Uplifting Experience (remedying stern squatting while on plane)

[grady]

Hey Guys not sure if this was the correct place to post this but seemed safe so here it goes.

I don't pretend to know anything about hull design or hydrodynamics. So with that said, heres the question. What considerations have to be made when modifying the running bottom of a hull that has a undesirable character trait. And when modifying the hull hows does one evaluate the down sides of said modifications.

The boat is a deep vee sport fisher designed by a very well known and respected designer (trying to keep names out of this if possible). The trait is stern squatting while on plane. this appears to be a result of too much weight and maybe too little power? I do know that the hull were built a lot heavier than the designer had planned. There is also more bow rise than would be optimal.

Could this trait be easily remedied by larger tabs or maybe a hook or ramp built into the bottom to assist in stern lift.

There is not always room for larger power plants not to mention the cost.


Thank you

[gonzo]

You are asking a question and withholding the most important information.

[eyschulman]

Try replaceing the v berth with a water bed.

[gonzo]

You may not be trimming the outboard, or maybe it is an outdrive, or maybe inboard. How can we answer without a make, model and description of the boat?

[PAR]

As you eluded to, there are extenuating circumstances that have probably left the designer pissed at the manufacture and an owner pissed at a previous owner.

In short, there are a number of things you can do, but as the questions are currently posed, so generically, not much of an answer can be offered. What boat is it? How under powered is it? How over weight is it? What has been tried so far? Etc., etc., etc.

[Mr Efficiency]

In the absence of further detail, I would think trim tabs the best bet, choose larger ones rather than smaller sized. They won't cost a bomb, and they can be experimented with as regards settings, which you can't do with wedges, at least on the run.

[daiquiri]

Originally Posted by grady

The trait is stern squatting while on plane. this appears to be a result of too much weight and maybe too little power? I do know that the hull were built a lot heavier than the designer had planned. There is also more bow rise than would be optimal.
Could this trait be easily remedied by larger tabs or maybe a hook or ramp built into the bottom to assist in stern lift.
There is not always room for larger power plants not to mention the cost.

Larger power plant is not necessarily a solution if the hull shape is not suitable to attain the speed you'd like to attain, or if the boat is too heavy. By installing larger and more powerful engine(s) you will be adding more weight (engine, tanks, drivetrain, propellers - everything grows up proportionally), ending up with the exactly opposite results to what you'd hoped to see - more fuel consumption and bigger wave left behind the boat, pluss possibly a cavitating and vibrating prop.
Same is valid for the trim tabs. They need speed in order to work.

You have given no numbers so far (speed, hull dimensions and type, engine installed, weight etc.) which could help us to help you. How about giving some?

Cheers

[grady]

OK, sorry to have tried this without the make and other details.

The Designer is C. Jannace, and the boats are the low to mid 30 ish Blackfins of the late 80's to mid 90's.
I believe the 31, 32, 33, 36, and maybe the 38 all suffer from this affliction. Some more than others.

They are all twin inboard, mostly diesel powered & very deep vee. I'm not sure if the flybridge, and the combi's are affected equally.

I do know that some of the smaller models used transom mounted rudders (Ellis), not sure if this was because of a lack of running surface or a design advantage.

If I've left anything out I'm sure you'll let me know.


Thanks

[grady]

No Thoughts at all?

[baeckmo]

Originally Posted by grady

No Thoughts at all?

Gotta have a nap first......

[grady]

If any additional info needed please reply.

Seems as this would be a very easily identified problem, with a choice of no so simple solutions.

[PAR]

Originally Posted by PAR

. . . How under powered is it? How over weight is it? What has been tried so far? Etc., etc., etc.

Deep V hulls tend to be tender, particularly if asked to do things they're not well suited to do, such as serving as a solid fishing platform.

We still don't know what has been tried, what worked, what hasn't, etc. Has the designer been contacted, the manufacture, other owners?

Tabs could be a solution, but only under certain circumstances.

[tom28571]

Your problem is an affliction that is characteristic of deep V boats in general. Deep V boats are heavy and they have to be in order to give them adequate stability. Otherwise the chines would be high out of the water and the boat would be unacceptably tender, which they are compared to shallow V boats anyway. The combination of heavy weight and a more inefficient bottom shape makes them stick their nose in the air in attempting to get on plane. The universal solutions are to add planing strakes and chine flats which give some flat areas to help them generate more lift to get on plane and another thing is to add more power. Adding more power adds more weight and fuel and the result is a vicious circle.

The plain truth is that a deep V is not a good hull shape for other than what it was intended for. That purpose is the ability to run at higher speed than flatter hulls in rough or offshore water. Otherwise, they are a poor choice of hull shape. As is often the case, certain styles catch the novice public's attention and the builders build what the public will buy.

If trim tabs won't make your boat acceptable, there is not much else that will make a significant improvement. Other attempts at a solution like moving weight forward are less successful. These are my opinions although others may have differing thoughts.

[IMP-ish]

What is the current power? Also what top end and cruise speeds are you are running and what speed does she plane at?

[grady]

Unfortunately, This is all research for a soon to be purchased boat. The ones I am leaning toward are the 33 combi's (Blackfins express) powered with cat 3208 ta's @ 375 hp. with 340 gals of fuel which weigh in at just under 20,000 lbs. There are a few with 435 hp 3208's (which I've heard was a tad unreliable) and some with 420 Volvo's although rare.

They are a bit tenter on the drift but great in a head sea. Would like to use for a weekly tuna fishery with a couple of canyon trips mixed in.

All in all this hull will get the job done, but have heard complaints of some small undesirable traits and thought they might be easily mitigated.

The original design called for much less weight, and lower power, no gen sets, no ac. Like stated earlier, builders will build what sells. The one piece cabin liner added considerable to the weight problem. But Blackfin wanted to court a higher end consumer. So more power, more amenities, more fuel, more weight.