13ft running skinny?

Discussion in 'Prop Shop' started by wrv993, Feb 24, 2011.

  1. wrv993

    wrv993 I Love microskiff.com!

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    Hope i posted this in the right place. Just to start out... i really dont know much about outboards, but am learning slowly... (fished most my life from yak and wading) I have a 13ft gheenoe with a 2001 15hp 2 stroke yam. stock prop. Most people know these things float pretty damn skinny and you can pole them across anything. I just feel it should run shallower. I'm def not running in a foot. Probably at 16 inches i can run. Right now i have a mini jacker and a doel fin set up. If i mount the plate up any higher, i start to get prop blow out while running WO. I was wondering if there's any thing i could do to prevent this, or just any suggestions that would help me run skinnier? I've heard cupped props help cavitation? Again, im new to this so any information will help. Thanks
     
  2. Brett

    Brett > PRO STAFF <

    Those numbers sound about right.
    The distance from the cavitation plate to the bottom of the skeg is 12 inches.
    Hull draft on plane is about the same as when at rest, figure 4 inches.
    So add 12 to 4 and you get 16 inches. It is what it is.
    If you want to run shallower, plan on spending a lot of cash.

    Welcome to the forum... ;)
     

  3. cutrunner

    cutrunner Cert. Yamaha technician

    Ohh how i want to find a 13 ft gheenoe and put front- rear decks in it with a 25 merc 2 stroke, power trim n tilt and put a tunnel in it.... Can anyone say run in 3inches? ;D ;)
     
  4. wrv993

    wrv993 I Love microskiff.com!

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    Thanks for the response. Yeah i figured there wasn't too much i could do, at least for a reasonable price. My buddies dad does have a CMC tilt and trim layin around the shop. Never heard of a 13 with one though, it seems to weigh a little over 20lbs... any ideas?
     
  5. WhiteDog70810

    WhiteDog70810 Mostly Harmless

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    The Mini-Jacker is fixed at 4" setback.  Other than that, I am basing everything else on assumptions, so I am going to ask a series of apparently condescending questions.  That is not my intent, but I need to know my assumptions are correct.  A pic from the side of your hull while floating and a pic from the side of your stern/motor/Mini-Jacker taken level with the bottom of the boat with the motor in operating position while on the trailer would be helpful.

    1.)  You have a standard shaft motor (the cavitation plate is roughly 15" below the top of whatever surface the motor is clamped to).  True or false?
    2.)  You have a standard Gheenoe 15" transom.  True or false? 
    3.)  Your cavitation plate is currently level with the bottom of the boat.  True or false?
    4.)  If all the above are true, the top of the clamping board of the Mini-Jacker should be level with the top of the transom.  True or false? 
    5.)  Your motor blows out if you move the Mini-Jacker up the transom 1 bolt hole.  You've already said this is true, but I want to clarify you are adjusting the Mini-Jacker up the transom and not the pin that adjusts the trim of the motor.
    6.)  The top of the cowling is roughly parallel with the surface of the water while on plane.  True or false?
    7.)  If any are false, please elaborate.

    If all the above are true, you will need more setback to run shallower.  You could get creative and shim what you have using 1" aluminum stock.  This has my vote since you already own it.  You also could look into other options like the Bob's Mini Jack.  It will get you a max of 6 1/8" setback for 12# of weight.  The CMC T&T will get you 5 1/2" setback for 24# of weight. 

    However, you can't get too crazy with setback without moving the COG aft, squatting down the stern and decreasing resting draft, which is more important to me than running draft.  Tuning a motor/hull combo for shallow running is a balancing act specific to your boat.  Regarding the CMC T&T, I wouldn't want to add 24# to the back of a 13 footer with a tiller motor, but since your motor is pretty light you could make it happen if you move your batteries and fuel forward.

    A trim and tilt will not get you shallower by itself.  It will help you get on plane quicker and maintain perfect trim at slower speeds regardless of the balance of the boat.  With small boats that are balanced so they float level, you don't need one as much, but they are definitely nice to have. 

    The setback is what allows shallow running because it allows the motor to be mounted higher to take advantage of the hump of water which wells up behind the hull at speed.  As a result, the cavitation plate is higher than the bottom of the hull.  T&Ts are frequently confused with hydraulic jack plates, which are a whole other creature.  The super sexy flats boats that run in inches usually have tunnels, T&Ts, hydraulic jack plates and power trim tabs.  They also have the size and speed necessary to make the system work and keep all that heavy shtuff afloat.

    If you want the shallowest performance possible with your hull, you are looking at getting a cupped prop.  A cupped prop, often with 4 blades, can be ran higher than a standard prop because it can work in aerated water.  However, even cupped props still have to be far enough under the water surface to avoid blowing out.  You'd have to talk to someone who knows props better than me to figure out the appropriate pitch to start with.

