Porpoising flat boat

**97yamaha** · 04-09-2017, 01:09 PM

@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** · 04-10-2017, 07:36 PM

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** · 04-25-2017, 02:26 PM

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** · 04-25-2017, 02:33 PM

Originally Posted by 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.

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** · 04-25-2017, 02:52 PM

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

**Nicklab** · 04-25-2017, 05:13 PM

Originally Posted by 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.

Post some pictures of your setup

**97yamaha** · 04-25-2017, 08:12 PM

@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.
@xtreme Vking I have trim tabs but not sure what ya mean by wedges under motor.

**sharpeye Mike** · 04-26-2017, 07:38 AM

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** · 04-26-2017, 08:36 AM

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

**97yamaha** · 04-26-2017, 09:47 AM

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** · 04-26-2017, 11:06 AM

BOTTOM is flexing when under power

**phillnjack** · 04-26-2017, 11:19 AM

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** · 04-26-2017, 01:06 PM

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** · 04-26-2017, 01:24 PM

Hope you are a good swimmer...

**phillnjack** · 04-26-2017, 04:26 PM

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.