Jack plate and (How to Hook bottom of my aluminum boat) info needed!!

[MarshHawk83]

Hey guys! I need some help! I have a 1999 Fisher MarshHawk 180 with a brand new Mercury 4 stroke 115 efi on the back. Porpoising is my problem! And also I added a 6" pro high jacker manual Jack plate Friday and the porpoising is twice as bad now! BUT I haven't attempted to move the plate up any yet. It's down as far as it can go right now. What is the formula I need? I have a 19p power tech stainless 3 blade on the motor. ALSO....I saw on here the other day a guy was talking about "HOOKING" the bottom of the boat to make it run like a fiberglass rig. I need to find out more about that!

[pointer]

Porpoising can be caused by a few things.
Where is the cavitation plate relative the the bottom of the hull?
What do you have behind the seats - batteries, fuel etc? Thinking weight distribution here.
The jack plate isn't on the boat in the pics above, but if adding it made it worse I'd suspect you'll have to balance weight for and aft.

pointer

[MarshHawk83]

Yea I know I have way too much behind the seat. P*ss poor design by manufacturer. Have 18 gallon gas tank two batteries on board charger and a big live well. I put my trolling battery in front but didn't help much.

[David - WI]

Could be the angle the photos were taken at, but it looks like the motor is WAY too low... it looks like the cavitation plate is even with the bottom of the boat?

[MarshHawk83]

The bottom of the transom isn't actually the bottom of the boat....I have a drain plug and my live well drains underneath. Is that considered a pad or no? I'm sorry to sound ignorant but I really don't know the difference. The boat is a 1999 Fisher MarshHawk 180

[MarshHawk83]

If it's too low wil it porpoise? I just installed the 6" jack plate and it's set at its lowest position right now.i ran it like that on Saturday and it porpoises badly. Haven't changed position on anything yet.

[David - WI]

If it's too low wil it porpoise? I just installed the 6" jack plate and it's set at its lowest position right now.i ran it like that on Saturday and it porpoises badly. Haven't changed position on anything yet.

Yeah, it can... I would sure start there before messing with the bottom of the boat or moving major items like fuel tanks & batteries.

[capnzee]

You might try an aftermarket add-on to the cav plate of the engine (sometimes called a "whale's tail". Or you might try a couple of trim plates with manual trim adjustment added to the transom just slightly above or even with the bottom. Trimmed down, this should stop the hopping. Is your engine trimmed all the way under? or is there a trim stop preventing the engine from reaching a negative trim? Don't try and build a hook into your boat, period. Shifting engine back 6 inches actually moves your Center of Gravity back and could cause an increase in porpoising. Before doing a lot of moving things, take a "rider" or two out with you and place him/them in the front of the boat, if this stops the porpoising, move him/them back a little (you get the idea) If you can stop the bouncing by changing your C.G. with humans then replace your human ballast with boat stuff.

[outasite]

This is a case where a pair of manual tabs, could be just the ticket.

[Jimboat]

Hey guys! I need some help! I have a 1999 Fisher MarshHawk 180 with a brand new Mercury 4 stroke 115 efi on the back. Porpoising is my problem!

MarshHawk83 - If it's the porpoising problem that you're trying to solve, here's some ideas.

Porpoising is pretty common...any vee hull (or tunnel hull) can be susceptible to porpoising, depending on design and setup. Flatter bottom vees are more prone to porpoising than steeper deadrise vee hulls, but there are several contributors to the occurance and any vee hull can find the problem caused by dynamic instability.

The "bouncing" or porpoising comes from a change in the location of the center of Lift as the boat accelerates. The relocation of static weights is one way of dampening the rate of change of the CofL...so it's not always obvious whether to move weight fore or aft in order to cause the "dampening". The solution can be calculated, but we use boat performance software for that. It's not too difficult for you to find out through testing, whether moving weight fore or aft will help your particular problem.

The resolution to a porpoising problem with a hull design is most always addressed by causing the boat to run with less trim. There are many different ways of achieving this.
Porpoising is a function of the lift generated by your hull, the deadrise of your running surfaces, and the trim angle that is needed to get that lift. The onset of porpoising is, in part, influenced by the weight balance in the boat. Altering the deadweights in your boat (fuel, payloads, etc) can affect the speed and trim angle that porpoising will initiate. If a boat is porpoising at a given speed and load, lowering the trim angle will reduce or eliminate the porpoising. There are several ways to get there, but the bottom line is to reduce trim at the velocity of porpoising onset. (Higher deadrise hulls are less suseptible to porpoising.) Even if the hull design is operating in the "Porpoising Regime" through a full range of velocities, reducing trim in some way will improve or resolve the problem.

For your hull design and setup, the speed and trim angle at which you will see the onset of porpoising can be determined. Changing trim angle, changing motor height, changing propellers, moving weight foreward, will all help to reduce porpoising at a given velocity. Changing design variables such as running surfaces design, deadrise, engine setback, etc. can help maintain the hull in stable planing regime, avoiding porpoise regime.

Trim tabs (or more extreme whale fins) will also improve a porpoising problem, but these will also affect overall performance of the hull. Adding transom wedges can often help too, since they will allow for more negative trim travel, if it's required. Raising the prop shaft higher will shift CofG foreward and reduce trim angle. More HP also reduces trim angle. Reducing your trim angle in some way will always help a porpoising problem.

Consideration of your design and setup arrangement can predict whether you will see porpoising and at what speed you will experience porpoising with the TBDP Porpoising Regime Analysis for Vee Hulls, Vee-Pad hulls, Tunnel boats, Power catamarans and Mod-VP type hulls.
Check out this thread... there is more discussion there on porpoising.

Also, have a look at this article on "Hump Zone/Why does my Boat Porpoise?". It may be of some help to understand some of the solutions to porpoising problems.

[capnzee]

You might also check the bottom of the boat. The last 3 or 4 feet of the boat should be perfectly flat. Whether intentional or not, there may be a "rocker" built into the bottom of the boat. Even a rocker as small as 1/4 inch can cause a boat to porpoise. Use a long straight edge or "string" the bottom to check for a rocker in the bottom. (a "rocker" in the bottom of a boat is the opposite of a "hook"). As the boat accelerates across the water, it tries to climb up on the trailing edge of the last few feet of the boat bottom but cannot "hold" it there and the bow drops this sequence is then repeated over and over. Manual trim plates mounted on the transom and flush with or very near the bottom should stop this action.

[MarshHawk83]

Very good information guys! Thanks a lot and I'll Keep y'all posted on my results!

[MarshHawk83]

Oh and I almost forgot! When I installed my jack plate I noticed that my original bolt holes in the transom are a little out of square. I couldn't even tell until I got the plate on but there is a 3/16 difference in one side to the other when measuring from the top of transom. Is this a big deal?

[capnzee]

Some "finicky" engineer will argue this one, but I have seen this in the past and it really didn't cause a problem--couldn't tell the difference.

[Jimboat]

You might also check the bottom of the boat. The last 3 or 4 feet of the boat should be perfectly flat. Whether intentional or not, there may be a "rocker" built into the bottom of the boat. Even a rocker as small as 1/4 inch can cause a boat to porpoise.

Agree 100%! if there is ANY rocker, it could definitely cause onset of porpoising....even much less than a 1/4 inch could cause enough instability to start porpoise! [see article on Porpoising]