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Thread: 2 questions from the new guy

  1. #1

    Default 2 questions from the new guy

    I just came across this forum, and it seems like it may be a good place to get some input on a couple questions that I have. After running my boat for 3 years, I'm considering making some changes to try and get better performance. If I didn't like the boat so much, I'd probably buy a new one, but it works great for me and it's very versatile (and paid for). It was made in BC, and I use it equally on lakes and rivers. I have 2 motors, a 50 prop and a 60/40 jet.

    Boat is 15' long, 62" at bottom and 76" max beam, with a 6 degree vee and a wooldridge style tunnel. It's quite heavily built, weighs around 1200lbs as pictured, fully loaded and fueled. According to USCG formulas, it is rated for about 90-100HP, but no rating from the manufacturer is on the boat.

    Although it runs fairly well with a prop motor, there are some handling issues with my jet. I believe part of the solution would be a bigger jet, but I'd like to tinker with setup before I spend any more $$.

    1st question -- The boat seems to both sit, and ride fairly low in the water, causing a lot of spray, and the jet will eventually cavitate and fall off step in a sharp turn -- it does not slide in corners. I believe these things might be due to the tunnel. It would be fairly easy for me to cover over the tunnel with an aluminum delta pad. Probably better for the prop too... Any thoughts on if this would improve handling? The boat presently runs very shallow, but overall handling is more important to me.

    2nd question -- If I can get the handling figured out, I've contemplated replacing my jet with something more powerful, quiet, and efficient. Based on the presence of local service, I'm considering either a 90/65 yamaha four stroke or a 115/80, and then modifying my boat to accept a small centre console. These motors would add about 130lbs more stern weight, but would move me (220lbs) away from the tiller.

    thanks for any advice you may have.

    regards from Canada

    Shane
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  2. #2
    Member alaskanmoosehunter's Avatar
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    Is your tunnel cut at 90 degree angles like a box? I have noticed that strait tunnels in a sharp turn, when the boat is moving in a sideways motion tends to "churn" the water which mixes air bubbles and water then the jet sucks up air. (Cavitation)

    If this is the case you can weld an inward 45 degree angle into your tunnel to force the water up into the jet.

    AMH
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  3. #3

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    AMH, the sides of the tunnel are cut inwards at approximately 45 degree angle. Very similar to a wooldridge design. I'm not sure where the air is getting introduced, but in a wide corner, the pump starts sucking air and the boat falls off step.

  4. #4
    Member Akgramps's Avatar
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    As you already know thats a heavy boat for its size. It sounds like it could be bow heavy, but if that was the case it should slide easily.
    Do you have fuel tanks in the bow? Do you feel it is bow heavy?
    Does the tunnel extend past the transom very far? Like the Wooldridge?
    Is the top or roof of the tunnel parellel to the bottom of the boat?
    Are there any ribs or strakes on the bottom of the boat?
    “Nothing worth doing is easy”
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  5. #5
    Member Akgramps's Avatar
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    [QUOTE=1st question -- The boat seems to both sit, and ride fairly low in the water, causing a lot of spray,

    Shane[/QUOTE]

    Another thought I just had, probably stupid but gonna ask anyway, do you have as much spray when running the prop as you do the jet?
    The 64$ question is the 40hp jet getting the boat on step?
    Having you tried trimming the motor?
    “Nothing worth doing is easy”
    TR

  6. #6

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    Hi AK gramps, to answer your questions....

    The boat does not feel like it is bow heavy. Fuel tanks are just forward of the mid-point of the boat, and yes, the boat definitely planes. It will go about 28mph with the jet WoT and about 31 with the prop WoT. I can waterski behind it (220lbs) with the 50 horse prop motor! The prop motor lifts the boat a little higher out of the water than the jet does, with slightly less spray.

    The tunnel extends about 2" behind the back of the hull, and on both sides of the tunnel, the 3/16" making up the bottom of the boat was left to overhang about 10" on each side. This effectively increases the bottom profile of the boat, when on plane. Based on the way that the boat likes to ride with the stern sitting so low, I think that the tunnel is really influencing the ride. The top of the tunnel is about 11" wide, but the sides taper down to make the overall tunnel opening about 19" wide on the very bottom of the boat. This 19" opening is 36" long, and the top of the tunnel is not parallel to the bottom of the boat. Ie. it tapers up from the keel to the top of the tunnel at the stern.

    There are four strakes that extend along the bottom, but they would be easy to remove from the back half of the boat.

    Yes, it is a heavy boat for it's size, but buying a different boat isn't really an option right now. The hull works well for what we use it for, but I would like it to handle a jet better. Looking at the way that tunnel was installed in the boat, I'm asking myself how much of a project it would be to remove it, and replace it with a normal keel or a delta pad.

