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Thread: Boat Planing Issues

  1. #1

    Default Boat Planing Issues

    I have a 16' Sprint Marine boat that started life as one that had an inboard 115HP inboard OMC Turbojet. I purchased the boat after it had been converted to an outboard with a 50HP Johnson. The boat worked fine and did not have a tunnel on it. The boat hull has a semi-V hull design. I wanted to put more power on the boat, so added a 75HP Johnson to it and had a tunnel put on the boat. After this change, the boat has tended to want to ride bow-low when I hit the 3/4 power and higher level. I've tilted the motor back and it helps, but a friend has an almost identical boat and his flat out performs far better and planes perfectly with the motor put into the normal, straight/horizontal position that Outboard Jet recommends. I've studied this and looked at what could be causing the problem to no avail. The tunnel doesn't angle wrong and I actually bent up the 3/4" of metal up beyond the transom, to make sure it wasn't causing the issue. The only thing I see that could possibly be doing this are two areas just off the sides at the beginning of the tunnel towards the bow. These areas looked almost cupped upwards, as if two large boulders were hit (which they weren't). I can maybe see this causing the boat to make the bow force down in the water at 3/4 and above power, but figured I'd check on here to see what you all think. I can strip out the console and floor to pound or use a ram to push the metal down, but decided to check in here and see if anyone has had this issue before. I took the boat over to Karold's welding and he looked at it with me for awhile. He didn't see anything that stuck out as a problem though. If I stick absolutely all of my fuel and weight near the rear, it'll plane out alright at higher powers, but I can't always run like that. And....when I need the power, it'll be in skinny water where the bow riding low is a real problem. I love this little boat, but fighting this planing issue has driven me nuts for a couple years now. Thanks for any tips and ideas.

  2. #2
    Member DRIFTER_016's Avatar
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    Post up some pics for us will ya.
    Would especially like to see a rear view with the motor down.
    It may be possible that the motor is a little high.
    Do you have a jack plate?

  3. #3

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    Where are your batteries located? How much extra weight do you have in the front?

  4. #4

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    Here are a few pictures. I have my battery and almost all weight in the rear of the boat as well, in attempts to get it to plane out better and ride lower in the stern.



  5. #5

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    Been tinkering around with this more this week and made a few modifications that have helped. I removed the intake fins after reading a little online about those potentially causing lift. Sure enough....that helped keep the stern riding a little lower. Then, I lowered the motor about 1/4-3/8" from its current position and that as well helped a little bit more. The piece of UHMW I have on there was taken off and again, another slight improvement. I will likely put a smaller, flat piece on down the road, but for now will keep trying little things here and there to try and get it really dialed in. I am still thinking that the rear of the hull needs to be pounded out near the front of the tunnel to try and get those indented areas pushed out. That involves a little bit more work, but I'll give it a try here soon enough. Overall though, there has been some decent improvement lately and I'm pretty happy about that.

  6. #6
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    I'm glad you are making head way. If you were having this problem with an airboat I would suggest you place washers between the motor mount and the engine support tilting the prop thrust. By adding or removing washers the bow will move up or down when your running.

    I don't think you can add washers, what you can do is place a piece of metal starting with increment of 1/16 or 1/8 inch between the motor mount and the transom.

    It looked like the front of the jet is pointing up. I would start by adding a ¼ inch metal strip on the bottom of the motor mount if it makes it worse put the strip on the top.

    From the picture it looked someone already added a strip on the top.

  7. #7

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    I believe adding or removing the washers would be almost similar to me just tilting the motor further or closer to the transom with the pin that can be moved in the holes on the mount. I've even used a piece of pipe on that rod to go between the pre-determined OMC holes to try and dial it in. It does help the boat plane better, however it gets to a point where it wants to cavitate when it's tilted out that far. The strip on the top was just a filler piece put in when I had the tunnel installed, it doesn't really push the motor mount out.

  8. #8
    Member Akgramps's Avatar
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    Does it bow steer?

    I had some issue with bow steering on a OB jet a few years back, the original tunnel never flattened out at the top. When I had that fixed and extended the extension was not parallel to the boat of the boat. It ran down hill a few degrees and acted like a big ol trim tab and just held the bow down. This is how it looked after that,

    I decided to experiment and used a piece of sheet metal to angle the extension up a few degrees just to try it.....


    HUGE improvement, I was able to move the motor up a set of holes, it was as if I gained power steering. It was so bad before it was scary and hard to drive, it would start to slide and then come around as the bow was digging in.

    Small changes can make big improvements, try one at time.....However with all that being said it doesn't look like your extension flap is angling down.......?

    What I see in your pictures is the bottom dented up on each side of the tunnel, I can see the aluminum angle strake is deformed. I would think this would have a tendency to push the bow down as the bottom is "hooked". Damage closer to the bow or even at the front of the tunnel would have less effect, the critical area is the last several feet before the transom.

