Help Needed 33 ft Centre Console planing problem

Discussion in 'Boat Design' started by Bull GT, May 9, 2008.

  1. Bull GT
    Joined: May 2008
    Posts: 4
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    Location: Andaman Islands

    Bull GT New Member

    Hi Guys,
    I am new to this forum and have recently purchased a custom boat from an American boat builder, this boat was shipped to us in the Andaman Islands, India and despite repeated requests the builder did not come either for the final fit out, when we tried the boat it just did not plane more it acted like it was going to flip over backwards, the bow was sticking up so high in the air. What the builder then told us was to fit trim tabs as this would solve the problem, we fitted Bennett 16 inch sport tabs but that did not help.

    All the while the builder promised he would come and rectify the planing problem which has not happened till date 5 months after the boat was delivered, this brings me to the reason I post on your forum is to ask for some guidance and help as to how we can rectify the planing problem or worse case yet to know if we have been supplied with a lemon!!

    I have put down all the information we have on the boat along with some pictures so you can actually see what has been happening:

    Type: 33 foot Custom Hercules Centre Console Sport fishing boat, deep V
    Length:9.3m
    Breath : 2.9m
    Draft : 0.8m
    Dry Weight: 3600kg
    Fuel capacity: 300gallons

    Engine: F225 Yamaha 4 stroke Counter Rotation Set
    Weight: 265Kg
    Prop: 19 pitch Standard Yamaha, 3 blade
    HP: 450

    [​IMG]

    We initially tried to put the bow down by using sand ballast, we put upto 800kg in front with no change we were doing 13 knots burning 140lph at max 4600 rpm and still had the bow so high there was no driver side visibility. At 2400rpm the boat did 8 knots at 40lph cons the moment we tried to accelerate it went to crap. Tank 75% full, Pax 4 all in front

    [​IMG]

    We then had the boat builder come over to see the boat and how it actually performed, this is when he said trim tabs would solve all the problems, we ordered 16 inch Bennett sport tabs which are recommended for a boat this size. The only position which these tabs could be installed on was either side of the engine bracket and at fully retracted would always have a negative angle.

    The result of this was the boat only improved to about 15.6kts at 4600rpm and the same 140lph consumption, this is with the tabs fully lowered. While at 2400 rpm it performed the same as above. Tank 75% full, Pax 4 all in front

    We then called a Yamaha technician in to check the engine mounting, he said it was fine but the hull was defective and brought our attention to the fact that the rear of the hull was not a V throughout but had been flattened (see picture).

    [​IMG]

    He also suggested moving the fuel tank further forward to change the CG, see picture of general arrangement, we did this by removing the toilet and waste holding tank and fabricated a new 650 litre tank which was a drop in, we did not want to fiddle with the original as the hull was supposed to be rectified by the builder.

    This improved the performance substantially:
    Speed 26kts
    RPM 5200
    Cons 140lph
    Tank Full
    Persons on board: 2
    Tabs retracted fully

    However if the weight of the passengers went to the back the speed would drop to 20kts, boat performed at this rate for 1 week. Then speed dropped back to 16kts, tabs full down did not improve this. We discovered the engine bracket was full of water, pumped it out speed improved again to 22-24 kts, however the swim platform is not moulded to the engine bracket and this problem keeps arising as it has hairline cracks which allow water through to fill the engine bracket.

    The engine bracket is flat at the bottom see picture.

    [​IMG]

    [​IMG]

    The boat is very difficult to get on the plane at any speed less than 20kts it is not on the plane see pic, we also have to run it most time with the tabs fully down to get any speed out of it, we are always burning 120-130lph of gas to get any decent speed (14+ knots) and have to put all passengers in front to get it to plane.

    Any help/ advice would be much appreciated so please do let us know it there is any other information we can give you.

    Thanks

    Darran.
     
  2. kmorin
    Joined: Apr 2005
    Posts: 185
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    Location: Alaska

    kmorin Senior Member

    Checking the Hull Lines

    Bull GT,
    The performance sounds like the after butt lines are not straight- it sounds like they rise in the after 1/3 of the bottom and that allows the bow to pitch up where engine thrust is pushing the transom down instead of forward.

    To check if the hull needs reshaping, which is sounds as though it does, hold a 8-10 straight edge to the bottom while on the trailer. The lines should be completely straight at the transom and for about 8 to 14 feet forward depending on how far out board you check.

    If the hull is curved all along the after end, where the bottom rises upward from a straight line, then the lowest point of the hull is not the transom. If the boat is to perform at the best speeds possible, generally, the butt lines need to descend all the way to the transom or at the very least remain flat and straight in the after 1/3 of the bottom.

    This check will allow you to learn if the hull needs to be "wedged" Essentially, this would imply that you fill in the bottom so the after 1/3 of the hull is a uniform prism.

    If there is any rise in the butts, and a very small amount won't hurt, the bow rises and the stern drops.

    ON a hull this size I'd suggest 30" tabs.

    Engine depth in the water is also critical to performance on this hull. Because the wake rises aft the hull and the engine mount is not above the planing bottom, the engine depth while running becomes very critical. I'd suggest a hydraulic transom slide so you can raise and lower the engines to run at the proper depth. If they are too deep they won't reach RPM and since they're only rated 225 HP at 95% of the usable RPM, you're trying to push a very heavy boat out of the hole with 200 hp total.

