Hull bounce?

[stuee]

Hi.
I purchased my 21ft Fiberglass halfcab a year ago.
It had no floor in and was a mess.
I replaced all the stringers, floor (added ander floor tank) thanks to users from this site.
i did it all correct using glass, and marine ply etc.
I completed it all about 2 months ago.
I purchased a old 100hp merc which wouldn't move the boat much till i discovered the leg was too short (again thanks to this site for the help)
I now have a 235HP johnson on it. It all works fine and been out a few times now.
I do however have a huge issue.
When i motor over 55kph it stats hull bouncing at the front and i have to pull back.
I know the motor is a little higher than rated on the sticker (200hp) but figure 235 would only be perhaps an extra 10kg more.
I have tried messing with the whole weight thing, things ive done.
Moved both batteries to the front now under the seats.
Added 2 full bags of sandpit sand (thanks kids) to the very front near the hatch.
Thrown in some extra chain just for weight at the front.

but it didn't do anything and im at my whits end.
Even when its slight choppy or wavy and im doing 50kph i dont seem to cut through them or skip but more ride up them and slam the other side.

Any ideas, changes or things to try, its very uncomfortable while traveling.

thanks

 

[Scott Danforth]

Re: Hull bounce?

it sounds like porpoising. can you trim the motor down a bit? do you have trim tabs?

 

[stuee]

Re: Hull bounce?

Hi Scott,
i have trimmed it down to max, i also removed the stop bar to try get further but seems it stops there itself anyway.
No i dont have trim tabs. I actually purchased off ebay the tabs and hose / fittings but it didnt have a pump which i cant find a cheap one, they all seem to be the same price as with a kit.

 

[dan02gt]

Re: Hull bounce?

I think trim tabs are the only way your going to fix the porpoising issue. Smart Tabs would by far be the cheapest option or you could go with some helm adjustable tabs like Bennett.

Here's some links:

Smart Tabs Trim Tabs

Bennett Marine: The world leader in trim tab innovation, quality and service

 

[JoLin]

Re: Hull bounce?

What's the shape of the hull? 50kph = 31mph. 21' boat, choppy water... if the hull doesn't have much deadrise your running problems are pretty much what I'd expect.

My .02

 

[idonthaveaname]

Re: Hull bounce?

Trim tabs could help but defining the reason for the instability is more important. Hull design, weight ratios etc. all play into this. Here is a link that may assist you:

Donald L. Blount and Associates, Incorporated

 

[Frank Acampora]

Re: Hull bounce?

Go over to the prop forum. Describe your boat weight, load, power, and prop accurately. Then see if you have a bow lifting prop. If you do, ask for examples of stern lifting props. It may cost a couple of hundred but it just MIGHT help the porpoising.

 

[smokeonthewater]

Re: Hull bounce?

Nauticus makes a smart tab conversion for hydraulic tabs

You COULD get that and use it with your tabs and you wouldn't need the pump.

 

[crabby captain john]

Re: Hull bounce?

More than enough HP without adding weight forward. If you decide on tabs a 21' glass boat deserves Lenco or Bennett. You need to be able to adjust.......

 

[stuee]

Re: Hull bounce?

thanks for all your replies.
Would hydrofoil on the motor make any difference?
also are the smart tabs as good as the proper trim tabs, I have found a couple of pumps around for about $350 delivered from USA or the smart tab conversion which is basically swapping over the actuators and will cost about $170 delivered.
Does anyone know anywhere else doing cheap trim kits?
thanks everyone

 

[dan02gt]

Re: Hull bounce?

thanks for all your replies.
Would hydrofoil on the motor make any difference?
also are the smart tabs as good as the proper trim tabs, I have found a couple of pumps around for about $350 delivered from USA or the smart tab conversion which is basically swapping over the actuators and will cost about $170 delivered.
Does anyone know anywhere else doing cheap trim kits?
thanks everyone

A hydrofoil will help you get on plane but probably will not help with the porpoising issue as it should be out of the water when you're on plane.

A lot of people love Smart Tabs and they do what Nauticus claims. However they don't give you the control that a helm adjustable tab can. That may or may not be a problem for you. I wouldn't trade my helm adjustable tabs for Smart Tabs for many reasons, but that's just me. Sense you already have the trim tab kit I would get a pump for it.

