. 

Dammit Curtis!  You can't be super knowledgeable about everything and be a master of subtle humor as well.  You are setting the bar too high for the rest of us!

To see how lightly built those jet drag boats are check out some of Finnegans drag boat build videos.   The hull is so thin he can’t put weight on the bottom when it’s on the trailer, he’d punch right through it. 

X2 on a relatively small flat bottom boat for your shallow water project.  It might be easier to modify a small fiberglass hull to suit the jet if you can find one. 

We were at our sons lake house recently and I was watching a approximately 15 foot jet boat. He was pulling a couple kids on a tube. Boat struggled to get on plane pulling kids on tube but once on plane seemed to be just fine. It seemed jet skis were having no problems pulling kids on tube so there must be something to the horsepower to weight ratio. I don't know anything about ability to trim nozzle which is probably also a factor. I do know Sea Do made some small (13 or 14 foot) jet boats for while but you hardly ever see them so that probably indicates the performance they could achieve.

Thinking about this one. It's already beat, so if I botch it, no big loss. $650 asking price with the trailer. This is probably the best bet.

The Tri-hulls keep popping up, too. They're a bit wider and heavier.

It will be much easier to work with a mostly flat bottom. 

Toyman01 (Moderately Supportive Dude) said:

It will be much easier to work with a mostly flat bottom. 

 

How do you keep the intake from suffering cavitation with a flat bottom? I've seen some people develop "spoons" for this, but since you've already been there and done that, I figured you might have more info.

My only cavitation issues are due to wave action. On flat or mostly flat water it sucks no air. If I had it to do again, I would slide the engine and pump back about 12 inches and let the pump stick out past the transom. That would give the air between the waves more time to get out from under the hull before the intake. Also the transom is always deeper than the rest of the bottom. Moving my seating position forward solved 90% of the problem though I do get some air in the pump if the water is very choppy. Slowing down a little solves that problem. 

As someone who just happily got rid of a seadoo challenger 1800 with twin 787's I thing you're crazy to do this. The drivetrains were the worst part of that boat and were always giving me issues.  Now I've got an inboard with a 351 and it's so much more reliable. I actually get to have fun on the water and not just work on my boat. 

In reply to mw :

I'm using a Yamaha engine. It has been extremely reliable. 

Brett_Murphy (Forum Patrón) said:

Curtis73 (Forum Supporter) said:

Depending on hull design, an 80hp jet wouldn't even get that thing on plane.  A prop would, but doubtful a jet would.

I shoot for about 15 lbs per hp as a minimum with a prop.  Jets would need less weight per hp.  Current boat, for instance is an 18' I/O that weighs 2130 empty (probably north of 2500 with me, gas, anchor, etc) and has a 140hp engine with a prop.  Top speed is about 35, and it takes a bit of time to get on plane.  That ratio is about 18 lb/hp.  Can't pull a skier out of the water unless they're very light.

I've got a 20' Key West dual console for family boating, this would be a build just for me. I already own the jet ski, having picked it up for $150, and I don't really want a jet ski.  So this is more of an exercise in potentially getting use out of this vs. selling it.


The goal is a shallow water 2 person fishing skiff. I'm not looking for it to be fast. It'll probably spend most of it's life getting poled around really shallow water, but I want it to be able to get on a plane to and from the boat ramp. To this end, I'm looking at boats around the NC area. There are some tri-hull boats for cheap, but they're dogs. There are some fiberglass row boats available, but the hull design of those isn't optimal. A old bass boat might work. There is a 16' Key West Center console available, but that seems too big.

I love where you're going with this.

I can't really jump inside your head and think of what you would like, but (as others have said) I would personally go with something like a John Boat.  Flat bottoms tend to plane super easy, and they also draft less for a given weight.  The aluminum might be a little tougher to graft in your jet drive, but epoxy is a beautiful thing.

Also, many of the cookie-cutter 14' vee aluminum boats (alumacraft, sylvan, etc) taper the vee to a flat/barrel bottom at the back.

Finding a lightweight fiberglass boat with a flat bottom isn't common, but it can be found.  Have you thought about building one?  It wouldn't be as cheap, but plywood and fiberglass/epoxy you could make exactly what you want.

Around me, I found these that are ideas of what is in my brain, but maybe not yours.

https://www.facebook.com/marketplace/item/137301934328218/ I don't really like the freeboard/chine design of this one.  I minimizes floor space and adds a couple inches of draft, but it's a fiberglass flat bottom for the most part.

https://www.facebook.com/marketplace/item/314317839927961/ Here is one of the ubiquitous random 12' aluminum boats.  For this price you could buy it, sell the motor for $750 and have a $250 boat and trailer.  12' is a too dinky, but they're made in 14 and 16'

https://www.facebook.com/marketplace/item/287202945733599/ This is one that popped into my head.  It's a Sears Gamefisher from the 70s.  Pretty solid little fiberglass hulls.  By the 80s, I think they stopped making it in fiberglass and went to a more typical aluminum john boat style.  I think they made 12, 14, and 16' versions of this hull.

As an add to what Curtis posted. I would look for something with a single bottom. A lot of older boats are double bottomed for floatation and to keep the stringers under the floor. Dealing with a doubled bottom would be a pain. 

This Sears Gamefisher that Curtis linked looks like a great platform for a skiff, but it looks to be double bottom.

I contemplated using an aluminum semi-V for a buck and building a boat, but I think it's far enough outside of my area of expertise and working space that it's a non starter. If I had more room to store a buck and materials, I might give it a shot.

I do want to stick with fiberglass, since I can cut up the jet ski like Toyman did and just graft it in place. I've seen builds by people who can TIG well enough that they fabricate a mounting and intake system into an aluminum boat, but that's also beyond my skill set.

The search continues.

The pandemic has people digging around in the bushes on their property, posting things for sale that would've otherwise gone to the scrapper, and people are apparently buying them.

mw said:

As someone who just happily got rid of a seadoo challenger 1800 with twin 787's I thing you're crazy to do this. The drivetrains were the worst part of that boat and were always giving me issues.  Now I've got an inboard with a 351 and it's so much more reliable. I actually get to have fun on the water and not just work on my boat. 

Coming from a former Seadoo Speedster owner, this hits home.  Mo engines, mo problems.  Now I have a stern drive 19' Four Winns with an exotic Volvo Penta 4.3L (Chebby) engine and a Volvo SX (OMC) outdrive....pretty simple.   This was a 'free' boat, given from my FIL when he moved to the beach and now needs a bay boat.  Still always seems to be plenty to work on.

If I were buying another boat, it would have an outboard.

Pictured below is my most recent project boat and what I went with for propulsion.  Restored a low-hour 1976 4hp Johnson.  

Regarding the project in question -- make sure you consider room for fuel.  Pushing a bigger boat, that thing is going to be thirsty.