Porpoising flat boat

[97yamaha]

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]

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]

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]

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]

@flatbackcanoe (http://www.screamandfly.com/member.php?u=60970)

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]

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]

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]

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]

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]

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]

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]

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]

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' (http://www.screamandfly.com/content.php?325-Porpoising-Why-It-Happens-and-How-To-Fix-It) ...

[97yamaha]

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]

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-Digital-Hour-Meter-Tachometer-for-Marine-Spark-Engine-Motor-/391358517220?hash=item5b1ec90fe4:g:I-AAAOSw-0xYY0Ta

or this one , same thing just even cheaper
http://www.ebay.co.uk/itm/Spark-Plugs-Engine-Digital-Tach-Hour-Meter-Gauge-Tachometer-Motorcycle-Bike-F72/322019760358?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D40656%26meid%3D46dabb78014b4f99bec06e62886f c1f1%26pid%3D100005%26rk%3D6%26rkt%3D6%26sd%3D351545195769

[Flex]

Almost every flat bottom I have been in the porpoising came from a combination of the motors mounting and trim setting, and weight distribution. Move some weight up front. Try going out with an ice chest up front and move it around to see if you can get the bounce under control. Also, a jack plate makes a huge difference in the ability to set the motors and trim on these smaller rigs.

In my 16ft flat if I didn't have weight up front I would start bouncing at 25 also. Solution was either a person, my 110lb dog, or ice chest went into the front of the boat.


In my current rig we relocated the gas tank up under the bow of the boat to help with the heavier motor and set back I am running.

[97yamaha]

Yeah, I've mounted a 12 gallon tank up front but the weight varies with fuel consumption, obviously. With trim tabs bent down pretty aggressively I can power through the bounce if I catch it right, but then the nose digs and stern gets light with more speed. I'm having the 15pitch Yamaha stainless semi cleaver reworked at a reputable prop shop, and hunting down a jack plate to get my motor up and back, but dunno how long it's gonna be until I can give it a test.

I wonder if a hydrofin would help with hoping at low speeds, and at high speed it'll be out of the water?

I know two people with the exact same setup as me, but they have 6" jack plates and are running smooth at all speeds and hitting 40mph with their gear. Gives me hope for the jack plate working.

[Flex]

I have never seen anybody mess with tabs on one of those boats, but everybody has a jack plate. And there is one in every garage in SWLA. I think a jackplate will probably solve your issue. I scored my CMC for $100 on Clist local. but there is a ton on ebay as well. Just get a manual one. Where you located?

[Flex]

Just saw Slidell, add Cajun Boat Swap on FB. You will have no trouble finding a good used jack plate cheap and local!

[phillnjack]

I think the same , jack plate to get centre of gravity back a bit and raise the motor up.

[97yamaha]

Thanks guys, posted some on FB pages and CL but no hits yet. You're right, nearly every yard here in southeast Louisiana has a boat with no motor and a jack plate on it. I'm fixing to start knocking on doors.

Anyways, prop guy called yesterday and said he can't rebuild 3/8 inch diameter. He suggested leaving diameter, fixing the tips, and cupping it and it'll be "just as fast" for $160. Not sure what Ima do yet. I've tried several different props from friend and family and so far nothing runs as smooth and fast as the Yamaha semi cleaver. If nobody contacts me with a jack plate by Friday I'm ordering the Vance MFG jpl4500 and being done with it.

[Jimboat]

prop guy called yesterday and said he can't rebuild 3/8 inch diameter. He suggested leaving diameter, fixing the tips, and cupping it and it'll be "just as fast".
That doesn't sound right at all. Cutting a prop down that much definitely will not perform same. Suggest another prop guy. is your prop aluminum or ss?

[Flex]

https://lafayette.craigslist.org/bpo/6065335242.html
https://neworleans.craigslist.org/boa/6025704281.html

There ya go! Good luck! Dude that's too much on the prop, can order new aluminum props online cheaper than that!

[97yamaha]

@ Jimboat that's what I thought. The prop is SS and very expensive to replace ($470 cheapest I've found.) Powertech makes a SCB3 version of it for around $300.
Flex I saw the one in NO but it's a static plate and they still want $200. The one in Lafayette is a 3 hour drive. I know I sound picky. My local marine shop has 6" HiJacker for $200 but I'm cheap and was hoping to score a used one for cheap like everyone else. My bank account says yeah, but my wife says no, so I'm trying to compromise.