Here's a used Bennett pump that could stand a good cleaning: 12 V Hydraulic Power Bennett Trim Tab Unit Motor Pump Model V351 Free Shipping | eBay

 

[Teamster]

Re: Hull bounce?

I think the first thing to look into is whether or not your running the proper prop,.........

Everything else is only a band-aid until the prop is as good as it can get,.......

 

[Fed]

Re: Hull bounce?

Make sure your hull doesn't have a rocker in it after all the work you did.

 

[smokeonthewater]

Re: Hull bounce?

neither helm adjustable or smart tabs are "better" than the other.. they do different things... each does things that the other can not do.

I am a freshwater boater and I would prefer the smart tabs though I have the hydraulic tabs.... You can pick up a pump used for under $170 and MAYBE way under if you are patient and willing to shop for the right deal.

Oh and IMHO whale tails are for underpowered boats at best.... you certainly are not underpowered

 

[stuee]

Re: Hull bounce?

thanks for all the advice.
ok how can i find out if i have the correct prop? the nearest boat place is over 2 hrs drive to take it there and they are pretty useless anyway
also what is a rocker and how can i tell if i have one, another thing too, if i have one is it possible to get rid of it?

I was reading up on the SE 300 foil which says removed or dramatically reduces proposing.
ive also follower the link from above for the tabs pump the the seller has helped by giving me advice and also throwing in a switch and wiring loom for free.
I highly recommend him if anyone is after used parts abcmotorsonline1 on eBay

 

[Scott Danforth]

Re: Hull bounce?

the SE300 will be out of water on plane and not help at all.

before you go spending money on a foil. try the following:

take a cooler, fill it with ice, put it in the bow area (a few # in the bow may be enough to settle it down)
try a stern lifting prop
install tabs. I recommend bennetts as they have the best customer service out there.

 

[ondarvr]

Re: Hull bounce?

Take a pic of the stern and post it.

Rocker is a curve in the hull, it can create all kinds off problems on a planing hull.

How low is the motor mounted on the transom, the lower it's mounted, the more chance you'll experience porpoising.

Used correctly a foil doesn't stop porpoising by dragging in the water at speed, but can help by allowing you to possibly raise the motor higher on the transom than without it.

 

[QC]

Re: Hull bounce?

IMH, and experienced, O, Trimtabs are never a "band-aid" and the best tool you can add to a v-hull other than a motor. Here's what I would do:

1) Check hull for Rocker, if convex, reassess everything, if flat, move to 2,

2) Post pics showing motor height, respond to suggestions

3) Investigate stern lifting prop, may make a difference, but as bad as your porpoise situation sounds I doubt it would "solve" it.

4) Buy the pump!!!! and install the adjustable tabs. SmartTabs are cool, but helm adjustables cannot be beat for their ultimate flexibility and adjust-ability. They will solve any remaining Porpoising.

5) remove any excess weight from anywhere (pet peeve), and distribute anything heavy to the bow that you actually need and can move.

6) Go boating and rejoice in the many solutions that adjustable tabs have now brought to your boating experience

7) report back!

 

[Tail_Gunner]

Re: Hull bounce?

Hi Scott,
i have trimmed it down to max, i also removed the stop bar to try get further but seems it stops there itself anyway.
No i dont have trim tabs. I actually purchased off ebay the tabs and hose / fittings but it didnt have a pump which i cant find a cheap one, they all seem to be the same price as with a kit.

Here is where the answer lie's.....perhaps that is. Any 21' glass boat with a 200hp outboard motor tucked in and still bounces...It should be nosing and plowing , post pics of your setup up that is the motor on the transom and its relationship..tuck the engine in all the way take a pic.....if you have a 8' 2x4 lay it flat of the bottom hull and take a pic

Your running very deep in that pic i would assume no bouncing there...what have you done to your transom

http://forums.iboats.com/non-repair...long-where-should-sit-630475.html#post4402214

 

[Scott Danforth]

Re: Hull bounce?

since your motor has tilt and trim, why is the tilt bar still being used? as Tail_gunner stated, take a pic of your motor and transom as it currently sits with the motor tilted in.