Project 15' Skiff

Discussion in 'Boat Building' started by saqa, Oct 14, 2013.

  1. saqa

    saqa Jonny TooBad

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    Madday, Its all pangas here. either ply or copies of the old Yamaha glass. Not bad boats at all. Just not all that well suited for standup fishing like heavy popping as they not that much wider then my boat. Next build will be a 30' cat with 14' beams, dedicated GT chaser :)

    Mate she is only 1.1m waterline beam and the heel is as I had expected it to be from the calculations when designing the hull. My 100kg leaning on the gunnel is actually not bad for 1m beam as it doesnt get into the tipping zone. I am thinking that a pair of pods will totally eliminate that if required an dprovide other benefits like increased planing surface. I am currently playing with adding the pods on as a stabilised monohull platform

    Pods like these, fitted using wings like the second pic instead of outrigger types beams. Dennis suggested full length pods which I will look at closely. A converting the boat to a tri like this should let me run around the boat without heeling it much
    [​IMG]pwr tri 35 p2 by jonny.toobad, on Flickr
    [​IMG]00_FF_Custom_34_AtSpeed by jonny.toobad, on Flickr

    I dont know if I actually need to go to the bother. As is the slender hull works well with the small outboard while underway. Once I hook into some decent fishys then will learn of any shortcomings
     
  2. madday

    madday Fishing Guru

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    just one suggestion,
    why not adding pair of retractable outrigger pods to the panga? you can retract it while otw to fishing ground so it will not retard the panga maneuverability, and deploy it once fishing... plus it will be easier to tow...

    here, some big pangas were modified that ways, without retractable feature though.... :)
     

  3. saqa

    saqa Jonny TooBad

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    G'day fellas
    Spent yesterday trouble shooting the motor. Changed out the spark plugs. A fellow helped adjust fuel flow and that. Left the motor running while I did some other work on the boat at the mooring. Took the canopy off, moving around the boat is easier now. I dont have to lean on the gunnels to get around the console. Changed out the steering cable with thicker diameter jobs. By the time I finished all that the motor had smoothed up a lot. She does need a service. Starts easy now though

    Took the boat out after midday today. She runs well. Did a few laps of the bay. Towed a lure around a bit but that was boring, I was keen to just hoon around and learn the boats ways a bit. Standing up and driving at trolling as full throttle was easy. The chop was up again today and I was pretty comfy. That might change with bigger hp though. The boat feels like she is begging for more power

    Have fishing session planned for tomoz. Hopefully all be good and I get to throw some poppers around if feeling active or just sit on anchor a bit soaking bait
     
  4. Popfly

    Popfly Junior member

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    Saqa,

    After re reading this, if you get your hands on the 25 you are going to really move with your boat. You already get on plane nicely with the 8HP & the 25 will make you really move! I am really impressed with your fabrication skills as you have created a totally useable craft with minimal resources. You better start cleaning out the in laws barn to build the Cat...:)
     
  5. saqa

    saqa Jonny TooBad

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    She has been blooded :)
     
  6. saqa

    saqa Jonny TooBad

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    Madday, if I was already stuck with pangas I would be looking that route. I just have this idea stuck in the head of a 30 x 14' fishing deck, extremely fuel efficient at around 25 knot top speed. Be a fun project some time in a near future

    Thanks mate. I reckon even a 15hp will be more then plenty and maybe quite efficient too. Looking at most brands, seems the 8, 10 and 15hp units share the same motor with just different carbies. Motor weights and displacements seem the same in their own brands. With a 15hp I would have the same weight as now but almost twice the power which would mean improved power to weight ratio
     
  7. saqa

    saqa Jonny TooBad

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    Dennis mate
    Does having a small amount of rocker mean a nicer cruise in the 3-6knot trolling range in chop? Will having lifting foils on the cav plate stabilise the pitch from the rocker and offset it for raising throttle further and getting on the plane? This is some bit of logic stuck in my head that might be totally incorrect. Can it explain why a small slender craft can just glide through without feeling any bother?
     
  8. DenisB

    DenisB Senior Member

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    Rocker allows the boat to change its vertical attitude more easily to ride over waves/chop & down the other side without digging the bow or stern in heavilly.
    A keel without rocker typically has a stiff action.......... one with rocker has a looser action.

