Severe Porpoising Need help fast

[82rude]

Re: Severe Porpoising Need help fast

90hp 306lbs. 120hp 365lbs. weights not the issue . nauticus trim tabs will at least control the problem.

 

[QC]

Re: Severe Porpoising Need help fast

None of us can say that "weight is not the issue". At a minimum it is part of the issue. I suspect multiple causes. While Frank's description of porpoise is accurate, the ability to lift the bow, or the ease at which the bow lifts is absolutely affected by weight balance. I did not say caused by weight imbalance, but again, at a minimum it is a contributing factor. That would be along with prop, hull shape, trim angle, motor height, etc.

With that said, the OP has not responded with anything regarding the hull. And indeed, the simplest "fix" for porpoise if it cannot be controlled by trim is trim tabs. Sorry, but I do not deal well with absolutes in this sloppy medium we operate in. AKA H2O, DHMO, water etc.

 

[Budget33]

Re: Severe Porpoising Need help fast

Sorry guys havent had a chance to check the hull, I will get on that this afternoon.

My next question is with only being 59lbs over weight ( 90hp - 120hp) how come all the weight i added ( up to 500lbs!) didnt solve the problem?

Sorry for being so new

 

[roscoe]

Re: Severe Porpoising Need help fast

Retired Guy, The cavitation pate is the adjustable plate above the prop correct? the fin on the lower unit that plate is mounted to is 1" below the bottom of the boat.

Captain John, I agree to finding the cause. Could the motor be too big? ( is there such a thing? haha) its rated for 90hp and i think the weight difference is about 80lbs at most

There is part of your problem.

THe large horizontal plate, just above the prop, is your anti- ventilation plate.

It should be 1" ABOVE the bottom of the hull when the motor is trimmed down

Need to raise the motor 2".

 

[QC]

Re: Severe Porpoising Need help fast

Man, I always biotch when someone uses Cavitation when we are discussing Ventilation, and I went and called it an Anti-Cavitation plate. Doh!

One of the reasons I believe there is a hull issue, is that you can't fix it with weight in the bow. I would never add weight to a boat, but I do believe that is indicative of additional problems. Motor height may indeed be part of it.

 

[dazk14]

Re: Severe Porpoising Need help fast

What specific size and brand prop are you using? Raising the motor will help, since it shortens the torque lever that naturally lifts the bow, vs. driving the boat directly forward.

Adding to that, some models of the 120 on that vintage had the small gearcase(not V6), so not much diameter/blade area to help hold the bow steady. Correct me if you have the V6 gearcase). I can tell via your prop size.

The motor weight IS a contributing factor. We see this a lot when adding a kicker to a boat with a 3 blade prop. We also see this when the boat is holding water - typically more aft - since that is the most common place for it to accumulate.

An of course as mentioned, the hull bottom may have deformed.

 

[H20Rat]

Re: Severe Porpoising Need help fast

Sorry guys havent had a chance to check the hull, I will get on that this afternoon.

My next question is with only being 59lbs over weight ( 90hp - 120hp) how come all the weight i added ( up to 500lbs!) didnt solve the problem?

Sorry for being so new

You aren't overweight... There are 4 stroke 90's that are heavier than that, and would still be in spec for horsepower.

 

[QC]

Re: Severe Porpoising Need help fast

Good thing we are all singing the same tune :facepalm:

Feels like a mini oil thread. Ultimate answer? Do what you want...

 

[Budget33]

Re: Severe Porpoising Need help fast

I hear ya QC on singing the same tune, except I'm not sure what direction to go

I checked the hull with a straight edge and its straight, no hook or rocker. Also, I cant raise the motor anymore, its mounted as high as it can

Dazk14, the prop is a 13 1/4 x 17.

So should I get Trim tabs, like the Nauticus ones? or switch props? 4 blade? I dont have a tach so i have to be careful about over-reving

Thanks for the help, keep it coming
Budget

 

[82rude]

Re: Severe Porpoising Need help fast

I had a boat with the same issues .a 4 blade did not help .trim tabs cured it pronto.now for the rest of the story!when I scraped the boat a few years later I tore apart the floor to find tons of wet foam .the boat weighed 650 lbs more than it should of!have you checked for wet foam or a spongy transom?a flexing transom will give you lots of issues also.

