New Boat Porpoising

New Boat Purposing

l Have purchased a brand new fiberglass cubby cabin 4.85 mtr vessel, It has a variable dead rise hull, & a 75 hp mercury outboard. with 85 ltr inboard fuel tank, Dual battery system, With 11.25 hours on the clock so far.
My Problem is purposing constantly, l have to trim it every two mins or so, This boat can not handle any kind of water condition's without it purposing at all.
We Have done the following
Moved the motor up and down, The motor is on the second hole from the top now,
The cav plate on the motor is just be low the hull of the boat,
Props pitch used 17 Deg 18 Deg 19 Deg
Bolted on SE 300 high performance hydrofoil cavitation plate to motor now off no diffidence.
Put in 5 deg aluminium wedge Moved the purposing from 4600 RPM to 4200 RPM, Moved wettered area from 1 mtr to 1.5 mtr's still purposing Now Removed.
l keep 20 ltr's of spare fuel in the anker well, we run out of fuel so we pored the fuel in to the 85 ltr fuel tank and got going again, The boat started behaved differently again, They have put a strait edge over the hull, They tell me it's OK Strait.
Just wondering how much of a difference the weight of the motor has on the situation
Max motor weight 170 kgs Max
Motor weight Merc 75 hp 137 kgs
Motor weight Suzk 70 hp 158 kgs
Motor weight Yam 75 hp 168 kgs

They tell me that about 100 of these boats have been sold, and no problems, this is the first one sold with a Merc on it and first to have problems
My other problem is now the boat has started to lean to to RHS heavily

Any help would be much appreciated

 

If 20 litres of fuel in the anchor well makes a significant difference to the porpoising behaviour, I'd say the centre of gravity is too far toward the stern than optimal. You can get a rough idea of where the COG is by pushing the boat back along the trailer on level ground and noting the point it starts to tip. ( you need to support the back of the trailer so it can't drop) That will be different with crew and gear in the boat, of course. Another clue would be how it trims at rest, if it is sitting down at the rear rather than floating more or less flat, chances are it is bum heavy. ABout all you can do is shift weight forward or trim the motor in. I have a 20 foot hull with the COG about 6 feet forward of the transom waterline, and it has a slight tendency to porpoise but fitting tabs fixed that. What is the hull make of yours ?

 

If its new take it back and tell them. I don't like the listing bit at all.

If "they" say they have never had a problem, well they have now.

At least get one of there engineers out to try it with you.

Thing is you've done all the right things, the only stuff left is serious stuff.

 

Purposing

Thank you,
Boat Brand, Haines Traveler TF 157, it dose not seam to sit heavy in the water, top speed is about 63 kph, when they put the 5 deg wedge it droped to 57 kph, it moved the wettable area from 1 mtr to 1.5 mtrs, So do-sent that mean that the COG has moved forward by .5 mtr by doing this it should eliminate the purposing. but it has not.

Cheers Wheelybin.

 

That's a highly respected brand, so hull design is not an issue. The wedge was a transom wedge to get greater trim range, I assume ? That doesn't change the COG. I can only assume you have a fore-aft weight distribution problem, there is no water trapped underfloor ? Can't see what else it could be, if the fuel tank is a built-in one and not something sitting on the floor at the stern. You can only keep moving weight forward to see if it helps, but it seems a bit mysterious to me.

 

Purposing

l must admit that the Boat dealer and company rep have been great, They have taken it back to do more, l dont know whats left tho.

Cheers Wheelybin.

 

"Starting to lean to the RHS" ? That does sound like free water hidden in there, or maybe there was just a cross wind the day you noticed it.

 

The particular hull may be more sensitive to stern weight than others, which makes me wonder about the weight of the twin battery set-up, which is a bit unusual for a small boat. And I assume they sit right aft.

 

Purposing

it is my first boat, And l asked for the dual batterie to be install, they are both on the RHS, the same side as it has started to leaning to, The problem is that it did not start leaning to the RHS until they put the 5deg wedge in, they have the boat at the moment undertaking more tests, As for the free water in the boat l don't know where it might come from as it has covers on it when not in use, it gets drained every time it has been out the most that we have only got a cup of water out of it ever (Water skiing) l don't hear any water sloshing around.

Cheers Wheelybin.

 

Purposing

Sorry forgot about the location of the battery, yes the battery sit right aft.

If the boat is recommended to use the Suzuki which is heaver by 19 kg than the Merc, if l steer the boat, and place the top of the steering wheel between the 10 & 11 on a clock it steers & sits flat.

Cheers Wheelybin.

 

Next time you're out, mark this with a piece of tape on the wheel and the dash. With the boat on trailer and steering wheel in this "sits flat & steers straight" position, put a small straight edge along the skeg on the outboard. How does it align with the centerline of the boat?

 

Is the wedge still on the boat or is it removed now?

Does the list to starboard get worse as power is increased?

 

If the dealers selling them can't figure it out, it is a bit hard. I assume they have water tested it with you. If you have to hold the wheel over to stay on track, it suggests the tab on the outboard behind the prop is not set right, or you are using a lot of in-trim forcing the bow down, resulting in steering torque, but that should kill porpoising. I don't understand how a name brand boat like this in the hands of experienced people has an unresolved problem, really. If you are wrestling with the wheel, that will cause heel, but how it is still porpoising mystifies me.

 

All you need to do its to tuck the motor under more ..trim down ..any boat will porpoise if you trim it up too much...dual batteries normal ..tank at front normal ...my 16 ft Haines hunter was identical and no porpoise ....if its pulling right move the trim tab right its like an aerofoil not a rudder

 

i am surprised the haines dealer can't solve the problem, there has never been a bad haines hunter and are a lot of haines travellers on the water, its odd no one else has trouble, what revs does it get to, it may be over propped. i over propped an 18 ft mustang once and it caused the boat to lean and porpoise if trimmed out a bit.