    At least that's my thoughts......!
    “Nothing worth doing is easy”
    TR

  9. #9
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    It's obvious my lack of experience with outboards shows. Oh well I can't know everything...... just think I do. LOL

    With my airboat I adjusted the prop angle just below the point of cavitation and I use trim tab to find tune the bow depending on the load. Some airboats have a 2-3 inch aluminum strip added to the back of the boat to act like a fix trim tab. You might try using C clamps to hold the strip on to see what happen.

  10. #10
    Moderator bkmail's Avatar
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    I'd say you need to flatten out the dents on either side of the tunnel in the last foot or two. It's acting like a hook and regardless what you do past (aft of) the transmon it won't make a whole lot of difference. The hook in the last foot or so is keeping the bow down.
    Pull the floorboards, support the boat from underneath with dunnage and beat the bottom flat again, won't take long.
    My thoughts anyway.
    BK

  11. #11

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    Akgramps,
    When I took the boat to Karold's to check out and get his ideas, we both noticed that the tunnel on mine actually goes up a little bit, which should really help make the bow ride higher (which it doesn't). The bow will definitely steer if I go to full throttle, which is both scary and dangerous.

    I tore everything out of the boat this weekend and am looking at it more and there is definitely a hook to the hull beside the tunnel. When I lay a 2x4 from the transom to the middle of the boat, the hull has about an inch upward dent on both sides of the tunnel. I'm almost thinking that when I had that tunnel put on, the welder might have pushed the hull up to meet his tunnel and inadvertently made those bug bumps. I don't see any real rock damage that caused it, so I'm just not certain.

    Here's a shot of the hull from above with everything out of the boat. I'm going to drag the boat off the trailer and put it on sandbags around that tunnel. Then I'll get the 5lb dead blow hammer out and start trying to reform the metal. If that fixes the problem, I think I'm going to have more structural supports/ribs put into this section to beef it up and keep it more rigid.


  12. #12
    Moderator bkmail's Avatar
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    You don't need to pull it off the trailer to whack the bottom flat.
    Simply crib up from underneath, work around a trailer crossmember if need be. Use solid lumber underneath and it won't take very long at all.
    Prepping for the job will likely take longer than the chore itself.

    If you have an inch tall hook in the floor just before the transom, you have found your problem, as I mentioned in the earlier post.
    This is a very common problem on airboats that have been running ground and beating the hull, they want to run on the bow.
    BK

  13. #13

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    bkmail,
    Thanks for the info and input, I appreciate it. I messed around cribbing a little, but it was a serious pain. This boat is light enough that I can pretty easily lift if up and move bags around. My trailer cross members and supports were in the way of cribbing and it's so low, that getting up under it is a real nightmare. I'll let you know how it goes...I'm also thinking this might be the problem..I hope.

  14. #14
    Moderator bkmail's Avatar
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    Any chance you could just drag the last couple feet of the boat off the trailer so you didn't have to unload the whole darn thing? Just a thought.....
    Regardless, I think your on the right track, keep us posted on how it went.
    BK

  15. #15

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    I was thinking of doing that and just cribbing it up on some stands with 3/4" plywood, some 2x4's and the sandbags. That'd be a lot easier to get back on the trailer that way. I'll give that a shot at first. I have a nice big spruce by my driveway to tie onto and just pull ahead with the trailer to get it to come off a little at a time. The good thing now is that I'll just throw the motor and console back in after I have it beat back into shape and see how it performs. No need to put the entire floor back in just yet until I have it exactly where I want it. I might still get some structural members welded on in that area to keep it stiffened up so that this doesn't happen down the road by the incidental rock.

  16. #16

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    Just an up date on results. The pounding out of those sections solved the problem!!! The boat handles and rides superb now. I can't believe how great it rides and steers. I can make much sharper turns without cavitating as well, since that bow isn't trying to dig in and throw the stern up higher, which makes me really happy. I appreciate the input everyone, thank you. Now it's time to get some beefed up structure in the rear of the hull and put it back together.

  17. #17
    Member Akgramps's Avatar
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    Hey, that's great news...thanks for the update and am glad it worked out......!
    “Nothing worth doing is easy”
    TR

  18. #18
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    That is really good news.

    From the pictures it did not look good, but sometimes pictures are miss leading.
    What I don't understand is why Karold's Welding did not see the problem. Let me guess....was he the one that welded up your boat?????????

  19. #19

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    Karold's didn't weld my boat. I can see how it wasn't apparent though. When you looked at the boat from the rear, you had to get at the exact level to see the upward bow in it. From the front it was more apparent and even then, it still wasn't totally obvious that that could create such an extreme change in the planing performance. I'm still really happy that it's resolved. I told myself that if that didn't fix it, the boat was going up for sale and a new, custom hull was going to get ordered. Guess I saved some serious money with the old hammer.

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