Help finding the right size pump

I am planning to build about a 16-20 foot aluminum jet boat this summer an I'm looking for advice for the jet pump. I want about 100-150 hp. I'm gonna use it for river and lake fishing. I'm mainly wondering what size pump I would need. And what kind of top speed might I get?

 

There is a large difference on what is practical between a 16 foot jet boat and a 20 foot jet boat as far as what jets you might want to use.
Additionally, you need to provide a bit more information as for the chine width and deadrise of the hull. Around 20 feet, with say a 7 foot chine beam, (this is on the higher end) and a 6- 12 degree v, an inboard jet becomes feasible as far
as having some space inside the boat to fish comfortably.
You will find that an outboard jet completely opens up the cockpit and with the proper layout will provide ample room in around the 17 foot length. Transom to bow.
If you are running rivers with the odd rock, you should really look at a welded hull with a thicker bottom no less than 5/32" with adequate stringers.

 

100-150 hp jet is going to be tough unless you have access to lots of old stock. In that range of hp your almost exclusively outboard or repurposed jet skis setups.

A 16 foot Jon boat is a big difference than a welded 20 foot welded jet boat.

 

Yamaha has 3 sizes that would be appropriate, 90, 115 and 150. Note that these are relatively equivalent to an outboard prop. Ie the motor manufacturers uses say a 115 hp head (ie prop rated) add a jet on it and rate it as an 80 hp
At least Merc does.

 

Yeah it robs a tremendous amount of power especially if the boat has some weight to it.

 

Thanks for the responses. I want to do an inboard set up and I'm fine with using a jetskis drive train. I likely won't reuse the engine though because I'm not a fan of 2stroke jetskis motors and if I spent the money on a four stroke I would probably not want to rob the parts off it. I think 20 foot will be ideal mabe even 22, I have a 22 fiberglass center console now and like the size of it. What jet skis have would have the pump I'm looking for?

 

So for the motor what will you use?

A jetski motor/jet assembly runs extremely high rpm as compared to say an automotive engine. An issue with running rivers where there is a chance of ingesting a small rock is that the high rpm and a small hard object can result in a catastrophic instantaneous destruction of the impellor. I doubt that a single jetski pump motor combination will give you satisfactory performance. If you are considering a 20 - 22 foot boat, you should research the jet boat forums
to find anyone who has built a river boat in that size with a jetski powertrain set up.

Our normal build was a 21 foot, 1/4 inch bottom as boulder to hull interface interactions were anticipated, plywood floor covered with duradeck, a 350 cu. in. (300 plus HP )engine with a 212 Hamilton ahead of it, 75 gallons of fuel capacity, windshield and that power was adequate.

With a 20- 22 foot boat, you have room for the engine and room to walk around.

If your build is on a light weight Jon boat style hull, ie Tracker, Smokercraft etc, perhaps the jet will work, but a lot of work will be required to reinforce the bottom. Particularly in front of the intake as you need to provide
stability, to ensure that when you try to crush a rock that the hull can withstand the bump before it hits the actual intake. If you think that you can build a 21 foot boat and "beat the weight" parameter, you should do an analysis of what you will carry.

I tamped down a boulder on a remote river in the Pacific northwest, 120 miles from assistance, (not one of my better days, but in my defense - excuse it was obscured by silt laden water). We had to hire a K-Max, twin rotored helicopter, to get us off the river down to where we had another boat in the ocean that would tow us back to base. 3 guys, camping gear, additional fuel, stand up camper canvas, food, back up remote winch etc, came in around 4- 4,500 lbs. The helicopter, used for logging, had a scale on the tether.

A jet ski power train designed for maybe 1000- 1200 pounds would not have been adequate, though our trip was definitely overweight due to the purpose of the trip.

Instead, you can look at a two stage style pump if they are still available, Kem equipment built (not sure now) a 2 stage copy of a Hamilton, 772 and a V-6 marinized engine, ( they used to sell it as a package) would be a good fit for a light weight
combination and the V-6 saves , maybe 6 inches of longitudinal inside cockpit space.

Suggestion, look on the internet for jetski powered aluminum boats and focus on the weight of the hull, chine width and deadrise as these will be the prime design parameters with a low horsepower motor.

 

Jetboat design 101!

 

Realistically a jet ski that doesn't exist unless your going Jon boat thin on the aluminum. Most common re used jet in the mini boat applications is the Yamaha 155, I has pretty good aftermarket support for grates and buckets. As does the sparc and a few other common seadoo skis. Problem is that's s 6 inch jet designed around high rpm and low weight vessels.

Stepping up to an alamarin jet the 160 is made for 2200 lbds and 136 hp the 185 3700 lbds and 150 hp both are better suited for something 20+ feet.

As pointed out from above jets rob a fair whack of power and weight is the enemy of jets to a greater degree than other propulsion methods. Aside from a fairly thin Jon boat style 22 footer it's going to be hard to get weight down low enough.

Barry nailed it, his response went out while typing mine.

My brother in law had a 21 footer built a few years ago, he sea trialed a demo boat with the sport jet package and described it as "good for a shallow lake if I don't bring friends". They opted for whatever is the current iteration of the Hamilton 212 and a 350 hp v8 in front of it. It's now 4 or 5 years old and coming up on time to re power or rebuild the v8 (they use it a tremendous amount to access a cabin and v8s attached to jets are not long lived). Last time we spoke of it he had saved pennies for a bigger pump and an ls1 setup. They are running rapids and the aeration kills even more power.

 

I was wanting to use around a 2-3 liter 4 cylinder to power it, but I think I knew all along it would be under powered. My main reason for it was fuel efficiency. I'm not against using a small block Chevy or a 4.3 liter Chevy I think I want about a 6 foot beam and 10 degree hull. I planned on 3/16 bottom and 1/8 inch sides. What top speeds are usually obtained with a set up like this? I know it also depends on weight front stringers and other things but is there a ball park?

 

Need to know the hp. Also, is it marinized? Big bucks to take an automotive engine and convert it to meet safety requirements.

I had suggested Kem equipment as I think that they have a 2 stage pump but you can look at American Turbine, a mixed flow pump, and if it is like the old Berkeley's you can machine the outer diameter of the impellor to reduce the
hp required.
Beam is 6 feet? One of the most important measurements besides deadrise is the chine width. The boat that I am reworking is 84 at the gunwale and only 60 inches at the chine. It will not be a load carrier as you should try to build for at least a 72 inch chine width or even a bit more. As I did not want to start a complete build, I had to take what was available.