    Another method of helping to trim an outboard hull is to lean the engines aft. When they're bolted to the transom all the trim and tilt it outboard/aft/bow up. Loosen all the bolts and install a 1" tube around each top stud so the engines are leaned aft - several degres more than the transom plate. If you need to trim OUT- well that's where all the engine trim goes. But now the bow down/stern up vector of the engines will act to counter the bow pitch.

    You may find that a permanent wedge on the after side of the engine mount will allow both outboards to 'tuck' under the hull more and help get the boat up on step with less effort. The increased angle aft, combined with a vertical transom lift is a good solution for almost any planing problems that involve a skying the bow.

    Lots of engine maker think 15 degrees aft lean/rake/tilt of the engine mount is good practice. I've used 25 degrees for almost all boats, (a few hundred outboard powered in the last 3 decades) and they can all trim out- but if they need to 'tuck'- well-with an increased transom angle they can.

    If the engine cantilever won't hold up to normal use, this indicates you need a lawyer not a boat builder- leaks in a 'new' boat sound like a shady deal.

    Just a view from afar.

    Cheers,
     
  3. afrhydro
    Joined: Mar 2008
    Posts: 243
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    Location: port charlotte fl

    afrhydro Senior Member

    the motors are to deep
    the caveatting plate needs to be 1 and 1/2 inch above the brkt
    this should put the motors prop shaft about flush with the bottom
    or just a inch below would be good
    i need a picture of the motors on the brkt to tell for sure
     
  4. Bull GT
    Joined: May 2008
    Posts: 4
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    Location: Andaman Islands

    Bull GT New Member

    Engine position


    Afrhydro,
    Thanks I have attached a picture though not the best, the anticavitation plates of the motors are in line with the base of the hull

    [​IMG]

    Can you based on the picture let me know if they should be higher up and if so by how much.

    thanks.
     
  5. Bull GT
    Joined: May 2008
    Posts: 4
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    Location: Andaman Islands

    Bull GT New Member

    Re; Boat lines

    Kmorin,
    Thanks for the advice can you please send me a diagram of what we need to measure, I am not a builder or designer but and trying as a customer to make the boat work.

    We will try the wedge as the current mounting bracket rake angle is 12 degrees, what is a good material to use for the wedge? Stainless plate??

    30 inch tabs will mean adjusting the position of the swim platform as there will not be enough room for 30in tabs and the rams. K-plane tabs have been recommended do you know where these are available and the model number for 30in in the k-plane series?

    Sorry for the questions but if we can easily rectify the problem the issue of a lawyer may be avoided if possible,

    Thanks again.
     
  6. afrhydro
    Joined: Mar 2008
    Posts: 243
    Likes: 12, Points: 0, Legacy Rep: 245
    Location: port charlotte fl

    afrhydro Senior Member

    yea i can tell nothing from the picture
    but they need to be 1 /1/2 inches above the hull
     
  7. kmorin
    Joined: Apr 2005
    Posts: 185
    Likes: 18, Points: 0, Legacy Rep: 231
    Location: Alaska

    kmorin Senior Member

    Planing hulls versus semi displacement hulls

    Bull GT,
    the attached sketches try to illustrate what (I think) we're discussing first a look at a generic planing hull's bottom, where the waterlines and buttock lines of the hull are all straight and parallel aft the mid point of the bottom.

    If these lines aren't fairly straight, if they rise toward the stern when a straight edge is level to the waterline; then the hull will 'squat' or pitch up the bow when you run. In the two images under the hull (green bottom) straight edges are laid along the hull to see how the bottom is shaped. Here the hull is straight - BUT the butt line does rise at the bow. If this rise happens at the stern(?) the straight edge will not be parallel to the waterline, it will instead be pointed down into the trailer when you hold it to the aft most bottom section.

    The idea here is that a hull might have rising lines aft causing your pitch/bow up attitude and trim planes will help counter this effect. Better still would be to have the lines repaired, if needed, and that implies glassing in a wedge to allow the butt lines to be straight as shown.

    The last two images were included to show a boat that is designed to go 6 knots and has rising buttock lines. If you held an edge to this bottom that rule would point downward at the middle of the hull= rising butt lines aft.

    All views are intended to illustrate the possibility the hull is not well formed, and to help make my previous remarks more clear.

    Now to the issue of the cantilever versus engine cavitation plate depth of immersion.

    Notice the cantilever section's lower surface rises aft so the hull's wake is released at least 2', perhaps 3', forward of the engines. This guarantees that engine cav. plates are immersed 6 to 12" in water! That is why this hull form often has the vertical rising hydraulic transom plate - to adjust the engine immersion as speeds increase the wake's rise does too.

    You cannot align the cav plate to the hull as the engines are aft the release point. You must either make the engines adjustable OR get a clear view of the wake as it hits the outboard drive leg and adjust the heights of the engines from that observation.

    I'd use plywood and resin for a wedge of the engine mounts' aft surface.

    Hope this helps make some sense of the hull shape issue.

    I don't have any suggestions for planes except that wide transversely and narrow fore and aft are what I've built in the past. If they already made a difference and they're large enough to get the boat on step- don't bother with them until you have more running time on the boat.

    Next, lift the outboards, lean them aft and see what happens.

    Cheers,
    kmorin
     

    Attached Files:


  8. Bull GT
    Joined: May 2008
    Posts: 4
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    Location: Andaman Islands

    Bull GT New Member

    Kmorin, thanks for the inputs as soon as we get the boat out of the water i will remeasure and let update on the findings and progress

    Cheers
     
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