15ft StarCraft V8 and jet conversion

Your comment below--- The only thing I'm trying to accomplish is travel at the slowest speed possible while still on plane and be able to withstand the occasional contact with a log,

Without being to wordy but you are on the forum asking for opinion and or experience

The first jet boat that I had, I purchased, 21 foot, 7 foot beam (5 and a bit at the chine- ie narrow) 460 naturally aspirated, Berkeley A cut impellor, 320- 330 hp. With 100 gallons of fuel, camping gear, 3 guys (we made long excursion trips) I had issues even getting the boat on plane. High RPM, 4400 - 4600, and terrible fuel economy, maybe 2 1/2 miles per gallon. In order to keep this boat on step, the cruise rpm was maybe 4000 - 4200 rpm. At the time Berkeley only had the A impellor available, 1979, in a packajet

At that time we started building boats to keep our aluminum manufacturing guys busy during the winter and installed the 460 and then the 454, 330hp. The first boat had an A impellor and while the boat performed better than the one that I had bought, (wider chine width and downturned lift strakes) the boat loaded still took quite a bit of rpm and poor economy to
give the CRUISE performance that we were looking for. Our target was 32mph and about 8 gph or 4 mpg.
The next hull got an A2, which gave us better fuel economy and a lower rpm cruise. The next hull got an A3 which gave us 30 - 32 mph with an 8 gph cruise AT ABOUT 3000 rpm.
We purchased a Flowscan fuel flow meter to record fuel consumption as we were interested in more precise numbers

The A impellor was not a good fit with the 460 -454. Nor is it the best fit for your goal above
At 270 hp, I am guessing that the A2 might prove to be the best match but if you are buying an impellor, I would purchase an A3, try it, then if necessary, cut it back to an A2.

Regarding your thoughts about peak rpms, more torque from the big blocks. More torque at low rpm is more horsepower at low rpm. Leave torque out of it and look at the Berkeley
impellor selection guides. The fuel consumption figures reflect the amount of horsepower that your engine is producing to move your boat at your goal, on plane, cruise speed.
Ie when we were using 8 gallons per hour, the engine would have been producing about 100 horsepower. IF you choose the impellor that will let your engine produce the
max horsepower that it can produce, say it is 270 hp at 5500 rpm, you will be using about 22- 25 gph, and possible doing in the 50 to 60 mph range. (or more)
And at lower rpm cruise, your goal, you will have poor acceleration (you have already identified this from other people) and fuel consumption will be less than optimum.

Regarding my request to see your horsepower curve for you engine. People, not saying you, often get a little confused with the curves. The horsepower curve shows the max horsepower that an engine CAN generate for a particular rpm. At these upper limits that define the graph, the engine is not running with the best fuel efficiency, (efficiency defined here as fuel burn wrt to horsepower produced)

The point that I am trying to make is that you want to match the engine and impellor to optimize your goal. Running it at 5500 rpm or above to realize the absolute maximum that the
engine can produce would never be my goal unless I was building a sprint boat and don't care how much fuel I burned in a 5 minute race.

As a shoot from the hip estimate. With your light boat, A2 or A3 cut, and light fuel load, with a couple of people on board, I would GUESS that you would easily run 30 mph, and burn around 6 1/2 - 7 gph. The impellor will be a cut that will never let your engine attain ITS peak horsepower output which I maintain is an rpm and loading that you never really want to
run it at.
4.25 mpg fuel burn and still have some acceleration left to provide for collision avoidance steering. Your

 

Can't find a stock 4.8 dyno graph but this is the 5.3. The 4.8 would follow these curves precisely just 10-15 hp less. The numbers are inflated because the test mules typically have no accessories, electric water pump and long tube headers. When I say 270 HP that is on the factory truck tune with full truck accessories, truck intake and cast exhaust manifolds. So in all reality my 4.8 should be closer to 330-340 hp after its final tune in the boat. As you can see these engines have a smooth power curve and a very flat torque curve. I understand where you're coming from with the larger impellers now. The problem is all the experts I've talked to deal with Berkeleys exclusively and what is a Berkeley- it's a race pump originally bred for competition against the New Zealanders and their Hamiltons. They all immediately assume I'm trying to go as fast as possible, so the engine needs to reach peak power at all costs. Imo the majority of the Berkeley community has evolved in to bunch of mouth breathers building boats with hideous paint schemes and running archaic short lived oversized power plants but that's another conversation. A larger impeller with more thrust at lower speeds makes more sense in this application as long as the wee little engine can turn it

 

It certainly won't be a boat with a low clean plane speed, too much weight on too little bottom.

 

With the CG as far forward as it is, it will likely hop right up on a plane very easily and be able to plane flat at speeds a lot lower that it would with an outboard. If he trims the jet to provide lift it'll jump on a plane... Watch the Aussie Jets.. they are up on a plane in one boat length...

 

I don't know the dimensions of the boat, but a V8 in something like that is all wrong, it is a boat that suits a 25 hp outboard, and the shape just isn't suitable for a jet boat anyway.

 

Been awhile I had a busy summer guys but it's cold again so I'm back in garage working on this boat. We got the engine and pump in place and lined up with each other. Rear structure is pretty well done we're gonna run stringers the full length but they'll only be 3" tall tubing instead of 6" like in the engine compartment. At 3" tall the floor will go right back in place where it was originally. Let me know what you think I guess

 

Front floor structure somewhat mocked up

 

I went the opposite direction.

I bought an inboard jet and converted it to an outboard jet. The smaller jet boats have almost no room with an inboard. I used it for fishing and camping, so I wanted as much open space as possible.

Mine was welded and slightly thicker aluminum, but I gutted it and needed to add supports, plus have sections rewelded.

It’s been about 20 years since I finished it and still works very well for what I need it to do.

I lived on a river at the time that required a jet, you will hit stuff.

My friend had a hull like yours and used an outboard jet on it. It worked well, but he needed to be careful. Hitting a rock or log can open the hull like a can opener due to the thin aluminum.

Now I live on a river that doesn’t require a jet, I may repower to a prop if I find one one cheap.

 

Structure is all finally welded what a nightmare. Progress should hasten somewhat from this point forward

 

You have side stiffeners welded in. I suspect that they will be stitch welded. Ensure that for every bead above the stiffener that you have an exactly bead opposite and below.
We found that over time if you alternated beads, ie say starting at 0 inches to 4 inches upper, skip 4 - 8, then weld 8 - 12 under, that over time there was enough flex in the side of the hull
to cause these welds to crack. We then changed, upon the advice of another boat builder, to do opposing welds and the problem went away.