    All that being said, I have a suspicion that your problem is that the 15 HP is too much motor to maximize shallow running in that hull.  The faster you move, the bigger the hump behind the hull, so hypothetically you should be able to run shallower.  However, as speed increases, that bigger hump gets further and further from the hull, so you need more and more setback take advantage of it.  You can only increase setback to a certain extent on a 13' hull that is engineered for a 5 HP motor until you will damage the transom (longer lever arm, increased torque, etc, blah, blah, blah).  That 15 HP probably makes that hull scoot and at WOT, your dinky little 4" of setback doesn't amount to nothing.  The hump is probably 6-8" further back.  At your slowest planing speed, you probably can set the plate 3-4 bolt holes higher and motor along pretty easily.  Once you hammer it, your prop ventilates.

    Nate
     
  6. cutrunner

    cutrunner Cert. Yamaha technician

    13ft, 12 inches of setback, 25 hp, 15 pitch cupped and "worked" prop. I run with my buddies pathfinder tunnel..[​IMG]
     
  7. WhiteDog70810

    WhiteDog70810 Mostly Harmless

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    Cut Runner,

    With 12" of setback and using a cupped prop, how high is your cavitation plate above the bottom of your hull?

    Nate
     
  8. cutrunner

    cutrunner Cert. Yamaha technician

    Bout 2.5 inches maybe a tad more
     
  9. Brett

    Brett > PRO STAFF <

    I'm bored and feel like fussing... ;)

    Do you really think the cav plate is 2.5 inches above the bottom of the hull
    when running on plane? Gotta remember to compensate for running angle!

    I know...picky, picky, picky! ;D
     
  10. cutrunner

    cutrunner Cert. Yamaha technician

    Lol yep! My good friend helped me set up the running heights and angles. He has a 112mph allison that he built amongst many other boats, so i took his word. He said measuring height from the bottom of the boat cant be done with a straight edge due to hook, and hull indifferences. He came with all knds of angles and levels and was measuring the ground.. At that point i went inside lol. But thats what he said it came out to. And when im running and look back the antiventilation plate is clearly dry and out of water while undr speed. Reason we got so into it was he was really setting my boat up to run on a 18 pitch ron hill chopper.. But it only turn up enough rpm for that prop when completely empty.
     
  11. cutrunner

    cutrunner Cert. Yamaha technician

    And yes i know its on kill.. :D probably not getting enough water.. Blah blah.. If its not pretty flat, even all the way trimmed down it will suck air
     
  12. WhiteDog70810

    WhiteDog70810 Mostly Harmless

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    Cut Runner,

    Sounds like you've got your rig dialed in tight. However, would you recommend APflyFL hang a 12" setback T&T unit with a 15 HP motor on the back of a stock 13' Gheenoe? I was guessing it would take around 10-12" of setback for him to see a significant decrease in running draft using that motor, so he would need a similar set up to what you have. I don't think a stock 13' Gheenoe is built to withstand that. I would want add stringers and beef up the transom.

    Nate
     
  13. cutrunner

    cutrunner Cert. Yamaha technician

    I wouldnt recommend it.. If shallow is what he has to have to get to where he wants to be he could look into one of tjose TomC jackplate/trim n tilt built into one. Now that would be the way to go power trim and power jackplate built into one small light package but at about $700(i believe) it depends on if hes got a fire burnin in his pocket... Or for waaay less than 700$ and some elbow grease he could convert the boat to a tunnel, and while doing it add some strength and customization and run where people in kayaks dream of getting. ;D
     
  14. Flyline

    Flyline Won &quot;Do More With Less&quot; Award!

    y'all wasting time! ;D

    for a 13ft gheenoe running shallow all you need is a adjustable trim tabs from TomC, then get a tiller extension to move your butt forward, add a caviation plate and cupped s.s prop. the trim tabs help your gheenoe running more level with stern lift and channel the water higher to the propeller. Caviation plate helps the water in and around the propeller without blowing out.

    on my 15.4 highsider set up with tabs, jackplate, caviation plate, tiller extension, grab bar, and battery forward with fuel tank. it can run 10" of water at the most without touching the bottom. ;)
     
  15. WhiteDog70810

    WhiteDog70810 Mostly Harmless

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    APflyFL,

    You mentioned you were new to this, so...

    A cav plate like Whitesnooky is suggesting: http://www.stiffypushpoles.com/shawwing.htm

    Your Doel Fin mounts on the lower unit's cavitation plate also, but it is designed to give you a better hole shot and keep the boat on plane at slower speeds not prevent cavitation. Notice how the edges of the linked cavitation plate cup down and contain the "hump" around the prop. It will allow you to mount the motor a little bit higher, but you need a cupped prop to fully take advantage of it.

