Transom Improvements for rough water

Discussion in 'Powerboats' started by DickT, Jul 13, 2014.

  1. DickT
    Joined: Apr 2008
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    DickT Junior Member

    Old 14' Aluminum 4' beam at transom runs great on plane in smooth water and mild chop. Ventilation plate to hull bottom angle is 4 degrees.
    -In rougher water it will occasionally suck air.
    -When I slow down because of pounding I lose efficient plane.
    -No wake displacement speed is pretty pitiful.
    I realize all this is to be expected given what I've got, but wondered what kind of hull extensions, trim tabs, interceptor, etc might help. I've got a piece of 16 gauge stainless 11"x10' I'd love to weld into something.

    Thanks, I promise to clean it up when it's running right.
     

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  2. tom28571
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    tom28571 Senior Member

    Couple of questions,

    What is the trim angle of the boat with you on the tiller going from idle to plane?

    Is the boat loaded at all up forward?

    What is the reason you are setting the motor tilt at 4 degrees up angle and have you tried other tilt angles?

    What is the relative vertical distance from anti ventilation plate and transom when the motor is vertical?
     
  3. DickT
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    DickT Junior Member

    Thanks for the quick reply.

    What is the trim angle of the boat with you on the tiller going from idle to plane?
    -Not more than 10-15 degrees at the most. No porpoising unless I'm alone and way
    aft.

    Is the boat loaded at all up forward?
    -I have extended the tiller 27" over stock, sit on a board between the two aft thwarts, have the fuel tank right about midships. My wife sits in the bow when it's calm and back one seat to raise the bow a bit when its rough.

    What is the reason you are setting the motor tilt at 4 degrees up angle and have you tried other tilt angles?
    -I've been through them all and its most happy there. It's the fifth out of six holes out. One hole towards vertical has the plate parallel to the bottom. It's a little squirrelly there at high speed and gets more spray.

    What is the relative vertical distance from anti ventilation plate and transom when the motor is vertical?
    -In that position the top of the plate is even with and parallel to the bottom of the hull.

    At full throttle 25-30mph the boat requires concentration and a good grip on the tiller (I loved that 50 years ago). The extension helped a lot. Backing off to two thirds throttle it holds trim nicely but is too fast for chop. Boat's a little small for Lake Champlain but we stay close to shore.
    __________________
     
  4. daiquiri
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    daiquiri Engineering and Design

    Ehm... The attached drawing shows a boat at 15° trim. Are you sure about your data?
     

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  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Your engine will "suck air" when it is a little rough because there is no deadrise in the bottom that would tend to push it (aerated water) away to each side. Not much you can do, except possibly lower the engine on the transom, which could create problems of it's own. When you slow any planing boat down because of rough water, efficiency usually suffers. Not much you can do there either ! Your no-wake speed is also not something you can alter. So, your boat is just doing what boats like that do. There is no fix that will turn it into something it isn't. Except possibly alter the disposition of weights in the boat, but even then you are limited, you cannot expect the wife to suffer a harsh ride at the bow because it improves trim ! A little boat has little boat limitations, and you basically have to live with them, or get into something larger, you will not turn a VW beetle into a Cadillac by any tinkering, you just have to accept that comfort in boats costs money, especially planing boats.
     
  6. tom28571
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    tom28571 Senior Member

    Dick,

    Anything over 5 degrees is too much on a boat like that. A deep V runs higher but that is definitely not one of those.

    When you say spray, is it spray that curls over the side or is it thrown out and blown back. The spray rails on the side are supposed to stop the first which curls up along the round bilges and nothing stops the second, other than a cabin, especially on a course that is even a point or so off the wind. The boat is probably going to be a bit wet in any kind of a chop.

    If those Tohatsu horses are real horses andthe boat is set up right, speed in the upper 20's is possible but my guess is that it is less than that now.

