Porpoising flat boat

**97yamaha** · 03-28-2017, 01:00 PM

Picked up a 1442 alweld flat boat with 97 Yamaha 40esrv tiller with manual trim. Motor is mounted directly to transom with a 10 1/4x15p chopper style prop.As soon as I plane it starts to bounce horribly. I took it home, mounted the tank and battery up front and welded on some 3x12 inch trim tabs. I've got it to where I can run decently with 2 people at lower speed, but still trying to bounce. Could it be the chopper prop?

Someone loaned me a 14p REB prop I'm gonna try this evening. But I'm thinking I might try and setup with a jackplate and a SCB3 prop. What you guys think?

Any suggestions on setup and prop selection would be much appreciated.

**vnemous** · 03-28-2017, 01:50 PM

The chopper is a bow lift prop, I would suspect that the prop is trying to fly the bow but the motor doesnt have enough HP to hold it up hence the porpoise JMO

**97yamaha** · 03-28-2017, 02:17 PM

I'm new to boats but I figured a 40hp 2 stroke was plenty enough for a 14' aluminum flat. The boat came with this prop, but the previous owner never mentioned this issue. I'm hoping this borrowed prop can give me some indication if I'm on the right path thinking a lower rake prop.

**flatbackcanoe** · 03-28-2017, 06:22 PM

My opinion.
Ditch the trim tabs things. You need a jack plate.
Jack the motor up and see what happens, took the bounce out of my 14 foot fiberglass V hull.
Put on a prop that doesn't lift the bow.
As long as the RPM of the motor is where it should be the prop pitch is o.k.

More pitch = more distance traveled per revolution of the propeller, more speed.
A smaller pitch will get out of the hole faster but will also turn the engine faster for a given boat speed.

40 hp on a 14 foot tin boat should go like a scalded cat, but the engine is probably on the heavy side.

**97yamaha** · 03-28-2017, 07:27 PM

@flatbackcanoe

Thanks, That's kinda what I was thinking. I just tacked the tabs in case I wanted to take em off. I'm gonna check into a jack plate. I think a little lift and setback will go a long ways. Right now my cavitation plate is a hair below the hull. I tried the 14pitch round ear prop this evening and it's a big no go. Barely helped with bounce and dropped my top speed from 37mph to 33mph with 2 people. Being a fishing boat, I don't need it to necessarily scream and fly, but I feel I should easily be able to get 38-40mph with the right set up.

**flatbackcanoe** · 03-28-2017, 08:19 PM

Most of the people on here are talking propeller shaft, not cavitation plate, when they tell you engine height.
I'd try to get the cavitation plate out of the water once you're on plane.

**97yamaha** · 03-28-2017, 08:44 PM

Oh okay, well... in that case, the prop shaft is well below the hull. I've got to find a reasonably priced jack plate.

**XstreamVking** · 03-29-2017, 06:05 AM

Your bottom is rockered. It has a hi point at the middle (or near) of the length of the hull. Run a string from front to back and see what I am talking about. I could be wrong, but what I hear is pointing to this. Trim tabs are not a bad thing, they are a way to get transom lift. You will need this lift to overcome the bulge in the middle of the hull with more force at the stern. Plus you already have a good bit of weight on the trans with the big motor.

**97yamaha** · 03-29-2017, 07:10 AM

I'll put a string on it when I get home this evening. This is a legitimate question, but aren't all flat bottoms kinda designed that way? To have a high point near the middle where it begins to taper to the front?

**XstreamVking** · 03-29-2017, 08:11 AM

Most will have a slight concave from the front to the back. This prevents the problem you are having. All alum boats are built in a jig. Start out straight but can be bent by having a trailer that supports just the bow and the stern letting the middle of the hull just hang and sag down. The string will tell a lot about the shape.

**97yamaha** · 03-29-2017, 12:19 PM

Ah, I see.... not designed that way but happens over time. I'm gonna try a few more things before I just sell hull and trailer and get something bigger. Really frustrated.

I was gonna try a mini jacker just to see, but it's only rated for a 35hp. Ill keep looking. I'm thinking of trying a semi cleaver or something with even less rake. What y'all think? If so, what pitch? A 10 1/4x15p chopper had me at 37mph. A 10 1/4x14p round ear had me at 34 mph.

**XstreamVking** · 03-29-2017, 12:53 PM

A prop with a hub gives stern lift. A set of tabs set correctly should "cure" the problem. But, you could use the chopper and the tabs and tune it to run w/o the bounce. Might be faster with tabs. Try and tune it some before selling.

**Jimboat** · 03-29-2017, 01:27 PM

Originally Posted by 97yamaha

As soon as I plane it starts to bounce horribly. Any suggestions on setup and prop selection would be much appreciated.

Check out this article on 'Porpoising' ...

**97yamaha** · 03-30-2017, 07:15 AM

@Jimboat Thanks, very informative. Helps to diagnose a problem when you learn a little of the physics involved.

Well, I made a rookie mistake. I miss identified the prop it came with as a chopper prop (thanks google). After a little homework and some help from my local marine shop, The prop is indeed the original Yamaha 15pitch semi cleaver. It's missing the black paint which I blame for some of the confusion. I'm gonna have it refurbed since nearly 3/8 inch diameter is missing. This still doesn't explain the low speed porpoise but explains the digging and light back end at higher speeds with the tabs bent down.

I still need to check for rocker this evening. Also, the marine shop said there was no chance a jack plate would help, only make it worse.

**phillnjack** · 04-01-2017, 11:36 AM

I hope your not going to have the prop refurbed by the same people who says a jackplate wont help ???

the trim tabs wont help much, tabs are not to drop the bow, they are meant to keep the boat level left and right. they cause tons of drag and can upset any
boat if not used correctly
if you need engine lift for holeshot then simply add a foil to the cavitation plate on the motor, atleast then it will only operate at holeshot and just before planning.
once up on plane the foil will then be out the water by around an inch or more (depending on prop used).

your boat being a dead flat hull will always bounce a bit over about 25mph, you need to just alter weight around a bit and try lots of different heights with motor.

once on plane slowly bring down the speed to where the bounce stops, its probably about 4mph over minimum plane speed.

a jackplate will move centre of gravity, and could be exactly what you need. most marine shops will say they dont work if they dont stock them.
just make up a home built one and try it. if it works then buy or build a real nice one..

if your prop is alloy then dont bother getting it repaired, put that money into buying a decent used stainless prop and keep the original as a get you home/spare.
cheap props come up on fleaby and on here too.

what is your max rpm when running ? if you aint got a tacho again you can buy a very simple copy tiny tach on the bay for under $20 delivered to your door.
they do work and will give a decent idea of what the motor is reaching at top end.

something like this, dead easy to fit and even stick on the front of the motor with a bit of double sided tape if you want.

http://www.ebay.co.uk/itm/Tiny-Tach-...AAAOSw-0xYY0Ta

or this one , same thing just even cheaper
http://www.ebay.co.uk/itm/Spark-Plug...3D351545195769