 

[dazk14]

Re: Severe Porpoising Need help fast

I hear ya QC on singing the same tune, except I'm not sure what direction to go

I checked the hull with a straight edge and its straight, no hook or rocker. Also, I cant raise the motor anymore, its mounted as high as it can

Dazk14, the prop is a 13 1/4 x 17.

So should I get Trim tabs, like the Nauticus ones? or switch props? 4 blade? I dont have a tach so i have to be careful about over-reving

Thanks for the help, keep it coming
Budget

Who raised the motor so high such that it, "can't be raised any higher", and why. You also said the motor is mounted as low as it can go..

You also indicated the AV plate is 1" below the bottom of boat.

If the motor is mounted as high as it can go and the AV plate is 1" below the hull, is your 120 a 25" shaft motor?

A 25" shaft motor would certainly help to explain your issue.

What is the specific prop model. At a minimum, is it stainless steel and polished?

Pics never hurt.

Let's start with clarifying where the AV plate is and where the motor's mounted.

 

[Budget33]

Re: Severe Porpoising Need help fast

Dazk14, Sorry for the confusion, it looks like the motor could be lowered 1 hole but not raised, I will take a pic soon and post back.
As for the prop, its aluminium for sure and stock as I can tell, I will take a pic of it as well. How do i tell if I have a 25" shaft, according to the model # its should be a 20"

Whats the quick and easy as to tell if I'm water logged? the transom seems solid

Thanks
Budget

 

[Outsider]

Re: Severe Porpoising Need help fast

should be 1" ABOVE the bottom of the hull when the motor is trimmed down


Uhhh, regardless of what it's called, it should be on the surface of the water when at decent planing speed. Relationship to the hull at that point is irrelevant. That, of course, involves a motor with the proper shaft length, doesn't sound like the case here ... :facepalm:

 

[Budget33]

Re: Severe Porpoising Need help fast

Here are the pics, I also included a pic with the engine trimmed all the way down to see ho much is below the boat. The numbers on the prop are 391199

 

[JimS123]

Re: Severe Porpoising Need help fast

Your motor is the proper shaft length. It's one hole up from the bottom and you have 2 more holes to go. Adjust the motor up to the next hole (up 3/4") and try it there. Note that you have to remove the motor from the boat to do that.

Also, please be advised that I'm trying to help you, and I'm not suggesting that you run an engine that is 30% over the boat's recommended capacity.

 

[NYBo]

Re: Severe Porpoising Need help fast

It's hard to tell for sure from the fourth picture, but motor height looks to be about right. Take a straightedge and put it along the keel and extend to the lower unit with the motor perpendicular to the keel. Post a picture of that.

 

[Budget33]

Re: Severe Porpoising Need help fast

Jim, do you think 3/4" will make that much of a difference? I'm not disagreeing just its a small amount of travel, and i'm new at this so please explain.

 

[sphelps]

Re: Severe Porpoising Need help fast

It looks like the bottom bolt is bottomed out . You would have to drill that hole higher to lift the motor . I think ..
As mentioned straight edge the hull to the vent plate and see exactly what ya got .. I have a little porpoising problem myself so I am very interested on how you get this solved ..

 

[dazk14]

Re: Severe Porpoising Need help fast

If you have a dump/transfer station, it would be worth the trip to weigh your rig.

In 89 Maxum's came standard with Bimini tops, faded maroon gelcoats and factory wet foam...sorry couldn't resist.

They actually had some nice deep V hulls.

We'll just work with what you have to get you out on the water.

The prop that is currently installed has no holding power, although fully trimmed in, it should not be porpoising like crazy.

Shooting from the hip here. I think the tools to settle it down are... in order of cost. A hydrofoil, a 4 blade Solas Amita, Smart tabs.

 

[Budget33]

Re: Severe Porpoising Need help fast

Thanks Dazk14, finally some direction, haha. I have a line on a hydrofoil for free, the tabs are about 250 here, is that prop stainless?

Do I stay with the same size?

don't have the bimini top but i sure have the faded maroon, haha. Can you tell me more about these boats, haven't found too much

I also may be able to get it weighed, i'll look into that