    As everyone has mentioned, your hull needs to float pretty level, even when you are in the back steering it. Fuel and batteries are often moved forward to balance it out and a tiller extension allows you to move yourself forward. This allows you to stay on plane with less downward trim of the motor. Since the motor is not trimmed down so far, you can run shallower.

    Whitesnooky is right. Tabs are awesome. I didn't give them much thought. They trim the hull separate from your motor. This allows you to run your motor at perfect trim (cav plate parallel with the water surface). This is your motor's shallowest operational depth. You will not only run shallower, you will also run a bit faster (all force directed straight back with no wasted upward vector and less lower unit drag). Tabs also channel water to the center making the hump a little bit bigger and closer to the stern so you can reach it with less setback. Hydraulic tabs are the gold standard, but for small skiffs many guys use manual tabs and set them once at "perfect trim" and then distribute the load to keep the hull within operation parameters. There are some excellent pics on this forum somewhere, but I can't remember whose Gheenoe it was. Manual tabs are very reasonably priced. They are definitely worth serious consideration.

    The cool thing is that you can slowly add mods over time. You don't have to dump all the money in at once. Start out by moving your fuel and batteries around (cheap), add a little more setback (fairly cheap-reasonable), a tiller extension (reasonable) and cup your factory prop (reasonable) . Dial in your motor mount height and see what you get. Add tabs later (reasonable-expensive). Tweak your setback and height. Get a cav plate (reasonable). Dial in you motor mount height again. Then find the perfect cupped prop to make it really sing (expensive). I feel like the guys that slowly tweak their rigs over time are the ones that really have the dialed-in rigs.

    Nate
     
  16. WhiteDog70810

    WhiteDog70810 Mostly Harmless

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    I found the manual trim tag pics!  I had to save Brett the work.  This whole rig is gorgeous.

    http://www.microskiff.com/cgi-bin/yabb2/YaBB.pl?num=1232931658/0

    Note the Bob's Mini Jack.

    The transom on a 13' is wider than a NMZ, so you could mount wider tabs, but TomC probably has the best input regarding optimal size.

    Nate
     
  17. wrv993

    wrv993 I Love microskiff.com!

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    Thanks everyone i appreciate all the input. I have looked into the manual tabs and cav plate some time ago, and it seems like the best route to go, and that cupped prop... as soon i get that extra money i can spend. Slowly i hope to build this little boat. Me and my buddy plan on doing some fiberglass work soon just strengthening the hull, putting some hatches, and maybe a small livewell. Don't really have many pics, but heres what I am working with so far...[​IMG]

    [​IMG]
     
  18. Brett

    Brett > PRO STAFF <

    Get a 14' aluminum jon and bolt this on the back.

    ;)

    Then go play in the shallows...let us know how low you can go...
     
  19. cutrunner

    cutrunner Cert. Yamaha technician

    Thats a nice 13'!! Like the setup. Ultimately all the answers from everyone were right. Tabs, trim, prop etc etc. Im still not fully sold on dol-fins tho... I had a customers dinghy 14ft and it had 60 yamaha 4 stroke. Big motor right? (yes newer dinghys are heavy)Well he had us service it and we were talkin to him an he was complaining the boat was so slow. Well we gave it a quick check and ran it.24mph the motor itself ran fine. I was curious so i took the dolphin off and gods honest truth picked up 10 mph. He was ecstatic. Now the motor was mounte a bit low but not terribly low. I know im gonna get burned at the stake for that but oh well, i even asked one of the Yamaha engineers of N America when i was at school last november and he gave me a look an said "think about, with the million upon millions of dollars we spend in engineering, and research and development if those things worked we would make the factory anti ventilation plate that way. ". I dunno makes good sense to me, considering the lengths manufactures are going thru to be better than each other. But no matter what, if extreme shallow is your goal unless you have Bretts jet drive or a tunnel hull , about all of your prop but maaayyyybbeee an inch of the top all the way down to your skeg WILL be in the water while running and that WILL determine running draft. And i feel your pain.. When i first took my skiff to the keys i got a quick realization of how skinny i can really run without trim, tabs, etc.
     
  20. WhiteDog70810

    WhiteDog70810 Mostly Harmless

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    Doel Fins have been getting a bad rap for years because they add a lot of drag. A 10 MPH difference is pretty spectacular. Did the customer have power T&T? We actually had one on our 70 HP Yamaha until it fractured the factory cav plate . It was great for getting the bow down, popping out of the hole and planing as slow speeds. I know it slowed the boat a bit on the top end, but we had T&T, so it was minimal. I think we gained 3-4 MPH on the top end when we took it off. The Doel Fin does not prevent cavitation when running a cupped prob on a jacked motor.

    Nate