    Rocker causes the bow to rise higher at speed . Highspeed hulls have a straight keel & a fine entry bow with progressive vee flattening towards the middle/stern.

    Having a cavitation plate foil will counteract the effect of rocker ( but its a wetted area thing ............foil area vs hull ) the most significant effect of cav plate foils is stopping the bow from bouncing in chop................ much less effect in swell. (one could call this an inertial effect ...........where small higher frequency impacts are insufficient to overpower the inertial stability of the hull weight/ momentum & cav plate forces ...............but larger more infrequent wave events can.).

    The relationship between wave pitch & vessel length determines the type of ride.
    - vessel length significantly less than wave pitch allows boat to ride with the swell in a smooth curve.
    - vessel length nearing equal length to wave pitch causes bow to crash into each approaching wave front as its still coming down the back of the last wave as the bow encounters the front of the next wave.( vessel bow momentum is opposite to wave forces( = crash ))
    - vessel length significantly longer than wave has a fairly constant path with multiple waves sliding along its hull length....................the downside of this situation is that the hull must be rigid enough & strong enough to support itself above the trough of the waves as they pass down the boat length. & take the strain as wave peaks hold the boat up in only a small proportion of the boat length ( when the swell is passing the middle of the boat).
    It is this effect that causes large shipping to fail in cyclonic/hurricane conditions with high swell height & long pitch, causing cyclic loads from bow & stern supported on successive waves & then vessel centre riding on a swell peak in the middle.............with modest length recreational vessels this situation is replicated in the poor ride experienced in "short chop" ( typically experienced in offshore wind conditions where the wind has only a short fetch to generate a wave pattern with a relatively short pitch /wavelength ).

    Hopefully that helps you understand the conditions when the boat length produces a smooth gliding ride relative to the wave pitch.

    FWIW
     
  9. saqa

    saqa Jonny TooBad

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    Cheers for that mate, I am trying to understand if what I put into the skiff was wasted and flawed logic or if that is what is making it perform so. When I first approached you for assistance with the light power/rowing skiff I could really imagine your advice on having some rocker and keeping it slender would work with water. I read somewhere about having to keep rear straight to plane properly and elsewhere good sea keeping and slow speed performance of slender displacement hulls. I thot that a stingray type job on the motor might help like a spoiler types job ploughing water both above and below the flat surface keeping the boat level on the water under power and at whater top speed I could coax with the motor. Thot I might give it a go since I prolly wouldnt plane with the 8hp I had anyway I might go for some comfy cruise speed

    After cutting the ply to the final CAD design I posted I kept on thinking back to the small motor and the regular choppy conditions in the bay here. So I took out the cable ties and cut it again for a waterline beam of 1.1m and a sharper and longer blade at the bow as well as modified the forward frames for a concave flare from the edge of the blade to chine. I put 30mm of rocker in the middle of where the flat part of the keel used to be. Instead of fitting the motor to the transom

    The boat is total length 4.3m, waterline beam 1.1m. Had to really torture the 9mm ply with quadrupled binding wire twisted with a long nail to bring the sheet to meet the forward frame

    The performance I have had so far. I am 95kg I can stand behind the boat in knee deep water and place my left foot on the left boarding step and then right foot on the middle of the deck with my right hand on the motor cowl. She doesnt heel enough to pose any danger of me ending the drink. I think the foil on the cav plate is helping a bit there but I dont know for sure

    The boat just glides through the chop which has been getting over a foot in the middle of the afternoon. Just like a ocean kayak there is no crashing or impact on hitting the face of the chop at any of the speed I have driven so far with the 8hp. With just myself the best spot is trim down one notch, with one passenger up front its motor hanging level trim. With best the 8hp can deliver, the water being cut by the keel at the bow flows back, and rolls down like soil on a plough. Today we went out and crossed a bit of the white stuff to get to the closest broken bommie grounds in about 2m of water. We were heading up wind at about 45 degree to the wind at full throttle I was copping a touch of spray blowing in at the drivers position. Mrs and bubs were dry. The lil one was enjoying it and relaxed. Every chop I would cringe in anticipation but it just flowed along and glided along I didnt stay out in the channel for too long with family onboard. Came back in the lee of the hilly shoreline and back towards the mooring. I made a mistake when I let go of the wheel to fit a bomber to the split ring and looked up to see that the boat had turned minus 90 degrees. I righted it quickly without throwing the girls off balance. Mrs had the camera going at that moment and here is a short vid showing the speed I was going and the turn. I didnt adjust throttle, boat washed off some speed in the turn and quickly picked it back up moments after the end of vid

    https://flic.kr/p/pK3yKq

    I really want to work out if the changes I made to the hull before gluing are the reasons for the positive behavior. I have been in smaller and larger regular planing hulls in similar conditions and it wasnt all that comfy. I want to make a displacement cat next and wondering on the use of a large stingray shape foils
     
