low top speed

I don't know if anyone can help but here we go. I purchased an old ski boat. I removed the 140hp mercruiser and stern drive and mounted then in my 16-48 flat bottomed jon. My old jon was a 14-48 with 25hp outboard. It would run 24mph. This boat has 115 more hp and top speed is only 35mph. Both boats have floors and front decks. I was expecting around 45mph. Motor is running good. I also trimmed it up as high as I could. The lower unit is in the water exactly the same as it was in the ski boat. any ideas? I run 4000 rpm with a 14.25 x 17 prop. I tried my buddies 14.25 x 19 with no difference in top speed. It just dropped the rpm about 800. :shock

you ought to be running closer to 4800 rpms, maybe look at a 12 or 13 pitch.

motor is a 1984. I thought redline was 4100.

the 3.0L 4 cyl is rated at 4400-4800 rpm at WOT.

What does the motor and outdrive weigh?

I will look for a lower pitch prop thanks.
I don't know what the weight is but you can see the waterline in the pics.

after some searching it looks like the motor/drive weighs around 600 lbs.

I would think scary fast would be happening, over 50 mph minimum. Maybe check the compression on the cylinders. I don't get it, no other ideas at this time.
Tim

Motor and outdrive are 600 lbs. Therein lies most of the problem. Also consider that most jon boats have a TON of hydrodynamic drag. The more weight we put in the boat, the more the boat is pushed into the water, which means a LOT more drag. Also, where is the AV plate in relation to the bottom of the hull? I'd want to see it about an inch or so above with that much weight in the back end. Maybe more.

More power isn't always faster, and you're finding this out.

Once knew a guy who had a 1648 and put an inline 90hp outboard on it. Originally had a 40. With the 40, it went around 34 mph. With the 115, about 40 mph is all he could get out of it. A 175 lb 40hp motor vs a 300 lbs 90.

Needs a better hull to go fast. That said, it is neat, and great job of doing something different. Looks awesome.

Thanks fellas. How could I modify the hull for more speed? I could fabricate floats on each side of the lower unit. I've seen that on some boats that are heavy in the aft. What do you think?

I mean stern :LOL2:

Pods are a possibility.

One could add 'speed rails' to reduce the wetted surface/contact area, as that is what is causing the drag - wetted surface areas on boats is a parasitic effect.

FWIW, we once took a 140hp V4 OB (crankshaft rated motor) off a boat and put on a 225hp (propshaft rated motor) in its place ... and left the dock, fully expecting it to run like a raped ape! Not ... we only achieved maybe 5mph more ... and learned the valuable lesson that more HP doesn't always equate to more speed - due to the inherent design limitations of the said hull under test.

Translation - so there was a valid reason why that hull mfg'r shipped them out with only 140 or 150hp motors on the stern ...

Thank you I will look into "speed rails". Not familure with that term.

if your motor is rated to 4400-4800 WOT and you're at 4000 you need to be going down in pitch not up - your friends 19P prop wasn't going to do you any good - the fact that you lost 800 rpm seems a bit extreme. If that was truly accurate a 12 or 13 pitch is going to over rev. the engine in a bad way. If you're at 17P now rule of thumb is ~200rpm per 1P so you'd want to be looking to 14 and 15 - if you saw an 800rpm change for 2" of pitch then I'd be trying the 15P prop first and see if that lands you at 4800, but I'd be inclined to think it won't and you'll need the 14P.

Look around, several prop makers have a return/exchange program to allow you to play around a bit and find the prop that works best for your application.

Just my .02, but dropping a 600lb motor in a tin boat seems to totally defeat the POU behind running a tinny in the first place. These boats aren't designed with a chine hull to operate at those types of speeds - you're really inviting horrible stability and safety issues. Your flat bottom won't bite in turns and is so *** heavy that flipping in chop should be a real concern.

I would look into making some float pods along with getting the correct prop on it.

The previous owner of the donor ski boat, which was a 17' fiberglass tri hull, said it ran 38mph. My boat is WAY lighter than the ski boat. This is why I expected more speed.

Has anyone out there actually modified the hull of a flat bottom?

Know a guy with a 16' "dead skiff" (heavily built flat welded alum. 1/4" bottom)approx. 500lbs.It has alum Ibeams welded on the bottom he runs a 30hp Yamaha four stroke and he gets mid 30's.I have the same boat but a 14' version much lighter and can only get 30mph with a 30hp 2-stroke. I'm thinking the I beams are acting like "speed rails" or it might be the extra 2' displacing the weight.I have seen flat bottom bow-pickers of 20+ feet that run fast with heavy 200hp four strokes so maybe length is the factor that is preventing you from achieving speed.

I'm thinking a different prop will help you, but it might be more of a prop angle or prop depth issue. No doubt you can hit the desired rpm with the right prop, but that won't mean it's the most efficient design for your rig just because you hit the right rpm.

You'll want a prop that hits your rpm level with the least amount of slippage. A prop guru will be able to make some recommendations, but it will take a real guru. Someone matching "this prop to that outboard on this hull" because it worked for ten other people might not be the right person to talk to. You'll want someone who understands the dynamics involved. I'm sure there are many out there, but Steve's Custom Props in Quitman, Tx comes to mind. Steve is a bonified guru when it comes to props, and can customize to meet a particular need.