  7. #7
    Member Akgramps's Avatar
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    Longshanks,
    The tunnel on my boat is similar, it is wider at the front and tapers back and up towards the transom. It works.
    The critical part is just in front of the foot, that last 6-10 inches should flatten out and be parellel to the bottom of the boat. If it continues running up at an angle, it can be hard to get the foot set just right and create a lot of spray. If it runs down (and it doesnt sound like this is the case w/ your boat) it can act like a big trim tab and push the bow down.
    The hull where it extends past the bottom: is it bent up or down by chance?
    The strakes if they run full lenght alongside the tunnel and all the way forward could be catching some air and funneling in to the jet, particuarily when turning.
    Is the motor trimmed all the way down/forward when running the jet?
    Performance wise the boat sounds like it does ok, just need to sort out the handling.
    Just some thoughts.
    “Nothing worth doing is easy”
    TR

  8. #8

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    thanks AK Gramps for your help. I would have responded earlier but I was fishin

    Based on what you describe as the 'critical' part, my tunnel is different. It tapers from the bottom of the keel at the front of the tunnel, in a straight line up to the back of the tunnel. I suspect that if I were able to modify the tunnel so that the last 10" of tunnel was parallel to the keel, the boat trim position may be better (rides stern low now). Not sure how I feel about modifying the tunnel, but it just isn't working right now, and I've tried all the potential adjustments with limited improvements.

    The hull as it extends beyond the stern got bent once, but my sledgehammer fixed that

    The motor is trimmed in about the middle of it's trim settings. any closer in towards the stern and it pushes the bow low into the water and creates too much spray/drag.

    I may try cutting off those strakes for the last 4' or so of the hull and see if they make a difference.

  9. #9
    Member Akgramps's Avatar
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    can you post a picture of your tunnel where it exits the transom?
    “Nothing worth doing is easy”
    TR

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    You might try lowering your engine by a few inches 2-3, then trimming it up more...

    I had the same sort of problem you describe - Felt like my boat was plowing.....The only way I could fix this was to trim my jet up....Then it would cavitate really easily...especially in the turns...

    I lowered my engine about two inches, and this allowed me to run it trimmed higher without sucking air...

    Fixed all my problems...

  11. #11

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    the way the shoe presently sits, I can't make it cavitate, in any condition, unless I try to 'slide' the boat through a wide corner. The bottom of the shoe is high enough that I can jump logs, etc, without kicking the motor up.

    thanks for the hint about lowering the motor, I think I'll plan on dropping it down hole by hole, and eventually welding in 10-12" of flat plate in the top/rear of the tunnel, parallel to the keel strip.

    I attached a photo that shows the boat from the rear. The leading edge of the intake is flush with the top of the tunnel, and I modified the shoe with intake fins, which helped bring the bow of the boat down considerably in most conditions.
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  12. #12
    Member Akgramps's Avatar
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    Its hard to tell in the picture, but it sounds like your jet is set at the right height,"The leading edge of the intake is flush with the top of the tunnel, and I modified the shoe with intake fins, which helped bring the bow of the boat down considerably in most conditions. "

    lMO, lowering it will cause a lot more spray, most jet boats will start to cavitate in sharp corners, one of the problems you mentioned in your very first post.

    Most tunneled jets seem to like a piece of flat plastic (UHMW) bolted to the top of the tunnel to minimize srpay at the foot, it sounds like this is something you do not currently have? I removed it on my boat briefly and water would boil all the way up to the powerhead.!

    For some reason that I do not know, most flat bottom boats w/o a tunnel can get away with out a piece of uhmw, and the water comes off the bottom in a nice flat "apron" with the edge of the foot set just at that point.

    If your tunnel is still angling up when it exits the transom, it may need to be flattened out, then install a UHMW spray guard. You could always try a piece and see what happens.

    Heres a photo of my setup, not the best, but what I have at the moment.
    Last edited by Akgramps; 03-21-2010 at 22:55.
    “Nothing worth doing is easy”
    TR

  13. #13

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    thanks again AK Gramps for your thoughts. At one time, I installed a UHMW skirt over top of the tunnel, and found that it had little effect. The jet shoe is right at the top of the tunnel, as you say, and there isn't a lot of spray coming from the tunnel. I think that the main hinderance is the design of the tunnel; the entire length (36") of the tunnel runs in a straight line, on an angle from the keel up to the back of the tunnel. I believe you when you say that the last 10" or more of the tunnel should be parallel to the keel, to make the boat plane properly. It now sits pretty low in the water on plane, and I get a lot of spray off the chines when running.

    Unless somebody convinces me otherwise my plan is to have the last 10" or so of the tunnel partially covered in with a plate that spans the entire width of the tunnel, but is parallel to the keel. I'll have to lower my motor about an inch to work with this setup.

    If this doesn't improve handling, I am going to try covering up the whole thing, and basically leave a 19" by 3' flat pad at the back of the hull. I would expect this to ride nicely with a prop, which I use at least 50% of the time, and also would lift the hull out of the water a little more when using the jet. Only downside would be that I'd lose some of the shallow handling of the rig.

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