  10. madday

    madday Fishing Guru

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    the little one looks litle bit scared.... now you really need bigger boat my man.... :D
     
  11. DenisB

    DenisB Senior Member

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    Rohit
    As above
    "Rocker allows the boat to change its vertical attitude more easily to ride over waves/chop & down the other side without digging the bow or stern in heavilly.
    A keel without rocker typically has a stiff action.......... one with rocker has a looser action.

    Rocker causes the bow to rise higher at speed . Highspeed hulls have a straight keel & a fine entry bow with progressive vee flattening towards the middle/stern."

    What adjustments you did before gluing made the hull more sea-kindly at moderate speeds................no doubt about it.
    A flat keel would have made the boat action much stiffer.

    at higher speeds the boat will ride more bow-up. ............it will still ride quite sea-kindly in conditions other than "short chop" & no boat handles that particularly well ( when wave pitch is close to boat length).

    The cav plate hydro will hold the bow down & make the boat follow the swell contour ...............whether the keel was straight or has rocker.
    BUT
    it can only hold the bow down under modest wave impacts....................larger wave impacts will over-ride the stabilising effect of the cav plate hydro............but the hydro will reduce the bow lift under impact.

    Have no doubts that what you did before gluing has improved the ride, not just luck etc, was not wasted & was good logic.............it was the right move & especially with 2 lads in it with gear totalling (say) 200kg in that 4.3M..................its going to be sitting deep in the water with a fair bit of inertia ................that 30mm of rocker is going to quickly be your best friend.
    As an aside ...............
    driftboats have lots of rocker ( lots & lots) to deal with pressure waves in rapids..................which present to a hull like "short chop " on the ocean or estuary..............with the pressure waves in rapids being a lot steeper , hence the need for lots of rocker.

    FWIW
     
  12. saqa

    saqa Jonny TooBad

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    Thanks for explaining that mate, much appreciated. I am trying to determine what speeds I am getting with the 8hp and whether if I am actually getting on the plane or traveling efficiently in displacement mode. I dont have a GPS. I dont think I am getting a bow wave at full throttle and with motor trimmed correctly, I dont have much in the way of wake or wash. With the trim I ran on first launch I had quite a bit of wake

    What I am trying to make sense of is if I am actually running in semi displacement with wetted area reduced by the lift of the foil. What clues should I be looking at?
     
  13. saqa

    saqa Jonny TooBad

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    Madday, she lets us know if she is scared mate :) She gets that look of intense concentration and when you let her be she tries to do what we doing. But one always needs a bigger boat :D
     
  14. DenisB

    DenisB Senior Member

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    the main clue is clear water separating from the transom . At just barely planing speed this will not be constant as the 'tumbledown' of the wake will roll forward in a cyclic pattern to meet the transom..................as speed increases the tumbledown can't roll forward to reach the transom & then stops tumbling & becomes the classic highspeed V-wake ( that is a consequence of the boat rising in the water as the speed increases)

    @ post#123...............pics 012762, 763, & 765 seem to indicate reaching planing speed with 1 up.
     
  15. madday

    madday Fishing Guru

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    cant agree more... :D
    so when the catamaran project will start...?
     
  16. saqa

    saqa Jonny TooBad

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    Thanks Denis, looks like I am just getting on the plane. I have managed to source a 10hp carby for the same motor. Will try to get it serviced and fitted

    Madday, within one year mate ;)
     
    madday likes this.
  17. saqa

    saqa Jonny TooBad

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    Hi fellas, unfortunately I had to pull the boat out and put it in the shed for now. The stupid old 8hp Mercury has developed another prob where it will run for a while and then lose revs to almost idle. Looking at buying a new 15hp for it in the near future. Design work on the cat is just starting, will post up some lines for it soon. I still am learning the design part and have a few small issues to sort out as I currently do not know enough about certain aspects