A SS prop is a must running the Pearl River here. Aluminums don't last very long.

[phillnjack]

I would not trust the prop guy who cant put the prop back to original.
there are people on here who have had some very bad props made great again by those who know their stuff.

I have seen props that I thought were no way repairable that have come back better than new, balanced sharpened and just looking great.

you just need to keep watching the adds and a decent prop will come up, same with the jack plates.
and dont forget to watch the adds on this forum too, often some very nice bits for sale cheap.

.

[Jimboat]

The prop is SS and very expensive to replace ($470 cheapest I've found.)
A good prop man can restore a SS propeller. if your guy says he can't do it, I suggest that you find a proven propeller repair shop - they'll do it for you.

[FMP]

www.propcopropellers.com

[97yamaha]

Well, I was going to go get the prop back, but he already started reworking it. Reckon I'll let him finish, and If I don't like it i'll sell it and go with something different. I found a deal on a Vance JPL 4800 jack plate but I'm thinking 8" of setback might be a tad much for a 14ft flat. What do ya'll think?

[cyhuff]

Hopkins Propeller was the place to go to down there in the '80s. In Covington about 15 minutes from Slidell. Don't know if they're the same since Floyd passed away a while back.

[phillnjack]

well I have a 5 inch home made jack plate on my 13ft and I need atleast another 2 inches, .
what are other people with similar boats using ? and would any of them prefer the extra 2 inches ? would it benefit the boat

[97yamaha]

cyhuff Hopkins is who's doing it. Everyone told me that in the past they were THE prop guys, but I think the guy that runs it now is down in his health and doesn't feel like messing with em. Which is understable, but... if you don't want to be bothered doing your sole profession anymore, it might be time to close shop.
phillnjack most run a 4-6" on 14&15 feet boats, but I'm one of the few that have a big tiller, so I might could afford the extra 2 inches back.

[Nicklab]

like stated already, (assuming that you have the correct shaft length for your transom) simply play with your trim setting and weight placement. I have been on plenty of flat bottoms without jackplates that don't porpoise. consistency is key, every slight change of weight distribution is going to change your outcome on these boats

[97yamaha]

Well, installed a Vance MFG 6" jack plate, put the "reworked prop" back on and..... it's freakin worse. Kept jacking motor up until it started cavitating then dropped it back down a 1/2 inch. This reworked 15g Yamaha prop killed my Speed. All Hopkins did was fix the tips and add slight cup to the tip. The porpoising is intolerable now. Gas tank, battery, and a person up front, with a jack plate, a stern lifting prop, and trim tabs, and still wanting to porpoise. Beats all I've ever seen. Spent $350 to make the bouncing worse and lose 4mph.

[Flex]

Well, installed a Vance MFG 6" jack plate, put the "reworked prop" back on and..... it's freakin worse. Kept jacking motor up until it started cavitating then dropped it back down a 1/2 inch. This reworked 15g Yamaha prop killed my Speed. All Hopkins did was fix the tips and add slight cup to the tip. The porpoising is intolerable now. Gas tank, battery, and a person up front, with a jack plate, a stern lifting prop, and trim tabs, and still wanting to porpoise. Beats all I've ever seen. Spent $350 to make the bouncing worse and lose 4mph.

That is so bizarre. Did you try removing trim tabs and trimming down? Could be fighting too many things at once.

I would let Jack plate as high as possible. Remove or hold up tabs, and try each trim setting over again.

[XstreamVking]

Trim tabs and a set of wedges under the motor for more tuck under.....jm2cts....

[Nicklab]

Well, installed a Vance MFG 6" jack plate, put the "reworked prop" back on and..... it's freakin worse. Kept jacking motor up until it started cavitating then dropped it back down a 1/2 inch. This reworked 15g Yamaha prop killed my Speed. All Hopkins did was fix the tips and add slight cup to the tip. The porpoising is intolerable now. Gas tank, battery, and a person up front, with a jack plate, a stern lifting prop, and trim tabs, and still wanting to porpoise. Beats all I've ever seen. Spent $350 to make the bouncing worse and lose 4mph.

Post some pictures of your setup

[97yamaha]

Nicklab, I'm trying and it logs me out every time I try.
Flex, bizarre indeed. That's what everyone is saying. I'm gonna put everything back straight and start again. My father in law has a very similar setup. 14' alweld with 6" jack plate and trim tabs, With a tohatsu 40tldi, 15 pitch prop and runs 40 with 2 people. That's the setup I was going for.
xtremeVking I have trim tabs but not sure what ya mean by wedges under motor.