    Squirrels are dizzy creatures on the loose and it's not nice to have a boat with those characteristics. The lack of a keel, if there isn't one, could be the cause of that. Probably all aluminum boats have bit of keel just for strength at the joint.

    I agree with what Mr Efficiency said other than the V bottom bit which I don't find to be true at all.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Having vee in the boat bottom puts the outboard deeper, and the deeper it is, the less chance the prop will "let go" due to aerated water. Bubbles rise. I have had aluminium boats that were pretty well flat at the centreline aft, and with swagings in the bottom, that would channel aerated water into the prop stream, racing the engine, this would be especially noticeable when running with chop, when you ran through a "white cap". Dropping the engine height fixed that, but slowed the boat and generated spray off the leg.
     
  8. DickT
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    DickT Junior Member

    Thank you all for your comments and my apologies for my off the cuff angle and speed estimates. I may have hit over 10 degrees in some porpoising episode but half the transom immersed and half the keel out of the water would be about 7. This may be the case getting on plane. When planing I would guess I'm at 4 degrees degrees because that's the difference between the hull and ventilation plate at the best trim setting. I did see 27mph on my GPS once, but shouldn't make such claims. The spray is as Tom says thrown out and blown back.

    Now suitably humbled I can say you've confirmed what I suspected and apparently there aren't any magic fixes. I think I'm managing weight distribution OK. I have a Doel fin but don't like it and am assuming there's not much to gain with trim tabs, hull extensions, or interceptors. Would you recommend a small skeg or parallel skis to improve tracking? Would adding to the chines help lift?
     
  9. tom28571
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    tom28571 Senior Member

    My poor brain read your comment opposite to what you meant. My apologies. However, I have never had that problem with flat bottom boats aerating the prop either. I can see your reasoning but have not had that experience.
     
  10. HJS
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    HJS Member

     
  11. DickT
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    DickT Junior Member

    I think as the center of the hull is raised by a crest of the short chop the bow drops into a trough and raises the prop in the trough behind. It behaved worse with the Doel fin. I'm speculating that as the fin rises it creates negative pressure around the prop.

    Tom and others do a good job of explaining how a well designed hull functions through the speed ranges. I've read some of your posts and have seen your website describing your experimental work. Would be curious if anything could be added on to my hull as I'm stuck with it and don't mind tinkering.
     
  12. HJS
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    HJS Member

    [
    Tom and others do a good job of explaining how a well designed hull functions through the speed ranges. I've read some of your posts and have seen your website describing your experimental work. Would be curious if anything could be added on to my hull as I'm stuck with it and don't mind tinkering.[/QUOTE]

    I've tried Doel Fins and found them insufficient for this purpose.
    I would try a delta-shaped wing that extends to the transom.
    I would make it slightly larger than necessary to begin with and then cut it down until I'm satisfied.

    js
     
  13. DickT
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    DickT Junior Member

    Do you mean a larger version of a Doel fin attached to the lower unit?
    I'd rather attach something to the hull.
     
  14. tom28571
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    tom28571 Senior Member

    One thing I notice is that there appears to have been quite a bit of modification around the sheer and across the transom. Maybe done to stabilize a less than rigid structure of the aluminum boat with no side deck. What has this done to the transom height?

    Lowering the motor in steps while diddling with the tilt may lead to an acceptable result. That is, lowering the prop may allow making the tilt more vertical, This may maintain the same trim and prevent ventilation at the same time. That is, assuming the current trim of the boat is acceptable.

    The Duel Fin basically lifts the stern and that brings the prop closer to the surface and may lead to worse ventilation, which is what you experienced.

    In other words, play around with it and see what happens. Best way to learn and probably better than listening to us pontificators.
     

  15. DickT
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    DickT Junior Member

    I added the sheer strake and outboard well and stiffened up the transom several years ago.
    I set the motor height with the plate even with the hull bottom. Haven't used the boat that much since. I can easily cut it down and add back in if I go too far. I think Ill take an inch off and see what happens.

    I enjoy reading others opinions. It expands my understanding
     
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