[sharpeye Mike]

Have you looked at the bottom of the boat? Have you put a 4' level or a straight edge on the bottom to see how flat it is? A slight banana shape will cause you the problem you are describing. If the bottom of the boat is out a shape you can bring it back with a rubber mallet and a piece of wood. What kind of prop is your father in law using? That could be the other culprit.

[XstreamVking]

Post # 8 recommended checking the bottom. Never heard if flat or not......

[97yamaha]

Sorry for the lack of feedback on the bottom. I placed a string on the bottom and it was very straight. There was a few smaller dents (in and out) but nothing that gave a curvature of the entire bottom. And I did tap those out. My water seals and impeller should be coming in today to get it peeing again, and I'm gonna restart adding single forces at a time. If I can't get it this time, I'm selling the hull, buy a 1548, and give up on the magical 40mph.

[Magcat 62]

BOTTOM is flexing when under power

[phillnjack]

get rid of the trim tabs, they will do absolutely nothing for you and just slow the boat down, they are a waste of money on a boat like that.

jack it to the moon and get a decent prop, by adding cup to the tips means more bow lift.
forget about stern lift that only makes things far worse.

put impeller in and go start checking again, jack plate right up, engine up as high as it will go, have the cav plate parallel with the bottom
and hit the throttle and hold on, dont try adjusting anything until the rpm stops rising.
if it still porpoises while at speed move something forward a touch. then a bit more etc. but get rid of the tabs or tie them right up out
the way.
they only keep side to side level and your not worried about that yet.
if the boat is still the same then trim under 1 hole or use power trim, just a quick dab and wait 2 seconds then same again until it
levels out..
I think you need to get someone to video this and post it, then we can see if your actually porpoising or just bouncing due to speed.

[97yamaha]

Sounds like a plan. I'll have someone video.

It it is a very light boat! 0.080 aluminum instead of 0.100.

Someone told me something similar, "bend those tabs up and power through it, peg the throttle and hang on", but I haven't developed the cohones for acting crazy on the water yet. What's the worst that could happen right? It's warm enough to go for a swim if it bucks me out I guess.

[XstreamVking]

Hope you are a good swimmer...

[phillnjack]

make sure you have the kill switch lead attached to yourself and dont panic. nothing bad will happen.

once you get past the porpoising youl be fine, 10 mins flat out and youl be way used to it, doing flat out dounuts and
drifting around the corners.
just keep that throttle pushed all the way forward.
2 hours bashing about and youl be looking for a larger motor ha ha

when doing the new water pump use some dish soap on the impeller to help get it in easy, it will wash right out as soon as water hits it.
and dont forget to grease the main drive shaft but not on the very top of it.

[97yamaha]

Thanks, didn't get a chance to try it, new impeller and water seals/dampers didn't solve my water circulation issue, i'll tackle that tomorrow.

Here's pictures of my setup:

373608373609

[phillnjack]

I cant see any reason why that wont go very well,

I would trim the motor out a touch more, as in take the prop away from boat another inch or 2. to have it straight down .
But that looks like it should get a move on with the right propeller.

.

[Flex]

Thanks, didn't get a chance to try it, new impeller and water seals/dampers didn't solve my water circulation issue, i'll tackle that tomorrow.

Here's pictures of my setup:

373608373609

Set up looks pretty good to my uneducated eyeballs. I also just assumed, but a boat that small and light, and flat bottomed, is always going to be slapping when you get going fast. They are not known for smooth ride. If the water is absolute glass you can scream though. 40 isn't too fast, I'm a big boy I have to barefoot about 44!

[phillnjack]

if running with it high it spins out on the bends or loses grip get the cavitation plate repaired ....
that skeg looks like a fair amount has either worn off or been taken off the bottom.

[XstreamVking]

I wonder if he survived the drive thru it advice.....

[sharpeye Mike]

Yeah, same here.

[Flex]

Maybe it goes real fast now.

[phillnjack]

its a shame when people dont post the follow up to the problem.
I have often read very long topics on the net and you get real into it waiting to find out how it went then nothing, zip, zero Grrrrrrrrr

[Flex]

its a shame when people dont post the follow up to the problem.
I have often read very long topics on the net and you get real into it waiting to find out how it went then nothing, zip, zero Grrrrrrrrr

I do too! Especially some of the threads that go several pages!

[Flex]

Well, looks like we shall never know.